Manual de Destilacion

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The Distillation Group, Inc. Technology in D istil lat ion

P.O. Box 10105, College Station, TX 77842-0105 USA Phone 979-764-3975

[email protected] Fax 979-764-1449

Distillation Operations Manual

This public document has been converted to HTML format by Andrew W. Sloley.

Converted document Copyright © 2001 Andrew W. Sloley.All rights reserved.

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Distributed by

The Distillation Group, Inc.P.O. Box 10105

College Station, TX 77842-0105USA

[1]-(979)-764-3975[1]-(979)-764-1449 fax

[email protected]

www.distillationgroup.com

mailto:[email protected]://www.distillationgroup.com/http://www.distillationgroup.com/mailto:[email protected]


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The Distillation Group, Inc. Technology in D istil lat ion

Conversion Notes

The original document was Energy Conservat ion Seminars for Industry: T exas

Energy Conservat ion Program: D isti ll at ion Column Operat ions by J. E Sirrine Company.

Within the confines of H TM L, the text has been converted to an approximation of

the material. At some points it is unclear in the original document if a typographic

change was made to organize the text or to simply make text fit better on a page. As

closely as possible, the organization of the material has been maintained. Very few

corrections have been made to the original text, even where errors may be present.

The intent has been to maintain the original document.

No warranty is made as to the accuracy of the material, the conversion to

electronic form, or to the applicability of the techniques discussed to any given plant.

Editorial comments added to the text are shown with text in [ it alics] .


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Energy Conservation SeminarsFor Industry

presented byThe Energy Utilization Department

of theTexas Industrial Commission

410 East Fifth StreetAustin, Texas(512) 472-5059

Gerald R. Brown, Executive Director Lance E. dePlante, Manager

Energy Utilization Department


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The information presented herein isintended to enhance knowledge of industrial energy conservation andto provide the necessary tools toimplement an energy conservationprogram in an industrial plant.References to specific products or

ideas should not be consideredendorsements of said products or ideas by the Texas IndustrialCommission.

This workbook and other projects of the Industrial Energy UtilizationDepartment are funded through a

U.S. Department of Energy grantadministered by the Governor’sOffice of Energy Resources.


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TEXAS ENERGY CONSERVATION PROGRAM

DISTILLATION COLUMN OPERATIONS

Prepared By

J. E. SIRRINE COMPANY Houston, Texas

For

TEXAS INDUSTRIAL COMMISSION

FUNDED BY GRANT FROM THE GOVERNOR'S

OFFICE OF ENERGY RESOURCES

THROUGH THE DEPARTMENT OF ENERGY

1978


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DISCLAIMER

These materials were prepared as a result of work

sponsored by the Governor’s Office of Energy

Resources through funds provided by the Department

Energy. Neither the Texas Industrial Commission,

nor the sponsoring agencies, nor any of their

employees, nor any of their contractors, subcon-

tractors, or their employees, makes any warranty,

expressed or implied, or assumes any legal liability

for the successfulness of the implementation of energy

conservation techniques described. References to

specific ideas, products, and services should not be

construed as endorsements. It is hoped that the infor-

mation provided through these materials will be useful

in your efforts to explore opportunities available for

energy conservation.


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TABLE OF CONTENTS

ENERGY CONSERVATION MANUAL

DISTILLATION COLUMN OPERATIONS

PAGE

TITLE

DISCLAIMER

TABLE OF CONTENTS

ABSTRACT

LIST OF TABLES

LIST OF FIGURES

SECTION 1 - INTRODUCTION 1 - 1

SECTION 2 – DESIGN REVIEW, AUDIT OF ENERGY AND MATERIAL BALANCE 2 – 1

A. REVIEW OF PLANT DESIGN 2 – 1

B. AUDIT OF ACTUAL PLANT OPERATION 2 – 1

C. DATA COLLECTION DURING PLANT OPERATION 2 - 3

SECTION 3 – ENERGY SAVING IMPROVEMENTS WITH MINIMAL CAPITAL IN-

VESTMENTS 3 – 1

A. OPERATING PROCEDURE REVISIONS 3 – 1

(1) Reducing the Reflux Ratio of Columns 3 – 1

(2) Lowering Product Specifications 3 – 3

(3) Lowering Pumping Costs 3 – 4

(4) Lowering Steam Usage 3 – 9

(5) Process Heaters 3 –11

B. SCHEDULING SHUTDOWNS TO MAXIMIZE ENERGY RECOVERY

AND PROFITS 3 –13


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PAGE

SECTION 4 - ENERGY SAVING IMPROVEMENTS WITH CAPITAL INVESTMENTS 4 - 1

A. OPTIMIZATION OF HEAT RECOVERY - HEAT EXCHANGERS 4 - 1

B. COLUMN REVISIONS 4 - 5

(l) Additional or More Efficient Trays 4 - 5

(2) Additional Column Draw 4 - 7

C. OPTIMIZATION OF RECOVERY AND USE OF ENERGY 4 - 7

(1) Introduction 4 - 7

(2) Column Heat Utilization 4 - 9

2.1 Bottoms Product 4 -10

2.2 Distillate Product 4 -11

2.3 Condenser Duty 4 -11

2.4 Reboiler Duty 4 -11

2.5 Feed Preheating 4 -12

(3) Changing the Column's Temperature 4 -12

(4) Two-Stage Condensation 4 -12

(5) Waste Heat Boilers 4 -13

(6) Multiple Effect Heat Cascading For

Distillation Columns 4 -13

(7) Split Tower 4 -15

(8) Interreboilers, Intercondensers, and

Feed Preheating 4 -19

(9) Feed Preheating 4 -21

(10) InterreboiLER 4 –21

(11) Intercoolers and Feed Precoolers 4 –21

(12) Circulating Refluxes 4 –22


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PAGE

D. USE OF VAPOR RECOMPRESSION AND HEAT PUMPS FOR

DISTILLATION 4 -24

(1) Introduction 4 –24

(2) Distillation Column's Reflux and Heat Balance 4 –25

(3) Vapor Recompression 4 –26

(4) Heat Pump 4 –27

(5) Theory Behind Vapor-Recompression and Heat Pumps 4 –28

5.1 The Carnot Cycle 4 –28

5.2 The Refrigeration Cycle 4 –33

(6) Vapor Recompression 4 –37

6.1 Situations 4 –37

6.2 Auxiliary Heat Transfer Equipment 4 –38

6.3 Compressor Drives and Their Energy Costs 4 –40

6.4 Insulation of Columns Using Vapor Recom-

pression or Heat Pumps 4 –41

6.5 Vapor Recompression for Interreboilers,

Other Columns 4 –41

(7) Reasons For Conversion of an Existing Column 4 –42

(8) Conversion of an Existing Column 4 –43

(9) Advantages of Vapor Recompression 4 –44

(10) Disadvantages of Vapor Recompression 4 –46

(11) Advantages and Disadvantages of the Heat Pump 4 –49

(12) Guidelines for Considering Vapor Recompression 4 –50

(13) Procedure for Vapor Recompression Evaluation 4 –51

(14) Example, Propane-Propylene Splitter 4 –54

14.1 Situation Statement 4 –54

14.2 Solution 4 -55


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PAGE

(15) Work Problem Propane-Propylene Splitter With

Bottoms Vapor Recompression 4 –60

E. IMPROVING CONTROL OF DISTILLATION COLUMNS 4 –61

F. REDUCING HEAT LOSSES USING INSULATION 4 –64

SECTION 5 – ECONOMICS 5 - 1

A. DEFINITION OF ECONOMIC TERMS 5 – 2

(1) Profit 5 – 2

(2) Net Back 5 – 2

(3) Depreciation 5 – 3

(4) Investment Tax Credit 5 – 5

(5) Fixed Costs 5 – 5

(6) Variable Costs 5 – 6

(7) Cash Flow 5 – 6

(8) Discounted Cash Flow 5 – 6

(9) Return on Investment (R.O.I.) 5 – 7

B. CONCEPT OF INVESTMENT EQUIVALENCE TO SAVE ENERGY 5 – 8

C. ECONOMIC INTERPRETATIONS FOR ENERGY SAVINGS 5 – 9

D. STEAM ECONOMICS 5 –11

E. COOLING WATER 5 –13

F. COMPRESSED AIR 5 –14

G. VACUUM PUMPS AND STEAM EJECTORS 5 –14

H. EXCHANGERS USED FOR HEAT RECOVERY 5 –15

I. CONCLUSION 5 –15

SECTION 6 – BIBLIOGRAPHY WITH ABSTRACTS 6 – 1


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PAGE

SECTION 7 – APPENDICES 7 – 1

A. ENERGY SAVINGS CHECLKIST – GENERAL 7 – 1

B. PROCESS ENERGY CHECKLIST 7 – 6

C. REFERENCES – TECHNICAL ARTICLES 7 –10

D. SOLUTION TO WORK PROBLEM 4-F-15 7 –14


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ABSTRACT

Distillation operations have been branded as high energy users. An estimate

3% of the total energy used in the United States in 1976 was for distillation.

Energy conservation is indicated. This manual is addressed to the small or medium

sized chemical or refining company. It is structured to guide these people on how

to analyze and reduce energy requirements. The criteria of no reduction or

increased profitability of the process are stressed in analyzing any energy saving

proposals.

Information for writing the sections came from technical articles, design and

operating experience, and seminars on energy conservation.

This manual is divided into seven sections. The contents of the sections are

discussed in the following paragraphs.

Before any energy conservation steps can be logically taken, a knowledge of

energy usage of the existing facility must be known. Section 2 of this manual

describes a procedure for reviewing the original plant design, auditing the energy

usage as presently operated, and collecting plant data if required for the audit.

After the distillation process is analyzed for energy usage, the first step

is to study energy saving improvements needing minimal capital investments and

quickly implementable. Section 3 covers this, giving ideas on changing the

operating procedure and scheduling shutdowns to maximize profits and minimize

energy usage.

Capital investments to save energy are generally longer term projects. These

projects include the optimization of heat recovery and revisions of


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the column. Capital intensive and complex systems using vapor recompression or heat

pumps are possible energy savers. These are covered in Section 4 along with heat

losses and column control.

For distillation processes, the energy used per pound of product is a simple

ratio for evaluating the performance of the program to reduce energy usage.

Similarly, an economic guideline is helpful in requesting management to make

decisions concerning capital investments. In Section 5, the concept of investment

equivalence to save a unit of energy is developed for use as an economic guideline.

The economic interpretations of several energy savings proposals are discussed.

Potential conflicts in placing a cost value on various steam pressures by

accountants compared to its value from a thermodynamic or energy level viewpoint

are discussed.

The appendices include reprints of technical articles pertinent to

distillation columns, a general energy savings checklist, a process energy

checklist, and the results of a sample work problem on vapor recommendation.


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LIST OF TABLES

TABLE

NO. TITLE PAGE

2 – 1 Electric Motor Study 2 – 5

4 – 1 Process Data for Column Shown in Figure 4-13 4 –65

4 – 2 Process Results for Column in Figure 4-14 4 –66

4 – 3 Process Data for Column in Figure 4-15 4 –67

4 – 4 Process Data for Splitter in Figure 4-16 4 –68

4 – 5 Nomenclature of Symbols Used in Section 4 4 –69

7 – 1 Results for Splitter in Figure 2-1 7 –16


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LIST OF FIGURES

FIGURE NO.

NO. TITLE PAGE

2 – 1 Feed Fractionator with Preheat 2 – 6

2 – 2 Depropanizer Unit 2 – 7

3 – 1 Centrifugal Pump Characteristics and System Curve 3 –17

3 – 2 Expansion of Pumping System 3 –18

4 – 1 Heat Availability and Requirements For Crude Tower 4 –72

4 – 2 Heat Cascading Distillation Train 4 –73

4 – 3 Split Tower Arrangement 4 –74

4 – 4 McCabe - Thiele Diagram for System with Intermed-

iate Condenser and Reboiler 4 –75

4 – 5 Recirculating Reflux or Pumparound Tower 4 –76

4 – 6 Example of Conventional Distillation Column, No

Side Draw 4 –77

4 – 7 Vapor Recompression Examples 4 –78

4 – 8 Example of Heat Pump System 4 –79

4 – 9 The Refrigeration and Carnot Cycles 4 –80

4 –10 Column Using Vapor Recompression 4 –81

4 –11 Hot Columns with Vapor Recompression 4 –82

4 –12 Refrigerated Columns with Vapor Recompression 4 –83

4 –13 Propane Propylene Splitter 4 –84

4 –14 Results of Example of Propane Propylene Splitter 4 –85

4 –15 Splitter with Bottoms Vapor Compression 4 –86

4 –16 Splitter of Figure 4 - 15 with Data 4 –87


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PAGE

4 –17 Vapor Pressure of Olefin Hydrocarbons 4 –88

4 –18 Vapor Pressure of Normal Paraffin Hydrocarbons 4 –89

4 –19 Enthalpy Temperature Diagram for Propylene 4 –90

4 –20 Enthalpy Temperature Diagram for Propane 4 –91

4 –21 Control of Column Reflux to Maximize Profit and

Energy Conservation 4 –92

5 – 1 Revenue and Expense Variation with Production –

Ideal Case 5 –17

5 – 2 Variation of Profit with Production 5 –17

5 – 3 Revenue and Expense Variation With Production –

Real Economic Case 5 –18

7 – 1 Work Problem 7 –17


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SECTION 1

INTRODUCTION

Many words and phrases may have more than one meaning. In energy discussions,

the expression “energy conservation” is presently spoken with two meanings. The

original meaning is related to the first law of thermodynamics, which states that

energy is always conserved, never destroyed, but changes from one form and level to

another. Now that the United States is no longer endowed with new sources of low

cost energy fuels, energy conservation has taken on the meaning of reducing the

amount of energy used either increasing the efficiency of performing a certain

task, or using a substitute requiring less energy. Examples of conservation are the

use of higher efficiency air conditioning units, lighter weight automobiles, and

handwashing dishes.

In the chemical industry, the meaning of energy conservation includes

conserving the temperature level of the energy and in consequent the availability

of the energy to produce work. Since distillation processes require large amounts

of work and heat energy to perform the required separations, these processes are

prime areas for better energy utilization.

Many Americans are skeptical about the United States being in an energy

crisis. They say that energy is plentiful, but have they considered the cost to

produce it? Russell E. Train, formerly administrator of the EPA, made the following

comment in an address upon receiving the $150,000 Tyler Ecology Award:

“...the artificially low prices for more conventional energy maintainedby subsidy and regulation. In 1976 the average weighted price of theindustrial use of energy per million Btu was $2.55,

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whereas the average replacement cost---the cost of finding and producing newenergy resources---was $3.74. Thus, the replacement cost of natural gas isnow more than 70% above the average price, that of oil about 45% above, andthat of electricity nearly 40% above. Only in the case of coal didreplacement cost approximate actual price. Since our political processes have

so far proven unequal to the task of achieving more economically realisticprices for energy, whether by taxes, pricing policy, or by deregulation, orany combination of these, ...”

If his costs are realistic, then the United States is living on previously

developed resources. When they are depleted, the cost of energy will soar.

If the decision is made by management to reduce the energy requirements

of the processes, it implies that long term profits or return on the company’s

investment must not decrease. This economic viewpoint is a prerequisite to the

writing of any energy conservation manual.

This manual is divided into seven sections, it is assumed that the reader has

sufficient technical knowledge to understand the principles of heat transfer,

separation operations, and thermodynamics. After information is presented on how to

conduct an energy audit of the distillation process, energy saving ideas that

require minimal capital investments are given. Similarly, ideas for long term

capital investments are discussed. Finally, economics and the concept of investment

equivalence to save a unit of energy are detailed.

The appendices include copies of technical articles pertinent to distillation

processes. It also lists ideas on energy savings in general and specific to

distillation operations. It is the purpose of this manual to aid the chemical

company in reducing the energy requirements of the distillation units without a

reduction in profitability of the process.

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SECTION 2

DESIGN REVIEW AND AUDIT OF ENERGY AND MATERIAL BALANCE

Before proceeding with a detailed energy analysis on your distillation unit

as presently operated, you should find out the energy consumption of the same type

of separation by the industry. Your sources of information are: (1) similar

distillation columns within the company, (2) contact with the original engineering

design company, (3) contact with technical people from your professional groups or

college or professional friends, and (4) the technical literature. For example,

Mix, Dweck, and Weinberg estimated and reported specific consumptions in Btu/lb of

product for various product

separations in the CEP April, 1978 issue (see Appendix 7-C). They believe that a

large percentage of the columns in operation can be retrofitted for energy

conservation with attractive economic benefits.

2-A. REVIEW OF PLANT DESIGN

Your plant engineering files should contain all the design information for

the process. If it is not available, this information should be requested from the

original design company. In particular, process flow sheets, design calculations,

piping and instrumentation drawings, specifications of the equipment purchased,

performance characteristics of the equipment, utility usage tabulations, and

revisions since the original installation are very valuable for the analysis.

Examples of process flow sheets are found in Figures 2-1 and 2-2.

Design values for fuel, steam, and electrical usage should be found on the

utility summary forms. Calculated values for specific operating conditions

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should be in the process calculations. Values for fuel and steam usage should be

indicated on the process flow sheet. For example, if the design values showed

30,000 lbs per hr. of 75 psig saturated steam to produce 6000 lbs per hr of

product, the ratio of the pounds of 75 psig saturated steam to pounds of product is

5. If the condensate is not recovered, the energy usage is (1185 - 48)5 or 5685 Btu

per lb. If a competitor operated with the same ratio of steam to product, but

recovered the condensate at 200º F, his energy usage is (1185 - 188)5 = 4985 Btu

per lb. This is an energy saving of 12%.

Specifications of purchased equipment and their performance are valuable for

any plant study. They must be used with caution because revisions may have been

made since the original installation. If the changes were not documented (not

uncommon in small plants) or simply given verbally to the present unit supervisor,

you may not know that revisions occurred.

2-B. AUDIT OF ACTUAL PLANT OPERATION

After the background information is compiled and the energy information

extracted, the present energy usage of the unit should be determined. Plant

accounting records should be checked for present and past usage of steam, fuel,

electricity, etc. This information may be reported on a monthly basis on “value

added” sheets or “production cost” sheets. All values reported by accounting should

be considered questionable until they can be verified for accuracy. Instruments may

be broken. Flow meters may measure usage for more than one unit, and the flow split

guesstimated. If the guess was wrong, the estimated values recorded by accounting

are in error and could incorrectly bias your decision on a proposed energy

conservation project.

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Plant inspections should be made of the measuring instruments. An orifice

meter may have been calibrated for 100 psig line pressure, but the actual gas

pressure found in the plant is 150. The meter’s conversion factor and reported

usage will be incorrect.

Production rates reported by accounting should be confirmed. Production

figures are based upon meter readings and/or product shipments plus storage tank

content changes. A level indicator on a storage tank may be based upon a 0.800

gravity liquid, but the actual gravity is 0.750. The production figure is not

correct.

A heat and material balance can be made of the existing operation after the

plant instrumentation has been corrected. This information will be compared with

the original design balance and other energy figures found.

2-C. DATA COLLECTION DURING PLANT OPERATION

When developing a heat and material balance for the existing operation, you

may have insufficient information recorded on daily operating and laboratory logs

to compile the balance. Since distillation units are generally well instrumented,

the only expense burdens for a plant data collection test are the manpower to

collect the data and laboratory charges to perform the analyses on the special

samples. Of course, if one flow meter measures steam usage to two different units,

an additional meter must be added to separate the units.

The degree of success of a plant data collection test is influenced by

the preparation and planning stages. Step one is to list the data required for

calculating the heat and material balance. Measuring locations are marked on the

engineering flow diagram. Step two requires a tour of the unit, confirming

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and having calibration checks made of critical measuring instruments. Dial

thermometers, pressure gauges, and dp cells are examples of these instruments.

Table 2-1 is an example of a data collection sheet for electric motors in the unit.

When reading pressure drops across an exchanger, it is preferable to use the same

pressure gauge to read up stream and downstream pressures. A three way selector

valve such as made by D/A Manufacturing Co., Tulia, Texas is a very convenient

option for making two readings with the same pressure gauge. A more expensive

option is to use a pressure differential transmitter.

The accuracy of flow meters can be checked by the use of a prover, if the

necessary piping manifold is in place or installed. Otherwise, the meter design

calculations and test results made by the instrument department should be studied

and checked. If an orifice meter is in use, you can visually confirm that the

upstream side of the orifice plate is inserted in the line correctly and that the

orifice size stamped on the plate agrees with specifications. The condition of the

orifice opening cannot be checked unless it is removed.

After all instruments are checked, you can take one data set of readings,

noting time to make readings, and problems in collecting readings or samples. A

heat and material balance can be calculated and inconsistencies noted. For example,

in making an energy balance across an exchanger, the heat transferred to the colder

stream is found higher than the cold stream. An incorrect temperature reading or

flow rate may be the reason. When this “dry run” is completed and changes made, the

plant test and evaluation are performed.

A data collection run for the electrical usage is determined by reading

amperage loads on each motor and reading the wattmeter for the unit over the test

period. Electric motors connected to instrument air and plant air compressors

should be included in the energy audit.

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2 - 5


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FIGURE 2-1

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FIGURE 2-2

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SECTION 3

ENERGY SAVINGS IMPROVEMENTS WITH MINIMAL CAPITAL INVESTMENTS

Process units built prior to 1973, the year of the drastic rise in energy

costs, were generally designed on a low capital cost investment basis for maximum

rates of return. Energy saving equipment was included in the investment if it

obviously improved the return on investment. No extensive engineering was directed

at energy in the design phase.

In the current period of high energy costs, economics still dictates how much

energy a new plant design can conserve. But the incentive to expend more engineer-

ing time in the design phase to optimize the process with maximum energy conserva-

tion has increased. Likewise, there is the economic incentive to return to older

operating plants and retrofit them with additional energy saving equipment.

Similarly, years ago, plant operators had been instructed to minimize off

specification production. They achieved this and reduced the amount of scrutiny and

effort needed to operate the unit by producing a purer product than necessary. This

results in an increase in energy usage. This section of the manual will cover

changes in plant operation with minimal capital investments to reduce the energy

required to produce one pound of product.

3-A. OPERATING PROCEDURE REVISIONS

Your operating procedures were probably written before the large increase in

energy cost drew attention to energy conservation as one primary objective. In

addition, the operators are probably using the procedures only as a guide and have

developed their own procedures based upon ease of operation.

3-A-1. Reducing the Reflux Ratio of Columns

The optimization of the reflux ratio of the distillation column can

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produce significant energy savings. The investigation can start by checking the

operating manual and column performance specifications for the design conditions,

including the reflux ratio. If the design conditions are no longer valid due to

changes in feed composition or product requirements, it is recommended that a

vigorous distillation calculation be made. If the calculations are very difficult,

you can make use of commercial computer programs made available through various

computing service bureaus (see section 4-B). The design reflux should be compared

with the actual ratios controlled by each shift operator. The daily laboratory

analyses of the column products are compiled and compared with the design

specifications. If the column is operated at a reduced production rate, the design

reflux rate should be calculated for this reduced rate.

It is extremely difficult to change people, even more difficult when it

requires more work effort without visually seeing the results. If one operator was

found who operated the column at a lower reflux ratio than the others, you might

get the confidence of the operators by getting all the operators to maintain this

ratio. If you merely write a note in the unit's operating log leaving instructions,

you will probably not be successful in lowering the reflux ratio. You must work

closely with the superintendent, foremen, and operators instilling confidence as

you show the energy savings resulting from their efforts. If the operating depart-

ment has monthly meetings for the supervisory people, you can use it as a forum to

present your objectives, how you plan to approach them, and request their support

and assistance. Later you can report progress and discuss problem areas.

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Steam or fuel usage per pound of product can be tabulated daily along with

reflux ratio, product purity, etc. and compared with column performance before the

change. The savings in energy can be converted to a monetary value and reported to

the operating people. As an alternate you might represent the energy savings as

barrels of imported oil per year.

As the reflux ratio is reduced, a point will be reached at which the

operators are overworked and having difficulty in maintaining product purity. This

is the opportunity to show your concern to the operators by backing off on the

ratio.

3-A-2. Lowering Product Specifications

Sometimes, product specifications can be lowered. Who decided on the present

product specifications? Are they justifiable? For example, the sales group may have

had the product purity increased to justify selling more product and beating the

competitors. The buyer may require a purity in excess of his real needs. Higher

purity product requires more energy to be consumed per pound of product. Since the

sales department has probably expressed an optimistic opinion as to the value of

higher product specifications in the market place, an economic analysis based upon

their opinions would most likely say to make no specification changes. A better

approach may be to analyze the specification requirements for each type of user of

the chemicals and determine if the higher specification is required. A different

selling technique may retain the customer even if product specifications are

lowered to save energy.

If the product from the column is feed to another unit in the plant, then the

effect of lowering the purity on the other unit must be determined. Thus, the

energy conservation project requires the additional collection

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and tabulation of operating data. A statistical approach may be required to fully

interpret the results of changes due to the variability of the processes by changes

in other parameters.

3-A-3. Lowering Pumping Costs.

When making an inspection of the unit for an energy audit, you should note

any operation of two centrifugal pumps in parallel. Within the distillation unit,

you can have reflux pumps, product pumps, feed pumps, pumpa-rounds, etc. with

spares. Other examples are cooling water pumps in the water cooling tower and

cooling pond systems.

If the pumping system was designed for one pump and the operator places the

spare pump in service, too, he has not doubled the flow rate. Instead, each pump

provides one half of the developed system flow rate and each operates at the

identical head. To understand this, let us assume a centrifugal pump characteristic

curve as shown in Figure 3-1. At 100 gpm of flow, one pump produces 130 ft of head.

If identical pumps are on stream, the flow is 100 + 100 or 200 gpm at 130 ft of

head. The characteristic curve for two pumps was developed this way and is also

shown in Figure 3-1. The actual flow rate through the piping system is set by the

intersection of the pump curve with the system head curve. Referring to Figure 3-1,

the flow rate is 160 gpm with one pump operating and 172 gpm with two pumps on

stream. In the latter case, each pump is handling one half the flow or 86 gpm.

The efficiency of centrifugal pumps varies with flow rate. Thus, pumps are

selected in the design phase to operate at or near their highest efficiency. As

seen in Figure 3-1, the pumping efficiency decreased from 46.5% at 160 gpm to 34%

at 86 gpm. Assuming an electric motor efficiency

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of 95%, the energy used in both cases is determined as follows:

p m

(gpm)(TDH)(S)Hp

(3960)(E )(E )=

For one pump operating

(160)(119)(1.0)Hp 10.0

(3960)(0.465)(.95)= =

For two pumps in parallel

(172)(129.5)(1.0)Hp 17.4

(3960)(0.34)(.95)= =

By increasing the flow 7.5%, the energy requirements increased 60%. As an

alternate to two pumps, the size of the impellers could be increased to handle the

172 gpm of flow with one pump. Assuming an efficiency of 47%, the energy required

is:

(172)(129.5)(1.0)Hp 12.6

(3960)(0.47)(.95)= =

Thus, 17.4 - 12.6 or 4.8 Hp was conserved. In section 5 of this report, the

concept of investment equivalent for energy savings is developed. This is the

amount of capital that can be invested to save a unit of energy. If new impellers

were placed in the two pumps (one pump is the spare), the impellers would likely be

expensed (if the motors were changed, the new motors would probably be

capitalized). How long would it take to recover the expense of purchase and

installation of the two impellers if the pump operated at 172 gpm with 0.95 on

stream time? Assuming the cost of electricity at 3.0 cents per KWHr and the

replacement expense of $800, the payout is:

(X) (.95)(4.8)(.746)(.030)= $800

where X = hrs

X = 7839 hrs or 0.9 years

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Management should be receptive to this expenditure.

As chemical plants expand by adding more process units, additional cooling

water is probably required. Usually, the existing cooling water lines are not

replaced with larger lines, but additional pumps are added to handle the increased

flow requirements. Suppose a new pump was purchased with an impeller that gives a

higher head to compensate for the higher system pressure drop. The impellers of the

existing pumps are replaced with larger diameter impellers. This is a minimal

capital cost pumping installation, but what about energy usage?

As an example, Figure 3-2 shows the pump characteristics and system curves

for a cooling water pumping system before and after expansion. Flow was increased

from 1500 gpm to 2250 gpm. At the original flowrate, pumps A and B operated at 750

gpm each at 70 ft of head and probably at the best efficiency for these pumps. With

the expansion, flowrate is at 2250 gpm at a head of 108 ft. At 108 ft of head,

pumps A and B handle 1150 gpm or 575 gpm each. The efficiency of the two pumps

probably dropped. Frictional energy increased 38 ft. The following calculations

assume a $0.03 per KWHr of electricity:

Operating cost before change

(1500)(70)(1.0)Hp 55.8

(3960)(0.50)(.95)= =

Pumping cost = (55.8)(.746)(0.03)(24) = $29.98/day

Pumping cost per day per gpm = $.020

Operating cost after change

( )

(2)(575)(108)(1.0) (1100)(108)(1.0)Hp 136.5

(3960)(0.45)(.95) (3960)(.50) .95= + =

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Pumping cost = (136.5)(.746)(.03)(24) = $73.33

Pumping cost per day per gpm = $.033

The pumping cost per gpm has increased 65% in addition to the capital costs, not a

very efficient modification. Before making the pumping change, it may be possible

to reduce frictional energy losses. The existing distribution system should be

traced and pressure drop calculations made for sections of the system that appear

to have high pressure drops. Maybe a short section of pipe could be replaced with a

larger size. Maybe the proposed tie-in point for the cooling water to the new

process could be moved closer with a small increase in piping costs, but a

significant lowering of frictional energy losses.

Another possible way to cut energy usage is to limit cooling water flow

through the exchangers. It is doubtful that the operating procedure covered this

aspect. If flow is not throttled, the flow through an exchanger is determined by

the ÄP available from the pumping system and the frictional energy losses in the

exchanger and piping. For example, an unthrottled flow showed 8 psi across the

exchanger. Design flow was for 800 gpm with a 5 psi

drop across the exchanger. Since flow is approximately proportional to the square

root of the pressure drops, the flow rate is8

8005 or 1000 gpm. An inexpensive

type butterfly valve with a manual lock positioner could be installed to throttle

the flow to 800 gpm, saving 200 gpm of cooling water.

If a cooling water system operated at 6000 gpm and 50 psig before the

exchanger flows were throttled and 5000 gpm at 55 psig after the throttling,

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how much energy was saved? Let us assume there is an improvement in efficiency from

0.50 to 0.52.

Horespower before change

(6000)(50)(2.31)(1.0)Hp 368(3960)(.50)(.95)= =

Horespower after change

(5000)(55)(2.31)(1.0)Hp 325

(3960)(.52)(.95)= =

Electrical savings

Savings = (43) (.746) (24) (365) (.95) (.03)

= $8000 per year

Even better savings may be gained by changing impellers, etc. to give 5000

gpm at 50 psig or less. If a process fluid is being cooled by cooling water to

lO0ºF, but a fluid temperature of 120ºF is acceptable, it may be possible to use

less cooling water or cooling water preheated by another source, thereby reducing

cooling water flow.

Flow of liquids through piping transfer lines is generally controlled by the

use of throttling valves. Past design practice has been to design the control valve

to take from 25% to 50% of the system pressure drop. This gives the control valve a

rangeability of approximately 50 to 1. The valve has converted work energy derived

from electricity into frictional heat. Most processes don't require this much

rangeability so a larger control valve with less pressure drop could replace the

original valve, the rangeability being reduced say to ll to 1. Of course, energy

savings can only occur if the pressure in the line is reduced, possibly by reducing

the diameter of the pump impeller. The electric motor should also be replaced with

one of lower horsepower that

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meets power requirements. Just installing a new control valve will be useless as

the valve will throttle down until flow is controlled to the original point.

Shinskey, in the “Control Systems Can Save Energy” article graphically discusses

this energy saving idea.

3-A-4. Lowering Steam Usaqe

One of the most talked about energy wasters is steam leakage from “bad” steam

traps and leaking fittings. Steam traps are blamed for being inefficient or worn

out and causing as much as 10% of the generated heat from steam to be lost. Is this

true or just a sales method to sell more traps? It turns out that steam leaks cause

a significant energy loss.

Mr. Goyette, in his article “Estimating the Costs of Steam Leaks”, (see

appendix 7-C) shows the cost effect of steam leakage from various size holes (1/8”,

1/16”, and 1/32”) in a 150 psig steam system. The cost was based upon incremented

steam costs. An example showed that a 1-inch union was found leaking at a loss of

$3000 per year. The repair cost was $50 or a six day payout. Of all the energy

savings steps that the Tenneco plant did, Mr Goyette said the single largest

contributor was steam-leak repairs. Steam traps will wear out. Armstrong Machine

Works claim that the inexpensive disk type steam trap wears out in 6 months and

should be replaced that frequently. If condensate is recovered, leaking traps can

cause an excessive return temperature and cause failure of the condensate return

pumps. Severe water hammer can occur as hot steam contacts condensate that has

cooled below the temperature of the steam.

The following steps are recommended for saving energy in your steam

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condensate distribution system and starting an effective steam energy management

program:

1) Develop an estimate of the cost of steam leaks based upon your plant costs

similar to the Goyette article described above. A method for demonstrating

visually to plant people what these losses are can be made.

2) Run a survey, recording all leaks, size, cost, and location.

3) Check the operation of all installed disc traps used for drips and steam

tracing. If found leaking, consider replacing with a more efficient type

trap. Before replacing, check installation design and confirm trap size

(not over or undersized).

4) Check installation and operation of steam traps used on equipment using

the sound detection method, the pyrometer method, or the glove method. The

installation should be checked for proper trapping. Items checked include

strainer, check valve, back pressure, orifice, and inert gas venting.

Improper venting can cause a severe reduction in heat transfer rate.

5) Check vent valves on steam jacketed equipment and kettles for proper

operation (removal of inerts without steam loss).

6) Start a preventative maintenance program to maintain the steam

distribution system in excellent condition. If manpower is not available

in maintenance, you can have the operating people maintain a simple log

for their area of responsibility.

7) Steam trap manufacturers will be happy to furnish information to assist in

your energy saving program to reduce steam losses, but use your own

economic costs to decide whether to replace, repair, or redesign the

system.

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There is insufficient published information to say that 10% of the steam is

wasted by steam traps, but some major chemical companies have invested large

amounts of manpower and money to replace or revise steam trapping systems in their

plants.

3-A-5. Process Heaters

The Texas Industrial Commission has developed a manual specific to boiler and

process heater efficiency. Consequently, our discussion of process heaters will be

very limited, briefly covering the reduction of excess combustion air and reduction

of stack temperature with small capital investment.

Control of Excess Air

According to Mr. A. M. Woodard, (see article, “Reduce Process Heater Fuel”,

in appendix 7-C), over half the total fuel consumption for refineries is for

process heaters, the remaining for steam generation. These fired heaters can be

improved from an energy efficiency viewpoint by reducing the amount of heat in the

stack discharge. With the advent of the more accurate and simpler oxygen analyzers,

the control of excess air in a fired heater can be automatically or manually

controlled by the operator. Mr. Woodard’s article details a method of sampling the

flue gas, monitoring and controlling the system. Four systems are described, but

system 3 is recommended. This consists of locating the draft and oxygen analyzer

readouts in the control room, too. The operator can then monitor and control the

operation of the heater or heaters with ease and comfort. Two safeguards are built

into the system. stop installed to prevent full closure. failure, the positioner

opens the damper. The damper has a mechanical If there is an instrument air

A simple stepwise

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procedure for heater adjustment is given on the last page of the article.

A target excess oxygen for the oxygen recorder with remote manual damper

control was given in the article as 4.0% for gas and 4.5% for oil firing. More

recently, manufacturers are indicating the oxygen can be controlled at 2%. The

decision to go this low must be based upon the risk of temporarily going below

stoichiometric conditions with possible explosion when the heater returns to excess

oxygen conditions. Based upon figure 1 of Mr. Woodard’s article, substantial

reductions in heat input are accomplished by this approach. This modification will

probably cost less than $5000, yet show considerable savings.

Recovery of Heat from Stack Gases

The amount of heat extracted from burning a fuel can be related to the flue

gas or stack temperature. The extracted heat is defined as the heat absorbed by the

process stream being heated and the losses from the furnace casing (generally

around 2%). Thus the percent heat extracted is:

Heat available in Btu/lb of fuel at the Flue Gastemperature (FGI), divided by the Heat Content of theCombustion Fuel in Btu/lb times 100.

The lower heating value (LHV) of the fuel is used for efficiency

calculations. The flue gas temperature depends upon the design condition of the

convection section of the heater and the physical condition of the convective

tubes. A reasonable FGT is the inlet process fluid temperature plus approximately

150º F. If your inlet fluid is at 300º F, the FGT is approximately 450º. A check of

the FGT for your heater may show 500º F. Thus, your convective tube section may

have lost some of its heat transfer ability by loss of fins on the tubes. This

becomes a replacement expense.

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3-B. SCHEDULING SHUTDOWNS TO MAXIMIZE ENERGY RECOVERY OR PROFITS

If an exchanger (or reboiler or condenser) used to recover heat from a hot

stream is slowly losing the amount of heat recovered because of fouling, when do

you shutdown? This decision can be based upon maximizing heat recovered or

minimizing the loss in profits. Three cases are described below:

Case 1---Decision based upon energy conservation

Given: An exchanger used to recover waste heat is rated at 11,000,000 Btu/Hr

when clean before fouling. This exchanger slowly loses its heat

transfer capability and the loss is estimated to be 10,000 Btu/Hr per

day. A 12 hour shutdown is required to replace the tube bundle.

Find: Frequency of shutdown to maximize the energy recovery. Assume a 3500

day period of time.

A) At start of day l, heat transfer rate =611 x 10 At the end of day l,

heat transfer rate is6 411 x 10 1 x 10− or 10,990,000 Btu/Hr

B) Let C = number of repairs during the 3500 day period. The heat

recovered for any given day, X of the cycle is

6 4

DE 24(11 x 10 1 x 10 X)= −

The heat recovered for an entire cycle is

3500

C6 4 6 4

C

0

3500E 24(11 x 10 1 x 10 X)dx (12)(11 x 10 1 x 10 )

C= − − −∫

46 6

C

1x103500 3500 3500E 24[(11 x 10 )( ) ( )] (12)(11 x 10 x )

C 2 C C= − −

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For the 3500 day period, total heat recovered is

= − − −2C3500 3500 3500

E 1000C{24[(11,000)( ) 5( )] 12[11,000 ( )10]}C C C

= − − −23500 3500 3500

24000 C[(11,000)( ) 5( ) 5,500 5( )]

C C C= − −2

3500 350024000 C[(10995)( ) 5( ) 5,500]

C C

= − −7

7 6.125 x 1024000[3.848 x 10 5500C ]C

= − −3

6 3 61.25 x 1024 x 10 (38.48 x 10 5.5C )C

−= − −

16 3 2

dE24 x 10 [0 61.25 x 10 x(c ) 5.5]

dC

= −

3

2

61.25 x 10

0 5.5C

= =3

2 61.25 x 10C 11,1365.5

= =3500

C 105.5 cycles or 33 days / cycle1055

= 11TE 8.96 x 10 Btu

Case 2---Decision based upon maximum profit, production rate not affected.

Given: Same conditions as Case 1

Each61x10 Btu is worth $2

Each shutdown costs $10,000 in maintenance and $20,000 in profits.

Find: Frequency of shutdowns to maximize dollar savings

A) Savings =

36 3 661.25 x 1024 x 10 (38.48 x 10 5.5C)(2 x 10 ) (10,000 20,000)(C

C

−− − − +

=

33 361.25 x 1048(38.48 x 10 5.5C) 30 x 10 C

C= − − −

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B)

13 32

ds48[0 61.25 x 10 ( c ) 5.5] 30 x 10

dc

−= − − −

63

2

2.94 x 100 264 30 x 10

C= − −

62

3

2.94 x 10C 97.15

30 x 10= =

3500 days

C 9.86 cycles or 3559.86 cycle

= =

C)

36 3 11

T

61.25x10E 24x10(38.48x10 5.5C) 7.72x10 Btu

C= − − =

Case 3---Decision based upon maximum profit, production rate affected by loss of

heat transfer.

Given: Same conditions as Case 1 and 2, but production capacity is reduced by

.05% per day. Each .05% loss in rate is $20 per day (20,000 x24

12 x

.0005) in profits.

Find: Frequency of shutdowns to maximize dollar savings.

A) Savings =

6 3 3 6 261.25 20 350024 x 10(34.48 x 10 x 10 5.5C)(2 x 10 ) (10000 20,000)C ( )C 2 C

−− − − + −

3 83 3

2

61.25 x 10 1.225 x 1048(38.48 x 10 5.5C) 30 x 10 C

C C= − − − −

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B)

3 83

2 3

61.25 x 10 (1.225)(3)x 10ds48[0 5.5] 30 x 10

dc C C= − − + −

6 83

2 3

2.94 x 10 3.68 x 100 264 30 x 10

C C= − − +

3 3 6 80 30.264 x 10 C 2.94 x 10 C 3.68 x 10= − −

C 24.4 cycles=

or3500

143 days24.4

=

C)

36 3 11

T

61.25x10E 24x10(38.48x10 5.5C) 8.6x10 Btu

C= − − =

SUMMARY

CASE 1MAX ENERGY

CASE 2MAX PROFIT

CASE 3MAX PROFIT

Btu’s total 118.96 x 10 117.73 x 10 118.6 x 10

No. of cycles 105.5 9.9 24.4

Days per cycle` 33 355 143

It is doubtful that management would agree to shutdowns every 33 days to

maximize energy savings when the maximum profit occurs at 355 days (Case 2), or 143

days (Case 3). However, as the energy cost increases, the frequency of exchanger

cleaning will increase for Cases 2 and 3. In a real plant, the assumptions of

linear losses of heat transfer and production may not be true, but the principles

of handling the decision making are still valid.

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FIGURE 3–1

CENTRIFUGAL PUMP CHARACTERISTIC AND SYSTEM CURVE

3 - 17


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FIGURE 3–2

EXPANSION OF PUMPING SYSTEM

3 - 18

0 500 1000 1500 2000 2500 3000 350040

60

80

100

120

140

160

180

Two PumpsPumps AB New ImpellersNew Pump C

Pumps ABCSystem Curve

Total head - Ft of fluid

Pumping EfficiencyA or B at 750 gpm = 0.50

A or B at 575 gpm = 0.45C at 1100 gpm = 0.50

Flow - gpm


44/173

SECTION 4

ENERGY SAVING IMPROVEMENTS WITH CAPITAL INVESTMENTS

Energy consumption for all distillation processes in the United States in

1976 was estimated at 3% of the entire national energy usage. Since distillation is

considered a low efficiency process, it should be possible to improve efficiency

with investments of capital and still receive a reasonable return on investment.

Investments may be made in additional exchangers for heat recovery, column

revisions, better insulation, or column control. In contrast to these simple

changes not requiring capital investments, the more complicated vapor recompression

or heat pump changes are reviewed.

4-A. OPTIMIZATION OF HEAT RECOVERY - HEAT EXCHANGERS

The basis for optimizing heat recovery involves the first and second laws of

thermodynamics. The first law covers the energy balance, the conservation of energy

and the energy equivalence of work and heat. The second law develops the concept of

energy level, the irreversible process, and the conversion of heat to work energy.

If one process stream must be heated and can be heated using another process

stream without using energy from steam or electricity, the heat recovered saves

fossil fuels. The cost savings in energy must exceed the capital investment

equivalence of energy for the heat exchangers and ancillary equipment to be worthy

of installation.

It is easier to design a new facility with the objective of optimizing energy

use than an existing plant. According to Steinmeyer (Seminar on energy

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conservation in the AIChE today Series), "----the existing plant cannot

economically achieve the same low (energy) usage as a new plant. The cost

to return to an existing plant and reinsulate a vessel, add heat exchangers,

or increase the number of distillation trays on the basis of energy conser-

vation alone is much higher than starting out in the design phase of a new

plant. Thus, any proposed changes in an existing unit must be carefully

analyzed so that no expenditure for making the change is overlooked. Changes

that reduce profit because certain expenditures were overlooked will be re-

membered by management when additional changes are recommended.

The amount of heat that can be exchanged depends upon the fluid’s temperature

level and the amount available. The optimization of heat recovery involves

exchanging Btu’s at as high a temperature as possible. For example, a vapor product

stream is condensing at 350ºF in an exchanger using cooling water to remove the

heat. The cooling water temperature discharges at 110ºF. At this temperature level,

the energy in the cooling water has no use and is totally wasted.

To give an example of the amount of heat available, assume liquid stream A is

flowing at 10,000 lbs/hr at 400ºF, liquid stream B is flowing at 600 lbs/hr, and

400ºF too. If both streams must be cooled to 300°F, stream A has the greater

availability of heat. If liquid stream C is flowing at 100,000 lbs/hr at 300ºF, the

heat available above 300ºF for transfer is zero. Stream C could be used to heat up

a cooler stream, D, to 280ºF and then stream A could heat up stream D to 380°F. The

method for optimizing heat recovery is described in the technical article by Huang

and Elshout (see Appendix 5-C)

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A heat availability diagram is shown as Figure 2 in their technical article,

“Optimizing the Heat Recovery of Crude Units”, by Huang and Elshout. Four streams,

the overhead reflux, kerosene pump around, gas oil product, and the residuum are

available for exchanging heat with the crude in a 130,000 bbl per stream day crude

unit. Each exchange stream has restrictions as to the temperature range that heat

can be removed, and the rate of flow. Huang and Elshout had plotted the heat

available in “Enthalpy Times Mass Rate”, as millions of Btu’s per hour for each

stream using 0 enthalpy as the lower restriction temperature for the stream

available for heat exchange. Figure 4-1 is the same drawing as found in Figure 2 of

the Huang and Elshout except the total heat availability curve was returned to its

unshifted position.

The total heat availability curve is determined by summing the enthalpy rate

for each stream at a given selected temperature. For example, at 300º, the enthalpy

rate is 0 (kerosine PA) + 37 (G 0 Product) + 55 (residuum) + 320 (OVH) : 412

million Btu/hr. At 400ºF, the enthalpy rate is 55 (kerosine) + 60 (G 0 Product) +

106 (residuum) + 320 (OVHD) = 541 million Btu/hr.

The total heat exchange curve as plotted is right of the crude oil heat

requirement curve. At first, this would indicate that the crude can be heated to

645ºF and have an excess of 60 million Btu/hr excess (675 x 106 Btu/hr at 645ºF -

515 x 106 Btu/hr crude requirement). This is not true because heat must be available

at the required temperature level. Below 370°F, the slope of the total heat

availability curve is less than the crude requirement curve. This means that

sufficient heat is available at the proper temperature to heat up the crude. Above

370°F, the slope is greater than the crude curve and insufficient heat is

available. Even with infinite heat transfer, the final crude

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heat exchange temperature must be below 645ºF.

Haung and Elshout shifted the total heat availability curve to the left until

the two curves touched. They said this represented the maximum amount of heat that

can be exchanged with infinite heat transfer. Below the pinch point, we have

already concluded that more than enough heat is available at the proper temperature

to heat up the crude. Thus, the maximum amount must be represented by the end point

of the total available with the shifted curve or 420 million Btu's per hr. The

maximum crude temperature is 530ºF. When Huang and Elshout studied the heat

optimization of this unit, they studied four cases and the maximum temperature

reached was 480ºF. (See Case D, their Figure 4).

Bannon and Marple of Shell Oil Company presented a paper on "Heat Recovery In

Hydrocarbon Distillation" (see Appendix 7-C for paper), in November 1977. They show

two ways to improve the thermal efficiency of distillation columns based upon the

concepts just discussed. If the overhead vapor from a column is at a temperature

high enough to be useful and produces a boiling range top product, the overhead can

be condensed into two stages. First, heat is removed to condense only enough of the

overhead vapors to produce column reflex. The temperature of the condensation stage

is at a higher level than if the entire overhead vapors were condensed in one step.

Then, the remaining vapors are condensed and cooled to product conditions. Bannon

and Marple described a crude oil distilling column at one of their manufacturing

complexes. This column used the two stage condensation approach and transferred 203

million Btu/hr to the crude oil feed. If one stage operation, the heat recovered

would only be 122 million Btu/hr, a loss of 81 million Btu/hr.

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If heat can be withdrawn from a column to balance column vapor loads and

improve separation, the temperature level of the heat removed and made available

for exchange can be increased by designing at high circulating rates. The three

factors for designing circulating reflux systems are the number of systems, the

placement of the systems, and the circulation rate. These factors are described in

the Bannon and Marple article.

The heat recovery efficiency of your distillation columns can be checked for

possible improvements. This can be done by using the Elshout “Heat Exchanger

Network Simulator” program available on the computing service bureau, United

Computing Systems (UCS) or other similar programs. You can also develop your own

available heat curves. Using the exchangers available in the plant as well as new

exchangers, you may be able to hand calculate a fairly good heat recovery system

that is economically feasible.

4-B. COLUMN REVISIONS

Many options are available for conserving energy in distillation processes.

Mix, et al have outlined and also placed in tabular form guidelines for selecting

energy saving options. The more attractive options found in their table and article

are discussed below.

4-B-1. Additional or More Efficient Trays - According to Mix, et al, tray

changes are economically feasible if:

4-B-1)

2 2 ln PRN 150ln s R 1

αξ− <

−

Where N = Number of trays in the column

ξ = Murphie Plate efficiency

α = Relative volatility (light to heavy)

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S = Separation Factor

DLK BHK

BLK DHK

(x )(x )

(x )(x )

D = Distillate

B = Bottoms

LK = Light Key

HK = Heavy Key

x = Concentration, mole fraction

P = Column pressure in ATM

K = Reflux ratio R/RM

Before one proceeds, it is recommended that a rigorous distillation

calculation be performed on the existing column using the actual temperature,

pressures, compositions, etc. of the column. Distillation programs that have

been developed by Chemshare, Simulation Sciences, Phillips 66, and others for

simulating your column are available through various computing service

bureaus.

A plot can be made of the distribution of the various components tray

by tray. This plot may indicate the feed tray may be changed or additional

trays may be beneficial if entirely in the rectification or stripping section

of the column.

If equation 4-B-1 shows the column may benefit from more trays, you can

run several cases with reflux as the variable (heat load changes) and

determine the saving in energy.

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You have the option of adding more trays or replacing existing trays

with more efficient type trays. For example, Kirpatrick, in his article, "M D

Trays Can Provide Savings In Propylene Purification", (see Appendix 7-C),

describes the design of propylene-propane splitters and the application of

trays with 13” spacings compared to the usual 18 to 24” spacing. With the

shorter spacing and more efficient design, a single column 13.25 ft in

diameter and 265 ft tall, using 196 M D trays was installed and producing

polymer grade propylene.

4-B-2. Additional Column Draw - Three possible column draw options are

pasteurization, intermediate product and intermediate impurity.

Pasteurization means the removal of light ends from the distillate by venting

off the accumulator and removing the distillate product several trays below

the top. Six criteria are listed by Mix, et al.

The intermediate product is considered when the temperature difference

between bottom and top exceeds l00ºF, and when the split of one key between

two products is desired. The intermediate impurity drawoff is useful for

removing impurity buildup under high reflux operations. The impurity flow

rate must be less than .01 times the feed rate in lb moles per hr, and the

relative volatility between the light and heavy key less than 1.5.

4-C. OPTIMIZATION OF RECOVERY AND USE OF ENERGY

4-C-1. Introduction - The maximizing of the overall plant energy efficiency

is our purpose in utilizing waste heat possibilities and energy conservation

methods. Distillation columns consume and reject large

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amounts of heat. Much of this heat is lost and not recovered. By a

proper reevaluation it might be possible to greatly increase the re-

covered heat and reduce the input requirements. Several items will be

of major importance in this reevaluation.

(1) The temperature and heat flows within the column.

(2) The changes that can be made within the column, including

changed upstream and downstream requirements.

(3) The plant utilities, heat and cooling sources.

(4) The needs of the nearby surrounding processes.

The relation of the efficiency of the distillation column to the

overall energy efficiency of a plant cannot be optimized without

knowledge of the requirements of the other processes of the plant.

Integration of the overall plant is the key to maximum energy savings.

To evaluate the available options for this purpose, the following

needs to be known about the plant.

(1) The process streams that require heating. The beginning and

ending temperature, total heat capacity, and the current

heating methods of each stream is required.

(2) The process streams that can be cooled. The beginning high

temperature, any low temperature bound, the total heat

capacity, and current cooling method of each stream is

required. Remember that every Btu that can be usefully

recovered replaces a Btu that would otherwise have to come

from a fuel. Any part of heat recovered from a cooled

stream is useful.

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(3) Any reboilers or evaporators on neighboring units are of interest

where a potential use of the distillation columns condensing

vapor exists. For this, the temperature, duty, continuity of

operating parameters, and the current heating method of the

nearby reboilers needs to be known.

(4) The overall plants steam system. The steam header pressures,

capacities, flows, and overall stream balance (amount letdown,

excesses, etc.) is needed.

(5) Any units requiring large amounts of low pressure steam or low

quality heat for some purpose. The requirements of duty and

temperature is needed, in addition to the distances from the

column to the unit. As low pressure steam requires large lines,

long distance transport is costly.

(6) All heat sources from nearby equipment, condensers, etc. that can

be used by the distillation column for its re-boiler and feed

preheating duties. Note that some or all of the distillation

column reboiler duty can be supplied by a high temperature liquid

stream.

4-C-2. Column Heat Utilization - A distillation column has three basic

sources of reject heat, the bottoms product, the condensing overhead vapor,

and the distillate product. The two basic heat inputs are the reboiler and

the feed.

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4-C-2.1 Bottoms Product - The bottoms product liquid is the hottest

source of heat and the obvious heat source. Due to its temperature and liquid

form, the bottoms stream will probably already have some use on an existing

column, such as feed preheating. The most efficient use of the bottom product

is made by maximizing the temperature at which heat is recovered, and by

maximizing the total heat recovery. This situation is reflected by perfect

countercurrent exchange with equal heat capacity on both sides, where there

are only a few degrees driving force throughout the exchanger. As heat is

more valuable at higher temperatures, we must try to recover the heat at as

high a temperature as possible. As even low temperature heat can be valuable,

our aim must also be to recover as much heat as possible. Use a number of

exchangers in series, instead of a single exchanger, is also useful. For

example, assume we have a 700º stream. (and assume heat capacity = 1,000

Btu/ºF, and all steam at 1000 Btu/lb). We could use a single waste heat

boiler to produce 50 psig steam, causing the stream to cool 700 - 350 = 350

lb of 50 psig steam. A better system would be to use a series of waste heat

boilers. We could produce 200 lb of 400 psig steam (700 - 500), 100 lb of 150

psig steam, (500 - 400), 50 lb of 50 psig steam (400 - 350), and 100 lb of

1ATM steam (350 -250). By using a series we have recovered higher value steam

(400 and 150 psig), and more steam overall (450 lb versus 350 lb).

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The ability to use the heat in the bottoms product will depend on its

requirements for downstream processing. If the product is required hot

downstream, it is impractical to cool it and then to reheat the bottoms

stream. If the stream does not need to remain hot, the following represent

possible uses of the bottoms liquid heat.

(1) Preheating the column feed.

(2) Use to run all or a portion of another column's reboiler.

(3) Exchange with another process liquid stream.

(4) Steam generation and boiler feedwater heating.

4-C-2.2 Distillate Product - The options that apply to recovering heat apply

equally well to the distillate product. The opportunities of heat recovery

differ as the distillate product is at a lower temperature than the bottom,

and the distillate product may be a vapor, therefore containing a large

amount of heat in its vaporized condition.

4-C-2.3 Condenser Duty - The largest potential reject heat source of the

distillation column is the condenser. All this heat is available at

essentially a single temperature, and all the heat duty must be removed.

Possible uses of the condenser duty could be to supply heat to a neighboring

column’s reboiler, to produce waste steam, or to heat large liquid streams at

low levels, such as supplying hot water for a building.

4-C-2.4 Reboiler Duty - The reboiler represents the largest heat input to the

distillation column. The reboiler requires heat at a

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single high temperature. It is desirable to minimize the steam consumption if

possible by using condensing vapors from other columns, hot process streams,

or special very low pressure steam.

4-C-2.5 Feed Preheating- The bottoms product or another hot liquid stream is

often used to preheat the feed. Whatever source used should cause the maximum

overall energy efficiently for the plant.

4-C-3. Changing the Columns Temperature - The existing or proposed column

does not necessarily have to operate on the design conditions. (Do not

operate existing columns over the allowable pressure). By changing the

temperature in the column a small amount, we may be able to obtain a valuable

energy recovery. Lowering the temperature might allow a less valuable steam

to be used. Raising the temperature may allow a waste heat boiler to be used,

or the vapor used to provide reboiling in another column. Note that changing

the temperature will effect the column’s operation (different pressure) and

raise or lower both the reboiler and condenser temperatures.

4-C-4. Two-Stage Condensation - For some multicomponent distillation columns

there is a broad range over which the overhead vapors condense (dew point to

bubble point). By using more than one condenser instead of a single total

condenser, we have the opportunity to recover some of the heat at a higher

temperature. For example, we could have two condensers, the first condenser

condensing part of the overhead to provide reflux, the second condensing the

distillate product. This situation is effectively a partial condenser

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with vapors later condensed. The items to be emphasized on a multi-stage

condensation column are to avoid subcooling as much as possible, and the

recovery of the waste heat by steam generation in the high temperature

condenser.

4-C-5. Waste Heat Boilers - The use of a condenser as a waste heat boiler is

simple. The condenser is operated in a partially flooded situation, where the

level changes as the heat duty is changed. The water is boiled at constant

pressure in the tube side, and all steam produced sent to steam headers or

its ultimate use. The temperature at which the condenser operates is

important. For a temperature of less than 200ºF, no sort of steam can be

produced. For higher temperatures the steam produced is determined by the re-

quired pressure (1ATM, 40#, 150#, etc.). The condenser will have to be larger

as the temperature driving force goes down, so the economics should be looked

at. It may be necessary, in a case where low pressure steam could be produced

but no use exists at this low pressure, although one does at a slightly

higher pressure, to mechanically compress the low pressure steam to a higher

pressure, say from 25 psig to 40 psig. Remember the true values given to the

different steam pressures during the evaluation of different waste heat steam

generator possibilities.

4-C-6. Multiple Effect Heat Cascading for Distillation Columns - The

condensing overhead vapors of one distillation column can be used to provide

the reboiling duty of another column, where the condensing temperature is

higher than the reboiling temperature. This

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creates in effect the equivalent of a multieffect evaporator system, except

that the distillation columns is used, rather than the direct evaporation.

Any number of distillation columns can be placed in series, such as the three

column example of Figure 4-2. Note that different materials are being split,

and the columns are disimilar, except for the heat duties.

The columns run by using the overhead of one column to provide the

reboiling of the other will probably not have the same heat duties, therefore

any excess duty can be carried by an auxiliary system. Where the hotter

column is smaller than the cooler column, an auxiliary reboiler will be

needed for the cooler column. In the other case where the hotter column is

larger, an auxiliary condenser on the hot column will be used, with all the

cooler columns duty carried by the hot column’s vapor. With the proper

auxiliaries, the heat cascaded columns can be operated almost independently,

therefore little control problems will be met. The heat cascaded distillation

columns are different from a split tower arrangement, because the split tower

has the same feed and products.

The heat savings by use of heat cascading are obvious as each reboiler

run by the overhead vapor of another column removes that much of an external

heat input. The costs are for a slightly more complex system, and the piping

and extra heat exchanger surface for the condenser reboiler. The heat

cascaded system work best where nearby columns exist, these columns having

different temperatures, and each column has a fairly narrow range of

temperature between the top to bottom of the column. In some cases, it may be

desirable to operate the hot column at a higher pressure and the cool column

at a lower

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pressure than optimum in order to increase the temperature difference between

them. The use of heat cascading will interfere with other possible uses of

the hot condenser duty, such as in producing waste heat steam, so that the

various cases must be evaluated for the optimum case.

4-C-7 Split Tower - The use of a split tower can afford significant energy

savings over a conventional distillation column. A split tower arrangement

consists of splitting the feed into two equivalent streams and distilling in

two smaller columns. The two columns operate at different pressures, one

higher than the other, resulting in its overhead vapor having a condensing

temperature high enough to be able to use the condensing vapor to provide the

reboiling duty in the lower pressure column. The bubble point temperature of

the overhead vapor must be high enough over the bubble point of the lower

pressure reboiling bottoms to provide a sufficient delta T for the condenser-

reboiler. The feed stream will be split so that the condenser duties of the

high pressure column approximately matches the required reboiler duty of the

low pressure column. (See Figure 4-3 for an example split tower arrangement).

The heat, input to the reboiler, of the high pressure column rises to

the condenser where it then provides the reboiling duty of the other column.

By use of the split tower arrangement, we have cut our energy use almost in

half. Note that instead of two columns, any number of columns can be used in

the split tower fashion. However, For each additional tower, an extra delta T

must be supplied, plus the temperature drop across the column. In addition,

the energy savings drops

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as each column is added. The two tower system saves 50% of the energy.

Another tower saves (50 - 33) or only 17%. A fourth tower will save

only 8.3%. So our writeup will deal with only the two column

arrangement.

The split tower system has a single reboiler and single con-

denser. The temperature difference between the reboiler and condenser

will be much greater than that of an ordinary column. This occurs

because the two columns each have their own temperature diffence to be

met from the top to bottom, and the driving force for the condenser-

reboiler must be supplied. As a result of this for the split tower

arrangement to work, the following factors must be present:

(1) The temperature and pressure in the high pressure column

must be below the critical points.

(2) The pressure must not be so large as to require too heavy

column walls.

(3) The low pressure column must not be too low, so low a

vacuum as to cause trouble.

(4) The products must not be degraded by the highest tem-

perature or frozen, or too viscous at the lowest tem-

perature.

(5) The heat source must be able to supply heat at a tem-

perature above that of the reboiler.

(6) The condenser temperature must not be below that obtainable

by conventional air and water cooling. Re-

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frigeration cannot be tolerated, unless the conventional column

would also need refrigeration.

The split tower arrangement has a large temperature difference between

the reboiler and condenser, thus it will probably be desirable to minimize

this by using small delta T’s across the reboiler, condenser-reboiler, and

condenser. This will mean a large heat exchanger surface being required. Even

so, it is likely a higher temperature heat source will be needed for the

reboiler. As it is at a higher temperature, the heat will be more expensive,

such as a higher pressure steam. This means we are saving energy, but using a

more costly source.

The feed to a single tower will be split in two for the two column

arrangement. Therefore, the individual columns will be about one-half the

size of the single column. However, the relative volatility and the mass

flowrate/area through the columns will change with the pressure, resulting in

a differently sized tower than just one-half the size.

From an economical viewpoint a split tower arrangement will require two

columns, instead of one larger one. Each column will require its own

instrumentation, causing twice the instrument costs. The higher pressure

column will need thicker walls, and its size may be larger than expected.

(See preceding paragraph). A larger exchange surface is needed for the

various exchangers. Various auxiliary exchangers may be required for column

control. The savings of the split tower arrangement come from the reduced

heat requirement. However, the value of the heat used should be higher per

Btu used than in the case of a single column. In many respects, a split

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tower will be similar in economic desirability to a vapor re-compression

column. The key is to have a low temperature difference from the top to the

bottom of the column.

In designing the split tower arrangment, the low pressure column should

be set by the achievable condenser temperature. Then the split tower should

be worked backwards from this point, a reasonable temperature drive given for

the condenser-reboiler, then the high pressure column found, finally

resulting in a temperature for the reboiler. With this temperature the

available heat sources should be examined, for example, the various steam

pressures, and one chosen. The delta T available should then be distributed

between the reboiler, condenser-reboiler, and condenser to obtain the minimum

required heat exchanger surface area. The feed between the towers should be

split in order to approximately give equal duties for the high pressure

condensation and low pressure reboiling under design operating conditions.

The control of a split column will be more complex than that of a

single column. The object of the control system will be to decouple the two

towers to a certain extent. The use of an auxiliary condenser on the high

pressure column and a auxiliary reboiler on the low pressure column will give

energy efficiency and good control. Control can also be had by having only

one auxiliary exchanger, and by having one of the columns run at a higher

duty than the other. The feed split ratio between the columns can be used as

part of the control. Note that the bottoms of the high pressure

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tower can be mixed with the low pressure bottoms and flashed in the low

pressure tower. This would result in a uniform bottoms composition.

The split tower design offers a good possibility of energy savings with

a new installation. Where an existing column exists already, it would be

possible to increase capacity by adding another tower next to the existing

one and installing a new condenser-reboiler so that the existing column will

become one-half of a split tower arrangement. In cases where no capacity

increase is desired, but the column original size was such that two towers

were used, it may be possible to convert it to a split tower operation by

installing a more efficient column internal trays and by adding a condenser-

reboiler, new piping, and new instrumentation. The savings that can result

from a split tower design are very much afftected by the cost of energy to

the reboiler, so the true energy cost should be evaluated before using a

split tower.

4-C-8. Interreboilers, Intercondensers, and Feed Preheating – The reboiler is

at the highest temperature of any part of the distillation column, therefore

it is the worst place to add heat as a high temperature (and therefore more

valuable) heat source must be used. Likewise, the condenser represents the

worst place to remove heat as its temperature is the lowest, and any

recovered heat will be of low value. If heat can be added at another part of

the column in place of heat added at the bottom, we can use a less valuable

heat source (i.e. lower pressure steam) or have a smaller heat exchanger

surface area due to the increased delta T available. In the case of the

condenser heat re-

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jection being replaced by rejection at another part of the column, a smaller heat

exchange surface could be used, or the heat recovered (example waste steam

generation), or a refrigeration requirement for the condenser reduced. Thus if we

can shift some of the re-boiler or condenser duty to another part of the column, we

may be able to save money.

The reboiler duty can be reduced by using one or more inter-reboilers and

feed preheating. The condenser duty can be reduced by use of intercondensers and

feed precooling (i.e. condensation of a vapor feed). Note that if we hold the total

heat duties constant, and use interreboilers and intercondensers, then the number

of trays in the column will have to be increased at the top and bottom sections,

although the column cross-sectional area can be reduced. The key to proper use of

feed preheating and interreboilers is to make sure the reboiler duty goes down

correspondingly with the increased auxilliary duty, hold overall energy use

constant while less valuable

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