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Liquefaction Process Evaluation for-FLNG

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8/10/2019 Liquefaction Process Evaluation for-FLNG http://slidepdf.com/reader/full/liquefaction-process-evaluation-for-flng 1/50 Liquefaction Process Evaluation for Floating LNG Maya Kusmaya, Trondheim 2014
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Page 1: Liquefaction Process Evaluation for-FLNG

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Liquefaction Process Evaluation

for Floating LNG

Maya Kusmaya, Trondheim 2014

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Outline

6/4/2014TGTC 20142

Introduction

Expander processes for natural gas liquefaction

Proposed process schemes for FLNG

Basis for process evaluation and comparison

Result and discussion

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Introduction

6/4/2014TGTC 20143

• A challenge to find the optimal liquefaction process

technology for use in offshore environment• Growing interest to apply expander-based liquefaction

processes for FLNG

• A challenge to get objective comparison between thevarious technology efficiencies

Background

• To do a comparative evaluation of several expander-based processes for FLNG on a identical basis focusingon capacity, efficiency, integration into energy system,complexity, and hydrocarbon inventory

Objective

• Literature study, establishment of a identical evaluationbasis, HYSYS model development, case studies andsystematic comparative analysis of performance dataresulted from HYSYS

Scope of work

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A typical expansion-based liquefaction

processes

6/4/2014TGTC 20144

N2

N2 N2

Natural

Gas

LNG

Flash gas

Natural Gas/LNG

Low Pressure N2

Medium Pressure N2

High Pressure N2

N2 Compressor

N2 Compander

N2 N2

N2

N2

N2

N2

Natural

Gas

LNG

Flash gas

Natural Gas/LNG

Low Pressure N2

Medium Pressure N2High Pressure N2

WarmCompander

Cold

Compander

Cycle Compressor

N2 N2

N2

N2

N2

N2

CO2

Natural

Gas

LNG

Flash gas

Natural Gas/LNG

Low Pressure N2

Medium Pressure N2

High Pressure N2

CO2 refrigerant

Single Expansion Double Expansion Double Expansion + Precooling

Efficiency and Capacity Improvement

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 The Cooling Curves

6/4/2014TGTC 20145

Single Expansion Double Expansion Double Expansion + Precooling

Efficiency and Capacity Improvement

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Proposed Expander Process Schemes for

Floating LNG

6/4/2014TGTC 20146

Process A• Based on US Patent 2010/0122551

A1

• N2 expander-based process with

two pressure levels and three

expander temperatures

• ARS (LiBr/water) is used for

precooling system

• The LiBr process driven by gas

turbine waste heat and provides

cooling of feed gas, N2 loop andgas turbine air intakes.

ARS : Absorption Refrigeration System 

N2

N2N2

N2

N2

Natural

Gas

LNG

Flash gas

Natural Gas/LNG

Nitrogen

N2 N2

N2

SCHE

MCHE

N2 Compressor

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Proposed Expander Process Schemes for

Floating LNG

6/4/2014TGTC 20147

Process B• Based on US Patent 5,768,912

• a CO2 precooled dual nitrogen

expander liquefaction process

• the CO2 cooling is also used for

gas turbine air intake cooling.

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Proposed Expander Process Schemes for

Floating LNG

Process C Process C with precooling

6/4/2014TGTC 20148

CH4

Natural Gas

LNG

Flash gas

Natural Gas/LNG

Methane

Nitrogen

CH4CH4

N2 N2 N2

CH4

Compander

CH4

Compressor

N2

Compander

N2

Compressor

Based on US Patent 6,412,302

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6/4/2014TGTC 20149

Simulation scope• The pretreating units are out of the scope

• Covers the process of the treated feed gas to LNG product

Feed gas• Same condition (60 bar, 22oC)

• Medium gas (91% of methane)

Site conditions• 27 oC of air temperature

• 17 oC of cooling water temperature

• 5 oC min temperature approach of cooling water cooled HX

Drivers• GT GE LM6000

• At 27 oC air GT gives 35 MW

Cryogenic HX• 3 oC min temperature approach

• 85 bar max pressure

Componentefficiency

• Compressor polytropic efficiency of 78%

• The compander polytropic efficiency was assumed of 73% for thecompressor and 83% for the expander

LNG product• Same condition (1.38 bar, -149 oC)

• At LNG spec

Evaluation and Comparison Basis

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Result & Discussion

6/4/2014TGTC 201410

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Key Parameter

6/4/2014TGTC 201411

CycleProcess

A

Process

B

Process C

Basic CO2 precooled

Cycle compression

power, MW35 35 35 35

Precooling power, MW - 1.3 - 1.4Precooling heat duty 1,

MW10 - - -

 Total power, MW 35 36.3 35 36.4

LNG production, MTPA 0.83 0.83 0.82 0.91

Specific power, kWh/tonLNG 336 347 339 315

Compander size  –  the

biggest in a single train,

MW

9.83 13.23 11.45 8.53

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Production and Efficiency Comparison

6/4/2014TGTC 201412

0%

10%

20%

30%40%

50%

60%

70%

80%

90%

100%

0.00

0.50

1.00

1.502.00

2.50

3.00

3.50

4.00

4.50

5.00

Process A Process B Process C Process C +

precool

DMR

   L   N   G   P   r   o    d   u   c   t   i   o   n ,   M   T   P   A

Production (MTPA) Relative process efficiency (%)

Note: At the same given power about 140 MW (4 GE LM6000) for liquefaction and

precooling cycle

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Specific Power Comparison

6/4/2014TGTC 201413

200

220

240

260

280

300

320

340

360

Process A Process B Process C Process C +

precool

DMR

   S  p  e  c   i   f   i  c  p  o  w  e  r ,   k   W

   h   /   t  o  n   L   N   G

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Effect of Cooling Water Temperature on the

Relative Production to the DMR

6/4/2014TGTC 201415

Each case was compared to the DMR and represented as a percentage of the

corresponding DMR production (100% = production for each DMR case) The difference in production between expanders vs DMR at the same cooling

temperature is smaller at high cooling temperature and vice versa.

High ambient temperature (e.g. tropical) reduces the advantage of the DMR over

the expanders in term of efficiency

60%

70%

80%

90%

100%

Process A Process B Process C DMR

Low temperature Base case High temperature

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Cooling Water System

6/4/2014TGTC 201416

A higher specific power of the process give a higher need for CW Process A is the highest as a consequence of introducing a large amount of

heat into the precooling (ARS)

Process A need a chilled water loop in addition to CW system for theprecooling

CycleProcess

A

Process

B

Process C

DMRWithout

precooling

With CO2 

precooling

Cooling water flow, m3/h

- For precooling cycle per

train

- Total per train

- Total in a 3 MTPA plant

1710

5,448

21,792

336

4,434

17,736

-

4,371

17,484

418

4,181

16,724

-

13,180

13,180

In/Out =17 oC/30 oC

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 Train Configuration and Equipment Count

6/4/2014TGTC 201417

CycleProcess

A

Process

B

Process C

DMRWithout

precooling

With CO2

 

precooling

Number of train per 3 MTPA plant 2 4 4 4 1

The key equipment count per train:

- Compressors

- Pumps

- Compander

- Heat exchanger

(CWHE/PFHE)- Separators

- Water cooled exchanger

Total equipment per train

Common precooling:

- Compressors

- Separator

- Heat exchanger

- Water cooled exchanger

- ARS package (@ 7 MW)

- Chilled water pump (plus back up)

- HRSG unit (additional)

Total equipment per plant

6

0

6

4

(2/2)1

8

25

YES

-

-

-

6

8

2

2

50 + 18

2

0

2

1

(-/1)1

3

9

YES

2

-

1

2

-

-

-

36 + 5

4

0

2

1

(-/1)2

6

15

NO

-

-

-

-

-

-

-

60

4

0

2

1

(-/1)2

6

15

YES

2

1

1

2

-

-

-

60 + 6

6

1

-

2

(2/-)4

6

19

NO

-

-

-

-

-

-

-

19Note: numbers are indicative based on the equipment units shown in the HYSYS simulation and did not consider the size and the duty of the units which may

result in several units in parallel in actual plant. 

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Complexity

6/4/2014TGTC 201418

The number of equipment units indicates complexity ofthe facilities.

Process A is more complex and Process B is the simplest

Even though the DMR has less number of equipment, the

DMR is still considered more complex particularly ifoperational complexity when the plant start up/shut

down and or dealing with feed gas condition changes are

taken into account

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Footprint and weight

6/4/2014TGTC 201419

Footprint is a function of equipment count and dimension

The actual volume flow indicating how large the suction piping needs to beand it may represent facility dimension

Mass and actual volume flow rate of refrigerants into LP compressor inlets:

Weight of a topside processing facility is a function of number ofequipment, thickness and the material use, including structural material

The process with higher number of equipment and operates at higherpressure will be heavier

Process A is considered as the heaviest and having the largest footprint

CycleProcess

A

Process

B

Process C

DMRWithout

precooling

With CO2 

precooling

Refrigerant flow, ton/h 1412 1062 706 (C1+N2) 891 (C1 + N2) 2227

Actual volume flow at

compressor suction1):

- in m3/s

- in m3/h

10.5

37,800

11.9

42,840

8.7 (C1)

31,320 (C1)

6.4 (C1)

23,040 (C1)

25.1

90,360

Niche processes is the smallest

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Hydrocarbon inventory

6/4/2014TGTC 201420

All the expander-based processes evaluated use of a safenon-flammable refrigerant, i.e. nitrogen, or a minimum use

gaseous methane as in Process C

The expander-based processes are therefore ideal for

FLNG where a small hydrocarbon inventory is preferablefrom a safety point of view.

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Motion

6/4/2014TGTC 201421

Except the basic Process C, all the expander basedprocesses subjected to two-phase operation in their

precooling system

They are all subjected to the vessel motion to some

extent.

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Comparison Summary

6/4/2014TGTC 201422

Criteria Process A Process BProcess C

Basic Precooled

Process efficiency Medium Low Medium High 

Complexity/Equipment count High Low  Medium Medium

Footprint High Medium Low  Medium

Weight High Low  Medium Medium

Safety High  Medium Low Low

Sensitivity to motion Medium Medium Low  Medium

Note:

The process, which is considered the most suited for a certain criteria, is highlighted

(bold letter )

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6/4/2014TGTC 201423

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FLNG Overview

6/4/2014TGTC 201424

Combining advance technology

in land-based LNG and

offshore FPSO

Used for monetizing strandedoffshore natural gas

No FLNG currently exist

Has different requirement

compared to land-based LNG

i.e. safety, simplicity, motion,

low weight and small footprint(ExxonMobile, 2013)

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Absorption Refrigeration System

6/4/2014TGTC 201425

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Evaluation and Comparison Basis

6/4/2014TGTC 201426

The same set of conditions were applied to the proposedprocess schemes:

Simulation scope

Feed gas condition and composition

Site conditions (air/cooling water temperature)

Drivers (gas turbines)

Heat exchanger sizing (min temperature approach)

Component efficiency (compressor polytrophic eff.)

LNG product condition (i.e. end flash vapor quantity)

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Simulation scope

6/4/2014TGTC 201427

1) The pretreating units, i.e. CO2 removal, dehydration, mercury removal, are out of the scope of the simulation in this study. The

feed gas is processed, sweet and dry natural gas coming from the pretreating units at the upstream.

2) NGL/LPG extracted in NGL recovery unit (modeled as turboexpander unit) is fully re-injected into LNG feed stream

(assumes that no LPG production and no make up refrigerant required). The NGL/LPG extraction is only for removing BTX

components in the feed gas stream. Those aromatic components are assumed to leave the LNG feed stream in the

condensate (C5+) product.

3) Lean gas leaving the NGL recovery unit enters into the liquefaction circuit at the same temperature/pressure condition as

when it enters the NGL recovery unit (it is recompressed by the booster and cooled by cooling water).

4) The proposed expander-based liquefaction process schemes and the APCI DMR were simulated in this thesis for analysis and

comparison.

5) The LNG condition is set to provide constant end flash vapor quantity. The nitrogen content is assumed to be moderate and

does not necessitate the implementation of a dedicated nitrogen rejection unit.

NGL RecoveryLiquefaction

CycleLNG

NGL/LPG

CO2-free and

dried Feed Gas

Fuel gas for GT

Treated Feed Gas

(C5+ and BTX-free)

C5+

1

2

3

4

5

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Feed Gas

Condition Composition

6/4/2014TGTC 201428

Properties  Feed Gas Stream 

Pressure  60 bar abs 

Temperature  22 oC 

ComponentComposition

(in %-mole)

Nitrogen 1.00

Methane 91.00

Ethane 4.90

Propane 1.70

i-Butane 0.35

n-Butane 0.40

i-Pentane 0.15

n-Pentane 0.15

n-Hexane 0.13

n-Heptane 0.10

n-Octane 0.04

n-Nonane 0.01

n-Decane 0.01

CO2  0.00

H2O 0.00

Benzene 0.03

Toluene 0.02

m-Xylene 0.01

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 The Driver (Gas Turbine)

6/4/2014TGTC 201430

Gas turbines (GT) used as mechanical driver of main refrigerant

compressors and electrical power generation considered are GeneralElectric (GE) LM6000 models

Air temperature was assumed at 27 oC

ISO rated

power (MW)

TIT

(oC)

Exhaust

(oC)

Air flow

(kg/s)Pressure ratio

Efficiency

(%LHV)

42.9 1260 456 124 30 41.7

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Heat Exchanger Sizing

6/4/2014TGTC 201431

A minimum temperature approach of 3 oC in the

cryogenic heat exchangers was assumed

The refrigerant pressure of expander-based processes

was limited to 85 bar

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Component Efficiency

6/4/2014TGTC 201432

Compressors used in the simulation were assumed

centrifugal type that has moderate polytropic efficiency of

78%

The compander polytropic efficiency was based on GE

(Byrne and Mariotti 2010), it was assumed of 73% for thecompressor and 83% for the expander

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Production and Product Quality

6/4/2014TGTC 201433

The intended LNG production is about 3 MTPA with 330 days in a year for

the plant availability.

LNG product was assumed at -149 oC at the exit of cryogenic heat

exchanger and was expanded in end flash column to pressure of 1.38 bar

before going to the storage tanks.

By this condition, the end flash vapor generated is about 8%-mass of LNG

product and nitrogen content is within the LNG specification.

It was assumed that the end flash gas from a single expander process train

covers fuel needed for a gas turbine

LNG Quality

Parameters  LNG Nitrogen, %-mole  < 1 

C5+, %-mole  < 0.1 

BTX, ppm  10 

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HYSYS Model Development

6/4/2014TGTC 201434

Methodology

1. The literature review on the patents and publications of each proposed

process scheme was used as basis information for model development

2. Peng-Robinson EOS was used for calculation of thermodynamic

properties

3. A steady state mode calculation4. Optimizing by varying refrigerant flow rate to obtain the selected

assumption of minimum approach in LNG heat exchangers.

5. The production was determined based on the given gas turbine power as

a mechanical driver for the refrigerant compressors.

6. The key parameters recorded after optimization was LNG production,

UA value of LNG heat exchangers, refrigerant flow rates and specific

power.

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DMR process as basis for comparison

6/4/2014TGTC 201435

This is a modification on an established HYSYS model from Statoil, which was adopted by the author during his previous

work on the specialization project

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HYSYS Model Development

6/4/2014TGTC 201436

DMR Process

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HYSYS Model Development

6/4/2014TGTC 201438

Process B

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HYSYS Model Development

6/4/2014TGTC 201439

Process C

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HYSYS Model Development

6/4/2014TGTC 201440

Process C + precooling

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HYSYS Model Development

6/4/2014TGTC 201441

NGL Extraction Unit

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Sensitivity Analysis

6/4/2014TGTC 201442

Effect of feed gas composition

Effect of treated feed gas pressure

Effect of refrigerant pressure

Effect of gas turbine intake air cooling

Effect of cooling water temperature (heat rejection)

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Effect of Feed Gas Composition

6/4/2014TGTC 201443

A study case with lean feed gas was performed

Production of all processes drops at the same given power (lower efficiency)

The lean gas has a lower condensation temperature i.e. larger temperature lift thus

higher work requirement

90.0%

91.0%

92.0%

93.0%

94.0%

95.0%

96.0%

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Process A Process B Process C Process C +

precool

DMR

   L   N   G   p   r   o    d   u   c   t   i   o   n    (   M   T   P   A

    )

Base Case Production Lean Gas Case Production Drop from base case (%)

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Effect of Feed Gas Composition (cont.)

6/4/2014TGTC 201444

The process C without

precooling tends to suffermore than the other processes(production drops about 9%from its base case)

The production drops is due tothe feed gas is also the basis for

refrigerant in one of thecircuits i.e. the methane richrefrigerant circuit.

Using a leaner feed gas for therefrigerant increases thespecific compression powersince the gas has a highercompressibility (z) and a lowermolecular weight (M)

90.0%

91.0%

92.0%

93.0%

94.0%

95.0%

96.0%

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.004.50

5.00

   L   N   G

   p   r   o    d   u   c   t   i   o   n    (   M   T   P   A    )

Base Case Production

Lean Gas Case Production

Drop from base case (%)

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Effect of treated feed gas pressure

6/4/2014TGTC 201445

Pressure was varied to the limit that

the main HX still withstand For all processes, higher pressure

increases the efficiency

How the pressure effects to the

efficiency in a T-S diagram At higher pressure, the min work and

heat load to liquefy the gas is reduced

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

4.50

4.75

60 70 80 90

   P   r   o    d   u   c   t   i   o   n ,   M   T   P   A

Pressure, bar

DMR Process A Process B Process C

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Effect of refrigerant pressure

6/4/2014TGTC 201446

The refrigerant pressure was limited up to a practical

limit of the cryogenic heat exchangers, which is in this

study limited to 85 bar

200

220240

260

280

300

320

340

360

380

400

Process A Process B

   S  p  e  c   i   f   i  c  p  o  w  e  r ,   k   W   h   /   t  o  n   L   N   G

55 bar 85 bar

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6/4/2014TGTC 201447

Lower air temperature

higher gas turbine power

output

The production capacity

for all processes

potentially increases over

15% when the precooling

system is utilized to cool

gas turbine air intake.

Effect of gas turbine intake air cooling

CycleProcess

A

Process

B

Process C+

CO2 

precooling

Total power (MW) 4 x 45 4 x 451

  4 x 451

 LNG production (MTPA) 4.22  4.13  4.5

% increased in

production 97% 95% 105%

Compander size (MW) 12.6 16.3 10.5

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Integration into Energy System

6/4/2014TGTC 201448

Power and Heat Balance (correspond to the Process A

only since it requires larger amount of heat to drive its

precooling)

150

160

170

180

190

200

210

220

230

240

250

3.3 3.5 3.7 3.9

   M   W

LNG Production

Total heat required Total power required

Heat produced

from 6 GTs

Effect of Cooling Water Temperature to the

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Effect of Cooling Water Temperature to the

ARS system used as Precooling in Process A

6/4/2014TGTC 201449

The COP of the system is lower at higher cooling water temperature

To provide the same amount of cooling duty from this system, increased in heatsupply to the system is required. There will be not enough waste heat to provide

that requirement.

And at higher cooling water temperature, crystallization of the solution in the ARS

is more likely occurs

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

20 22 24 26 28 30

   C   O   P

Cooling Water temperature (oC)

1

2

43

5

6

7

8

9

10

1112

13

14

15

16

1718

Desorber

(generator)

Solution HeatExchanger

 AbsorberEvaporator

Condenser

PumpThrottling Valve

Throttling Valve

Concentrated Solution Vulnerable tocrystallization

Refrigerant

Solution

1

2

43

5

6

7

8

9

10

1112

13

14

15

16

1718

Desorber

(generator)

Solution HeatExchanger

 AbsorberEvaporator

Condenser

PumpThrottling Valve

Throttling Valve

Concentrated Solution Vulnerable tocrystallization

Refrigerant

Solution

 

Effect of Cooling Water Temperature to the

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Effect of Cooling Water Temperature to the

Process A Heat Balance

100

120

140

160

180

200

220

10 15 20 25 30

   M   W

Cooling Water temperature (oC)

Total heat requirement Total heat supply


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