Removal of Benzene From Gasoline

University of Cape Town

Removal of Benzene From Gasoline CHE4049F Project 1: Flowsheet Development and Description

Table of Contents 1. Process Summary ............................................................................................ 1

2. Process Synthesis ............................................................................................ 2

Eliminate Differences in Molecular Types ........................................................ 2

Distribute Chemicals by Matching Sources and Sinks (Mixing and Recycling) 3

Eliminate Differences in Composition (Separations) ........................................ 4

Eliminate Differences in Temperatures, Pressures and Phases ...................... 5

Integration of Tasks (Unit Operations) ............................................................. 6

3. Process Flow Diagram for Benzene Extraction Process using GTC Extraction

Technologies .............................................................................................................. 8

Equipment List ................................................................................................. 9

Stream Table ................................................................................................. 10

4. Rationale ........................................................................................................ 12

Pre-Distillation Column (PDC) ....................................................................... 12

Extractive Distillation Column (EDC) ............................................................. 12

Solvent Recovery (SRC) and Aromatic Gasoline Distillation (AGDC) Columns

...................................................................................................................... 12

5. Thinking about the Benzene Extraction Process ............................................ 13

6. Health, Safety and Environmental Impact Evaluation ..................................... 14

7. Appendix A1: Various Extractive Distillation Options ...................................... 15

8. Appendix A2: Criterion for Separation of Components ................................... 16

A2.1. Specifications for Distillation columns: ................................................. 17

A2.2. Final Product Stream Information ......................................................... 18

9. Appendix A3: Additional Information for Temperature, Pressure and Phase

Changes ................................................................................................................... 19

10. Appendix A4: Enlarged Diagram of Integration of Tasks (Unit Operations) .... 21

11. References ..................................................................................................... 22

1

1. Process Summary

The benzene extraction process begins with combining a Naphtha and C5+ Catalytic

Reformate gasoline stream at a high pressure to ensure adequate mixing. The mixed

stream's pressure is then reduced to 1 atm. before it enters Distillation Column 1 to

separate the C5 and C6 molecules from the heavier components. The distillate from

Column 1 is sent to an Extractive Distillation unit (Column 2) to separate benzene

from the C5 and C6 molecules using a polar solvent.

Benzene leaves with the solvent via the bottoms of Column 2 and is separated at

Distillation Column 4 to yield a high purity stream. The solvent is recycled in a closed

loop back to Column 2. The bottoms product from Column 1 goes to Distillation

Column 3 to separate an aromatic gasoline stream containing C7 and C8’s from the

heavy (C9+) molecules. The heavy molecules exit the column via the bottoms with

the gasoline stream leaving as the distillate of Column 3.

2

2. Process Synthesis

Eliminate Differences in Molecular Types

The benzene extraction process involves no chemical reactions; it comprises only of

the separation of benzene from the gasoline stream. There are, however, a variety of

methods available to achieve this separation. These include:

Sulfolane Extractive Distillation Process

Morphylane Extraction Technology

Distapex Extraction Process

GTC Extractive Distillation Technology

Sulfolane Process:

The Sulfolane system is the most widely used process (in terms of market share)

utilised to extract Benzene from gasoline.

Morphylane Process:

Utilises a single-tower configuration to extract the aromatic compounds from

gasoline.

Distapex Process:

Solvent is non-corrosive and, along with modest operating conditions, means that

the overall plant can be constructed from carbon steel.

GTC Extractive Distillation Process:

Requires fewer, and simpler, pieces of equipment (lower capital requirements) than

other extractive distillation systems. The solvent (Techtiv-100) has a lower toxicity

and is less corrosive than solvents utilised in other systems whilst still increasing the

boiling point of benzene by the greatest factor.

There are many more advantages for each of these systems. However, the process

chosen to separate benzene from the given gasoline stream is that of GTC

Extractive Distillation. This is because GTC Technologies offers the most feasible

choice in terms of required capital investment. It also has the smallest impact on the

environment (most energy efficient) due to its solvent-to-feed ratio being the lowest

(See Appendix A1).

3

Naphtha25 000 kg/hr40.0 °C9.88 atm

Catalytic Reforming Gasoline34 300 kg/hr182 °C8.89 atm

Benzene Recovery

Unit

Benzene10 900 kg/hr

44 ° C

Raffinate Stream8400 kg/hr

44 °C

Aromatic Gasoline32 800 kg/hr

44 °C10.5 atm

Heavy Aromatics7200 kg/hr

44 °C10.5 atm

Distribute Chemicals by Matching Sources and Sinks (Mixing and Recycling)

The only mixing point in the system is between that of the two feed streams. The

naphtha and reformate streams are mixed at high pressure to ensure that the

various volatile components remain as liquids in the stream.

The solvent (Techtiv-100) is loaded into the Extractive Distillation section as a batch

unit of 30 000 kg. This mass is circulated internally around the system via a recycle

loop between the Solvent Recovery unit and the Extractive Distillation unit with

negligible losses to the respective product streams.

Figure 1: Overall summary of the Benzene Extraction system

4

Pre-Distillation

Column

Extractive Distillation

Column

Aromatic Gasoline

Distillation Column

Heavy Aromatics7200 kg/hr

44 °C10.5 atm

Aromatic Gasoline32 800 kg/hr

44 °C10.5 atm

Solvent Recovery Column

Benzene10 900 kg/hr

44 ° C

Raffinate Stream8400 kg/hr

44 °C

Catalytic Reforming Gasoline34 300 kg/hr182 °C8.89 atm

Naphtha25 000 kg/hr40.0 °C9.88 atm

Eliminate Differences in Composition (Separations)

There are significant differences in the boiling points of the various components in

the naphtha and reformate streams (See Appendix A2, Table 6). Thus, a series of

distillation columns can be used to separate the lighter components from the heavier

ones. These columns are to operate at 1 atm. and moderate temperatures to negate

the need to use more exotic materials of construction required when operating a

distillation column at high pressures.

Column 1 (Pre-Distillation unit) operates such that all the C5 components and the

majority of the C6 compounds are recovered to the top of the column. It was noted

that because some C7 isomers have boiling points close to the feed temperature

(85 °C) a certain fraction will leave the column in the distillate stream. The heavier

C7+ molecules all exit Column 1 via the bottoms.

Column 2 is the GTC Extractive Distillation column. Here, benzene is separated from

the remaining C5, C6 and C7 compounds due to the presence of the polar solvent.

The solvent (Techtiv-100) increases benzene's boiling point (See Appendix A2,

Table 7) to allow it to be recovered as the bottoms product of Column 2. Benzene is

then separated from the solvent in Column 4 (Solvent Recovery unit) to produce a

bottoms stream consisting of pure solvent which is recycled back to the Extractive

Distillation column.

In Column 3 (Aromatic Gasoline recovery unit), separation occurs between the C8

and C9 hydrocarbons. The distillate of Column 3 is the gasoline stream with a

benzene specification that meets the new regulations (maximum of 1 volume %).

Figure 2: Proposed separation process to remove benzene from the gasoline stream

5

Eliminate Differences in Temperatures, Pressures and Phases

The two feed streams entering the Benzene Extraction system are at different

temperatures and pressures. Due to the naphtha stream being at a greater pressure,

its pressure is reduced to 8.89 atm. The resultant mixed stream's temperature of

122 °C was approximated by calculating a weighted average stream temperature

(See Appendix A3, Eqn 2). The pressure of the mixed stream is then reduced to 1

atm. The following T, P or Phase changes occur throughout the rest of the process:

Table 1: Summary of Changes in Phase, Temperature and Pressure for the Major streams in Benzene Extraction Process

*Column 1 Feed is cooled to 85 °C Column 2 Distillate is pressurised to 3.5 atm.

*Column 1 Distillate (DC2 Feed) is heated to 100 °C Column 4 Distillate is pressurised to 3 atm.

*Column 1 Bottoms (DC3 Feed) is heated to 145 °C Column 3 Distillate is pressurised to 10.5 atm.

Column 2 Distillate is cooled to 44 °C Column 3 Bottoms is pressurised to 10.5 atm.

*Column 2 Bottoms (DC4 Feed) is heated to 120 °C Condensers on all columns change vapour phase

distillate to saturated liquid phase Column 3 Distillate is cooled to 44 °C

Column 3 Bottoms is cooled to 44 °C Reboilers on all columns partially vaporise liquid

phase bottoms stream to vapour phase *Above temperatures for column feed streams are determined as

approximate bubble point temperatures (See Appendix A3, Eqn 3)

Figure 3: Flowsheet with Temperature, Pressure and Phase-change operations in Benzene Extraction process (See Appendix A3 for enlarged diagram)

6

Integration of Tasks (Unit Operations)

1. Pre-Distillation Column (PDC). Operating at 85 °C and 1 atm, it separates the C5

and C6 compounds from the heavier C7+ molecules. This column is of paramount

importance as it reduces the load placed on the Extractive Distillation Column. This

is due to there being other heavier aromatic compounds present in the feed stream.

The PDC thus aids in producing a final benzene stream which meets the required

specifications.

2. Extractive Distillation Column (EDC). It is at this point that benzene actually

gets removed from the system. In the EDC a polar solvent (Techtiv-100) flows from

the top of the column to absorb benzene from the non-aromatic hydrocarbons. The

GTC process is able to remove 99.9 wt-% of the benzene entering the column. The

aromatic-lean raffinate flows out the top of the column where it is condensed, cooled

to 44 °C and then pressurised to 3.5 atm to ensure the most volatile components

remain as a saturated liquid.

3. Aromatic Gasoline Distillation Column (AGDC). The AGDC is responsible for

recovering the majority (99 wt-% min.) of the C8 aromatics to the distillate along with

the C7 components from the heavier C9+ molecules.

Figure 4: Flowsheet showing Task Integration for Benzene Extraction process (See Appendix A4 for enlarged diagram)

7

4. Solvent Recovery Column (SRC). A 99.9 wt-% pure benzene stream is

recovered via the distillate of the SRC while the pure solvent stream is sent back to

the EDC. The operating temperature was found to be 120 °C at a pressure of 1 atm.

This temperature was calculated using the enhanced relative volatility between n-C7

and benzene and estimating the new boiling point of benzene after it is absorbed into

the solvent stream (See Appendix A2, table 7). Because its boiling point is so much

greater than the hydrocarbons, it was assumed that no solvent exits with the distillate

stream. Thus the solvent loaded into the system initially is operating in a closed loop

manner.

5. Condensers and Coolers. Since none of the overhead streams required cooling

to below 40 °C, it was deemed unnecessary to utilise refrigerated water to provide

cooling duty to the condensers. These systems are more energy intensive than

traditional cooling water streams (re-cooling utilities stream to 10 °C vs. 30 °C). The

same logic was applied to the other various cooling units.

6. Reboilers and Heaters. The type of steam used in the reboiler was dependent on

the required temperature of the stream. If a stream required heating to below 135 °C,

low pressure steam was used. Between 135 and 170 °C, medium pressure is to be

used and greater than 170 °C would require high pressure stream for adequate

heating duty. The use of steam is more feasible and ecological than the use of fuel

gas heater.

8

CHE4049F Project 1PFD for Benzene Recovery Unit [Area 101]Nicholas Munsami [MNSNIC002]01 March 2013

P-15

Naphtha

ReformateGasoline

101-L-01 101-L-02

101-C-01

101-H-02

cw

101-V-01101-P-01

101-C-02

101-H-06

101-V-02101-P-03

101-C-03

101-H-08

101-V-03101-P-05

101-C-04

101-H-11

101-V-04101-P-07

101-P-08101-P-02

101-P-04

101-H-01

101-H-03

101-H-07

101-H-12

101-H-09

cw

cw

cw

HPS

LPS

LPS

HPS

101-H-04

101-H-05

101-H-13

101-H-14

101-H-15

101-H-16

Raffinate

Benzene

AromaticGasoline

Heavy Aromatics

1

2 4 5

7

8

9

10

101-H-10

11

12

14

15

16

cw

LPS

6

cw

LPS

cw

MPS

cw

cw

101-P-06

101-L-03

13

17

18

19

3

3. Process Flow Diagram for Benzene Extraction Process using GTC Extraction Technologies

Figure 5: PFD of Proposed Benzene Extraction Process

9

Equipment List

Table 2: Equipment List for Benzene Extraction Process

Equipment ID Name Description

101-C-01 Pre-Distillation Column (PDC) Carbon steel shell

316 Stainless steel trays

101-C-02 Extractive Distillation Column (EDC)

Carbon steel shell 316 Stainless steel trays

101-C-03 Aromatic Gasoline Distillation Column (AGDC)


101-C-04 Solvent Recovery Column (SRC)


101-H-01 PDC Feed cooler Carbon steel (85 °C) 101-H-02 PDC Distillate condenser Carbon steel (68.4 °C)

101-H-03 PDC Bottoms reboiler 316 Stainless steel (111 °C)

101-H-04 EDC Feed heater 316 Stainless steel (100 °C)

101-H-05 AGDC Feed heater 316 Stainless steel (145 °C)

101-H-06 EDC Distillate condenser Carbon steel (55.3 °C)

101-H-07 EDC Bottoms reboiler 316 Stainless steel (80.1 °C)

101-H-08 AGDC Distillate condenser Carbon steel (116 °C)

101-H-09 AGDC Bottoms reboiler 316 Stainless steel (170 °C)

101-H-10 SRC Feed heater 316 Stainless steel (120 °C)

101-H-11 SRC Distillate condenser Carbon steel (80 °C)

101-H-12 SRC Bottoms reboiler 316 Stainless steel (200 °C)

101-H-13 EDC Raffinate cooler Carbon steel (44 °C)

101-H-14 Benzene Product cooler Carbon steel (44 °C)

101-H-15 Aromatic Gasoline cooler Carbon steel (44 °C)

101-H-16 Heavy Aromatics cooler Carbon steel (44 °C)

101-L-01 Naphtha Feed Pressure reduction valve

High strength steel alloy 9.88 - 8.89 atm

101-L-02 PDC Feed Pressure reduction valve


101-L-03 AGDC Distillate Pressure reduction valve


101-P-01 PDC Reflux pump Stainless steel centrifugal pump

1.2 atm. Operating P.

101-P-02 PDC Bottoms pump Stainless steel centrifugal pump


101-P-03 EDC Reflux pump Stainless steel centrifugal pump


101-P-04 EDC Bottoms pump Stainless steel centrifugal pump


101-P-05 AGDC Reflux pump Alloy 20 centrifugal pump 10.5 atm. Operating P.

101-P-06 AGDC Bottoms pump Alloy 20 centrifugal pump 10.5 atm. Operating P.

101-P-07 SRC Reflux pump Stainless steel centrifugal pump

3 atm. Operating P.

101-P-08 SRC Bottom pump Alloy 20 centrifugal pump


101-V-01 PDC Reflux drum Carbon steel

101-V-02 EDC Reflux drum Carbon steel

101-V-03 AGDC Reflux drum Carbon steel

101-V-04 SRC Reflux drum Carbon steel

10

Stream Table

Table 3: Stream Table for Benzene Extraction Process

Stream 1 2 3 4 5 6 7 8 9 10

Pressure 8.89 atm 8.89 atm 8.89 atm 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm 3.5 atm

Temperature 182 °C 40 °C 122 °C 122 °C 85 °C 68.4 °C 111 °C 100 °C 145 °C 55.3 °C

Total Mass Flow 34300 Kg/hr 25000 Kg/hr 59300 Kg/hr 59300 Kg/hr 59300 Kg/hr 20300 Kg/hr 39000 Kg/hr 20300 Kg/hr 39000 Kg/hr 9750 Kg/hr

Total Molar Flow 370 Kmol/hr 262 Kmol/hr 637 Kmol/hr 637 Kmol/hr 637 Kmol/hr 257 Kmol/hr 379 Kmol/hr 257 Kmol/hr 379 Kmol/hr 122 Kmol/hr

Component Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole %

1-Pentene 5.50 7.27 0 0 3.18 4.22 3.18 4.22 3.18 4.22 9.28 10.5 0 0 9.28 10.48 0 0 19.3 22.1

Branched Paraffins 1.20 1.54 3 3.98 1.96 2.53 1.96 2.53 1.96 2.53 5.72 6.28 0 0 5.72 6.28 0 0 11.9 13.3

Cyclopentane 2.50 3.30 0 0 1.44 1.92 1.44 1.92 1.44 1.92 4.22 4.76 0 0 4.22 4.76 0 0 8.78 10.1

C6 Paraffins 5.60 4.71 0 0 3.24 3.50 3.24 3.50 3.24 3.50 9.45 8.69 0 0 9.45 8.69 0 0 19.7 18.4

Cyclohexane 0 0 10.2 11.6 4.30 4.76 4.30 4.76 4.30 4.76 11.9 11.2 0.327 0.400 11.9 11.2 0.327 0.400 24.9 23.7

Benzene 14.6 17.3 23.5 28.7 18.4 21.9 18.4 21.9 18.4 21.9 52.0 52.7 0.838 1.10 52.0 52.7 0.838 1.10 0.108 0.111

C7 Paraffins 4.60 4.26 10.5 10.0 7.09 6.59 7.09 6.59 7.09 6.59 7.24 5.73 7.01 7.20 7.24 5.73 7.01 7.20 15.0 12.0

C7 Dienes 0.35 0.338 0 0 0.203 0.196 0.203 0.196 0.20 0.20 0.177 0.146 0.216 0.231 0.177 0.146 0.216 0.231 0.366 0.306

Toluene 39.5 39.7 16.2 16.8 29.7 30.0 29.7 30.0 29.7 30.0 0 0 45.1 50.4 0 0 45.1 50.4 0 0

C8 Paraffins 0.25 0.204 7 5.86 3.10 2.53 3.10 2.53 3.10 2.53 0 0 4.71 4.25 0 0 4.71 4.25 0 0

Ethyl Benzene 20.0 17.4 0 0 11.56 10.14 11.6 10.1 11.6 10.1 0 0 17.6 17.0 0 0 17.6 17.0 0 0

Styrene 0.05 0.044 0 0 0.0287 0.0256 0.0287 0.0256 0.0287 0.0256 0 0 0.0436 0.0431 0 0 0.0436 0.0431 0 0

Xylene 0 0 8.9 8.01 3.76 3.29 3.76 3.29 3.76 3.29 0 0 5.71 5.54 0 0 5.71 5.54 0 0

C9 Paraffins 1.90 1.37 6.2 4.62 3.71 2.70 3.71 2.70 3.71 2.70 0 0 5.65 4.54 0 0 5.65 4.54 0 0

Cumene 0 0 3.6 2.86 1.52 1.18 1.52 1.18 1.52 1.18 0 0 2.31 1.98 0 0 2.31 1.98 0 0

C10s 2.75 1.79 0 0 1.59 1.04 1.59 1.04 1.59 1.04 0 0 2.42 1.75 0 0 2.42 1.75 0 0

Cyclic Paraffins 0 0 9.3 6.34 3.92 2.61 3.92 2.61 3.92 2.61 0 0 5.97 4.38 0 0 5.97 4.38 0 0

Naphthalene 0 0 1.6 1.19 0.675 0.490 0.675 0.490 0.68 0.49 0 0 1.03 0.824 0 0 1.03 0.824 0 0

C11 HCs 1.20 0.711 0 0 0.694 0.413 0.694 0.413 0.69 0.41 0 0 1.06 0.695 0 0 1.06 0.695 0 0

11

Table 4: Stream Table for Benzene Extraction Process continued

Stream 11 12 13 14 15 16 17 18 19

Pressure 1 atm 1 atm 10.5 atm 10.5 atm 3.5 atm 3 atm 3 atm 10.5 atm 10.5 atm

Temperature 80.1 °C 120 °C 116 °C 170 °C 44 °C 80 °C 44 °C 44 °C 44 °C

Total Mass Flow 10550 Kg/hr 10550 Kg/hr 32800 Kg/hr 6200 Kg/hr 9750 Kg/hr 10550 Kg/hr 10550 Kg/hr 32800 Kg/hr 6200 Kg/hr

Total Molar Flow 135 Kmol/hr 135 Kmol/hr 335 Kmol/hr 46.0 Kmol/hr 122 Kmol/hr 135 Kmol/hr 135 Kmol/hr 335 Kmol/hr 46.0 Kmol/hr

Component Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole % Wt-% Mole %

1-Pentene 0 0 0 0 0 0 0 0 19.3 22.1 0 0 0 0 0 0 0 0

Branched Paraffins 0 0 0 0 0 0 0 0 11.9 13.3 0 0 0 0 0 0 0 0

Cyclopentane 0 0 0 0 0 0 0 0 8.78 10.1 0 0 0 0 0 0 0 0

C6 Paraffins 0 0 0 0 0 0 0 0 19.7 18.4 0 0 0 0 0 0 0 0

Cyclohexane 0 0 0 0 0.389 0.454 0 0 24.9 23.7 0 0 0 0 0.389 0.454 0 0

Benzene 99.9 99.9 99.9 99.9 0.995 1.25 0 0 0.108 0.111 99.9 99.9 99.9 99.9 0.995 1.25 0 0

C7 Paraffins 0.0976 0.0761 0.0976 0.0761 8.33 8.16 0 0 15.0 12.0 0.0976 0.0761 0.0976 0.076 8.33 8.16 0 0

C7 Dienes 0.00341 0.00277 0.00341 0.00277 0.256 0.262 0 0 0.366 0.306 0.00341 0.00277 0.00341 0.00277 0.256 0.262 0 0

Toluene 0 0 0 0 53.6 57.1 0 0 0 0 0 0 0 0 53.6 57.1 0 0

C8 Paraffins 0 0 0 0 5.60 4.81 0 0 0 0 0 0 0 0 5.60 4.81 0 0

Ethyl Benzene 0 0 0 0 20.9 19.3 0 0 0 0 0 0 0 0 20.9 19.3 0 0

Styrene 0 0 0 0 0.0477 0.0449 0.0220 0.0285 0 0 0 0 0 0 0.0477 0.0449 0.0220 0.0285

Xylene 0 0 0 0 6.52 6.02 1.44 1.83 0 0 0 0 0 0 6.52 6.02 1.44 1.83

C9 Paraffins 0 0 0 0 3.36 2.57 17.8 18.7 0 0 0 0 0 0 3.36 2.57 17.8 18.7

Cumene 0 0 0 0 0 0 14.6 16.3 0 0 0 0 0 0 0 0 14.6 16.3

C10s 0 0 0 0 0 0 15.3 14.4 0 0 0 0 0 0 0 0 15.3 14.4

Cyclic Paraffins 0 0 0 0 0 0 37.7 36.1 0 0 0 0 0 0 0 0 37.7 36.1

Naphthalene 0 0 0 0 0 0 6.48 6.80 0 0 0 0 0 0 0 0 6.48 6.80

C11 HCs 0 0 0 0 0 0 6.66 5.73 0 0 0 0 0 0 0 0 6.66 5.73

12

4. Rationale

Solving the mass balance for the Benzene Extraction unit involved making various

assumptions with regards to the degree of separation occurring in the respective

distillation columns.

Pre-Distillation Column (PDC)

The feed temperature of the column was taken as 85 °C because of cyclohexane. This

ensured that all C5 and C6 compounds will be in the vapour phase inside the column

since cyclohexane has the highest boiling point.

Because the C7 paraffins in the feed stream boil at temperatures close to the feed

temperature (90 - 98 °C), it was assumed that a fraction of the C7s exits in the distillate

of the column (35 % to top). The components that did not distribute were those which

had boiling points 15+°C higher or lower than the feed temperature as they will either be

all vapour or liquid in the column. The final distribution of C7 was found such that the

minimum specification for the benzene product stream was achieved.

Extractive Distillation Column (EDC)

In a traditional GTC Extraction process, benzene is separated relative to n-heptane (n-

C7). However, in this EDC system, benzene is separated relative to cyclohexane. Since

these compounds have near-identical boiling points, the solvent would have increase

the relatively volatility substantially. To approximate this difference, the relative volatility

between n-C7 and benzene was found over a range of temperatures. These factors

were compared to the enhanced separation factor (α = 2.44) when using the Techtiv-

100 solvent (See Appendix A1) to determine the EDC feed temperature (T = 100 °C).

Solvent Recovery (SRC) and Aromatic Gasoline Distillation (AGDC) Columns

The SRC was the simplest column to solve since the solvent has a boiling very much

higher than benzene. Thus, perfect separation was assumed to occur in the SRC. In the

AGDC, C8 was separated (at 145 °C) from C9+ with some C9 (50 %) being recovered to

the distillate due to its boiling point range (145 to 151 °C). All chosen splits were based

on the boiling points of the respective components.

13

5. Thinking about the Benzene Extraction Process

The current design of the system does not show any emergency relief/dump valves.

In reality these will be readily available if the benzene stream is to be purged for

some safety concerns. This purge stream will not, however, simply be vented to the

atmosphere since benzene is carcinogenic in nature.

A safer option would be to send the purge stream to a flare to carefully combust the

vented benzene, under strictly-controlled conditions, to release carbon dioxide and

water into the atmosphere rather.

The GTC Extraction process ensures that the maximum amount of benzene can be

recovered from the gasoline stream. Ideally one would like to be operating the

system well above the minimum required specifications in order to maximise product

production. However, this may not be feasible in reality.

A better approach would be to operate the system such that the minimum

specifications are met. Hence the plant would be operating at its most efficient state

to produce quality product. This was the type of design philosophy considered when

configuring the proposed system.

14

6. Health, Safety and Environmental Impact Evaluation

With all chemical plants, there is the inherent danger associated with the

production/separation of various chemical species. The Petrochemical industry

specifically, utilises highly volatile organic compounds (VOCs) in the production and

refinement of various fuels as is the case with the proposed design.

To make the proposed design inherently safer, streams containing highly volatile

components are pressurised above the minimum pressure required for the stream to

be a saturated liquid when stored. This ensures that the vapour content in the

system, and hence flammability limits, are not reached in the system.

Furthermore, the distillation columns are operated at 1 atm to avoid utilising multiple

high-pressure vessels on a single plant unit. Once again, this decreases the risk

associated with having multiple volatile compounds in the system (reduced

explosion risk due to potential vessel ruptures when operating at high pressures).

Benzene, specifically, is of grave concern as it is both flammable and carcinogenic.

Thus extra care has to be taken to ensure that pipelines neither leak nor have

benzene vapours forming inside them. To avoid vapours in the pipelines, the

benzene product stream is pressurised to 3 atm at a temperature of 44 °C even

though it would be a liquid at 1 atm and 44 °C.

15

7. Appendix A1: Various Extractive Distillation Options

Sulfolane Process:

Utilises both liquid-liquid extraction and extractive distillation techniques to

achieve separation.

Technology has been available since the 1960s (well-established operating

procedures).

Can achieve a benzene recovery of 99.9 wt-% with levels of non-aromatics being

less than 100 ppm in the benzene product stream.

Solvent has some corrosive properties in the solvent extraction unit.

Morphylane Process:

Does not require a raffinate wash stream (unlike Sulfolane process)

Single column configuration reduces capital costs for plant

Single column design is more complicated than other process designs (less

inherently safe plant design).

Distapex Process:

Information on this process not readily available (proprietary information)

Requires benzene feed concentrations of above 80% to achieve 99.5% benzene

recovery.

GTC Extractive Distillation Technology

Table 5: Comparison of solvent effect on the relative volatility of n-heptane to benzene

Solvent Solvent-to-Feed ratio Relative Volatility (nC7/Benzene)

Techtiv-100 3.00 2.44

Sulfolane 3.00 2.00

N-formyl Morpholine 3.00 1.89

CAROM 3.00 1.35

The table above (Kolmetz et al.) shows that for a given solvent-to-feed ratio, Techtiv-

100 (GTC solvent) would require the least amount of solvent as it has the greatest effect

on the boiling point of benzene. This, along with the simple, yet effective process

operation results in GTC extraction technologies being the process of choice.

16

8. Appendix A2: Criterion for Separation of Components

Table 6: Boiling point temperatures of gasoline feed stream components at 1 atm.

Component Boiling Point (°C)

Component Boiling Point (°C)

Tlow Thigh Tlow Thigh

1-Pentene 30.0 Ethyl Benzene 136

Branched Paraffins 9.50 27.7 Styrene 145

Cyclopentane 44.3 Xylene 140

C6 Paraffins 49.7 68.8 C9 Paraffins 143 151

Cyclohexane 80.7 Cumene 152

Benzene 80.1 C10s 174

C7 Paraffins 90.1 98.5 Cyclic Paraffins 171 181

C7 Dienes 90 108 Naphthalene 218

Toluene 111 C11 HCs 196

C8 Paraffins 99.3 126

Note that for the unknown hydrocarbons, the isomers with the lowest and highest boiling

points were used as a range to approximate the distribution of those various

components.

The following formula was used to account for the effect that the solvent has on

benzene's boiling point:

..................................................................................................Equation 1

Where theta is an enhancement factor. Theta was calculated over a range of

temperatures (65-95 °C) to yield and average value of 4.16 (since αij is constant). This

enhancement factor was then utilised to determine benzene's vapour pressure when it

is dissolved in the solvent by dividing benzene's normal vapour pressure by theta.

Table 7: Comparison of vapour pressures for Benzene before and after absorption into solvent between 65 and 95 °C

Temperature (°C) Pvap (with solvent)

atm Pvap (normal)

atm Temperature

(°C)

Pvap (with solvent)

atm

Pvap (normal)

atm 65 0.144 0.613

70 0.170 0.725 91 0.364 1.38

75 0.200 0.852 92 0.417 1.42

80 0.234 0.997 93 0.477 1.46

85 0.272 1.16 94 0.543 1.51

90 0.316 1.34 95 0.782 1.55

17

A2.1. Specifications for Distillation columns:

The following specifications were used to meet all the specifications of the design brief:

Pre-Distillation Column

Operating Temperature : 85°C

Operating Pressure : 1 atm (for all columns)

Benzene recovery : 98 wt-% to distillate

Cyclohexane recovery : 97 wt-% to distillate

C7 paraffins recovery : 35 wt-% to distillate

C7 dienes recovery : 30 wt-% to distillate

All other components were assumed to be non-distributing (light components all went to

the distillate while the heavier compounds all went to the bottoms).

Extractive Distillation Column

Operating Temperature : 100 °C

All non-aromatic C6 and C5 to distillate stream

C7s recovery : 99 wt-% to distillate

Benzene recovery : 0.1 wt-% to distillate

Aromatic Gasoline Distillation Column


Xylene recovery : 96 wt-% to distillate

C9s recovery : 50 wt-% to distillate

All other C7/C8 compounds to distillate and remaining heavy C9+ leaves via bottoms.



Perfect separation occurs between benzene (distillate) and the heavy solvent (bottoms).

18

A2.2. Final Product Stream Information

Table 8: Mass and Volumetric flowrates for the Aromatic Gasoline stream (Benzene vol-% < 1%)

(Aromatic Gasoline Stream)

Compound Volumetric Flow (m3/hr) Volume % Density (kg/m3) Mass Flowrate (Kg/hr)

Cyclohexane 0.164 0.414 779 128

Benzene 0.372 0.941 877 326

C7 Paraffins 4.03 10.2 677 2730

C7 Dienes 0.118 0.299 710 84.0

Toluene 20.3 51.3 867 17600

C8 Paraffins 2.64 6.67 696 1840

Ethyl Benzene 7.91 20.0 867 6850

Styrene 0.0172 0.0435 909 15.6

Xylene 2.48 6.27 861 2140

C9s 1.54 3.89 716 1100

Total 39.5 32800

Recovery of feed C8 aromatics to gasoline stream : 99.0 wt-%

Purity of benzene product stream : 99.9 wt-%

Non-aromatics in benzene product stream : 0.10 wt-%

19

P-15Pre-

Distillation Column


Column

Aromatic Gasoline

Distillation Column


? P ? P ? T

?Phase

? T

?Phase

? P ? T

?Phase

?T

? Phase

?T?Phase

?Phase

? T

?T

?P

?P

? Phase

?T

?Phase

Naphtha25 000 kg/hr40 °C, 9.88 atm

CR Gasoline34 300 kg/hr182 °C, 8.89 atm


Column 1 Feed59 300 kg/hr122 °C, 1 atm


Column 1 Distillate20 300 kg/hr68.4 °C, 1 atm


Column 1 Bottoms39 000 kg/hr111 °C, 1 atm


Column 2 Distillate9750 kg/hr55.3 °C, 1 atm

Raffinate Stream9750 kg/hr55.3 °C, 3.50 atm

Raffinate Stream9750 kg/hr44 °C, 3.50 atm

Aromatic Gasoline32 800 kg/hr116 °C, 1 atm

Aromatic Gasoline32 800 kg/hr44 °C, 1 atm

Aromatic Gasoline32 800 kg/hr44 °C, 10.5 atm

Heavy Aromatics6200 kg/hr170 °C, 1 atm


Heavy Aromatics6200 kg/hr44 °C, 10.5 atm

Column 2 Bottoms10 550 kg/hr80.1 °C, 1 atm


Column 4 Distillate10 550 kg/hr80 °C, 3 atm

Purified Benzene10 550 kg/hr44 °C, 3 atmP P

P

P

P

P

P ?P

P

9. Appendix A3: Additional Information for Temperature, Pressure and Phase Changes

Figure 6: Enlarged Diagram for Temperature, Pressure and Phase Changes

20

The temperature of the feed stream was estimated by taking a weighted average of

the temperatures of the two streams in terms of mass flow rates. It is given by the

following:

....................................................................................................Equation 2

A similar method was used to estimate the temperatures of the distillate and bottoms

streams leaving the various distillation columns; except that the weighted average

was now in terms of mole fractions and boiling point temperatures.

∑ ...............................................................................................Equation 3

A more accurate method to determine these stream temperatures is to use the

Antoine equation for vapour pressures to solve for the temperature at which the

Bubble/Dew point pressure is equal to the system's pressure.

However, due to time constraints and insufficient information with regards the actual

composition of the gasoline stream, it was not possible to calculate these

temperatures more accurately at this time using the previously mentioned method.

21

Pre-Distillation

Column1 atm


Column1 atm

Aromatic Gasoline

Distillation Column

1 atm


1 atm

CR Gasoline34 300 kg/hr182 °C, 8.89 atm




Column 1 Distillate20 300 kg/hr68.4 °C, 1 atm


Column 1 Bottoms39 000 kg/hr111 °C, 1 atm


Raffinate Stream9750 kg/hr55.3 °C, 3.50 atm

Raffinate Stream9750 kg/hr44 °C, 3.50 atm





Heavy Aromatics6200 kg/hr44 °C, 10.5 atm

Column 2 Bottoms10 550 kg/hr80.1 °C, 1 atm


Column 4 Distillate10 550 kg/hr80 °C, 3 atm

Purified Benzene10 550 kg/hr44 °C, 3 atm

Cooler

Reboiler

Condenser

Reflux Drum

Heater Condenser

Reboiler

Reboiler

Reboiler

CoolingWater

Heater

LowPressureSteam

CoolingWater

Condenser

CoolingWater

Reflux Drum

LowPressureSteam

LowPressureSteam

Heater

Condenser

CoolingWater

CoolingWater

MediumPressureSteam

LowPressureSteam

HighPressureSteam

Condenser

CoolingWater

HighPressureSteam

Condenser

CoolingWater

Condenser

CoolingWater

Condenser

CoolingWater

Reflux Drum

Reflux Drum

Pump

Pump

Pump

Pump

Pump

Pump

Pump

Pump

10. Appendix A4: Enlarged Diagram of Integration of Tasks (Unit Operations)

Figure 7: Enlarged Diagram for Integration of Tasks (Unit Operations)

22

11. References

Kolmetz, K., et.al. (2008). "Guidelines for BTX Distillation Revamps." GTC

Proprietary Licensed Technologies.

UOP LLC, (1999). "Sulfolane Process." Aromatics and Derivatives.

Colwell, R.F, (2010). "Benzene in Gasoline: Regulations and Remedies". Process

Engineering Associates, LLC

Petrochemical Processes 2005, Hydrocarbon Processing

Netzer, D, et.al. (2002). "Improve benzene production from refinery sources".

Process Technology-Petrochemical, 71 - 78.

Biaohua, C., Zhigang, L., and Jianwei, L. (2003). "Separation on Aromatics and Non-

Aromatics by Extractive Distillation with NMP."Journal of Chemical Engineering of

Japan 36(1), 20 - 24

Yee, C.F, et.al. (2000)."Application of Extractive Distillation for the Separation of

Benzene and Cyclohexane Mixture." Symposium of Malaysian Chemical Engineers

(SOMChE 2000)

Duvekot, C., (2008). "Fast Detailed Hydrocarbon Analysis of Naphtha". Varian, Inc.

Mehrotra, A.K., Tiwary, D., (2006). "Understand Temperature Change in Process

Stream Mixing". Heat Transfer, 33 - 38

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Removal of Benzene From Gasoline

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