Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes Small

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes Muneeb Nawaz Supervisor: Dr Megan Jobson

L07- 1

PIRC Annual Research Meeting 2009

Centre for Process Integration © 2009

When a design problem dealing with low temperature separations is approached in a systematic way, the number of separation alternatives to be studied is generally very large. The assessment of all possible flowsheets with numerous options is a time consuming task with a lot of simulations required to select the economically best option. Therefore, a systematic approach for synthesis is required to generate effective and economic design of such processes like demethanizer system with minimal time and effort. To define an optimal configuration problem, it is necessary to devise an adequate superstructure of the integrated process with new demethanization processes that is rich enough to account for all potential configurations and connectivity. This aim of this research is to extend an existing platform for the design of heat integrated separation systems to address the demethanizer subsystem which is characterized by complex interactions between the distillation column and other flowsheet components, including turbo expander, flash units, multistream exchangers and refrigeration cycles. Opportunities for heat-integration within the subflowsheet and also in the overall flowsheet are identified to improve the economics of the process. Case study demonstrates the strength of the methodology developed.

Outline 1. Problem Statement 2. Overview of Previous Research a. Introduction b. Limitations

3. System Parameter Identification 4. Column Design Method a. Need for a new method b. Boundary Value Design Method c. Case Study

5. Heat Recovery in Multistream Exchanger 6. Overall Flowsheet Synthesis and Optimization 7. Conclusions & Future Work

Centre for Process Integration © 2009

L07- 2

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

1. Problem Statement

Centre for Process Integration © 2009

L07- 3

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

A Typical Low Temperature Gas Separation Process

Products

Feed Cold Box

Centre for Process Integration © 2009

L07- 4

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Low temperature processes usually consist of three main components; the process i.e distillation, the heat exchange system (cold box) and the refrigeration system. It is important to note that compression and refrigeration constitute the main factors that regulate both capital and operating costs in this type of plant. The continuous emphasis on increasing plant efficiency, driven by the increased cost of energy and feedstock, has led designers and owners to drive down the operating and installation cost.

Demethanizer Subsection

For Example

Ethane recycle Drying & chilling

Ethane

Cold box & Expander

Ethane cracking furnaces LPG E/P

Ethylene sales Storage

Acethylene reactions

LPG splitter

Propane Buy

Off-gas sales

Propylene sales

Propane cracking furnaces

Caustic Wash

Sell

Propane C4+ sales Propane recycle

Ethane/Propane cracking process diagram for ethylene production * * The Dow Chemical Company, 2005, 17th Annual Ethylene Producer’s Conference Centre

L07- 5

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The figure illustrates Dow’s process for an ethylene plant based on a gas feedstock (ethane and propane). After cracking the feed in the furnace it is sent to the demethanizer section where the methane\ethane separation takes place at low temperatures. The C2 splitter follows the demethanizer. The recovered ethane and propane from deethanizer are recycled to the cracking furnaces.

ABB Lummus Global Residual Recycle High Ethane Recovery Process *

* Patel et al., US 7,107,788 (2006) Centre

L07- 6

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The figure illustrates ABB Lummus residual recycle process for high ethane recovery from the natural gas. The process is flexible to recover ethane or propane depending upon the market conditions. The process utilizes an additional reflux steam leaner in ethane and propane compared to the demethanizer overhead stream. The additional reflux stream is taken as a side stream of the vapour stream from the cold separator.

Self or External Refrigeration? How many Feed Streams? Number of Flash Columns Multistream Heat Exchangers

There are well-established process packages for low temperature separation systems ... But which choice is the best ? And can we produce novel designs? Centre

L07- 7

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Demethanizer Process Issues Process Selection Issues P Complex interactions among different components of flowsheet P Various options for refrigeration P Structural and parametric degrees of freedom.

Process Design Issues P Different designs offered by licensors P Can licensed flowsheets be further optimized? P Selection of flowsheet package for specific feed and products specifications

Centre

L07- 8

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The heart of every cold train is the demethanizer system and this is where various technology licensors have different designs. There are many possible options for optimizing the design of a single column. Depending on the scenario it may be beneficial to cool the feed, or to split the feed in multiple separators at different pressures before entering into the column at different stages. The feed cooling is usually done by heat exchange with the top product in a multistream exchanger. Similarly the heat for distillation can be provided by side reboilers taking the energy from the feed stream. As the separation requires very low temperatures, so there is a need for refrigeration which can be provided by using turboexpander or using external refrigeration cycles.

Objective At preliminary design stage provide a high level screening and scoping methodology that would allow energy-efficient and costeffective demethanizer flowsheets to be generated and evaluated.

Centre

L07- 9

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

2. Overview of Previous Research

Centre

L07- 10

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Previous Research P Synthesis and optimization of low-temperature gas separation (Jiona Wang, Robin Smith,2005). P Operating conditions optimization in natural gas processing plants (Diaz, Serrani, Be Deistegui, 1995) P Energy expenditure in the thermal separation of hydrocarbon mixtures using a sequence of heat-integrated distillation columns (Markowski, M., Trafczynski,M., 2007) P Automatic design and debottlenecking of ethane extraction plants (Diaz, Serrani, Bandoni Brignole, 1997)

Centre

L07- 11

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Limitations of the Previous Work

P Multiple feed columns not modelled P No model for column with side reboilers P Heat integration through multistream exchangers not adressed P No systematic methodology to screen and optimize such highly integrated flowsheets

Centre

L07- 12

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

3. System Parameter Identification

Centre

L07- 13

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

COMMERCIAL DESIGNS *

P P P P

GSP (Gas Subcooled Process) CRR( Cold Residual Reflux Process) RSV (Recycle-Split Vapor Process) RSVE (Recycle-Split Vapor with Enrichment Process)

* Next Generation Processes for NGL/LPG Recovery by ORTLOFF (77th Annual GPA Conference) Centre

L07- 14

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

COMMERCIAL DESIGNS *

GSP Process

CRR Process

* Next Generation Processes for NGL/LPG Recovery by ORTLOFF (77th Annual GPA Conference) Centre

L07- 15

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

For ethane recovery the most widely used process is the gas subcooled process. In this process a portion of the feed gas is condensed and subcooled, fash down to the tower operating pressure and supplied as an external reflux stream. The remainder is also expanded by employing a turboexpander) and fed to the tower at one or more intermediate feed points. The Cold Residue Recycle (CRR) process is a modification of the GSP process to achieve higher ethane recovery levels. The process flow sheet is similar to the GSP except that a compressor and condenser have been added to the overhead system to take a portion of the residue gas and provide additional reflux for the demethanizer. This process is attractive for extremely high ethane recovery. Recovery levels above 98% are achievable with this process.

COMMERCIAL DESIGNS *

RSV Process

RSVE Process

* Next Generation Processes for NGL/LPG Recovery by ORTLOFF (77th Annual GPA Conference) Centre

L07- 16

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The Recycle Split-Vapour (RSV) process uses the split vapour feed to provide the bulk ethane recovery in the tower. The external reflux stream is produced by withdrawing a small portion of the residue gas, condensing and subcooling and flashing it down to the demethanizer pressure. The additional cost of adding a compressor in the CRR process is avoided as well as the shaft power is reduced. A slight variation of the RSV process is the recycle split-vapor with enrichment (RSVE) process in which the reflux stream from the residue gas is mixed with the split vapour feed before being condensed and subcooled. Since the ethane content of the reflux stream is richer as compared to the RSV process, but the lower capital cost and simplicity justify the slight loss of ethane recovery.

Comparison of NGL Recovery Processes Process

Recovery %

Main feature

Comment

Gas Subcooled Process (GSP)

90%

Subcooled feed gas as reflux at demethanizer

When switching to LPG recovery, propane recovery decreased

Cold Residue Reflux (CRR).

97%

Uses GSP advantages, while creating a reflux stream of nearly pure methane.

Can be operated for near complete rejection of ethane while maintain high propane recovery.

As CRR uses the split-vapour feed to provide bulk ethane recovery, but produced by withdrawing a small portion of the residue gas

Requires less capital investment. A separate compressor is not needed for the recycle stream. Can be operated as GSP.

Variation of RSV. The recycle stream is mixed with the split vapour feed before being condensed and subcooled.

Do not require a separate exchanger as RSV. Ethane recovery limited. Lower capital cost

Recycle Split-Vapour Process (RSV)

Recycle Split-Vapour with Enrichment (RSVE)

97%

94%

Centre

L07- 17

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Base case for Sensitivity Analysis*

Process and Apparatus for Hydrocarbon Separation * * Ohara et al. U.S 7,357,003 (2008) Centre

L07- 18

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

HYSYS SIMULATION OF BASE CASE

* Ohara et al. U.S 7,357,003 (2008) Centre

L07- 19

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Process Data Process Conditions Feed Gas Composition Com ponent

Mole Fraction

CO2 N2 Methane Ethane Propane i-Butane i-Pentane

0.010 0.005 0.894 0.049 0.022 0.013 0.006

Feed Conditions Parameter Temperature (°C) Pressure (bar) Molar Flow Rate (kmol/h)

Value 17 62.4 13700

Sales Gas Conditions Param eter Temperature (°C) P ressure (bar)

V alue 35 60

Process Specifications P Minimum P Maximum methane in the demethanizer bottoms: 1.5% mol. P Apply 2 freeze point in the coldest area of the plant. P 75% compressor efficiency and expander efficiency * Ohara et al. U.S 7,357,003 (2008) L07- 20

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Centre

Sensitivity Analysis Sensitivity analysis is being performed to identify the key parameters which affect the flowsheet performance. These parameters are then examined in order to find the optimal range of operation conditions. P P P P P

Pressure of column Feed stage location Side reboiler location Temperature of high pressure separator Vapour split ratio

Centre

L07- 21

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Column Operating Pressure Effect of Pressure on Compression Power

P High operating pressure is required in order to decrease the compression power but this in turn affects the volatility of the key components and therefore the recovery P The column pressure is varied from 2000 kPa to 4000 kPa keeping a constant pressure drop of 50 kPa across the column

16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 2,000

2,500

3,000

3,500

4,000

4,500

Pressure (kPa)

Effect of Pressure on Reboiler Duty 16,000.0 15,000.0 14,000.0 13,000.0

With the increase in pressure

12,000.0 11,000.0 10,000.0 2,000

P The total power consumption decreased by 58% P Ethane recovery also decreased by 8.8% P Reboiler duty increased by 20%

2,500

3,000

3,500

4,000

4,500

Pressure (kPa)

Effect of Pressure on Ethane recovery 98.00 97.00 96.00 95.00 94.00 93.00 92.00 91.00 90.00 89.00 88.00 2,000

2,500

3,000

3,500

Pressure (kPa)

Centre

L07- 22

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

4,000

4,500

Side Reboiler Location Side reboilers in low-temperature distillation columns are particularly important because they enable: P Recovery of refrigeration at temperatures lower than the temperature of the column reboiler P The cost of generating refrigeration is lowered as the temperature decreases. P For example, the generation of a BTU of refrigeration at -35 F requires roughly twice as much compression horsepower as a BTU of refrigeration at P The side reboiler is studied for a fixed temperature difference between the outlet and inlet stream

* Elliot,D.G., Chen, J.J., and Brown, T.S.,(1996), Fluid Phase Equilibria 116, 27-38 Centre

L07- 23

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Vapour Split Ratio The split ratio from the low pressure separator is an important factor as it directly affects the amount of external reflux entering the column. The split ratio between the column upper feed and the external reflux stream has been varied from 0.3 to 0.7 to yield the following results

Effect of Vapor Split on Reb duty

Effect of Vapor Split ratio on Refrigerant Power 6000

18000 16000

5000

14000

4000

12000 10000

3000

8000

2000

6000

1000

4000 2000

0

0 0.2

0.3

0.4

0.5 Split Ratio

0.6

0.7

0.8

0.2

0.3

0.4

0.5

0.6

Split Ratio

Centre

L07- 24

30

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

0.7

0.8

Demethanizer Feed Stage Effect of Feed stage on Total Compression Power

The top feed point is moved from Stage 2 to Stage 14. This range of feed points is studied as the column simulation converges for the given specifications. As the feed stage moves down the column the results observed are listed below:

8125 8120 8115 8110 8105 8100 8095 8090 0

2

4

6

8

10

12

14

16

14

16

14

16

Feed Stage Location

Effect of feed Stage on Reboiler Duty 14200 14100

P Ethane recovery increases. Larger

14000 13900

changes are seen in the first stages of the

13800 13700

column, and then the changes are very

13600 13500 0

small after the middle of the column. P There is very slight decrease (0.4%) on the total power consumption P The reboiler duty decreases by 4.3%

2

4

6

8

10

12

Feed Stage Location

Effect of Feed Stage on Ethane Recovery 95.0 94.5 94.0 93.5 93.0 92.5 92.0 91.5 91.0 90.5 0

2

4

6

8

10

Feed Stage Location

Centre

L07- 25

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

12

Temperature of High Pressure Separator Effect of Separator Temperature on Compression Power

The temperature has a significant effect as it

9000

affects the ratio of vapor to liquid leaving the separator, which in turn affects the performance of the column and the rest of

8600 8400 8200 8000

the –60

8800

7800

The C.feasible operation range is

7600 7400

above

-65

-55

-50

-45

-40

-35

-30

LTS Temperature (C)

minimum approach temperature criterion of 2

-60

C

Effect of Separator Temperature on Ethane Recovery 97.0 96.5 96.0

The results are:

95.5 95.0 94.5 94.0

P Ethane recovery diminishes from 95 % to

93.5

91.5 %. P Compression power decreases by 14%

92.0

93.0 92.5 91.5 -65

-60

-55

-50

Centre

L07- 26

-45

Temperature (C)

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

-40

-35

-30

Conclusion of Sensitivity Analysis At this stage of the process analysis we can conclude that in general the key parameters that have the most impact on the demethanizer process performance are: P The column operating pressure P The upper feed location in the column P The location of side reboiler P Temperature of the high pressure separator, and P The split ratio between the upper feed and the external reflux stream

Centre

L07- 27

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

4. Column Design Method a. Need for the new method

Centre

L07- 28

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Design Challenges

P Multiple feed points P Side reboilers P External reflux stream P Large change in relative volatility over the column

Centre

L07- 29

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Modelling Challenges Shortcut Models P Constant molar overflow and constant relative volatility assumptions P Unable to address complex column configurations P Give inaccurate design of demethanizer

Rigorous Models P Solve MESH equations on each stage P Requires complex iterative methods to solve P Requires good initial guess P Difficult to employ in optimization framework

Centre

L07- 30

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

At present, there is no shortcut method for determining the minimum ratio for a complex column like demethanizer. The major characteristics of this column are the multiple feed, side reboilers and reflux from an external source. The rigorous methods can be applied to solve the column but the computational time to solve the model will be increased and that will further increase when this model will be employed in a flowsheet superstructure and optimized.

Simulation Models of Separation Options - Shortcut vs Rigorous Input Shortcut model

P Feed information P Recovery specification P Constraints

Rigorous model

Output

Feature

P Product profile P Overall heating/cooling requirement P Approxiamte sizing of device P Cost estimation

P Simple P Acceptable accuracy P Fast P Suited to preliminary design

P Product profile P Local heating/cooling requirement P Final design of device P Rigorous cost estimation

P Complicated P Sensitive to process specifications P Slow and unavoidable iteration P Suited to device optimization

Centre

L07- 31

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

4. Column Design Method b. Boundary Value Design Method i) Introduction

Centre

L07- 32

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Introduction P Analogous to McCabe-Thiele method for column design P Composition profiles starting from distillate and bottom product compositions P Gives column design details e.g. number of stages, heat duty of condenser and reboiler, minimun reflux etc.

McCabe- Thiele Method

Boundary Value Method

Centre

L07- 33

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The boundary value method or BVM is first developed for simple column. It requires the calculation of liquid composition profiles at a given reflux ratio. Composition profiles of a column show liquid phase compositions on each tray, which vary from stage to stage because of material and energy balances between trays and phase equilibrium of the mixture. At a given reflux ratio, the composition profile of the rectifying section of the column starts from a specified distillate composition. For the stripping section of the column, the composition profile starts from a specified bottom composition.

Modeling Column Profile Continuous Differential Equation P Develop continuous differential equation for evolution of column profile (Doherty, 1985) P Taylor expansion truncated after first term xi, n+1 = xi,n + x i, n + 1 =

dxi dh

2

h=n

(∆h) + 1 d x2i 2 dh

h=n

(∆h)2 + .....

s 1 y i, n + x i, B s + 1 s + 1

Continuous column profile with independent variable h Rectifying Section dx dh

i

= x

i

−

r y r + 1

i

+

1 x r

i, D

Stripping Section dx i s 1 = yi − xi + x i, B dh s +1 s +1

* Levy, S.G., Van Dongen, D.B., and Doherty, M.F., (1985), Ind. Eng. Chem. Fundam. 24, 1463. Centre

L07- 34

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The method is based on finite difference approximations of column profiles in ordinary differential equation form under the assumption of constant molar overflow (CMO). Conditions such as minimum reflux are determined using a boundary value method, in which the rectifying profile for the liquid compositions is integrated from top to the feed stage while the stripping profile is integrated from bottom to the feed stage. Thus a feasible column configuration is one in which the rectifying and stripping profiles intersect and the reflux ratio for which these profiles just touch each other corresponds to minimum reflux.

Original Boundary Value Design Method (BVM) V2, y2

P For ternary mixtures P First developed for homogeneous azeotropic distillation* P Calculation of composition profiles

D, xD, hD

Stripping profile

Vm ym V hm

0.8

Rectifying profile

0.6

profiles at a given reflux ratio starting from

P Composition

n Vn+1 yn+1 V hn+1

Hexane 1

L1, x1

2

Ln xn hnL

< distillate < bottoms

Lm+1 xm+1 L hm+1

P Calculate column profiles

m

from

B

material and energy balances < Vapour Liquid equilibrium

0.4

F

2

<

V1, y1, hV 1

0.2

0

0

Nonane

0.2

0.4

0.6

0.8

1

D

Pentane

composition (xD) composition (xB)

L2, x2, h2L

B, xB, hB

* Levy, S.G., Van Dongen, D.B., and Doherty, M.F., (1985), Ind. Eng. Chem. Fundam. 24, 1463. Centre

L07- 35

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The BVM requires the specification of the product and feed compositions, reflux ratio and either feed condition or reboil ratio. The next step is to calculate composition profiles of each column section from material and energy balances together with vapour liquid. The feasible design is identified from the intersection between the striping profile and the rectifying profile. Then, we get column design information such as the number of stages, feed location, condenser and reboiler duty. Finally, the column designs can be used to evaluate their capital and operating costs.

4. Column Design Method b. Boundary Value Design Method ii) New Boundary Value Design Method

Centre

L07- 36

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

New Boundary Value Design Method Boundary value method has been extended to # Multicomponent feed mixtures # Double-feed columns # Column with side reboilers # Reboiled absorber column with an

external reflux stream

Software:

" Matlab " HYSYS < Physical properties

Centre

L07- 37

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The major problem with the original BVM method proposed by Doherty et al. is the visualization of mixtures of more than four components. For this purpose the concept of “minimum distance” between a pair of is employed instead of the strict criterion of the intersection of the composition profiles. The intersection of the profiles is assessed by calculating the shortest distance in composition space between the two profiles. If this distance is the same or smaller than the “minimum distance” the lines are considered to intersect.

Model Formulation & Implementation P Model developed in MATLAB involving mass and energy balance P Stage by stage VLE calculations to avoid constant relative volatility assumption P Complex column configuration addressed in the model P Physical and thermodynamic properties retrieved automatically from HYSYS VLE and Enthalpy

MATLAB

HYSYS Mass and Energy

Balance

Centre

L07- 38

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The model is originally formulated in MATLAB with the mass and energy balance original equations. For the calculations of physical properties and vapour liquid equilibrium, the extensive physical property packages available in ASPEN HYSYS are used. This is done by developing a link between HYSYS and MATLAB where HYSYS gives respective equilibrium and enthalpy data for respective composition from MATLAB

Double-feed Columns V, y2 I II

Upper Feed x FU

D, xD

L, xR

Lower Feed xFL

B, xB

P Double-feed column may be more economic than a single-feed column < Require fewer stages

P BVM has been developed for double-feed columns separating homogeneous mixtures*

* Levy, S.G., Van Dongen, D.B., and Doherty, M.F., (1985), Ind. Eng. Chem. Fundam. 24, 1463. Centre

L07- 39

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

New boundary value method Double-feed column â Initialisation steps • •

Specifications - compositions of upper and lower feeds - compositions of products Specify - reflux ratio and reboil ratio or feed quality

ä Identify feasible designs from the intersection of rectifying and middle profiles

•

ã Composition profile calculation of stripping, rectifying and middle sections from < Material and energy balances < Phase equilibrium

•

Calculate middle profiles starting from various stages of stripping section

• •

The location of lower feed is chosen such that the smallest number of stages is required The intersection point is the upper feed location Design parameters are obtained < Number of stages in each section < Upper feed location < Feed condition or reboil ratio < Condenser and reboiler duty

Centre

L07- 40

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

To get the number of stages and location of both feeds in the column, all products and feeds are first specified. At a given reflux ratio the stripping and rectifying profiles are calculated. The middle profile can be generated starting from a stage either in the stripping section or the rectifying section. The middle profile calculation from a stage on the stripping section is easier. The lower feed location becomes a degree of freedom. To choose the lower feed location, middle profiles are generated starting from different stages in the stripping profiles. The feasible designs are identified from the intersection between the middle profiles and the rectifying profiles. Then, the most attractive designs can be chosen.

Double-feed column design by BVM BVM

FU

FL

10 D

22 30

Composition : Methane Ethane Propane Flow rate (kmol/h) Condenser duty (MJ/h) Reboiler duty (MJ/h)

FU

FL

D

B

0.8 0.15 0.05 100

0.64 0.26 0.9 100 202.8 1368

0.997 0.0028 0 145.5

0.98 0.012 0 54.5

B

HYSYS

Reflux ratio = 0.22 Composition : Methane Ethane Propane Flow rate (kmol/h) Condenser duty (MJ/h) Reboiler duty (MJ/h)

FU

FL

D

B

0.8 0.15 0.05 100

0.64 0.26 0.9 100 205.3 1372

0.997 0.0028 0 145.5

0.98 0.012 0 54.5

P Results from HYSYS are in good agreement with the specified compositions for the design method Centre

L07- 41

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

4. Column Design Method C. Case Study

Centre

L07- 42

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Pressure : 30 bar, Reflux ratio =0.2516, Column Specificaion : 94.5% Recovery of Ethane in Bottom product Feed Data Upper Feed Flowrate, FU = 1000kmol/hr Upper Feed Composition (Mole Fraction) Methane: 0.8595, Ethane: 0.102, Propane : 0.023, i-Butane: 0.0155 Lower Feed Flowrate, FL = 1000kmol/hr Lower Feed Composition (Mole Fraction) Methane: 0.62, Ethane: 0.21,Propane : 0.10, i-Butane: 0.07

Top Product

FU

FL

Q1 Q2

Bottom Product

Side Heaters Q1at 11th stage = 1200 MJ/hr Q2 at 16th stage = 1000 MJ/hr

Ref: Foglietta et al. US 7,159,417 (2004) Centre

L07- 43

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Validation of Model

BVM Resu lts

HYSYS Results

% Difference

Num ber of Stages

23

23

-

Upper Fee d Stage Location

6

6

-

Lower Fee d Stage Location

12

12

-

2075

2087

0.6

11 079

11 083

0.035

Condenser Duty (MJ /hr) Reboile r Duty (M J/hr)

Centre

L07- 44

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Comparison of Models Column Composition Profiles 1.0 0.9

Methane(HYSYS)

0.8

Ethane(HYSYS)

0.7

Propane(HYSYS)

0.6

i-Butane (HYSYS)

0.5

Methane (BVM)

0.4

Ethane (BVM) Propane (BVM)

0.3

i-Butane (BVM)

0.2 0.1 0.0 Top 1

3

5

7

9

11

13

15

17

19

21

Bottom

Stages

Comparison of composition profiles obtained from BVM with HYSYS Centre

L07- 45

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Summary P Simplified model for demethanizer has been developed. Model is suitable for multicomponent systems P Model represents process behaviour satisfactorily P Model accommodates complex column features such as multiple feed, intermediate heating and external reflux stream P Complex column simulation has been tested against HYSYS simulation. Model represents column behaviour well P New BVM can be used for assessing the feasibility of a proposed specification and setting up designs < Number of stages < Feed locations < Reboiler/condenser duties Centre

L07- 46

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

5. Heat Recovery in Multistream Exchangers

Centre

L07- 47

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Heat Recovery in Multistream Exchanger P Obtain Process Heat and Material Balances < Analysis of the entire process to obtain a heat and material balance for each operation. The heat and material balances should be performed as if all of the exchanger duties were independent. For this step, refrigeration and external loads are supplied independent of the process and should be ignored. P Tabulate Duties < The second step is to list the inlet and outlet temperatures and duties for all units involving heat exchange.

Centre

L07- 48

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Heat Recovery in Multistream Exchanger P Match Duties and Temperatures < Matching the duties by starting with the largest cooling loads. < If heating loads with temperatures lower than the exit temperature of the cooling load are available, these should be accumulated until the duty approximately matches the cooling load. Any excess cooling load will have to be handled by refrigeration. P Balance Loads < Heating and cooling loads balanced by separately summing the duties on each side of the exchanger < Residual duty must be reduced to zero by refrigeration or a heat source P Temperature cross within the exchanger < Phase change or large flow rates across a narrow temperature range results in the cumulative cooling load temperature drops below the cumulative heating load temperature Centre

L07- 49

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Multi-steam exchanger design according to the balanced composite curves

T

H1

H2 H3

H

C1 C2

H3

H3 Cold Utility

H2

H1

H3

H1 H2 Hot Utility

C1 C2

C1

C2

C2 Centre

L07- 50

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

6. Overall Flowsheet Synthesis and Optimization

Centre

L07- 51

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Synthesis Methodology • Develop simplified model for demethanizer < New model compares well with rigorous simulation results for simple columns

• Optimise column design parameters with simplified model • Develop systematic approach for generation and evaluation of flowsheets • Develop optimisation framework for overall flowsheet synthesis and optimization

Centre

L07- 52

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Base Case Simulation Process Conditions Feed Gas Composition Com pone nt Methane

M ole Fraction 0.865

Ethane

0.075

Propane

0.035

i-Butane

0.015

n-Butane

0.006

i-Pentane

0.004

Feed Gas Conditions Param e ter

Condition

Temperature (°C)

25

Pressure (bar)

60

Molar Flow Rate (kmol/h)

36000

Demethanizer Conditions Parameter Number of trays Operating pressure (kPa) Pressure drop (kPa)

Condition 25 3000 50

Product Specifications: methane to ethane in bottom =1.5% Ethane Recovery = 90% * Campbell et al. US 4,157,904 (1979) L07- 53

Centre

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

The feed to the gas subcooled process for ethane recovery based on US patent 4,157,904 is selected as the case study. The objective has set to be minimising the operating cost of the system which is essentially reducing the compression power and reboiler duty for the system. Steam and cooling water next to refrigeration system are provided as utility for the system. Overall ethane recovery of more than 90% is used as a specification for the process. The process is selected as it is the most widely employed process for NGL recovery, with all the necessary flowsheet components present.

Base Case Simulation Gas subcooled process* is selected as the base case for the study and simulated with the shortcut models developed in MATLAB Reflux Exchanger

Sales Gas

Expander Inlet Gas

Flash Column

Multistream Exchanger

Bottom Product * Campbell et al. US 4,157,904 (1979) Centre

L07- 54

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Comparison of Model with Rigorous Simulation

Model

HYSYS

Methane Recovery in Sales Gas

99.40

99.7

Ethane Recovery in NGL

90.2

Total Shaft Power Reboiler Duty Number of Trays of Demethanizer

90.4

38.12MW

38.4 MW

57420 MJ/hr

58870 MJ/hr

25

25

Model shows good agreement with rigorous HYSYS simulation

Centre

L07- 55

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Optimization of the Process P The total shaft power required for the recompressor and refrigeration need to be optimized and compared against base case P Design variables < Column operating pressure (10 ~40 bar) < Flash column temperature (-20 ~ -40 C) < Side reboiler location ( 5th ~ 20th stage) < Upper feed to reflux ratio (0.2 ~ 0.8)

P Objective function: Minimize total shaft power P Constraint: Ethane recocery in NGL > 90%

Centre

L07- 56

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Optimization Approach Base Case Initial conditions and structure

Objective Function

Minimize total shaft power

Optimization Algorithm (SQP)

Variables Constraints

Optimum demethanizer flowsheet Centre

L07- 57

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Optimization Results Condition

Power (MW)

Base case without optimization

38.12

Base case with optimization

33.6

Optimized Variables P Column pressure = 25 bar P Side reboiler location = 8th stage P Flash column temperature = -30 C P Upper feed to reflux ratio = 0.6 Centre

L07- 58

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Now Let’s try a different configuration to increase the ethane recovery

Centre

L07- 59

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

RSVE Process Optimization

* Campbell et al. US 4,854,955 (1989) Centre

L07- 60

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Comparison of Results with Rigorous Simulation Model

HYSYS

Methane Recovery in Sales Gas

99.8

99.8

Ethane Recovery in NGL

94.3

Total Shaft Power Reboiler Duty Number of Trays of Demethanizer

94.2

42.8 MW

42.4 MW

70280 MJ/hr

69820 MJ/hr

30

30

P Increase of ethane recovery compared to GSP at the expense of increased shaft power and reboiler duty P The model shows good agreement with rigorous HYSYS simulation Centre

L07- 61

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Optimization Results Optimization condition

Power (MW)

Total shaft power without optimization

42.8

Total shaft power with optimization

36.6

Optimum values of variables P Column pressure = 30 bar P 1st side reboiler location = 12th stage P 2nd side reboiler location = 20th stage P Flash column temperature = -42 C P Upper feed to reflux ratio = 0.75 Centre

L07- 62

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Summary GSP (90.2% Ethane Recovery )

Total Power Requirement (MW)

RSVE (94.3% Ethane Recovery )

Without Optimization

38.2

With Optimization

33.6

Without Optimization

42.8

With Optimization

36.7

P Optimization brings significant benefits P Trade-off between power input and ethane recovery P Need to optimize total cost Centre

L07- 63

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

7. Conclusions and Future work

Centre

L07- 64

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Conclusions P Shortcut models developed for demethanizer, flash column, turbo-expander and compressor P Heat recovery issues with multistream exchangers addressed P Different flowsheet configurations are modelled and validated against rigorous simulation P Important variables affecting the overall process are identified P Different flowsheet configurations are optimized to decrease the total power requirement

Centre

L07- 65

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Future Work

P Development of superstructure to accommodate all important features for synthesis and optimization of flowsheet P Development of systematic methodology to create heat integration opportunities P Application of stochastic optimization methods for structural and parametric optimization P Application of developed methodology to industrial case studies

Centre

L07- 66

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes

Centre

L07- 67

Synthesis of Demethanizer Flowsheets for Low Temperature Separation Processes