kherbeck2014

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http://slidepdf.com/reader/full/kherbeck2014 1/6

Optimizing

ethane

recovery

in

turboexpander

processes

Laura Kherbeck, Rachid Chebbi*

Department of Chemical Engineering, American University of Sharjah, PO Box 26666, Sharjah, United Arab Emirates

1. Introduction

There are many extraction processes for natural gas liquids

which include Joule–Thompson (JT) expansion, refrigeration using

propane in a chiller, and turboexpansion. More often than not, all

three processes are used at once. Mixed refrigerants can also be

used [1] but the most popular process in the natural gas liquids

(NGL)-recovery industry is turboexpansion. A review of NGL

recovery can be found in Manning and Thompson [1], McKee [2],

Pitman et al. [3], GPSA [4], Chebbi et al. [5,6], Mehrpooya et al. [7]

and in the references therein. The optimized conventional process

for ethane recovery [5], to which the present results are compared,

is shown in Fig. 1. The cold residue recycle (CRR) process, examined

in this paper (Fig. 2), is claimed to provide very high ethane

recovery, above 98% [4]. The CRR process [3,4,8] is built upon the

gas subcooled process (GSP). In the GSP [4], the gas leaving the

separator is split, with one fraction subcooled by heat exchange

with the overhead stream from the demethanizer, and the otherfraction entering the turboexpander. The fraction subcooled by the

demethanizer overhead stream is flashed in a valve and fed to the

tower as reflux [4]. The GSP process is considered in the present

work (Fig. 3). The process in Fig. 4, referred to as GSP with cold

separator in the rest of the manuscript, has a cold separator

operating at a lower temperature than the chiller temperature. The

CRR process has one addition when compared to the GSP process

(Fig. 3): a reflux stream to rectify the vapors in the demethanizer

tower in order to minimize the amount of ethane and other heavier

hydrocarbons that leave with the overhead. A compressor is used

to boost part of the demethanizer overhead stream to a slightly

higher pressure so that a fraction of the methane could be liquefied

by the flashed stream and sent to the top stage of the demethanizer

(see Fig. 2). The flashed feed to the demethanizer would condense

some of the ethane from the turboexpander outlet vapor and the

liquid reflux stream would condense some of the remaining ethane

vapors at the top of the tower.

Maximum ethane recovery can be carried out by changing a

select number of design variables. Ethane and NGL recovery

problems are characterized by a large number of design variables

affecting ethane and NGL recovery that include, but are not limited

to demethanizer pressure and split ratio(s) if any.

The present paper considers the effect of demethanizer

pressure on maximum ethane recovery for the CRR process ascompared to a conventional turboexpander process [5]. Further-

more, GSP (without or with cold separator) is considered and its

performance compared to both the CRR process and the

conventional turboexpander process [5] for a lean and a rich feed

gas at different demethanizer pressures. Optimization is per-

formed by maximizing the percent ethane recovery. Ethane

recovery as a function of demethanizer pressure is then reported

and analyzed for the two types of feed. Feed composition, flow

rates, temperature and pressure are identical to the values used in

Bandoni

et

al.

[9],

and

Chebbi

et

al.

[5,6]

for

feeds

A

and

D.

Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

A R T I C L E I N F O

Article history:

Received 5 October 2013

Accepted 19 February 2014

Available online xxx

Keywords:

Simulation

Ethane recovery

Natural gas liquids (NGL)

Turboexpander

Cold residue recycle (CRR)

Gas subcooled process (GSP)

A B S T R A C T

Optimization of ethane recovery usingthe CRR process shows that, except for thecase of lean gas at low

demethanizer pressure, theCRRprocess reduces to GSP, in which there is no reflux streamand therefore

no added cryogenic compression and heat exchange equipment. Adding a second cold separator,

operating at lower temperature, in GSP is found to lead to more or less recovery depending on the NGL content of the feed gas and the demethanizer pressure. GSP is also compared with the conventional

turboexpander process. Optimization shows that adding more equipment or even flow splitting may

lead to less ethane recovery.

2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

* Corresponding author. Tel.: +971 65152983.

E-mail address: [email protected] (R. Chebbi).

G Model

JIEC-1925; No. of Pages 6

Please cite this article in press as: L. Kherbeck, R. Chebbi, J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.02.035

Contents

lists

available

at

ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mepag e: w ww.elsev ier .co m / locate / j iec

http://dx.doi.org/10.1016/j.jiec.2014.02.035

1226-086X/ 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.


















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2.

Simulation

and

optimization

2.1. CRR process

The

study

was

conducted

by

first

simulating

the

CRR

process.

Fig. 2 demonstrates the process flow sheet for the CRR process. The

figure

does

not

depict

the

refrigeration

loop,

which

is

connected

to

the main process through the chiller. The feed is first cooled by

providing the duty necessary for the reboiler, and further cooling of

the

feed

is

achieved

by

heat

exchange

with

the

residue

gas.

The

four

demethanizer

pressures

considered

are

100,

215,

335,

and

450 psia as in [5,6]. The pressures are grouped as low (100 psia),

Fig. 1. Conventional ethane recovery process optimized for maximum ethane recovery in [5].

Fig. 2. CRR process flow sheet.

Fig. 3. Gas subcooled process (GSP) flow sheet.

L. Kherbeck, R. Chebbi / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx2

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intermediate

(215

psia),

and

high

(335

and

450

psia)

and

cover

the

typical

range

of

demethanizer

operating

pressures,

100–450

psia

[1]. The reboiler duty cannot be provided through heat integration

at high demethanizer pressures (335 and 450 psia) due to the fact

that

the

temperature

profile

in

the

column

is

shifted

up

and

the

feed

gas

temperature

ceases

to

be

enough

to

provide

the

reboiler

duty, as also indicated in [6].

The pre-cooled feed is senttoa chiller where propane is usedto

reduce its temperature to

31 8F.

This

temperature

was

selected

to maximizecooling; taking into account the lowest temperature

allowed in the chiller of 40

8F required to avoid air leakage into

the

system [1],

and temperature approach in

the

chiller. The cold

feed from the chiller enters a separator where thegas is separatedfrom the liquid. A portion of the separated gas is cooled by heat

exchange with a fraction of the overhead stream leaving the

demethanizer column. It is

then

expanded through

JT

expansion

and enters another

heat exchanger designed

to cool

a

portion of

the recycled overhead, following which the stream enters the

demethanizer. The other portion of the separated gas is expanded

in

a

turboexpander

andsent to thedemethanizerat

a

lower

stage.

A

fraction of

the

liquid

leaving

the

separator is

mixed with

the

separator gas outlet that goes into heat exchange, while the

remainder undergoes JT expansionto column pressureand enters

the

demethanizer at

a

lower

stage than

the

feed stream from the

turboexpander.

The

flashed

split-vapor

stream is

not cold

enough

to condense partially the overhead methane reflux stream at the

operating pressure of

the

demethanizer. Thus, a

cryogeniccompressor

is

used to

boost

part

of

the demethanizer

overhead

to

a

slightly

higher

pressure so that a

fraction

of

the methane

can

then be condensed [3]. The compressed overhead is cooled, and

then expanded through a valve before being suppliedto the topof

the

tower. The fractionof

the overhead

that is

notrefluxedback is

termed the

residue

gas. Part

of

the power

required

to

recompress

the residue gas is provided by the turboexpander, but a

recompressor is needed to bring the residue gas pressure up to

882

psia. Two different

feeds

are considered:

feed A

and feed D

as

in

[5,6,9].

The compositions

in

terms

of

mole

fractions

are given in

Table 1. FeedA is a lean gaswith 6% C2+ content, andD is a rich feed

with 30% C2+ content. Thefeedgasenters the NGLrecovery unit at

100 8F

and

882 psia,

the residue

gas is

recompressed to

882 psia,

and the

molar ratio

of

C1 to

C2 in

the NGL stream is

set to

0.02

in

consistency with the typical range in [1]. In all cases, the feed gas

flow rate is 10,980 lbmol/h. The maximum conventional tur-

boexpander ethane

recovery

values were

obtained from

[5].

2.2. GSP and GSP with cold separator

The

gas

subcooled

process

(GSP)

and

GSP

with

cold

separator

are

shown

in

Figs.

3

and

4, respectively.

3.

Results

and

discussion

The simulation was performed using Aspen HYSYS with the

Peng-Robinson

thermodynamic

package.

The

optimization

in-volved

changing

the

design

variables

affecting

ethane

and

NGL

recovery to maximize the objective function, defined as the ratio of

ethane in the NGL stream to the ethane in the feed. The design

variables

selected

were

the

split

ratios

in

all

of

the

splitters,

the

temperatures

at

the

outlet

(higher

temperature

side)

of

the

heat

exchangers following the mixer, and the cryogenic compressor

outlet pressure. The constraints were set so as to prevent

temperature

cross

in

all

the

heat

exchangers

and

to

ensure

that

the

reflux

stream

is

colder

than

the

flashed

split-vapor

stream

as

they enter the demethanizer. Optimization thus yielded the

optimum values of the design variables affecting ethane and

NGL

recovery

for

each

demethanizer

pressure

for

both

lean

and

rich

feeds,

i.e.

the

values

that

contributed

to

the

highest

ethane

recovery.

Fig. 4. GSP with cold separator.

Table 1

Feed gas composition.

Component Feed A Feed D

Nitrogen 0.01 0.01

Methane 0.93 0.69

Ethane 0.03 0.15

Propane 0.015 0.075

Butanes 0.009 0.045

Pentanes 0.003 0.015

Hexanes 0.003 0.015

C2+ (%) 6 30

L. Kherbeck, R. Chebbi / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3

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3.1.

Feed

A

The HYSYS optimizer tool indicated that the CRR process indeedmade recoveries of 99.9% possible only for a demethanizer

pressure

of

100

psia

and

for

the

lean

feed

gas

A.

For

all

the

higher

demethanizer

pressures,

results

indicated

that

for

feed

A,

the

optimum process would not include the reflux. Initially, these

results were suspected after a sensitivity analysis was carried out

on

the

split

ratio

in

the

overhead

splitter

and

the

cryogenic

compressor

outlet

pressure.

Later,

optimization

using

the

HYSYS

optimizer tool confirmed the results. The configuration in Fig. 2 has

hence been altered to discard the reflux splitter, the cryogenic

compressor,

heat

exchanger,

and

expansion

valve,

while

retaining

the two splitters at the top and bottom outlets of the first separator.

The results summarized in Fig. 5, show how ethane recovery with

the

CRR/GSP

compares

to

the

recovery

from

the

conventional

turboexpander process [5]. It can be seen that the CRR process, andhence the GSP, are not as effective at high demethanizer pressures,

while the conventional turboexpander process is not as effective as

the

other

two

processes

at

low

and

intermediate

demethanizer

pressures.Some of the differences between the GSP (Fig. 3), and the

conventional

turboexpander

process

in

[5]

(also

shown

in

Fig.

1),

lie

in

the

two

splitters

at

the

outlet

of

the

separator

operating

at

31

8F in the GSP case, and also in the additional cold separator in

the conventional turboexpander process [5]. However, since the

GSP

has

been

shown

to

give

superior

recovery

at

low

and

intermediate

pressures,

and

lower

recovery

at

high

pressures,

it

was postulated that the difference could lie in the additional cold

separator utilized in the conventional turboexpander process.

Thus,

the

GSP

was

modified

by

adding

a

cold

separator

operating

at

a temperature lower than the chiller temperature. At high

demethanizer pressures, GSP with cold separator provided higher

recoveries

than

GSP,

and

slightly

higher

ethane

recoveries

than

the

conventional turboexpander values [5]. The separator overheadstream splitter was found unnecessary, leading to the simplified

flow sheet in Fig. 6. At low and intermediate pressures, GSP with

Demethanizer pressure, psia

50 100 150 200 250 300 350 400 450 500

C 2

r e c o v e r y ,

%

0

10

20

30

40

50

60

70

80

90

100

CRR/GSP (CRR at 100 psia, GSP at higher P)

Conventional turboexpander

GSP with cold separator (GSP at 100 & 215 psia)

Feed A

Fig. 5. Effect of demethanizer pressure on ethane recovery for the lean gas feed A;

conventional turboexpander results from [5].

Fig. 6. GSP with cold separator process (feed A at high demethanizer pressure configuration or feed D at all demethanizer pressures).


50 100 150 200 250 300 350 400 450 500

C 2

r e c o v e r y ,

%

50

60

70

80

90

100

GSP

Conventionel turboexpander

GSP with cold separator

Feed D

Fig. 7. Effect of demethanizer pressure on ethane recovery for the rich gas feed D;

conventional turboexpander results from [5].


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cold separator was found to reduce to GSP, with zero vapor flow

rate

leaving

the

cold

separator.

3.2. Feed D

In

the

optimization

for

rich

gas

D,

the

CRR

process

was

tested

first

and

found

to

reduce

to

GSP.

The

ethane

recovery

from

GSP

was

compared to the recovery from the conventional turboexpander

process

[5].

The

results

are

shown

in

Fig.

7. At

low

and

intermediate

demethanizer pressures, GSP and the conventional turboexpander

process were found to give close ethane recoveries. The deviations

in recovery were observed at high demethanizer pressures at

which

the

GSP

gave

better

recovery

than

the

conventional

turboexpander

process.

For

this

particular

feed,

the

GSP

with

cold

separator was also tested and found to give recovery values similar

to the other processes tested for low and intermediate pressures. In

the

high

pressure

range,

the

recovery

from

the

GSP

with

a

cold

separator

fell between

the

values

from

the

GSP

and

the

conventional

turboexpander process [5]. At all demethanizer pressures, GSP with

cold separator (Fig. 4) reduced to the configuration in Fig. 6, with no

need

for

the

separator

overhead

splitter.

3.3. Ethane recovery

Although

the

CRR

process

is

frequently

stated

to

allow

for

ethane

recoveries

of

99%

or

higher

values

[8],

the

process

optimization yielded significantly lower recovery values for

intermediate and high demethanizer pressures in the case of feed

A,

and

all

pressures

for

feed

D.

At

low

demethanizer

pressures

(100

psia),

the

reflux

stream

entering

the

tower

is

80.5

8F colder

than the turboexpander outlet stream entering the tower. Forintermediate pressure (215 psia), the temperature difference is

56.5

8F. It

is

speculated

that

it

is

the

large

temperature

gap

observed

at

low

demethanizer

pressure

that

is

responsible

for

the

superior ethane recovery as compression not only enhances

pressure but also temperature. The lowest recovery values for both

feeds

A

and

D

were

the

ones

at

the

highest

demethanizer

pressure

(450

psia).

Fig. 8 shows the effect of demethanizer pressure on ethane

recovery for feeds A and D. For all the processes, the impact of the

demethanizer

pressure

on

ethane

recovery

is

significantly

less

for

the rich gas D compared to the lean gas A over the range 215–

450 psia. On the other hand, higher ethane recoveries are obtained

for

the

lean

gas

A

at

low

demethanizer

pressure,

whereas

higher

C2

recoveries are attained for feed D at intermediate and higherpressures.

For the lean gas A, the GSP with cold separator was found to be

the

most

viable

process,

reaping

the

benefits

of

the

split-stream

configuration

at

low

and

intermediate

pressures,

and

those

of

the

conventional turboexpander process at high demethanizer pres-

sures. However, the CRR process remains the process of choice for

the

lean

gas

at

low

demethanizer

pressure.

With

the

exception

of

low

demethanizer

pressure,

the

process

that yields the highest recovery for feed D is the GSP, which

excludes additional cryogenic compressor and heat exchanger

from

the

CRR

process.

4. Conclusion

The

cold

residue

recycle

(CRR)

process

was

simulated

to

maximize

ethane

recovery

at

different

demethanizer

pressures.

The optimized design variables included all split ratios, the outlet

temperatures (higher temperature side) from the heat exchangers

(downstream

of

the

chiller)

and

the

cryogenic

compressor

outlet

pressure.

The

CRR

process

is

the

most

viable

option

for

low

demethanizer pressure with ethane recovery of 99.9% for the lean

gas considered. However, adding more complexity to the process

may

lead

to

lower

ethane

recovery.

This

result

concurs

with

the

finding

in

Chebbi

et

al.

[10]. In

particular

(i)

the

only

case

where

a

cryogenic compressor is needed is for feed A (lean gas) at low

demethanizer pressure (100 psia). In all other cases, the CRR

process

reduces

to

GSP

where

the

cryogenic

compressor

followed

by

the

heat

exchanger

and

JT

valve

following

it

are

all

not

needed


50 100 150 200 250 300 350 400 450 500

C 2 r e c o v e r y ,

%

0

10

20

30

40

50

60

70

80

90

100

Feed A (GSP at 100 and 215 psia)

Feed D

GSP with cold separator


50 100 150 200 250 300 350 400 450 500

C 2 r e c o v e r y ,

%

0

10

20

30

40

50

60

70

80

90

100

Feed A

Feed D

Conventional turboexpander (Chebbi et al., 2008)


50 100 150 200 250 300 350 400 450 500

C 2 r e

c o v e r y ,

%

0

10

20

30

40

50

60

70

80

90

100

Feed A (CRR at 100 psia, GSP at higher P)

Feed D (GSP)

CRR/GSP

(a)

(b)

(c)

Fig. 8. Effect of demethanizer pressure on ethane recovery for (a) CRR/GSP, (b)

conventional turboexpander process [5] and (c) GSP with cold separator for feeds A

and D.

L. Kherbeck, R. Chebbi / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 5

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for higher ethane recovery. On the other hand (ii) adding a cold

separator

to

GSP,

operating

at

a

lower

temperature

than

the

chiller

temperature (in addition to the separator operating at 31

8F),

yields less recovery in the case of the rich feed gas D, except at

100 psia. Also (iii) the GSP with cold separator reduces to GSP in the

case

of

the

lean

feed

gas

A

at

low

and

intermediate

pressures,

making

the

cold

separator

unnecessary.

Furthermore

(iv)

in

case

GSP with cold separator provides higher recovery (lean gas A at

high demethanizer pressures), only one splitter is required: the

separator

liquid

outlet

splitter,

with

the

separator

vapor

outlet

splitter

discarded.

For feeds containing CO2, care should be taken to make sure CO2

frost [1] will not occur; this point is not addressed in the present

investigation

since

the

two

feeds

considered

are

free

from

CO2.

On

the

other

hand,

the

objective

in

this

work

is

to

maximize

ethane

recovery; therefore costing is not required. Further investigations

could address the abovementioned two points.

References

[1] F.S.Manning,R.E. Thompson, OilfieldProcessing of Petroleum, firsted., PennWellPublishing Company, Tulsa, 1991.

[2] R.L. McKee, Evolution in design, in:Proceedings of the56th Annual GPAConven-tion, Dallas, 1977.

[3] R.N.Pitman,H.M.Hudson, J.D.Wilkinson,K.T. Cuellar,Next generationprocessesforNGL/LPG recovery,in:Proceedingsofthe77th AnnualGPAConvention,Dallas, 1998.

[4] GPSA, Engineering Data Book, Sec. 16, twelfth ed., Gas Processors SuppliersAssociation, 2004.

[5] R.Chebbi,K.A.Al Mazroui,N.M.Abdel Jabbar, Oil& GasJournal106(46)(2008)50.

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R. Chebbi, N.S. Al-Amoodi, N.M. Abdel Jabbar, G.A. Husseini, K.A. Al Mazroui,Chemical Engineering Research and Design 88 (2010) 779.

[7] M.Mehrpooya, A. Vatani, S.M.A. Mousavian, Chemical EngineeringandProcessing49 (4) (2010) 376.

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