methods of ethane liquid recovery from natural gas

7/18/2019 Methods of Ethane Liquid Recovery From Natural Gas

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There are three general methods which can be used to

achieve the conditions necessary to attain high ethane

recovery levels.

1. J-T Expansion

2. Turboexpander

3. Mechanical rerigeration

Each o these processes has been used successully! withthe turboexpander being the predominant process o

choice or ethane recovery acilities.

METHODS OF ETHANE RECOVERY



J-T EXPANSION PROCESS

" The use o the Joule-Thomson #J-T$ eect to recover

li%uids is an attractive alternative in many applications." The general concept is to chill the gas by expanding the gas

across a J-T valve. &ith appropriate heat exchange and

large pressure dierential across the J-T valve! cryogenic

temperatures can be achieved resulting in high extraction

eiciencies.

" The main dierence between the J-T design and turbo

expanders is that the gas expansion is adiabatic across the

valve and is nearly isentropic path. Thus the J-T design

tends to be less eicient per unit o energy expended than

the turbo expander.



The J-T process does oer some advantages over the

turboexpander and rerigeration processes in the ollowingsituations'

1. (ow gas rates and modest ethane recovery.

2. The process can be designed with no rotating

e%uipment.

3. )road range o lows.

*. +implicity o design and operation.




" ,n order to eectively use the J-T process! the gas must

be at a high inlet pressure #over /0a $. " , the gas pressure is too low! inlet compression is

necessary or insuicient expansion chilling will be

attained." The gas must irst be dried to ensure that no water

enters the cold portion o the process.

" Molecular sieves or lumina are used or the drying." Methanol inection has been used in a ew plants

successully but can be an operating problem.




" ,n some cases the eed gas is not at high enough

pressure or the gas is rich in li%ueiable hydrocarbons.

Then mechanical rerigeration can be added to the J-T

process to enhance recovery eiciencies.

" The J-T process with rerigeration added to aid in

chilling the eed gas.

" The gas in this design is expanded downstream o thecold separator.

REFRIGERATED J-T PROCESS



" The location o the J-T valve is dependent on the gas

pressure and composition involved.

" The advantage o rerigeration is that lower eed pressure

can be used or alternatively! the demethanier can be

operated at a higher pressure thus reducing residue

compression." The J-T process! whether rerigerated or non-rerigerated!

oers a simple! lexible process or moderate ethane

recovery.

" ,t is usually applied to smaller gas lows where some

ineiciency can be tolerated or reduction in capital and

operating costs.




TURBOEXPANDER PROCESS

" The turboexpander process dominates ethane recovery

acility design." This process uses the eed gas pressure to produce

needed rerigeration by expansion across a turbine

#turboexpander$. The turboexpander recovers useulwor/ rom this gas expansion.

" The expander is lin/ed to a centriugal compressor to

recompress the residue gas rom the process. )ecausethe expansion is near isentropic! the turboexpander

lowers the gas temperature signiicantly more than

expansion across a J-T valve.



" 4ry eed gas is irst cooled against the residue gas and

used or side heating o the demethanier.

" dditionally! with richer gas eeds! mechanical

rerigeration is oten needed to supplement the gas

chilling.

" The chilled gas is sent to the cold separator where the

condensed li%uid is separated! lashed and ed to the

middle part o the demethanier.

" The vapor lows through the turboexpander and eeds the

top o the column.




" J-T valve is installed in parallel with the expander.

This valve can be used to handle excess gas low beyond the design o the expander or can be used or the

ull low i the expander is out o service.

" ,n this coniguration the ethane recovery is limited to

about 56 or less.

" lso! the cold separator is operated at a low temperature

to maximie recovery. 7ten the high pressure and low

temperature conditions are near the critical point o the

gas ma/ing the operation unstable.

" nother problem with this design is the presence o

872! which can solidiy at operating temperatures.




To increase the ethane recovery beyond the 56 achievable with the

conventional design! ollowing methods were developed'

9esidue 9ecycle #99$

:as +ubcooled 0rocess #:+0$

8old 9esidue 9ecycle #899$

+ide 4raw 9elux #+49$ 0rocess




RESIDUE RECYCLE

" To increase the ethane recovery beyond the 56

achievable with the conventional design! a source o

relux must be developed or the demethanier.

" 7ne o the methods is to recycle a portion o the residuegas! ater recompression! bac/ to the top o the column.



RESIDUE RECYCLE



" The process low is similar to the conventional design

except that a portion o the residue is brought bac/

through the inlet heat exchange.

" t this point the stream is totally condensed and is at

the residue gas pipeline pressure.

" The stream is then lashed to the top o the

demethanier to provide relux. The expander outlet

stream is sent a ew trays down in the tower rather

than to the top o the column.

" The relux provides more rerigeration to the system

and allows very high ethane recovery to be realied.

RESIDUE RECYCLE



" The residue recycle #99$ system has been used

successully in numerous acilities.

" ,t is 872 tolerant and the recovery can be adusted by

the %uantity o recycle used.

" The recovery level is a unction o the %uantity o

recycle in the design.

" The 99 process can be used or very high ethane

recoveries limited only by the %uantity o horsepower

provided.

RESIDUE RECYCLE



GAS SUBCOOLED PROCESS



" The :as +ubcooled 0rocess #:+0$ was developed to

overcome the problems encountered with theconventional expander process.

" portion o the gas rom the cold separator is sent to

a heat exchanger where it is totally condensed withthe overhead stream.

" This stream is then lashed to top o the demethanier

providing relux to the demethanier.




" s with the 99 process! the expander eed is sent to the

tower several stages below the top o the column.

" )ecause o this modiication! the cold separator

operates at much warmer conditions well away rom

the system critical.

" dditionally! the residue recompression is less than

with the conventional expander process.

" The horsepower is typically lower than the 99 process

at recovery levels below ;26.




" The :+0 design has several modiications.

" 7ne is to ta/e a portion o the li%uid rom the cold

separator along with the gas to the overhead exchanger.

:enerally! this can help to urther reduce the horsepower

re%uired or recompression.

" lso! the process can be designed to ust use a portion o

the cold separator li%uid or relux. This modiication is

typically used or gases richer than 3 :0M.




" The :+0 design is very 872 tolerant< many designs

re%uire no up ront 872 removal to achieve highrecovery.

" 872 levels are very composition and operating

pressure dependent! but levels up to 26 can usually

be tolerated with the :+0 design.




" process scheme has been developed to combine the

:+0 and 99 processes into an integrated process

scheme.

" This concept is based on applying the best eatures o

each process to the integrated design.

" This combination can result in higher ethane recoveryeiciency than can be achieved with :+0.

COLD RESIDUE RECYCLE (CRR) PROCESS



CRR PROCESS

CRR PROCESS



" The 8old 9esidue 9ecycle #899$ process is a

modiication o the :+0 process to achieve higher ethane

recovery levels.

" The process is similar to the :+0 except that a

compressor and condenser have been added to the

overhead system to ta/e a portion o the residue gas and provide additional relux or the demethanier.

" This process is attractive or extremely high ethane

recovery. 9ecovery levels above ;56 are achievable

with this process.

" This process is also excellent or extremely high propane

recovery while reecting essentially all the ethane.

CRR PROCESS



SIDE DRAW REFLUX (SDR) PROCESS

" The +ide 4raw 9elux #+49$ process is another

modiication o the :+0.

" ,n this design! a stream is ta/en o the demethanier!

boosted in pressure and condensed to provide relux.

" This design is interesting in cases where the residue gas

stream will contain inerts such as =2 which ma/e the

subcooling o the cold separator overhead ineasible.

" The stream ta/en rom the side o the demethanier is

ree o the inert components and condenses easily.



" s with the 899 process! the extra e%uipment

associated with +49 system must be ustiied on

additional li%uid recovery.




" The use o a mixed rerigerant process is an interesting

alternative to the turboexpander process. +uch processes

have been used widely in (=: processing and to a lesser

extent in =:( recovery.

" 7ne o the characteristics o the process is that low

temperatures can be achieved with signiicantly reducedinlet gas pressure.

" The chilling can be achieved totally with mechanical

rerigeration or with a mixture o rerigeration and

expansion.

" , inlet compression is contemplated or a turboexpander

plant! then mixed rerigerant processing can be an

economic alternative.

MIXED REFRIGERANT PROCESS



" ,n this process! the eed gas is chilled to cold separator

temperature where the li%uid is sent to the demethanier as

in an expander process.

" The overhead vapor is split and the maority sent

through an expander to the upper part o the demethanier.

" portion o the gas is cooled urther in the main heat

exchanger and sent to the top o the demethanier as

relux.

" lternatively! the turboexpander can be eliminated and thetotal stream cooled in the main exchanger and ed to the

demethanier.




" The residue gas would be exchanged with the eed in the

main heat exchanger. The rerigeration is provided by asingle mixed rerigerant system designed to provide the

necessary low temperature conditions.

" The rerigerant would typically be a methane! ethane!

propane mixture with some heavier components as

dictated by the design conditions.

" critical aspect o the design is to maintain the desired

rerigerant composition during plant operation.




" ,n all =:( recovery processes! one o the inal steps in the

plant is the production o the desired li%uid product by use

o a ractionation column.

" This column produces the speciication product as a bottom

product with the overhead stream being recycled to the

process or sent out o the plant as residue gas product.

" This mixed product then needs to be separated into usable

products in a series o one or more ractionation columns.

" The number and arrangement o these columns is dependent

on the desired product.

FRACTIONATION CONSIDERATIONS



" , the =:( stream is an ethane plus stream the irst step

is to separate the ethane rom the propane and heaviercomponents in a deethanier.

" The propane is then separated rom the butane and

heavier components in a depropanier." , urther processing is desired the butane may be

separated in a debutanier and the butanes urther

separated in a butane splitter column." The butane splitter is only used when a dierential value

can be realied or the isobutane versus the mixed

butane stream.


methods of ethane liquid recovery from natural gas

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