Synthesis Gas Treating with PhysicalSolvent Process Using Selexol

Process Technology

The Selexol process has proven to offer economic advantage, energysavings, and operating reliability for gas treating. The use of Selexol-II solvent lowers the ammonia production cost even further.

Vinod A. ShahNorton Company, Akron, OH 44309

T.L. HuurdemanDSM Fertilizers, 6160 McGleen, The Netherlands

The selection of a gas treating processtoday is more complex than in the past.The high cost of energy which hasprevailed since the 70's has brought manyenergy saving processes to the field ofgas treating industry. Because ammoniaproduction is a high energy intensiveprocess and because future energy costis uncertain and expected to be higher,ammonia producers and process licensorswere forced to seek more energy-efficient production methods. Thisbrought many new synthesis gas treatingprocesses to the ammonia industry.Some of these processes may be moreenergy intensive, some have moremaintenance, some have economics forremoving carbon dioxide while some haveeconomics for removing hydrogen sulfidefrom coal and partial oxidation (POX)gasified gas streams.

This paper describes the process thatcovers efficient treatment of allvarieties of synthesis gases. Thisprocess is a physical solvent processcalled the SELEXOL(R) Solvent Process.The paper is presented into two sections :

SELEXOL SOLVENT PROCESS - This sectiondescribes the SELEXOL solvent, itsapplications for synthesis gas treatingand an illustration of a plant andits performance and operating history;and

NEW SOLVENT - This section presents anew solvent which is specially formu-lated for synthesis gas treatment.The solvent is named SELEXOL IIsolvent. The solvent and its economicbenefits are discussed and compared withthe present SELEXOL solvent.

SELEXOL SOLVENT PROCESS;

General

The solvent is a homologue of dimethylether of polyethylene glycol, and hasbeen commercially proven in over 40licensed facilities throughout theworld. The process is owned and licensedby Norton Company Chemical ProcessProducts. The process and solventcharacteristics of the solvent arehighlighted in Table 1.

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TABLE 1

SELEXOL solvent, processCharacteristics

* Non-toxic, non-polluting, nonreactive

* Non-corrosive - biodegradable* 151° C (304° F) flash point* 0.0007 mm Hg vapor pressure at 25°C (77° F) - low solvent losses

* Selectively absorbs H2S, COS, RSH,CS2 in preference to C(>2

* Little or no heat required forsolvent regeneration

* Selectively removes heavy paraf-fins, olefins, naphthenes andaromatic byproducts ofgasification

* no change of solvent is necessary

Its application and experience in thesynthesis gas treating area includes:

* Selective sulfur compound and car-bon dioxide removal from gasesoriginated from:* natural gas* partial oxidation* coal gasification* coak gasification

* Removal of coal gas heavy hydro-carbon byproduct

The SELEXOL Process was firstapplied to the treatment of ammoniasynthesis gas in 1965. Since that timethe process has proven its applicabilityfor synthesis gas treatment in 16installations, including a total of 10ammonia plants. These facilities rangein size from demonstration scale to theworld-scale 1,360 t/d DSM ammoniafacility. Several of these installa-tions are integral parts of "low energy"ammonia plants.

Synthesis Gas Treating OperatingPhilosophy

SELEXOL Solvent is a physical solventwhich means that its capacity to absorbgases is based on the physical solubi-lities of gases in the solvent and noton any chemical reactions with thesolvent.

Table 2 summarizes the relative sol-ubilities of various gases in thesolvent. The actual solubility data isproprietary, however the relative sol-ubilities can be compared for theapplication (Basis: Ü2 = 1.0).

TABLE 2Relative Solubility of Gases in

SELEXOL Solvent

Component

H2 (low solubility)N2COCH4C02COSH2SSO2

H20C8H10 (high solubility)

SolubilityRatio

1.01.52.25761756707000

19,00055,00070,000

As shown in Table 2, the H2S is muchmore soluble than H2, N2, and CO2- IfH2S is present, the solubility differ-ences allows the selective removal ofhydrogen sulfide from the gas streamleaving most of the carbon dioxideunabsorbed.

In the case of carbon dioxide removal,because it is more soluble than hydrogen,it allows selective removal of carbondioxide while leaving hydrogen andnitrogen essentially unabsorbed.

In the case of physical solvents,because the solubilities are enhanced athigher pressures and lower tempera-tures, in order to keep the solventcirculation rates low, the absorbers areoperated at feed pressures and cold -12 °C (10°F) to ambient temperatures. Sincethe solubility decreases at lowerpressure, the desorption of the carbondioxide from the solvent is achieved bysimply reducing the pressure of thesolvent. This characteristic eliminatesthe need of heat for solvent regenera-tion, and that is he primary reason whythe process is an "Energy SavingProcess".

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Energy Saving Process - Highlights

* Application: Steam reforming ofnatural gas or naphtha:

- The heat requirement for solventregeneration is essentially zero.

- Reformer steam-to-carbon ratio isindependent of treating require-ment; allows minimum steam-to-carbon ratio to the primaryreformer.

- Allows substantial heat recoverydownstream of low temperatureshift. Heat can be used for purposesother than solvent regeneration.

- Product carbon dioxide is availableat high pressures and at low watercontent.

- Moderate operating temperatures 10° Fto ambient.

* Application: POX or coal basedsynthesis gas:

H2S removal section- highly selective for H2S over carbon

dioxide which results in sub-stantial capital and energy costsavings for sulfur production

- lower heat of regeneration- low solvent circulation- ambient temperatures operation- no solvent degradationCO2 removal section- The heat requirement for solvent

regeneration is essentially zero.- Product carbon dioxide is available

at higher pressures and at lowwater content.

- Operating temperatures -12° C(10°F) to ambient.High carbon dioxide productrecovery.

- No solvent degradation.

Process Variables

For process optimization, the mainprocess variable is solvent circulation.The solvent circulation, however isdependent upon

- the temperature of the solvent, and

- absorber height

In the case of the SELEXOL Process,the temperature could be any temperaturein the range of refrigerated at -12" C(10° F) to water cooled at 49° C (120°F) . If the refrigeration route isselected, the requirements are small andusually supplied by the ammonia plantrefrigeration compression system. Noseparate refrigeration unit is required.

Normally the height of the absorberis moderate in the range of +/- 36 mto 46 m (120 feet to 150 feet) .

Process

H2S removal from POX and coal basedsynthesis gas. The process schematicdiagram is shown in Figure 1.

MODUCIHjS REMOVAL

FEED

ACID GAS

HEAT

Figure 1.

The process unit is a simple operatingunit which consists of a conventionalabsorber, a recycle flash drum systemand a reboiled stripper column.

The higher solubility of hydrogensulfide and other sulfur compounds insolvent allows selective removal ofthese compounds from the synthesisgas stream. The resultant acid gasstream is sufficiently concentrated inhydrogen sulfide for Claus processing.

CO2 Removal From Synthesis Gas. Theprocess flow diagram and processdescription for CÛ2 removal unit isdiscussed in greater detail in the nextsection.

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Synthesis Sas Treating - A CaseHistory

In order to best evaluate the processapplication, reliability, and solventcapabilities, it is best to evaluatea specific plant performance and itsseveral years of operating history.The SELEXOL unit at DSM - Ammonia plant(formerly UCAM plant) is selected forthis purpose.

The DSM Ammonia plant, a part of DSMFertilizers division, is a 1,360 tonsper day ammonia plant located at Geleen,the Netherlands. As of July 1989, theplant has completed five years ofsuccessful operation.

The plant was designed and constructedby Kellogg Continental, b.v., based onM. W. Kellogg's reduced-energy ammoniatechnology for steam reforming ofnatural gas. The plant has achieved anactual energy consumption of 7.0 millionkcal per metric ton of ammonia (25.2million Btu/s.ton) without purge gasrecovery. The plant was commissionedin July 1984 and was producing ammoniaonly 3 1/2 weeks after initial start-up.

Process Description. The SELEXOL unitschematic process flow diagram is shownin Figure 2.

HIGH CO2 RECOVERYFLOW DIAGRAM

PRODUCT

FEED

VENT

AIR

Figure 2.

Raw synthesis gas from the lowtemperature shift effluent, followingheat recovery and steam condensateremoval, enters the carbon dioxideabsorber column at the bottom. The gasis counter-currently contacted in the

packed absorber column with the leansolvent entering at the top of theabsorber. The carbon dioxide isabsorbed by the solvent and collected inthe bottom.

Carbon dioxide-rich solvent from thebottom of the absorber then flowsthrough a power recovery hydraulicturbine, which converts the pressureenergy of the solvent to mechanicalenergy through reduction of pressure.This energy is utilized by the maincirculating pump and reduces its energyrequirement by approximately 50%.

Rich solvent from turbine flowsinto the recycle flash drum whereessentially all of the co-absorbedhydrogen and nitrogen is flashed.The flashed gas is separated, compressedand recycled back to the absorber.

The rich solvent from the recycle flashdrum V-l flows into the low pressureflash drum V-2 where more than 70% ofthe carbon dioxide is flashed andrecovered.

In order to achieve the additionalcarbon dioxide recovery required at DSMAmmonia Plant (DSM), the solvent fromthe low pressure drum is further flashedin a vacuum flash drum V-3. Vacuum isgenerated by the vacuum compressor whichsimultaneously compresses the vacuumflash gases up to the desired carbondioxide product pressure.

Should higher carbon dioxied productrecovery (up to 97%) be desired, thesolvent could be flashed to lowerpressures. The actual operatingpressure of the vacuum flash drumdepends upon the desired carbon dioxiderecovery, i.e. the lower the flash drumpressure, the higher the carbon dioxiderecovery. The solvent is very stable ata wide range of pressures, therefore,vacuum operation has no detrimentaleffect on the solvent or on the process.Furthermore, because the solvent vaporpressure is extremely low (1. x 10~4

kPa or 0.0007 mmHg at 25°C), the vaporlosses at vacuum pressure are minimaland are not a concern.

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It should be noted here that theSELEXOL solvent process is also capableof essentially 100% carbon dioxiderecovery. A patented process schemewhich accomplishes this has been dis-cussed in references 1 and 3.

Following the carbon dioxide removaland recovery, the solvent is regeneratedin an air stripping column. The air,supplied through an air blower, entersthe packed stripping column at thebottom. It is counter-currentlycontacted with the solvent containinga residual amount of carbon dioxide.The air with the stripped carbon dioxideis vented to atmosphere. The regeneratedsolvent from the bottom of the strippercolumn is refrigerated and recycled backto the top of the absorber.

To keep the unit in water balance,a small amount of the solvent takenfrom the stripper bottom is continuouslydehydrated. During winter, when theambient stripping air is very dry,dehydration is not required and thedehydrator is taken off the service.

Since the SELEXOL solvent is regen-erated using air as a stripping medium,the ikeat which is contained in theprocess gas downstream of the lowtemperature shift effluent is availablefor other in-plant uses. At DSM, morethan 90%̂ of this heat energy is recoveredand supplied to the following plantservices;

High pressure boiler feedwaterpreheat

- Low pressure steam generation

- Process condensate stripper reboiler

- Low pressure boiler feedwater pre-heat.

By sensible use of the energy,DSM/Kellogg has taken full advantage ofthe energy saving feature of the SELEXOLprocess and has achieved high energyefficiency. This energy recovery issubstantial; it represents 0.862 millionkcal/ton of ammonia.

Equipment. All of the equipment inthe SELEXOL unit is conventional. Nospecial designs or materials of con-struction unique to SELEXOL service arerequired. Primarily all carbon steelconstruction is used in the plant,including absorber and stripper towers.These towers have Norton's high effi-ciency distributors and redistribu-tors. Use of stainless steel towerpackings were specified by DSM, however,carbon steel packings have been used inother SELEXOL units for similar serviceswith no reported corrosion problems.The pumps and vacuum compressors arespared, however, the recycle compressoris not spared.

unit Startup. startup is conven-tional, very easy, and can generallybe accomplished in less than 30 hours.During start-up, because the refriger-ation is not in service, the solventcooling can be achieved by a water cooledexchanger. At about 50% of the gasflow, the unit will produce the desiredtreated gas. Alternatively, the ammoniacould be imported for initial startupand refrigeration unit started priorto the plant start-up. DSM had selectedthis route.

Operating Expérience. The SELEXOLunit on-stream efficiency of the DSMplant has been excellent - approaching100%. The solvent losses have beennegligible - approximately 5,000 kg peryear, which is only 0.01 kg per ton ofammonia.

The SELEXOL unit of the DSM ammoniaplant is very easy to start and easy tooperate. There is no mixing of solventor composition control. Also there areno stringent process control require-ments, nor any need of additives such ascorrosion inhibitors or promoters. Asa result, the unit does not require muchof the operator's attention. The onlysolvent analysis required, which isnormally checked only once a week, isthe water content of the solvent.

Operating History. During more thanfive (5) years of operation, only threeSELEXOL unit plant shutdowns haveoccurred. These shutdowns were caused bythe mechanical problems not attributedto the SELEXOL process itself. Thesemechanical problems were pump seal

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failure, low oil pressure to thehydraulic governor of the steam turbinedriver, and a flow instrument malfunc-tion.

On rare occasions, the vacuum flashdrum showed signs of froth/foamingresulting in an overflow of the solventinto the storage tank. The exact causeof the froth/foaming is not determined,however when it occurs, it is controlledby adding 0.5 liters of anti^foam agentdirectly into the flash drum. Aninstalled slip-stream filter unit alsohelps suppress foaming by eliminatingaccumulation of foreign materials.

Plant Performance Data Table 3summarizes DSM's SELEXOL unit designspecifications and performance.

TABLE 3

SELEXOL Unit Performance DataDSM Fertilizer Ammonia Plant

Spec. Operation1360 (106%)

18.2 18.1

1000 400

NH3 Capacity, t/d

CO2 Removal Unit:

- Feed CO2, mol%

- Treated Gas CO2

- C02 Product:

- Recovery, % 81* 82*

- Purity, mol% 99.0 99.4

- Temperature, °C 17.0 6.3max.

- Pressure, kPa. 150 162min.

- This was the recovery desired. Muchhigher recovery - to essentially 100% -can be achieved with the SELEXOL process.

Energy Requirement - summary

Table 4 summarizes energy requirementfor SELEXOL unit at DSM Fertilizer plant.The energy efficiency at the DSM

Fertilizer plant has been excellent.As indicated earlier, the plant hasachieved an actual energy consumptionof 29.3 GJ per ton of ammonia (25.2million Btu per ton of ammonia) withoutpurge gas recovery. In fact the portionattributed to the battery limit SELEXOL

TABLE 4

SELEXOL Unit Energy Requirement

Energy Consumption (for 1,442 t/d NH3)

CO2 removal unit*, GJ/hr

LTS effl. heat recovery, GJ/hr

Net energy surplus, GJ/hr

Energy surplus, GJ/t(198.3)(3.3)

*Includes electric power, steam andrefrigeration •

unit is established to be only 0.3 GJ/t.If credit is taken for the upstream lowtemperature shift effluent heat recoveryof 3.62 GJ/t, the net result is a surplusof 3.3 GJ/metric ton.

To summarize SELEXOL unit operationat DSM, it can be said that it hasperformed to expectations. Besidesbeing a low energy process, it alsoproves to be a very reliable process.

NEW SOLVENT FOR SYNTHESIS GAS TREATING

With the success of the SELEXOL processin the area of synthesis gas treating,Norton Company initiated a researchprogram to further enhance the SELEXOLprocess economics.For this solvent, our goals were:

- increase the carbon dioxide solu-bility

- reduce coabsorption of gases

- decrease the solvent circulationrequirement

- increase the mass transfer effi-ciency

The result of our research is asolvent, named SELEXOL II solvent. LikeSELEXOL, it is a glycol based solvent,but with much higher solubility forcarbon dioxide. Table 5 summarizes andcompares the property of the SELEXOL IIsolvent with present SELEXOL solvent.

The SELEXOL II solvent has beentested successfully in the pilot plant.The results are very encouraging. Itproves to be a more energy efficient andcost effective solvent.

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TABLE 5

Physical Properties

SELEXOL SELEXOL IISOLVENT SOLVENT

Vapor Pressure@ 25"C (kPa) 9.3 x 10~5 1.2 x 10~3

Density @25 °C,

Vise. @ 25°Ccp

Boiling Pt. °c

Freezing Pt.°C

Odor

1.05

5.9

215

-22

mild

0.88

1.5

209

-58

mild

The SELEXOL II solvent has thefollowing applications for the synthesisgas treatment:

- Grass root ammonia plant

- SELEXOL unit expansion from presentSELEXOL solvent to new SELEXOL IIsolvent

- Revamp of amine units and hotcarbonate unit for expansion andenergy savings.

Process Description

The process flow diagram and processdescription for use of SELEXOL IIsolvent is the same as discussed pre-viously for (present) SELEXOL system.Economie Comparison

Table 6 summarizes and compareseconomics of using SELEXOL II solventfor the treatment of ammonia synthesisgas.CONCLUSION

The physical solvent process has animportant place in the ammonia synthesisgas treatment. The experience of theSELEXOL process proves its economicadvantage, energy savings, and oper-ating reliability. All this results inone thing - a lower ammonia productioncost.

The use of SELEXOL II solvent will evensurpass the economic advantage ofSELEXOL and prove to be as reliable andflexible as the SELEXOL solvent process.

TABLE 6

Economic Comparison ; SEISELEXOL II

SELEXOL SELEXOL II

Basis

Treated Gas, CO2 500ppm

CO2 Recovery % 91

Economics

I

Capital Cost 100

Utilities 100

Absorber Height 100

SolventCirculation 100

Recycle GasVolume 100

SolventInventory 100

500

92

i

75

80

45

93

104

42

References:

1. V. A. Shah and J. McFarland, "LowCost Ammonia and CO2 Recovery", Hydro-carbon Processing, March 1988

2.R. J. Hernandez and T. L. Huurdeman,"Solvent Unit Cleans Synthesis Gas",Chemical Engineering, P-154-156, Feb-ruary 1989

3. Vinod A. Shah, "CO2 Removal fromAmmonia Synthesis Gas", Energy Progress,8 (2), 67-70, (1988)

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DISCUSSIONMAX APPL, BASF: I understand that Selexol has quitea high solubility for hydrocarbons too. How is the qualityof COa in your process? Can you properly separate fromCÛ2 such organic impurities as propane or butane tomeet food-grade specification for liquid CÛ2?

SHAH: Max, on the separation of carbon dioxide fromsuch absorbed hydrocarbons as propane and butanes,I presented a paper at the Gas Processors Associationconvention in March, 1989. This paper also provideseconomic comparison of this process with conventionalscheme. For the feed gas containing carbon dioxide andheavy hydrocarbons such as benzene and toluene, andrequiring the recovery of high-purity carbon dioxide, wepropose the installation of a Selexol prewash unit(upstream of CCfe removal unit) to remove these heavyhydrocarbons prior to carbon dioxide removal. Oncethese hydrocarbons are removed, CO2 can be extractedin the COz removal unit. Max, did I answer your question?

APPL: Not exactly, but I understood your point. I wasactually referring to traces of propane or butane and otherhydrocarbons in the recovered CÛ2. Removing COz inan ammonia plant from syngas hydrocarbon leaves somesmall traces of methane and higher hydrocarbons, mostlybeing formed in the high-temperature shift conversion.These hydrocarbons will probably remain in the CÛ2;however, to process the CÛ2 for liquid CÛ2 for foodgrade, impurities like hydrocarbons should not be allowed.How can you remove them, or do you have problemswith that?

SHAH: The recovered carbon dioxide from Selexol unithas been liquified and used for food processing. Ifhydrocarbons are present in the CÛ2 stream, the bestway is to inject oxygen over catalyst and oxidize thesehydrocarbons to produce COs- l believe this is a veryinexpensive way to handle the hydrocarbons.

K.S. RAGHURAMAN, KTI: What's the minimum partialpressure of CÛ2 needed in the syngas of the feed gasto the Selexol absorber for its economic operation, inlight of the fact that it is operated with a pressure swingand the partial pressure is being used as the motivatingforce? What's the minimum pressure for Selexol to beeconomical?

SHAH: Normally, we like to have a minimum of 150-psi (1,033-kPa) absorber pressure which for 18% CÜ2in the synthesis gas feed represents a 27-psi (186-kPa)partial pressure.

RAGHURAMAN: Partial pressure?

SHAH: No. As a feed pressure in ammonia will have

approximately 17 to 18%, 18 times 150 will look veryattractive.

RAGHURAMAN: I assume that below that level it won'tbe very interesting.

SHAH: That's correct, since there will be a largercirculation. But remember the pumping requirements.Since it is not pumped all the way to 400-500 Ib (182-227 kg), more circulation can be allowed to pick up allthe CO2.

RAGHURAMAN: Thank you. What's the minimumpartial pressure needed for COz to be economical?

SHAH: We checked up to 150 Ib (68 kg), and it lookslike an attractive way of doing the job.

RAGHURAMAN: Are you talking about the 18% CO2in the gas?

SHAH: Correct.

RAGHURAMAN: You also mentioned that it could bea good alternative for revamping existing plants with otherprocesses. Have you ever done that? Most of the unwieldyproblems I see stem from taking the heat out and doingsomething else with the same plant. Have you workedon any plant which was revamped with Selexol to properlyuse available heat.

SHAH: One plant in Sweden, designed and constructedby Haldor Topsoe, had a lot of excess heat availablesince the conversion to the Selexol unit. The availableheat was converted to produce low-pressure steam, whichthen was converted to high-pressure steam that wasworth a lot of money. I believe this steam was compressedto approximately 150 Ib (68 kg).

HARRY VAN PRAAG, CIL: You mentioned that forexpansion, you could switch from existing Selexol andSelexol II. But could you gradually change over and installSelexol II to improve performance?

SHAH: The only new solvent we have used is Selexoln solvent. To date we haven't operated a unit with amixture of Selexol and Selexol II solvent. Until we performpilot testing, I would not know the answer to that question.

A. ILYAN, Asean Fertilizer, Indonesia: If the feed werenatural gas containing COz and high sulfur (H2S) bySelexol solvent, is it possible to produce pure COz gas?CÛ2 gas used for raw material of urea which containsmaximum of l ppm H2S.

SHAH: In fact, we do this kind of application all thetime. In the EOR-type (enhanced oil recovery) application,

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the CO2 content is always very, very high (95 - 97%) orall the way to 99%. And there is always a trace amountof sulfur (FbS) present. We circulate a small amount ofSelexol solvent which will pick up I-feS out of the CÛ2-rich stream, and several Selexol plants do this.

ILYAS: How do you separate CC»2 gas from PfeS gas,when both of the gases are diluted in the Selexol solvent?How many absorption towers and regeneration towersare needed to treat the gas?

SHAH: The simplest towers you can buy.

ILYAS: Are the two towers required? Is there oneabsorber and one stripper?

SHAH: One needs to have just H2S absorber and astripper.

SATISH NIRULA, S.R.I. International: You illustratedthe application of Selexol to ammonia production. Hasit ever been applied to hydrogen production?

SHAH: Yes. We have a unit at Shell Carson Refineryin California for a hydrogen cleanup. Another hydrogencleanup unit was where Selexol was to remove andrecover a bulk amount of CÛ2 for CÛ2 production only —the remaining COa was removed by the PSA unit, because

they needed hydrogen purity with CÛ2 content around50-100 ppm.

NIRULA: In hydrogen production, the reformerpressures are generally low, which means that the feedgas to the Selexol unit would be low. What effect doesthat have on the economics of the Selexol process,compared with chemical solvent processes?

SHAH: It depends on the energy cost. There was aquestion raised earlier as to what partial pressure levelSelexol unit would be justified. The hydrogen plant feedwould usually be around 200 Ib (91 kg), and at this pressureit proves to be economical. I'd like to indicate that wefrequently have a misconception about the physicalsolvent because we always think of a partial pressure.Yes, it's a factor. But, remember, pressure is a correctionto the circulation. The lower the feed pressure, the morecirculation may be required. But the pressure differentialof the pump is not that high for low feed pressureoperation. The 200 Ib (91 kg) operation may require twicethe circulation compared to the 400 Ib (182 kg) operation.But, overall pump horsepower requirements are thesame. So even at low feed pressures, installation of Selexolunit may be justified.

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