Selection of a Hydrogen Separation Process

Selection of a Hydrogen Separation ProcessGeoffrey Q Miller and Joerg Stoumlcker

IntroductionThe three main processes for hydrogen upgrading in refineries are the pressure swing adsorption processthe selective permeation process using polymer membranes and the cryogenic separation process Each ofthese processes is based on a different separation principle and consequently the process characteristicsdiffer significantly Selection of an appropriate hydrogen separation process depends not only oneconomics but also other project considerations such as process flexibility reliability and ease of futureexpansion A review of the separation process characteristics and equipment is given below followed bydiscussions of other project considerations General and application-specific selection guidelines are thenpresented along with process integrations which take advantage of the complementary characteristics of thethree processes

Process and Equipment DescriptionPressure Swing Adsorption (PSA) SystemsThe PSA process for hydrogen purification is based on the capacity of adsorbents to adsorb moreimpurities at high gas-phase partial pressure than at low partial pressure The principle of the process isillustrated in Figure 1A Impurities are adsorbed in an adsorber at higher partial pressure and then desorbedat lower partial pressure The impurity partial pressure is lowered by ldquoswingingrdquo the adsorber pressure fromthe feed pressure to the tail gas pressure and by using a high-purity hydrogen purge Hydrogen is adsorbedin only small amounts The process operates on a cyclic basis Multiple adsorbers are used in order toprovide constant feed product and tail gas flows Each adsorber undergoes the same process steps in thesame sequence but the steps are staggered with respect to time A simple cycle sequence chart is shown inFigure 1B for a system with four adsorbersThe driving force for the separation is the impurity partial pressure difference between the feed and the tailgas A minimum pressure ratio of approximately 41 between the feed and tail gas pressure is usuallyrequired for hydrogen separations However the absolute pressures of the feed and tail gas are alsoimportant particularly to hydrogen recovery The optimum feed pressure range for PSA units in refineryapplications is 200-400 psig The optimum tail gas pressure is as low as possible Since vacuum isnormally avoided tail gas pressure between 2 and 5 psig are typically used when high recovery is needed2Figure 1AAdsorption IsothermsWt- Loading(lbs impurity100 lb adsorbent)HeavyComponent(C5 H12)IntermediateComponent(CH4)LightComponent(N2)Pressure SwingImpurity Partial Pressure1048577X3

1048577X1

1048577X2UOP 3111-1

Figure 1BPSA Cycle Sequence ChartAdsorptionE =PP =P =

BD =R =E1BDE1RPPPE1E1BDAdsorptionPPRPAdsorptionBDE1PRE1E1RPPAdsorption2 E1 PP BD P341Vessel 411 CycleTimeEqualization (COCurrent Depressurization)Provide Purge (COCurrent Depressurization)PurgeBlowdown (CounterCurrent Depressurization)Final RepressurizationUOP 3111-2

3The 2 psig pressure can be used when the tail gas is to be compressed and 5 psig can be used when the tailgas is sent directly to fuel burners as in steam reforming applications Process performance is much moresensitive to tail gas pressure than feed gas pressure as shown in Figure 2 which shows the influence of asystem pressure levels on hydrogen recovery for a fixed feedstock and process configurationFigure 2Effect of Pressure Levels on PSA System RecoveryFeed Pressure psigBase-2-4-6200 350HydrogenRecoveryTail Gas Pressure = 5 psigTail Gas Pressure psigBase

-5-10-155 60HydrogenRecoveryFeed Pressure = 300 psig-20UOP 3111-3

The product hydrogen from a PSA unit is available at essentially feed pressure There is a nominal 10 psipressure drop between feed and product The product gas will contain only the less-strongly adsorbedcomponents of the feed gas in detectable levels The PSA separation is chromatographic in nature meaningthat the lightest impurities will appear first in the product followed by the more strongly adsorbedimpurities Relative adsorptivity of typical feed impurities is given in Figure 3Two of the advantages of the PSA process are its ability to remove impurities to any level (eg ppmvlevels if desired) and to produce a very high purity hydrogen product Typical PSA hydrogen productpurities range from 99 to 99999 vol- Removal of CO and CO2 to 01-10 ppmv levels is common andeasily achieved The impact of product purity on hydrogen recovery for PSA systems is small as shown inFigure 4 High hydrogen purity is often of benefit to downstream processes and because of this mostunits are designed to produce the high purities mentioned above4Figure 3Relative Strength of Adsorption of Typical ImpuritiesNon-Adsorbed Light Intermediate HeavyH2 O2 CO C3H6

He N2 CH4 C4H10

Ar C2H6 C5+CO2 H2SC3H8 NH3

C2H4 BTXH2OFigure 4Effect of Product Purity on PSA System Hydrogen RecoveryProduct H2 Purity vol--1-2-397 999Hydrogen Recovery (Impurity = CH4)BaseUOP 3111-4

The tail gas from a PSA unit is almost always used as fuel The economics of the PSA process dependstrongly on the ability to use the tail gas at low pressure When the tail gas must be compressed to therefinery fuel gas system pressure the cost of the compression equipment can equal the cost of the PSAunit For this reason selection of the appropriate tail gas pressure is extremely important5The hydrogen recovery achievable by PSA units is moderate typically 80-92 at optimum conditions and60-80 when the tail gas is delivered at higher (40-80 psig) pressure The system configuration is varied tooptimize the recovery for specific pressure levels flow rates and stream compositionsFigure 5 shows a typical UOP POLYBED PSA unit These units use skid-mounted construction whichimproves quality control and simplifies installation and commissioning The adsorbers and piping arecarbon steel The automatic valves are the only moving parts in the system and through properspecification of these valves high reliability is achieved Control of the process is effected by amicroprocessor-based control system with a CRT operator interface Alternate modes of operation are preprogrammedinto the control system in order to allow uninterrupted operation in the event of a valve or

instrument malfunctionFigure 5UOP POLYBEDreg PSA UnitCommercial units normally use between 4 and 12 adsorbers More adsorbers are used to provide higherhydrogen recovery andor for higher capacity Adsorbents are individually selected for each application andare loaded into the adsorbers on site Proper selection of the adsorbents is critical to both the performanceof the unit and adsorbent life The adsorbers are good for the life of the equipment in properly designed andoperating units6

Membrane SystemsSelective permeation through polymer membranes is a relatively recent and rapidly evolving commercialseparation development The process is based on the difference in permeation rates between hydrogen andimpurities Permeation involves two sequential mechanisms the gas phase component must first dissolveinto the membrane then diffuse through it to the permeate side Different components have differentsolubilities and permeation rates The former depends primarily on the chemical composition of themembrane and the latter on the structure of the membrane Gases can have high permeation rates due tohigh solubilities high diffusivities or both Relative permeabilities of typical components are given inTable 1 Absolute permeation rates will vary depending on the type of membraneTable 1Relative Membrane PermeabilitiesRelative Permeability High Medium LowComponents H2 C1 C2+H2O O2 N2

H2SCO2

The driving force for both solution and diffusion is the partial pressure difference across the membranebetween the feed and permeate sides Gases with higher permeabilities such as hydrogen enrich on thepermeate side of the membrane while gases with lower permeabilities enrich on the non-permeate side ofthe membrane due to depletion of components with permeabilities The first fraction of gas to permeatethrough the membrane will consist primarily of the components with the highest permeability As a largerfraction of the feed gas is allowed to permeate there is an increase in the relative amount of thecomponents with lower permeabilities in the permeate stream Hence in hydrogen separations higherpurity hydrogen is associated with lower recovery and lower purity hydrogen is associated with higherrecovery The effect of hydrogen purity on recovery with membrane systems is much more dramatic thanfor PSA or cryogenic systems as illustrated in Figure 6 Inspection of this figure shows that a change inhydrogen product purity from 98 mol- to 95 vol- can result in a 25 increase in hydrogen recoveryHigher hydrogen recovery also requires that more membrane area be provided The membrane area requiredunder fixed feed composition and system pressure levels increases exponentially at high hydrogen recoveryThe performance of a specific membrane system ie the recovery vs product purity relationship for agiven feedstock is primarily dependent upon the feed to permeate ratio and is only slightly dependent on7the absolute pressure levels However the area requirement is inversely proportional to the feed pressureFor this reason it is often preferable to compress the feed gas to achieve the required pressure ratio ratherthan permeate even though the permeate flow is always smallerFigure 6Hydrogen Recovery vs Product Purity Membrane SystemsHydrogen Recovery 96929450Hydrogen Recovery Relative Permeability 309840 60 70 80 90 100100Pressure Ratio 51

Pressure Ratio 251UOP 3111-5

There are two types of membranes currently in commercial use for hydrogen upgrading asymmetric andcomposite Asymmetric membranes are composed of two layers of a single polymer There is a dense layerwhich performs the separation and a microporous substrate layer whose function is to provide mechanicalsupport The polymers used for this type of membrane must have both acceptable permeation characteristicsand mechanical properties Hence the ability to tailor membrane properties to specific applications islimited for this type of membrane Composite membranes consist of two different polymers the separationpolymer is coated on the substrate polymer This allows selection of a separation polymer based on itspermeation characteristics without regard for its mechanical properties Furthermore since only very smallquantities of separation polymer are required even for large commercial installations expensive polymersof limited availability can be used without significantly increasing cost or delivery timeMembranes are manufactured as hollow fibers or as flat sheets Both types of membranes are packaged asmodules which allows standardization of equipment Hollow fiber membrane systems have the advantagethat a larger surface area can be packaged in a given number of modules The membrane modules areskid-mounted along with the pretreatment section of the system All equipment is carbon steel and the8control components are straightforward In large installations requiring many modules care must be takento provide for even flow distribution among modules A typical UOP Advanced Membrane System isshown in Figure 7Figure 7UOP Advanced Membrane Systems

Cryogenic SystemsThe cryogenic process is a low temperature separation process which uses the difference in boilingtemperatures (relative volatilities) of the feed components to effect the separation Hydrogen has a highrelative volatility compared to hydrocarbons The simplest and most common version of the cryogenicprocess is the partial condensation process The partial condensation process condenses the required amountof feed impurities by cooling the feed stream against the warming product and tail gas streams in brazedaluminum multipass heat exchangers The refrigeration required for the process is obtained byJoule-Thomson refrigeration derived from throttling the condensed liquid hydrocarbons Additionalrefrigeration if required can be obtained by external refrigeration packages or by turboexpansion of thehydrogen productThe partial condensation process is normally applied to hydrogenhydrocarbon streams and is bestillustrated by example A typical flow schematic for the process is shown in Figure 8 A suitably pretreatedhydrogenhydrocarbon feed at 300-1200 psig and ambient temperature is fed to the cryogenic unit The unitis cooled in exchanger X-1 to a temperature which condenses the majority of the C2+ hydrocarbons9Figure 8Partial Condensation Cryogenic ProcessS-1S-4S-2S-3X-2X-1InsulationFeedLPFuelMPFuelHydrogenProductCold BoxCasingS =X =

SeparatorExchangerUOP 3111-6

The two-phase stream is then separated in separator S-1 The hydrogen-methane vapor from separator S-1 issent to exchanger X-2 where it is cooled to a temperature low enough to provide the required hydrogenproduct purity The cooled stream enters separator S-2 and the vapor from S-2 becomes the hydrogenproduct after it is rewarmed The C2+-rich hydrocarbon liquids from separator S-1 are throttled to a pressurewhich will result in vaporization when exchanged against the incoming feed stream in exchanger X-1 Thisstream can be withdrawn separately at its highest pressure as a by-product or mixed with the membranereject stream at a lower pressure The methane-rich liquid from separator S-2 is throttled to a pressure atwhich it will boil while providing the necessary temperature differential to cool the feed to the S-2temperature The S-2 temperature sets the hydrogen product purity by controlling the amount of methanewhich remains in the vapor phase It is usually necessary to bleed a portion of the hydrogen product streaminto the fuel stream This has the effect of reducing the methane partial pressure in the fuel stream andthereby lowering its boiling point The amount of hydrogen product bleed required depends on the requiredhydrogen product purity and the required fuel gas pressure Thus there is a trade-off between hydrogenpurity recovery and tail gas pressure The relationship among these variables is shown in Figure 910Figure 9Hydrogen Purity vs Recovery ndash Cryogenic SystemHydrogen Recovery 88808492Hydrogen Producy Purity vol-9690 94 96 99 10010010 psig Fuel Gas9260 psig Fuel GasUOP 3111-7

The additional separators S-3 and S-4 shown in the cryogenic process flow schematic are used to providethe proper distribution of liquid and vapor into the multiple passes of the heat exchangersSeparate hydrocarbon streams rich in C4+ ethanepropane etc can easily be obtained through the use ofmore separators at intermediate temperatures The ability to produce hydrocarbon by-product streams is animportant advantage of the cryogenic processIf there are insufficient hydrocarbons or insufficient pressure available to provide the required amount ofrefrigeration by the Joule-Thomson effect alone then additional refrigeration can be obtained by expansionof the hydrogen product or by a packaged refrigeration system Expansion of the hydrogen product ofcourse lowers the product hydrogen pressure Refrigeration at 0 to -40degF is normally used if externalrefrigeration is chosenIf the feed to the cryogenic unit contains significant amounts of low boiling constituents such as CO andnitrogen and these are to be removed by the cryogenic unit a methane wash column is used This columnuses liquid methane to wash the impurities from the hydrogen product stream It is necessary if CO is tobe reduced to ppmv levels in the hydrogen product11The cryogenic process is thermodynamically more efficient than the other hydrogen upgrading processesHigh hydrogen recovery (92-97) is easily achieved at 95+ hydrogen purity and the penalty in recoveryfor increased hydrogen purity is less than for membrane systems as illustrated in Figure 9The equipment used in a partial condensation cryogenic unit consists of multipass heat exchangersthrottling valves separators and piping These components are normally grouped together in an insulatedlow pressure carbon steel vessel (cold box) The valves are the only system components requiringmaintenance and access to these valves from outside the cold box casing is provided A UOP cryogenicunit is shown in Figure 10

Project Considerations Operating Flexibility

In this context operating flexibility will be defined as the ability to operate under variable feed qualityconditions either on a short-term or long-term basis Variations in feed composition are common inrefinery applications When the source of the feed is a catalytic process there is often a gradual change infeed quality as the catalyst activity changes More sudden changes occur when the feedstock to theupstream unit changes In addition in many cases a combination of streams are sent to a hydrogenupgrading system and feed quality changes can be rapid and substantial as one or more streams are addedor subtracted from the combination Thus operating flexibility should consider not only ultimate responseto feed quality changes but also the response timeThe PSA process is the most flexible process of the three under consideration in its ability to maintainhydrogen purity and recovery under changing conditions As the concentration of a feed impurity increasesin the feed (at constant feed pressure) the partial pressure of the impurity also increases As shown inFigure 1A an increase in impurity partial pressure normally results in an increase in the amount of theimpurity which will be adsorbed Thus the process is to a large extent self-compensating and evenrelatively large changes in feed impurity concentrations have little impact on performanceIf the feed impurity concentration to a PSA unit changes product purity can be maintained by a simplecycle time adjustment to maintain product purity UOP has introduced a patented product purity controlsystem for its POLYBED PSA units which automatically adjusts cycle time to account for feed qualitychanges eliminating the need for operator intervention The response time for a PSA unit is rapid but not12Figure 10UOP Cryogenic Unitabrupt the system will respond to a step change in feed quality in 5-15 minutes The time to reach steadystateupon restart following shutdown is also short on the order of one hour for most systemsIn membrane processes increases in feed impurity concentrations tend to cause increases in productimpurity concentrations Product purity can be maintained for small feed composition changes by adjustingthe feed-to-permeate pressure ratio but the relatively strong impact of product purity on recovery for thesesystems means that hydrogen recovery can be significantly affected In most refinery membraneapplications however the major product impurity is methane and this can be allowed to increase in the13product slightly without major downstream impact The response time of membrane systems is essentiallyinstantaneous and corrective action has immediate results but operators have less time to react The startuptime required by the process is extremely shortThe cryogenic process is less flexible than the other processes Particular attention must be given to thehigh freezing point constituents which are removed in the pretreatment system since failure to removethese contaminants can result in plugging of the heat exchangers Changes in the concentration of the lowerboiling components of the feed affect the product purity directly Recovery is not strongly affectedChanges in feed quality are compensated for by operator adjustment of the hydrogen-methane separatortemperature (separator S-2 in Figure 8) by increasing or decreasing the amount of product hydrogen bleedResponse time is not as rapid as for PSA or membrane systems Start-up is 8-24 hours depending on theprocedure used

TurndownTurndown can be an important factor in process economics because many installations are expected tooperate at reduced capacity for the first few years after commissioning Large operating penalties due topoor turndown performance can significantly reduce the return on investment and should be accounted forin economic comparisonsAll three processes have good turndown characteristics when properly designed PSA units can maintainboth recovery and product purity at throughputs ranging from approximately 30-100+ of design byadjustment of the cycle time Between 0 and 30 of design product rates purity can be maintained butrecovery is reduced unless special provisions are made in the designMembrane systems are also normally capable of maintaining product purity rates from 30-100+ ofdesign Recovery will be sacrificed at turndown to a varying extent Turndown is accomplished by eitherreducing feed pressure increasing permeate pressure or by isolating modules from the system The firsttwo methods can be used for short-term operation and the latter when operating at significantly reducedcapacity for extended periods The number of modules in the system is also a consideration when decidingwhether isolation is appropriateThe turndown capability of cryogenic systems depends on the design Partial condensation units canmaintain product purity at slightly reduced recovery at flows down to 30-50 of design The theoreticallower limit of turndown is determined by the heat leak to ambient which is typically less than 30 ofdesign throughput If the cryogenic system contains a methane wash column turndown is limited to about

50 of design throughput by column hydraulics14

ReliabilityReliability of the hydrogen separation processes is an important project consideration particularly if theprocess is a primary source of make-up hydrogen to a hydroprocessor or other mainstream refinery processReliability is normally measured by on-stream factor relative to unscheduled shutdowns However theability to produce usable product is also a reliability consideration since off-specification product affects thedownstream processes even if a shutdown of the separation process is avoidedMembrane systems are extremely reliable with respect to on-stream factor The process is continuousand has few control components which can cause a shutdown On-stream factors approaching 100 withrespect to unscheduled shutdowns are possible However if product purity is of critical importance (thisis rare in most membrane applications) a relatively high degree of operator attention may be required toassure production of usable product under varying feed conditions and this should be considered in theselection processPSA systems are also highly reliable While there are numerous valves associated with the process theseare the only moving parts of the system and field experience has proven that with proper valve selectionon-stream factors of 998+ can be achieved Furthermore UOPrsquos POLYBED PSA units are designedwith alternate modes of operation in which 100 of design capacity can be achieved while bypassing anyfailed valve or instrument with only a slight loss of recovery Product purity is maintained and there isno interruption of hydrogen supply Failures are automatically detected and bypassed by themicroprocessor-based control system The PSA process can also maintain product purity under changingfeed conditions adding to its effective reliabilityThe cryogenic process has been considered by refiners to be less reliable than the PSA or membraneprocesses This is not due to the process itself which is relatively simple and has few moving partsRather it is due to the need for feed pretreatment and the types of refinery streams normally processedFeed pretreatment as previously stated is critical to the process reliability Failure of the pretreatmentsystem will usually result in contaminants freezing in the cold box leading to shutdown In this case aldquothawrdquo is necessary prior to restart The pretreatment system itself is often more complex than thecryogenic system Also cryogenic systems normally are used to process combined refinery streams ofrelatively low hydrogen content which can result in high feed quality variations An exception to this isthe TDA application discussed in a later section which is an established cryogenic application andillustrates the inherent reliability of the process15

Ease of Future ExpansionIn some case future expansion is contemplated even during the initial phase of a project Membranesystems are ideally suited for expansion since this only requires the addition of identical modules (thepretreatment system can normally be sized for future expansion without a significant cost increase) PSAsystems can also be expanded but a true expansion as opposed to a debottleneck requires additionaldesign considerations and adds cost in the initial phase of the projectFuture capacity increases from cryogenic units can often be obtained without modification to the cold boxitself through addition of a tail gas compressor Lowering the pressure of the tail gas provides results inlarger heat exchanger temperature driving forces The resulting increase in heat exchange capacity can beused to condense additional feed impurities The cost impact of designing in this way for future expansionis smallPSA units can be easily debottlenecked by cycle modification Changing the number of hydrogen recoverysteps allows significant increases in throughput with relatively minor recovery losses Cycle modificationsare easily performed by changing the control system software UOP has debottled several PSA units in thismanner Typically capacity increases of up to 25 can be achieved with only a few percentage points lossin hydrogen recovery PSA unit operating at a high tail gas pressure can also be expanded significantly bythe addition of a tail gas compressor which allows operation at lower tail gas pressure In this case bothcapacity and recovery can be increased substantially

By-Product RecoveryIn many refinery hydrogen upgrading processes the impurities to be rejected include hydrocarbons whichif separated can have value significantly above fuel value This is particularly true for olefin-containingstreams The relative amounts of high-value hydrocarbons and the incremental cost of further separationdetermine whether by-product recovery should be consideredThe cryogenic process is best suited to applications involving by-product hydrocarbon recovery Even witha simple partial condensation process it is possible to recover separate hydrocarbon streams containing

C2C3 and C4+ components Recovery of these components is generally quite high with typical overallvalues of 90 for the C2C3 components and up to 100 of the C4+ These streams can be delivered atmoderate pressure to existing refinery processing units for final processingThe membrane process is also used in these applications since the hydrocarbon-rich stream is availableat high pressure However the membrane process is not capable of providing separate streams rich inspecific hydrocarbon fractions This must be done in downstream units such as a cryogenic system16The non-permeate stream from a membrane system can also be sent to a turboexpander plant forfurther processingBy-product hydrocarbon recovery is normally not economical when the PSA process is used becausethe single hydrocarbon-rich tail gas is delivered at too low a pressure PSA processes producingseparate tail gas streams rich in specific components have been designed but have not found widecommercial applicationCombinations and separation processes such as cryogenic and PSA processes or membrane and cryogenicprocesses may be applicable to these applications This topic is covered in more detail in a later section

General Process Selection Guidelines Pretreatment RequirementsPretreatment considerations are important to the selection of a hydrogen separation process The extent towhich feed pretreatment is required affects cost operating flexibility and ease of operation The PSAprocess usually requires the least pretreatment The process can treat most vapor phase feedstocks withoutadditional conditioning Entrained liquids including water and condensed hydrocarbons must not bepresent in the feed as these will permanently damage the adsorbents used in the process A well-designedknockout drum with mist eliminator located as close as possible to the unit suffices for most refineryapplications In cold climates heat tracing and insulation of the feed piping is used to avoid condensationPSA units commonly process feedstocks saturated with water andor hydrocarbons If the feed iscompressed upstream of the PSA unit a non-lubricated feed compressor should be used (multi-stage oilseparators followed by an activated carbon bed to reduce oil levels to the ppb range have also providedsatisfactory results when lubricated compressors were used) If liquids do enter with the feed gas underupset conditions only the bottom portion of the adsorbent bed will be affected which usually results in aslight loss of capacity This capacity can often be recovered by cycle time adjustmentSince essentially all feed contaminants in refinery applications including HCl H2S BTX and wateramong others are more strongly adsorbed than hydrogen they will not appear in the product stream Thecontaminants will be concentrated in the tail gas at a lower partial pressure than in the feed A knowledgeof which contaminants and their maximum concentration in the feed is important to the design of a PSAunit since strongly adsorbed components can permanently deactivate some adsorbents intended forremoval of light components Experience in the application and in adsorbent selection are essential toavoiding this problemMembrane systems can also be permanently damaged by liquids Entrained liquids in the feed evenfrom relatively short-term upsets can damage an entire unit because of the large amount of feed gasprocessed per module Membrane units cannot directly process saturated feeds because the non-permeate17remains at feed pressure and the condensibles are concentrated The dew point of the non-permeate stream ishigher than that of the feed gas Pretreatment of saturated feedstocks for membrane applications normallyconsists of a knock-out drum with mist eliminator coalescing filter and a heater The feed is normallyheated to approximately 20degF above the tail gas dew point The extent of superheat required depends onthe design recovery and purity in addition to the feed properties Units operating at higher hydrogenrecoveries require more heating Good feed temperature control is important because operation at highertemperatures increases permeability at the expense of selectivity and the membrane itself can be damagedby high temperaturesCertain feed contaminants cannot be completely removed by the membrane process and pre- orpost-treatment is necessary if these contaminants are not acceptable in the product steam Importantexamples are H2S CO and CO2 These components cannot normally be reduced to ppmv levels bymembrane purification alonePretreatment for cryogenic units involves removal of those components which would freeze out at lowoperating temperatures Water in the feed should be removed to less than 1 ppmv CO2 is normallyremoved to less than 100 ppmv Molecular sieve prepurifiers are commonly used for this purpose Othercomponents such as H2S and heavy hydrocarbons are tolerable to a limited extent An extensiveknowledge of the solubility of contaminants at the system operating temperatures as well as experiencewith normal fluctuations of feed contaminant levels is required for proper design Many operatingproblems in cryogenic units are attributed to failure of the pretreatment system usually due to operation

outside design limits

Feed CompositionFeed composition has a large impact on the selection of a hydrogen separation process Most refineryhydrogen streams available for upgrading are comprised primarily of hydrogen and C1-C6+ hydrocarbonswith other contaminants such as H2S water aromatics HCl etc being present in some cases Otherstreams contain carbon oxides and nitrogen The composition of the feed and its variability affect theperformance reliability and pretreatment required by the three upgrading processesHigher hydrogen content of the feed favors the PSA and membrane processes and lower hydrogen contentfavors cryogenic separation Streams with 75-90 vol- hydrogen are most economically upgraded by PSAor membrane processes with the selection being based on flow pressure and pretreatment requirementsCryogenic upgrading is applicable to large streams with 30-75 vol- hydrogen The larger the methanecontent of a stream the more Joule-Thomson refrigeration is available to the cryogenic process18Heavier hydrocarbons (C5+) are important factors in the design of all three processes Membrane systemswill remove these components but higher concentrations increase the dew point of the non-permeatewhich makes the reliability of the system more dependent on the proper operation of the pretreatmentsystem PSA systems will remove heavier hydrocarbons but increased concentrations result in largersystems with lower recovery due to the difficulty in stripping these impurities from the adsorbents Heavierhydrocarbons concentrations in the feed to a cryogenic unit must be limited in order to avoid freezing inthe process which makes this system also more dependent on the operation of the pretreatment facilitiesBenzene is of particular concern in the membrane and cryogenic processes because of its high boiling pointStreams with significant quantities of CO CO2 and nitrogen such as the effluent from a stream reformerare almost always upgraded by the PSA process as this is the only process which can remove thesecomponents easily and completely In particular the removal of CO and CO2 to 10-15 ppmv is often arequirement and this can only be achieved by the PSA process in a single stepIf H2S is present and must be removed the membrane process is not suitable because H2S has a relativelyhigh permeation rate and will leave with the hydrogen product If H2S is present at concentrations above afew hundred ppmv the PSA process is favored because the cryogenic process will need separate upstreamH2S removal

Feed Pressure and Product FlowFeed pressure and product flow rates are best considered together when selecting a hydrogen purificationprocess because the three processes have drastically different economies of scale Thus although highavailable feed pressure favors both the membrane and cryogenic processes there are few cases where theselection between these processes is difficult because membrane systems are economical for smaller flowrates and cryogenic systems for much large flow ratesMembrane systems are the lowest capital cost alternative for small (less than 5 MMSCFD product) flowrates excluding the cost of compressors for hydrogenhydrocarbon separations Since the cost of amembrane system is proportional to the number of modules required there is little economy of scale forlarger flow rates and the process is rarely economical for large (gt25 MMSCFD product) flow ratesExceptions are if the feed gas is already at high pressure or if there is downstream hydrocarbon recoveryfrom the non-permeate where the high pressure of this stream can be used to advantage For small flowrates at high pressure such as a purge stream from the high pressure separator of a hydroprocessormembrane systems are the most economical This is the most common refinery membrane applicationPSA systems have moderate capital costs in the small flow range and have good economies of scale PSAsystems can be economical for flow rates from 05-90+ MMSCFD for hydrogenhydrocarbon steams at19200-450 psig Many refinery streams are available at these pressures For very small flow rates less thanabout 1 MMSCFD of product hydrogen PSA systems will compete economically with membrane systemsif recovery is not important andor if the available feed pressure is low If there is an excess of feedavailable and only a small quantity of upgraded hydrogen is required the PSA can be designed for hightail gas pressure and tail gas compression will not be required PSA units cannot take advantage of highavailable feed pressures (700-1000 psig) and in many cases the feed will be lowered to a more optimumpressure Thus the PSA product delivery pressure will be comparable to the delivery pressure from amembrane unit and the tail gas will be at lower pressureThe capital and operating costs associated with feed product andor tail gas compression are almostalways a significant portion of the total separation system costs Compressor requirements often determinewhich process is most economicalCryogenic systems have high capital costs at low product rates but have excellent economies of scale Forexample the capital cost of a cryogenic system producing 80 MMSCFD of product hydrogen is only about

three times higher than one producing only 10 MMSCFD Cryogenic units are most applicable for feedpressures of 300 psig or greater and for larger systems Credit for by-product recovery makes the systemmore economical at lower flow rates Unfortunately there are few large high pressure raw hydrogenstreams available in most refineriesIn general then small systems with high available feed pressure favor membrane systems small systemswith low feed pressure favor PSA and membrane systems (compression requirements usually dictatingselection) and large systems at high pressure favor cryogenic systems when hydrogen upgrading aloneis considered

Product PurityHydrogen product purity and the levels of specific product impurities are of course critical to processselection As previously mentioned cryogenic and membrane processes normally produce hydrogen at 90-98 vol- whereas the PSA process normally produces hydrogen at 99+ vol- If the upgraded hydrogen isto be used as a primary source of make-up hydrogen to a high pressure hydrotreater or hydrocracker theimpact of high purity hydrogen on the design and economics of the hydroprocessor can be significantbecause additional impurities in the make-up hydrogen must be purged from the system or the operatingpressure increased in order to dissolve them in the liquid hydrocarbons Purging results in increasedhydrogen losses and increased operating pressure increases the hydroprocessor investment costs andmake-up compression utility requirements The PSA process is most often used to upgrade the bulk of themake-up hydrogen for new hydroprocessor installations20If the upgraded hydrogen is only an incremental portion of the make-up hydrogen to a hydroprocessorlower product purity is usually acceptable because the relative amount of inerts entering with theincremental hydrogen is small Membrane and PSA systems are commonly used in these applicationsIf the feedstocks available for upgrading contain only hydrogen and hydrocarbons then the principleimpurity in the hydrogen product will be methane The amount of methane which can be tolerated in theproduct hydrogen should be carefully determined since it has an important effect on the recoveryobtainable with membrane and cryogenic systems Allowing 3-10 vol- methane in the hydrogen producttypically results in the most economical design of membrane and cryogenic systems if the downstreamhydrogen-consuming process is not overly sensitiveIf the feed to the upgrading process contains substantial quantities of CO and CO2 then PSA units aremost often selected The major stream in this category is the effluent from a steam reformer where thecryogenic and membrane processes are not suitable It is possible to remove CO by using a methane washin a cryogenic system and CO and CO2 can be removed using a methanator However both of theseoptions add considerable cost and operating complexityIf nitrogen is to be removed then the PSA process is also favored because nitrogen can be removed to anydesired level Both the membrane and partial condensation cryogenic processes will remove a portion of thefeed nitrogen but the extent of nitrogen removal is typically less than 50 and is not easily controlledNitrogen is much less soluble in hydrocarbon liquids than methane and builds up rapidly inhydroprocessor recycle loops its removal can have a significant impact on the hydroprocessor

Selection Guidelines for Specific ApplicationsCatalytic Reformer Off-GasThe largest source of easily recovered hydrogen in a refinery is the off-gas from catalytic reforming Thisoff-gas typically contains from 70-90+ vol- hydrogen with the balance being C1 to C6+ hydrocarbonsThe stream also contains small amounts of aromatics and ppmv levels of HCl It is usually available atpressures from 250-400 psig and near ambient temperatureIf large quantities of reformer off-gas are to be upgraded for use as a primary source of make-up hydrogenfor a hydrocracker or hydrotreater the PSA process is normally used The pressure is optimum for the PSAprocess and the typical flow rates (10-80 MMSCFD) allow good economies of scale The high hydrogencontent makes the adsorbent requirements low and the hydrocarbons in the tail gas sent to fuel arerelatively small with respect to the feed gas The high purity hydrogen produced is of benefit The aromaticand HCl content of the feed is not of concern and no special pretreatment is required The PSA tail gas is21normally provided at 2-5 psig and compressed to the fuel header pressure in order to increase hydrogenrecovery to approximately 82-90 depending on the PSA system configurationIn some cases there is a requirement for relatively small quantities of hydrogen at 98+ vol- purity foruses such as catalyst regeneration Membrane systems using feed compression may be most economical forthese applications requiring less than about 5 MMSCFD of hydrogen product due to low capital costs Iflow recovery is acceptable because of feed gas being available in excess then PSA systems should also

be consideredFor large flows cryogenic upgrading has sometimes been used However the feed hydrogen purity is oftentoo high and external refrigeration is required If the reformer is of the semi-regenerative type differencesbetween start-of-run and end-of-run conditions can have a significant effect on the heat exchanger arearequired with the high feed hydrogen case controlling Aromatic removal in the pretreatment section isalso an important design consideration Unless recovered hydrocarbons are of high value the cryogenicprocess is not normally used to process reformer off-gas alone

Hydroprocessor Purge GasesThe high pressure and low pressure purge gases from high pressure hydrocrackers and hydrotreaters aregood candidates for hydrogen upgrading The recovered hydrogen is returned to the hydroprocessor with themake-up hydrogen High pressure product hydrogen is of value Many hydroprocessors have both high andlow pressure purge streams The high pressure purge streams are available at 800-2500 psig and contain 75-90 vol- hydrogen with the balance being hydrocarbons Low pressure purge streams are available atmuch lower pressures typically 100-250 psig and have hydrogen contents ranging from 50-75 vol- Thetotal amount of hydrogen contained in the low pressure purge stream is usually much larger than that in thehigh pressure purge stream when both are presentAs mentioned previously the membrane process is the most economical process for high pressure purgegas upgrading The membrane system is normally designed to produce hydrogen at 300-600 psig 92-98vol- purity and 85-95 hydrogen recovery The product delivery pressure is chosen to allow the productto enter one of the stages of the make-up hydrogen compressorsLow pressure purge gases are usually upgraded by the PSA process The PSA process is better suited thanthe cryogenic process because the flow rates are relatively small and the stream composition can be highlyvariable The combination of lower pressure and lower hydrogen content makes the membrane system lesseconomical than the PSA system22Upgrading low pressure purge gases at pressures below 180-200 psig normally requires feed compressionupstream of the PSA system and may not be economically viable Operation at high tail gas pressure willusually give the highest rate of return on investmentIf both high pressure and low pressure purge gases are available for upgrading consideration must be givento whether to upgrade them together This will depend on the pressure levels and relative flow rates It isusually better to combine the streams by letting down the high pressure stream rather than compressing thelow pressure stream

TDA Purge GasThe purge gas from the toluene hydrodealkylation (TDA) process contains about 55 vol- hydrogen withthe remainder being mostly methane This gas also contains approximately 05 vol- of valuablearomatics Feed gas pressure is 400-500 psig This stream can be upgraded by the cryogenic or PSAprocess with the cryogenic process normally being preferredThe cryogenic process is well-suited for this application because the feed pressure is optimum andthe major impurity to be rejected is methane The removal of aromatics from the feed which isaccomplished in a toluene wash column is not a penalty because of the value of the recovered aromaticsThe cryogenic unit is normally designed to produce a hydrogen purity of 90 vol- at a hydrogenrecovery of approximately 90 Impure make-up hydrogen can be combined with the purge stream beforecryogenic processingFor smaller streams a PSA unit may be used The PSA unit would have a lower capital cost in theseinstances which would be offset by a lesser hydrogen recovery and the consequential increase in make-uphydrogen requirements The PSA would normally operate at a high tail gas pressure to avoid tail gascompression and would produce a 99+ vol- purity product at a recovery of about 65-70 The impact oflower recovery is reduced somewhat by the increased product purity the ratio of hydrogen recovered toimpurities rejected is the appropriate basis for comparison and the PSA unit rejects a much larger fractionof the impurities entering the feed A separate aromatics recovery step can be used if cost-effective but isnot required by the processMembrane systems are not normally economical because of the loss of hydrogen pressure and the low feedhydrogen content

FCC Off-Gas and Other Refinery Purge SteamsHydrogen can be recovered from FCC off-gas and other low pressure refinery purge streams of lowhydrogen content These streams typically contain 15-50 vol- hydrogen and are available at 100-250 psig23Depending on flow rate feed composition and variability feed pressure and required hydrogen product

purity either the cryogenic or membrane process can be used In almost all cases the stream is notupgraded for its hydrogen content alone the tail gas is also of valueIf there are valuable hydrocarbons particularly olefins that can be recovered in addition to hydrogen or if ahydrogen product purity in excess of 90 vol- is required the cryogenic process is normally used Thecryogenic process can recover a mixed C2 stream containing gt99 vol- C2+ components and producehydrogen at 95 vol- purity Typical feed pressures are 250-400 psig and feed compression may berequired Feed quality variations and contaminant levels are important considerations in determiningwhether the cryogenic process is appropriateThe membrane process can efficiently recover hydrogen from these streams at a hydrogen purity 80-90vol- The tail gas can be sent to downstream hydrogen recovery units The low purity hydrogen productcan be used effectively in some applications such as low pressure hydrotreaters and the tail gas can besometimes sent to downstream hydrocarbon recovery units If the tail gas is to be used as fuel extractionof the hydrogen will upgrade the heating value of the stream When the membrane process is used the feedis usually compressed to 400+ psig and the hydrogen product it produces at 100-250 psig Both the feedand hydrogen product may require further compression Hydrogen recovery is often not critical and rangesfrom 50-85Impurities in the hydrogen product from either process can include nitrogen CO and H2S The amounts ofthese impurities in the product depend on numerous factors and must be determined on a case-by-case basis

Steam ReformingHydrogen plants employing the steam reforming of hydrocarbons are necessary to support manyhydroprocessing projects because there is an insufficient amount of hydrogen available from other sourcesThe effluent from stream reformers is most often processed in a PSA unit The tail gas is sent to thereformer furnace as fuel gas and is used directly at low pressure The PSA-based hydrogen plant process ismore thermally efficient and economical in most cases compared to the traditional process which uses theseparate steps of low temperature shift CO2 scrubbing and methanation instead of the PSA unit Themembrane and cryogenic processes are not economical for this application due to the large amounts of COCO2 and water in the feedIt should be noted however that the tail gas streams from either a membrane system or a cryogenic systemshould be considered as candidates for the hydrocarbon feed to the reformer When this is done thepreviously unrecovered hydrogen in the tail gas is returned in the steam reformer effluent reducing nethydrogen losses24

Ethylene Off-GasWhile not a refinery process the manufacture of ethylene produces large amounts of hydrogen and thishydrogen can be easily upgraded for refinery use if the ethylene plant is in close proximity to the refineryThe ethylene process uses a series of cryogenic units to separate the products from the ethylene furnace anda high purity hydrogen stream is produced Typically only a small fraction of the available hydrogen isused in the ethylene plant (for acetylene hydrogenation) and the balance is sent to fuel unless it canbe exportedThe hydrogen-rich ethylene off-gas from the ethylene plantrsquos cryogenic separation train normally contains80-90 vol- hydrogen with CO CH4 ethylene and nitrogen as impurities In some cases the cryogenicsystem will produce a hydrogen purity as high as 95 vol- with the same impurities through use ofexternal refrigeration The hydrogen stream is available at pressures from 250-400 psig and ambienttemperatureRegardless of the hydrogen purity the ethylene off-gas must be processed to remove CO to ppmv levelsfor general refinery use For off-gas with 80-90 vol- hydrogen the PSA process is most often used forupgrading The pressure is optimum and the PSA system will produce a very high purity product eg9995 vol- Tail gas compression from 2-5 psig to fuel system pressure is used in large systems whenhigh recovery is desiredFor off-gas at 95 vol- hydrogen purity methanation of the CO is possible In this case the hydrogenproduct contains 5 vol- hydrocarbons as well as the water produced by methanation which can effect thehydrogen-consuming processesOccasionally methanated hydrogen is available from an ethylene plant at 80-90 vol- hydrogen purity Inthese cases membrane upgrading can be considered The choice between the membrane and PSA processeswill primarily depend upon the cost of compressing the membrane hydrogen product compared to the costof compressing the PSA tail gas assuming high hydrogen recovery is desired in both cases The productpurity from the membrane system will be lower (95-97 vol-) than for the PSA system25

Combinations of Upgrading ProcessesPotential combinations of the three hydrogen separation processes under consideration have been receivingincreased attention in recent years particularly combinations involving the more recently developedmembrane process Integrations of the processes are designed to take advantage of the different processcharacteristics eg10485771048577The ability of the PSA process to produce a high purity hydrogen product and to completely removelow boiling components10485771048577The ability of the membrane process to achieve high hydrogen recovery and to provide tail gas atfeed pressure10485771048577The ability of the cryogenic process to efficiently separate the feed stream into multiple streams at highrecoveryThis being said there are still relatively few commercial applications involving combined processes Onereason for this is that the combined processes typically have significantly higher capital costs than any ofthe individual processes The value of the improved separation performance does not often justify thesehigher costs at current product values High capital costs are sometimes unavoidable because manyapplications require high hydrogen recovery from each process and the capital costs of the membrane andPSA processes increase significantly with recoveryOne of the more common integrations is the combined PSAcryogenic process The cryogenic process isused to make a bulk separation usually with by-product production and the PSA process is used to furtherupgrade the product hydrogen from the cryogenic unit either to increase hydrogen purity or to removespecific impurities The tail gas may be recycled back to the cryogenic unit In this integration thecryogenic unit can be made simple and less expensive (by avoiding for example a methane wash column)and the PSA unit can be made smaller and achieve higher hydrogen recovery because of the reduced amountof impurities to be adsorbed This approach can be used for upgrading ethylene off-gas or low hydrogencontent streams such as FCC off-gas The generalized flow scheme is illustrated in Figure 11A membrane unit can replace the cryogenic unit upstream of the PSA unit in some applications Themembrane unit is used to reject the bulk of the impurities at high hydrogen recovery and the PSA unitupgrades the relatively low purity hydrogen produced by the membrane system Hydrocarbons in the nonpermeatefrom the membrane system can be recovered The combined system achieves both high recoveryand high hydrogen product purity and low pressure tail gas is minimized This process combination canbe used when there are two hydrogen streams available one at high pressure and one at low pressure In26this case the low pressure stream is combined with the membrane hydrogen product upstream of the PSAunit An example of this application is hydrogen recovery from both high pressure and low pressurehydrocracker purge gasesOther refinery applications of combined processes exist but apply only to special situationsFigure 11Flow SchematicIntegrated Cryogenic and PSA Systems(Optional Recycle)Product H2

(99+ vol-)FeedPSAUnitCryogenicUnitBy-productTail GasTail GasH2 Rich StreamUOP 3111-8

ConclusionsHydrogen upgrading in refinery applications can be achieved by the PSA membrane or cryogenicseparation processes Each of these processes has different characteristics which are of advantage in different

situations Selection of the appropriate hydrogen separation process can depend on several factors inaddition to design-point economics Consideration of other project requirements such as turndownreliability flexibility in processing variable feedstocks pretreatment requirements etc are often importantto the selection and may help in deciding between processes with similar economicsGeneral selection guidelines can be helpful in process selection or at least in eliminating an inappropriateprocess In order to use such guidelines feed characteristics contaminant levels required product purityallowable product impurities pressure levels and flow rates must be known These parameters can be usedin conjunction with experience-based application-specific guidelines to select the optimum process27Combinations of these three separation processes can be used in certain applications to take advantage ofthe complementary properties of the different processes However these combinations typically have highercapital costs than single-process alternatives and are usually justified only when by-product recovery isalso practicedThis paper was presented at the 1989 NPRA Annual Meeting held March 19-21 1989 at the San Francisco Hilton in San Francisco California

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1 wwwuopcomobjectsSelOfHydroSepProcpdf

BD =R =E1BDE1RPPPE1E1BDAdsorptionPPRPAdsorptionBDE1PRE1E1RPPAdsorption2 E1 PP BD P341Vessel 411 CycleTimeEqualization (COCurrent Depressurization)Provide Purge (COCurrent Depressurization)PurgeBlowdown (CounterCurrent Depressurization)Final RepressurizationUOP 3111-2

3The 2 psig pressure can be used when the tail gas is to be compressed and 5 psig can be used when the tailgas is sent directly to fuel burners as in steam reforming applications Process performance is much moresensitive to tail gas pressure than feed gas pressure as shown in Figure 2 which shows the influence of asystem pressure levels on hydrogen recovery for a fixed feedstock and process configurationFigure 2Effect of Pressure Levels on PSA System RecoveryFeed Pressure psigBase-2-4-6200 350HydrogenRecoveryTail Gas Pressure = 5 psigTail Gas Pressure psigBase



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Selection of a Hydrogen Separation Process

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