gas treating fundamentals

Understanding gas treatingfundamentals

While much is being placed on the study of conversion and hydroprocessing units,refiners are finding that primary amine units and associated sulphur plants requirestudy and optimisation to meet the new environmental challenges

James L Jenkins and Randy HawsCCR Technologies Inc

Reprinted from PTQ Winter 2001-2 issue

ptqREFININGGAS PROCESSINGPETROCHEMICALS

Winter 2001/2002

PETROLEUM TECHNOLOGY QUARTERLY

SPECIAL FEATURES

OUTLOOK 22002

CONTROL AAND IINSTRUMENTATION

CCR Reprint_8PP 3/27/02 12:58 PM 0

At a recent symposium regarding thechallenges of the new clean fuelsregulations, refiners were particu-

larly interested in ways to improve capac-ity, efficiency and reliability of aminesystems and sulphur units. Much of thefocus on improving the operation of theamine unit, and the associated affects onthe downstream sulphur plant, has beenin the area of amine solvent conversion.Some of these solvent conversions haveimproved plant operation dramatically,while others have not been so successful.

When comparing successful cases tothose less so, it may be found that focuson the basic fundamentals of gas treatinghas a significant role to play in the out-come of any new plant amine selection orexisting plant amine conversion. Under-standing the fundamentals of gas treatingand applying them to any evaluation mayseem commonplace, while in actualitymarketing efforts of suppliers and lack offunds for detailed engineering evalua-tions has led to deviation from funda-mental best practice.

A review of the fundamentals of gastreating with an evaluation of an actualcase study of amine selection and furtheroptimisation gives an excellent basis ofunderstanding for refiners to use in eval-uating their own systems.

Considerations for evaluating anamine type in refinery main system ser-vice are numerous. It is important to con-sider all aspects of the amine chemistry

and type since the omission of a singleissue may lead to operational issues.While studying each issue, it is importantto understand each amine type basedsolely on fundamentals and measureddata instead of making decisions basedsolely on marketing information fromsuppliers or service companies.

Common amines Amines used in refinery main system ser-vice may be categorised into three groups,primary, secondary, and tertiary. Primaryamines react directly with hydrogen sul-phide (H2S), carbon dioxide (CO2) andcarbonyl sulphide (COS). Examples of pri-mary amines used in refineries includesmonoethanolamine (MEA) and the pro-prietary Diglycolamine agent (DGA).

Secondary amines react directly withH2S and CO2, and react directly with someCOS. The most common secondaryamine used in refineries isdiethanolamine (DEA), while diiso-propanolamine (DIPA) is another exam-ple of a secondary amine which is not ascommon any more in refinery main treat-ing systems.

Tertiary amines react directly with H2S,react indirectly with CO2, and react indi-rectly with little COS. The most commonexample of tertiary amine used in refiner-ies is methyldiethanolamine (MDEA).

Focus will be placed during the reviewof selection fundamentals on the mostcommon amines employed in refinerymain system service. These amines areused by engineering companies for grassroots design evaluations, and are theamines used in comparison for solventconversion studies and include MEA,DGA, DEA, and MDEA.

Fundamentals The fundamentals for amine selection inrefinery main system service are based onthe features of each amine type. Thesefeatures are based on industry recognised

guidelines and actual measured data orphysical properties. Evaluation of thesefundamentals will lead to proper solventselection and serious issues may arisewhen solvent selection is based solely onmarketing information and word ofmouth.CapacityCapacity is generally the first item evalu-ated for a new plant design or solventconversion. Capacity of the circulatingsolution is one of the most basic and crit-ical principals in a treating plant, but isoften misunderstood. Plant operatorsmeasure the strength of the circulatingsolution on a very frequent basis toensure that they have enough capacity tomeet treating specifications.

Solvent strength is measured on aweight per cent of solution basis due tothe ease of measurement and interpreta-tion of results. However, when comparingthe relative strengths of the amines toeach other the relative molecular weightsof the amines need to be accounted for tofully understand what their actual capac-ities towards acid gas really is. For exam-ple, each mole of H2S reacts with onemole of amine, so the actual acid gasremoval capacity of each amine is relatedto how many moles of amine are con-tained in one unit volume of circulatingsolution. Molarity (moles/litre) is theactual measure of amine strength from areaction (or chemistry) standpoint andproperly expresses how potent each circu-lating volume of circulating solution is.

Actual capacities of the various aminetypes based on typical maximum usestrengths in refinery primary systems arelisted in Table 1.

This concept is important to under-stand in light of much of the publicationsand claims in the industry that may leadto confusion. For example, MDEA hasbeen stated to have many advantagesover MEA due to the fact that it is operat-ed at higher concentrations in solution.

Solvent MEA DGA DEA MDEA

Wt% 20 45 30 45MW 61 105 105 119Molarity 3.3 4.3 2.9 3.8

DEA < MEA < MDEA < DGA Increasing capacity

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Understanding gas treatingfundamentals

While much is being placed on the study of conversion and hydroprocessing units,refiners are finding that primary amine units and associated sulphur plants requirestudy and optimisation to meet the new environmental challenges

James L Jenkins and Randy HawsCCR Technologies Inc

Table 1

CCR Reprint_8PP 3/27/02 12:58 PM Page 1

While this statement is true, one mustunderstand what the true capacity differ-ence is between the two amines. If onecompares 20 wt% MEA to 45 wt% MDEA,on the surface it looks like MDEA mayeffectively more than double the capacityof the MEA if only the weight per cent ofcirculating solution is considered. How-ever, the above shows that the molarityof the two solutions are the true expres-sion of the capacity of the circulatingsolutions with MEA circulating at 3.3molar and MDEA circulating at 3.8molar in truly optimised plants. There-fore, MDEA will give an increase incapacity over MEA of approximately 15per cent (on a molar basis), which ismuch lower than the perceived increasewhen looking at the weight percent ofsolution strength only.Relative base strengthAnother key component to understand-ing the potency of unit of circulating vol-ume is the relative base strength of thecirculating solution. A higher basestrength indicates a higher affinity for theacid gas to be removed. Relative basestrengths of various gas treating solutionsis often referred to as the pKa of the sol-vent. Ka is the acidity constant andchemists often express it as its negativelogarithm, pKa (pKa = -log Ka). The largerthe value of the pKa, the weaker the acid(or stronger the base). The pKa values forthe gas treating solvents under discussionare listed in Table 2.

It is vital to understand this concept andit affects the maximum loading guidelinesof the solution and may prove criticalwhen comparing solvents types treatinggas at relatively low partial pressures andelevated temperatures. A stronger base willgive better performance with regard to acidgas removal when process conditions limitthe driving force of partial pressure andtemperature. Maximum loadingIn order to achieve maximum capacity ofeach volume of solution circulated, refin-ers will often set the circulation rate toachieve the recommended maximumloading guideline. While this is an excel-lent practice for unit optimisation, it isimportant to understand what the maxi-mum acid gas loading (or rich loading) of

the solution really should be. The indus-try likes to talk about maximum richloadings on a mole-to-mole basis. This isdue to the fact that the measurement isrelatively easy to accomplish in the laband the units of measurement makesense.

However, rich loadings on a mole-to-mole basis need to account for the rela-tive partial pressure of the acid gas andthe relative base strength of the gas treat-ing solution. A rich loading of 0.4 mole-to-mole means one thing in a systemoperating at 10 bar (145psi) versus a sys-tem operating at 70 bar (1015psi). It iscritical to understand that constant mole-to mole loadings mean different thingsfor different amines at different pressures.Therefore, a unit of measurement for

maximum loading must be used thataccounts for process conditions andamine type.

Vapour-liquid equilibrium (VLE) data isthe basis for selecting the maximum load-ing guidelines for the various amines andis probably the most important piece ofdata required to design and optimise anyamine treating plant. These data accountfor the pressure and temperature of thetreating application as well as the charac-teristics of the individual amine types.The data in these curves represent theconcentration of acid gas in the liquidphase as moles of acid gas per mole ofamine (the mole-to mole representationthat we mentioned earlier).

The value of these data varies with thepartial pressure of the acid gas, tempera-

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REFINING

Solvent MEA DGA DEA MDEAPKa, 77ºF 9.5 9.5 8.8 8.6

MDEA < DEA < MEA = DGA Increasing base strength

Table 2

Figure 1 Comparison of produced acid gas composition

Figure 2 Acid gas quality from simulation results


ture, type of amine, and amine concen-tration. When plants are designed, typi-cally the maximum loading guidelines areset or represented as an approach to theequilibrium set by the VLE data. Forexample, some engineering companiesand operating companies will specify thatthe plant will be designed with a maxi-mum loading of 80 per cent of equilibri-um. This means that they will set thedesign around the process conditions toload the solvent to 80 per cent of thevalue of the equilibrium set by the VLEdata.

While this may sound confusing, thesevalues are quite easily calculated withcomputer simulation models. Once againit is critical to design the plant based onthese data versus a concrete mole-to-moleloading, since it will vary from applica-tion to application and will also vary byamine type. The concept of per cent ofequilibrium loading is well illustrated inthe following example: 2 molar MDEAand DEA will be compared at an H2S par-tial pressure of 20kPa at 20ºC in theabsence of CO2 (Table 3).

This example shows that a quick checkof available curves indicates that percentof equilibrium shows a dramatic differ-ence between these two amines at similarconditions.

Amine suppliers have published thatamines may be loaded to an average of0.475 (mole/mole) in refinery service, butthis example shows that under certain cir-cumstances the mole-to-mole loadingguideline, set as an absolute, gives a sol-vent loading above the recommended 80per cent of equilibrium. Under these con-ditions the 0.475 mole/mole loading tar-get is well within the equilibriumguideline for DEA, while the target is out-side the guideline for MDEA.

Therefore, instead of setting an arbi-trary rich loading on a mole-to-molebasis, the maximum rich loading needs tobe set as a percent of equilibrium andthen translated back to a mole-to-molebasis for monitoring purposes.CO2 slipThe literature published by some of theamine suppliers has claimed benefit tothe utilisation of MDEA in refinery mainsystem service due to its ability to slip

CO2. This benefit is reported to increasethe capacity of the main amine systemand helps unload the front end of the sul-phur plant. While MDEA is a tertiaryamine, does not react directly with CO2,and provides excellent slip characteristicsin tail-gas treater and gas plant service,the benefits of this characteristic have notproven itself in refinery main system ser-vice. Actual measured produced acid gasfrom various refinery main amine sys-tems and third party computer simula-tions show that there is very littledifferentiation in acid gas quality to thesulphur plant between the variousamines, as shown in Figure 1 and Figure 2(on previous page).

These figures show that an amine con-version for CO2 slip to improve the capac-ity of the amine system or to improve thecapacity of the sulphur plant has very lit-

tle promise. The money spent on thestudy and conversion of the amine plantin these cases would be better spent onstudy and implementation of proven andemerging technologies to de-bottleneckexisting sulphur plants.

Another important aspect of consider-ing a solvent for CO2 slip is the CO2 spec-ification that needs to be met in thetreated gas. Some refineries sell part oftheir treated gas to chemical plants as afeedstock and removal of CO2 to a partic-ular level is often required in these cases.Some gas and many liquid treated prod-ucts from the amine plants are also fur-ther treated with once through orregenerative caustic systems. CO2 left inthese product streams will increase theuse of caustic or will increase the size ofthe regenerative caustic systememployed.

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Solvent DEA MDEAEquilibrium loading

(mole/mole) 0.69 0.5280% of Equilibrium

(mole/mole) 0.55 0.42

Table 3

Figure 3 Pseudo-first order rate constant for COS at room temperature

Figure 4 COS and CO2: Reaction rate constants with amines


Trace sulphurThe removal of COS is very important inrefinery applications when the crude slateis sour and conversion units are in therefinery scheme (FCC and coking). Withtougher clean air regulations being con-stantly implemented, trace sulphurremoval is a key component in new unitdesign, and has also been the driver insome amine conversions.

Figure 3 shows the relative reactionrates of the various amines with COS.Reaction of COS with amines closely fol-lows the kinetics of CO2 for the variousamines as is shown in Figure 4.

These data show the effectiveness ofthe various gas treating amines in theremoval of COS.

COS is often overlooked in some refin-ery gas treating evaluations, and later wewill look at a case study where this issueessentially drove the amine selection forthe process. It is also important to remem-ber this issue when reviewing amine sup-plier information that often tries to driveconversions to tertiary amines. Theseamines, specifically MDEA is not effectiveat removing COS as shown in the reactionrate kinetic data and the fact that COSremoval kinetics are closely related to CO2

removal.Energy consumptionThe industry has tried to use energy con-sumption as a selling point for MDEAbased on the heats of reaction comparedto the other amines. While the differences

in heats of reactions may be as much as 15to 30 per cent, the energy required revers-ing heat of reaction is rather small whencompared to the amount of heat requiredto raise the bulk solution temperature inthe regenerator.

For example, if we look at a typical 4molar gas treating solution with 0.5 mole-to-mole loading (for ease of calculation,the solution will contain 4 moles ofamine, and 2 moles of acid gas loading).The balance of the solution will be waterand will equate to about 30 moles. Table 4shows the relative amount of each compo-nent when compared to the total.

This table shows that the heat of reac-tion associated with the acid gas pick upis 5.6 per cent of the total solution on amolar basis, so the effect of the relativeheat of reactions and its effect of theregeneration energy requirements will beminimal and is largely offset by the watercontent. The data presented here showsthat from a fundamental standpointamine type plays a minor role in the ener-gy requirements of an amine system andalso shows that optimisation of circula-tion rate (maximising the potency of eachunit volume circulated) is the key to ener-gy optimisation.

This point is further stressed by actualdata taken from a multitude of gas treat-ing plants shown in Figure 5. These datashow that there is very little difference inregeneration heat requirements for thevarious gas treating solvents, which vali-dates the earlier discussion based on fun-damentals.

In order to appropriately interpret thegraph in this figure, remember to com-pare the steam consumption as a functionof the actual amine strength on a molarbasis; meaning that if a 3.5 molar solutionis required as the capacity, look at thesteam consumption for the variousamines in that range and then verify thatit is acceptable amine strength.

It is also important to note that whileenergy consumption may be an issue inoffshore and remote gas plant operations,many refiners are venting low-pressuresteam and energy savings typically do notfactor into amine selection.

Another aspect to evaluate surround-ing this issue is the fact that if the plant isfully optimised from an energy stand-point, it may not be able to handle upsetsand changes in composition common inrefinery primary treating systems. TheAmine Best Practices group has also statedthat those plants surveyed with the high-est energy consumption were also theplants with the least amount of off-specproduct incidents [Scott B, ABPG cost surveyupdate; 1997 Brimstone Sulfur Recovery Sym-posium].

Hydrocarbon solubilitySome data have been published on hydro-carbon solubility and the various gastreating solvents. The only thing consis-tent with them thus far has been the factthat they did not use consistent reportingmethods, and often based conclusions onword of mouth [Plaumann D, Stewart E,Kuroda R; Performance of specialty amines ingas processing; Petroleum Technology Quarterly,Summer 1999]. However, recently gooddata was published on hydrocarbon solu-bility in gas treating amines [Critchfield J etal, Solubility of hydrocarbons in aqueous solu-tions of gas treating amines; 2001 LauranceReid Gas Conditioning Conference].

The data does summarise hydrocarbonsolubility as the following:

The reason why this recent data is con-sidered and listed here is due to the factthat this is the first time that the hydro-carbon solubility work was completed forthe whole range of amines and aminestrengths that encompassed typical usestrengths. These data also directly mea-sured the solubility via a well-respectedthird party laboratory, reported theresults on a consistent basis, and did notreport any data based on word of mouth.

While hydrocarbon solubility in thevarious amines has not always been fac-tored, or factored well, in solvent selec-tion or solvent conversion studies, itneeds at least some review. Changes insolubility may not affect plant operationsgreatly, but flash gas rate changes and sul-phur plant pressure drop may need to bemonitored if switching amines from oneend of the hydrocarbon solubility spec-trum to the other (MEA to MDEA forexample).

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Moles Percent of total

Solvent, moles 4 11Gas loading, moles 2 5.6Water, moles 30 83.4Total, moles 36 100

Table 4

MEA =< DEA <DGA < MDEA Increasing solubility of hydrocarbons

Figure 5 Steam consumption in refinery treating at maximum m/m load


Hydrocarbon handlingHydrocarbon handling and solubility inthe amine plant greatly affects the perfor-mance of the upstream sulphur plant. Ifhydrocarbons carryover from the amineplant to the sulphur plant, oxygendemand will increase and the potentialexists to “soot up” the first converter bed.

If the upset is large enough, theincrease in pressure drop or a drop in bedconversion may bring the sulphur plantdown for a catalyst change out. Therefore,hydrocarbons need to be handled in theamine plant with proper flash drum size,design and operation. The flash drumneeds to be designed as a three-phase sep-

arator as opposed to single baffle designsthat were employed in early treatingplants.

The flash drum is the best and only wayto handle hydrocarbons in the aminesince the vessel sees the whole flow of thestream and can effectively dispense thehydrocarbons continuously until thehydrocarbon ingress is brought undercontrol. There have been efforts to use car-bon as a way to remove hydrocarbonsfrom the amine solution but this hasproven to be inefficient. While the carbondoes effectively remove the hydrocarbonsfrom the amine solution it is quite expen-sive due to its limited capacity.

Carbon filtration was employed toremove small amounts of foam promotersfrom the amine solution and this hasproven effective and is the basic funda-mental reason to employ carbon filtration.

Degradation/quality controlUnderstanding the nature of degradationspecific to each amine type and the effec-tiveness of available reclaiming technologyon removing these compounds is also criti-cal. The Amine Best Practices Group has stat-ed that corrosion is the biggest cost elementin amine plant operations, and proper sol-vent quality control is one way to controlthis cost.

There are many compounds present ingas treating solutions circulating in processplants. Throughout the industry there hasbeen a lot of focus on HSS while often therest of the contaminants and degradationproducts are ignored. However, these HSSanions may often be only a portion of thetotal contaminants present in the solutionand it is prudent to look at the total level ofcontaminants and degradation products[Haws R and Jenkins J, Contaminant reporting inamine gas treating service; 2000 Brimstone SulfurRecovery Symp].

The total level of contaminants and degra-dation products in the solution may bereferred to as the residue of the solution. Theresidue of the solution is the amount of mate-rial that is generally not considered part of ahealthy gas treating solution, meaning any-thing that is not free (active) amine or water.This residue, as the equation below shows, iseasy to calculate and encompasses all of thecontaminants and degradation products pre-sent in the sample:

wt% residue =100 – wt% free amine (FA) – wt% water

The residue is the total level of the con-taminants and degradation products pre-sent in the sample. Contaminants anddegradation products may be defined asfollows:

Contaminants: items that enter the pro-cess and “pollute” the amine. These itemswould generally include solids/particu-lates, hydrocarbon, process chemicals,strong cations (sodium), HSS (from theirprecursors entering with the gas), anddegradation products.

Degradation products: contaminants insolution that are derived from reactionswith the base amine molecule itself,where the molecule is broken down orchanges chemical form. Many of thesecompounds are the result of irreversibledegradation of the base amine molecule;ethylenediamine derivatives (THEED inthe case of DEA and HEEU in the case ofMEA) would be examples of this.

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REFINING

MEAFree amine (alkalinity) 20 wt% solution maxWater 70 wt% solution minHSS anions reporting basis**(1) Expressed as wt%t of solution <1.2 wt% solution - or(2) Expressed as wt% amine <2.5 Expressed as wt% as MEA - or(3) Expressed as percent amine capacity <8.0 Percent amine capacity

Formamides (MEAF) <3.0 wt% solutionHEED <0.5 wt% solutionHEEU <1.0 wt% solution

DGAFree amine (alkalinity) 50 wt% solution maxWater 40 wt% solution minHSS anions reporting basis**(1) Expressed as wt% of solution <1.2 wt% solution - or(2) Expressed as wt% amine <2.5 Expressed as wt% as DGA - or(3) Expressed as percent amine capacity <8.0 Percent amine capacity

Formamides (DGAF) <3.0 wt% solutionBHEEU <6.0 wt% solution

DEAFree amine (alkalinity) 30 wt% solution maxWater 60 wt% solution minHSS anions reporting basis**(1) Expressed as wt% of solution <1.2 wt% solution - or(2) Expressed as wt% amine <2.5 Expressed as wt% as DEA - or(3) Expressed as percent amine capacity <8.0 Percent amine capacity

Formamides (DEAF) <3.0 wt% solutionTHEED <1.5 wt% solutionBicine <1.0 wt% solution

MDEAFree amine (alkalinity) 50 wt% solution maxWater 40 wt% solution minHSS anions reporting basis**(1) Expressed as wt%t of solution <1.2 wt% solution - or(2) Expressed as wt% amine <2.5 Expressed as wt% as MDEA - or(3) Expressed as percent amine capacity <8.0 Percent amine capacity

MDEA fragments <2.5 wt% solutionBicine <0.4 wt% solution

**Since there are three bases in common use for reporting HSS anion levels, guidelines are provided for all three reporting bases.

Common contaminants and degradationproducts: solvent quality guidelines


Some of these compounds are theresults of a reversible reaction or chemicalequilibrium with the base aminemolecule; formamides in the case of pri-mary and secondary amines and BHEEUin the case of DGA would be examples ofthis. For optimised unit operations, it isimportant to know and understand thetotal level of residue including all contam-inants and degradation products. It is alsoimportant to understand the characteris-tics of the residue for evaluating concernsfor unit operation, concerns for corrosion,and for evaluating merchant reclaimingoptions when necessary.

The adjoining panel carries a list of themost common contaminants and degra-dation products and recommendedguidelines for total solvent quality con-trol for the amines most commonly usedin refinery treating systems.

Since it was stated earlier that corro-sion is the biggest cost element associatedwith amine system operations, it isimportant to understand how the con-taminants and degradation products list-ed above affect the solution from acorrosion or physical property stand-point. Care must also be taken whenchoosing a reclaiming technology to con-trol these contaminants and degradationproducts since the available technologiesdo not all remove the same types of mate-rials from the solution.

A list of the common contaminantsand degradation compounds for the vari-ous amines used are listed in the panel onthe next page, along with the effective-ness of the various reclaiming technolo-gies on removing them.

MDEA contamination/degradation: Spe-cial considerations based on whether

generic or formulated. One aspect ofMDEA that may be cause for concern isdue to the fact that since it is a tertiaryamine is does not form formamides inthe presence of formates. The primaryand secondary amines in the presence offormates under go hydrolysis where theamine formate HSS forms a formamide[Koike et al, N-formyldiethanolamine: a newartefact in diethanolamine solutions; Chemistry& I ndustry, 1987].

This compound is not considered cor-rosive and effectively acts as a way to"hide" some of the corrosive formateanions in solution. This may be one ofthe main reasons that MDEA is just as cor-rosive in refinery treating systems despiteearly claims that its corrosion rates wouldbe much lower than those for the prima-ry and secondary amines.

Case studyWhile many of the papers published late-ly concerning amine selection or amineconversions have focused on the utilisa-tion of MDEA over the “older genericamines”, there are many recent caseswhere these “older generic amines” havebeen the best and, even perhaps, the onlychoice for recent new plant design.MEA – refinery serviceA plant operator selected MEA for a rela-tively new refinery primary amine treat-ing system for specific reasons. The gasbeing treated in this unit had high levelsof COS, so MEA was selected due to itsexcellent reaction rate kinetics with COS.The gas being treated also had variablelevels of oxygen, so MEA was also select-ed since it is a simple amine with slowerdegradation rates in the presence of oxy-gen.

Because of MEA degradation due tooxygen and to the level of HSS precursorsin the gas, a slipstream thermal reclaimerwas necessary for optimal unit operation.Based on this information, it may be seenthat most if not all of the fundamentalswere considered from performance of thesolvent to understanding the need forsolvent quality control to ensure reliabili-ty and efficiency.

When this unit was started up it per-formed well and fuel gas specificationscould be met. Unfortunately, over time itbecame apparent that the reclaimer wasundersized, since it had a hard time con-trolling the level of HSS anions in solu-tion. Also, gas rates dropped since therewere some fouling issues surrounding theprocess equipment. Some equipmentchanges were made and fouling issueswere somewhat minimised, but were stilla concern.

Since the plant was using HSS anions as

REFINING

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MEAControl of HSS AllControl of MEAF (slipstream processing) AllControl of MEAF (batch processing) Vacuum distillation onlyControl of HEED Vacuum distillation onlyControl of HEEU Vacuum distillation onlyControl of polymeric material Vacuum distillation only

DGAControl of HSS AllControl of DGAF (slipstream processing) AllControl of DGAF (batch processing) Vacuum distillation onlyControl of BHEEU Vacuum distillation onlyControl of polymeric material Vacuum distillation only

DEAControl of HSS AllControl of DEAF (slipstream processing) AllControl of DEAF (batch processing) Vacuum distillation onlyControl of THEED Vacuum distillation onlyControl of bis-HEP Vacuum distillation onlyControl of MEA Vacuum distillation onlyControl of bicine Ion exchange - partial

vacuum distillationControl of polymeric material Vacuum distillation only

MDEAControl of HSS AllControl of MMEA Vacuum distillation onlyControl of DEA Vacuum distillation onlyControl of bicine Ion exchange - partial

vacuum distillationControl of HE-sarcosine Ion exchange - partial

vacuum distillationControl of Polymeric Material Vacuum distillation only

* Best efficiency = Batch processing

Common contaminants and degradationcompounds: removal by reclaiming type


a control point for its reclaimer designand operations, not much else was mea-sured in the solution on a frequent basis.It was decided to try using ion exchangeto remove the HSS anions, thinking thatthis would greatly improve unit opera-tions. Ion exchange was effective atremoving the HSS anions, but, with time,the unit fouling became worse and unitoperation began to suffer. A decision wasmade to look at a complete sample analy-sis, which ended up surprising the plantoperator.

As can be seen from the complete sam-ple analysis, the sample recovery is verylow at around 80 per cent, meaning thateven a fairly complete sample analysisstill could not account for 20 per cent ofthe contaminants and degradation prod-ucts in the sample. As mentioned before,the plant operator was very surprisedsince it was thought that there wasbetween 6 and 8 per cent HSS and noth-ing else in the sample. The amount ofresidue in this sample was about 47 percent, so that while the plant had account-ed for 7 per cent HSS there was a signifi-cant number of other contaminants anddegradation products accounting forabout 40 per cent of the circulating solu-tion.

The analytical results, and residue cal-culation and amine and nitrogen bal-ances, all showed that HSS was only asmall portion of the concern surrounding

the solvent quality. HEEU was high due tothe fact that there was a high level of COSin the gas. Even though HEEU has a highboiling point and may be controlled in aconventional thermal reclaimer, theundersized reclaimer was not able toremove it adequately since this com-pound was not considered during thedesign of the unit.

While HEEU should have been consid-ered in the design of the unit, it was notdue to the fact that the amine supplierswere hesitant to supply technical service tothe design of a new MEA plant, and mostof the amine suppliers do not even analysefor HEEU. As the level of contaminantsand degradation products increased in thissample, the water content of the solutiondropped. As the water level decreased, thesolution properties began to change (vis-cosity, mass transfer rates, boiling point)and the reaction rates of the degradationproducts increased since they are dehydra-tion reactions catalysed by heat. Thisexacerbated unit operations problemsand the degradation of the solvent overtime. Since the nitrogen imbalance wasvery high, there were also increased con-cerns over the fouling potential of the sol-vent due to polymeric material. Throughthe use of vacuum distillation technologyand the dumping and replacement of thesolvent during plant outages, the planthas restored the solution quality to anacceptable level and has efficient and reli-

able operations.This case shows that even when a

detailed evaluation is completed for newplant design, it is possible to miss a fun-damental issue that will effect the opera-tion of the plant. While often notconsidered in full detail, more often thannot it is very important to understand thetotal level of contaminants and degrada-tion products that may form in the circu-lating solution. Also it is important tounderstand how conventional thermalslipstream reclaiming or merchantreclaiming technologies may or may notimprove the total quality of the solvent.

ConclusionWhen dealing with the capacity, efficien-cy and reliability of the amine plant andits effect on the upstream sulphur plant itis important to look at and understandthe fundamentals of the amine solution.When the fundamentals are studied andunderstood, then the salesmanship ormarketing of the amine supplier or ser-vice company cannot lead the plantowner in the wrong direction. Whenthinking of these fundaments, think ofthe following key points:.Capacity

1.Capacity – study on a molar basis.2.Maximum loading – study on a per-

cent of equilibrium basis.Efficiency

1.CO2 slip – generally unachievable inrefining systems.

2.Trace sulphur removal – often over-looked but becoming very important.

3.Energy savings – function of circula-tion rate, not amine type.Reliability

1.Hydrocarbon solubility – understandwhere the solvents rank.

2.Hydrocarbon handling – flash drumhandles full flow effectively and continu-ously.

3. Solvent quality – key for reliability, issolvent specific.

4.Solvent quality control – understandfeatures and benefits of reclaimingoptions.

The figures in this article are by courtesy ofJenkins J L, from his paper, Use amine fea-tures to guide selection for refinery applica-tion; Brimstone Sulfur RecoverySymposium, 1999.

James L Jenkins and Randy Haws are busi-ness development managers with CCR Tech-nologies Inc, Houston, Texas, USA. Jenkinshas a BS in chemical and petroleum engi-neering from the University of Pittsburgh,and Haws has a BS in chemical engineeringfrom the University of Texas.

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gas treating fundamentals

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