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Journal of Natural Gas Science and Engineering 20 (2014) 208e213

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Journal of Natural Gas Science and Engineering

journal homepage: www.elsevier .com/locate/ jngse

Optimization of MDEA concentration in flow of input solvent to theabsorption tower and its effect on the performance of other processingfacilities of gas treatment unit in Sarakhs refinery

Hassan Ghanbarabadi a, 1, Fatemeh Karimi Zad Gohari b, *

a Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iranb Department of Chemical Engineering, College of Engineering, Lahijan Branch, Islamic Azad University, Lahijan, Iran

a r t i c l e i n f o

Article history:Received 10 March 2014Received in revised form11 April 2014Accepted 12 April 2014Available online

Keywords:SolventMDEAConcentrationTemperatureSimulationAspen Hysys

* Corresponding author. Tel.: þ98 142 4222028; faE-mail addresses: h.ghanbarabadi@yahoo.com (H.

com (F.K. Zad Gohari).1 Tel.: þ98 933 4845460 (mobile).

http://dx.doi.org/10.1016/j.jngse.2014.04.0061875-5100/© 2014 Published by Elsevier B.V.

a b s t r a c t

Among the popular and important processes to remove the acidic gases (hydrogen sulfide and carbondioxide) is using amine solvents.

The duty of gas refinery unit, besides the separation of acidic gases from the natural gas, is alsocontrolling the dew point of water and hydrocarbons in the output gas. Selection of suitable processingconditions in gas refinement unit to reach the standard characteristics of sweet gas significantly affectson the operation costs. Present study has simulated the gas softening unit of Sarakhs refinery usingAspen Hysys software with MDEA aqueous solvent to optimize the concentration, MDEA solvent’s flowstream and thermal load of restoration unit and other processing conditions. Results indicate that theoptimum performance of MDEA solvent is 45e50% wt concentration at 55e63 �C.

According to the input feed flow and market demand, solvent concentration and input solvent’stemperature vary in above-mentioned efficiency. As well, in present study, the effect of temperature,flow stream and concentration of input solvent on contactor and other processing sections is discussed.

© 2014 Published by Elsevier B.V.

1. Introduction

Amine solutions are weak organic bases. They adsorb the acidicgases in the normal temperature, and repulse them in the highertemperature. Alkanole amines are nitrogenized organic materialsobtained from combination of specific organic materials andammonia (NH3).

In the major reaction, one hydrogen is replaced to the freeradical of an organic chemical material. Organic alkanoles areclassified based on the number of organic groups attached to thenitrogen atom.

- The first type of amines such as mono-ethanol amine MEA anddi-glycolamine DGA.

- The second type of amines such as di-ethanol amine DEA and di-isopropanol amine DIPA.

x: þ98 142 4226228.Ghanbarabadi), fkzg8@yahoo.

- The third type of amines such as tri-ethanol amine TEA andmethyl di-ethanol amine MDEA.

The first kind of amines is stronger bases compared to the sec-ond kind of amines and has more tendency to react to H2S and CO2and form stronger bonds with acidic gases.

Generally, alkalinity and reactivity of the first kind of amines aremore than the second kind and those of the second kind are morethan the third kind of amines.

The heat resulting from the reaction between the acidic gasesand the first kind of amines is about 25%more than the second kindof amines (Campbell et al., 1976; Douglas et al., 1978; Bolhar-Nordenkampf et al., 2004).

Solvents such as methanol, carbonate propylene and sulfolane,adsorbing the gases physically without and chemical reaction, arecalled physical solvents and the solvents such as AMP, MDEA,chemically reacting to the gases, are called chemical solvents. Thecombination of physical and chemical solvents is called thesolvent mixture. At present, MDEA solvent, due to the ability toselective adsorption of H2S in the presence of CO2 and low enthalpyevaporation, has wide spread application in gas softening processas chemical solvent. Solvability of acidic gases in physical solvents isalmost linear. Thus, in low partial pressures, it does not have much

Fig. 1. Schematic of gas refinement unit by Aspen Hysys software.

Table 1Analysis of gas refinement unit of feed sour gas.

Parameter Inlet Out

Gas flow rate (kgmol/h) 7718.524 6932.958Solvent aqueous MDEA (kgmol/h) 3.540e�5 3.552e�5Gas temperature (�C) 53 56.58Solvent temperature (�C) 55 78.4063Gas in press (psia) 1050 1050Solvent in press (psia) 1100 1102H2S inlet gas composition (%mole) 3.85 3.01e�6CO2 inlet gas composition (%mole) 6.41 2e�3

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adsorption power. But in low partial pressures, solvability of acidicgases in the chemical solvents is due to the above-mentioned re-action. However, in high partial pressures, chemical solvents havestoichiometric limitations. In these conditions, acidic gas to amineratio is about 1 or more (a� 1) and the concentration of free (un-decomposed) amine concentration is low for the reaction.Adsorption of acidic gases in these conditions increases the

Fig. 2. The effect of MDEA concentration entering to the absorption tower on thetemperature distribution across the absorption tower.

concentration of undecomposed gas in the aquatic phase. In thehigh pressures, due to reaction stoichiometric limitation, capacityof physical solvents is higher than chemical solvents. Solvabilitydata for H2S, CO2 in the solvent mixtures in different temperaturesand concentrations are necessary for planning acidic gas separationprocess with these solvents. Limited studies have been performedon solvents mixture compared to the great solvability data given foracidic gases solvability in chemical solvents. Accuracy of laboratorydata is a function of precision in the measurement of amine solu-tion concentration, acidic gas amount in the liquid, solution den-sity, mole fraction of evaporation phase and the system pressure. Toadsorb a fixed amount of gas, it is possible to use the first andsecond kind of amine solutions with lower concentration (per wt %)compared to the third kind of amine solution. So, amine flow in thefirst and second kind of circulation is lower than amine flow in thethird kind of circulation. But carbamate formationwill decrease theadsorption capacity of the first and second kind of amines. Someamines, due to the formation of stable carbamate, have CO2adsorption thermodynamic limitation. The third kind of amines

Fig. 3. The effect of concentration of amine entering to the absorption tower on thetemperature distribution mode in trays 18, 19, 20 of absorption tower.

Fig. 5. The effect of flow of amine entering to the absorption tower on the temperaturedistribution across the absorption tower.

H. Ghanbarabadi, F.K. Zad Gohari / Journal of Natural Gas Science and Engineering 20 (2014) 208e213210

(MDEA, TEA) lacks the ability to form the carbamate and bicar-bonate is produced as a result of their reaction to CO2 (Campbellet al., 1976; Douglas et al., 1978; Bolhar-Nordenkampf et al., 2004).

2. Description and simulation of gas softening unit ofSarakhs refinery whit MDEA process and study on the results

Gas refinement units’ duty is separation of acidic gases fromnatural gas and controlling the dew point of the water and hy-drocarbon in the output gas. In this unit, firstly, sour gas enters theknock out Drum; suspended liquid particles, water and heavy hy-drocarbons will change their phases and are separated as liquidfrom its end. This flow is sent to stabilizer unit to adsorb thehydrogen sulfide to the sour water unite and its organic phase. Theoutput gas inters acidic gas adsorption tower from above the knockout Drum, which is usually divided between two similar adsorptiontowers. In the gas refinement unit of Sarakhs refinery, MDEA with45wt% is used in two parallel adsorption tower where saturatedsolvent is regenerated in two parallel towers (Shahsavand andGarmroodi, 2010; Klinkenbij et al., 1999)

Gas enters from beneath the tray 20 and amine enters fromabove first tray to MDEA contactor. The clean amine enters fromabove the first contact tower tray with a temperature above 5 �Cabove the input gas temperature and after passing the 20 trays, itexit from beneath the tray. H2S, CO2 existing in the gas will beabsorbed by MDEA and sweat gas exits from upper part of thetower. Output gas will be heated due to receiving amounts of theheat resulting from H2S and CO2 absorption as by MDEA. Thus, forcooling, it enters the treated gas cooler and its temperature willdecrease. Then, the gas enters to treated gas separator so thatamine (MDEA) with gas separate from it. To facilitate this process,treated gas water spray pump is dispersed to the input line to thesweet gas separator.

After passing through the treated gas separator the gas entersthe treated gas filler separator. This way any possible particles,amine or heavy hydrocarbons are separated and will be prepared toenter the dew point control system. The liquids separated in thisfilter accompanied to the liquids separated in the treated gasseparator are sent to the Flash Drum. Sweet gas is guided towardthe dehumidification and to control the dew point (Fig. 1). Aminepackage of Aspen Hysys software was used to simulate the gas

Fig. 4. The effect of concentration of amine entering to the absorption tower on thetemperature of trays 1e10 of absorption tower.

refinement unit of Sarakhs refinery with MDEA aqueous solvent toreach the global standards of refined gas, and mixture of inputmaterials to the gas refinement unit in Table 1 was used forsimulation (Shahsavand and Garmroodi, 2010).

According to the general explanation of the gas refinement unit,in this section, the results of simulation are studied.

2.1. The effect of MDEA concentration on the adsorption towertray's temperature

By increasing the concentration of the clean amine solutionentering to the adsorption tower, lower acidic gases can penetrateduring the tower toward the trays above the tower.

So that the reaction region will be transferred toward the lowerpart of the tower. As mentioned above, the gas will be heated after

Fig. 6. The effect of flow of input amine on H2S absorption over each tray of absorptiontower.

Fig. 9. The effect of amine flow entering to the absorption tower on the temperature ofthe flow entering to the torch.

Fig. 7. The effect of input amine flow on the CO2 absorption over the tray of absorptiontower.

H. Ghanbarabadi, F.K. Zad Gohari / Journal of Natural Gas Science and Engineering 20 (2014) 208e213 211

entering to the tower by amine solution and after this preheatingstep, it will be entered to the reaction region. After the reaction,output gases convey from thermal reaction to the amine solution.But in the absorption tower, by increased concentration of inputamine, pre-heating region will be reduced in size, due to the re-action area. So, maximum temperature inside the tower will bedecreased (Fig. 2). Figs. 3 and 4 indicate that the temperature overthe tray will be decreased by increasing the amine concentration.Fig. 2 indicates that not only maximum temperature is decreasingbut also maximum temperature region is transferred toward thelower part of the tower. This indicates the transportation of thereaction region toward the lower part of the tower (Shahsavandand Garmroodi, 2010; Mccab et al., 2001; Rodríguez et al., 2011) .

2.2. Study on the MDEA solvent flow

Increasing the circulating rate causes that in the adsorptiontower each mole of acidic gas per time unit has contact with more

Fig. 8. The effect of amine flow entering to the absorption tower on the thermal load oflean amine cooler entering to the absorption tower.

mole numbers of amine. So, adsorption rate per time unit will in-crease. In other words, by increased rate of clean amine flow, thenumber of amine moles in the solution will increase and accordingto the Le chatelier’s principle, increased amine mole leads to theprogress of reaction equation toward the acidic gases adsorption.So, acidic gases’ concentration in gas phase will decrease on eachtray. Figs. 6 and 7 indicate the concentration of Co2 and H2S in gasphase over each tray in respect of change in lean amine flowentering to the tower. As observed in these figures, by increasingthe lean amine flow, acidic gases will penetrate to the trays over thetower with lower rate. According to Fig. 5, as the tower’s temper-ature decreases, lower flow of the solvent is needed to reach the

Fig. 10. The effect of amine flow entering to the absorption tower on the makeupmolar flow.

Fig. 13. The effect of amine flow entering to the absorption tower on the thermal loadof repulsion tower’s condenser.

Fig. 11. The effect of amine flow entering to the absorption tower on the thermal loadof cooler of gas exiting from absorption tower.

H. Ghanbarabadi, F.K. Zad Gohari / Journal of Natural Gas Science and Engineering 20 (2014) 208e213212

standard characteristic of refined gas (Shahsavand and Garmroodi,2010; Mccab et al., 2001; Romano et al., 2010).

2.3. The effect of solvent flow on the thermal load of gas refinementunit's installations

According to the increased circulating amine’s flow, based onFig. 8, thermal load in the lean amine cooler will also decrease. Asobserved, rich amine’s temperature exiting the repulsion towerwill decrease with the increase of amine flow.

Thus, according to Fig. 9, the temperature of the separator aswell as the temperature of the flow 13 entering to the torch willdecrease. Thus the evaporation rate in this part will decrease. Butthe evaporation rate in the exit section 19 in the repulsion towerwill be such that according to Fig. 10, more recovery water with

Fig. 12. The effect of amine flow entering to the absorption tower on the thermal loadof repulsion tower’s boiler.

increased circulating amine flow is needed (Mccab et al., 2001;Rodríguez et al., 2011).

In addition, if MDEA concentration decrease, more flow streamis needed to reach the standard characteristics of refined gas, sothat in addition to the increase in the investment cost in the solventsection, investment cost in gas refinement unit’s energy sectionwill increase. Figs. 11e13 indicate the effect of solvent’s flow effecton the thermal load of different equipments of gas refinement unit(Mccab et al., 2001).

3. Conclusion

In the present study, performance and reaction of MDEA solventin the adsorption tower and other processing equipment wereaddressed using Aspen Hysys, so that obtained results have lowestcost rate to be applied on the process.

Thus, considerable increase of circulating amine flow in theprocess compared to the flows available in the PFDS of manufactureis not recommended, Because changes in the piping are includingthe change in the tower’s diameter and other sections of the pro-cess. Increased pressure does not have significant changes on theprocess performance. Thus, tower pressure according to its char-acteristics, must be recommended by its manufacturer in thehighest pressure tolerable for the absorption tower. Despite the factthat by increased amine temperature, operation costs will decrease,but the amounts of acidic gases available in the tower output gasflow will increase. Thus output amine temperature will be set suchthat tower output gas reach the desired quality. Increased con-centration of circulating amine also increases the acidic gasadsorption, thus decreasing the tower output acidic gases. But thisincreased output gas purity will lead to increased operation costs aswell as increased corrosion of the equipment. For this purpose, itsrate must be arranged according to the needs of consumer as wellas the characteristics of equipment utilized in the refineries by themanufacturer.

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