design and simulation of ethane recovery process in an extractive dwc

8/18/2019 Design and Simulation of Ethane Recovery Process in an Extractive DWC

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Design and simulation of ethane recovery process in an extractivedividing wall column

Yadollah Tavan a,*, Shahrokh Shahhosseini b, Seyyed Hossein Hosseini c

a National Iranian Gas Company (NIGC), Tehran, Iranb Process Simulation and Control Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, P.O. Box 16765-163,

Narmak, Tehran, Iranc Chemical Engineering Department, Faculty of Engineering, Ilam University, 69315-516 Ilam, Iran

a r t i c l e i n f o

Article history:

Received 9 May 2013Received in revised form1 March 2014Accepted 3 March 2014Available online 14 March 2014

Keywords:

Dividing wall columnDistillationSeparationSimulationAzeotropic processDesign

a b s t r a c t

Separation of CO2 from hydrocarbons in the natural gas is complicated due to the existence of anazeotrope between ethane and CO2 at the cryogenic temperatures. The key issues to break this azeotropeare high investment costs for the unit equipments and the associated high energy requirements.Accordingly, an innovative process based on the dividing-wall column (DWC) technology is designedusing short-cut methods and relevant rigorous simulations. The energy demand and some environ-mental factors such as CO2 removal ef ciency and CO2 emission reduction are studied for the conven-tional and DWC processes. It is found that the process including DWC is a better choice than theconventional one from economical and environmental point of views. Remarkably, this technology re-duces the energy demand up to 51.6%.

2014 Elsevier Ltd. All rights reserved.

1. Introduction

The increase in global energy demand has led to widespreadinvestigations on alternative sources of primary energy even at themost remote areas of the earth. Natural gas is the most sought, afterliquid fuel source, due to its cleaner combustion and less ue gasemission into the atmosphere (Alfadala and Al-Musleh, 2009).Natural gas contains impurities such as carbon dioxide, hydrogensulde, carbon disulde, mercaptans and sometimes traces of carbonyl sulde. The removal of acid gases, H2S and CO2 from gasstream is essential due to environmental, operational and health

reasons (Maddox, 1982). Generally, the acid gas pipeline speci-cations are 4.0 ppm H2Sand2vol%ofCO2 with the dew point of lessthan 263 K at 4500 kPa. Since H2S is extremely corrosive and toxic,it is removed from the gas before its consumption. Apart frommeeting customer’s contract specications and successful lique-faction process, removal of CO2 from natural gas at high pressurehas currently become a global issue (Tavan and Hosseini, 2013a).Despite several researches done on CO2 capturing in chemical

processes (Sun and Smith, 2013; Harkin et al., 2010; Câmara et al.,2013), the existence of the minimum boiling CO2-ethane azeo-trope in natural gas process could causes certain problems. Highconcentrations of carbon dioxide in natural gas occur when carbondioxide is used for enhanced oil recovery. An azeotrope betweenethane (C2H6) and CO2 complicates separation of CO2 from naturalgas. Accordingly, using natural gas liquid (NGL) as extractivecomponent, Lastari et al. (2012) proposed low temperature distil-lation process in a series of distillation columns. The system gen-erates high pressure CO2, pure ethane and some amounts of NGL.However, the conventional extractive distillation process typically

includes two serial distillation columns. The main disadvantage of this separation process is its high capital investment and highamount of energy required to fulll the desired purication.Therefore, to overcome this drawback, advanced intensication andintegration process techniques such as thermally coupled distilla-tion columns, dividing-wall columns (DWC), heat-integrateddistillation columns and reactive distillation (RD) were employed(Yildirim et al., 2011). In a DWC,the middle section of a single vesselis split into two sections by inserting a vertical wall into anappropriate position of the column (Bravo-Bravo et al., 2010;Gutiérrez-Guerra et al., 2009). The DWCs have attracted moreattention in the chemical industries recently due to separation of

* Corresponding author. Tel./fax: þ98 21 88912525.E-mail address: [email protected] (Y. Tavan).

Contents lists available at ScienceDirect

Journal of Cleaner Production

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / j c l e p r o

http://dx.doi.org/10.1016/j.jclepro.2014.03.015

0959-6526/

2014 Elsevier Ltd. All rights reserved.

Journal of Cleaner Production 72 (2014) 222e229

mailto:[email protected]://www.sciencedirect.com/science/journal/09596526http://www.elsevier.com/locate/jcleprohttp://dx.doi.org/10.1016/j.jclepro.2014.03.015http://dx.doi.org/10.1016/j.jclepro.2014.03.015http://dx.doi.org/10.1016/j.jclepro.2014.03.015http://dx.doi.org/10.1016/j.jclepro.2014.03.015http://dx.doi.org/10.1016/j.jclepro.2014.03.015http://dx.doi.org/10.1016/j.jclepro.2014.03.015http://www.elsevier.com/locate/jcleprohttp://www.sciencedirect.com/science/journal/09596526http://crossmark.crossref.org/dialog/?doi=10.1016/j.jclepro.2014.03.015&domain=pdfmailto:[email protected]


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more components in one single distillation unit, thereby savingboth energy and capital costs. The theoretical studies have shownthat DWCs could lead to at least 30% reduction in energy costscompared to conventional schemes (Sangal et al., 2012; Gómez-Castro et al., 2011). It is notable that DWC technology is notlimited to ternary separations; it can also be used in azeotropicseparations, extractive distillation, and reactive distillation (Kissand Suszwalak, 2012a).

Although several researches have conducted about extractivedividing-wall columns (Ignat and Kiss, 2012; Sangal et al., 2014; Wuet al., 2013), the process being studied is different from conven-tional extractive distillation columns. In conventional extractivecolumns a third component is added to the system and solvent lossin the product streams requires a make-up stream; while in thepresent study, the solvent is a mixture of propane and heaviercomponents (NGL) in which the solvent stream is quite similar tothe light key (ethane). These distinct features of the process leads tosome convergence problems. Furthermore, the present study hassome advantages such as existence of no water in the solventstream and accordingly non-corrosive behavior of the solvent ascompared with conventional extractive processes. In addition, inorderto reduce the energy requirements and number of trays in the

extractive distillation process of separating the CO2/ethane azeo-trope, possibility of using DWC is examined by HYSYS3.1 (www.aspentech.com) for the rst time using top-wall conguration.Furthermore, the rates of the interconnecting streams are opti-mized in order to minimize energy requirement of the process.Eventually, energy requirements and some environmental param-eters of the novel DWC (improved) process and the conventionalone are compared with each other tonda morebenecial process.

2. Simulation

2.1. Thermodynamic analysis of the extractive column

Several strategies have been used in industries in order to

separate the azeotropic mixtures. Some of them require addition of a third chemical component for shifting the vaporeliquid equilib-rium such as extractive distillation, which uses a higher boilingsolvent and azeotropic heterogeneous distillation for entrainingchemical component. Another method for breaking azeotropes,which does not require addition of a third component, is pressureswing azeotropic distillation, wherein two columns operate at twodifferent pressures (Doherty and Malone, 2001; Luyben, 2013). Thethermodynamic analysis should be done prior to choosing the bestmethod for separating the azeotropes. For this purpose, a systemcontaining CO2/ethane and n-pentane as an agent of extraction isconsidered. CO2 and ethane are dissimilar molecules and havedifferent boiling points of 78 and 88 C and molecular weightsof 44 and 30 g/mol, respectively. These molecules have a strong

repulsion towards each other which leads to existence of a mini-mum boiling azeotrope in the system as shown in Fig. 1(a, b). Fromthe gure, the azeotrope compositions are 0.67 and 0.64 at 2400and 1500 kPa, respectively. Therefore, the relative volatility of CO2-ethane azeotrope does not signicantly change with pressure andconrms that the pressure swing distillation is not appropriate forthe present study. Fig. 1b clearly indicates that the phase envelopeof the system drastically changes with n-pentane mole fraction.Furthermore, addition of n-pentane decreases CO2 freezing tem-peratureto 75.9 C (preventing formation of solid CO2). Therefore,using extraction distillation is a promising process for the presentsystem. The residue curve of CO2/ethane/n-pentane can be helpfulin elucidating the distillation part. Accordingly, the residue curvemap of the mixture at the pressure of 2400 kPa is illustrated in

Fig. 2. As shown in Fig. 2, the ternary mixture presents a single

Fig. 1. The properties of CO2-ethane azeotrope process in terms of (a) binary diagram

and (b) phase envelope.

Fig. 2. The residue curve map of CO2, ethane and normal pentane.

Y. Tavan et al. / Journal of Cleaner Production 72 (2014) 222e 229 223

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binary azeotrope at 19.11 C with CO2 (mole fraction of 0.67) andethane without any liquid phase splitting. This gure also showsthe residue curve lines from the lowest azeotrope boiling point upto the highest one (n-pentane). The results indicate that extractivedistillation can be used in presence of a miscible mixture and ahomogeneous azeotrope. Moreover, this azeotrope limits theattainable separation in a single column and addition of the secondcolumn is mandatory in order to cross the azeotrope.

2.2. Simulation of extractive process

Fig. 3a illustrates a direct sequence of extractive distillationcolumns (conventional scheme) used for separation of CO2 fromethane. The process contains two distillation columns, two con-densers and two reboilers. In the rst column, a fraction of NGL stream is served as high entrainer which preferentially carriesethane to the next column. The CO2 and ethane streams (mainproducts) are drawn from the top of the columns with composi-tions of 95 and 99.9 mol %, respectively. The bottom stream of thesecond column that contains higher hydrocarbons is divided into

two parts, one of which is pumped back into the rst column forbreaking the azeotrope. For simulation of the conventional processillustrated in Fig. 3a, two “Set” blocks are added to the system inorder to control the solvent/feed ow ratio (S/F) and rate of recy-cling stream. The conventional process is simulated by HYSYS 3.1with Peng-Robinson property model for the vaporeliquid equilib-rium of the system (Torres-Ortega et al., 2013). The two serial col-umns containing 50 ideal trays operate at 2400 kPa with a 100 kPapressure drop.The numberof stages, optimum inlet stages and feedrates are extracted from the Lastari et al. (2012) study. The runspecications of CO2 purity and the temperature of the condenserin the top stream of the rst column are 95% and 14 C, respec-tively. Furthermore, the condenser temperature of 5.5 C andethane purity more than 99% are used in the top stream of the

second column. The input data and simulation results are listed inTables 1 and 2.

The process under study is a challenging and complex simula-tion case and some considerations are essential during its designand optimization. Fig. 4 shows the temperature and compositionproles of CO2/ethane/propane along with the rst and the seconddistillation columns. This gure shows that the required speci-cations aresatised by the model. Theconcentrationsof ethane andCO2 increase oppositely along the rst column. In addition, theconcentration of CO2 is negligible in the second column and theNGL stream contains lower amounts of CO2 and ethane.

Fig. 3. The diagrams of (a) extractive distillation and (b) the dividing wall column

(improved) processes: (symbols used in Fig. 3b: RCY: “Recycle” operator; SET: “Set”

operator; E: Heat exchanger; TEE: Flow splitter; 1, 2, 3, 4, Vap and Liqq: Material

streams; Qa, Qcc, n and Qr: Energy streams).

Table 1Input data and simulation results of the conventional and DWC systems.

Stream Feed Solvent Conventional DWC

CO2 Ethane NGL CO2 Ethane NGL Vap Liqq

Property

Flow rate (mol/s) 3800 3024 1288 1720 792.1 1280 1772 760 3033 8545Pressure (kPa) 2415 2410 2400 2400 2600 2400 2400 2600 2500 2500Temperature (C) 30 40 13.05 5.53 96 13.38 5.99 90.07 33.01 30.36Mol%

CO2 32.25 0 94.96 0.14 0 95.42 0.21 0 0 0Ethane 46.23 0.50 3.30 99.66 0.50 0.14 99.01 0 83.3 51.29Propane 7.53 32.63 1.74 0.2 0 32.86 3.86 0.17 30.89 8.13 15.24i-C4 7.47 35.93 0 0 35.84 0.58 0 36.43 5.89 18.00n-C4 3.29 15.46 0 0 15.79 0 0 16.54 1.90 8.00i-C5 2.09 10.3 0 0 10.03 0 0 10.39 0.58 5.00n-C5 1.1 4 5.18 0 0 4.98 0 0 5.75 0.20 2.47

Table 2

Comparison of energy demands for the conventional and DWC systems.

Equipment Conventional DWC

Column1 Column2 DWC Main column

Property

Total trays 50 50 20 40Condenser duty (kW) 95,782 181,471 18,096 88,981Reboiler duty (kW) 592,44 197,272 e 150,855Total duty (kW) 155,026 378,742 18,096 239,835

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3. Results and discussion

3.1. Simulation of extractive DWC unit

A ow diagram of extractive DWC that is consisted of twocondensers, a reboiler and a main column containing sieve trays, isshown in Fig. 3b. Such conguration for a DWC has previously beendescribed by the researchers (Yildirim et al., 2011; Kiss andSuszwalak, 2012a,b; Tavan and Hosseini, 2013b) and usuallycalled top-wall conguration. In the DWC system, the solvent isseparated as a single bottom product, while two other distillateproducts (CO2 and ethane) are extracted from each side of the maincolumn. Since there is no off-the-shelf DWC unit in the currentcommercial process simulators, two coupled columns are used inHYSYS 3.1, as thermodynamically equivalent of the extractive DWC.The liquid coming out of the bottom of the prefractionator (PF) isfed into the main column (MC), while a vapor stream is withdrawn

from the MC and fed into the bottom of the PF (also known as thevapor split). Two distinct routes are used for decomposing the DWCsystem into shortcut columns as shown in Fig. 5(a, b). These guresare self-explanatory and the results reveal that the stages numberof 20 and 40 are accounted for the PF and MC, respectively.Therewith, tray rating feature of HYSYS is used to calculate pressuredrops in the columns. The “Vap” stream is drawn from the 20thstage of the MC column and introduced tothe bottom tray of the PF.The bottom stream of the PF column is also sent tothe 20th stage of the MC. Since the PF section has a condenser, CO2 purity in thedistillate stream (95%) is used as input data. Moreover, the MC has 3degrees of freedom, therefore, the condenser temperature of 6 Cand ethane purity in the distillate and bottom stream of 99.99 and0.1%, respectively, are used as the input parameters for easier

convergence of the solution. It is worth mentioning that the results

of the base simulation and shortcut design are used as the initialestimates for the feed tray locations and vapor splitting. In thisstudy, the design steps and implementation of the DWC are similarto the conventional process. The detailed information about theDWC simulation can be found elsewhere (Premkumar andRangaiah, 2009). The simulation results of the DWC unit are alsolisted in Tables 1 and 2.

Fig. 6(a, b) shows the composition proles in both sides of thewall. As can be seen in the gure, high purities of the valuableproducts are obtained in the extractive DWC. Furthermore, thereare large amounts of CO2 and propane in the MC and the ethaneconcentration increases gradually along the column. In the PF col-umn, concentration of ethane increases, while CO2 concentrationdecreases along the rst column and the same trend can beobserved in extractive sequence columns. A comparison betweenthe conventional scheme, which includes two sequence columnsand the proposed DWC, is made. It is found that energy saving of 13.7% is possible by using DWC unit. This value is roughly equiva-lent to the corresponding term obtained by Kiss and Suszwalak(2012a) for bioethanol dehydration process. It should be notedthat 13.7% energy saving is considerably low due to the fact that theoptimized parameters of the conventional scheme proposed by

Lastari et al. (2012) was used in this study and the DWC system isnot in its optimized state, yet. Consequently, the DWC system isoptimized in the next section in order to make a fair comparison.

3.2. Optimization of extractive DWC unit

Hereafter, it is assumed that energy optimization of a DWC withxed number of trays is equivalent to energy minimization of thewhole DWC process since the energy consumption strongly de-pends on the interconnection between the liquid and vapor rates

Fig. 4. The temperature and composition proles of CO2/ethane/propane along the (a)

rst and (b) second distillation columns.

Fig. 5. Decomposing of the dividing wall column system into shortcut columns based

on (a) removal of ethane and (b) removal of CO 2: (symbols used in gures: QC1, QC2

and QC3: Energy streams for condensers; Qr1, Qr2 and Qr3: Energy streams for

reboilers; Up-MC, Down-MC and PF: Shortcut columns).



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(Delgado-Delgado et al., 2012). Hence, optimization of the inter-connecting streams leads to an optimal design.In the present study,the effect of the interconnecting stream rates, “Vap” and “Solvent”streams, on the energy demands are investigated by xing thenumber of ideal stages for the DWC unit. Since the “solvent” and“Vap

” streams have signicant effects on the composition proles

and duties in the extractive DWC, sensitivity analysis must be car-ried out rst in order to determine initial values for the “solvent”and “Vap” streams (Xia et al., 2012). The results of sensitivityanalysis are used for optimization by HYSYS process software af-terwards. Furthermore, it is tried to keep the nal product streamsat the industrial condition (CO2 > 95mol%andethane >99mol%inthe top streams).

Fig. 7a shows the inuence of interconnecting stream (“Vap”ow rate) on the total energy requirement that is summation of allduties of the reboiler and condensers, for the DWC unit. This gureshows that ow rate of “Vap” stream has a critical impact on theprocess duty; consequently, “Vap” stream is an important param-eter in DWC design that should be optimized. As can be seen in

Fig. 7a, increasing “Vap”ow rate leads to a subtle decrease in totalduty till the ow rate reaches up to 4000 mol/s, which attributes to

reduction in the condenser duty of the PF. By increasing the owrate of “Vap” stream above the minimum point (4000 mol/s), en-ergy consumption increases signicantly due to the increase inreux ratio of the MC in order to attain the desired purication of ethane in the product stream, as shown in Fig. 7b. Fig. 7b also showsthat by increasing the “Vap” rate, the mole fraction of CO2 in the topproduct of PF column decreases from 0.97 to 0.84. This conrmsthat major part of the CO2 content appears in the MC. Therefore, theoptimum ow rate of “Vap” stream needed to attain a desired CO2purity in the system is 3000 mol/s; hence the total energy demandis minimized.

Fig. 8 shows the inuence of the solvent ow rate on total en-

ergy requirement of the DWC unit by

xing stage numbers of the

columns and “Vap” ow rate (3000 mol/s). It is found that thesolvent ow rate is a crucial design parameter that should also beoptimized. From Fig. 8a, when the solvent rate is increased from300 up to 6000 mol/s, duties of the units increase due toenhancement of the columns stream rates. When the solvent rate isincreased, concentrations of CO2 and ethane in the product streamsrise exponentially in the system and consequently, required reuxratio decreases as depicted in Fig. 8b. As a result, addition of solventto the system exhibits an advantage (reduction in reux ratio) and adisadvantage (increase in duties) simultaneously. Therefore, thesolvent amount should be determined optimally not only todecrease both duty and reux ratio, but also to achieve the indus-trial specication of the product streams. In the solvent rate of 3000e3300 mol/s, all desired specication of the product streams(CO2 > 95 mol % and ethane> 99 mol % in the top streams) aresatised and with further increase in the solvent rate, more energyis required due to increase of the column rates. Accordingly, theoptimum rate of the solvent stream is found to be 3024 mol/s.

Another simulation was carried out using optimal values of interconnecting streams (solvent rate of 3024 mol/s and “Vap” owrate of 3000 mol/s) and the energy results are listed in Table 2. Itshould be note that in order to minimize energy requirements in

the whole DWC unit, the sequential quadratic programming (SQP)method implemented in HYSYS is also used to conrm the opti-mized results tabulated in Table 2. The target is minimization of energy demand using the interconnectingows as search variableswhile assuming the pure CO2 and ethane at the top of the columnsas a constraint. During performance tests of this method, thenumber of trays remained constant. The optimal result of SQPmethod clearly conrms earlier ndings presented in Table 2.Moreover, the mole balance of the components in the systems ispresented in Table 3. It can be inferred from Table 2 that 51.6%

Fig. 6. The results of the preliminary design of dividing wall column system in terms of

(a) composition proles of CO2/ethane/propane along the PF and (b) the MC.

Fig. 7. Finding the optimal rate of “Vap” stream using (a) energy demand and (b)

product speci

cation.



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reduction in energy demand is possible for the optimal design of the extractive DWC conguration compared with the optimaldesign of conventional extractive distillation columns. This is in afair agreement with the ndings of other researchers of DWC

schemes (Yildirim et al., 2011; Delgado-Delgado et al., 2012; Pre-mkumar and Rangaiah, 2009).It is worth mentioning that the Petlyuk column is generally

more ef cient than other thermally coupled schemes and a DWC ispractically identical to a Petlyuk column if the heat transfer acrossthe column wall is neglected or the wall is insulated. Accordingly,Fig. 9 that shows the temperature difference between both sides of the wall is prepared. As can be seen in this gure, the difference isless than 20 C. Therefore, it can be assumed that there is no heattransfer between two sides of the wall and the modeled system isthermodynamically equivalent to the Petlyuk column. Moreover,Fig.10 shows the composition proles in both sides of the wall. Thegure shows that products with high purity are obtainable in theDWC conguration. As a main result, using a DWC is an interesting

and suitable choice in extractive CO2-ethane process for the goal of reducing operating and investment costs.

3.3. Comparison between alternatives

In order to make further comparison between the conventional

and DWC processes, in addition to energy demand, environmentalfactors such as ef ciency of CO2 removal and CO2 emission reduc-tion are investigated in the present research. Subsequently, resultsof energy demand and environmental impacts analyses arecompared with each other.

3.3.1. CO 2 removal ef ciency

Removal ef ciency (h) is dened as percentage of CO2 in the gasstream that is removed during absorption operation. It should benoted that removal ef ciency implies absorption performance of the system (Tavan et al., 2014). The removal ef ciency for CO2 issimply determined from the difference between the amounts of CO2 entering the column and the corresponding term leaving eachstage of the column, which can be expressed by the following

equation:

h ¼

"1

Y CO2 ;nth

1 Y CO2 ;nth

1 Y CO2;in

Y CO2;in

!# (1)

where yCO2;in and yCO2;nth stand for mole fractions of gas-phase CO2entering to the column and leaving from each tray, respectively.Fig. 11a displays CO2 removal performance for the introduced sys-tems. In addition, Fig.11b also shows distribution of overall removalef ciency of the processes. It is clearly shown in Fig. 11a thatremoval ef ciency of the conventional process is quite higher thanthe DWC in the middle trays of the absorption column. For the DWCprocess, higher removal ef ciencies are observed in the lowersection of the absorption column. Additionally, equal removal ef-

ciencies are observed in the upper section of the absorption col-umn, which indicates the same performances of the processes.

3.3.2. Estimation of CO 2 emission reduction

During fuel combustion, air is assumed to be in excess to ensurecomplete combustion, so that no carbon monoxide is formed. Theamount of emitted CO2, [CO2] Emiss (kg/s), is related to the energyequivalent of the fuel, Q Fuel (kW), in the heating device, as follows(Gadalla et al., 2005; Tavan et al., 2014):

½CO2Emiss ¼

Q FuelNHV

C %100

a (2)

where a (¼3.67) is the ratio of the molar masses of CO2 and C, while

NHV, which is equal to 39,771 (kJ/kg), stands for net heating value

Fig. 8. Finding the optimal rate of “Solvent” stream using (a) energy demand and (b)

product specication.

Table 3

Mole balance of the components in the proposed systems.

Component CO2 Ethane Propane i-C4 n-C4

Property

Conventional processInput (mol/s) 1225.50 1756.74 286.14 283.86 125.02Output (mol/s) 1225.49 1756.66 286.14 283.89 125.07DWC process

Input (mol/s) 1225.50 1756.74 286.14 283.86 125.02Output (mol/s) 1225.17 1756.25 286.04 284.29 125.07

Fig. 9. Temperature prole of the dividing wall column system.



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of heavy oil fuel with a carbon content of 86.5%.The ame tem-perature of a boiler is lower than the ame temperature of a

furnace, because combustion heat is removed immediately to thesteam. However, the same theoreticalame temperature of 1800 Cmay still be used. A stack temperature of 160 C is also used in thecalculations. The energy equivalent of fuel can be calculated asfollows.

Q Fuel ¼ Q ProclProc

ðhProc 419Þ T FTB T OT FTB T stack

(3)

where lProc (kJ/kg) and hProc (kJ/kg) are the specic latent heat andspecic enthalpy of steam delivered to the process, respectively,and T TFB (C) is the ame temperature of the boiler ue gases. Theabove equation is obtained from the simple steam balance aroundthe boiler required to relate the energy equivalent of fuel in theboiler to provide a heat duty of Q Proc. The boiler feed water isassumed to be at 100 C with a specic enthalpy of 419 kJ/kg. FromFig.11b, it is evident that when the base case (conventional process)emits one unit of CO2 (kg/s) to the environment, the improvedprocess (DWC) emits much lower CO2, by 0.59. Therefore, it can beseen that 41% reduction in carbon emission is possible with the newprocess and it comes as no surprise that the DWC alternative is inthe pole position with the lower carbon footprint.

3.4. Final comparison

By comparing energy demand of the conventional and DWCprocesses, it is evident that the DWC process reduces energy de-mand by 51.6%. In addition, the novel proposed DWC needs low

number of trays compared to the conventional process. Based on

these ndings and data about CO2 emission reduction and removalef ciencies, it is concluded that the novel DWC process can beconsidered as a serious alternative candidate for the CO2-ethaneazeotropic process.

4. Conclusion

In the current research, the use of DWC for extractive CO 2-ethane azeotropic process is demonstrated through rigorous sim-ulations. It is found that the rates of interconnecting streams have asignicant impact on total energy demand and column specica-tions. Therefore, in order to make a fair comparison with the con-ventional sequence distillation columns, the optimal values of interconnecting rates are determined based on sensitivity analysisand the SQP method. The simulation results indicated that byraising the Vap rate, the mole fraction of CO2 in the top product of DWC column decreases from 0.97 to 0.84 and a high amount of CO2appears in the main column. In addition, escalation of “Vap” owrate leads to enhancement in energy consumption rate due tohigher reux ratio. Accordingly, the simulation results show thatthe optimum rate of “Vap” stream is 3000 mol/s. The addition of the

solvent to the system exhibits an advantage (reduction of reuxratio) and a disadvantage (increase in duties) at the same time.Therefore, the solvent amount is determined not only to decreaseboth duty and reux ratio, but also to achieve the industrial spec-ications of the product streams. The results indicate that optimalvalue of the solvent rate is 3024 mol/s. The results clearly show thatDWC process is feasible and the novel proposed DWC reduces en-ergy demand by 51.6% and carbon footprint by 41%. In addition, thenovel proposed DWC needs low number of trays compared with

Fig.10. Optimal results of the improved system in terms of (a) temperature prole and

(b) composition proles.

Fig. 11. Comparison between the proposed processes in terms of (a) CO2 removal ef-

ciency and (b) environmental effects.



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design and simulation of ethane recovery process in an extractive dwc

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