ff

ssa,kha

Multiple effect evaporatorsCondensate ashingVapor bleedingPinch analysisOptimization

mbstet ri

purpose seven effect evaporator system of a typical Indian pulp and paper industry is considered. The

hich isly useaft pro

evaporator house of a Pulp and Paper industry consumes about 24e

evaporator system of typical Indian pulp and paper industry isconsidered for analysis based on above congurations.

The mathematical models of MEE systems [4e14] have beenused to analyze these complex systems since last many decades. Afew recent enhancements in the area are: Kumar et al. [15]

system in the sugar industry with several multiple effect parallel

non-linear equa-proposed a modelodel of an evapo-ifferent operatingther investigators

proposed models where the whole set of governing equations ofthe model needs to be changed to address the new operatingconguration.

These models also use complex transport phenomena basedmathematical models or empirical models for the prediction ofoverall heat transfer coefcients (U) of evaporators as a function ofliquor ow rate, liquor concentration, physico-thermal propertiesof liquor and type of evaporator employed. In contract to these,Khanam and Mohanty [17] proposed linear model based on* Corresponding author. Tel.: 91 1332 285157; fax: 91 1332 276535.

Contents lists availab

International Journal

w.e

International Journal of Thermal Sciences 76 (2014) 110e117E-mail addresses: shabinahai@gmail.com, shabina@iitr.ernet.in (S. Khanam).30% of its total energy and makes it as an energy intensive section.The energy recovery studies in different industrial systems such

as cement industry, pulp mill and potato crisp frying process arecarried out by many investigators [1e3]. The energy efciency ofMEE system can be enhanced by inducting condensate ashing,steam splitting and vapor bleeding. In the presentwork seven effect

lines and time decaying performance.These models are based on set of linear and

tions. Amongst these models Bhargava et al. [14]using generalized cascade algorithm in which mrator body is solved repeatedly to address the dcongurations of a MEE system. However, oeffect evaporators (MEE) system as one of the major section. The discrete time representation and applied to a typical evaporation1. Introduction

The pulp and paper industry, wpresent investigation, predominantconvert wood chips into pulp. The Kr1290-0729/$ e see front matter 2013 Elsevier Mashttp://dx.doi.org/10.1016/j.ijthermalsci.2013.08.016equation solver.Economic evaluation to optimize the number of ash tanks is carried out for seven effect evaporator

system. The two different types of congurations of vapor bleeding are considered and compared.Considering optimum number of ash tanks and best conguration of vapor bleeding, a system isdesigned. Further, a modied system is found considering optimum number of ash tanks and pre-heating of liquor using condensate. This modied design enhances the steam economy by 23.77% andreduces the steam consumption by 36.76% in comparison to base case and thus, it is selected as optimumdesign. Finally, Pinch analysis of the MEE network has also been carried out and it is found that predictedresults are compared well with base case.

2013 Elsevier Masson SAS. All rights reserved.

the main focus of thes the Kraft Process tocess consists of multiple

developed an unsteady-state model for the MEE system based ondynamic behavior of the system of a paper industry. The behavior isobserved by disturbing the feed ow rate, feed concentration, livesteam temperature and feed temperature. Heluane et al. [16] pro-posed mixed integer nonlinear programming model based on aKeywords:

thermodynamic properties such as vapor pressures and enthalpies. The model is solved using nonlinearAccepted 27 August 2013Available online 5 October 2013 model developed is a set of nonlinear algebraic equations that include total and solute mass balances,

energy balances, heat transfer rate equations, and the composition and temperature dependence ofSimulation of heat integrated multiple e

Ghoshna Jyoti a, Shabina Khanam b, *a Dept. of Chemical Engineering, National Institute of Technology, Rourkela 769008, Orib Dept. of Chemical Engineering, Indian Institute of Technology, Roorkee 247667, Uttara

a r t i c l e i n f o

Article history:Received 9 September 2012Received in revised form19 June 2013

a b s t r a c t

In the present work a nuashing, vapor bleeding,properties and boiling poin

journal homepage: wwson SAS. All rights reserved.ect evaporator system

Indiand, India

er of congurations and difculties of MEE system such as condensateam splitting, preheating of liquor using condensate, variable physicalse are taken into consideration to analyze the evaporation system. For this

le at ScienceDirect

of Thermal Sciences

lsevier .com/locate/ i j ts

conry

5 6 Effe

ct N

o 7

S

VaporfromLasteffect

V4 V5 V6 V7

e se

nal of Thermal Sciences 76 (2014) 110e117 111principles of process integration. This model worked on theassumption of equal DT in each effect and thus, eliminated therequirement of U in the model.

Though all these models account complexities of real MEE sys-tem such as variation in physical properties, ashing, splitting andbleeding these do not propose methodology to optimize the per-formance of the system considering different congurations ofashing as well as vapor bleeding. In other words, these modelswere developed with condensate ashing in which positions ofash tanks were xed. These did not account the optimum numberof ash tanks, its position in the MEE system, performance of eachash tank, etc. These also did not consider different congurations

PF1 PF3: Primary SF1 SF4: Seconda

Steam, V0

Effe

ct N

o 12 3 4

PF1 PF2

SF1

Steam Vap o ns

V1 V2

V3

Fig. 1. Schematic diagram of th

G. Jyoti, S. Khanam / International Jourfor vapor bleeding to optimize the economy of the system. Based onthe above discussions the present work emphasizes on developingmodel for seven effect evaporator systemwith variation in physicalproperties of liquor, condensate and vapor, boiling point rise (BPR)and for different operating conguration such as steam splitting,condensate ashing, vapor bleeding, etc. Further, the contributionof different ash tanks towards total evaporation is computed andthus the optimization of number of ash tanks in the system isdone based on economic analysis.

2. Problem statement

The MEE system that has been considered in the present workis seven effect falling lm evaporator operating in pulp and papermill [14]. It is used in an Indian paper mill for concentrating non-wood (straw) black liquor, which has steam economy of 4.99.Schematically the system is shown in Fig. 1 where seven ashtanks, PF1, PF2, PF3, SF1, SF2, SF3 and SF4, are employed. Amongstthese PF1 to PF3 are primary condensate ash tanks whereas SF1to SF4 are secondary condensate ash tanks. In the present work,condensate of live steam is denoted as primary condensatewhereas, condensates of other vapor streams that exit from vaporchests of effects 2 to 6, is referred as secondary condensates. Theseash tanks are used to generate auxiliary vapor through ashingwhich is then used to enhance overall steam economy of the seveneffect evaporator. The operating parameters of the system arepresented in Table 1 which shows that steam temperatureentering into 2nd effect is 7 C hotter than that of rst effect. Theplausible explanation is unequal distribution of steam from headerto these effects leading to two different pressures in steam side ofthese effects. It is the actual scenario in the industry and thus, hasbeen considered as it is.

3. Model development

A model for seven effect evaporator system, used for concen-trating black liquor solution is developed. For the present investi-gation a number of correlations for the prediction of physico-

densate flash tanks condensate flash tanks

Feed

PF3

SF4SF3F2

Condensate

ven effect evaporator system.thermal properties of black liquor and condensate are developedas shown below:

s 200:1 x2 (1)

l 0:003T2 2:062T 2493 (2)

CP 4:1871 0:54 x (3)Mathematical model for computation of U of different effects as

shown in Eq. (4) is used in the present work [14]. The computedvalues of U from pulp and paper plant data, for all the seven ef-fects, are used to estimate the value of unknown coefcients, a, b, cand d, of Eq. (4) using constrained minimization technique ofSigma Plot. The estimated values of coefcients are given inTable 2. Details of the correlations can be found in the work ofBhargava et al. [14].

Table 1Operating parameters for the seven effect evaporator system.

S. NO Parameter Values S. No Parameter Values

1 Total numberof effects

7 4 Liquor inlettemperature

64.7 C

2 Live steamtemperature

Effect 1 140 C 5 Black liquorfeed ow rate

56200 kg/hEffect 2 147 C

3 Black liquorinletconcentration

0.118 6 Last effecttemperature

52 C

U2000

aDT40

bxavg0:6

cFavg25

d(4)

The model for the system shown in Fig. 1 is derived using massand energy balance around each effect as well as ash tanks asgiven below:

The value of h0 is computed at temperature T0 whereas, h0L andH0v are predicted at T3.

Similarly, expression of V1V, generated in SF1, is derived andshown in Eq. (11). Here condensate of streams, V1 and V2, are

Table 2Value of coefcients.

Effect no. a b c d

1 and 2 0.0604 0.3717 1.2273 0.07483e7 0.1396 0.7949 0.0 0.1673

G. Jyoti, S. Khanam / International Journal o112In fact for each effect two equations are derived one is forevaporator side and another is for steam chest side. First to thirdeffects are operating similarly and thus, governing equations ofthese effects should also be similar as shown below:

For ith effect

Li1Cpi1Ti1 si1 Vi1li1 LiCpiTi si Vili 4:2Ti 0

(5)

UiAiTi1 Ti si Vi1li1 0 (6)

where, i 1,2,3In Eq. (5) the term 4.2Ti represents the enthalpy of solvent which

is CpT. As in evaporation solvent is water where Cp should be 4.2 kJ/kg C. Therefore, the term CpT is directly considered as 4.2Ti.

Further, Fig. 1 shows that in effects 4 to 7 ashed vapor are alsoused for evaporation along with vapor emerging from previouseffect. Thus, the governing equations for these effects should bedifferent than that for 1st to 3rd effect i.e. Eqs. (5) and (6). For 4theffect along with V3, vapor generated from ashing in ash tanks,PF1 to SF1, i.e. Vov and V1v are available for evaporation as shown inFig. 2.

Thus, equation for 4th effect is derived using energy balance as:

Sensible heat of liquor L5 Latent heat of vapor V3 Latent heat of vapor streams from PF1 and SF1 V0v V1v Sensible heat of liquor L4

Heat of vapor stream V4

If total amount of vapor generated through ashing andentering to an effect is referred as Vf then for 4th effect

4

Vapor stream V3inlet from effect 3

Liquor inlet L5from effect 5

Vapor stream V4outlet from effect 4

V0vPF1

SF1 Liquor outlet L4 from effect 4

V1v

Fig. 2. Material and energy balance around 4th effect with ashing.Vf V0v V1v (7)Thus, generalized equation for jth effect can be written as:

Lj1Cpj1Tj1 sj1

LjCpjTj sjVj1 Vfj

lj1

Vjlj 4:2Tj

0 (8)

UjAjTj1 Tj sj

Vj1 Vf jlj1 0 (9)

where, j 4 to 7For 7th effect

Lj1 F

Cpj1 CpF

and Tj1 TFThe term Vfj used for ashed vapor entering into jth effect can be

elaborated as shown in Eq. (7) for 4th effect.Further, the amount of vapor generated through ashing can be

computed based on material and energy balance around ash tank,PF1, shown in Fig. 3. Here V0 is amount of condensate of steamentering the ash tank PF1 at T0 which is ashed at T3. In fact,condensate and vapor streams associated with a ash tank areassumed to be at saturated condition. So, entering and exiting stateof a stream is represented with temperature instead of pressure.The temperature T3 is selected as vapor generated in PF1 is used insteam chest of 4th effect which is being operated at T3. V0v is theamount of vapor leaving the ash tank at temperature T3 and V0L isthe remaining condensate exiting PF1 that is led to ash tank, PF2.The expression of V0v is derived as:

Material balance around PF1 : V0 V0v V0L

Energy balance around PF1 : V0h0 V0vH0v V0Lh0LSolving above equations,

V0v V0 h0 h0LH0v h0L

(10)

PF1V0 at T0

V0V at T3

V0L at T3

Fig. 3. Schematic diagram PF1.

f Thermal Sciences 76 (2014) 110e117entering SF1 at average temperature of T1 and T2 and ashed at T3.Therefore, h1 is computed at average temperature (T1 T2/2)whereas, h1L and H1v are found at T3.

V1V V1 h1 h1LH1v h1L

(11)

The present model consists of fourteen equations (two equa-tions for each effect) with variables V0, L1 to L7 and T1eT6. Thus,

4.334, respectively. Further, the base case is analyzed considering

ow, L, decreases from 7th to 1st effect more vapor is generated due

This can be explained as: feed to 2nd effect is coming from 3rdeffect which is at lower temperature than 2nd effect. Therefore,steam of amount 0.9225 kg/s is rst used to heat-up the feed from89.5 C to 126.8 C and then evaporation takes place, which causes

3 4 5 6 7

0.2584 0.6955 0.839 0.9698 1.2241.524 1.586 1.534 1.508 1.9197.54 9.064 10.65 12.184 13.6920.2442 0.2032 0.1729 0.1512 0.13452.37 1.839 1.4899 1.2619 1.1002e 0.2585 0.1135 0.1199 0.10041.72 1.7825 1.6995 1.6539 1.6084e e e e e

89.5 77.2 67.2 58.7 52

nal of Thermal Sciences 76 (2014) 110e117 113to evaporation which causes value of concentration, x, increasesfrom 7th to 1st effect. The similar trend is observed for BPR, s.However, amount of vapor produced from ashing vary from effectto effect, which depends on amount of condensate as well astemperature difference available for ashing. Further, it is seenfrom Table 3 that total evaporation for this system is 9.79 kg/s,different heat recovery options as discussed in subsequentparagraphs.

4.1. Seven effect evaporator system with condensate ashing

Condensate leaving from an effect is ashed to lower tempera-ture to obtain vapor that can be used as heating medium is thesubsequent effects along with the vapor exiting for previous effect.In the seven effect evaporator system there are 7 condensate ashtanks, as shown in Fig. 1 and nal results of the model are shown inthe Table 3. The trends of U and L are increasing from 1st to 7theffect which is obvious for backward sequence. However, as liquorunique solution exists for the model. The solution of these modelsrequires an iterative approach as number of parameters such asvariable physical properties, s, U, etc., are involved in the modelwhich depend on unknown intermediate temperatures. A detailedalgorithm for solution of developedmodels is discussed in theworkof Gautami and Khanam [18].

4. Results and discussion

For seven effect evaporator system, shown in Fig. 1, model isdeveloped considering steam splitting and condensate ashing. Asa base case simple seven effect evaporator system with backwardfeed sequence is considered which accounts variations in l, Cp, sand steam splitting, however, it does not consider ashing. Theresults of base case model, which is obtained in 9 iterations, showthat steam consumption and economy are found as 2.296 kg/s and

Table 3Results of seven effect evaporator system with condensate ashing.

Effect 1 2

U, kW/m2C 0.296 0.4303V, kg/s 1.186 0.534L, kg/s 5.82 7.006x 0.3165 0.2629s, C 3.4697 2.6344Vapor from cond. ashing, kg/s e eTotal vapor consumed, kg/s 0.9225 0.9225Steam consumption, kg/s 0.9225 0.9225T, C 106.3 126.8

G. Jyoti, S. Khanam / International Jourwhich is summation of all vapor streams shown in Table 3. How-ever, evaporation is 9.95 kg/s for base case. Though total evapora-tion is decreased with condensate ashing, it is obtained only byconsuming 1.845 kg/s of steam, which is 21.78% less in comparisonto base case. This is due to availability of 0.5923 kg/s of additionalvapor, generated through condensate ashing as shown in Table 3.Thus, for present system steam economy is found as 5.3, which is22.4% more than that for base case. In fact, for the similar systemBhargava et al. [14] proposed a complex model which gives steameconomy as 5.00. It shows that though present model is simpliedit gives comparable results with rigorous model proposed byBhargava et al. [14].

Further, it can be observed from Table 3 that consuming equalamount of steam in rst two effects amount of vapor generated in1st effect is signicantly higher than that is produced in 2nd effect.lesser vapor to be generated in 2nd effect. However, in 1st effectfeed enters at 126.8 C and ashes to acquire the temperature ofeffect i.e.106.3 C. Further, in this effect steam of amount 0.9225 kg/s is used only for evaporation. Therefore, amount of vapor gener-ated in 1st effect is due to ashing as well as evaporation which issignicantly higher in comparison to 2nd effect.

4.1.1. Optimization of number of ash tanksTo optimize number of ash tanks, the model developed in this

network is solved with condensate ashing using different numberof ash tanks. Maximum possible number of ash tanks that can beused in the system, shown in Fig. 1, is eight. Considering eight ashtanks the model is solved, which consumes 1.8387 kg/s of steam.Consequently, steam economy increases 5.3198 which is 0.25%more in comparison to the system shown in Fig. 1.

For optimizing number of ash tanks it is necessary to computethe contribution of each ash tank towards total evaporationwhichis presented in Fig. 4. It shows that % contribution of PF3 is mini-mum amongst three primary ash tanks. However, PF3 cannot beeliminated as no ash tank is available ahead to PF3 to share itsload. Hence, ash tank, PF2, is eliminated and its load is shifted toPF3. Now the seven effect evaporator system consists of six ashtanks instead of seven. The solution of model of this system showsthat steam consumption and steam economy are 1.8485 kg/s and5.278, respectively. Steam economy is reduced by 0.55% than thatfor the systemwith seven ash tanks. This is due to less amount ofadditional vapor is generated using six ash tanks in comparison toseven tanks. Further, load of SF2 is shifted to SF3 as it is contributingleast amongst all four secondary ash tanks as indicated in Fig. 4.Fig. 4. Contribution of each ash tank in seven effect evaporator system.

Thus, system is incorporating only ve tanks. The steam con-sumption and economy for this system are 1.8755 kg/s and 5.1917,respectively.

Further, to choose the best system economic analysis of fourcongurations, which are seven effect evaporator system witheight, seven, six and ve ash tanks, is carried out. The total annualcost (TAC), net prot and payback period of four congurations arecompared in Table 4. The TAC is dened as:

TAC Annual operating cost Annualized capital cost(12)

For conguration with eight tanks annual operating cost is

with ve ash tanks can be selected as optimum as it gives less

Table 4Economic analysis of four congurations.

Parameter 8 Flashtanks

7 Flashtanks

6 Flashtanks

5 Flashtanks

TAC (million $/year) 2.326 2.333 2.338 2.371Net prot (million $/year) 0.578 0.569 0.565 0.531Payback (days) 20 18 17 15

G. Jyoti, S. Khanam / International Journal o114complex network in comparison to other congurations withoutcompromising the prot signicantly.

4.2. Seven effect evaporator system with vapor bleeding

Vapor bleeding is done to preheat the liquor near to tempera-ture of effect before it is entering into the effect so that liquor canquickly attain the boiling temperature inside the effect. In thepresent work two congurations of preheating are proposed. Inboth congurations four pre-heaters are placed between effects 3computed based on annual steam consumption of the congura-tion and per unit cost of steam. Whereas annualized capital cost isfound considering cost of eight ash tanks, life of each ash tanks as10 years and straight line depreciation. Table 4 shows that TAC ismaximum for system with ve ash tanks, however, it is only 1.9%more in comparison to the system with eight ash tanks which isnot signicant. In the similar lines net prot and payback period arealso not vary appreciably for four congurations. Therefore, systemFig. 5. Comparison between conguration-1 and -2 of system with vapor bleeding.and 7. For conguration-1 the vapor required for pre heaters placedbetween 2nd and 3rd, 3rd and 4th, 4th and 5th, 5th and 6th effectsare bled from V2, V3, V4,and V5, respectively. For conguration-2 thevapor required for pre heaters placed between 3rd and 4th, 4th and5th, 5th and 6th and 6th and 7th effects are bled from V3, V4, V5 andV6, respectively. Fig. 5 compares these two congurations, whichshows that conguration-2 is better than the rst one as it gives1.21%more steam economy. Enhancement in steam economy is dueto proper utilization of driving force available with low vaporpressure (with high latent heat) streams.

4.3. Seven effect evaporator system with vapor bleeding andcondensate ashing

In the present section seven effect system is considered whichincludes steam splitting, variation in physical properties, conden-sate ashing along with vapor bleeding as shown in Fig. 6. For thiscase it is found that steam consumption reduces to 1.8542 kg/s andsteam economy enhances to 5.549.

Further, steam economy as well as steam consumption of thepresent system is compared with that of system, shown in Fig. 1,and base case. It is found that the steam consumption of the presentsystem is 21.29% and 1.15% lesser in comparison to base case andsystem shown in Fig. 1, respectively.

4.4. Seven effect evaporator system with preheating of liquor usingsensible heat of condensate

In this section the liquor is preheated near to the temperatureof effect before it is entering into the effect. Under Section 4.2preheating of liquor is carried out through bled vapor, however,in this case condensate of steam/vapor is used to preheat theliquor, which is entering into that effect using a counter currentheat exchanger. Condensates exiting 1st, 2nd, 3rd, 4th and 5theffects are utilized in the process to preheat the liquor comingfrom the 4th, 5th, 6th and 7th effect. For this modication totalevaporation and steam consumption are found as 9.22 kg/s and1.676 kg/s, respectively, and hence the steam economy is pre-dicted as 5.503. Further, steam economy as well as steam con-sumption for base case and vapor bleeding system, discussed inSection 4.2, are compared with that of present model. It isobserved that with the addition of preheating of liquor usingcondensate steam consumption reduces by 31.22% and steameconomy enhances upto 23.77% compared to base case. Con-sumption of steam in the present system is 18.3% less and steameconomy is 11.5% more in comparison to system with vaporbleeding. The reason of reduction in steam consumption is thatafter preheating liquor is entering the effect at temperature ofeffect and thus steam/vapor is used only for evaporation insteadof sensible heating.

4.5. Seven effect evaporator system with preheating of liquor usingsensible heat of condensate and condensate ashing

The seven effects system is considered with preheating of li-quor through condensate and ashing. It is carried out as thecondensate leaving the exchanger after preheating is at signi-cantly higher temperature and its heat can further be utilizedthrough ashing in the effects, which are being operated at lowtemperature. For this purpose seven effect system with preheatingof liquor through condensate is modied to incorporate ve ashtanks, PF1 to PF3, SF1 and SF2, as shown in Fig. 7. In these tankscondensates of live steam, C01, C02, and condensate from vaporchest of third effect, C , are being ashed. Hence, ve ash tanks

f Thermal Sciences 76 (2014) 110e1171system with preheating of liquor through condensate is

nal of Thermal Sciences 76 (2014) 110e117 115G. Jyoti, S. Khanam / International Jourconsidered in the present section. It is observed that the modiedseven effect evaporator system consumes 1.583 kg/s of steam andsteam economy enhances to 5.807.

Further, steam economy, steam consumption and product con-centration of present system are compared with that of base caseand model with vapor bleeding and ashing. The results of com-parison are shown in Fig. 8. From this gure it is concluded thatwith the induction of preheating of liquor using condensate andashing in the system, steam consumption reduces and steameconomy enhances. For this case total evaporation rate is found as9.193 kg/s. Though evaporation rate is 8% less than that is for basecase, it is achieved using 36.76% less steam. Consequently, steameconomy of the system is increased upto 23.77% as compared tobase case. Further, it is observed that consumption of steam in thepresent system is also 15.78% less in comparison to system withashing and bleeding.

Fig. 6. Schematic diagram of seven effect system with vapor bleeding and condensate ashing.

C01 C02 C1

Steam, V0

Feed

T1 T2 T3 T4 T5 T6T7

VaporfromLasteffect

Black Liquor

Product

L1, T1

V1 V2 V3 V4 V5 V6 V7

PF1PF2 PF3

SF2SF1

C3

Fig. 7. Schematic diagram of seven effect system preheating of liquor using condensate and with ashing.

Fig. 8. Comparison of base case, system with vapor bleeding and ashing and systemwith preheating of liquor using condensate and ashing.

4.6. Pinch analysis of seven effect evaporator system

The heat integration options discussed under Section 4.2e4.5are considered for enhancing steam economy of MEE system.Another heat integration technology is pinch analysis [19,20],which may be applied to MEE system. The heat integration studieson sugar and desalination plants are carried out by a few in-vestigators [21,22].

For applying pinch analysis in MEE the base case system is

To consider the heat associated with sensible heating the streamdata of evaporator system are extracted and reported in Table 5. Thepinch analysis is applied to the stream data considering DTmin as10 C using ASPEN PINCH and composite curve is shown in Fig. 9.The minimum hot and cold utility of the system is 351.69 kW and

Table 5Stream data of seven effect evaporator system.

Streamno.

Streamname

Ts (C) Tt(C) Cp (kJ/kgC) m (kg/s) CP (kW/C)

1 Cold 53.088 61.745 3.886 13.818 53.6912 Cold 61.745 71.551 3.849 12.312 47.383 Cold 71.551 82.063 3.8013 10.799 41.054 Cold 82.063 93.639 3.738 9.286 34.715 Cold 93.64 130.18 3.653 7.796 28.476 Hot 84.84 83.84 2652.099 0.935 2479.717 Hot 127.72 126.72 2717.025 0.449 1219.948 Hot 91.38 90.38 2662.662 1.49 3967.3679 Hot 80.28 79.28 2644.593 1.513 4001.26910 Hot 70.08 69.08 2627.38 1.513 3975.22511 Hot 60.5 59.5 2610.662 1.506 3931.6512 Hot 52 51 2595.412 1.793 4653.86

MP

He

Table 6Comparison of results of all models for seven effect evaporator system.

S. No. System Section Steam economy

1 Base case 4 4.3342 System with

condensate ashing4.1 5.306

3 System withvapor bleeding

4.2 4.85

4 System withvapor bleedingand condensate ashing

4.3 5.549

5 System withpreheating of liquorusing condensate

4.4 5.503

6 System withpreheating of liquor usingcondensate and ashing

4.5 5.807

7 System withpinch analysis

4.6 5.246

G. Jyoti, S. Khanam / International Journal of Thermal Sciences 76 (2014) 110e117116considered. Based on equal driving force as well as equal vapor-ization in each effect, temperatures and concentration of each ef-fect is predicted. Using these parameters physical properties andBPR are computed. Further, to apply pinch analysis it is assumedthat total sensible heat required in the system is provided outsidethe effect and thus, liquor is entering the effect at boiling temper-ature. Consequently, only evaporation is taking place inside theeffect. To consider this fact the equations of system are modied asliquor entering and exiting the effect at same temperature i.e. effecttemperature plus BPR. The modied equations are solved to getnew values of temperatures, concentration as well as steamconsumption.

TEMPERATURE COCase: 1

260.0

280.0ENTHA

TE

MP

ER

AT

UR

E F

0.0 20.0 40.0 120.0

140.0

160.0

180.0

200.0

220.0

240.0

Hot composite c

Fig. 9. Composite curve for seve21775.18 kW, respectively. The hot composite curve shows totalheat available with different vapor streams. Similarly, cold com-posite curve represents the total heat required by all liquor streamswhich need to be preheated.

In Fig. 9 the shaded area shows that the amount of heat asso-ciated with vapor streams which is provided to liquor streamsunder the curve. The part of cold composite curve above the shadedarea is the heat provided through hot utility. Thus, if the heatavailable in shaded area as well as hot utility is fullled fromoutside then no sensible heating is required inside the effect.Consequently, liquor streams enter and exit the effect at boilingtemperature and inside the effect only evaporation takes place.Considering this fact the steam consumption is computed as

OSITES (Real T, No Utils)

DTMIN =10.00 at ImbalanceHOT COLD

LPY MMBTU/hr

60.0 80.0 100.0

urve Coldcompositecurve

n effect evaporator system.

0.416 kg/s. Fig. 9 shows that total amount of heat required forsensible heating of liquor is 2402.6 kW. Assuming this heat isprovided by steam, the amount of steam required is 1.337 kg/s.

s boiling point rise (C)Cp specic heat (kJ/kg C)V0 bled vapor ow rate (kg/s)

G. Jyoti, S. Khanam / International Journal of Thermal Sciences 76 (2014) 110e117 117Nomenclature

F feed ow rate (kg/s)V0 steam ow rate (kg/s)L ow rate of liquor stream (kg/s)V ow rate of vapor stream (kg/s)C condensate ow rate of steam/vapor (kg/s)l heat of vaporization/latent heat (kJ/kg)A heat transfer area of an effect (m2)U overall heat transfer coefcient (kW/m2 K)T temperature (C)DT temperature drop (C)x mass fraction of solute5. Conclusions

The salient conclusions are as follow:

Themodel based on set of nonlinear equations, directs almost alldifculties of real MEE system such as variable physical prop-erties, BPR, steam splitting, condensate ashing and vaporbleeding.

Based on economic analysis as well as steam economy it isconcluded that the seven effect evaporator system can runeffectively with ve ash tanks instead of seven. Thus, thisapproach gives simplied network for evaporator system.

The two different types of congurations of vapor bleeding areconsidered and compared. It is observed that steam economy forconguration-2 is more than that for conguration-1. Consid-ering the optimum number of ash tanks and conguration-2, asystem is designed which enhances the steam economy by24.6% and reduces the steam consumption by 21.3% in com-parison to base case.

Liquor heating using sensible heat of condensate contributesconsiderably towards steam consumption. Moreover, it pro-duces less complex MEE network in comparison to other sys-tem. Considering ashing and preheating of liquor withcondensate the steam economy is increased by 23.77% ascompared to simple system.

Pinch analysis of the MEE network has also been carried out andit is found that predicted results are compared well with basecase.Thus, total steam consumption in the system is found as 1.753 kg/s,which gives steam economy as 5.246. It is 21.04% more in com-parison to base case.

Comparison of all systems, proposed in the present work, isshown in Table 6 to visualize how individual conguration isaffecting the steam economy of the seven effect evaporator system.It shows that maximum steam economy is observed for systemwith preheating of liquor using sensible heat of condensate andcondensate ashing (S.No. 6 of Table 6). The reason of such highsteam economy is better recovery of heat availablewith condensatethrough sensible heating as well as ashing. Thus, the presentsystem improves steam economy by 16.4% in comparison to that ofreal plant.Subscripts1e7 effect numberF feed0 live steamL liquorV vapor

AbbreviationBPR boiling point riseS.No serial number

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Simulation of heat integrated multiple effect evaporator system1 Introduction2 Problem statement3 Model development4 Results and discussion4.1 Seven effect evaporator system with condensate flashing4.1.1 Optimization of number of flash tanks

4.2 Seven effect evaporator system with vapor bleeding4.3 Seven effect evaporator system with vapor bleeding and condensate flashing4.4 Seven effect evaporator system with preheating of liquor using sensible heat of condensate4.5 Seven effect evaporator system with preheating of liquor using sensible heat of condensate and condensate flashing4.6 Pinch analysis of seven effect evaporator system

5 ConclusionsNomenclatureReferences