Design strategy for heat pump assisted distillation system

Heat Recovery Systems Vol. 6, No. 6, pp. 469-476, 1986

0198-7593/86 $3.00+ .00 Pergamon Journals Lid

Printed in Great Britain

DESIGN STRATEGY FOR HEAT PUMP ASSISTED DISTILLATION SYSTEM I. M~SZkROS a n d Z. F o N Y 6 Technical University of Budapest, Department of Chemical Engineering, Budapest, H-1521 Hungary (Received 22 April 1986)

Atatract--Distiliation columns are the major energy-consuming units in the chemical and petrochemical industries. The heat integration concept [1] attempts to integrate the heating and cooling demands of a column within the heat network of the overall process. In case of stand-alone unit or severe restrictions on integrability of distillation column, the realization of heat pump assisted distillation seems to be one of the most promising energy saving techniques. Because of its less energy demand, the heat pump assisted distillation reduces the operating cost and occasionally can compensate the additional capital expenses of compressor. Based on the COP and energy costs, simple expressions are developed for preliminary economic analysis and design of heat pump misted distillation. A design strategy is proposed for selecting the most economical distillation system based on the energy cost factor and the estimated payback time of excess capital. The strategy is demonstrated by two examples.

NOMENCLATURE A c, c, c. C¢E Cce

CcR C~ CEx C~ CCO CHP COP COP o

coe~, coP~L coe~. ECF El. HPY OCO OHP PT P8

PBTE PBTE + PBTE,~

Q

ST

TT Tn UER W ACHE

ATcE ATc~ ATRc AWA

heat transfer area of heat exchanger [m2] unit cost of electric energy [$ GJ-i] unit cost of heating steam [$ G J- i] unit cost of cooling water [$ GJ -j] installed cost of condenser of closed cycle process [$] installed cost of compre~or [$] installed cost of condenser [$] installed cost of distillation column [$] installed cost of heat exchanger for compression heat removal [$] installed cost of reboiler-condenser [$] installed cost of reboiler of conventional distillation system [$] capital cost of conventional distillation system [$] capital cost of heat pump assisted distillation system [$] coefficient of performance idealized coefficient of performance idealized coefficient of performance of bottom flashing idealized coefficient of performance of closed cycle process idealized coefficient of performance of vapour recompression energy cost factor electric energy cost of heat pump process [$ yr -z] operating hours in a year [h yr-'] operating cost of conventional distillation system [$ yr-'] operating cost of heat pump assisted distillation system [$ yr-'] column top procure [bar] column bottom pressure [bar] payback time of excess capital [yr] estimated payback time of excess capital Lvr] maximum payback time of excess capital [yr] heat turnover of reboiler and condenser [GJ h -t] cost of heating steam of conventional distillation system [$ yr-'] column top temperature [K] column bottom temperature [K] ratio of unit costs of energies energy supplied to compressor shaft [kW] heat exchanger cost difference of conventional and heat pump assisted distillation systems [$] temperature difference in condenser-evaporator [K] temperature difference in condenser [K] temperature difference in reboiler-condenser [K] cooling water cost difference of conventional and heat pump assisted distillation systems [$ yr- i] INTRODUCTION

O w i n g to the h u g e a m o u n t o f e n e r g y r e q u i r e m e n t , e c o n o m i c s o f t h e r m a l s e p a r a t i o n processes largely d e p e n d o n t h e c o s t o f a p p l i e d e n e r g y . O p p o s i t e t o c o n v e n t i o n a l d i s t i l l a t i o n in h e a t p u m p 469

470

I. M ~ z ~ o s and Z. For,n¢6

T-r

Tr- ATcE

1"-

TT

TT-Z~T.c

i

Jil

T

'1 rT

~[J

Te~-Z~T~c CLosed cycLe process

T++ ATRc Vapour

Bottom flashing

recomprossion

Fig. l. Heat pump assisted distillation systems.

assisted distillation systems the energy passed through the column does not depreciate entirely, but becomes reusable by adding external mechanical work. The heat pump cycle can be connected to distillation column in three ways (Fig. 1). The simplest alteration is to replace steam and cooling water with refrigerant. The other two types of heat pump system apply column fluids as refrigerant [2]. When the distillate is a good refrigerant the vapour recompression can be used. In the vapour recompression process the vapour leaving column top is compressed to a pressure high enough could be able to satisfy heat demand of reboiler. If the bottom product is a good refrigerant the bottom flashing can be applied. The bottom liquid is flashed across expansion valve and evaporated in reboiler-condenser, then the vapour is compressed to column bottom pressure before entering the column. Detailed evaluation and design algorithm for closed cycle heat pump system was published by Omideyi et aL [3-6]. In this study the three types of heat pump assisted distillations are compared from the view-point of thermodynamic efficiency. Computer aided analysis is performed to develop simple expression for determining effectiveness and economic advantage of heat pump assisted distillation systems. An algorithmic strategy is proposed for selecting the most advantageous distillation system including heat pump assisted processes. C O M P A R I S O N OF HEAT PUMP ASSISTED DISTILLATION PROCESSES The heat pump requires a work input to remove heat from a low temperature and deliver it to a higher temperature [7]. The heat pump can be especially advantageous for that operation where both hot and cold sides of the thermodynamic cycle are attachable. In the case of distillation the heat supply of condenser and the heat demand of reboiler can be considered as cold and hot sides of heat pump respectively. In this way cooling and heating costs can be saved at the same time by introducing external work. In order to analyse the three types of heat pump assisted distillation systems, the idealized thermodynamic effectiveness, based on Carnot-cycle can be expressed as coP~

=

COP~ve =

TB + ATRc Tn + ATRc - ( T r - ATcE)

(1)

(2)

Te + ATRc TB + A TRc -- T r

coP~

=

T,

.

(3)

Tn -- ( T r - A T~c)

From equation (1-3) and from Fig. 2 it can be concluded, that vapour recompression seems to be the most efficient process thermodynamically and the least value of C O P D belongs to the closed

Design strategy for heat pump assisted distillation system i

471

,

Vopour recomprenlon Bottom ftashl~ll Closed cycle process

COP"

COP

0.--0 D---el

0.-.*0 O---.O

~

~--¢*

1.2--

I.I ~ O. 0 0

*4%%%

1.0--

"0 C 0

0.9

0 ¢.) ¢P o _l

0.8

%

-~O "~cJ -%'~'~%.

T v • 331 K

T e " 351 K

0.7

~TRc a n d

ATcl E (K)

Fig. 2. Comparision of C O P and C O P [] for i-butane--n-butane separation. The refrigerant for closed cycle process is R718. Thermodynamic data are obtained from [8].

cycle process, i.e. coP[:L < c o e ~ ¥ < c o e ~ , .

(4)

Nevertheless the order of real C O P s for the three types of heat pump processes could be changed owing to the latent heat of working fluids. When latent heat of bottom product is greater than that of top product, the bottom flashing process could prove more efficient. For example, separating/-butane and n-butane the thermodynamic efficiency of bottom flashing is greater by 16--25% (Fig. 2), so needs less compressor and could become more attractive than vapour recompression. With proper selection of refrigerant, the closed cycle system also could be advantageous. Considering the separation of butane isomers and using steam as refrigerant at low temperature differences in heat exchangers, the coefficient of performances are practically equal to that of vapour recompmssion. Summarizing the effect of temperature lift and latent heat of working fluids it can be established, that in the course of designing heat pump assisted distillation all the three types of heat pump processes should be judged since there is no unanimous thermodynamic reason to prefer vapour recompression. FACTORS FOR DECISION PREPARATION Although the heat pump assisted distillation systems consume less energy than conventional one, the significant cost of compressor has a decisive influence on its economic advantage. Considering electric motor driven compressor from thermodynamic efficiency of heat pump cycle and the ratio of unit costs of energies, the following energy cost factor of crucial importance can be formed ECF = C O P UER

(5)

C$ UER = - . Ce

(6)

where

The value of ECF expresses definitely, when heat pump assisted process can be economical. If ECF is less than unity the heat pump process cannot be economical since the greater capital cost

472

I. M~z~RoS and Z. FON'~6

Table 1. Capital costs of distillation systems

Table 2. Installedcosts of equipments

f.on~qn :iorlal system

= CDC + CRE * CCR

~dpeuv P¢compr~ssion

= CDC + CRC + CEX ÷ Ccp

CRF =

47{30 A 0 " 6 5

Bottom flashJr, g

= CDC + CRC + CEX + Ccp

CCR =

2790 A0"65

Clo~,~d c y c l e pro~ee8

= CDC + CRC + CEX + CCp + CCE

(;I{C =

470t? A0"65

CEX =

;279(3 A 0 " 6 5

I::,p

4630

=

WO ' B 2

cannot be paid back through operating cost. From equation (5) it is also obvious, that a system of higher thermodynamic efficiency can balance the higher cost of electric energy. In order to judge economics of heat pump process the payback time of excess capital through operating cost is observed according to equation 7 PBTE

CHP-CCO = OCO - OHP

(7)

For analysing P B T E the following assumptions are made: AI. Heat duties of condenser and reboiler are identical approximately. A2. The operational parameters and the cost of distillation column are the same for conventional and heat pump systems. The capital costs of distillation systems and installed costs of equipments are calculated according to Tables 1 and 2 respectively. The cost data are obtained from Guthrie [9] and are actualized by CE cost index from 1969 to the present. The capital cost difference between conventional and heat pump processes can be expressed as the sum of two terms, where the first is the cost of compressor and the second is the cost difference of heat exchangers C H P - C C O = Cce + A C H E .

(8)

The operating expenses are calculated from utility costs (Table 3). The operating cost difference between conventional and heat pump assisted systems consists of steam cost, electric energy cost and cooling water costs difference OCO - OHP = ST - EL + A WA.

(9)

According to equations (8) and (9) the P B T E can be rewritten as PBTE

=

Ccp+ ACHE ST - EL + AWA

(10)

Neglecting A C H E in the nominator and A W A in the denominator, P B T E can be estimated as PBTE + =

Ccp ST-EL

(11)

furthermore from Tables 2 and 3, 4630 W°'s2 0.0036 ce W)"

PBTE+ = HPY

(c, Q -

(12)

Expressing compressor power by coefficient of performance and rearranging equation 12, 4630

PBTE + = HPY

(c.)

(0.0036 C O P ) ° 8 ~ Q °'l~ c, - C O P

Table 3. Operating costs of distillation systems Conven¢ional 8y8t~m

=

Vapour recompreaBion Bottom f l a a h i n g Closed c y , l e

process

HPYCcsQ + CwQ ) HPY Co.o036CeW + O.O036CwW~ HPY CO.O036CeW + 0.0036¢wW ~

•

HPY CO.O036CeW + OoO036cwW~

(13)

473

Design strategy for heat pump assisted distillation system Table 4: Ranges of parameters for P B T E + analysis O ,, 1 - 1 0 0 0 GO/h

CO? = 5 -

20

ce . c. -

8 ~/G3 20 ~/0,3 6 F/GO

c s

2 10 2 -

The validity of P B T E + calculated according to equation (13) is analysed in wide range of several parameters (Table 4). For comparison in Fig. 4 the estimated P B T E (i.e. P B T E +) and the real P B T E determined according to equation (10) are illustrated as function of energy cost ratio for different C O P . Based on computer aided analysis, it can be observed, that in general the deviation of the estimated P B T E from the real value is less than one year and is within 15%. In this manner equation 13 is capable of approximating P B T E for analysing the economics of heat pump assisted distillation systems at the stage of preliminary design. Since the developed expression for P B T E + is based on installed cost of compressor the value of the constant in the nominator can be actualized with application of C E cost index. Analysing equation (13), the effects of several parameters on payback time of excess capital can be examined. (1) The increase of operating hours in a year makes heat pump processes more attractive. (2) The relatively low electric energy cost is favourable for the application of heat pump process. (3) With increasing column heat demand as the value of P B T E + is decreasing, the heat pump process becomes more economical (Fig. 3). The heat demand of column and economic advantage of heat pump process can be influenced by several parameters, such as column pressure, reflux ratio, feed rate, number of plates, pressure loss of column and column internals. I ! I

)1

~q Se_7_-

i~

Cs~6.0 $/GJ

~ 1 6 . 7

a • io GJ/h

C." 16.7 S/Gd 10.06 $ / kWh)

S/GJ (0.06 S/kWh)

D--~ PBTE* o---o PBTE v Ira 5 a. "o c o

~J 4 Ira ft. 3

w tin Q.

-

0

I I

I I0

I I00

I000

L.o@ Q (GJ/h) Fig. 3. T h e effect o f rcboilcr heat duty on PBTE for several COPs.

0

I

0.!

I

0.2

I

0.3

l

0.4

l

05

UER • CslCe Fig. 4. Comparison of PB TE and PBTE + as function of energy costs for several COPs. For PBTE calculation 75% polytropic efficiency is applied.

474

I. M#.szggosand Z. FoNY6

I Specify separation problem Other process streams for heat integration are ova table ~ >

;No

Yes

,_1AnaLyse feasibility of ] heat integration Heat integration is economical ? >

effect:ivenessof heat pump '~ processes I

aLcuLateECF according to Eq.5

I

IY.s

l

I

with heat Design integration I

I Yes I D'cide pBTEmax I

I

Estimote PBTE according to Eg.13

~Yes I Design with heat pump process

I LI Design without heat pump process

I

Fig. 5. Block diagram of design strategy for heat pump assisted distillation systems.

DESIGN STRATEGY A design strategy for selecting heat pump assisted distillation system of closed cycle type was developed by Omideyi et al. [3]. Their algorithm does not consider the feasibility of vapour rccomprcssion and bottom flashing, although vapour recompression is frequently applied in propane-propylene separation. A strategy is described here for selecting the most advantageous distillation system including all the three types of heat pump processes. The strategy is outlined in Fig. 5. At the first step the separation task is to be identified with product specifications. Afterwards other process streams, such as vapours of distillation columns are to be considered for heat integration. If other process streams as heat source are available, first the feasibility and economics of heat integration is to be analysed. Otherwise additional energy would be introduced into the system in vain by compressor [1]. When heat integration cannot be considered the attention turns to the application of heat pump processes. The coefficients of performance are to be determined for heat pump processes. Several column pressures and refrigerants should be considered with a pressure limit which allows to remove compression heat with cooling water. Based on ECF calculated according to equation (5) the feasibility of payback through operating cost is examined. If ECF is less than unity, beside capital cost the operating cost of heat pump process also exceeds that of conventional system, so the design of heat pump process should be avoided. When ECF is greater than unity, the heat pump process can be economical because results in reduction of operating cost. Then, depending on economic circumstances decide a maximum value for payback time of excess capital (PBTE,,x) which is a bounding criteria for accepting heat pump process. At the next steps the PBTE is approximated according to equation (13) and compared with PBTE,~x. If the estimated PBTE exceeds the bounding value, the heat pump process is rejected. The procedure for selecting heat pump assisted distillation system can be repeated for all types of heat pump processes and for several column parameters and refrigerants. An exhaustive analysis for selecting refrigerant is given by Reay and Macmichael [7] and Omideyi et al. [3].

Design strategy for heat pump assisted distillation system

475

42.9"C~~ 46.0"C

Ibor i-

55.1"C

Propane

Propane ~ I-Butane n- Butane n-Pentaneloo.8eC~

Butane

~2.7"C ~

fur

n-B~ane

140"2eC~~ -- n-Pen~ane Fig. 6. Optimalflowsheetfor Example1. Total annualcostis 9.567× 105$yr-'. EXAMPLES In order to illustrate the procedure for selecting heat pump processes according to the strategy described above, two examples are studied. Example 1 A four-component hydrocarbon mixture is separated into its components with amount of 368 kmol h-~ and molefractions Propane: 0.366 i-Butane:

0.185

n-Butane:

0.352

n-Pentane:

0.097.

To determine the optimal distillation system with heat integration total annual cost is considered as objective function, which is a combination of capital and operating expenses. Design and cost equations are obtained from the paper of Rathore et al. [10]. The unit costs of utilities are cw = 0.159 $ GJ -I cs = 3.98 $ G J - J c¢ = 22.22 $ G J- i. According to the strategy outlined in Fig. 5 first the feasibility of economical heat integrations is examined. Applying the predictor based ordered search technique [l 1] the optimal distillation system is determined and shown in Fig. 6. Since columns 2 and 3 are matched, those cannot be operated by heat pump process. In order to judge profitability of heat pump assisted column 1, first the COPs and ECFs are calculated and listed below. COP ECF vapour recompression 3.16 0.57 4.17 0.75 bottom flashing closed cycle process (R718) 4.81 0.86 Since none of the values among ECFs reaches unity, to run column I with heat p u m p process cannot be profitable.

Example 2 A mixture consisting of butane isomers with 990 kmol h-i flowrate and molefractions /-Butane:

0.345

n-Butane:

0.655

476

I. M ~ z ~ o s and Z. FONY6

is separated into pure products. T h e unit costs o f utilities are c. = 0.159 $ G J -l c s = 4.51 $ G J -I Ce = 16.67 $ G J -l

Because other process streams are not available for heat integration the C O P s and E C F s are calculated f r o m p a r a m e t e r s o f column 3 in E x a m p l e 1 (Fig. 6). COP ECF

v a p o u r recompression b o t t o m flashing closed cycle process R718

11.63 3.15 12.01 3.25 9.44 2.55

F r o m heat duty o f reboiler (Q = 73.1 G J h -m) and operating hours in a year ( H P Y = 8000 h yr -~) P B T E + s are calculated according to equation 13. F o r checking P B T E + s the real values o f P B T E s are also determined. PBTE ÷ PBTE

v a p o u r recompression b o t t o m flashing closed cycle process (R718)

1.52 1.46 2.11

1.67 1.59 2.32

Fixing the m a x i m u m value o f P B T E to two years it can be concluded, that P B T E + o f closed cycle process is greater than P B T E , , ~ so is rejected. Considering P B T E + o f the other two heat p u m p processes, the b o t t o m flashing seems to be the m o s t economical. Nevertheless there is no significant difference between the values o f P B T E + s for v a p o u r recompression and b o t t o m flashing, it is advisable to examine b o t h in detail. CONCLUSIONS T h e main features o f the p r o p o s e d strategy for preliminary economic analysis and design o f heat p u m p assisted distillation systems are the following. (1) (2) (3) (4) (5)

H e a t integration is preferred to heat p u m p process for heat recovery. All the three types o f heat p u m p processes are considered. Energy cost factor is used for recognizing the feasibility o f economical heat pumping. T h e attractivities o f heat p u m p processes are measured by p a y b a c k time o f excess capital. T h e p a y b a c k time o f excess capital is estimated by simple expression based on coefficient o f performance, energy costs and reboiler heat duty. REFERENCES

I. B. Linnhoff, H. Dunford and R. Smith, Heat integration of distillation columns into overall processes, Chem. Eng. Sci. ~ 1175-1188 (1983). 2. H. R. Null, Heat pumps in distillation, Chem. Eng. Progr. 72, 58--64 (1976). 3. T. O. Omideyi, J. Kasprzyeki and F. A. Watson, The economics of heat pump assisted distillation systems--I. A design and economic model, J. Heat Recovery Systems 4, 187-200 (1984). 4. S. Gopichand, T. O. Omideyi, J. Kasprzycki and S. Dcvotta, The economics of heat pump assisted distillation systems---II. Analysis of ethanol-water mixtures, J. Heat Recovery Systems 4, 271-280 (1984). 5. T. O. Omideyi, M. G. Parande, J. Kasprzycki and S. Dcvotta, The economies of heat pump assisted distillation systems---Ill. A comparative analysis on three alcohol mixtures. J. Heat Recovery Systems 4, 281-286 (1984). 6. T. O. Omideyi, M. (3. Parande, S. Supranto, J. Kasprzycki and S. Dcvotta, The economics of heat pump assisted distillation systems--IV. Experimental assessment with methanol-water mixtures, J. Heat Recovery Systems $, 51 I-518 (1985). 7. D. A. Reay and D. B. A. Macmichael, Heat Pumps, Design and Application, Pergamon Press, Oxford (1979). 8. F. A. Holland, F. A. Watson and S. Dcvotta, Thermodynamic Design Data for Heat Pump Systems, Pergamon Press, Oxford (1982). 9. K. M. Guthrie, Data and techniques for preliminary capital cost estimating, Chem. Engng. 76, 6, 114-142 (1969). 10. R. N. S. Rathore, K. A. Van Wormer and G. J. Powers, Synthesis strategies for muiticomponvnt separation systems with energy integration, AIChE J. 20, 491-502 (1974). I I. I. M/,'szArosand Z. Fony6, A new bounding strategy for synthesizing distillation systems with energy integration, Comp. Chem. Engng. (in press).