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Distillation column configurations in ammoniawater

absorption refrigeration systems

Jose Ferna ndez-Seara*, Jaime Sieres, Manuel Va zquez

Area de Maquinas y Motores Termicos, Escuela Tecnica Superior de Ingenieros Industriales, Universidad de Vigo,

Campus Lagoas-Marcosende, N 9, 36200 Vigo, Spain

Received 29 October 2001; received in revised form 30 May 2002; accepted 7 June 2002

Abstract

In ammoniawater absorption refrigeration systems a purification process to reduce the water content in the vapour

leaving the generator is required. During this process the water content in the vapour must be reduced to a minimum,

otherwise it tends to accumulate in the evaporator and strongly deteriorates the efficiency of the system. The vapour

purification can be carried out by partial condensation, by establishing a liquidvapour counter flow or by combining

both methods. In systems with partial condensation, the distillation column can be composed of one or more rectifiers

using different cooling mediums, and the rectifying and stripping sections. In complete condensation systems only the

rectifying and stripping sections can be used. Therefore different distillation column arrangements should be con-

sidered. This paper presents a study of several distillation column configurations for single stage ammoniawater

absorption refrigeration systems with partial and complete condensation. In order to evaluate and compare the dif-

ferent configurations, a parameter that indicates the ratio of the ammonia vapour concentration increase in each partof the column to the total ammonia purification has been defined. The analysis has been based on the system COP.

Finally the efficiency in each part of the column has been calculated to estimate its design requirements.

# 2002 Elsevier Science Ltd and IIR. All rights reserved.

Keywords: Refrigerating system; Absorption system; Ammoniawater; Distillation column; Design

Configurations des colonnes de distillation des syste` mes

frigorifiques a` absorption a` ammoniac/eau

Mots cles : Syste` me frigorifique ; Syste` me a` absorption ; Ammoniac-eau ; Colonne de distillation ; Conception

1. Introduction

This work is circumscribed to ammoniawater absorp-

tion systems. This technology, applied to refrigeration,

dates its first practical applications around 1860. From

this time on the understanding of the processes involved

in these systems has been increasing, to the point where

today the applied technology is considered well known.

However, some pitfalls must be avoided in designing

and building a prototype or an industrial system in

order to obtain an effective behaviour and performance.

Some of these drawbacks can derive from a deficient

ammonia vapour purification process of the regenerated

vapour from the system generator.

0140-7007/03/$20.00 # 2002 Elsevier Science Ltd and IIR. All rights reserved.

P I I : S 0 1 4 0 - 7 0 0 7 ( 0 2 ) 0 0 0 3 7 - 3

International Journal of Refrigeration 26 (2003) 2834

www.elsevier.com/locate/ijrefrig

* Corresponding author. Tel.: +34-986-812605; fax: +34-

986-812201.

E-mail address: [email protected] (J. Ferna ndez-Seara).
http://www.elsevier.com/locate/ijrefrig/a4.3dmailto:[email protected]:[email protected]://www.elsevier.com/locate/ijrefrig/a4.3d


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This paper deals with the ammonia purification process

in ammoniawater absorption refrigeration systems. In

this type of systems the vapour pressure of water (sor-

bent) is not negligible compared to the vapour pressure

of ammonia (refrigerant). Therefore the vapour leaving

the generator always contains a small part of water. The

water content in the vapour must be reduced, because it

tends to accumulate in the evaporator. The presence ofwater in the evaporator diminishes the evaporation pres-

sure and strongly deteriorates the efficiency of the system.

Ignoring the water content in the regenerated vapour

constitutes one of the major pitfalls in designing an

ammoniawater refrigeration system, as pointed out by

Bogart [1].

On the other hand, the processes used to reduce the

vapour water content always involve the condensation

and return of a vapour fraction back to the generator.

This means that part of the heat input in the generator is

used to generate vapour which does not produce refrig-

eration. Consequently, the system efficiency is reduced.

The effectiveness of the purification process has become

a crucial issue in order to guarantee a reliable and effi-

cient system operation.

The vapour purification process can be carried out by

partial condensation, by establishing a liquidvapour

counter-flow or by combining both methods. In partial

condensation the vapour is cooled, so that a small partwith a high water content condenses. The condensate is

called reflux. The cooling process can be carried out

using different cooling mediums. The heat exchanger

where the partial condensation takes place is usually

named as rectifier or reflux cooler. The counter flow

processes are established in a distillation column. The

vapour stream from the generator enters at the bottom of

the column and rises in counter-flow to a liquid stream,

so that heat and mass transfer processes between both

phases are enabled.

The liquid stream can be produced by condensation of

the vapour at the column top, or can be the strong solu-tion (or a fraction) from the absorber. The liquid stream

at the column top can be obtained by partial condensa-

tion or by returning part of the condensed liquid from the

condenser. The last is usually denominated complete

condensation. The contact zone between the vapour and

the reflux is known as the rectifying section. When the

liquid stream is the strong solution, then the vapour

liquid contact is established below the column feed entry

point. This contact zone is known as the stripping section.

If the distillation column is composed of rectifying and

stripping section, then the liquid stream in the stripping

section is the mixture of the reflux from the rectifying

section and the strong solution from the absorber.

In systems with partial condensation, the distillation

column can be composed of one or more rectifiers and

the rectifying and stripping sections. In complete con-

densation systems only the rectifying and stripping sec-

tions can be used. Therefore the ammonia purification

process can be carried out with different distillation

column configurations.

In this paper a single stage ammoniawater absorp-

tion refrigeration system has been studied. Different

possibilities of distillation column configurations have

been analysed for partial and complete condensation. In

partial condensation the rectifier cooling medium can beeither an external fluid or the strong solution when a

single stage cycle with rectifier heat integration is con-

sidered. The evaluation of the different column con-

figurations has been based on the system COP.

Moreover the efficiency in each part of the column is

calculated in order to estimate its design requirements.

2. System description

The single stage ammoniawater absorption refrig-

eration systems with partial and complete condensation

Nomenclature

COP coefficient of performance

Cp specific heat (J kg1 K1)

Gr temperature gradient (K)

h specific enthalpy (J kg

1

)m:

mass flow (kg s1)

P pressure (Pa)

%Pur purification ratio (%)

Refl internal reflux ratio

Q:

heat flow (W)

T temperature (K)

x ammonia mass concentration (kg kg1)

Greek symbols

" mass transfer efficiency

energy efficiency

SubscriptsA absorber

b bomb

C condenser

E evaporator

G generator

l liquid

R refrigerant

rect rectifying section

Rss strong solution cooled rectifier

Rw water cooled rectifier

ss strong solution

strip stripping section

v vapourw water

ws weak solution

J. Fernandez-Seara et al. / International Journal of Refrigeration 26 (2003) 2834 29


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examined in this paper are depicted schematically in

Fig. 1. The components of the distillation column in the

partial condensation system are the stripping and the rec-

tifying sections, a heat integrated rectifier with the strong

solution as coolant and a water cooled rectifier at the top

of the column. For the complete condensation system

only the rectifying and stripping sections are considered(dashed line in Fig. 1).

3. Mathematical model

A lumped steady state model has been developed in

order to study the vapour purification process in single

stage ammoniawater absorption refrigeration systems

with partial and complete condensation. The mathema-

tical model is based on the application of global mass,

species and energy balances and heat and mass transfer

equations.In formulating the model the following assumptions

have been made: heat losses to the environment are

negligible, pressure drops are considered only between

the evaporator and absorber, the condensed liquid from

the condenser and the weak solution leaving the gen-

erator are saturated, as well as the liquid and the vapour

currents inside the column. The irreversibilities in the

absorber, solution and liquidvapour heat exchangers

and in the solution pump are defined by means of their

efficiencies. State equations used for the ammoniawater

equilibrium and thermodynamic properties have been

calculated from Ziegler and Trepp [2].

The mathematical model has been used to determine

the thermodynamic state at every representative point of

the cycle, outside of the distillation column, for the

partial and complete condensation systems. As a result,

the conditions of the streams entering and leaving the

column are known. In order to determine the liquid and

vapour state conditions inside the column an appro-

priate parameter has been introduced. It has been

named ammonia purification ratio (%Pur) and is

defined as the quotient of the ammonia vapour enrich-ment achieved in any component of the column to the

ammonia concentration difference between the vapour

leaving the column and the vapour produced in the

generator.

The equations applied to the rectifying and stripping

sections of the column and to the rectifiers are reported

in the following sections. The model equations of the

remainder system components are not presented here

for the sake of simplicity and brevity, but they are

described in detail in Refs. [3,4].

3.1. Rectifying and stripping sections

Applying global mass, species and energy balances

over the rectifying section the corresponding equations

are obtained. The vapour ammonia purification ratio

for the rectifying section (%Purrect) is calculated using

Eq. (1), according to its definition and nomenclature in

Fig. 1.

%Purrect x19 x17

x23 x131

For the stripping section the corresponding equations

for the global mass, species and energy balances and for

the ammonia purification ratio have been derived. At

the feed entry point, it is assumed that the liquid and

vapour fractions of the feed flow mix with the down-

ward liquid from the rectifying section and with the

upward vapour from the stripping section, respectively.

Thus, the liquid and vapour conditions at the top of the

stripping section can be determined.

3.2. Rectifiers

Global mass, species and energy balances over the

water cooled rectifier are also obtained. For the watercooled rectifier, the purification ratio is defined in Eq.

(2).

%PurRw x23 x21

x23 x132

In the literature concerning mass transfer operations,

the liquid and vapour phases leaving a rectifier are

usually considered to be in thermodynamic equilibrium

and therefore the rectifier is assumed to be equivalent to

a theoretical stage [5]. However, in this paper this

assumption is not made and an efficiency term () thatFig. 1. Schematic diagram of the distillation column with par-

tial or complete condensation.

30 J. Fernandez-Seara et al. / International Journal of Refrigeration 26 (2003) 2834


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expresses the vapour concentration deviation from

thermodynamic equilibrium conditions is taken into

account. For the water cooled rectifier, the efficiency

used is given in Eq. (3).

"Rw x T23 T22 x21

x23 x21

3

In mass transfer operations, deviations from conditions

of a theoretical stage are generally expressed by the Mur-

phree efficiency [6], which is defined as the inverse of Eq.

(3). However, in a rectifier the liquid temperature at the

outlet is frequently greater than the vapour temperature

leaving the rectifier and, consequently, the Murphree

vapour efficiency exceeds unity. Therefore, the efficiency

expression in Eq. (3) has been chosen. For the strong

solution cooled rectifier corresponding equations to the

water cooled rectifier have been derived.

4. Results and discussion

The mathematical model described in the previous sec-

tion has been implemented in a computer program using

Fortran 90. The implemented model has been used to

simulate and analyse the performance of absorption

refrigeration systems considering partial and complete

condensation processes to purify the generated vapour.

The analysis has been performed varying the distillation

parameters one at a time while keeping the others para-

meters that define the operation of the absorption

refrigeration system at a constant value. The operating

conditions are indicated in Table 1, where the rectifier

efficiencies are given only for the partial condensation

system.

4.1. Partial condensation systems

The results presented here for the partial condensa-

tion systems examine the effect of diverse configurations

of the distillation column on the system COP, as well as

the required components efficiencies. This has been done

by varying the vapour purification ratio in the rectifiers

and in the stripping and rectifying sections, considering

also the cases of systems where some of these components

are not used.

4.1.1. Effect of vapour purification ratios distribution on

the system COP

Fig. 2 shows the system COP as a function of the

ammonia vapour purification ratio carried out in therectifiers. The rectifiers purification ratio was varied

from 0 to 90% of the total ammonia purification pro-

cess in the column (from x13 to x23) while a constant

value of 10% was established for the rectifying section

(%Purrect=10%). Thus, the special cases when one of

the rectifiers is not used (%PurRw=0% or%PurRss=

0%), none are used (%PurRw=0% and%PurRss= 0%)

or the stripping section is not used (%PurRw+%

PurRss=90%) are included in this figure.

Fig. 2 reveals that solutions can not be obtained for

the whole range considered. This means that for the

operating conditions in Table 1, the specified ammoniaconcentration can not always be attained. For a fixed

value of the ammonia purification ratio in the water

cooled rectifier %PurRw (different curves in Fig. 2) the

existing solutions are limited for small %PurRss values

since, in this case, the liquid reflux generated from the

condensation of the vapour stream in the rectifiers is not

enough to achieve the desired ammonia concentration

specified in the rectifying section (x19x17). However, if

%PurRw increases, solutions can be found for smaller

%PurRss values since the required reflux is generated in

the water cooled rectifier. On the other hand for a given

%PurRw value, the available solutions are also limited

to the %PurRw value for which the sum of the vapour

purification ratios in all components is 100%.

Fig. 2 shows that increasing %PurRss for a constant

%PurRw value decreases the system COP. The same

result is observed when increasing %PurRw for a con-

stant %PurRss value. However, in this case COP values

Table 1

Operating conditions

TE (K) 258 P (Pa) 10 000

TG (K) 398 "A 0.7

Tw (K) 293 "l-v 0.7

TC (K) 8 "ss-ws 0.8

TA (K) 10 "b 0.5

QE (W) 5000 "Rss 0.7

xR (kg kg1) 0.999 "Rw 0.7

Tevap (K) 10

Fig. 2. Partial condensation system COP as a function of the

purification ratio of the strong solution cooled rectifier for dif-

ferent values of the purification ratio of the water cooled recti-

fier and with a value of 10% for the purification ratio of the

rectifying section.



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are lower since the rectifier heat is rejected to a heat sink

outside the system (cooling water) instead of being used

within the absorption system [7].

If the rectifying section is not considered (%Purrect=

0%), the range of available solutions increases, since

restrictions are only imposed by the stripping section

efficiency which must be between 0 and 1. However, theCOP obtained is the same than in Fig. 2, since the rec-

tifying section is replaced with a larger stripping section

and both of them operate adiabatically. From the pre-

sented results, an appropriate configuration can be chosen

for a given application, based on the system COP.

4.1.2. Rectifying and stripping sections efficiencies

The achievement of a high purification ratio in a dis-

tillation column component usually means high efficiency

requirements. This implies that the vapour current con-

tacts a large surface area, resulting in a component of

higher dimensions. Thus, in order to complete the pre-vious analysis the efficiencies of the stripping and recti-

fying sections have also been studied when varying the

purification ratios of the column components.

The stripping and rectifying section efficiencies are

defined as the quotient of the ammonia vapour con-

centration enrichment value to the theoretical maximum

value for the given vapour and liquid inlet conditions. Eqs.

(4) and (5) represent the efficiencies of the rectifying and

stripping sections, according to nomenclature in Fig. 1.

"rect x19 x17

x T19T20 x174

"strip x15 x13

x T15T16 x135

Fig. 3 shows the efficiency of the rectifying section nee-

ded to achieve a purification ratio of 10% (%Purrect=

10%) versus the strong solution rectifier purification ratio

(%PurRss) and for different %PurRw values. The recti-

fying section efficiency is rather low, which suggests that

for the given conditions a higher vapour ammonia con-

centration could be obtained. However, the vapour

enrichment is limited for decreasing values of %PurRss

and %PurRw because the reflux solution is low and,consequently, the efficiency of the rectifying section

based on the liquid phase concentration which is defined

in Eq. (6) is close to unity.

"rect;l x20 x18

x20 x T18T17 6

Fig. 4 shows the necessary stripping section efficiency

for different values of %PurRss and %PurRw for the

most adverse case corresponding to not using the recti-

fying section (%Purrect=0%). Some of the curves in

Fig. 4 extend from zero to unity efficiency values, limit-ing %PurRss possible values. Since the liquid flow is

much higher than the vapour flow, it is usually possible to

achieve high purification ratios with a stripping section of

moderate dimensions.

4.1.3. Rectifier efficiencies

Fig. 5 shows the strong solution cooled rectifier effi-

ciency when %Purrect= 0%. The efficiencies are defined in

Eqs. (7) and (8) and considering the relation between the

heat capacity ratio of both fluids and the nomenclature

shown in Fig. 1.

Rss Q:

Q:

max

m:

7 h8 h7

min m:

19 h19 m:

21 h21 m:

20 h20 T21T7 ;

m:

7 hT8 T19 h7

7

Fig. 3. Rectifying section efficiency as a function of the purifica-

tion ratio of the strong solution cooled rectifier for different values

of the purification ratio of the water cooled rectifier and with a

value of 10% for the purification ratio of the rectifying section.

Fig. 4. Stripping section efficiency as a function of the pur-

ification ratio of the strong solution cooled rectifier for different

values of the purification ratio of the water cooled rectifier and

with the purification ratio of the rectifying section being zero.



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Rw m:

w Cpw Tw;out Tw;in

min m:

21 h21 m:

23 h23 m:

22 h22 T23 Tw;in ;h

m:

w Cpw TTw;outT21 Tw;in i

8

The efficiency of the water cooled rectifier increaseswhen increasing %PurRw but remains constant when

varying the purification ratio of the strong solution rec-

tifier (%PurRss), because the vapour inlet conditions are

set by %PurRw. However, Fig. 5 reveals that the strong

solution rectifier efficiency increases when increasing

%PurRss and when decreasing %PurRw, since in this

case, the condensate solution leaving the water cooled

rectifier enters the strong solution cooled rectifier and its

thermodynamic conditions vary with %PurRw.

As stated before, the system COP decreases when

increasing the vapour enrichment carried out in the

water cooled rectifier. Therefore, avoiding the use of thisrectifier seems a practical option attending to the system

COP results. However, according to Fig. 5 for %PurRw=

0% values, the required solution rectifier efficiency can be

as high as 0.95, hence for specific applications it would be

interesting to use the water cooled rectifier in order to

avoid the needs for a high efficiency in the solution rectifier.

4.2. Complete condensation systems

The parameters being considered in the analysis of the

purification process by means of complete condensation

are the internal reflux ratio and the purification ratios in

the rectifying and stripping sections. However, since the

column operates adiabatically only the reflux ratio

influences the system COP. The internal reflux ratio is

defined in Eq. (9), according to nomenclature in Fig. 1.

Reflm:

24

m:

23

m:

24

m:

1 m:

24

9

4.2.1. Effect of the internal reflux ratio on the system COP

Fig. 6 shows the system COP versus the reflux ratio,

for different purification ratios in the rectifying andstripping sections. When increasing the reflux ratio, the

vapour leaving the distillation column and the liquid

returned from the condenser increase. Thus, more liquid

returns to the generator and more vapour must be pro-

duced, which implies that the generation heat load must

increase and consequently the system COP decreases.

The COP values obtained are lower than for the partial

condensation system. This is due to the extra heat rejected

for cooling the column liquid reflux in the condenser,

where no vapour purification is performed.

4.2.2. Rectifying and stripping sections efficienciesAs mentioned, the purification ratios do not affect the

system COP, but rather influence the components effi-

ciency and the range of possible solutions. Results

obtained from the model developed have shown that the

ammonia refrigerant concentration specified in Table 1

can be achieved with a specific COP and a given reflux

ratio with different column configurations.

Fig. 7 shows the rectifying efficiency versus the reflux

ratio for different purification ratios of the rectifying

section (%Purrect). For large %Purrect values available

solutions are limited by small reflux ratio values. This

accounts for the small liquid flow returned to the column,

which is not enough to enrich the vapour current to the

desired ammonia concentration. Under these conditions

the liquid may come close to equilibrium with the vapour

and consequently the rectifying efficiency in terms of

liquid composition approaches unity.

Fig. 7 reveals that fewer solutions were found for

small purification ratio values, since according to Fig. 8,

the stripping efficiency approaches unity, meaning that the

vapour current is close to equilibrium with the liquid

Fig. 5. Strong solution cooled rectifier efficiency as a function

of the purification ratio of the strong solution cooled rectifier

for different values of the purification ratio of the water cooled

rectifier and with the purification ratio of the rectifying section

being zero.

Fig. 6. Complete condensation system COP as a function of

the internal reflux ratio.



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reflux. For large values of the reflux ratio, the liquid mass

flow returned from the condenser increases significantly,

thus, reflux conditions dominates in the stripping sec-

tion instead of the feed point conditions, which explains

the changes in the slope of the curves of Fig. 8.

Figs. 7 and 8 also show that when decreasing the

reflux ratio, fewer purification ratio combinations are

possible. The minimum reflux ratio is found when onlyone possible solution is obtained and the stripping effi-

ciency and the rectifying efficiency in terms of liquid

composition are unity [8] (extreme point of dashed curve

in Figs. 7 and 8).

5. Conclusions

This paper presents a study on the different distilla-

tion column configurations to carry out the ammonia

vapour purification process considering partial and

complete condensation for a single stage ammonia

water absorption refrigeration system. The analysis is

based on a mathematical model applied to different dis-

tillation column configurations. The model includes a

new parameter named as ammonia purification ratio. The

different column configurations are evaluated and com-pared based on the ammonia purification ratio carried out

in each column component and on the system perfor-

mance obtained. Moreover the mass and heat transfer

efficiencies in each part of the column are calculated in

order to evaluate their design requirements.

In the partial condensation system the liquid reflux is

generated in two rectifiers located at the top of the col-

umn over the rectifying section. Results have shown that

using the water cooled rectifier decreases significantly

the system COP. Therefore this rectifier should be avoi-

ded or reduced as much as possible. Moreover, the rec-

tifying section was shown to have low efficiency in termsof vapour concentration due to the small liquid reflux

from the rectifiers. Hence, the use of the rectifying sec-

tion could be avoided in some applications, given that

an appreciable benefit is not expected, which would also

simplify the column design. The complete condensation

system leads to simpler column designs, since the recti-

fiers are not used. However, the COP values predicted

are lower than for the partial condensation system.

References

[1] Bogart MJP. Pitfalls in ammonia absorption refrigeration.

Int J Refrigeration 1982;4:2038.

[2] Ziegler B, Trepp Ch. Equation of state of ammonia-water

mixtures. Int J Refrigeration 1984;7:1016.

[3] Ferna ndez-Seara J. Disen o, simulacio n, construccio n y

evaluacio n de un prototipo de sistema de refrigeracio n por

absorcio n con NH3-H2O para la produccio n de fro en

barcos de pesca, accionado con la energa te rmica residual

recuperada de los gases de escape. PhD thesis, A rea de

Ma quinas y Motores Te rmicos, E.T.S.I. Industriales y

Minas, Vigo University, Spain, 1999.

[4] Ferna ndez-Seara J, Sieres Atienza J, Va zquez Va zquez M.

Modelo de ca lculo para el estudio y ana lisis del compor-

tamiento de un prototipo de sistema de refrigeracio n porabsorcio n con NH3-H2O ya construido. Recliem2000, La

Habana, Cuba, July 2000.

[5] Henley EJ, Seader JD. Equilibrium-stage separation

operations in chemical engineering. New York: John

Wiley & Sons; 1981.

[6] Sherwood TK, Pigford RL, Wilke CR. Mass transfer.

McGraw-Hill; 1975.

[7] Herold KE, Radermacher R, Klein SA. Absorption chil-

lers and heat pumps. CRC Press; 1996.

[8] Bogart MJP. Ammonia absorption refrigeration in indus-

trial processes. Gulf Publishing Company; 1981.

Fig. 8. Stripping section efficiency as a function of the internal

reflux ratio for different values of the purification ratio of the

rectifying section.

Fig. 7. Rectifying section efficiency as a function of the internal

reflux ratio for different values of the purification ratio of the

rectifying section.


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11 destilação amonia (1)