Applying Vacuum Membrane Distillation to

Regenerate the Working Solutions of Absorption

Air-Conditioning Systems for Sustainability and

Energy Conservation

Tsair-Wang Chung Department of Chemical Engineering, R&D Center for Membrane Technology, Chung Yuan Christian University,

Chungli, Taoyuan 32023, Taiwan

Email: twchung@cycu.edu.tw

Shih-Hong Hsu and Amna Citra Farhani R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan 32023, Taiwan

Email: {brads1984, tinachencycu}@gmail.com

Abstract—The methods of reducing negative human impact

are environmentally-friendly chemical engineering,

environmental resources management and environmental

protection. Compared to the vapor absorption refrigeration

systems (VARS), the traditional vapor compression

refrigeration systems (VCRS) has negative effects on the

environment because of the refrigerant used in the system

and higher power consumption. Vacuum membrane

distillation (VMD) is an interesting process, which is gaining

attention in re-concentrating aqueous working solutions of

VARS in lower temperature for further reduce of the power

consumption. In this study, VMD experiment for VARS

application was conducted using response surface

methodology (RSM) with three parameters: initial feed

concentration (X1), feed inlet temperature (X2) and feed flow

rate (X3). Within the range of the selected operating

conditions, the optimal initial feed concentration, feed inlet

temperature and feed flow rate were found to be 36wt%,

70°C and 1.5L/min, respectively. The results showed that

the feed inlet temperature is the most significant factor in

this experiment, with a positive effect on the response.

Index Terms—membrane distillation, working solution,

regeneration, absorption refrigeration, experimental design

methodology, energy conservation

I. INTRODUCTION

Absorption refrigeration has been introduced since

1859 by the work of Ferdinand Carre [1]. This

technology flourished but then dimmed out, due to the

advanced development of vapor compression

refrigeration system (VCRS). However, it was found that

VCRS has a negative effect on the environment because

of the refrigerants used in the system. Therefore, the

application of vapor absorption refrigeration system

(VARS) is now taking back its place.

Manuscript received December 6, 2014; revised May 4, 2015.

The absorption cycle has its driving force from heat

energy. By using mixture of refrigerant and absorbent,

heat will be released to absorb refrigerant and will be

absorbed to separate refrigerant from the absorbent. The

most common absorbent – refrigerant mixtures are LiBr –

water and ammonia – water. Other potential pairs include

LiCl – water and CaCl2 – water. Since water is used as

the refrigerant and the application is always above 0°C,

the main application is at air-conditioning.

Figure 1. Basic absorption refrigeration system.

As shown in Fig. 1, the basic VARS consists of a

condenser, an evaporator, a generator, an absorber, a

pump, and two expansion valves. As for LiCl – water pair,

after the water had captured heat and evaporated, aqueous

LiCl solution will absorb water vapor causing the

concentration of working solution to decrease [2]. At a

certain concentration, LiCl cannot absorb water anymore,

and therefore, calls for a need to regenerate the LiCl

solution [3]. In the regenerator, as heat is applied, the

absorbent will release water in the vapor phase. The

water vapor later moves to the condenser and provides

International Journal of Electrical Energy, Vol. 3, No. 2, June 2015

©2015 International Journal of Electrical Energy 99doi: 10.12720/ijoee.3.2.99-104

the water to the evaporator. In the next cycle, the LiCl

solution returns to its working concentration, and is

pumped back to the absorber.

Using heat as the driving force makes VARS

performance (Coefficient of Performance, COP) less

compared to VCRS. The COP concept is based on the

first law of thermodynamics; but in order to have a better

investigation on VARS, the second law of

thermodynamics should be taken into account, so that the

inefficiency of the system will be detectable. This

concept, better known as energy concept, made point that

the regenerator is the most inefficient component [4].

High temperature difference between the regenerator and

the heat source is the major reason.

A separation process, which offers lower temperature

difference, can be proposed to address this matter.

Membrane distillation [5]-[7] is a separation process,

which uses thermal and pressure as its driving forces.

One mode of operation is by applying vacuum in the

permeate side, making the heat transfer negligible. The

vacuum membrane distillation (VMD) is also only

permeating water vapor. Therefore, the application of

VMD on VARS seems possible [8].

Conventional regenerator used in VARS is usually

bulky and requires a lot of space. VMD usage as

regenerator will overcome this problem. VMD system

made it possible to use low temperature heat source. It is

good the considerations of sustainability and energy

conservation. However, coupling VMD in VARS has not

been studied much. Hence, finding its system behavior is

important and should be compared to the traditional

regenerator which was already been established.

Coupling VARS with VMD will raise some questions

and problems, since each process has its own limitations

and considerations, such as pressure and temperature.

Some of the issues will be on operating parameters,

energy requirement and application aspect. To answer

them, the objectives of this study will be focused on 1)

discussing which parameter (temperature, pressure,

solution concentration, solution flow rate, etc.) is crucial

in the process; 2) developing optimal set of parameters;

and 3) predicting the system behavior.

II. EXPERIMENTAL DESIGN AND ANALYSIS METHOD

A. Materials and VMD Setup

In this study, a commercial hollow fiber (Microza)

polyvinylidene fluoride (PVDF) membrane was used in a

cross-flow module (as shown in Fig. 2) for doing vacuum

membrane distillation (VMD) to reconcentrate the

working solution in VARS.

Figure 2. Membrane module used in this research.

TABLE I. PROPERTIES OF THE MEMBRANE USED IN THIS WORK

Membrane material Polyvinylidenefluoride (PVDF)

Membrane configurations Hollow fiber

Number of fiber in module 21

Nominal pore size (μm) 0.2

Fiber ID/OD (mm) 1.4/2.2

Membrane area (m2) 0.02

Manufacturer Asahi – Kasei

The hollow fiber has a large area for regeneration

while remaining compact. PVDF membrane is

hydrophobic and so it is suitable for membrane

distillation (MD) applications. The membrane properties

are shown in Table I. Reconcentration of desiccant

solution is necessary for the continuous operation of all

vapor absorption refrigeration systems. One of the

alternative methods of VMD, which offers lower

temperature difference between the regenerator and the

heat source, was introduced in this study.

A schematic diagram of the VMD experimental set up

used is shown in Fig. 3. During the experiment, LiCl

solution from the feed bath was pumped through the tube

lumen, and vacuum was applied on the shell side in the

hollow fiber membrane module. A flowmeter (rotameter;

accuracy ±0.1L/min) was placed before the solution

entrance of the membrane module to measure the feed

flowrate. Temperature and pressure of the feed stream

entering and leaving the membrane module were also

measured. The K type thermocouple with accuracy of

±0.35°C and the mechanical vacuum pressure gauge with

accuracy 0.1% of span were used for the temperature and

pressure records, respectively. Since the membrane

module is designed as a cross-flow type, the permeate

(water vapor) pass through the hollow fiber membrane

tube to vacuum side and the retentate (concentrated LiCl

solution) flowed back to the feed bath, while the

permeate was condensed and sent to the collector (R)

using vacuum pump (VP). The vacuum on the shell side

was created using vacuum pump GAST DAA V 503 EB.

The temperature of feed bath was kept constant using

Thermostat Maxthermo MC-2438. Water-cooled

condenser was used as cold trap to recover the permeate.

Figure 3. Schematic diagram of system.

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B. Experimental Design for the VMD

As applied in the study, the experimental design was

carried out considering three factors, namely, the initial

feed concentration (LiCl solution), feed inlet temperature

and the feed flow rate. A Box-Behnken factorial model

was employed in this study requiring 15 experimental

runs. Parameters chosen were the initial feed

concentration, X1, in the range of 36%~40%; inlet

temperature, X2, in the range of 60°C~70°C; feed flow

rate, X3, in the range of 1.1~1.9L/min. Permeate flux Y

was chosen to be the target or response variable. The

design matrix employed, together with the results is given

in Table II. The experiments were carried out in

randomized order. Each experimental run was performed

for 2 hours and the VMD permeate flux was collected

after every run. The vacuum pressure for each experiment

was maintained at the maximum level, with values

between –715 to –730mbar.

TABLE II. THE BOX–BEHNKEN DESIGN MATRIX OF THE THREE

VARIABLES AND THE RESPONSE

Run

Pattern

Factors permeate flux

(g/m2h)

X1 (%)

X2 (°C)

X3 (L/min) Exp. Pred.

1 --0 36 60 1.5 91.76 89.89

2 -0- 36 65 1.1 123.75 125.09

3 -0+ 36 65 1.9 115.47 119.71

4 -+0 36 70 1.5 180.83 177.11

5 0-- 38 60 1.1 58.15 58.67

6 0-+ 38 60 1.9 68.37 65.99

7 000 38 65 1.5 113.44 112.32

8 000 38 65 1.5 114.45 112.32

9 000 38 65 1.5 109.07 112.32

10 0+- 38 70 1.1 169.99 172.37

11 0++ 38 70 1.9 162.22 161.70

12 +-0 40 60 1.5 37.04 40.76

13 +0- 40 65 1.1 93.99 89.74

14 +0+ 40 65 1.9 93.13 91.77

15 ++0 40 70 1.5 161.08 162.95

The permeate flux was determined gravimetrically by

weighting the distillate collected in the permeate tank for

a predetermined time. For each run, the permeation flux

value is calculated with the following equation.

(1)

where J is the permeate flux (kg/m2s), W is the quantity

of water (kg), S is the membrane area (m2), and t is time

(s). This permeation flux data and its running parameters

are then used for RSM analysis.

Experimental design and analysis of data were

performed using a commercial statistical package. A

mathematical model, describing the relationship between

the predicted response variable (permeate flux) in second-

order equation, has been developed and was applied in

this study. The experimental data were analyzed by the

response surface regression procedure to fit a second-

order polynomial equation. All data regression and

analysis were performed with the analysis of variances

(ANOVA). The discussions of the effect of the variables

and the interaction between the variables were using the

traditional statistical analysis of Student t-test. The

accuracy and general ability of the mathematical model,

describing the relationship between the response variable

and operating variables in a second-order polynomial

equation were evaluated using the coefficient of

determination (R2) and the least square method was

applied to do the regression of the above mathematical

model.

III. RESULTS AND DISCUSSION

A. Response Surface Methodology for VMD Experiments

Response surface methodology (RSM) was employed

in this study for maximizing and optimizing the VMD

process. The procedure consists of the following steps: a)

designing and conducting a series of experiment to obtain

the process response, b) developing mathematical model

of first or second order response surface with best fittings,

c) finding the optimal set of process variables that

guarantee an optimum value of the selected response, and

d) studying and representing the main and interaction

effects of the process variables on the response. Table III

shows the responses for each run. The range of

concentration was chosen to resemble the applied

solution in practical operation. The temperature range

was chosen so that the system could be applied with low

energy input.

TABLE III. ANALYSIS OF THE VARIANCES

Term Estimate Std Error t ratio Prob<0.05

Intercept 112.3175 2.615795 42.94 <.0001*

X1 -15.82156 1.601841 -9.88 0.0002*

X2 52.35031 1.601841 32.68 <.0001*

X3 -0.836656 1.601841 -0.52 0.6238

X1*X2 8.741875 2.265345 3.86 0.0119*

X1*X3 1.85375 2.265345 0.82 0.4504

X2*X3 -4.499563 2.265345 -1.99 0.1037

X1*X1 -1.366906 2.357846 -0.58 0.5864

X2*X2 6.7305312 2.357846 2.85 0.0356*

X3*X3 -4.365094 2.357846 -1.85 0.1233

Figure 4. Plot of actual results and predicted values.

Based on the results, a response surface model was

developed for permeate flux. Fig. 4 gives the plot of

experimental and predicted values. It uses standard least

square method to fit experimental and predicted ones.

Table III shows the summary of fit and analysis of

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variance. From the predicted equation, the initial feed

temperature (X2), has a strong positive effect on the

response and is also the most significant parameter. X1 is

the second most significant parameter, with a negative

effect on the response. The positive effect of the

temperature has been shown in most VMD experiments

[9]-[11], which is in accordance with mass transfer

phenomenon. The mass transfer that occurs in membrane

processes highly depends on temperature. Having a

higher temperature means having less temperature

polarization, enhancing the heat transfer process and

which in turn, will increase the permeate flux. The

significance of the regression coefficient of the models

written as function of the coded variables was also

subjected to statistical Student t-test. Table III shows the

t-test for the experiment. The test result shows that the

feed initial temperature is the most significant factor.

Feed temperature has a major influence on the water

vapor partial pressure and thus, the transmembrane

pressure difference. Increasing the temperature makes the

vapor pressure higher, leading to an increase of the mass

transfer driving force; despite the fact that the vacuum

pressure was set constant. In addition, the rise in

temperature reduces liquid viscosity. The improved

fluidity of the feed enhances turbulent movement. The

lower concentration polarization thus leads to a decrease

in mass transfer resistance. Furthermore, the interaction

effect between the feed initial concentration and

temperature is significant. The feed flow however, does

not affect the permeate flux in a significant way. In

relation to this, a lower flow rate will be favorable to

reduce the power consumption of the pump.

The response surface method for the given factors

obtaining the response is a saddle point one, which meant

for the range given, there is no optimal value. This saddle

point parameter is outside the data range which set is

concentration of 24.41%, feed temperature at 65.03°C

and feed flow 0.88L/min while its predicted permeate

flux value is 166.9g/m2h. The confirmation for this result

could not be conducted for the system scheme and

support the feed flow rate in the above set of parameters.

However, this saddle point result will have no real

meaning since the concentration is too low to be re-

concentrated. However, based on the experiment results,

the set of parameter of 36% of initial concentration, 70°C

of feed temperature and 1.5L/min of feed flow has the

best response. For practical purposes, this set of

parameter is sufficient to apply.

Figure 5. Prediction profile of experiment results.

To have a better understanding on the effect of each

parameter and the interaction between them on the

response, a prediction profile of the experiment is show

in Fig. 5. As can be seen, an increase in the feed inlet

temperature shows an increase in the permeate flux. An

increase in the initial feed concentration leads to a

decrease in the response, while the feed flow causes no

significant change in the flux.

The mutual effects of factors and their influence on the

response are shown in two-dimensional plots. Fig. 6

shows the influence of initial feed concentration and feed

inlet temperature on the permeate flux. As can be seen,

the increase in initial feed concentration leads to a

decrease in the permeate flux while the increase in feed

inlet temperature leads to an increase in the response. At

high concentration, the effect of temperature is stronger

than at low concentration. The graph also confirms the

strong effect of the feed inlet temperature over the initial

feed concentration.

Figure 6. Contour lines showing the response as function of initial feed concentration and feed inlet temperature for feed flow rate at 1.5L/min.

Figure 7. Contour lines showing the response as function of initial feed

concentration and feed flow rate for feed inlet temperature at 65°C.

Fig. 7 shows the effects of feed flow rate and initial

feed concentration on the VMD permeate flux for a fixed

International Journal of Electrical Energy, Vol. 3, No. 2, June 2015

©2015 International Journal of Electrical Energy 102

value of feed inlet temperature of 65°C. As can be

observed, for all feed flow rate values, there is a decrease

in the permeate flux with an increase in initial feed

concentration; whereas the increase of feed flow rate

value increases the permeate flux until its maximum, then

decreases. The figure also implies that the concentration

has a stronger effect on the permeate flux than that of the

feed flux.

Fig. 8 shows the effects of feed inlet temperature and

feed flow rate on the VMD permeate flux for a fixed

value of initial feed concentration of 38%. From the

figure, the increase of feed flow rate increases the

permeate flux up to the maximum. The increase in

temperature also increases the permeate flux. The effect

of temperature is more profound than that of feed flow

rate.

Figure 8. Contour lines showing the response as function of feed inlet temperature and feed flow rate for initial feed concentration at 38%.

The above-mentioned discussions are focused on the

VMD process where the permeate is more important. For

regeneration, the retentate of the process is more

important, since it is sent back to the absorber. Based on

the experiment, the VMD could reconcentrate the

solution with 1.3% rejection rate. This low result might

come from the time taken to perform the experiment. The

two-hour experiment has a considerable permeate flux

and maintains the membrane condition. Gryta and

Barancewicz (2010) conducted an experiment on PVDF

membrane material for NaCl experiment and found a flux

decrease or worse performance after 70h of usage, while

membrane drying could recover the membrane

performance. In this experiment, five liter of LiCl

solution was used to improve the rejection rate. To

increase the regeneration rate of the system, membrane

with larger contact area could be applied. It is difficult to

apply the proposed system on a continuous mode since

the achieved solution concentration was still dilute

enough to be sent back to the absorber. Putting the

permeate and feed in a separate chamber could be

proposed in order to have more concentrated solution. By

choosing the maximum set of parameter suggested by the

RSM method, the system will also have the maximum

rejection rate. High permeate flux in this set of parameter

will also indicate high refrigerant being sent to the

evaporator.

B. VMD as Regenerator of VARS

In this experiment, five liter of LiCl solution was used

to improve the rejection rate. To increase the regeneration

rate of the system, membrane with larger contact area

could be applied. It is difficult to apply the proposed

system on a continuous mode since the achieved solution

concentration was still dilute enough to be sent back to

the absorber. Putting the permeate and feed in a separate

chamber could be proposed in order to have more

concentrated solution. By choosing the maximum set of

parameter suggested by the RSM method, the system will

also have the maximum rejection rate. High permeate

flux in this set of parameter will also indicate high

refrigerant being sent to the evaporator.

IV. CONCLUSION

Vacuum membrane distillation (VMD) is an

interesting process which is gaining attention in

reconcentrating aqueous LiCl solution. In this research,

VMD experiment for VARS application has been

conducted using RSM with three parameters. Feed inlet

temperature is the most significant factor in this

experiment and it has a positive effect on the response.

Concentration is the second significant factor in the

process, with a negative effect on the response. Feed flow

rate shows different values in the experiment and

calculations, but it has the same positive effect on the

permeate flux as the temperature. The statistical result of

the experiment shows a good correlation, with an R2 of

0.99. The optimum operating parameter searching was a

saddle point. On the basis of the experimental data, the

set of parameter which will give the best permeate flux is

found to be at 36% of initial concentration, 70°C of feed

temperature and 1.5 L/min of feed flow.

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Tsair-Wang Chung was born in 1963 at Taipei City, Taiwan. He has received the M.S. and

Ph.D. degrees in Chemical Engineering from

University of Missouri-Columbia (USA) in 1991 and 1993, respectively. He was a

Research Scientist in Industrial Technology Research Institute in 1993. He started his

teaching carrier in Chung Yuan Christian

University (Taiwan) in 1995. Currently, he is the Distinguished Professor in the university

and is also the Chief Scientific Advisor of National Federation of Rural Cooperatives (Induk-KUD) in Indonesia. He has authored or co-author

more than 150 research papers in refereed journals and conference

proceedings. His current research interests include sustainable energy, bioresources, mass transfer, and separation technologies.

Shih-Hong Hsu has received the M.S. and Ph.D. degrees in Chemical Engineering from

Chung Yuan Christian University (Taiwan) in

2009 and 2012, respectively. From 2012 to 2015 he worked as a Research Scientist in the

R&D Center for Membrane Technology in Chung Yuan Christian University. His current

research interests include biomass energy,

mass transfer, and membrane technology.

Amna Citra Farhani has received the B.S.

degree in Agricultural Technology from Bogor Agricultural University (Indonesia) in 2007

and received the M.S degree in Chemical Engineering in Chung Yuan Christian

University (Taiwan) in 2012. She was the

graduate student and research assistant in the R&D Center for Membrane Technology in

Chung Yuan Christian University. Her current research interests include membrane

distillation, bioresources, and mass transfer.

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