Exergy Analysis of Membrane Utilization for Solution Separation in LiBr-H2O Absorbtion Refrigeration System

EXERGY ANALYSIS OF MEMBRANE UTILIZATION FOR
SOLUTION SEPARATION IN LiBr-H2O ABSORBTION
REFRIGERATION SYSTEM

BAYU RUDIYANTO

THE GRADUATE SCHOOL
BOGOR AGRICUTLURAL UNIVERSITY (IPB)
BOGOR
2013

STATEMENT OF RESEARCH ORIGINALITY
Hereby, I state that the dissertation entitled “Exergy Analysis in Membrane
Utilization for Solution Separation in LiBr-H2O Absorbtion Refrigeration System”
is my own work, which has never previously been published in any form and any
universities. All of incorporated originated from other published as well as
unpublished papers are stated clearly in the text as well as in the reference.
Bogor, November 2013
Bayu Rudiyanto
NIM F16409006

SUMMARY
BAYU RUDIYANTO. Exergy Analysis in Membrane Utilization for Solution
Separation in LiBr-H2O Absorbtion Refrigeration System. Supervised by
ARMANSYAH H TAMBUNAN, TSAIR-WANG CHUNG, LEOPOLD O
NELWAN and AEP SAEPUL UYUN.
Absorbtion refrigeration system provides prominent advantages in tems of
environmental impact and energy consumption. Absorbtion refrigeration system
uses natural refrigerant, such as water, which gives no harm to the environment.
In terms of energy consumption, it requires heat to regenerate refrigerant from its
absorber and provide the refrigeration effect. This form of energy is preferred
since heat can be supplied from various sources, including waste heat and
renewable energy, and thermodynamically categorized as low quality energy.
However, effectiveness of refrigerant regeneration could only be obtained if the
temperature is high enough. In this study, the heat utilization in the regeneration
process will be eliminated by introducing membranes for separating the
refrigerant from the absorber. Specifically, the objectives of this research are
to :1) to study the absorbtion mechanism of water vapor by LiBr-H2O solution in
a controlled condition, 2) to study the regeneration process of LiBr-H2O using
reverse osmosis (RO) membrane and vacuum membrane distillation (VMD) when
applied in LiBr-H2O absorbtion refrigeration system, 3) to perform the energy and

exergy analysis in LiBr-H2O absorbtion refrigeration system using RO membrane.
The membrane was used as a tool to separate the refrigerant from absorbent
which performs as regeneration process in conventional absorbtion system. As
separation component, RO membrane employs pressure to oppose the osmotic
pressure and pass certain component through the pores. While, VMD employs
vacuum pressure and thermal, so that the substance to be transported through the
membrane is in form of vapor. The result shows that absorbtion rate of water
vapor by LiBr-H2O solution was influenced by solution concentration,
temperature, humidity and pressure. The highest absorbtion rate was 0.031
g/minute which was occurred at temperature 40°C, RH 70% and solution
concentration 60%. Prediction model was developed to calculate the absorbtion
parameter inspite of P-T-X diagram.The highest water vapor absorbtion in
absorber using spiral wound module-based RO membrane occurred at initial
concentration of 30%, wich is the highest concentration used in the experiment.
Increasing of operation pressure during separation process will increase
permeation flux but decrease rejection factor.Unexpectedly, permeate of the RO
membrane still contain LiBr at concentration 7w/w %-9w/w%, which affect the
achievable cooling temperature and cooling capacity. Optimization of
regeneration using VMD was conducted with response surface methodology
(RSM) and the result show that highest permeate flux (0.93 kg.m-2.s-1) can be

achived at initial concentration of 47.5%, temperature of 80°C and flow rate of 1.9
L.min-1.COP obtained in absorbtion refrigeration system using RO membran with
initial concentration 20% was in the range of 1.147-1.171. Exergy analysis shows
that increasing pressure of RO membrane at concentration 20% increased the

irreversibity.On the other hand, exergy analysis on VMD shows that higher
concentration and temperature will increased irreversibility.
Keywords : refrigeration, absorbtion, reverse osmosis, vacuum membrane
distillation, energy, exergy

RINGKASAN
BAYU RUDIYANTO. Kajian Exergy Pemanfaatan Membran untuk Pemisahan
Larutan pada Sistem Pendingin Absorpsi. Dibimbing oleh ARMANSYAH H.
TAMBUNAN, TSAIR-WANG CHUNG, LEOPOLD O. NELWAN dan AEP
SAEPUL UYUN.
Sistem pendinginan absorpsi merupakan sistem pendingin yang ramah
lingkungan dan menguntungkan dari segi konsumsi energi. Sistem pendingin
absorpsi ini menggunakan refrigerant alami, seperti air, yang tidak berbahaya
terhadap lingkungan. Dalam hal konsumsi energi, proses pemisahan refrigeran
dari absorber membutuhkan energi panas. Bentuk energi panas lebih disukai

karena energi panas biasanya merupakan limbah dari proses konversi energi,
sehingga di dalam termodinamika panas dikategorikan sebagai energi kualitas
rendah, dan dapat diambil dari berbagai sumber, termasuk energi terbarukan.
Meskipun demikian, proses regenerasi dapat berlangsung dengan efektip jika
suhu digenerator cukup tinggi. Dalam studi ini, pemanfaatan panas untuk proses
regenerasi akan dihilangkan dan digantikan dengan teknologi membran.Secara
khusus, tujuan penelitianini adalah (1) Mempelajari proses penyerapan uap air
oleh larutan LiBr-H2O pada kondisi terkontrol(2) Mempelajari karakteristik
proses pemisahan refrigerant dari larutan LiBr-H2O menggunakan membran
reverse osmosis(RO) dan membrane vakum distilasi (VMD) pada proses
regenerasi serta aplikasi pada sistem pendingin absorpsi. (3) melakukan kajian
energi dan eksergi pada sistem pendingin absorpsi dengan menggunakan
membran RO dan VMD.
Dalam melakukan proses pemisahan, membran RO memerlukan tekanan
untuk melawan tekanan osmotik dan melewatkan komponen tertentu suatu larutan
melalui pori-porinya. Sementara itu, VMD menggunakan tekanan vakum dan
panas, sehingga hanya uap air yang dipindahkan melalui membran. Hasil
penelitian pada proses penyerapan LiBr-H2O didapatkan bahwa laju penyerapan
absorbat oleh absorban dipengaruhi oleh konsentrasi larutan, suhu ruang,
kelembaban dan tekanan. Laju penyerapan tertinggi dihasilkan pada kondisi suhu

40°C, RH 70% dan konsentrasi larutan 60% yaitu 0.031 gram/menit.Model
pendugaan telah dikembangkan untuk menentukan parameter proses absorpsi
menggantikan penggunaan diagram P-T-X.Penyerapan uap air tertinggi di
absorber pada proses pemisahan larutan LiBr-H2O menggunakan membran RO
terjadi pada konsentrasi awal 30%, yang merupakan konsentrasi tertinggi pada
penelitian ini. Peningkatan tekanan operasi pada proses pemisahan LiBr-H2O
akan meningkatkan fluks permeate tetapi akan menurunkan nilai rejeksi. Akan
tetapi, permeate hasil pemisahan menggunakan membran RO masih mengandung
garam LiBr sebesar 7(w/w %) -9(w/w %), dimana akan berpengaruh pada
temperatur pendinginan dan
kapasitas pendinginan. Optimisasi proses
regenerasi menggunakan VMD dengan response surface methodology (RSM)
menunjukkan bahwa permeate fluks tertinggi yaitu 0.93 kg.m-2.s-1didapatkan pada
konsentrasi awal 47.5%, temperatur 80°C dan laju alir 1.9 L.min-1. COP sistem
pendingin absorpsi menggunakan membran RO untuk konsentrasi awal 20%
berada pada kisaran nilai 1.147-1.171. Analisis exergy pada proses pemisahan
larutan dengan membran RO untuk konsentrasi awal 20%, menunjukkan bahwa

peningkatan tekanan akan meningkatkan irreversibilitas. Sedangkan analisis
exergy pada proses pemisahan dengan VMD menunjukkan bahwa peningkatan

konsentrasi dan temperatur akan meningkatkan irreversibilitas.
Kata kunci: refrigerasi, absorpsi, reverse osmosis, vacuum membrane distillation,
energi, eksergi

Copyright 2013 by IPB
All rights reserved
No part or this entire dissertation may be excerpted without inclusion or
mentioning the sources. Excerption only for research and education use, writing
for scientific papers, reporting, critical writing or reviewing of a problem.
Excerption doesn’t inflict a financial loss in the proper interest of IPB.
No part or all of this dissertation may be transmitted and reproduced in any forms
without a written permission from IPB.

EXERGY ANALYSIS OF MEMBRANE UTILIZATION
FOR SOLUTION SEPARATION IN LiBr-H2O
ABSORPTION REFRIGERATION SYSTEM

BAYU RUDIYANTO

THE GRADUATE SCHOOL

BOGOR AGRICUTLURAL UNIVERSITY (IPB)
BOGOR
2013

The external assessor for
close examination are :

The external assessor for
open examination are :

Dr.Ir. Irzaman, M.Sc
Dr.Ir.Lilik Pujiantoro, M.Sc

Dr. Ir. Y. Aris Purwanto, M.Agr
Dr. Ir. Lamhot P. Manalu, M.Si

Title of Dissertation

Name
Student Number

: Exergy Analysis of Membrane Utilization for Solution
Separation in LiBr-H2O Absorbtion Refrigeration
System
: Bayu Rudiyanto
: F164090061

Approved by,
Advisory Committee

Prof.Dr.Ir.Armansyah H.Tambunan
Chairman

Prof.Dr.Tsair-Wang Chung Dr.Leopold O.Nelwan, S.TP
Member
Member

Dr.Aep Saepul Uyun, S.TP,M.Sc
Member

Acknowledged by,

Chairman ofAgriculturalEngineeringSciences
Graduate Study Program

Dr.Ir.Wawan Hermawan, MS

Date of Examination:1 Nov 2013

Dean of Graduate School

Dr.Ir.Dahrul Syah, M.Sc, Agr

Date of Graduation:

Title of Dissertation
Name
NIM

: Exergy Analysis of Membrane Utilization for Solution

Separation in LiBr-H2 0 Absorption Refrigeration System
: Bayu Rudiyanto
: Fi64090061

Approved by,
Ad visory Committee

ｾｩｲＭｗ＠

Prof.Dr.Ir.Armansyah H.Tambunan
Chairman

Dr.Leopol O.Nelwan, S.TP
Member

Member

Acknowledged by,

Head of Study Program in

Agricultural Engineering Sciences

Date of Examination : 1 November 2013

Date of Graduation:

Ｎ ｾ＠

1 JAN 2U14

PREFACE
Gratitude for His glory and greatness, author prays to Allah SWT for His
grace and bless that let author to finish this draft dissertation.
For the completion of this draft dissertation author would like to express
my most profound gratitude to Prof.Dr.Ir.Armansyah H.Tambunan as the
chairman of the advisory commitee and all members of advisory commitee; Prof.
Tsair-Wang Chung, PhD, Dr. Leopold O. Nelwan, S.TP, M.Si and Dr.Aep Saepul
Uyun, S.TP, M.Sc for all valuable assistance, support and their tireless and
patient counsel. The author also say many thanks to Dr.Ir. Irzaman, M.Si and
Dr.Ir. Lilik Pujantoro, M.Agr over his willingness as the external assesor for
close examination, and Dr.Ir. Y. Aris Purwanto, M.Agr and Dr.Ir. Lamhot P.
Manalu, M.Si as the external assesor for open examination. Thanks the author
gave to Rector of Bogor Agricultural University (IPB), the Dean of the Graduate
School of IPB, the Chairman of Agricultural Engineering Graduate School
Program, and all the lecturer and staff over all the facilities and assistance in
studies and his research. The author also say thanks to the director of Politeknik
Negeri Jember (POLIJE), the Chairman of Agricultural Engineering Departement
of Politeknik Negeri Jember (POLIJE), and all lecturer of his friends in
Agricultural Engineering Departement of Politeknik Negeri Jember (POLIJE).
Then, all of his (author) friends in Heat and Mass Transfer of Laboratory
(Teti E. Nababan, Johanes FF Sipangkar, Dr. Kiman Siregar, Rosmeika, Agus
Susanto Ginting, Christian Solani, Angga Defrian, Gani, Ajen, Wahyudin, M.Sc,
Agustino Aritonang, Monalysa Harianca, Tiara Etika, Amalia, Deny F.
Situmorang and Ismi Idris), Agricultural Engineering Science Study Program
(2009, 2010, 2011), and the Graduate School of Bogor Agricultural University
(IPB).
Finally, the authorwould like to dedicate this research work to his family,
his wife (Indah Yuli Astuti), his daughter (Kartika Aulia Tsabitah, Sita Hanania
Dzakyah), specially for his mother (Hj. Siti Sumarsih) and his father (H.M.
Sunarno), his brothers (Gunawan Riyanto, SP and Dr. Bambang Piluharto) for
their love, continous encouragement and constant support in his life. The end of
the author hope my explain in what has been a writer of dissertations this could
be beneficial for writer and in need of them.

Bogor, November 2013
Bayu Rudiyanto

TABLE OF CONTENT
LIST OF TABLE
vi
LIST OF FIGURE
vii
LIST APPENDIX
viii
1
INTRODUCTION
1
Background
1
Problem Formulation
3
Research Objectives
3
Novelty
3
Sistematical Framework of the Research
4
Research Scope
5
Research Benefit
5
2
A STUDY ON THE WATER VAPOR ABSORBTION RATE BY LiBr
SOLUTION AS ABSORBENT IN ABSORBTION REFRIGERATION
SYSTEM
7
Introduction
7
Literature Review
9
Method
11
Result and Discussion
13
Conclusion
26
3
REGENERATION PROCESS BY REVERSE OSMOSIS (RO) AND
VACUUM MEMBRANE DISTILLATION (VMD)
27
Introduction
27
Literature Review
28
Method
33
Result and Discussion
37
Conclusion
47
4
EXERGY ANALYSIS ON LiBr-H2O ABSORBTION REFRIGERATION
SYSTEM BASED USING MEMBRANE FOR REGENERATION
PROCESS
49
Introduction
49
Literature Review
49
Method
57
Result and Discussion
58
Conclusion
66
5
GENERAL DISCUSSION
67
6
CONCLUSION AND SUGESSTION
71
Conclusion
71
Suggestion
71
Acknowledgment
71
REFERENCE
72
BIOGRAPY
89

LIST OF TABLE
Table2-1.
Table 2-2.
Table 2-3
Table 2-4.
Table 2-5.

Research procedure at temperature 40°C and 45°C
13
The effect of relative humidity onwater vapor absorbtion
15
The effect of water vapor pressure on absorbtion rate at 40oC
16
The effect of water vapor pressure on absorbtion rate at 45oC
16
Linear equation of each LiBr concentration
at T=40oC and RH=70%
17
Table 2-6. Linear equation of LiBr concentration
at T=45⁰C and RH=70%
19
Table 2-7. Equilibrium concentration at temperature 40oC
21
o
Table 2-8. Equilibrium concentration at temperature 45 C
21
Table 2-9. The result of Qecalculation and Qe Model BET at 40oC
21
Table 2-10. The result of Qecalculation and Qe Model BET at 45oC
21
Table 2-11. The result of Qecalculation and Qe Model Langmuir at 40oC
23
Table 2-12. The result of Qecalculation and Qe Model Langmuir at 45oC
23
Table 2-13. The result of Qecalculation and Qe Model Freundlich at 40oC
24
Table 2-14. The result of Qecalculation and Qe Model Freundlich at 45oC
24
o
Table 2-15. Percentage error at temperature 40 C
26
Table 2-16. Percentage error at temperature 45oC
26
Table 3-1. Summary of membrane distillation configuration
31
Table 3-2. Specification material constructions of membrane module
35
Table 3-3. Part number and technical specification of membrane module
35
Table 3-4. Coded and actual designed variables used
for experimental design
37
Table 3-5. Pressure difference in absorber and evaporator
41
Table 3-6. Box-Behnken design and experimental VMD
43
Table 3-7. Analysis of variance (ANOVA) of the RSM model
corresponding to the response: performance index (Y)
44
Table 3-8. Parameter estimates and t-test results
45
Table 4.1. Equation constant by Kim and infante Ferreira
56
Table 4.2. Net balance of energy flow rate in absorbtion refrigeration
system using RO membran at concentration 20%
59
Table 4.3. The energy required for LiBr-H2O separation process using
vacuum membrane distillation (VMD)
60
Table 4.4. Exergy in absorbtion refrigeration system using membrane
at concentration 20% and P=440 kPa
61
Table 4.5. Exergy in absorbtion refrigeration system using membrane
at concentration 20% and P=460 kPa
62
Table 4.6. Exergy in absorbtion refrigeration system using membrane
at concentration 20% and P=520 kPa
62
Table 4.7. Irreversibility in absorbtion refrigeration system using membrane
at concentration 20%
62
Table 4.8. Exergy efficiency in absorbtion refrigeration system using membrane
at concentration 20%
62
Table 4.9. Exergy in absorbtion refrigeration system using VMD
(Experiment 1: C=47.5%; T=60°C; flow rate=1.9 g/s
63

Table 4.10. Exergy in absorbtion refrigeration system using VMD
(Experiment 2: C=50%; T=60°C; flow rate=1.9 g/s
Table 4.11. Exergy in absorbtion refrigeration system using VMD
(Experiment 3: C=50%; T=80°C; flow rate=1.9 g/s
Table 4.12. Exergy in absorbtion refrigeration system using VMD
(Experiment 4: C=47.5%; T=60°C; flow rate=1.1 g/s
Table 4.13. Exergy in absorbtion refrigeration system using VMD
(Experiment 5: C=45%; T=70°C; flow rate=1.9 g/s
Table 4.14. Exergy in absorbtion refrigeration system using VMD
(Experiment 6: C=45%; T=60°C; flow rate=1.5 g/s
Table 4.15. Exergy in absorbtion refrigeration system using VMD
(Experiment 2: C=50; T=70°C; flow rate=1.1 g/s
Table 5.1.Concentration change during separation process using
RO membrane

64
64
65
65
65
66
68

LIST OF FIGURE
Figure 1.1 Sistematical framework of the research
Figure 2.1 Absorbtion process
Figure 2.2 A continous absorbtions refrigeration cycle
Figure 2-3. Functional scheme and climate chamber room
Figure 2-4. Water vapor absorbtion at each LiBr solution concentration
Figure 2-5. The effect of relative humidity to water vapor absorbtion
Figure 2-6. The effect of water vapor pressure toabsorbtion rate
at 40°C and 45°C
Figure 2.7 Graph of concentration decrease on LiBr-H2O solution
toward time at temperature 40°C and RH 70%
Figure 2-8. Relationship between gradient and concentration at T=40oC
Figure 2.9 Graph of concentration decrease on LiBr-H2O solution
toward time at temperature 45°C and RH 70%
Figure 2-10. Relationship between gradient and concentrationat T=45oC
Figure 2-11. Comparison between QeCalculation and Qe Model BET at 40oC
Figure 2-12. Comparison between QeCalculation and Qe Model BET at 45oC
Figure 2-13. Interaction between QeCalculation and Qe Model Langmuir at 40oC
Figure 2-14. Interaction between QeCalculation and Qe Model Langmuir at 45oC
Figure 2-15. Interaction between Qecalculation and Qe Model Freundlich at 40oC
Figure 2-16. Interaction between Qecalculation and Qe Model Freundlich at 45oC
Figure 3-1. Schematic of reverse osmosis mechanism
Figure 3-2. Vapour-liquid surface at each pore of membrane distillation
Figure 3-3. Absorbtion refrigeration system using RO membrane
Figure 3-4. Experimental VMD set-up
Figure 3-5. PVDF hollow fiber membrane module used in this research
Figure 3-6. Relationship among operation pressure,
permeateconcentration (%) and mass flux (gr.s-1.m-2)
Figure 3-7. Relationship among operation pressure, rejection factor (R)
and mass flux (gr.s-1.m-2)
Figure 3.8. Evaporator temperature during absorbtion process

4
9
10
13
14
15
16
17
18
19
20
22
22
23
24
25
25
29
30
34
36
36
38
38

at concentration 30%
Figure 3.9. Evaporator temperature during absorbtion process at
concentration 25%
Figure 3.10. Evaporator temperature during absorbtion process
at concentration 20%.
Figure 3.11. Mass solution changes at C=30%
Figure 3-12. Mass solution changes at C=25%
Figure 3-13.Mass solution changes at C=20%
Figure 3-14. Comparison between the experimental and predicted
VMD performance index (Y) determined by the RSM model
Figure 3-15. Prediction profiler of experiment result
Figure 3-16. Response surface plot and contour profiler showing
the VMD performance index (Y) as a function of
Temperature (°C) and Concentration (%)
for flow rate: 1.5 (L.min-1)
Figure 3-17. Response surface plot and contour profile showing
the VMD performance index (Y) as a function
of flow rate (L.min-1) and Concentration (%)
for temperature: 70 (°C)
Figure 3-18. Response surface plot and contour profile showing
the VMD performance index (Y) as a function
of flow rate (L.min-1) and temperature (°C)
for concentration: 47.5 (%)
Figure 4-1. System, surrondings and boundary
Figure 4-2. A process from state 1 to state 2
Figure 4-3.Thermodynamic cycle with two processes
Figure 4-4. Solubility of pure LiBr-H2O
Figure 4-5. Entropy of lithium-water solution as a function
of the concentration for different temperature
Figure 4-6.Flow diagram of absorbtion refrigeration system
using RO membrane

39
40
40
41
42
42
44
45

46

46

47
50
51
51
55
57
58

LIST OF APPENDIX
Appendix 1.Calculation of the surface area of the membrane
78
Appendix 2. Calculation of flux at concentration 30%
79
Appendix 3. Exergy in absorbtion refrigeration system using RO membrane
at concentration 20 (w/w %)
80
Appendix 4. Exergy in absorbtion refrigeration system using RO membrane
at concentration 25 (w/w %)
81
Appendix 5. Exergy in absorbtion refrigeration system using RO membrane
at concentration 30 (w/w %)
82
Appendix 6. Exergy in absorbtion refrigeration system using VMD at
concentration 30 (w/w %)
83
Appendix 7. Irreversibility and exergy efficiency in absorbtion refrigeration
system using RO membrane
88

1

1 INTRODUCTION
Background
Recent developments of refrigeration are primarily driven by environmental
problems especially the depletion of the ozone layer and global warming. Ozone
depletion reduces the ability of ozone layer to protect the earth from ultraviolet
radiation. Chlorofluorocarbon(CFC, HCFC), which is used as refrigerant in
refrigeration system is considered as substances that caused depletion of ozone
layer (ozone depleting substances, ODS). International community addressed this
issue in Montreal Protocol on Substances that Deplete the Ozone Layerwhich had
been approved in 1987, and ratified by Indonesia in 1992. This protocol declares
that international community agrees to stop the utilization of CFC and HCFC as
refrigerant. During its development on CFC and HCFC substitution,
hydrofluorocarbon (HFC) was believed as free chlorine substance and had similar
performance with those two substances. However, nowadays, it reveals that HFC
is also believed as a greenhouse gas which causes global warming. Its utilization
upon this substance should be decreased as declared in Kyoto Protocol on Climate
Change (Sihaloho and Tambunan, 2000). Furthermore, the other major problemis
energy crisis marked by the increasing of world oil prices due to the decreasing of
reserved fossil fuels especially petroleum fuel.
Considering to the long term effects on refrigerant (freon) utilization and
energy crisis, it is necessary to assess new alternatives refrigeration technology
which environmentally sound, lower energy consumption, lower cost and easily
produced. Absorbtion refrigeration system is reconsidered as a potential
alternative that complies with those requirements. Absorbtion refrigeration system
was developed in 1850s by Ferdinand Care and became the primary cooling
system at that time before the invention of vapor compression refrigeration
machine in 1880.
This absorbtion refrigeration system is environmentally friendly as it uses
safe refrigerant substance. This refrigeration system uses two different substances
i.e. absorbent and refrigerant. LiBr-H2O and NH3-H2O are widely used as
refrigerant-absorber pair in absorbtion refrigeration system.
Absorbtion refrigeration system uses heat energy to operate instead of
mechanical energy in the vapor compression system. Thermodynamically, heat is
considered as low-grade energy since it is normally generated as the by product of
any energy conversion. Heat can also be obtained from solar, biomass,
agricultural and animal husbandry waste, and industrial waste heat.
The use of heat in refrigeration cycle is for regeneration process to separate
the refrigerant from the absorbent. Separated refrigerant in vapor state will enter
condenser for condensation process to produce liquid refrigerant. The liquid
refrigerant in evaporator experiences evaporation process by absorbing heat from
the environment which produces cooling effect and the generated water vapor is
then absorbed by high concentration of LiBr-H2O in absorber. Regeneration
process in generator requires heat at temperature higher than 85oC (Ma, et al,
1998). Vargas et al. (2009) stated that regeneration process using LiBr-H2O in a
single effect absorbtion refrigeration system which operates at temperature below

2

80oC will produce un-effective process and low COP (Coeficient of Performance).
Furthermore, Sumathy et al. (2002) stated that regeneration process using twostage absorbtion refrigeration system with heater as the heat source operating at
70°C-85°C has COP as low as 0.39. Meanwhile, according to Gu et al. (2006),
regeneration process using LiBr-H2O solution in solar collector which operates at
80-93oC has COP value at 0.725. Moreover, the large contact area required to
separate water vapor from LiBr-H2O solution will end in large bulky generator.
Based on the problems, an alternative process such as membrane technology is
employed to overcome the utilization of high temperature for refrigerant and
absorbent separation.
Application of Membrane technology in absorbtion refrigeration system has
been previously reported by several researchers although still focus on the
membrane performance. Riffat and Su(1998) used centrifuge reverse osmosis
(RO) membrane in a refrigeration system to reduce the utilization of high pressure
pump. The research found that rate higher than 10.000 rpm at r=50 mm was
required in order to obtain 64% of solution concentration. The disadvantage of
this system was the uses of high velocity which corresponds to the increasing of
mechanical energy used in the system. Another research was conducted by Wang
et al. (2009) who used membrane distillation technology based on PVDF-hollow
fiber module for LiBr-H2O separation. In this research, several parameters i.e.
feed flux, temperature in lumen side and vacuum pressure in shell lumen were
observed. It was found that the increase of feed temperature and feed flux will
increase the water vapor permeation flux. Ahmed and Peter (2009) conducted an
experiment to analyze the effect of membrane characteristic towards the
absorbtion process in absorber. A good absorbtion performance was obtained
from membrane characteristic which has high permeability upon water vapor, uses
high pressure hydrophobic membrane to avoid membrane pore wetness, and no
water vapor capillary condensation to avoid membrane pore block. Mean while
the problem associates with low value of COP in absorbtion refrigeration system
should analyze the effectiveness of energy utilization and exergy analysis.
Energy analysis is normally carried out using the first law of
thermodynamic which states that energy is conserved. However, the energy
balance does not provide information about the quality of energy which enters and
exits the system boundary. In the second law of thermodynamic, the exergy
concept is introduced to analyze the thermal system. In term of thermal system,
exergy is the property or quality of energy and can be destroyed. According to
Moran and Saphiro (2003), exergy is defined as energy availability. The second
law of thermodynamic states that part of exergy which enters the thermal system
will be destroyed due to irreversibility of a system. This can be explained through
analysis on exergy balance in thermal system. Cengel (2002) mentioned that
energy cannot be created or destroyed but it can only be converted from one form
into another, while exergy is minimum required energy to do work. Thus, energy
and exergy are inter-correlated. Yumrutas (2002) stated that exergy analysis
method can be applied to determine the energy amount that can be converted, to
determine the location and the amount of energy which lost and unconsumed.

3

Problem Formulation
Based on the aforementioned back ground situations, it can be drawn that
LiBr-H2O separation is a complex problem as its separation process influenced by
the flux parameters.The problems associated with the utilization of membranes for
regeneration of refrigerant from its absorber in an absorbtion refrigeration system
can be formulated in question tenses, as follows.
1. What is the effect of separation condition using membrane on the characteristic
of produced refrigerant? On what conditions does the use of membrane will
give the best rejection factor?
2. How is the effect of the use of refrigerant produced from separation process
using RO membrane tothe performance of absorbtion refrigeration machine?
3. How does the exergy analysis method explain (a) the efficiency of each process
involved in absorbtion refrigeration system which uses membrane as separation
technique, and (b) the quality of energy loss during the entire process in
absorbtion refrigeration system?
Research Objectives
The general objective of this research is to analyse the energy and exergy
aspect of membrane utilization for regeneration process in a LiBr-H2O absorbtion
refrigeration system. The specific objectives of this research are as follow:
1. To study the absorbtion mechanism of water vapor by LiBr-H2O solution in a
controlled condition.
2. To study the regeneration process of LiBr-H2O using reverse osmosis (RO)
membrane and vacuum membrane distillation (VMD) when applied in LiBrH2O absorbtion refrigeration system
3. To perform the energy and exergy analysis in LiBr-H2O absorbtion
refrigeration system using RO membrane and VMD.
Novelty
According to the several literatures mentioned, the research of “Exergy
Analysis of Membrane Utilization for Solution Separation in LiBr-H2O
Absorbtion Refrigeration System” proposed novelty as follows:
1. This research was conducted to: (a) determine the influence of concentration,
temperature and water vapor pressure toward the water vapor absorbtion rate
by aqueous lithium bromide solution, and (b) determine the equilibrium
concentration of aqueous lithium bromide solution. These studies have not
been conducted before.
2. The application of membrane technology in LBARS has not been studied.
3. The exergy analysis on the application of membrane technology for separation
process in LBARS has not been studied.
Thus, the research of “Exergy Analysis of Membrane Utilization for
Solution Separation in LiBr-H2O Absorbtion Refrigeration System” had novelty
in terms of those three aspects. This research was expected to provide applicable
separation method used in LBARS.

4

Sistematical Framework of the Research
This research is focused on the utilization of membrane separation processes
for LiBr- H2O solution, water vapor absorbtion by LiBr-H2O solution, as well as
conducting studies on energy and exergy analysis utilization of membrane for
separation of absorbtion refrigeration system. Utilization of membrane for
separation processes of the LiBr-H2O using two types of membrane that is
Reverse Osmosis (RO) Membrane and Vacuum Membrane Distillation (VMD).
This research is carried out through several stages. The main scope of this study is
to test the absorbtion of water vapor by LiBr-H2O solution in the absorber, the
permeate flux did against the characteristics and value of rejection to the use of
RO membrane and VMD, conduct performance test the RO membrane utilization
on absorbtion cooling system, as well as conducting studies on energy and exergy
on absorbtion cooling system.
This research includes three parts. The first part of this research is to study
and examine the process of absorbtion of water vapor by LiBr-H2O solution in the
absorber. Studying the effects of concentration, temperature and water vapor
pressure to water vapor absorbtion by LiBr-H2O solution, to determine
equilibrium concentration of LiBr-H2O solution, to study sorption isotherm model
applicable to the LiBr-H2O absoption mechanism.
The third part of this study was to combine the results of the first and second
parts of the research by conducting analyses of energy and exergy as well as
conducting an analysis of the process of absorbtion of water vapor by LiBr-H2O
solution in the absorber based on separation process using a membrane. The
discussion thoroughly against all the things done in the first part until the latter is
expected to provide recommendations for the development of absorbtion
refrigeration system (Figure 1.1).
Reverse Osmosis Membrane
Vacuum Membrane Distillation

Membrane
Energy Analysis
Exergy Analysis
Absorber
Absorber

Evaporator

Water Vapor Absorption by LiBr-H2O Solution

Figure 1.1 Sistematical framework of the research

5

Research Scope
In order to focus on the research objectives, this study is limited to these
aspects:
1. Study and examine the process of absorbtion of water vapor by LiBr-H2O
solution in amodeled absorber. In this study, the effects of concentration,
temperature and water vapor pressure to water vapor absorbtion by LiBr-H2O
solution will be examined, and equilibrium concentration of LiBr-H2O
solution will be determined by evaluating sorption isotherm model applicable
to the LiBr-H2O absoption mechanism.
2. Study the characteristic and performance of RO membrane for separation
process in LiBr-H2O absorbtion refrigeration system.This study analyzes the
effect of separation condition such as pressure and concentration on the
characteristic of permeateion flux, rejection factor and permeate and retentatee
concentration that can be achieved. This study also analyzes the effect of RO
membrane as separation device on cooling process performance.
3. Study the characteristic and optimization condition of Vacuum Membrane
Distillation (VMD) for separation process in LiBr-H2O absorbtion
refrigeration system.This study analyzes the effect of separation condition i.e.
temperature, concentration and flow rate on the characteristic of permeateion
flux, rejection factor and permeate and retentatee concentration that can be
achieved. This study also identifies the optimum condition of temperature,
concentration and flow rate towards the achievable permeateion flux for
absorbtion refrigeration system application.
4. Perform the energy and exergy analysis on membrane utilization for
separation process in LiBr-H2O absorbtion refrigeration system.This study
analyzes (a) the thermodynamic model, (b) energy and exergy analysis to
determine the exergy efficiency of RO membrane utilization for separation
process in LiBr-H2O absorbtion refrigeration system. This study is also used
to determine the energy loss during the entire process in absorbtion
refrigeration system.
Research Benefits
The study of “exergy analysis of membrane utilization for solution
separation in LiBr-H2O absorbtion refrigeration system” was expected to generate
several advantages as follows:
1. The application of membrane in LBARS differs from the commonLBARS
which uses heat energy for the separation process. The application of
membrane technology resulted from this research could reduce the
consumption of heat energy and the utilization of large contact area required
for water vapor separation from aqueous solution occurred in generator.
2. The utilization of membrane for separation process was expected to produce
pure refrigerant.
3. The study of exergy analysis in absorbtion refrigeration system was expected
to determine the amount of convertible energy, the location and the amount of
energy loss and disused precisely.

6

7

2 WATER VAPOR ABSORBTION BY LiBr-H2O SOLUTION
Introduction
Recent developments of refrigeration are primarily driven by
environmental problems especially the depletion of the ozone layer and global
warming. Ozone depletion reduces the ability of ozone layer to protect the earth
from ultraviolet radiation. This problem has been caused by utilization
ofchlorofluorocarbon(CFC, HCFC) as refrigerant in refrigeration system.
International community addresses this issue in Montreal Protocol on Substances
that Deplete the Ozone Layer which had been approved in 1987. Indonesia then
ratified this protocol in 1992. This protocol declares that international community
agrees to stop the utilization of CFC and HCFC as refrigerant substances. During
its development on CFC and HCFC substitution,hydrofluorocarbon (HFC) was
believed as free chlorine substance and had similar performance with those two
substances. However, nowadays, it reveals that HFC is also believed as a
greenhouse gas which causes global warming. Its utilization upon this substance
should be decreased as declared in Kyoto Protocol on Climate Change (Sihaloho
and Tambunan, 2000). Furthermore, the other major problem is energy crisis
marked by the rising of world oil prices due to the decreasing of reserved fossil
fuels especially petroleum fuel.
Considering to the long term effects of refrigerant (freon) utilization and
energy crisis, it is necessary to asses new alternatives refrigeration technology
which environmentally more secure; lower cost and easily produced. Absorbtion
refrigerationsystemis a potential technology alternative that complies those
requirements. Absorbtion refrigerationsystem was developed in 1850s by
Ferdinand Care and became the primary cooling system at that time before the
invention of vapor compression refrigeration machine in 1880.As a power source,
absorbtion refrigeration system uses heat source to operate instead of mechanical
compressor.
Absorbtion is a process of absorbing a subtance by a solution. One of the
examples is H2O absorbtion process by LiBr-H2O solution which occurs in
absorbtion refrigeration system. LiBr is a solid salt crystal, which will change into
liquid by absorbing water vapor. LiBr-H2O solution which is usually used in
absorbtion refrigeration system is an absorbent. If absorber is in lower vapor
pressure than evaporator, it will absorb water vapor. Absorbtion process will stop
if absorbent could no longer absorb water vapor from the evaporator. In other
words, the system is in equilibrium state. In high concentration, LiBr-H2O
solution will absorb water vapor so the cooling process in evaporator will run
appropriately. The increase ofabsorbtion rate will improve the performance of
cooling process. The rate of water vapor absorbtion during cooling process is
affected by various factors such as absorbent concentration, water vapor
temperature that leaves evaporator, and absorbent temperature and water vapor
pressure that enters into absorber. In other words, water vapor absorbtion rate is a
function of concentration, temperature, humidity and water vapor pressure.
The process of water vapor absorbtion rate in absorber can be explained as
follows: the cooling process performance in evaporator is determined by the water
vapor absorbtion rate. At a certain concentration, high rate of water vapor

8

absorbtion produces good performance of cooling process which occurs in
evaporator. The increasing concentration of absorbent solution will decrease its
pressure which leads to increasing water vapor absorbtion rate. This condition
occurs when the water vapor pressure is higher than the pressure of absorbent
solution. In terms of temperature, the increasing temperature of absorbent solution
will affect the pressure of absorbent solution and water vapor.The increased
temperature along with the increased of absorbent and water vapor pressure will
decrease the water vapor absorbtion rate. Other factor i.e. relative humidity is also
one of the parameter affecting the water vapor absorbtion rate. The increases of
relative humidity will increase the water vapor amount in the air so the water
vapor absorbtion rate becomes higher. The whole interactions mentions that water
vapor absorbtion rate performance which occur in absorber should be supported
by low temperature and high relative humidity of water vapor which enter into
absorber from evaporator. Those conditions of water vapor will decrease its
pressure that enters into evaporator. The temperature of absorbent solution will
increase during water vapor absorbtion process. This temperature increment
occurs due to adiabatic process which eventually will terminate the water vapor
absorbtion process (Stoecker and Jerold, 1989). To overcome this unexpected
condition, cooling process should be performed through absorbing the produced
heat and releasing into the environment.
A correlation between humidity and moisture content at equal temperature is
termed as equilibrium moisture sorption isotherm (Bell and Labuza, 2000). Each
product has specific equilibrium moisture sorption due to different interaction
(including colligative effect, capillary effect and surface interaction) between
water and solid component of different water vapor content.
Generally, the term of absorbtion process is referred as absorbtion isotherm
which is defined as a curve which shows correlation between the absorbed
material concentrations at certain constant temperature. There are a number of
models which can be used to determine absorbtion isotherm, among which are: (1)
Freundlich model, (2) Langmuir model, and (3) BET (Brunauer, Emmett and
Teller) model.
The performance of sorption isotherm model depends on the ability of
mathematical and constants in determining sorption isotherm which are needed to
provide theoretical argumentation. Constructed model is generally cannot explain
all sorption isotherm model but only enable to predict sorption isotherm model on
one of three curves. Employing model is very dependent on the objective. For
example, simple model and less number of constantan could provide fittest model
(Bell and Labuza, 2000).
The objectives of this research are to: (a) study the effect of water vapor
concentration, temperature and pressure on its absorbtion rate performance in
absorbtion refrigeration system using LiBr-H2O solution, (b) determine the
equilibrium concentration of LiBr-H2O solution, and (c) determine the sorption
isotherm model.

9

LITERATURE REVIEW
Priciple of the Absorbtion Refrigeration Operation
The working fluid in an absorbtion refrigeration system is a binary solution
consisting of refrigerant and absorbent. In Figure 2-1(a), two vessels are
connected to each other. The left vessel contains liquid refrigerant while the right
vessel contains a binary solution of absorbent/refrigerant. The solution in the right
vessel will absorb refrigerant vapor from the left vessel causing pressure to reduce.
While the refrigerant vapor is being absorbed, the temperature of the remaining
refrigerant will reduce as a result of its vaporization. This causes a refrigeration
effect to occur inside the left vessel. At the same time, solution inside the right
vessel becomes more dilute because of the higher content of refrigerant absorbed.
This is called the absorbtion process. Normally, the absorbtion process is an
exothermic process, therefore, it must reject heat out to the surrounding in order to
maintain its absorbtion capability.

Figure 2.1 (a) Absorbtion process occurs in right vessel causing cooling effect in
the other; (b) Refrigerant separation process occurs in the right vessel as a
result of additional heat from outside heat source.
Whenever the solution cannot continue with the absorbtion process because
of saturation of the refrigerant, the refrigerant must be separated out from the
diluted solution. Heat is normally the key for this separation process. It is applied
to the right vessel in order to dry the refrigerant from the solution as shown in
Figure 2.1(b). The refrigerant vapor will be condensed by transferring heat to the
surroundings. With these processes, the refrigeration effect can be produced by
using heat energy. However, the cooling effect cannot be produced continuously
as the process cannot be done simultaneously. Therefore, an absorbtion
refrigeration cycle is a combination of these two processes as shown in Figure 2.2.
As the separation process occurs at a higher pressure than the absorbtion process,
a circulation pump is required to circulate the solution.

10

Figure 2.2 A continuous absorbtion refrigeration cycle composes of two processes
mentioned in the earlier figure.
Freundlich Sorption Isotherm Model
Freundlich sorption isotherm model assumes that monolayer and adsorbent
molecules will be constructed on the absorbent surface. However, this model
assumes that those active sites on the absorbent surface are heterogenic.
Freundlich sorption isotherm model is defined with equation (2.1) and (2.2):
⁄

(2.1)
(2.2)

Where Qe = adsorbent amount bound on the absorbent surface at
equilibrium state (gr adsorbent/gr absorbent), Ce = concentration at equilibrium
state (gr adsorbent/ml), K, n = Freunlich constant.
Langmuir Sorption Isotherm Model
This model is based on several assumptions, i.e.: (a) absorbtion occurs in
monolayer, (b) absorbtion heat doesn’t depend on covering surface, (c) all sites
and surfaces are homogen. This model can be theoretically derived based on the
assumption that there is an equilibrium state between absorbed molecules and unabsorbed molecules. Langmuir sorption isotherm model can be expressed as in
equations (2.3)–(2.5) :
e

KL Ce

(2.3)

KL Ce
KL Ce

e

KL Ce

e

KL

(2.4)
Ce

(2.5)

11

where Qe = adsorbentamount bound on the absorbentsurface at equilibrium state
(g adsorbent/g adsorbent), Ce = concentration at equilibrium state (g adsorbent/ml),
KL = Langmuir constant, Q0 = maximum adsorbentbinding capacity (g adsorbent/
g absorbent).
BET Sorption Isotherm Model (Brunauer, Emmet dan Teller)
BET model is the improvement of Langmuir model which is an approach on
multilayer absorbtion. The concept of this model is based on assumption that each
molecule on the first absorbtion layer covers the upper layer. This molecule
prefers to have a contact with adsorbent layer than absorbent layer. This condition
is due to the difference of equilibrium constants between the first contact layer
and absorbent layer. BET sorption isotherm model can be expressed as follows:
C
K e
C

e
Ce
C

[

C
- e ][
C

e[

C
- e]
C

e[

C
- e]
C

Ce
C

C
(K- ) e ]

(2.6)

C

C

(K- ) Ce
[

K

(K- ) Ce
K C

(2.7)

K

]

(2.8)

where Qe= adsorbent amount bound on the absorbent surfaceat equilibrium state
(gadsorbent/g absorbent), Q0 = maximum adsorbentbinding capacity (g adsorbent/
gabsorbent), K = absorbtion equilibrium constant, Ce = liquid
adsorbentconcentration at equilibrium state (gadsorbent/ml).
METHOD
This research was conducted by Nababan(2011), using a climate chamber
by utilizing the heated air through heater (Figure 2.3). Heated air which came into
drying chamber is controlled at certain temperature and relative humidity (RH) in
order to obtain required condition. Humidifier was used to control the humidity.
Wet hot air was forced into the climate chamber using blower. Incoming rate of
air into the chamber was controlled using flow controller. In order to maintain
temperature and moisture not higher than the set point, dehumidifier which works
on cooling and condensing process was used to discharge heat and water vapor.
Two units of independent control subsystem i.e. temperature and humidity
controler were implemented to obtain and maintain chamber in set point area. This
research used LiBr-H2O concentration at 45%, 50%, 55% and 60%.
Water vapor absorbtion by LiBr-H2O solution was measured at different
concentration. Solution was filled into climate chamber in similar condition with
absorbent in LiBr-H2O absorbtion refrigeration system. Water vapor rate was
calculated using equation (2.9).

12

-

(2.9)

Whereas:
LP : water vapor absorbtion rate (g/minute)
MGt : salt weight after absorbtion process (g)
MGa : salt weight before absorbtion process (g)
t
: absorbtion time (minute)
In absorber, required temperature was at 30°C-45°C, while in this research,
temperature was set at 40°C and 45°C. Water vapor condition in chamber was
controlled through setting up humidity and temperature in drying chamber
acquisitioned with climate chamber. Temperature sensor using C-C thermocouple
connected with Chino Recorder was set at absorbent solution to record
temperature change during absorbtion process.
The equilibrium concentration of LiBr-H2O solution was determined using
the gradient at each concentration of 45%, 50%, 55% and 60%. Each gradient was
then fitted into linear equation following this equation (2.10):
(2.10)
Where m is gradient magnitude, x is equilibrium concentration, a and b are
variable produced from the linear curve.
The determination of equilibrium concentration can also employ P-T-X
diagram (pressure-temperature-concentration). This diagram is used by fitting the
magnitude of air pressure and temperature of solution. The crossing point between
those two parameters is referred as equilibrium concentration of saturated LiBr
solution. The amount of bound adsorbent at equilibrium state (Qe) was
determined using this following equation:
-

(2.11)

Where,
Qe : adsorbent amount bound at equilibrium state (g adsorbent/g absorbent)
Ce : equilibrium concentration (g/ml)
C0 : initial concentration (g/ml)
m
: absorbent amount (g)
V
: tested solution volume (ml)
The calculation amount on bound adsorbent (QeCalculation) was then compared
with BET, Langmuir and Freundlich model result (QeModel). Moreover, error
generated from Qecalculation and Qemodel were also calculated so it can be
obtained the most appropriate sorption isotherm model which was defined from
the smallest error. The following equation was used to calculate the percentage
error:
|

-

|

(2.12)

13

Microprocessor
Controller

Humidifier

Airflow
Regulator
Electrical
Fan

Heating
Unit

PC

Climate
Chamber
room

Scale

Figure 2.3 Functional scheme and climate chamber room
Table 2.1 Research procedure at temperature 40°C and 45°C.
Concentration (%)
RH (%)

45

80

55

60

√

√

√

60
70

50

√

√
√

RESULT AND DISCUSSION
The Effect of Concentration to Water Vapor Absorbtion Rate
Figure 2-4 shows that the water vapor absorbtion rate is positively
correlated to the solution concentration. This condition was caused by increasing
number of salt molecules content in solution which affected the increased capacity
of absorbent compared to lower concentration. Moreover, solution with higher
concentration exerts lower pressure so high pressure water vapor will be absorbed
faster by low pressure of absorbent solution (Stoecker and Jones, 1992).

14

Figure 2.4 Water vapor absorbtion at each LiBr solution concentration
Another facts gained from previous figure is that treatment under constant
concentration and humidity with different temperature (40°C and 45°C) shows
increased water vapor absorbtion rate at lower temperature. This condition
occurred at each concentration. In term of temperature, lower absorbtion rate
occurred at higher temperature.
The coefficient of determination of the linear equation at 40°C and 45°C
shown in Figure 2.3 were 0.972 and 0.983,