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www.ierjournal.org International Engineering Research Journal (IERJ) Special Issue 236-1241, June 2016, ISSN 2395-1621

Investigation on thermal properties of composite

adsorber for use in Ammonia-Calcium chloride

refrigeration

#1Rahul R. Pawar,

#2Vaibhav N. Deshmukh,

#3S. Radhakrishnan

[email protected]

[email protected]

[email protected]

Mechanical Engineering Department, Maharashtra Institute of Technology

Pune- 411038, INDIA

Abstract: The adsorption performance of CaCl2/ activated carbon – NH3 working pair is studied under the

condition of different expansion spaces for adsorbent. The 4-compositions were prepared for solid-gas ammonia

adsorption refrigeration system, which utilizes the composite adsorbent of calcium chloride/activated carbon. The

percentage of activated carbon is varied to 20%, 30%, 40% and 50% in the composite adsorbent of CaCl2 and

activated carbon to improve the performance of adsorption refrigeration. The phenomenon of agglomeration and

swelling is studied during adsorption and desorption cycle for these different compositions. In this work, amount

of ammonia adsorbed, amount of ammonia desorbed is measured for all four compositions. Composite

adsorbents, which consists of chemical adsorbents and porous medium increases the adsorption quantity. The

result indicates that with increase in percentage of calcium chloride in the composite increases adsorption capacity

but also increases agglomeration and swelling phenomenon. For the composition ratio 80:20 (80% CaCl2), amount

of ammonia adsorbed was 7.4222 grams. The phenomenon of agglomeration is occurred in this composition as the

strong complex is formed between CaCl2 and ammonia. Agglomeration phenomenon will not occur if expansion

space is large enough i. e. activated carbon helps to avoid the agglomeration.

Keywords: chemical adsorption, composite adsorbent ammonia, activated carbon, refrigeration 1. Introduction Energy crisis and environmental damages are main challenges for the development of the society. For environmental benign and energy saving, adsorption refrigeration has advantages for utilization of low grade thermal energy [1, 2]. Attention has been paid to enhance the performance of adsorption refrigeration systems through improving the adsorbents properties and exploring some advanced cycles [7, 8, and 11]. It has pointed out that materials and sorption cycles are still very important for the sorption process. Adsorption working pairs includes zeolite-water, activated carbon-methanol, activated carbon-ammonia, CaCl2 – NH3 , silica gel-water etc. Many researchers make efforts to study the adsorption performance of the working pairs. Compared to the physical adsorption, the chemical adsorption has the advantages of larger adsorption quantity [1-6].

Due to the low driving temperature and low cost calcium chloride is frequently used as the chemical sorbent among metal chloride-ammonia working pairs. The working pair of CaCl2-NH3 has advantages as large sorption quantity and fast sorption rate. However, it has also disadvantages of swelling and agglomeration of reactive salts, which restricted its application [1, 2]. The chemical sorbents may agglomerate after consecutive adsorption cycles, which can reduce the reaction conversion capacity. To overcome these drawbacks, many efforts were devoted to developing the new composite sorbents using the porous inorganic host matrix materials with advanced structural and adsorption properties. The selection of a proper host matrix is most important. The composite sorbents using graphite or carbon fiber as porous additives have been investigated [2, 3]. The activated carbon fibers are highly porous materials and high adsorption capacity [1].

www.ierjournal.org International Engineering Research Journal (IERJ) Special Issue Page 1236-1241, June 2016, ISSN 2395-1621

In this work, the adsorption performance of ammonia by different composition ratios of calcium chloride and activated carbon was studied.

2. Materials and methods

2.1 Materials

The CaCl2 were obtained from Merch Life Science Pvt. Ltd. (India). The employed CaCl2 is with >98% purity. The activated charcoal were obtained from Loba Chemie Pvt. Ltd.

Fig.1 Images (a).Powder of CaCl2 (b).The

composite sorbent of CaCl2 and Activated carbon.

Fig. 1(a) shows photo of the calcium chloride. Fig. 1(b) shows composite sorbent of calcium chloride and activated carbon. The 4 types of composites were prepared by taking different ratios of calcium chloride to the activated carbon. In this work, the ratio of calcium chloride to the activated carbon was 50:50, 60:40, 70:30 and 80:20 (by weight) respectively.

2.2 Preparation of composite material

Fig. 2 shows photo of the mortar pestle which is used to prepare uniform mixture of ingredients or substances by crushing and grinding them into a fine

paste or powder. The pestle is a heavy and blunt club-shaped object, which is used for crushing or grinding.

The composite sorbent of CaCl2 and activated charcoal was mixed and prepared by using mortar pestle. Then the composite material was dried in the

oven at the temperature of 1300C to remove the moisture content from the mixture.

Fig.2 Mortar pestle.

3. Experimental work

3.1 System design

The test system is composed of the adsorption set, heating elements, magnetic flow pump and Rotameter. Fig. 3(a) shows the process flow diagram of Adsorption refrigeration system. The adsorption set is housed by the adsorber tube and temperature sensor sleeve. Material used to manufacture the adsorber bed is stainless steel. The adsorber bed is tube in tube type. Ammonia flows through the outer tube and cooling fluid flows in the inner tube.

The closed circuit for heating process is employed. In this experiment, the 9 heaters each of 3 KW are employed for the heating purpose. And a thermometer (grade A, PT100) is used for measurement of temperature having deviation of 0.5%. There are total 7 numbers of thermocouples. First is at inlet which gives inlet temperature of cooling fluid and second gives the outlet temperature of cooling fluid. Other thermocouples are used to measure the temperature of adsorber bed at different locations. All the temperature sensors are connected to the digital temperature indicators. Cooling circuit involves magnetic flow pump, Rotameter and Tank. Cooling fluid is pumped from the storage tank into inner tube. It absorbs the heat which is liberated during the adsorption process and then at last the cooling fluid flows back to the tank and the cooling cycle is completed. Outer tube is insulated to minimize heat transfer to the environment. The composite material of CaCl2 and activated carbon is filled in annular space. Bypass valve is placed before the inlet of adsorber bed to measure the flow rate of ammonia gas. Fig. 3(b) shows the


experimental setup and components of adsorption refrigeration system.

Fig.3 (a).Process flow diagram of Adsorption refrigeration system (b).The photograph of the experimental apparatus.

3.2 Adsorption cycle

The setup is initially run to ensure complete removal of air/water bubbles in the tubes as well as in the pump. The fluid to be tested is filled in the storage tank up to a certain required level. The small bucket is filled with water to measure the flow rate of ammonia. All the valves of ammonia cylinder and adsorption set are closed. Switch on the heater. Open exit valve for ammonia and water so that moisture content in the mixture will get removed.

Fig. 4 shows schematic flow diagram of adsorption refrigeration system. For measurement of ammonia flow rate samples are collected at bypass valve. Also, at the outlet of adsorber bed samples of ammonia solution are collected. For different samples take known amount of water in the sample bottles. The procedure for adsorption cycle is as follows.

i. Start coolant flow. ( open valve, connect tube and start pump)

ii. Ensure the tubes exits are dipped in water. iii. Ensure valve V1 (throttle for cylinder) is

closed. ix. Slightly turn the key on top of ammonia

cylinder. x. Dip the tube end in water in measuring jar.

xi. Turn on the valve V3 fixed to the TEE. xii. Slowly open valve V2 next to V1 and see if

bubbles are slowly appearing. xiii. Adjust the valve V2 to obtain 120-130 bubbles

per minute. xiv. Measure the actual number of bubbles per

minute. xv. Take Sample S1 of ammonia solution for

titration. xvi. Close the valve V3 and ammonia will start

flowing to the main tube (adsorber bed). xvii. The exit tube end should be in water.

xviii. Measure the number of bubbles per minute and monitor the same with transit time.

xix. Measure simultaneously ΔT for coolant flow with time and collect different samples of ammonia with time.

xx. Close the ammonia flow first by shutting of V1. xxi. Then close all valves. Close the key of

ammonia cylinder. xxii. Take Sample of ammonia solution for titration.

xxiii. Same procedure to be followed for all composition ratio material.

Fig.4 Schematic flow diagram of adsorption refrigeration system.

3.3 Desorption cycle

Desorption cycle is carried out after completion of adsorption cycle. Close all the valves of ammonia cylinder and adsorption set. Take known quantity of water in sample bottles. For each temperature collect 7 – 8 samples of ammonia in samples bottles with time. The following steps to be followed for desorption cycle.


i. Ensure the exit tube is dipped in water (sample bottle).

ii. Turn on the heater. iii. Set the controller temperature to 700C. iv. The coolant flow should be continued: Ensure

same flow rate as above. v. Record temperature for all sensors

continuously with time. vi. Simultaneously change the sample bottle

within time interval of 5 minutes. Collect 7 – 8 samples of ammonia solution.

vii. Record ΔT for coolant flow with time. viii. Measure bubble rate at exit with time.

ix. After 40-45 minute cycle, set the controller temperature to next temperature.

x. Repeat above procedure for different temperatures.

xi. Take the sample of ammonia solution for titration.

3.4 Concentration of ammonia

The purpose of this samples collection is to determine the concentration of ammonia in the solution using acid‐base titration. A known concentration and volume of titrant reacts with a solution of analyte or titrand to determine concentration. For titration following materials are used to determine the concentration of ammonia in the solution.

• 50‐mL Burette with clamp • Phenolphthalein indicator • Burette funnel • 250‐mL beaker • 25‐mL volumetric pipette • Pipette bulb

The hydrochloric acid is used as a titrant as its concentration is known and molar mass is approximately equal to the ammonia. Normality of hydrochloric acid was 0.5. The titrant and analyte follows the reaction as,

NH4OH + HCl NH4Cl + H2O (1)

Phenolphthalein (pH indicator) is used in basic solution of ammonia; the colour is a vibrant pink. From the known values i. e. normality of hydrochloric acid, molar mass of ammonia and hydrochloric acid, concentration of ammonium hydroxide can be determined. After calculating the molar mass of ammonium hydroxide and molar mass of ammonia the concentration of ammonia gas in the sample solution can be determined.

4. Results and discussion The adsorption capacity of ammonia on the 4 different compositions of calcium chloride and activated carbon was measured. The variation in percentage of calcium chloride in the composite and adsorption amount of ammonia is shown in Fig. 5. It can be seen that the amount of ammonia adsorbed on the composite is increased with increase in percentage of calcium chloride in the composite. At 50% of calcium chloride in composite, the amount of ammonia adsorbed was 4.7829 grams. Whereas, for 60% the amount adsorbed was 5.9321 grams. For 70% and 80% the amount of ammonia adsorbed was 6.7405 and 7.4222 grams respectively.

(gm

)

8

7.5 7.4222

Ad

so

rbe

d

7 6.7405

6.5

6 5.9321 A

mm

on

ia

5.5

5 4.7829

4.5

of

4

Am

ou

nt

3.5

3 40% 50% 60% 70% 80% 90%

Calcium Chloride % in Composite

Fig.5 The adsorption capacity of ammonia by 4-different compositions

The adsorption amount of ammonia on composites increased very fast during the initial stages for all experiments at the same operating conditions. Thereafter, the increase rate of adsorption amount dropped gradually, and then the adsorption amount became almost constant after about 22 min. This suggests that the adsorption process proceeds very quickly in the initial stages, and subsequently, it draws slowly near to the equilibrium state. This maybe attribute to the fact that there are a great many of accessible vacant surface sites for adsorption during the initial stage, and thereafter the remaining vacant surface sites becomes less and less with the lapse of the time, the residual vacant surface sites are difficult to be occupied due to repulsive forces between the ammonia molecules on the solid and bulk phases (Mall et al., 2006; Wu, 2007). And this can be seen from the bubble rate at the outlet of adsorber bed. In Fig.6 graph of bubble rate versus time is plotted.

It is observed that the adsorption quantity of ammonia on the composite decreased with the increase of the adsorption temperature under the same working conditions.


100

90

min

80

70

pe

r

60

bu

bb

les 50

40

30

of

20

No

.

10

0 0 5 10 15 20 25

Time (min)

200

min

) 180

160

(per

140

Bubb

les

120

100

80

of 60

40

No

.

20

0 0 10 20 30

Time (min) Fig.6 Ammonia bubble rate at the outlet of adsorber bed (a).For the composition 50:50 (b).For the composition 70:30.

It reveals that the adsorption of ammonia on CaCl2 and activated charcoal is exothermic in nature. This is accordant with the following

ΔT = T2-T1 (2)

It is the difference between outlet temperature of coolant and inlet temperature of coolant which is flowing through inner tube

Fig.7 Graph showing exothermic reaction of ammonia on composite.

From fig.8 it can be seen that ΔT increases as the ammonia reacts and adsorbed in the composite of CaCl2 and activated carbon. As the adsorption reach the equilibrium state value of ΔT decreases.

In desorption cycle as the percentage of calcium chloride in composite increases, desorption temperature is also increases. It reveals that the amount of ammonia desorbed decreases as the percentage of calcium chloride increases in composite for lower temperatures.

(gm

)

0.14

50%, 0.1326

0.12

deso

rbe

d

0.1

ofA

mm

onia

0.08

60%, 0.0612

0.06

Am

ou

nt

0.04

70%, 0.0289

0.02

80%, 0.0119 0

40% 50% 60% 70% 80% 90% Calcium Chloride % in Composite

Fig.8 Amount of ammonia desorbed at different compositions

From fig.8 it can be seen that at 50% of CaCl2 in composite the amount of ammonia desorbed is 0.1326 grams. It is decreases as the percentage of CaCl2 increases. For 60%, 70% and 80% of CaCl2 the desorbed amount was 0.0612 gram, 0.0289 gram and 0.0119 gram respectively.

Deso

rbed

0.16

0.14

0.12 70

amm

onia

0.1 0.06 80

0.08

( g m ) 90

of

0.04 100

Amou

nt

0.02 110

0 0 5 10 15 20 25 30 35 40 45

Time (min) Fig.9 Amount of ammonia desorbed at different temperatures for same composition Amount of ammonia desorbed increases as the temperature increases. Fig.9 shows desorption amount of ammonia for the 50:50 composition at different

temperatures. At 800C, it can be seen that desorption

amount is higher as compare to 700C. As the percentage of CaCl2 increases this desorption temperature also increases. The strong complex is


formed between the ammonia and CaCl2. To break this complex for desorption cycle, higher temperature should be achieved. As percentage of CaCl2 increases agglomeration and swelling occurs. The phenomenon of swelling and agglomeration seem to be related to the volume and the expansion space adsorbent. Agglomeration will not occur if the adsorbent is thin and space is large enough.

5. Conclusions

A new composite sorbent made of CaCl2 and activated carbon was prepared for solid-gas ammonia adsorption refrigeration. The ammonia adsorption capacity of the composite sorbent were measured by titrating the ammonia samples which were collected during experimentation of adsorption and desorption cycles. Experimental results showed that the adsorption capacity of ammonia on CaCl2 and activated carbon composite increases with the increase in percentage of calcium chloride in the composite. The maximum adsorption amount of ammonia by the composite sorbent is 7.4222 grams at 80% of CaCl2 in the composite under the experimental conditions. Whereas, in desorption cycle as the percentage of calcium chloride in composite increases, desorption temperature is also increases because higher temperature required to break complex between CaCl2 and ammonia. The results indicated that the additive of activated carbon has a small contribution to improving the adsorption capacity of ammonia on the composite sorbent, as it can avoid the agglomeration phenomenon. Thus, the CaCl2/activated carbon composite sorbent has potential application in the solid-gas ammonia adsorption refrigeration systems.

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