Composite adsorbents of CaCl₂ and sawdust prepared by carbonization for ammonia adsorption refrigeration

Abstract

Composite adsorbents of CaCl₂ and sawdust prepared by carbonization for adsorption refrigeration with NH₃ as refrigerant are tested, and the effects of carbonization temperature on the sorption capacity and rate are analyzed. The results show that the amount of pores in the sawdust of the composite adsorbents carbonized, apart from the content of CaCl₂, is the most dominant factor influencing the NH₃ sorption on composite adsorbents. The optimum carbonization temperature is 700°C, which gives the maximal NH₃ sorption capacity as high as 0.774 kg of NH₃ per kg of the composite, and the specific cooling power is approximately between 338 and 869 W/kg with the cycle duration varying from 5 to 20 minutes. The present study demonstrates that the composite absorbent of CaCl₂ and sawdust prepared by carbonization is more promising and competitive for adsorption refrigeration application.

Keywords

adsorption refrigeration / composite adsorbent / calcium chloride / sawdust / carbonization

Introduction

Adsorption refrigeration is well accepted for cold production in recent years [1]. This technology allows the utilization of renewable energy and the low-grade waste heat and is characterized by using environmentally friendly refrigerants, the absence of vibration, less sensitive to shocks and low operation and maintenances costs. However, the current state of the art is restricted by the poor properties of adsorbents. Aristov [2] indicated that the future progress of adsorption refrigeration is only dependent on the use of innovative adsorbents with advanced properties.

As one of the most promising working pairs for adsorption refrigeration, CaCl₂-NH₃ has been widely investigated. However, there are two challenges for the design of adsorption refrigerator using CaCl₂, viz. the low heat transfer properties of CaCl₂ powder and occurrence of the agglomeration phenomenon around the CaCl₂ particles [3]. Many purposeful modifications have been proposed to impregnate CaCl₂ into the pores of host matrices for overcoming the two challenges, leading to CaCl₂-based composite adsorbents. For example, Aristov et al. [4] investigated CaCl₂ impregnated into silica gel. Fujioka et al. [5] combined CaCl₂ with activated carbon fiber. And Oliveira and Wang [6] studied the consolidated composite adsorbent of CaCl₂ and expanded graphite.

The common preparation procedure of CaCl₂-based composite adsorbents is to soak the host matrices in CaCl₂ aqueous solutions or consolidate CaCl₂ with other physical adsorbents together. The problem existing is that the impregnation makes the content of CaCl₂ in the composite to be only 30%-40%, which greatly reduces the cycle uptake of the per unit mass of the composite. Also, the consolidation can improve the thermal conductivity, but the ability to transfer mass is reduced at the same time due to some pore cavities of the adsorbents being blocked. Besides, the common host matrices such as silica gel, activated carbon fiber and expanded graphite are industrial products with high cost, and the production process occupies large energy consumption.

Many waste materials and agricultural by-products with a low price or even no economic value used as adsorbents have attracted great attention in the past decade, of which, sawdust, as the timber and furniture industry waste, is relatively abundant and inexpensive and has been extensively investigated [7]. Sawdust used as the host matrix for composite adsorbents would benefit both wood agriculture and environment protection, viz. cold produced by a cheap adsorbent and thus a new market for sawdust and similar waste materials would be opened.

The present study reports a new CaCl₂-based absorbent with the sawdust as the host matrix, and the technology of carbonization is used to make pores in the sawdust of the composite with the aim to enrich the passages for mass transfer thus improving the adsorption performance. Experiment is conducted to test the sorption capacity and rate of the composite adsorbents. The effect of the carbonization temperature as well as the specific cooling power (SCP) on the sorption capacity is analyzed.

Adsorbent preparation

The composite adsorbent is prepared using pure CaCl₂ powder and fir sawdust within 20-35 meshes. The preparation procedure is shown as follows:

First, dry the fir sawdust in an oven at 120°C for 6 hours. Second, soak the sawdust in 50% CaCl₂ aqueous solutions for 24 hours. Third, filtrate the sawdust, let the sawdust dry in the oven at 120°C for 24 hours (the content of CaCl₂ in the composite is about 59% after drying). Next, make the composite, amylum and boiled water into paste according to a ratio of 1∶0.1∶0.25 by weight, and fabricate the paste into rotundity with approximately 3 mm in diameter. After that, dry the rotundity composite in the oven at 120°C for 6 hours. Finally, carbonize the rotundity composite in an electric furnace at a given temperature for one hour under vacuum conditions.

In the present study, eight samples of the composite adsorbents with different carbonization temperature are prepared, and detailed information is shown in Table 1. It is found that the content of CaCl₂ in the composite adsorbents increases monotonically with the rise of the carbonization temperature from 64% to 71%.

Experimental set-up and procedure

Experimental set-up

The experimental set-up illustrated in Fig. 1 is designed to test the adsorption performance of the composite adsorbent. The system consists of two main parts, the adsorber and the buffer tank of NH₃. The adsorber (10) with a volume of 80.99 cm³ is put in a constant temperature water bath (11) to maintain the composite adsorbent in the adsorber (10) at a constant temperature of 40°C, and a platinum resistance temperature sensor in the middle of the water bath (11) and near the wall to provide a direct reading of the temperature. The buffer tank of NH₃ (3) with a volume of 23091.58 cm³ (including the volumes associated with pipes and valves) is put in an additional constant temperature water bath (2) to maintain the NH₃ in the buffer tank (3) at a constant temperature of 25°C. Besides, an additional platinum resistance temperature sensor is provided for direct reading of the temperature. The temperature of the baths is maintained constant within a variation of±0.1°C. The ammonia pressures in the adsorber and the buffer tank are measured by the pressure sensors (9) and (6) respectively with an accuracy of±0.1 Pa.

Experimental procedure

Before the experiment, the composite adsorbent is dried at 120°C for 24 hours in an oven to make sure that all water vapor is eliminated from the sample. The experiment is performed with the evaporation temperature of -5°C and the condensation temperature of 40°C. The experimental procedure is as follows:

First, dismantle the adsorber (10) from the set-up, fill 10 g of the composite adsorbents in the adsorber (10), and connect the adsorber to the buffer tank (3). Second, evacuate the adsorber (10) for one hour at 90°C through the valve (8). Next, adjust the constant temperature water baths (11) and (2) to make the temperatures of the adsorber (10) and the buffer tank (3) to be respectively at 40°C and 25°C. After that, adjust the pressure in the buffer tank (3) to be 354.76 kPa, which is the saturation pressure of NH₃ at-5°C, through the valves (4) and (5). Finally, open the valve (7), the adsorption starts and runs for four hours. During the process, the pressures and temperatures of the adsorber (10) and the buffer tank (3) are collected by a computer.

The NH₃ uptake w of the composite adsorbent is calculated by

(1)w={\displaystyle \frac{(p_{\mathrm{0}}-p_{\mathrm{e}\mathrm{q}})V_{\mathrm{1}}}{R{T}_{\mathrm{1}}}}-{\displaystyle \frac{p_{\mathrm{e}\mathrm{q}}(V_{\mathrm{2}}-V_{\mathrm{a}\mathrm{d}})}{R{T}_{\mathrm{2}}}},

where T₁ and T₂ are the temperatures of the buffer tank and the adsorber (K), respectively; V₁ and V₂ are the volumes of the buffer tank (including the volumes associated with pipes and valves) and the adsorber (m³), respectively; V_ad is the volume occupied by the adsorbents (m³); p₀ is the initial pressure in the buffer tank (Pa); p_eq is the equilibrium pressure of reaction (Pa); and R is the gas constant of NH₃.

Results and discussion

Sorption capacity is an important indicator of adsorbents and also for adsorption refrigeration. A higher sorption capacity results in a better performance of the adsorption refrigeration [8]. The NH₃ sorption capacities of the eight composite adsorbents with different carbonization temperatures are depicted in Fig. 2. It can be found that with the carbonization temperature increasing from 400°C to 800°C, the NH₃ sorption capacities of the composite adsorbents increase first and then decrease. With the carbonization temperature between 400°C and 700°C, the increase of CaCl₂ content in the composite generally represents the increase in NH₃ sorption as expected. However, it is not the case when the carbonization temperature is over 700°C. For example, although the CaCl₂ content in Sample 8 is more than that in Sample 6, the NH₃ sorption capacity of Sample 8 is less than that of Sample 6 approximately by 2%. In addition, it should be mentioned that there is no occurrence of the agglomeration of CaCl₂ in all of the samples.

Sample 6 carbonized at 700°C gives the maximal NH₃ sorption capacity as high as 0.774 kg of NH₃ per kg of the composite. This can be explained by the scanning electron microscope (SEM) photographs presented in Fig. 3. It can be evidently found that the carbonization temperature of 700°C makes more pores in the composite, which provides rich passages for NH₃ transfer, thus leading to the maximal sorption capacity; the carbonization temperature below 700°C is relatively low to carbonize the sawdust and only a few pores are produced; on the other hand, although the carbonization of the sawdust is deepened at 800°C, the temperature over 782°C (melting point of CaCl₂) makes CaCl₂ melt and block the pores, reducing the mass transfer of NH₃. From this, it is concluded that, apart from the content of CaCl₂, the amount of pores of the sawdust carbonized is the most dominant factor influencing the NH₃ sorption on a composite adsorbent.

The performance of adsorbents not only depends on the sorption capacity but also the sorption rate. The sorption rates of the composite adsorbents with the carbonization temperature increasing from 400°C to 800°C against the adsorption duration are displayed in Fig. 4, which shows that the sorption rates of the eight samples drop with the reaction time lasting, and steep decrease can be found in the early stage. This characteristic fits the adsorption refrigeration well because the cycle duration of the adsorption refrigeration in practice is relatively short [9]. More importantly, Sample 6 has the highest sorption rate, which confirms the optimum carbonization temperature of 700°C.

Then, the SCP of Sample 6 carbonized at 700°C is calculated. SCP is an imperative indicator of an adsorption refrigerator reflecting the size of the system, and for a nominal cooling load, high SCP indicates the compactness of the system [10]. Figure 5 demonstrates that shortening the cycle duration will lead to a dramatic increase of SCP, especially when the cycle duration is less than 20 minutes. With the cycle duration between 5-20 minutes, the SCP approximately varies in the range of 338-869 W/kg. This is an improvement in the SCP as compared with the results reported in Refs. [9] and [11]. This finding argues the effectiveness of the composite absorbents of CaCl₂ and sawdust prepared by carbonization for ammonia adsorption refrigeration.

Conclusions

The present study investigates the composite absorbent of CaCl₂ and sawdust prepared by carbonization for adsorption refrigeration with NH₃ as the refrigerant. The pores carbonized in the sawdust of the composite absorbent effectively improve the adsorption performance and avoid the occurrence of the agglomeration of CaCl₂. The results of the experiment and analysis show that 700°C is the optimum temperature for carbonizing the composite absorbent, which makes the absorbent to have a good NH₃ sorption capacity as high as 0.774 kg of NH₃ per kg of the composite, and with the cycle duration varying from 5 to 20 minutes, the SCP is approximately between 338 and 869 W/kg.

Thus, the results of this study demonstrate that the composite absorbent of CaCl₂ and sawdust based on carbonization is more promising and competitive for ammonia adsorption refrigeration application. Meanwhile, the concept of using the sawdust as the host matrix for the production of composite adsorbents is of practical interest with respect to its quantity potential around the world, indicating that there would be a new market for sawdust and similar waste materials.

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