Metología empleada

4.3.1 Characterisation of Activated Carbon

Activated Carbon is a good adsorbent because of its highly porous surface. They are typically used for water treatment, industrial purification and protection against toxic gases. There are various types of activated carbon on sale commercially, the choice for this project was granular activated 208C carbon with a mesh (grain) size of 12 by 30 supplied by Chemviron Carbon. Usually, adsorption heat pumps are designed using off-the-shelf adsorbents. However, Tamainot-Telto [9] recently proposed a detailed methodology for obtaining optimum specifications of adsorbents for heat pump and refrigeration applications. These specifications may serve as a means of customising adsorbents in the future.

A Rubotherm magnetic suspension balance was used to characterise the porosity of the specific activated carbon sample used, and the following was obtained for the plot

of concentration (

x

sat T

T .

Figure 4-14: Plot of concentration (x) vs T/Tsat for the adsorbent used

As seen in Figure 4-14 above, the data has a poorer fit towards the left hand side of the graph. This is due to the presence of capillary condensation as the sample approaches saturation [10]. Capillary condensation is a phenomenon in which the

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pores of the sample are filled with condensed gas since condensation occurs below the saturation vapour pressure due to the pressure exerted by the meniscus between the liquid-vapour interfaces. Therefore, it is important to operate within the area of good curve fit with regards to the large temperature jump experiments. The values of the

Dubinin Astakhov equation constants (

x

n

characterisation were 0.3431, 4.4854 and 1.17 respectively. These constants were applied in the modified Dubinin Astakhov equation (Equation(4.13)) to obtain the adsorbate concentration in the adsorbent.

maxexp(- ( -1) ) sat T x x K T   (4.13)

This form of the Dubinin Astakhov equation is obtained from the original form by making a direct comparison between the adsorbed phase and a saturated liquid at the same temperature [11].

4.3.2 Large Temperature Jump Testing

The layout of the apparatus is shown in Figure 4-2. First, the adsorbent is placed in an

oven for 24 hours at 150◦C to remove all the moisture content of the adsorbent that may interfere with its adsorptive properties. Once the adsorbent is dried, it is quickly moved into the test cell. Once within the test cell, a vacuum pump is connected at V3 to evacuate the system for 5 hours. After this, V3 is shut and the system is filled with ammonia through V2. When pressurizing the large temperature jump system, it is important to add the correct mass of refrigerant (in this case ammonia) to stay within the reliable working range of the Dubinin Astakhov relation.

Once the sample adsorbent is in place and the system is filled with ammonia, water is run from thermal bath 1 at 40◦C through the heat exchanger circuit to the test cell containing the adsorbent material (active carbon) for 5 hours in order to reach equilibrium. This is achieved by setting three-way valve, V4 such that water flow is directed towards the test cell and opening valve V6 to allow for a circulation of the water.

Simultaneously, thermal bath 2 powers a short circuit through three-way valve V5 in order to prepare water needed for the temperature jump to 70◦C without interfering with the equilibrium in the pressurised ammonia system. Subsequently, valves V4 and

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V6 are closed and V5 is directed towards the test cell. This process facilitates the temperature jump by switching the heat transfer fluid (water) supply from one bath to the other.

There was no assessment of possible pressure differences between the two vessel. The temperature (T3) of the water was taken at 1 second intervals using a type K thermocouple. The same is done with the ammonia gas at T1 and T2 while a Danfoss AKS32 pressure transmitter was used to measure the pressure change in the system at P1. All of the data is logged by an Omega OMB-DAQ-2408-2AO 8-Channel temperature/voltage input USB data acquisition system throughout the duration of the experiments. The entire duration of each experiment was 3000 seconds which was adequate to reach sorption equilibrium. However, the analysis of the experimental data focused on the first 500s of the experiment as this timescale is representative of the cycle duration in a real AHP. Table 4-6 gives further the details on these experimental results and results of analysis. A sample pressure plot result for the large temperature jump test is shown in Figure 4-15.

Figure 4-15: Sample result for a large temperature jump test

0 50 100 150 200 250 300 350 400 450 500 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 Time(s) P re s s u re (b a r)

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4.3.3 Variable Density Testing

The adsorbent was compressed in order to investigate the effect of varying packing densities on the heat transfer properties. The compression was achieved with a

compression testing machine as shown in Figure 4-16. The 650 kg.m‾³ test which

required a compressive force of 19.3 kN was done in a displacement control mode of 1mm/min while the 705kg.m‾³ test was done on load control mode at 40kN.minˉ¹ with a load of 330 kN. The other densities were obtained by merely pouring the sample loosely into the test cell. The four combinations of bed thickness and density tested are shown in Table 4-6.

Figure 4-16: Adsorbent being compressed