The effect of temperature

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1: The effect of temperature

The temperature of 80oC is good because it aids in obtaining better results and it is below the highest set temperature of 90oC. Starting at a lower temperature of 79oC is better for the experiment because the permeation flux of the process of VMD tends to increase as the temperature of the liquid feed increase in the same conditions of operation. Further, temperature has the impact of increasing vapor pressures, which affects flux, the selection of 80oC as the optimum temperature ensures that temperature does not destabilize the experimental system (Zhao et al., 2013). Higher temperatures and flux also cause mass and heat transfer because of temperature polarization, which indicates a need for starting with a lower temperature. Such a temperature would ensure that quality data is gathered from the experiment and there no sharp differences or instabilities, which emerge from high temperatures at start.

2: The effect of pressure

Pressure has the effect of increasing permeate and rate of feed movement across the membrane. As such, an increment in the pressure results in the increment of permeates and feed rate movement across the membrane. Thus, the selection of the 5000 pa as permeate pressure has the basis of ensuring that there is no need to increase permeates. Hence, the experiment would have the optimum operation conditions at this pressure because the temperature is not being adjusted during experimental process. Furthermore, pressure acts as the key driving force towards ensuring that there is an effective feed movement in the membrane pores. It acts as the energy carrier for the feed movement.

3: The effect of energy

MD heat transfer involves three steps, which are convection, conduction and vaporization in the membrane pores. Thus, the feed boundary layer experiences convection heat transfer while the membrane absorbs the transfer of heat through conduction means. Membrane pores then allows heat to flow in the form of vapor, which is the latent heat of vaporization and the permeate boundary layer has heat transfer, which is the convection. Heat transfer resistance on the phase of liquid feed is dependent on thermodynamics and hydrodynamics while the membrane heat resistance depends on tortuosity, porosity, thickness and pore size (Lee & Kim, 2014). As such, the reduction of the heat transfer occurs as the number of stages increase during the experiment. Thus, there is a different Q value for each of the four stages, which occur during the experimental set-up. The variance in the Q value is because at each stage, there is a difference in the temperature, which heats the feed. The same temperature affects the heat transfer of the feed and membrane permeability, which influences the energy differences.

4: The number of stages

4 stages is the optimum number of stages because there are more than two components transfer, which occur through the membrane of the system. Furthermore, a single stage process would result only in warming the feed between the heater and the inlet and the there would be no condensation for the steam (Cipollina et al., 2012). As such, the four stages ensure that it is possible to maintain lower temperatures and pressures for the system, which results in collection of steam from the condensation process. In addition, four-stage process ensures that there is complete heating of the feed and condensation of the vapor without any trace elements being left behind.

References

Cipollina, A., Di Sparti, M. G., Tamburini, A., & Micale, G. (2012). Development of a

Membrane Distillation module for solar energy seawater desalination. Chemical Engineering Research & Design: Transactions of the Institution of Chemical Engineers Part A, 90(12), 2101-2121.

Lee, J., & Kim, W. (2014). Numerical study on multi-stage vacuum membrane distillation with

economic evaluation. Desalination, 33954-67.

Zhao, K., Heinzl, W., Wenzel, M., Büttner, S., Bollen, F., Lange, G., & … Sarda, N. (2013).

Experimental study of the memsys vacuum-multi-effect-membrane-distillation (V-MEMD) module. Desalination, 323150-160.