Abstract
The studies on the resistance to degradation of capillary polypropylene membranes assembled in a membrane crystallizer were performed. The supersaturation state of salt was achieved by evaporation of water from the NaCl saturated solutions using membrane distillation process. A high feed temperature (363 K) was used in order to enhance the degradation effects and to shorten the test times. Salt crystallization was carried out by the application of batch or fluidized bed crystallizer. A significant membrane scaling was observed regardless of the method of realized crystallization. The SEM-EDS, DSC, and FTIR methods were used for investigations of polypropylene degradation. The salt crystallization onto the membrane surface accelerated polypropylene degradation. Due to a polymer degradation, the presence of carbonyl groups on the membranes’ surface was identified. Besides the changes in the chemical structure a significant mechanical damage of the membranes, mainly caused by the internal scaling, was also found. As a result, the membranes were severely damaged after 150 h of process operation. A high level of salt rejection was maintained despite damage to the external membrane surface.
Highlights
In the membrane distillation (MD) process, pure water is separated from salt solutions that leads to the supersaturation state of solutes
A limited solubility of salts and increasing osmotic pressure results in fresh water obtained in the reverse osmosis (RO) process being constituted of about 40%–50% volume of desalinated seawater and the remaining RO retentate forms a brine, which is subsequently discharged as a secondary waste stream
It is a known fact that a fraction of the pores located on the surface of Accurel PP membranes underwent the wettability at the initial period of MD process, resulting in a decline of the permeate flux
Summary
In the membrane distillation (MD) process, pure water is separated from salt solutions that leads to the supersaturation state of solutes. The growth of salt crystals in the pores interior may cause the blockage of pores, enhancement of surface roughness, and even mechanical damage of the membrane walls [2,5,13] With regard to these phenomena, an elimination of crystallization in the MD module is a basic condition for an efficient operation of MDC. A fulfillment of the above mentioned condition is possible when the applied parameters of MDC operation prevent the formation of a supersaturation state at the membrane surface This can be achieved by selecting a temperature and the flow rate of feed and distillate through the MD module [5,11]. In order to accelerate the degradation process, a high feed temperature was used and a single-stage MDC, which facilitates the salt crystallization on the membrane surface [2,6]
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