Dynamic Research Of CaSO₄ Scaling And Inhibition In RO Systems

Scaling of reverse osmosis (RO) membrane is one of the main problems in desalination processes. So, scale inhabitation plays a vital part in promoting the application of RO technology. In the first part of the thesis detailed study is made to evaluate the inhibitors, and in the second part preliminary study is made to the use of a new type pHysical cleaning method.Currently,there is no satisfactory method for reliable evaluation of scaling limits and for confident selection of the most cost effective inhibitors. We attempted to establish a fast way to evaluate inhibitors in this thesis. Firstly,the mode of partial recycle was adopted, and inhibiting efficiency of LB0100, LB2000 and PTP0100 to gypsum are evaluated and the scaling process on the membrane is depicted. By comparing the turbidity and saturation index of bulk solution with the presence of three inhibitors, inhibiting effect of them on bulk solution are estimated; by comparing the permeate coefficient and saturation index of membrane surface, inhibiting effect on membrane surface are estimated; and taking three inhibitors'inhibiting effect on both bulk solution and membrane surface, their scaling upper-limits under various concentrations are concluded. Then LB0100 and LB2000 are compared under full recycle mode. Because no obvious concentration change is observed and the turbidity is kept under 1 NTU during experiment process under this mode, the inhibiting effect of inhibitors on the membrane surface can better be evaluated.A series of experiment are made to explore the effect of operating conditions such as temperature and concentrate flow on inhibiting effectiveness. It is proved by the experiment that temperature has a great impact on scaling process and decrease of temperature will alleviate scaling process. The effect on concentrate flow is more complicated: scaling is alleviated in one series of experiment while aggravated in the other experiment. So it is concluded that besides taking the effect of mass coefficient into account the effect of permeate flow must be considered which can directly affect the motion of crystallite on membrane surface. At last,conductivity method is adopted to validate the conclusion of dynamic method. The experiments show that the inhibiting capacity of LB2000 is far better than that of LB0100 which coincide with the conclusion of dynamic method.In addition,an attempt is made to remove scaling on the RO membrane by DOC (direct osmosis cleaning). It is proved that DOC is a good pHysical way to remove scaling, and the cleaning efficiency is better to be used with inhibitors. But it is only efficient immediately after the precipitation of scale and the membranes must have high pressure durability in both directions. Based on this, cleaning interval and duration must be chose carefully by take in to consideration permeate flux, cleaning cost and mechanical damage to membrane.