EVALUATION OF COKES AS FILTER AIDS IN DIRECT COAL-LIQUEFACTION

One of the major problems experienced in the development of filtration for coal liquefaction has been the rapid clogging of fine metal filte r screens. Tests have shown that the screens could be cleaned in situ, and the frequency of such cleaning could be reduced by using precoats . Standard diatomaceous precoats could be used, but these are expensiv e and their use could reduce the thermal value of the filter cake. Var ious carbonaceous products derived from coal and petroleum have been s uggested as alternatives. 15 different samples of coke have been asses sed in laboratory tests. All had a good performance in terms of low ca ke resistivities, screen bridging, and effected considerable protectio n for metal screens against dogging. When used in plant tests as a 5 m m precoat, the petroleum coke fines effected a doubling of interval be tween in-situ cleaning (sluicing) and was resistant to particle breakd own during slurry recycle. Coal extract coke and carbonized coal used as filter aids in plant tests also doubled the interval between in-sit u cleaning, but they behaved similarly to celite in the breakdown test s, being inferior to cyclone fines in this respect. The use of carbona ceous materials as body feed leads to a marked decrease in filter-cake resistivity and, therefore, to an increased efficiency of filtration area.