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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18,
No. 1,
1979
Constant Pressure Filtration of Liquefied Lignite Edward H. Hsu and L. T. Fan" Department of Chemical Engineering, Kansas State University, Manhattan, Kansas 66506
Experiments were performed to determine the effect of conditions of lignite liquefaction on the ease in separating the residual solid from the product liquid. Liquefied lignite slurry obtained through the solvolysis process under atmospheric pressure was filtered at 180 O C (356 O F ) and 13.8N/cm2 (20 psi) using a 200-mL capacity batch filter with an effective filter area of 9.62 cm2 (0.010ft'). The initial filtration rates were in the range of 0.05 to 1.56 mL/cm2 min (0.7to 23.0 gal/ft2 h) with the average specific cake resistance varying from lo9 to 10" M/kg (2-150 X lo9 ft/lb,). Solids concentration in the filtrate was below the detection levbl. The filterability of solid residues from the liquefied lignite slurry was dependent on the conditions during liquefaction.
Introduction It is known that the separation of solid residue from liquefied coal consitutes the bottleneck in the commercialization of liquefaction processes (Batchelor and Shih, 1975; Oak Ridge National Laboratory, 1975; Radgers and Westmoreland, 1976). Conventional solid-liquid separation systems, such as filtration, centrifugation, and sedimentation, have been found to be unsuitable because of the high cost and/or low efficiency in tHe removal of concentrated solids from the viscous coal liquid. Therefore, it is imperative that, among other possibilities: (1) a combination of the existing separation systems be determined so that the solid residues may be removed with relative ease; (2) a new separation system be developed; and/or (3) the conditions of liquefaction which will lead to easier separation be identified. The development of a hybrid or combined separation process containing more than one separation system by means of the available system synthesis technique is not likely because of the peculiar properties and complex structures of liquefied coal slurries and the lack of adequate design or perfortnance equations for the separation systems involved. New separation systems such as flotation of residues by injecting foaming gas (Kermode, 1975) and magnetic separation using filamentary magnetic material (Johnson, 1976; Oder, 1976) have been useful in removing a significant portion of the residue. However, the levels of solids removed with these new systems have not been satisfactory. In addition, no studies on the correlation of liquefaction conditions on the ease of separation have been found. In the present work, the effect of reaction conditions on the separability of solid residue from lignite liquefied through the solvolysis process (Honda and Yamada, 1974; Hsu and Fan, 1978) has been investigated. Filterability of liquefied lignite was chosen as the measure of the ease of separation. The theory of constant pressure filtration has been adequately developed (see, e.g., McCabe and Smith, 1956; Tiller, 1966). A general filtration equation can be derived by considering all drag forces created as the filtrate flows around the cake solids (McCabe and Smith, 1956). The resultant expression is 0 = !! ( y ) + Rm] V/A P 2 A
[
where B = filtration time, V = volume of filtrate collected, A = filter area, p = viscosity of filtrate, P = imposed filtration pressure, c = mass of solid particle deposited in the filter per unit volume of filtrate, a = average specific 0019-7890/79/ 1218-0076$01 .OO/O
cake resistance, and R , = resistance of the filter medium. Equation 1 indicates that e / ( V / A )is a linear function of V I A with a slope of p2a/2P. Therefore, the magnitude of the slope is a measure of the difficulty of separation.
Experimental Sectioh The experimental apparatus, shown in Figure 1, included a Gelman pressure filtration funnel, an electric heat tape, asbestos insulation, a variable auto transformer, a thermocouple, a temperature recorder, a volumetric filtrate receiving flask, and a hot plate. The stainless steel batch filter had a capacity of 200 mL (0.0071 ft3)and an effective filter area of 9.62 cm2 (0.010 ft2). The filter could be operated at a pressure as high as 137.2 N/cm2 (200 psi) and, because of the Viton O-ring seal, a t temperatures between -7 "C (20 OF) and 205 "C (400 O F ) . The filter medium used was made of glass fiber and had pore sizes ranging from 0.2 to 10 pm. The filter was wrapped with the electric heat tape and the asbestos insulating tape. The thermocouple, which was located near the filter medium, was connected to the temperature recorder to monitor the filter temperature. The liquefied lignite slurry used in the present study was obtained from a bench-scale investigation of the solvolysis liquefaction process (Hsu and Fan, 1979). In that investigation, experiments were carried out to determine the effects of the reaction temperature (300-410 "C), the reaction time (0-120 min) and the lignite-to-solvent (asphalt) ratio (1:2 and 1:l)on the conversion of solid lignite to gaseous and liquid products. The liquefied lignite slurry was heated to the filtration temperature of 180 "C (356 OF) and introduced to the filter. The filter was then pressurized to 20 psi (13.8 N/cm') with a nitrogen cylinder, and the accumulated filtrate volume was recorded as a function of time. Results and Discussion The results of the filtration of liquefied lignite at 180 "C (356 OF) and 13.8 N/cm2 (20 psi) are shown in Figures 2-9. The typical filtration data plotted according to eq 1 are shown in Figure 2. The plot will be henceforth referred to as a filtration curve. The highest (initial) filtration rates obtained in this study were in the range of 0.05-1.56 mL/cm2 min (0.7-23.0 gal/ft2 h). These rates were comparable to those obtained by other investigators with the Solvent Refined Coal, 4-14 gal/ft2 h (Schoemann et al., 1974; Katz et al., 1975; Rodgers et al., 1976). Concentration of the solid residue in the filtrate was below the detection level (
