Solvent de-ashing from heavy product of brown coal liquefaction using toluene .1. Solubility of heavy products and settling velocity of ash

The Brown Coal Liquefaction (BCL) process is a two-stage liquefaction (hydrogenation) process developed far Victorian brown coal in Australia. In the BCL process, the heavy product (vacuum residue) derived from the coal in the primary hydrogenation, which is named CLB (coal liquid bottom, boiling point > 420 degrees C), is treated in a solvent (CLB/solvent ratio, 1/8-1/4, wt./wt.) under high temperature (200-290 degrees C) and high pressure (4-5 MPa) to remove the ash and heavy preasphaltenes. This solvent de-ashing process uses toluene or coal-derived naphtha as a de-ashing solvent. After dissolving the CLB into the solvent, insoluble solid particles which consist of the ash and heavy preasphaltenes are settled in a settler by gravity and separated from the solution as an ash-concentrated slurry. The ash-concentrated slurry and the de-ashed solution are withdrawn from the settler as an underflow and overflow, respectively. The de-ashing solvent is recovered from both the overflow and underflow by distillation, and reused in the de-ashing process. The de-ashed heavy product recovered from the solution by eliminating the solvent is further hydrogenated in the secondary hydrogenation. This paper discusses the solubility of the CLB into toluene and settling velocity of the ash under the de-ashing conditions. The extraction experiments made clear that toluene dissolved similar to 60 wt.% of preasphaltenes (benzene insoluble-pyridine solubles) in the CLB under the following conditions, temperature 100-300 degrees C and CLB/toluene ratio (wt./wt.) of 1/3 or less. This solubility of CLB (epsilon(CLB)) under the de-ashing conditions is expressed as epsilon(CLB) = C-BS + 0.6C(Blorg), where C-BS and C-Blorg are the contents of benzene solubles and organic benzene insolubles, respectively, and organic CLB (ash-free CLB) = C-BS + C-Blorg. The de-ashing experiments confirmed that the insolubles including ash formed a boundary of concentration during the settling period, and the de-ashing efficiency was represented by the settling velocity of the boundary (V). V is expressed as V = A(CLB)(C-SA/2.5)(-0.91)(T/573)(9.5). Where, A(CLB), C-SA, and T are parameters expressing the properties of organic CLB, ash content in the feed slurry (wt.%) and temperature (K), respectively. The A(CLB) is also expressed by using the analytical results of the organic insolubles in the CLB under the de-asking conditions. The ash content of the de-asked heavy product recovered from the upper zone of the boundary is less than 3000 ppm, which is low enough to feed as a mixture with two times of the middle distillate to the secondary hydrogenation over Ni-Mo catalyst with fixed bed reactors. (C) 1997 Elsevier Science B.V.