Solvent de-ashing from heavy product of brown coal liquefaction using coal-derived naphtha

The brown coal liquefaction (BCL) process is a two-stage liquefaction (hydrogenation) process developed for Victorian brown coal in Australia. In this process, the heavy liquefaction product (vacuum residue) derived from the coal in primary hydrogenation, which is named CLB (coal liquid bottom), is treated in a solvent at high temperature (200-290 degrees C) and high pressure (5 MPa) to remove the ash and heavy preasphaltenes (solvent de-ashing). This solvent de-ashing step 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 by gravity and separated from the solution as an ash-concentrated slurry. The de-ashing efficiency (recovery of the heavy product and rate of ash removal) depends on the operating conditions and the properties of CLB and solvent, because they affect the extract yield from CLB, the settling velocity and concentration of the ash in the settler. This paper describes the stable operating conditions of the de-ashing plant using a coal-derived naphtha produced in the primary hydrogenation by discussing the effects of such parameters on the de-ashing efficiency. The de-ashing experiments with batch and continuous systems using the naphtha were carried out to determine the extract yield from CLB (epsilon(CLB)), the settling velocity of the ash boundary (V-NP) and the maximum ash content (Z(NP)) in underflow of the settler under the de-ashing conditions. According to the results of these experiments, the equations expressing epsilon(CLB), V-NP and Z(NP) are introduced by using the de-ashing conditions, the naphtha density and the properties of CLB expressed by analytical results of ash content and solvent extraction. The stable operating conditions of a continuous de-ashing system using coal-derived naphtha can be fixed by determining the upward velocity of solution in the settler, and the flow rates of ash in underflow and feed slurry based on the predictions of these equations. (C) 1999 Elsevier Science B.V. All rights reserved.