GB671334A - Improvements in or relating to the solvent extraction of hydrocarbon or fatty oils - Google Patents

Abstract

In a process of fractionating hydrocarbon or fatty oils, the oil is subjected to solvent extraction to give extract and raffinate phases, and at least one of these phases is separately distilled in the presence of a substantial amount of recycled water in a primary distillation zone. The liquid residue from this distillation consists of an oil fraction substantially free of solvent while the distillate consists of solvent containing small amounts of oil and water. This distillate is allowed to settle in a primary settling zone and forms a plurality of layers comprising a water-rich layer containing small amounts of solvent and oil and a solvent-rich layer containing small amounts of oil and water. A portion of the water-rich layer is recycled to the primary distillation zone and forms the recycled water referred to above. The remainder of the water-rich layer and the solvent-rich layer are subjected to further treatment, including separate fractional distillation, to recover solvent and residual oil, the recovered solvent being recycled to the extraction zone. The solvent employed is an organic liquid partially miscible with water and so chosen that the oil to be fractionated contains at least a small proportion of constituents having true boiling points in the range of the solvent boiling temperature or below. In the fractionation of kerosine, low-boiling gas oil or fatty oil (whether of animal or vegetable origin), a suitable solvent is furfural, but other solvents mentioned are other furan derivatives, other aldehydes such as benzaldehyde, ketones and nitrobenzene. Gas oils which may be fractionated include those derived from thermal or catalytic cracking but the process is particularly applicable to the preparation of diesel fuel from a gas oil obtained by catalytic cracking of petroleum oil. According to one form of the invention, the extract and raffinate phases are separately subjected to fractional distillation which may be azeotropic in character. The resulting distillates are separately subjected to settling in the presence of water with the formation of oil-rich, water-rich and solvent-rich layers, and liquid streams are continuously withdrawn from these separate layers for further treatment. The oil-rich liquids derived from both the extract and raffinate phases are preferably recycled at least in part to the raffinate phase fractionating zone, although they may be recycled to the extract phase fractionating zone. The water-rich liquid is recycled in part to the fractionating zone from which it is derived, while the remainder is passed to a secondary fractionating zone for recovery of solvent and elimination of water. The solvent-rich liquids are passed to a tertiary fractionating zone to dry the solvent. Provision may be made for recycling a portion of the solvent-rich liquid directly to the extraction zone to which is also returned the dried solvent. The portion recycled to the extraction zone preferably should come from the settling zone for the extract phase. The proportions of solvent-rich liquid recycled and passed to the tertiary fractionating zone are correlated to control the water or oil content of the solvent in the extraction zone. This water content is maintained not in excess of that required to saturate the solvent, but the extraction may be carried out with substantially dry solvent. If necessary, water may be introduced into the system at a point in advance of the extraction tower. The secondary and tertiary fractionating operations are operated so as to produce distillates which, upon settling in the presence of each other, separate into secondary oil-rich, water-rich and solvent-rich layers. The oil-rich layer is continuously withdrawn and, for the most part, recycled to the primary extract phase fractionating zone. The portion not recycled is treated to remove residual solvent. The secondary solvent-rich phase is advantageously passed to the tertiary fractionating zone wherein water and some oil is stripped from the solvent, the resulting recovered dry solvent being recycled for reuse. The secondary water-rich liquid is recycled as reflux to both the secondary and tertiary fractionating zones.