Adsorption Separation Of Normal Paraffins From Naphtha By Simulated Moving Bed And Desorbent Recycle

Based on the molecule-scale management, through the adsorption process using a simulated moving bed (SMB) coupled with the distillation process, SPGC naphtha was separated into the desorption oil which has high content of normal paraffins and the raffinate oil which has high content of non-normal paraffins. The ethylene yield for the steam cracking plant and the aromatic yields for the catalytic reforming plant using naphtha as feedstocks were improved simultaneously.Using5A molecular sieves as the absorbent and n-pentane as the desorbent, under the pressure of2.0MPa, temperature of170℃, switching time of900s, the effects of operation conditions and column distribution on separation performance of SMB were investigated. The optimal conditions were naphtha space velocity of0.024h-1, recycle ratio of2.25, distribution ratio of3, desorbent/naphtha ratio of4, SMB column distribution of7-2-6-1. The results showed that recycle ratio affected effective contact time and backmixing of liquid. The distribution of n-paraffins in extract and raffinate were decided by the distribution ratio. With the increasing columns in adsorption area, the n-paraffin content in raffinate was reduced; with the increasing columns in desorption area, the separation performance of extract and raffinate were improved simultaneously. The SGPC naphtha with the normal paraffins content of31.95%was separated into desorption oil and raffinate oil through SMB and distillation process steadily. The n-paraffin content in the desorption oil reached91.89%, and the content of non-normal paraffins in the raffinate oil reached96.74%. Compared to the naphtha feed, the ethylene yield increased by14.1%using the desorption oil as the feed of steam cracking process. The Reseach Octane Number of raffinate oil increased by21.9units and the potential aromatics content increased by10.1%.The experiments and simulations using Aspen Plus software were carried out to investigate the distillation process of desorbent in the extract and raffinate. The simulations of continuous distillation for extract indicated that the n-paraffin content in recycling desorpbent was99.25%and the recovery of n-pentane was98.28%with22theoretical plates when the reflux ratio was1.09. Batch distillation experiments of extract showed that the n-paraffin content in recycling desorpbent reached99.01%with22plates when the reflux ratio was1, which was close to the simulation results. The separation process using the recycling n-pentane as desorbent of SMB had the similar separation performance with the fresh absorbent.