Study On Optimal Utilization Of Naphtha By Simulated Moving Bed Based On Molecule-Scale Management

With molecule-scale management idea, a simulated moving bed (SMB) technology was developed and researched based on the previous fixed bed technology. Through SMB technology, SPC naphtha was separated into de-solvent extract oil (DEO) in which the content of normal paraffins was high and de-solvent raffinate oil (DRO) in which the content of non-normal paraffins was high. By this way. multi-object optimization on the ethylene cracking process and the catalytic reforming process with naphtha as the feed was achieved, the utilization efficiency of naphtha was increased and the level of molecule-scale management technology was developed to a new stage.An approach for determining the absolute liquid adsorption capacity of normal paraffins on 5A molecular sieves was designed, which was called "Inert-component method" in this paper. The wetting capacity of the sieves was firstly measured by using an inert component as the solution, in the next step when the sieves were immerged in the solution with n-paraffin as the solute and inert component as the solvent, the decreasing amount of the solution was called total adsorption capacity, then the absolute adsorption capacity was the difference between the total adsorption capacity and the wetting capacity. With this method, the absolute adsorption capacities of different normal paraffins on 5A sieves were determined, and it was found that the result from this method was more accurate than what's reported in literatures. With the increase of carbon number or decrease of adsorption temperature, the absolute adsorption capacity of n-paraffin on 5A sieves increased. The isotherm curves of n-paraffins on 5A sieves at different temperatures fit the Langmuir isotherm model. This experiment provided support for SMB technology.The investigation on the static adsorption/desorption process of n-paraffins on 5A sieves showed that n-pentane was the suitable desorbent for SMB. The phase diagrams of different normal paraffins on 5A sieves showed that with small difference of carbon number, the normal paraffin with less chain number would enter the adsorption pore easier, while with large difference of carbon number, the normal paraffin with more chain number would preferentially be adsorbed. Classic composition of naphtha could be divided into four groups, which were normal paraffins group, iso-paraffins group, cyclo-paraffins group and aromatics group. The influence of each non-normal paraffins group on the adsorption kinetics of normal paraffins on 5A sieves was investigated, and the result was fitted with the kinetics model and Arrhenius model. The result showed that the sequence of the influence on the adsorption rate was aromatics>cyclo-paraffins>iso-paraffins. Through the discussion on the adsorption mechanism, it was found that the aromatic solvent molecules were expected to be preferentially adsorbed on the external surface, forming an adsorbed layer which may act like a barrier to the diffusion of the sorbate molecules into the zeolite pores.For SPC naphtha, the optimal operation conditions were the operation temperature of 170℃, the flow rate of naphtha of 5ml/min, the flow rate of n-pentane of 20ml/min, the flow rate of extract oil of 10ml/min and the switch time of 900s. For the desorbent recycle process, the optimized conditions for D-D tower were 6 plates and the reflux ratio was 0.2, while the optimized conditions for R-D tower were 22 plates and the reflux ratio was 0.29. From the coupled technology of SMB with solvent recycle process, the n-paraffins content in DEO reached 98% and the non-normal paraffins content in DRO reached 92%. Compared with naphtha, with DEO as the feed for ethylene cracking process, the ethylene yield increased by 17%; while the aromatics potential content and octane number of DRO increased by 10 units and 20 units respectively. This technology improved the utilization efficiency of naphtha, accomplished the continuous operation and had the own property right. Compared with fixed bed technology. SMB technology had the advantages like lower temperature, liquid phase operation, higher handling capacity, higher adsorption capacity and smaller floor area.In the end, the heat integration for fixed bed and simulated moving bed technology was calculated, the result showed that the SMB heat-integration projectⅡcould save the energy cost by 9% as compared with the fixed bed technology, and by 77% as compared with the SMB heat-integration projectⅠ. And Microsoft Visual Basic language was used to make the two softwares for the two technologies.