(243j) Steady State Multiplicity Analysis of Reaction-Separation Sequences for Deep Hydrodesulfurization of Diesel

Stringent environmental regulations are exerting pressure on the oil industry to improve the quality of diesel fuel. In this sense the European Union and United States agreed to reduce the sulfur content of diesel to 350 wppm from the year 2000, and 50 wppm starting from the year 2005. The fulfillment of this legislation presents serious changes in the oil refineries in terms of modifying the catalyst used and/or in the technology involved in the hydrodesulfurization (HDS) process. That is, a higher activity of the commercial catalyst and structural changes in the reactor configuration to increase the sulfur-compounds conversion are needed. In addition this mandatory reduction of sulfur in diesel is promoting an intensive research of new catalytic systems and technological alternatives.

An analysis of the operating conditions to obtain ultra-low sulfur diesel in a conventional trickle bed hydrodesulfurization reactor suggests that reactive distillation could be an interesting technological alternative for deep HDS of diesel. When HDS of diesel is carried out in a reactive distillation column (RDC), it has the advantage of obtaining high sulfur conversion at moderate pressure (25-30 atm) with reduced hydrogen consumption. Also this integration has the potential for capital productivity improvements, selectivity improvements, energy savings, increased reactant conversion, and the reduction or elimination of solvents.

However, interactions between chemical reaction and separation difficult the design and control of reactive distillation columns (RDCs). Such complex interactions, primarily between chemical kinetics and thermodynamic models, lead to a highly nonlinear behavior in reactive distillation indicating the possible existence of multiple steady states (MSS). The analysis of existence of steady state multiplicity should give insight into the reactive distillation process, help to avoid unsafe operation conditions, and further facilitate various subsequent studies such as control, monitoring, data reconciliation, parameter estimation and optimization of existing plants.

In general, one way to avoid the hardware and controllability problems with RDCs, while maintaining the benefits of in-situ separation with reaction, is to employ the Distillation-Side Reactor Concept (DSRC) where the reactor feed is withdraw from the distillation column and the reactor effluent is returned back to the same column. The side reactor could be a conventional catalytic packed bed reactor.

Therefore, the main objective of the present work is to systematically define the operation conditions and parameter sensibility over the main variables to monitor or control hereinafter: the recalcitrant reactants conversion of the organo-sulfur compounds (as a measure of the variations in the composition of the sulfur-containing hydrocarbon feed stream) and the product purity (as a measure of the product quality: ultra-low sulfur diesel) and under which MSS may occur and to determine the effect of their presence for a ?conventional? reactive distillation column and, a reaction-separation schemes using the DSRC for the case study of deep HDS of diesel, which is carried out by means of two paths (hydrogenolisis and hydrogenation). The MSS analysis is done through bifurcation diagrams for several case scenarios, showing its implication on the design and operation of the RDCs.

It has been observed, that the main variables that affect the steady state behavior in the ?conventional? reactive distillation column are the hydrocarbon mixture (HC) feed stage, reflux ratio, HC feed flow, and holdup. In the obtained results through bifurcation diagrams, it was found that the holdup, sulfur composition in the HC feed stream and the H2/HC feed ratio determines the final steady state achieved. It could be noted that when the holdup is set at high values, a high conversion steady state is achieved independently of the HC feed flow, while if the reflux ratio is reduced drastically higher amounts of catalyst are required to achieve a high conversion steady state.

Keywords: Multiple steady states; reactive distillation column; deep hydrodesulfurization of diesel.