Design of isobutane-olefin alkylation process over the solid catalyst using catalytic distillation with side reaction section
Liquid acid catalyzed alkylation of isobutane with butenes is widely applied process of production high octane number gasoline component. However, utilization of sulphuric and hydrofluoric acid have a number of drawbacks related to utilization of refrigerating agents, corrosion, pollution and safety. Alkylation with solid acid catalysts has potential environmental and safety advantages over conventional liquid-acid alkylation.
The difficulty of solid catalysts application in alkylation is related to rapid catalyst deactivation. To prevent it an excess of isobutane to olefin is required. Consequently operating costs for solid-catalyzed alkylation are increased. In order to reduce heat consumption the catalytic distillation could be proposed.
This research is focused on the special design of catalytic distillation with external reaction section for solid catalyzed isobutane-olefin alkylation and its integration with butane dehydration and isomerisation processes.
The catalytic distillation approach can provide the utilization of reaction heat. The reaction is highly exothermic (on average 75-96 kJ/mol) but the low temperature (60-100ºC) and large excess of isobutane can not be effectively utilized in the state of the art liquid-catalyzed processes.
In the reactive distillation system the process parameters (temperature and pressure) are determined by the chemical reactions, and therefore could be non-optimal for the separation. Based on experimental data the optimal conditions for alkylation with zeolitic or sulfated zirconia oxide catalysts are 60-80°С and pressure above 10 bar. The most energy demanding process, separation of isobutane /n-butane, can be optimally done at 4-7 bar. Therefore in the combined process of catalytic distillation the conditions of separation are close to the optimal. The temperature at the top of column would be close to 65 °С at a pressure level 10 bar.
There are several possible structures of catalytic distillation which differ in reaction and separation sections position. It has been established in our previous research that the reaction section should be located above the butane separation zone (top of the column) and alkylate stabilization (n-butane separation) should be performed in a separate conventional distillation column (Figure 1, case «A»).
Figure 1. Possible catalytic distillation design for alkylation with solid catalysts
Simulations of the described processes were perfomed using Aspen Hysys. Distillation modeling was based on MESH (material balance, phase-equilibrium, mole balance and energy balance) equations approach. A set of seven alkylation reactions was simulated using conversion type equations. The simulation of the reactions were based on experimental data obtained in laboratory scale experiments for two different catalysts: zeolitic and sulfated zirconia oxide, which are considered for final design.
The traditional catalytic distillation design has a significant drawback concerning with catalyst regeneration challenge. At the current stage of solid catalyst alkylation technology, the process efficiency is mainly determined by catalyst deactivation and subsequent regeneration. In the case of severe deactivation the technological scheme should allow for continuous or semi- continuous regeneration of catalyst activity. The most promising for alkylation process is a two-step regeneration with a mild regeneration by hydrogen dissolved in liquid isobutane and subsequent regeneration with hydrogen at elevated temperature.
Therefore, for catalytic distillation the most feasible option is to have side (external) reactor sections, which number depends on reaction and regeneration time. A potential scheme of such arrangement is shown on Figure 2. Distributing the reactor is efficient method of increasing the P/O ratio on the economical basis. The temperature control of reactions can be established using different pressure of reactor and distillation column.
Figure 2. Possible catalytic distillation design for alkylation with solid catalysts
Another question, which is considered in the study, is the integration of alkylation with butane dehydration and isomerisation processes. The state-of-the-art condition of these processes are very different, but its modification, including catalyst, and single separation section utilization can be efficient and provide the extension of process feedstock by relatively cheap olefin free streams from refinery plant.