(164c) Membrane Reactor and Crystallization-Based Process Intensification Strategy for Para-Xylene Recovery

Para-xylene recovery process is highly energy-intensive due to limited equilibrium conversion of other isomers to p-xylene resulting in a recycle loop of 3-4-fold feed rate, and phase change during isomerization and recovery. Process improvements which can reduce this recycle ratio and/or eliminate the need of phase change operation/vapor phase isomerization can lead to significant energy savings. A novel process intensification concept is proposed for xylene recovery that overcomes the isomerization equilibrium restriction by using a membrane-reactor isomerization unit and a crystallization as a secondary separation process to achieve final p-xylene product.

Introduction

Para-xylene (p-xylene) is a key petrochemical intermediate and is used almost exclusively for the production of synthetic fibers and poly(ethylene terephthalate) resins, which are precursors for various textiles and packaging materials. However, p-xylene is produced along with other isomers, namely ortho-xylene (o-xylene), meta-xylene (m-xylene) and ethylbenzene, and is recovered using adsorption and/or crystallization processes. The other xylene isomers are then converted into p-xylene via equilibrium isomerization process and this mixed xylene stream is again separated to obtain pure p-xylene generating a loop more commonly known as xylene recovery loop. This overall process is highly energy-intensive due to 1) limited equilibrium conversion of other isomers to p-xylene resulting in a recycle loop of 3-4-fold feed rate, and 2) phase change during isomerization and recovery as isomerization happens in vapor phase while adsorption- and crystallization- based process requires liquid phase. In this study, we describe a novel process which can reduce this recycle ratio and/or eliminate the need of phase change operation/vapor phase isomerization and can lead to significant energy savings.

Methodology

A novel process intensification concept based on a membrane reactor is proposed to mitigate isomerization equilibrium restriction and using a crystallization as a secondary separation process to achieve final p-xylene product. The membrane reactor overcomes the equilibrium by simultaneously converting m-xylene and o-xylene into p-xylene and selectively permeating p-xylene over m-xylene and o-xylene on the permeate side. This selective permeation also leads to a higher purity of p-xylene enriched stream which enables the use of crystallization as a secondary separation process.

A heat and mass balance model for conventional adsorption-based and the proposed membrane-reactor/crystallization process is simulated in Aspen plus. Energy calculations for crystallization are performed based on the reference [Wilsak, 2001]. The membrane reactor performance is based on the zeolite membranes prepared in Tsapatsis Lab [Liu et al., 2021].

In the conventional process, mixed xylene and heavies stream is fed to xylene column which separates C8 aromatics fraction from the heavies. This C8 aromatics fraction obtained from the overhead is then sent to an adsorption-based p-xylene separation unit. The adsorption unit separates the feed into a p-xylene rich extract and a p-xylene depleted raffinate stream. The p-xylene depleted raffinate is sent to a raffinate column (for separation of desorbent) and is recycled back to xylene loop after reestablishing equilibrium composition via isomerization process. The p-xylene enriched extract is then sent to an extract column (separates p-xylene from the desorbent) and further to a finishing column to remove the residual toluene and other lights, and obtain final p-xylene product.

In the proposed novel process, overhead from xylene column is fed to a membrane reactor. The feed/retentate product is recycled back as feed to reactor. Except for heavies, other components will be either consumed (m-xylene, o-xylene, ethyl-benzene) or permeated (p-xylene, benzene) through the membrane. Thus, a small purge is required for heavies in the retentate. p-Xylene enriched permeate is fed to crystallization unit after distilling benzene in benzene column. Permeate hydrogen is recycled back to the permeate side – a purge or make-up hydrogen can be used to maintain permeate hydrogen based upon the direction of permeation of hydrogen through the membrane-reactor. The filtrate stream from the crystallization is recycled back to membrane reactor. Further, benzene column can be completely heat-integrated with the condenser of the rerun column. Thus, the total energy required constitute the rerun column reboiler, phase change which is associated with membrane-reactor, and crystallization energy.

Accounting for the heat integration, the base case adsorption process results in a total of 245 kW/kTA which is in agreement with 249 kW/kTA based on the utility consumption provided in one of the literatures on adsorption technology [Commissaris, 2003]. The proposed process intensifications including membrane reactor and crystallization suggest that energy requirement can be lowered down to ~135 kW/kTA.

Conclusion

A novel process intensification concept is proposed for xylene recovery that overcomes the isomerization equilibrium restriction by using a membrane-reactor isomerization unit and a crystallization as a secondary separation process to achieve final p-xylene product. A heat and mass balance model for conventional adsorption-based and the proposed membrane-reactor/crystallization process is simulated in Aspen plus. A significant advantage associated with proposed case is the reduction of the recycle ratio. While an adsorption/reactor base-case configuration recycles 3-4 times the amount of feed, a membrane-reactor/ crystallization recycles only 1.2 times the feed within the membrane and another 1.3 times the feed from the crystallization. This reduction in recycle has potential to lower both the operational and capital costs associated with p-xylene recovery.

References

Wilsak, R. (2001). Energy efficient process for producing high purity paraxylene. US6565653B2.

Liu, J., McCool, B., Johnson, J. R., Rangnekar, N., Daoutidis, P, Tsapatsis, M., (2021). Mathematical modeling and parameter estimation of MFI membranes for para/ortho-xylene separation. AIChE Journal., 67, e17232

Commissaris, S. E., (2003). UOP Parex Process. In Handbook of Petroleum Refining Processes. 2.47-2.54, McGraw-Hill.