15-Jul-09 9:00 AM CST

Paraxylene Production

Crystallization technology has improved dramatically over the last three decades. Recent advances in equipment and process control have answered many of the criticisms that have limited its application. Previous designs relied on small units arranged in multiple processing trains and were therefore deemed maintenance intensive. The new equipment is larger, more reliable, and capable of extended run lengths without maintenance.

Paraxylene Crystallization

The C₈ aromatic isomers are difficult to separate by distillation due to their close boiling points. In particular, the boiling points of paraxylene and metaxylene differ by less than 1°C. However, because paraxylene has a markedly higher freezing point than the other isomers, crystallization can be used to facilitate its separation.

Separation of paraxylene by freezing can be accomplished either by suspension crystallization or layer crystallization. For feedstocks containing low concentrations of paraxylene (i.e. 20-24%), suspension crystallization using two or more stages of separation is the only feasible option. For enriched feedstocks, crystallization is a very attractive method of recovery.

CrystPX^SM

Suspension crystallization of paraxylene in the xylene isomer mixture is used to produce paraxylene crystals. The design uses an optimized arrangement of equipment to obtain the required recovery and product purity. Washing the paraxylene crystal with the final product in a high efficiency pusher-centrifuge system produces the paraxylene product. Figure 1 shows a general flow scheme for the technology.

Figure 1. CrystPX Process Flow Scheme

When paraxylene content in the feed is enriched above equilibrium in the case of streams originating from selective toluene conversion processes, GTC’s crystallization process technology is one of the most economical means to produce high purity paraxylene product at high recoveries. The company takes advantage of recent advances in crystallization techniques and equipment to create this attractive method for paraxylene recovery and purification.

The process advantages of this new technology include:

High paraxylene purity and recovery (99.8+ wt% purity at 95% recovery).
Crystallization equipment is simple, easy to procure, and operationally trouble free.
Simple design requires small plot size and low capital investment.
The system is flexible, enabling it to meet market requirements for paraxylene purity.
The system is easily designed to allow for future incremental capacity increases.
Feed concentration of paraxylene is used efficiently.
Flexible technology allows a range of feed concentrations (75-95 wt% paraxylene) to be processed in a single stage refrigeration system.
Design variations are used to recover paraxylene efficiently from dilute mixed xylene feedstocks (22% PX) in a multistage system.

The design uses only crystallizers and centrifuges in the primary operation. This simplicity of equipment promotes low maintenance costs, easy incremental expansions, and controlled flexibility. High purity paraxylene is produced in the front section of the process at warm temperatures, taking advantage of the high concentration of paraxylene already in the feed. At the back end of the process, high paraxylene recovery is obtained through a series of crystallizers operated successively at colder temperatures. This scheme minimizes the need for recycling excessive amounts of filtrate, thus reducing overall energy requirements.

Case Study

LG-Caltex Oil Corporation will use CrystPX crystallization process technology for its new 400,000 tpy paraxylene production unit in Yosu, Korea.

The LG-Caltex Yosu Complex currently operates a world scale paraxylene production facility with a nominal capacity of 700 tpy. This expansion catapults LG-Caltex into the position of number one merchant producer of paraxylene from a single site in the world.

The award marks GTC’s full entry into the paraxylene technology licensing business. In today’s market, with changing feedstocks and market constraints, crystallization technology gives a producer the most flexibility, reliability, and lowest investment cost.

LG-Caltex believes that the CrystPX process is the most simple and reliable technology for its application. It provides and efficient recovery of PX from the feed and the flexibility to adjust process conditions to suit market requirements. The design and equipment are simple and easily procured. A fast startup and trouble-free operation is expected in January 2003.

LG-Caltex, a joint venture between Caltex and the LG Group in Korea, is one of the world’s largest oil refiners and petrochemical producers with a refinery capacity of 650,000 bpd crude oil.

*This article also appeared in Hydrocarbon Engineering.