Mar-2012

Optimisation of integrated aromatic complexes

A novel class of purification catalysts provides improved selectivity in aromatic complex service. Benzene, toluene and xylene (BTX) are typically produced by distillation and/or solvent extraction processes from reformate streams.

Axel Düker
Süd-Chemie AG

Article Summary

The various raw aromatic streams contain small amounts of non-aromatic olefins and other impurities, which are removed by Tonsil products. This article discusses a new generation of Tonsil catalysts that are already in use in the industry. The feedback from large unit operations confirms superior performance compared to standard, granular-activated clay products.

The general flow scheme of an aromatic complex is shown in Figure 1. To simplify the overview, only three major sections are shown: the treatment of the reformate bottom; the purification of the benzene/toluene stream; and the fine purification of the p-xylene rerun. All three services are different with regard to catalyst activity, which is important for the reduction in the amount of olefins, and catalyst selectivity, which is critical for the purification of the p-xylene rerun.

Before having a closer look at the services, an understanding of the mechanism and the specific differences of available catalytic systems is needed. The conversion of naphtha into BTX streams in the reformer is always accompanied by the formation of non-aromatic olefins (see Figure 2).

These olefins are either poisons to downstream units, such as p-xylene extraction units or benzene alkylation units, or they make the produced BTX streams off spec and lower their value. The removal of olefins is done by acid catalysed alkylation with aromatic compounds, mainly benzene and toluene, yielding molecules with higher boiling range, which are then separated in the next downstream distillation unit. Granular Tonsil CO products have been widely used in this application for many years. Activation with mineral acid converts natural clay into highly effective catalysts of the Tonsil CO series.

The alkylation reaction of olefins and benzene or toluene needs primarily Lewis acids and little Brønsted acids. An excess of Brønsted acids catalyses the polymerisation reaction of olefins into gum, which blocks the catalyst pores and reduces its effective operating lifetime (see Figure 3). Depending on the operating temperature, the catalysts can form both species of acids. At temperatures below 140°C, mainly Brønsted acidity prevails, whereas at temperatures above 165°C the desired Lewis acidity is active. During normal operation at temperatures well above 170°C, the reaction mechanism favours alkylation catalysed by Lewis acids (see Figure 4). During startup conditions, especially during the dry-out phase, the operating temperature remains below 140°C and the undesired polymerisation reaction occurs. This can only be avoided by increasing the operating temperature during the dry-out phase as fast as possible. The use of a special Tonsil product can help to considerably shorten the dry-out phase.

The characteristics of three major services in integrated aromatic complexes are determined by the concentration and molecular weight of non-aromatic olefins and the necessity of providing high selectivity. The olefin concentration is measured as bromine index (BI); the selectivity is mainly determined by the need to minimise the formation of benzene by the dealkylation of toluene and xylene. This is particularly crucial in the fine purification of p-xylene rerun. Table 1 summarises the different features and categorises the three services from high-activity service to high-selectivity service.

Table 1 lists the Tonsil CO products that are the most suitable for the preferred services, with Tonsil CO 616GS the most active product and Tonsil CO 610G the most selective one.

Standard catalytic clay products are granules of 0.5 mm on average, as are almost all competitive products. A typical reactor loading requires sand grading to assure proper loading without any loss of product during operation and for the avoidance of channelling. Due to the small particle size, the pressure drop over the reactors is not negligible.

Tonsil APT are new types of catalysts that not only look different, but also show more active and more selective performance. They are in the shape of 4 mm extrusions.

In Table 1, the most suitable Tonsil APT products for the three major services in aromatic complexes are shown in comparison to the standard products. For easier reference, the designations of the products reflect the major service they are designed for: BT for benzene/toluene; mX for mixed xylenes; and pX for para-xylene.

The use of Tonsil APT products is beneficial with regard to loading pattern, performance and cycle life. A typical loading pattern of a reactor loaded with Tonsil CO compared to one loaded with Tonsil APT is shown in Figure 6. The most striking difference is the replacement of the sand grading by Tonsil APT extrusions, which leads to approximately 30% more catalyst volume loaded in the reactor.

The major advantages of Tonsil APT products over Tonsil CO products can be demonstrated by the example of the treatment of reformate bottom cuts. This service is the most demanding (see Table 1) and the catalyst lifetime is normally the shortest of all services.

It is therefore of utmost importance to extend the lifetime in this service, as each change-out is a costly operation. The reactor system is a lead/lag system that allows for changing the catalyst of one reactor while having the entire aromatic plant on stream. The reactor that needs the replacement is placed off line and is purged by nitrogen and steam free of aromatics. Once that is done, the catalyst is unloaded, disposed and fresh material is loaded. Thereafter, time-consuming heating up and drying of the catalyst is necessary.

Cost for this operation can be as high as 30% of the costs of the catalytic clay. An approximate breakdown of the major costs is shown in Figure 7. The cost blocks may vary with plant location, but the message is clear: any catalyst replacement is accompanied by high costs.

With a typical lifetime for Tonsil CO 616 GS of four months, the costs of change-out have to be paid, on average, three times per year. That means that in only one year, more than the cost of one charge of  catalyst is spent by replacement of the same. Tonsil APT mX can significantly help to reduce the costs of the treatment of reformate bottoms.