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Apr-2004

Alternative routes to paraxylene production

An integrated refining and petrochemical approach to paraxylene manufacture from various feedstocks, such as excess aromatics in reformate or pyrolysis gasoline

Jacques Rault, Christian Dupraz and Françoise Montecot, Axens
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Article Summary
An annual market growth of 6% to 8%  over the next decade is forecast for paraxylene (PX), which is the main building block for an increasingly diverse range of polyethylene terephthalate (PET) resin and fibre consumer products. Increasing demand is a welcome change for a PX industry recovering from years of overcapacity and severe economic woes in Asia. Although the way to demand growth is clear, the industry wants reduced investment routes to PX and benzene. More importantly, the industry will be examining potential synergies between petrochemicals and refining.

In recent years, PX overcapacity and economic turmoil in Asia combined to diminish PX industry profitability. However, according to multi-client studies, PX demand should be applying pressure on existing or scheduled capacity in the coming years. In configuring the emerging new grassroots projects, refiners and petrochemical producers are benefiting from new developments and the experience acquired over the past few years.

High, single-train capacity is a straightforward way to reduce capital cost and benefit from economies of scale. By examining aromatics availability and needs in other areas of refinery or petrochemical complexes, a PX manufacturer can find ways to divert the investment burden towards several value-added applications. Further into this article, a representative paraxylene production complex is reviewed and the features of its component units are described.

An examination of three alternative PX process schemes follows. The first two alternatives take advantage of existing captive aromatics streams, whereas the third alternative reviews an 
integrated grassroots refinery-petrochemicals scheme.

The paraxylene production scheme displayed in Figure 1 can be considered as representative of complexes erected in the mid-1990s. The principal operating units perform the following functions: aromatics “backbone” production from naphtha by high severity reforming, PX separation and purification, C8 raffinate isomerization, toluene and C9-C10 transalkylation, and benzene-toluene extractive distillation. The principal products are PX, benzene and hydrogen.

Processing commences with a hydrotreated naphtha heart cut feed to the proprietary Aromizing reformer, which yields a full range of aromatics from benzene to C10+ aromatics. The final product distribution depends on naphtha composition and cut points.

Reforming
This high-severity, continuous-catalyst-regeneration (CCR) reforming process produces aromatics and hydrogen, an important, if not the only, source in the complex. To maximise aromatics production, the reforming reactions take place at high temperatures and low pressures using a highly selective and robust catalyst (AR 501) and Axens’ CCR technology. This unit is similar in function and layout to its gasoline CCR reforming counterpart known as Octanizing. The continuous catalyst circulation loop employs a series of lift lines that convey catalyst beads from the bottom of one reactor to the top of the next reactor (Figure 2).

Many attractive features arise from the side-by-side reactor arrangement: simpler reactor designs, low-profile structures, reduced engineering costs and easier construction. For example, reactor wall thickness depends on process conditions rather than on thermal expansion or mechanical constraints. The side-by-side arrangement also allows the heaters to be positioned directly in front of each reactor, resulting in short transfer lines, reduced mechanical and thermal constraints and a compact, low-cost implantation.

The catalyst circulation and regeneration system is fully automated and ensures very low catalyst consumption from attrition as well as stable performance during catalyst life. Catalyst life up to nine years has been industrially achieved.

PX separation/purification and C8 isomerisation
The effluent from the aromatics unit is distilled into C7- and C8+ fractions. The C8+ cut is further distilled into C8 aromatics and C9+ streams. The xylenes are sent to the xylenes loop, which uses the proprietary Eluxyl technology for PX separation and purification. There are two process versions, stand-alone and hybrid, each able to produce the general PX purity requirement of 99.9%. The stand-alone version employs two adsorption columns whereas the hybrid version uses one column followed by a single stage performance crystallisation step.

The hybrid configuration is suitable for existing crystallisation-based complexes to be revamped: the implementation of a hybrid Eluxyl step allows at least a doubling of the capacity of an existing crystallisation plant.

The process for separating high purity PX from a C8 aromatics mixture uses the concept of simulated countercurrent adsorption. Eluxyl was designed to expand single train capacity limits using advanced concepts that allow flexibility and near-ideal hydraulic control. The proven ability to reach single train capacities as high as 750000 tons/year enables the operator to bring economy-of-scale advantages to each unit in the complex.

High, single-train capacities raise a number of developmental challenges in the scale-up of liquid-full adsorption sections. These issues were addressed during the early stages of development and have inspired some of the technology features. These features are now completely industrially proven, including:
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