Debottlenecking product recovery using product pair distillation: Part I

Advantages of using a thermodynamically efficient method to debottleneck existing distillation trains using fewer new columns than traditional methods.

David Kockler
Dividing Wall Distillation and Separations Consulting, LLC

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Article Summary

Chemical processes that produce many products often rely on a series of distillation columns to separate each of the products from reactor effluent streams. Product recovery may take place using direct sequence distillation and/or indirect sequence distillation. In a direct sequence arrangement, individual products are progressively removed from each distillation column as distillate products, whereas in indirect sequence distillation, individual products are removed from each column as bottoms products. Direct sequence distillation is generally favoured over indirect sequence distillation because, in most cases, direct sequence distillation is more energy efficient.

The fractionation sections of chemical plants with large numbers of distillation columns can pose major challenges in plant capacity expansions. Plant capacity can be expanded to the point where one or more columns in a distillation train reach their hydraulic limits. Up to this point, the only option that has been available to plant owners is to replace existing columns or add a new parallel train of distillation columns to allow further expansion of plant capacity.

New parallel distillation trains are not only capital intensive but also require additional plant operators and increased equipment maintenance. In capacity expansions, revamping processes with large distillation trains is likely to result in the fractionation section of the plant incurring a disproportionate share of the capital and operating costs associated with plant expansion projects. Against this backdrop, the product pair distillation (PPD) method for debottlenecking existing distillation trains that utilise direct sequence distillation will be discussed.

Having the means to debottleneck an existing distillation train provides several advantages to plant owners. First, it reduces the capital cost of implementing a plant expansion and avoids large increases in plant manpower required by new parallel distillation trains. A third benefit of PPD is savings in plot space. Many plant sites have limited space for plant expansions, and plot space requirements can be reduced by operating a single distillation train vs two parallel distillation trains.

PPD applications in the chemical industry
Fischer-Tropsch processes

The PPD process for debottlenecking a series of distillation columns is especially well suited to chemical processes that utilise a distillation train to recover many products sequentially. Fischer-Tropsch processes may be used to synthesise a wide range of chemical products and synthetic fuels. The chemical products are generally recovered in large distillation systems. Sasol Synfuels has commercialised Fischer-Tropsch technology on a large scale and produces many chemical products and synthetic fuels at its Sasol II and Sasol III plants in Secunda, South Africa.

Ethylene oligomerisation
A second example of a chemical process that produces many reactor effluent products is the production of linear alpha olefins (LAOs) by ethylene oligomerisation. Several ethylene oligomerisation processes were developed 60 years ago by Gulf (currently owned by Chevron Phillips Chemical), Ethyl (currently owned by Ineos), and Shell to produce a broad slate of LAO products. An overview of typical processes for recovering individual LAO products and product blends from ethylene oligomerisation processes is presented in the following paragraphs as background information. Part II of this article will present a case study demonstrating the usefulness of PPD in debottlenecking LAO product recovery sections.

Ethylene oligomerisation processes produce LAO molecules in increments of two carbon numbers. The products recovered from the reactor effluent (after removal of ethylene) consist of molecules with carbon chain lengths ranging from 4 carbon atoms to more than 30 carbon atoms. The distribution of different carbon number products from ethylene oligomerisation varies from one process to another. Stochiometric ethylene oligomerisation processes produce Poisson product distributions, and catalytic ethylene oligomerisation processes produce Schultz-Flory distributions. The typical product distribution curves published in technical literature for the original Gulf, Ethyl, and Shell ethylene oligomerisation processes have characteristic peak product yields in the range of 6 to 8 carbon numbers.1

Individual carbon number LAO products are recovered from ethylene oligomerisation processes by fractional distillation in large distillation trains. The LAO distillation processes separate individual products by carbon number. Small amounts of branched olefins, internal olefins, and paraffin impurities are found in distilled LAO products.1

For the most part, it is desirable to recover each carbon number LAO as a separate product because LAO products of different carbon chain lengths are used in different applications. However, the major producers of LAOs also offer blends of LAOs, which may consist of carbon chain length pairs, such as C12/C14 and C14/C16, or blends of several carbon chain lengths, such as C20-C24.

Sequential distillation of LAO products
Process flow diagrams that depict several examples of distillation systems used by major LAO producers to separate LAO products can be found in the technical literature.2 A typical distillation train for the recovery of LAO products from ethylene oligomerisation processes by direct sequence distillation is shown in Figure 1. In this separation process, individual LAO products are separated for C4, C6, C8, C10, C12, C14, C16, and C18, and LAO product blends are produced for C20-C24 and C26+ mixtures. Some LAO producers also distil the C26+ bottoms stream shown in Figure 1, but distillation of the C26+ product stream is not considered in this article.

A deethanised LAO reactor product stream consisting of C4+ linear alpha olefins is sent to the product recovery section of the plant, where it is fed to a DeC4 column to recover a C4 distillate product. The DeC4 column is pressurised to allow the C4 distillate product to be condensed using plant cooling tower water. In the remainder of the distillation train, operating pressures are progressively reduced to prevent excessive thermal degradation from taking place because of high column bottoms temperatures. Towards the end of the distillation train, deep vacuum levels are required to control the rate of thermal degradation.

Some column feed flashing takes place in the feed to each column in the distillation train shown in Figure 1 because of the progressive reduction in column operating pressures. The largest amount of feed flashing takes place in the DeC6-C10 column downstream of the DeC4 column because of the substantial reduction in operating pressure between these two columns.

The DeC6-C10 column is intentionally designed to recover a large molar flow rate of distillate product to make efficient use of the high percentage of feed that is flashed upon entry to the column. The C6-C10 distillate product is then further fractionated in downstream columns to recover individual C6, C8, and C10 LAO products. The C12+ bottoms from the DeC6-C10 column are sent to a series of distillation columns where different carbon number LAO products are recovered using direct sequence distillation.

Is there a place for dividing wall distillation?  
In principle, advanced distillation techniques that are in commercial use today could be used to debottleneck large distillation trains or to reduce the number of required columns. One possible distillation technology to consider as a candidate for debottlenecking large distillation trains or for replacing conventional two-product distillation columns in new designs is dividing wall distillation. However, many reasons make dividing wall distillation unsuitable for recovering multiple distillation products from processes like ethylene oligomerisation.

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