PTQ Q2 2023 Issue

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

C hemical 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 progres- sively removed from each distillation column as distillate products, whereas in indirect sequence distillation, indi- vidual 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 expan- sion 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 sequen- tially. 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 distilla- tion 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 typ - ical 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 mol - ecules in increments of two carbon numbers. The products recovered from the reactor effluent (after removal of ethyl - ene) 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 eth- ylene oligomerisation varies from one process to another. Stochiometric ethylene oligomerisation processes produce Poisson product distributions, and catalytic ethylene oligo- merisation processes produce Schultz-Flory distributions. The typical product distribution curves published in techni- cal 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

PTQ Q2 2023