side-draw column products and the side-draw/bottoms column products. Product pair distillation PPD was developed to debottleneck all or part of a series of product recovery distillation columns without replacing the entire set of columns in the bottlenecked section or adding a new parallel distillation train. PPD provides a method for debottlenecking a series of columns designed to progres- sively remove individual LAO products (for example, C12) as overhead products in each column. Instead of replac- ing an existing series of columns with new columns, the bottlenecked columns are converted to a series of prefrac- tionation columns in a PPD design. Each prefractionation column produces an intermediate overhead product (for example, C12/C14) that is sent to a new main column, where the intermediate products are separated efficiently into individual products. The term ‘main column’ is used in this article to refer to distillation columns that receive two or more feed streams, each consisting of intermediate product pairs. The prod- uct pair feeds are separated into at least three individual products in the main column. A main column produces a minimum of three distillation products: an overhead prod- uct, a bottoms product, and at least one side-draw product. Using this definition, a main column can be distinguished from a main fractionator by the ability of the former to pro- duce higher purity side-draw products than the latter. The process of converting a bottlenecked series of col- umns into a series of prefractionation columns results in a PPD revamp design that requires fewer new distillation columns than the alternative of replacing the bottlenecked series of columns. In some applications, as will be dis- cussed further, the reduction in the number of new columns required in a PPD design can be remarkable, particularly in product yield distribution shift applications and in small increases in plant capacity. The PPD method of debottlenecking a series of columns can be applied to any section of a distillation train if the targeted bottleneck is located in consecutive columns in the distillation train. For example, PPD could be used to debottleneck the DeC12-DeC18 section of an LAO prod- uct recovery section, but it would not be useful in remov- ing individual bottlenecks that may exist in the DeC12 and DeC18 columns. Prefractionation columns One key concept of PPD is the conversion of an existing series of direct sequence distillation columns that recover individual LAO products into a series of prefractionation columns. The purpose of converting the existing series of columns to prefractionation columns is to permit the series of prefractionation columns to process a larger throughput than the original design throughput. Higher throughput is possible in prefractionation mode of operation because the quality of distillation performed in the prefractionation columns is substantially ‘sloppier’ than the quality of distil- lation performed when the series of columns is operated as originally designed.
In the original design, each column in the series of direct sequence distillation columns makes sharp cuts between the lightest LAO product and the next lightest LAO product. Using the first column in the series as an example, a sharp split is made between the lightest LAO product (having N carbon atoms) and the next lightest LAO product (having N+2 carbon atoms). The sharp separations between adja- cent carbon number LAO products allow each individual LAO product recovered in the series of columns to meet LAO product specifications. However, the drawback to a design that requires sharp separations is that large inter- nal vapour/liquid traffic is required throughout the series of distillation columns. After conversion to prefractionation service, however, the quality of distillation is relaxed, and a sharp split is not made between adjacent carbon number LAO products. Using the first column in the series as an example again, the LAO product consisting of N+2 carbon atoms is allowed to distribute between the first column that produces a dis - tillate product consisting of N and N+2 carbon atoms and the second column in the series that produces a distillate product consisting of N+2 and N+4 carbon atoms. The bot- toms from each prefractionation column are fed to the next column in the series. The process of removing product pairs from each column continues in the same fashion until the terminal column in the series is reached. In the terminal col- umn, an individual LAO product is removed as a distillate product. Additional key PPD features A second key feature of PPD is the use of one or more main columns to efficiently separate the product pairs that are produced in the series of prefractionation columns. Figure 2 depicts an implementation of PPD to separate the LAO reactor effluent into the same individual products and prod - uct blends as those produced in the flow scheme shown in Figure 1. The flow schemes for Figures 1 and 2 are iden - tical in the section of the distillation train used to recover C4-C10 products, except that in Figure 2, a new DWC is used to recover C6-C10 products in lieu of separate DeC6 and DeC8 columns. An explanation will be provided in Part II of this article as to why a DWC can be implemented suc- cessfully (albeit with marginal benefits) in this part of the product recovery section. In the PPD scheme, the bottoms product from the DeC6-C10 column is sent to a series of columns, where product pairs are progressively removed as distillate prod- ucts from each column up until the terminal column, in which a separation is made between remaining C20-C24 product and C26+ bottoms product. Each product pair produced in the series of prefractionation columns is sent to one of two main columns, where all product pairs are separated into individual LAO products. In column MC1, for example, C12/C14 and C14/16 product pairs are separated into a C12 overhead product, a C14 side-draw product, and a C16 bottoms product. The main columns used in PPD represent a very ther- modynamically efficient way to separate product pairs into individual products. Unlike conventional two-product
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