PTQ Q2 2023 Issue

C12

Feed to product recovery

C14

C6-C10

MC1

DeC4

DeC6-10

DWC

C6+

C16

C4 to C10 recovery section

C10

C12/14

C12+

C14/16

C16/18

C18/20-24

C16

C20-24

C18

MC2

C26+

C20-24

DeC12/14

DeC14/16

DeC16/18

DeC18/20-24

DeC20-24

Figure 2 PPD distillation scheme

distillation columns, main columns perform two or more separations using the reflux from a single column. Main columns used to separate two or more intermediate product pairs are well understood and extensively utilised and can therefore be considered established distillation technology. The concept of using main columns to separate intermediate product pairs dates to the 1960s, when three Russian researchers proposed what later came to be known as a ‘Petlyuk column’ for separating ternary mixtures into three products.3 A Petlyuk column utilises a fully integrated two-column arrangement to separate ternary mixtures. In the first column, an A/B/C mixture is separated into an A/B distillate product and a B/C bottoms product. The A/B and B/C mixtures are further separated in a second (side-draw) column into A, B, and C products. The use of main columns has expanded in recent years as a result of increased use of dividing wall distillation technol - ogy. Petlyuk columns are identical in concept to DWCs. A DWC represents a mechanical integration of the two col - umns that comprise a Petlyuk column, in which the product side of a DWC performs the function of a main column. Dividing wall distillation is now considered a mature tech - nology, with hundreds of DWCs currently in operation. 4 Processing multiple feeds Main columns are also efficient in terms of the amount of equipment required. In Figure 2 , two new main columns are shown as part of a conversion of a series of columns from individual product distillation to PPD. Each of the two main columns shown in Figure 2 accomplishes the same separations that would require two separate conventional two-product distillation columns. The scheme shown in Figure 2 shows two main col - umns, each with two feeds and three distillation products. However, main columns can process multiple feeds and produce more than one side-draw product. It would be possible, for example, to integrate the two main columns

shown in Figure 2 into a single main column with three side-draw products. This would result in a design that is even more thermodynamically efficient than the scheme shown in Figure 2. In Part II of this article, the improve - ment in the energy efficiency obtained by integrating the two main columns shown in Figure 2 will be demonstrated. The use of main columns requires that the product pairs, produced in the series of prefractionation columns, meet certain specifications with regards to LAO compositions. Using the example shown in Figure 2, it can be readily seen that the C14 side-draw product could potentially be contaminated by heavy LAO products that may be present in the C12/C14 column feed and may also be contaminated by light LAO products that may be present in the C14/C16 column feed. To avoid this problem, the distillations in the series of columns that produce product pairs are performed in such a way that excludes C16+ molecules from the C12/C14 product pair and excludes C12- molecules from the C14/16 product pair. The term ‘exclude’ used in this description means that the undesired molecules are excluded from the product pairs to the extent that individual products meeting all LAO product specifications can be recovered efficiently in the main column(s). Thermodynamic efficiency The PPD process is an energy-efficient process for recov - ering LAO products for two reasons. The thermodynamic efficiency associated with using main columns to sepa - rate intermediate product pairs was discussed earlier in the article. A second advantage of using PPD is that the prefractionation columns produce product pairs that meet the sales specifications for the LAO product blends (such as C14/C16 blend). This gives LAO producers the opportu - nity to separate the product pairs into a first fraction that is fed to a main column to separate into individual LAO prod - ucts (such as C14 product and C16 product) and a second

PTQ Q2 2023