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+ bot- toms 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 col- umns. 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 recov - ering multiple distillation products from processes like eth - ylene oligomerisation. Dividing wall distillation is generally not well suited for debottlenecking existing distillation systems for one sim - ple reason: three-product dividing wall columns (DWCs) require a larger number of theoretical stages than conven- tional two-product distillation columns. This makes it infea - sible to convert existing two-product distillation columns into DWCs by replacing the original internals with dividing wall distillation internals. Ethylene oligomerisation processes generate a high- pressure and high-temperature reactor product stream that is fed to a product recovery section. It is advantageous to feed high enthalpy reactor product streams to distillation trains and allow the pressures of the columns to cascade progressively to lower pressures. This will allow the col- umn feeds to flash partially, which results in lower column reboiler duties and lower energy consumption for distilla- tion. Sending high enthalpy feeds to DWCs, however, is counterproductive. Flashing feeds increase the reflux rate on the feed side of DWCs, which drives up the vapour rate (and reboiler duty) on the feed side of DWCs. Dividing wall distillation does not lend itself to replacing conventional two-product distillation columns in large dis- tillation trains that recover individual products using direct sequence distillation. Dividing wall distillation is generally efficient when the separation between the overhead prod - uct and the side-draw product is of comparable difficulty with respect to the separation between the side-draw product and the bottoms product. Conversely, dividing wall distillation is inefficient when a large difference exists between the degree of difficulty of these two separations. Attempting to implement a direct sequence distillation process in which a series of DWCs is used to remove indi- vidual LAO products as overhead products and as side- draw products (and allowing the balance of the column feeds to be withdrawn as bottoms products) will result in a sizable imbalance between the molar flow rates of the overhead, side-draw, and bottoms products in each DWC. The imbalances between flow rates of column product streams removed from each DWC in turn will create a wide disparity in the difficulty of separating the overhead/
C4
C6
C8
Feed to product recovery
C6-C10
DeC4
DeC6-10
DeC6
DeC8
C6+
C10
C4 to C10 recovery section
C12
C14
C16
C18
C20-24
C12+
C26+
DeC12
DeC14
DeC16 DeC18 DeC20-24
Figure 1 Typical distillation train for recovering LAO products
number LAO as a separate product because LAO products of different carbon chain lengths are used in different appli- cations. 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 sepa- rate LAO products can be found in the technical literature. 2 A typical distillation train for the recovery of LAO prod- ucts 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 sec - tion 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 distil- lation 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
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PTQ Q2 2023
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