PTQ Q3 2023 Issue

C12

Feed to product recovery

C14

C6-C10

MC1

DeC4

DeC8

DeC6-10

DeC6

C6+

C10

C16

C4 to C10 recovery section

C12/14

C16

C12+

C14/16

3.8

2.3

C18

1.1

0.6

C20-24

0.1

Pressure (psia)

C26+

DeC12/14

DeC14/16

DeC16

DeC18

DeC20-24

Figure 3 PPD distillation scheme for small expansion

Increased ethylene oligomerisation capacity The tabular simulation results presented in this article cor- respond to a large throughput expansion in the product recovery section that allows ethylene oligomerisation plant capacity to be expanded to 168% of the former operating capacity. In a large throughput expansion, it is necessary to replace all distillation columns in the C4 to C10 recovery area. It is possible, however, to replace the existing DeC6 and DeC8 columns with a dividing wall column (DWC). A DWC is (only) suitable in this section of an LAO product recovery area in part because the saturation pressure of the DeC6-C10 feed to the DWC, unlike all other feeds to columns in the distillation train, is lower than the pressure of the destination column. The major reason that a DWC can be implemented with any degree of success in the C4 to C10 part of the product recovery section is that the three DWC column products are individual LAO products. In all the distillation columns in C12+ section of the product recovery area, the column bottoms products consist of multiple LAO products. For the most part, the bottoms product molar flow rates in the series of columns exceed the molar flow rates of distillate products by a

large margin. Because of the large difference between the molar flow rate of bottoms products and distillate products for columns in the C12+ section, attempting to replace a series of two product columns with a series of DWCs is destined to fail. In this application, a DWC will provide small savings in energy usage compared with the existing two-column design. Using a basis of 589k metric t/y plant capacity, the DWC requires an energy input of 13.91 MMBtu/hr vs a combined energy input of 14.62 MMBtu/hr (9.21 MMBtu/ hr for the DeC6 column and 5.41 MMBtu/hr for the DeC8 column) for the DeC6/DeC8 design. In this case, the dis- appointing energy savings produced by the DWC can be attributed to the significantly greater difficulty in the C6/ C8 separation compared with the C8/C10 separation. In a new PPD configuration for a large throughput increase, the existing DeC12 through DeC18 columns are converted to prefractionation columns, and the existing DeC20-C24 column remains in its original service in which a single LAO product (C20-C24 mixture) is removed as an overhead product. The product pairs from the existing DeC12 through DeC18 columns are sent to the main col- umns for separation into individual LAO products.

Base case column operation for 350k metric t/y existing train and new 239k metric t/y parallel train

Existing distillation train

New parallel distillation train

Column

Column pressure,

Column

Reboiler

Column

Reboiler

name

bott temp,

diameter,

feed rate,

duty,

diameter,

feed rate,

duty,

psia

°C

klb/hr 40.43 31.59 24.41 18.65 14.22

MMBtu/hr

klb/hr 27.59 21.56 16.66 12.73

MMBtu/hr

DeC12 DeC14 DeC16 DeC18

3.8 2.3 1.1 0.6 0.1

459 482 497 502 539

5.0 5.0 5.5 5.0 6.5

3.08 2.64 2.15 1.42 1.16

4.25 4.25 4.75 4.25

2.10 1.80 1.47 0.97 0.79

DeC20-C24

5.5

9.70

Table 3

PTQ Q3 2023