PTQ Q3 2023 Issue

PPD case column operation for large expansion (589k metric t/y)

New or existing column

Column pressure,

Column

Required

Column

Reboiler

Column

bott temp,

column dia.,

feed rate,

duty,

name

psia 3.8 1

°C

klb/hr 68.03 47.45 36.85 27.42 18.42

MMBtu/hr

DeC12/C14 DeC14/C16 DeC16/C18

DeC6-C10

475.9 496.1 511.8 517.8 539.2 462.4 498.0

5.5 5.0 5.5 4.5 7.5 5.5 6.0

4.13 2.61 2.05 0.97 1.59 4.25 2.94

DeC14 DeC16 DeC18

2.3 1.1 0.6 0.1 4.0 1.0

DeC18/C20-24

DeC20-C24

NEW NEW NEW

MC1 (C14/C16/C18) MC2 (C16/C18/C20-24)

31.2 18.4

Notes: 1. DeC6-C10 vacuum system is designed for 8 psia operating pressure vs 3.8 psia required.

Table 4

The suitability of each existing column for use in a con- version to a PPD design can be seen by comparing Tables 3 and 4 . Table 3 shows the operating conditions and results obtained from the base case simulation for the existing distillation train at the 350k metric t/y original design rate. Separate columns are provided in Table 3 to show the requirements for a new parallel train of distillation columns that would provide an additional 239k metric t/y capacity (68% increase in existing capacity) to the existing product recovery system capacity. Table 4 shows the operating conditions and results obtained from the PPD case simula- tion for operation at a 589k metric t/y design rate. Column diameters shown in Table 4 represent the column diame- ters required to achieve the 589k metric t/y capacity. Comparison of the column diameters and reboiler duties shown in Tables 3 and 4 indicate that the existing DeC12 and DeC20-C24 columns must be replaced to achieve a 68% capacity expansion of the existing distillation train. The existing DeC14, DeC16, and DeC18 columns can all be converted to DeC14/C16, DeC16/C18, and DeC18/C20-24 service, respectively, without any changes to the column internals, column operating pressures, or column condens- ers. The existing column reboilers would need to be eval- uated for new prefractionation service because of slightly higher column bottoms temperature (discussed later). The location of the column feed nozzles and the number of stages in existing columns converted to prefractiona- tion service are also unchanged. Since the original DeC14, DeC16, and DeC18 columns remain in the same positions in the distillation sequence, it may be possible to reuse the inter-column piping associated with these columns. Large throughput increase The existing DeC12 and DeC20-24 columns are undersized for a large throughput expansion for different reasons. The DeC20-24 column is the terminal column in the series of columns in the product recovery area and requires a sharp split between the C20-C24 distillate product and C26+ bottoms products. In a conversion to a PPD design, a lower molar flow rate of C20-C24 distillate product is recovered in the DeC20-24 column. However, the existing column is undersized in a large throughput increase because of the large increase in the molar flow rate of C26+ product.

The existing DeC12 is the first in a series of columns con - verted to prefractionation service. The DeC12/C14 shown in Figure 2 produces a larger molar volume of distillate product than the remaining prefractionation columns because all the C12 reactor product is recovered in the DeC12/C14. The large molar volume of distillate product produced in the DeC12/ C14 column restricts the throughput capacity increase that can be obtained in comparison with the remaining columns that are converted to prefractionation service. In a large throughput expansion, the existing DeC12/C14 column can be replaced by one of the following two existing columns: the DeC6-C10 or the DeC20-C24. Either of these two columns meets the hydraulic capacity requirements for the new DeC12/14 service, and both are made redun- dant in a large throughput increase because the existing DeC6-C10 and DeC20-C24 columns need to be replaced with larger columns to accommodate a 68% expansion. One of the reasons existing columns DeC6-C10 and DeC20-C24 are suitable for replacing the DeC12 column is that there is a surprising degree of similarity in column design throughout the product recovery section. Most of the distillation columns in the product recovery section (the exceptions being the DeC4, DeC6, and DeC8 columns) show striking similarities in the column diameters, number of stages, and feed stage location. Except for the DeC4, DeC6, and DeC8, all the existing col- umns in the product recovery distillation train are designed with 18-20 theoretical stages, and with the exception of the DeC20-24 column, the feed stage in all columns is located at the midpoint of the column stages. The only significant differences in the designs of the DeC12, DeC6-C10, and DeC20-C24 columns are that the DeC12 has the small- est diameter of the group of columns, and the operating pressure of the DeC12 column is slightly lower than that of the DeC6-C10 and somewhat higher than that of the DeC20-C24. Thermal degradation impact Examination of the column bottoms temperatures for the PPD case shows that the column bottoms temperatures in the columns upstream of the DeC20-C24 column are slightly higher than in the base case. However, the maximum col- umn bottoms temperatures for the entire distillation train

PTQ Q3 2023