PTQ Q3 2023 Issue

Debottlenecking product recovery using product pair distillation: Part II

Advantages of using a thermodynamically efficient method to debottleneck existing distillation trains using fewer columns than traditional methods

David Kockler Dividing Wall Distillation and Separations Consulting, LLC

T he challenges associated with debottlenecking dis- tillation trains in the product recovery section of chemical plants were outlined in Part I of this article. Product pair distillation (PPD) was introduced as an effi - cient method for debottlenecking existing distillation trains. Part II will present a detailed case study of the use of PPD to implement small and large distillation capacity expansions in the product recovery area of an ethylene oligomerisation plant for production of linear alpha olefins (LAOs). Design basis for case study A case study was developed to demonstrate the effective - ness of PPD in debottlenecking an existing series of distilla- tion columns. Process simulations formed the basis for the evaluation. Two process simulations were developed for the case study: a first process simulation that reflects the original design of an ‘existing’ distillation train and a second process simulation that reflects the PPD mode of operation. A base case simulation was developed to model a typ - ical LAO product distillation train operating at maximum capacity. The vapour/liquid traffic modelled in the base case simulation for each of the distillation columns define

the upper limit of the hydraulic capacity of each column in the distillation train. The base case simulation uses the flow scheme depicted in Figure 1 to recover a slate of individual products and product blends from a large series of distilla- tion columns. The feed rate selected for the base case was 350K metric t/y, which represents a typical production rate for a world-class ethylene oligomerisation production plant. A second simulation case titled ‘PPD case’ was developed to determine how much capacity increase can be obtained by converting the existing series of columns to PPD oper- ation. The PPD scheme for a large capacity expansion is shown in Figure 2 . In a PPD configuration, a series of distil - lation columns originally designed to recover individual LAO products by direct sequence distillation are converted into prefractionation columns that recover LAO product pairs. Main column(s) are used to separate the product pairs into individual LAO products. A realistic design basis is critical for demonstrating that a new product pair distillation scheme can debottleneck a large distillation train for recovering LAO products. The most important design criteria developed to evaluate the new concept were the composition of the de-ethanised

Feed to product recovery

C6-C10

DeC4

DeC8

DeC6-10

DeC6

C6+

C10

C4 to C10 recovery section

C12

C14

C16

C18

C20-24

3.8

2.3

1.1

0.6

0.1

C12+

Pressure (psia)

C26+

DeC12

DeC14

DeC16

DeC18

DeC20-24

Figure 1 Typical distillation train for recovering LAO products

PTQ Q3 2023