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.
Dividing Wall Distillation and Separations Consulting, LLC
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The challenges associated with debottlenecking distillation 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 efficient 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 effectiveness of PPD in debottlenecking an existing series of distillation 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 typical 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 distillation 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 operation. The PPD scheme for a large capacity expansion is shown in Figure 2. In a PPD configuration, a series of distillation 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 LAO reactor product stream sent to the product recovery section and the distillation specifications selected for each of the final LAO products. Selection of optimal distillation column operating pressures and temperatures rounded out the design basis used to develop the process simulations.
The composition of the LAO reactor product stream sent to the product recovery area was determined using data published in chapter three of Alpha Olefins Application Handbook, which includes estimates of ethylene oligomerisation reactor yields from several major producers of LAOs.1 Graphical yield data for the Chevron/Gulf (currently Chevron Phillips Chemical) process using a Q factor of 0.7 was selected as the basis for the composition of the LAO reactor product stream. This data set was selected because the reactor yield distribution corresponding to the Chevron/Gulf process was judged to be the closest to median values for LAO reactor yield distributions from among all of the graphical data sets provided in Alpha Olefins Application Handbook. The compositions of the LAO reactor product stream used to develop the simulations for this evaluation are shown in Table 1. In this case study, the reactor product stream sent to product recovery was specified to contain 100% linear alpha olefins.
The product specifications used in this study are derived from LAO product information provided on the websites of three major manufacturers that produce LAOs by ethylene oligomerisation. Two of the major producers of LAOs provide typical compositions for each LAO product as well as sales specifications for each product. A third major producer of LAOs provides only sales specifications for each product. The typical compositions are more useful as distillation specifications in the process simulations because they more closely approximate the actual performance achieved by distillation columns in operating LAO plants. Because of significant variations between major LAO producers in both typical product compositions and sales specifications, composite distillation specifications were selected for use in the process simulations. The product specifications used as the basis for the simulations are shown in Table 2.
As mentioned in Part I of this article, the operating pressure and temperature of distillation columns used to separate LAO products are constrained by the thermal degradation of LAOs that takes place at elevated temperatures in the column bottoms and column reboilers. Minimising the risk of thermal degradation became a primary objective in selecting column operating conditions for the simulations.
Very little information has been published about the thermal degradation temperatures of LAOs produced by ethylene oligomerisation. One published patent document, US 6271434, makes references to ‘preferred operating temperatures’ ranging from 210°C to 280°C (410°F to 536°F) and ‘more preferred operating temperatures’ ranging from 230°C to 270°C (446°F to 518°F) for separations equipment associated with ethylene oligomerisation processes.²
Further complicating the issue of selecting column operating conditions for this study is the steady increase in molecular weight of the bottoms products from each column as one progresses downstream in the series of columns. The increasingly higher molecular weights of column bottoms require an increasingly deeper vacuum to be maintained in distillation columns to stay within the temperature ranges cited in US 6271434. Since the cost of achieving ultra-low vacuum levels throughout the entire series of columns is prohibitive, column operating pressures were allowed to cascade downward from column to column. Column operating pressures for the base case were selected so that the bottoms temperatures in the series of columns started at the low end of the temperature ranges cited in US 6271434 in the upstream columns and gradually increased to the upper end of the cited temperature ranges as one progresses downstream in the distillation train.
Once column operating pressures were selected for the base case, which represents the original mode of operation for the series of distillation columns, the same operating pressures were applied to the PPD simulation case. The logic for maintaining the same operating pressures for both simulation cases is that minimal changes to the ancillary equipment (heat exchangers and vacuum systems) associated with the existing distillation columns would be required as part of the conversion to prefractionation service. This is the case if the same operating pressures used in the base case were determined to be suitable in the PPD case.
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