PTQ Q2 2023 Issue

fraction that is sent to product storage to be sold as an LAO product blend. Splitting the product pair streams into prod- uct blend fractions and main column feed fractions reduces the energy usage of the main columns. Debottlenecking distillation trains Chemical manufacturers may have various reasons to debottleneck existing distillation trains. Three scenarios in which PPD may be used advantageously to debottleneck existing distillation trains include plant capacity expan- sions, modification of reactor operation to obtain a shift in product yield distribution, and switching to tighter product specifications. Plant capacity Plant capacity expansions represent the most obvious application for using PPD to debottleneck an existing dis- tillation train. In the case study that will be presented in Part II of this series, implementation of PPD in small plant capacity increases and large plant capacity increases are both discussed. PPD may also be employed to great advantage in sce- narios that do not involve capacity expansions. One such application arises when LAO producers wish to adjust process conditions to alter the LAO product yield distribu- tion intentionally. The three largest producers of LAOs can adjust product yield distributions and make use of this abil- ity to respond to periodic changes in demand for different carbon number LAOs. 1 Shifting LAO product yield distributions will reduce loads on some distillation columns in the distillation train and increase loads on other columns. This will cause a relo- cation of the bottleneck to a new position in the product recovery section. Without knowing the details of individual column sizing and capacities, it is impossible to determine where a new bottleneck will appear after a product yield shift takes place. It is safe to say, however, that the new bottleneck will constrain the production capacity of the plant. The new bottleneck will create a lost profit opportu - nity if the bottleneck prevents the plant from operating at maximum achievable production capacity. Shifting product yield distribution PPD can be applied to address bottlenecks created by shifts in product yield distribution regardless of whether the shift increases the yield of low molecular weight (C6-C10) LAO products or higher molecular weight (C12+) LAO prod- ucts. The approach to implementing PPD in applications that shift product yields towards lower molecular weight LAO products is quite different from the approach that is taken when the product yield distribution is shifted towards higher molecular weight products. When it is desirable to shift the product yield distribu- tion to make more C6-C10 LAO products, a new column is added in a PPD reconfiguration to the C4-C10 product recovery section to debottleneck the existing DeC6-C10 column. In a PPD reconfigured distillation scheme, both the new column and existing DeC6-C10 column operate as prefractionation columns, which remove intermediate

product pairs as overhead products. One of the two col- umns removes a C6/C8 intermediate product pair and the second column removes a C8/C10 intermediate product pair. Reconfiguring the design of the C4-C10 product recovery section in this manner also debottlenecks the existing DeC6 and DeC8 columns by permitting these col- umns to be operated in a parallel configuration instead of the original series configuration. The implementation of PPD in scenarios in which a prod- uct yield distribution shift increases the production of higher molecular weight LAO products is similar to the approach taken in plant capacity expansions. Consider a hypothetical scenario in which a shift in product yield distribution results in a new bottleneck that is concentrated in the range of col- umns beginning with the DeC14 column and ending with the DeC18 column. In this scenario, the simplest and most cost-effective way to remove the bottleneck would be to convert the DeC14 and DeC16 columns to DeC14/16 and DeC16/18 prefractionation columns, respectively. A new main column would be used to separate the C14/16 and C16/18 product pairs into individual LAO products. This scheme would also increase the available capacity of the DeC18 column since the scheme reduces the molar flow rate of distillate product recovered in the DeC18 column. The outcome of applying PPD in this scenario is that three columns in the distillation train are debottlenecked with the addition of a single new column. Tighter product specifications A second example of a debottlenecking application that does not involve a capacity expansion is a scenario in which there is an incentive to produce higher purity prod- ucts. Switching from looser product specifications to tighter specifications in the product recovery area will result in higher internal vapour-liquid traffic in affected distillation columns. This creates a bottleneck in the affected columns, which are no longer capable of operating at the previous throughput because of increased reflux resulting from tighter specifications. Implementation of PPD to debottleneck a series of col- umns in which higher purity products are recovered uses an approach that is very similar to the use of PPD in small capacity expansions or in product yield distribution shifts. The major difference between applications for produc- ing higher purity products and other applications is in the sharpness of splits made in the series of columns converted to prefractionation service. Using LAO product recovery as an example, a sharper split is required in higher product purity applications between the lightest LAO product found in the feed to the prefractionation column (such as LAO product with carbon number N) and the lightest LAO product that is excluded from the intermediate overhead product (such as LAO product with carbon number N+4). Diverse applications From the three categories of applications for debottleneck- ing existing distillation trains, one could make a good case that PPD is best suited for debottlenecking distillation

PTQ Q2 2023