Abstract
In the present study, we set out to examine an energy-reduction method whereby the reflux ratio of the iso-/normal-butane gas-splitting process is optimised. The splitting process used in this study was designed to be capable of handling 4,000 kg/h of mixed butane as a feedstock, to produce 2,600 kg/h of n-butane as the major product. Conventionally, three different feedstock compositions are used in the process. The n-butane concentration of the feedstock can vary from 60 to 98%, depending on the supplier. If a feedstock with a high n-butane concentration is fed into the splitter, the top product would have a high n-butane content, such that it cannot be sold as i-butane. To prevent this, in conventional processing, the operator must manually operate a valve to recirculate the overhead flow to the splitter. As a result, the reflux ratio of the splitter exceeds 90. Although a feedstock with a high n-butane content is fed into the splitter for only 2 h/day, the heat duty of the reboiler and condenser in the splitter increases due to the increased reflux rate. In addition, the amount of gas in the splitter increases due to the unequal mass balance, which imposes an extra load on the splitter. If the process could be modified to satisfy product specifications by minimising the reflux rate and thus reducing the unnecessary heat duty, energy optimisation would be possible. Therefore, to reduce the heat duty, in this study, we considered the use of an additional overhead buffer tank to store the top product, instead of recirculating the product to the splitter. In addition, we defined the following three constraint functions to identify the objective function, thus minimising the heat duty: 1) The concentration of n-butane in the major product is always > 99%; 2) When a feedstock with a high n-butane concentration is used, the n-butane concentration in the overhead flow is > 98.5%; 3) When a feedstock with a low concentration of n-butane is used, the concentration of the i-butane overhead flow is > 75%. As a result, when a feedstock with a high concentration of n-butane was used, the composition of n-butane in the overhead flow was 98.683% and the heat duty reduced to 7,242.017 kW. For a feedstock with a low n-butane concentration, the i-butane concentration of the overhead flow was 88.448% and the heat duty was 1116.469 kW. We assumed that the process would run for 8,000 h/year and that feedstocks with low and high n-butane contents would be fed to the splitter for 22 and 2 h/day, respectively. The reduction in the energy consumption as a result of this optimisation was determined to be 13.015 GW/year, corresponding to a financial saving of $1.124 million/year.
