Optimization of technological regimes of the deisopentanizer column for the preparation of the feed stream of the low-temperature isomerization unit PGI-DIG/280-NK

Raw fractions used in isomerization plants contain isopentane, which is the target product, and its presence in the feed as “ballast” occupies the useful volume of the reactor. To optimize the composition of processed raw materials, a number of enterprises have included a deisopentanizer column in the technological scheme. The paper evaluates the possibility of optimizing the technological regimes and design parameters of a deisopentanizer for a low-temperature isomerization unit that does not have a column in the technological scheme for extracting isopentane from the composition of the feedstock. The studies were carried out using the Honeywell UniSim Design modeling system. The properties of the fraction components were calculated using the Peng-Robinson method. Calculations were made for two types of raw materials containing 8.7÷19.7% wt. isopentane. The performed calculations established that in order to achieve almost complete recovery of isopentane in the feed stream from the hydrotreating section, the distillation column should contain 63 theoretical plates, and the minimum reflux ratio should be Rmin = 12.3. At the same time, the presence of normal pentane in the distillate of the column will not exceed 0.07% wt., and in the bottom product, the content of isopentane will also not exceed 0.07% wt. The operation of a distillation column is simulated, containing 80 plates (the efficiency of the contact device is 0.8), a feed plate is 33. The feedstock consumption is 62550 kg/h, the temperature is 40°C and the pressure is 294.2 kPa. The convergence of iterative processes by the Honeywell UniSim Design environment was obtained using active specifications: the content of isopentane in the cube is 0.0007wt. share and pentane in the distillate 0.0007wt. share. The obtained technological modes provide almost complete extraction of isopentane from the composition of the raw material, namely by 99.37÷99.7%, while for industrial conditions it is essential to implement the calculated temperature profile along the height of the column. The possibility of using a column with the same diameter in its exhaustive and strengthening parts is shown.