Feasibility Analysis, Dynamics, and Control of Distillation Columns With Vapor Recompression.

Abstract

The conventional distillation system owes its existence to its simplicity, low investment, and inexpensive energy. Inexpensive energy is something that belongs to the past. Therefore, vapor recompression has indeed become more attractive despite the more complex flow arrangements. In this research, a steady state model was developed to study the effect of different variables on the feasibility of heat pump installations. As expected, the heat pump system favored the close boiling mixture against the wide boiling mixture. A relationship between the savings and the thermodynamic availability of the mixture was devised. The savings were also found to be very sensitive to the relative cost of steam and electricity. A possible way of predicting the pressure effect on the heat pump feasibility based on the Antoine equation for the vapor pressure of the key components was also devised. A dynamic model of a distillation column with a heat pump system was developed which includes pressure dynamics in the compressor suction and in the compressor discharge. Pressure dynamics are included in order to study the effect of pressure variations on the overall control strategy. This model was used to perform dynamic and control studies. The presence of stiff differential equations in the dynamic model justified a search for a more efficient integration technique. The LSODE computer software package was found to be the most effective numerical integration package for our system. Basic distillation control schemes which utilize pressure to manipulate the trim cooler outflow were successfully implemented. No interaction was found between the pressure control loop and the product composition control loop. On the other hand, some interaction was observed when the temperature was used to indirectly control the composition. This interaction was found to be closely related to the relative magnitude of the sample time of the pressure controller to the sample time of the temperature controller. Vapor recompression is a feasible way of economizing energy on a distillation unit. The outlook for the use of the heat pump concept has now induced process engineers to consider its applications.