Study On Control Method Of Slightly Positive Pressure For Distillation Columns

Operating pressure is an important variable in distillation process, which impactson product quality, separation efficiency and operating costs. According to thepressure control mechanism, control methods can be classified into two categories;one is based on energy balance, the other is by directly controlling the top pressure ofcolumn. In practical work, some particular material can be perishable and explosivewhen exposed to air, so positive pressure nitrogen protection is need to isolate theparticular material from the air. In order to solve this problem, a method of nitrogencharging system was proposed as a slightly positive pressure control method fordistillation. The control system consists of three parts, a constant pressure tank, apositive pressure tank and a negative pressure tank, and the constant pressure tank isconnected to the top of distillation column.First this paper illustrates the adjustment process and flow process, and thenanalyzed the relationship between constant pressure tank pressure, negative pressuretank pressure, positive pressure tank pressure and constant pressure tank pressuredeviation, and the response time of pressure disturbance. The results showed that theresponse time was less than1s, which indicated a good sensitivity.Secondly, the equilibration time of three tanks under different pressure weresimulated by chemical process simulation software-Fluent. The results showed thatthe response time of three tanks were far less than1s under the experimental pressures,which also indicated a good sensitivity of the control method.Lastly, the experiment was conducted on a slight-positive pressure batchdistillation system with a mixture of methanol and water, and the process wascontrolled by the proposed control method. The results showed that the top pressureof the column can be effectively controlled by the control system. During actualmanufacturing process, the pressure of the constant pressure tank should be chosenproperly according to different mixtures, as well as the coefficients of positive andnegative pressure tanks, just to reduce the pressure deviation of constant pressure tankduring controlling process. Another pressure control method by changing the flowrate of the cooling water is also researched experimentally. The results show that the method of nitrogen charging system is more sensitive than the method by changingthe flow rate of the cooling water.