Instrumentation and Safety Circuits

Although a batch distillation column can be run manually by an attentive and experienced technician, the dynamic nature of operation requires extensive instrumentation for all but the simplest mode of operation. To gain the flexibility necessary for the operation at constant distillate composition, flow rates of the reflux and distillate must be independently controlled. In small units devoted to research, flow control can be easily accomplished by varying the speed of a gear pump through a control panel displayed on a computer screen. The connection of the pump to the computer consists of a driver board wired to a screw terminal connector. The latter is attached to an analog output board. The output signal to the driver board can be varied based on the results of calculations performed by external application programs. If the top composition is to be kept constant, the update on the reflux rate can be calculated by a user routine with the aid of operating charts such as those described later. The value of the reflux rate translated to flow rates (typically current values) controls the distillate and reflux pumps. The product and reflux flow rates must be constrained to balance the rate of condensation. This is conveniently accomplished by monitoring the condenser level as an indicator of the difference between boil-up and distillate and product flow rates. The direct use of the heat load output to control the condenser level is not recommended due to the large dead-time between a change in the reboiler conditions and the resulting effect in the liquid level. The result of the reflux calculation is stored in a data buffer from which it can be retrieved by another application and used to update the corresponding analog output channel.

Analog and digital I/O boards can also be installed to retrieve information such as cooling water temperature and flow rate, steam pressure, flow rate to the reboiler and the temperature profile of the column. The stage temperature is a direct indicator of stage composition; however, it is only useful as a control variable when the temperature variation between successive stages is significant. The greatest variation in temperature during the batch run will take place in the reboiler which is an excellent means of monitoring overall progress of the separation.

Variation of the heat load to the column can be actuated remotely by fitting the steam valve with a stepper motor. A variable heat load adds operation flexibility and can be used in conjunction with other strategies to maximize recovery and purity of a desired component at a lower energy cost. For example, the feed drum contains the highest fraction of the volatile component at the beginning of the process. Therefore, the column can be started up at a lower boil-up rate, which will be gradually increased to match the enrichment of the charge in the heavier component. Figure 3 shows typical column instrumentation, and data acquisition and control.

Three operating conditions are monitored constantly and may independently activate the safety system, stopping steam delivery to the reboiler by closing a steam safety valve that precedes the steam controller valve. These monitored conditions are the column pressure, cooling water flow and ventilation fans. The pressure transducers, flow transmitter and power to the fans are set up as a logic relay where loss of any one of the direct current voltages is sufficient to actuate the steam safety valve. It is important to choose the logic of these circuits such that electrical and mechanical failures will default to termination of the batch run. The safety system circuitry is depicted in Figure 4.

Solar Panel Basics

Solar Panel Basics

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