Floating pressure

To prevent this rapid drop, Shinskey developed a floating-pressure control system, sketched in Fig. 8.S. A conventional PI pressure controller is used. The output of the pressure controller goes to the cooling water valve, which is AC so that it will fail open. The pressure controller output is also scot to another controller, the valve-position controller (VPC). This controller looks at the signal to the valve, compares it with the VPC setpoint signal, and sends out a signal which is the setpoint of the pressure controller. Since the valve is AC, the setpoint of the VPC is about 5 percent of scale so as to keep the cooling water valve almost wide open. 

Note that in the floating-pressure application, there was only one manipulated variable (cooling-water flow) and one primary controlled variable (valve position). In the reactor temperature-control application, there are two manipulated variables and two controlled variables (temperature and refrigerant valve position). 

We want to analyze the floating pressure valve position control system proposed by Shinskcy. Let the transfer function between cooling water flow and column pressure be... 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Optimization and vacuum control strategies (a) minimizing (floating) pressure by maximizing coolant valve opening, (b) floating pressure control of partial condenser with vapor distillate, (c) floating pressure control when the distillate is both vapor and liquid. 

Figure 17.7 Miscellaneous pressure control methods, (a) Recirculation temperature control (b) Column overhead control (c) inerts pressure control (d) drained condenser bypass (e) pressure-reboil control (/) floating pressure control, combined with coolant flow manipulation.

Floating pressure control. While tight pressure control is mandatory in distillation columns, it may be desirable to slowly manipulate the set point of the pressure controller. This is the essence of Shinskey s "floating pressure control (362). The method minimizes column pressure without violating constraints such as maximum condenser or column capacity, or the ability to send column products to downstream units. 

The objective of the preheat control system is to supply the column with a feed of consistent specific enthalpy (enthalpy per unit mass). With a single-phase feed, this translates into a constant feed temperature with a partially vaporized feed, this translates into a constant fractional vaporization. Maximizing feed temperature (if desired) is usually performed manually, by an advanced control system, or by a valve position controller similar to that used in floating pressure control (Sec. 17.2.4). 

Kister, H. Z., and I. D. Doig "When Would Floating Pressure Strategy Save Energy , Chem. Eng. Prog. 77(9), 1981, p. 55. 

However, the steam flow set point signal coming from the TC is also sent as the process variable signal to a valve-position controller (VPC) whose set point signal is the minimum steam flow rate. Valve-position control is a type of optimizing control suggested over three decades ago by Shinskey who used it to achieve floating pressure control in distillation columns. Many other practical applications have been described. ... 

Fig. 16.1. Sketch and environmental model of distillation column with floating pressure. L<,oB

Since, in most cases, columns with floating pressure have a higher pressure at high throughput and lower pressure at lower throughput, letdown valves will have highest APp at high flows and lowest APp at low flows. The prcxluct will usually be hot, so anticavitation trim may be required. 

Level control is cascaded to flow control. With floating pressure columns and with the trend toward small control-valve pressure drops for energy conservation, this is virtually mandatory to counteract the effect of control valve up- and downstream pressure variations. The flow measurement must be linear if an orifice flow meter is used, it must be followed by a sqiaare-root extractor. 

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