Condensing pressure control

In all forms of condenser pressure control, the minimum maintained pressure should he the lowest which will give satisfactory operation, in the interests of running economy. An indication of the relative electricity costs for a 350 kW air-conditioning plant is given in Table 9.1. 

Improved condenser pressure control. If the expansion valve is too tightlyrated to accept lower condenser pressure, change the expansion valve, possibly for the electronic type. 

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 3.1, was operating between 5 and 15 percent open, and hence it was responding in a nonlinear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows. 

Figure 13.8 Flooded condenser pressure control the preferred method.

In general, flooded condenser pressure control is the preferred method to control a tower s pressure. This is so because it is simpler and cheaper than hot-vapor bypass pressure control. Also, the potential problem of a leaking hot-vapor bypass control valve cannot occur. Many thousands of hot-vapor bypass designs have eventually been converted—at no cost—to flooded condenser pressure control. 

Sometimes we see tower pressure control based on feeding a small amount of inert or natural gas into the reflux drum. This is bad. The natural gas dissolves in the overhead liquid product and typically flashes out of the product storage tanks. The correct way to control tower pressure in the absence of noncondensable vapors is to employ flooded condenser pressure control. If, for some external reason, a variable level in the reflux drum is required, then the correct design for tower pressure control is a hot-vapor bypass. 

Figure 3.11. Condenser pressure control with inert gas.

FIGURE S.11 2 Sdiemes for control of column pressure (a) pressure control for an atmospheric column (vent bleed to atmosphere) (b) split range valves in a block and bleed anangement (vent Weed to vacuum) . (c) hot vapor bypass pressure control and (flooded condenser pressure control. 

The condenser pressure controls the tower pressure and thus the feed tray pressure. There is a pressure valve in the overhead, which can be used to control tower pressure. The lower limit of the tower pressure is defined by the column overhead condensing duty, net gas compressor capacity, and column flood condition. During extended turndown periods, reducing pressure up against an equipment limit can avoid dumping. Many of the new APC systems have pressure control implemented. 

Second, changes in column pressure have other impacts such as changes in the off-gas rate, the amount of reboiler duty, and hydraulic profile of the plant. In the case of partial condensation, pressure control can interact with the overhead receiver level. While these effects are real, their magnitude is sometimes exaggerated and cited as reasons for not making any changes. 

One problem with flooded condenser pressure control is related to the need to occasionally vent non-condensables. This vent valve must not leak when closed. Since the drum is normally full of liquid, a leaking vent valve will pass liquid. Many pounds of product can be lost in this way. A butterfly control valve with a soft, rubberized seat is a good choice for a remotely controlled, non-condensable vent valve. 

Air-cooled condensers seem to work as well as shell-and-tube water coolers in flooded condenser pressure control service. Some air coolers are sloped toward the outlet to reduce the ratio of the delta condenser surface area to delta height change of condensate backup. I don t think this is necessary, but it may help in fine-tuning pressure control with air-cooled flooded condensers. 

But don t just take my word for it. Try running on flooded condenser pressure control rather than hot-vapor bypass, and I think you will be quite pleased with the result. 

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