Tower pressure controls flooded condenser

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. 

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. 

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. 

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. 

The purpose of the hot vapor bypass controller is to pump heat into the reflux drum. Obviously, if one is limited by condensing capacity, introduction of extra heat to the reflux drum aggravates the limitation. Usually, the rubber type seat in the hot vapor bypass butterfly control valve dries out with age and needs to be renewed. My experience is to eliminate the hot vapor bypass control scheme entirely and convert the tower to flooded condenser-type pressure control. 

Scheme (2) is very effective in the absenee of noneondensable gases. Figure 11.10 shows how the heat transfer area ean be ehanged by flooding the condenser tubes with liquid. The reflux flow essentially sets the separation factor for the tower. If vapor flow into the condenser exceeds liquid flow out, condensate will rise to eover more heat transfer surface. This will cause a pressure rise, whieh in turn will reduce the heat input through the pressure controller. Beeause of the rapid response of vapor flow to heat input, this is a fairly fast loop. 

Trays usually flood because of excessive vapor flow. The vapor flowing to tray 4 is essentially the heating steam flow. Certainly, most of this steam condenses on the trays, as the heating steam comes into contact with the 120°F softened water. It takes about 1 lb of 50-psig steam to heat 7 lb of water from 120 to 250°F. The uncondensed steam is then vented from the top of our tower through the atmospheric vent. But the full steam flow, from the pressure-control valve, does flow up through tray 4. Obviously, when this pressure-control valve is 100 percent open, the weight of vapor flow to tray 4 is at its maximum. 

Flooded condenser control A method of pressure control in a distillation tower. 

The answer has something to do with women s clothes. Perfectly good, sensible designs go out of fashion. The perfectly good, sensible design used to control a tower s pressure,/Iooded condenser control, shown in Fig. 16.8, has gone out of fashion. In a flooded... 

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