Pressure control hot-vapor bypass

Hot-vapor bypass pressure control. A more modern way of controlling a tower s pressure is shown in Fig. 13.6. This is the hot-vapor bypass method. When the control valve on the vapor bypass line opens, hot vapors flow directly into the reflux drum. These vapors are now bypassing the condenser. The hot vapors must condense in the reflux drum. This is because there are no vapors vented from the reflux drum. So, at equilibrium, the hot vapors must condense to a liquid on entering the reflux drum. They have no other place to go. 

Figure 13.6 Hot-vapor bypass pressure control. Note condensate backup.

You should have concluded by now that hot-vapor bypass pressure control actually works by varying the surface area of the condenser exposed to the saturated vapor. But why do this indirectly Why don t we simply and directly vary the liquid level in the condenser, as shown in Fig. 13.8 ... 

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. 

Problem A hot-vapor bypass pressure control system for a new debutanizer failed to work. 

FIGURE 5.11-2 Schemes Tor control of columa pressure (a) pressure control for na atmospheric column (vent bleed to atmosphere) (b) split ranga valves in a block and bleed arrangement (vent bleed to vacuum) . (c) hot vapor bypass pressure control and (d) flooded condeaser pressure control. 

A hot vapor bypass pressure control system is used on a distillation column. Some ol the vapor from the top of the column passes through a control valve and is added to the vapor space in the top of the reflux drum. The column operates at 7 atm and the overhead vapor is essentially pure isobutane. The vapor pressure of isobutane is given by tlic following equation ... 

Figure 6.3 Isobutane stripper with hot-vapor bypass pressure controller.

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This, then, is how opening the hot-vapor bypass pressure-control valve increases the tower pressure. 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the reflux drum. As the liquid in the reflux drum is at its bubble point, reducing the reflux drum temperature will reduce the reflux drum pressure. As the stripper tower pressure floats on the reflux drum pressure, the pressure in the tower will also decline. 

Figure 17.5 Pressure control by condenser flooding, (a) Control valve in condenser outlet ib) flooded reflux drum (c) flooded reflux drum with automatic noncondensables venting [d) hot vapor bypass (c) a poorly pip hot vapor bypass if) control valve in condenser inlet. (Part c from "Unusual Operating Histories of Gas Processing and Olefins Plant Columns, H. Z. Kister and T. C. Hower, Jr., Plant/Operations Progi a, vol. 6. no. 3, p. 153 (July 1987). Reproduced by permis-...

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. 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the... 

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. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

For column pressure control there are Ihrea genaial approaches vent bleed (to arraosphere or to vacuum system), hot vapor bypass, and flooded condenser. These approaches are illustrated in Fig. 5.11-2.3 For an atmospheric column, the vent approach is quite simple. The vapor bypass represents a temperature bleading method. Partial flooding of the condenser suiface adjusts the bent transfer capability or the condenser. The schemes are generally self-explanatory. 

Column pressure was controlled using a hot vapor bypass scheme (partially flooded condenser). Severe pressure and reflux drum level upsets occurred whenever the reflux drum surface was inadvertently agitated. 

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. 

MISCELLANEOUS PRESSURE-CONTROL TECHNIQUES Hot-Vapor Bypass... 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

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