Control condensate outlet

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

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. 

Condensation is normally used for the recovery of organic compounds from process or tank vent gases or from releases during loading. Condensation is used to recover valuable compounds prior to incineration, or to reduce the organic load entering other control systems, such as adsorbers or absorbers. Adsorption and absorption processes benefit from low condenser outlet temperatures. 

The flow of steam to a reboiler can be controlled by using a control valve on either (1) the steam inlet line or (2) the condensate outlet line. 

The oldest, most direct method of pressure control is throttling on the cooling-water supply. This scheme is shown in Fig. 13.5. Closing the water valve to the tube side of the condenser increases the condenser outlet temperature. This makes the reflux drum hotter. The hotter liquid in the reflux drum creates a higher vapor pressure. The higher pressure in the reflux drum increases the pressure in the tower. The tower pressure is the pressure in the reflux drum, plus the pressure drop through the condenser. 

I, drying chamber 2, condenser 3, condenser coil 4, jet pump 5, heat exchanger 6, throttle valve 7, pressure controlled N2 outlet 8, LN inlet (from [2.6] page 342). 

Figure 6.2 Schematic diagram of a vacuum system with a condenser between the vacuum vessel and the pump (1) vessel at working pressure p (2) condenser, coolant at T°C (3) vacuum pump (Seff at condenser outlet) (4) throttle valve (5) pressure controller...

Another correlation for vapor-shear-controlled condensation is the Boyko-Kruzhilin correlation [Inf. J. Heat Mass Transfer, 10, 361 (1967)], which gives the mean condensing coefficient for a stream between inlet quality x, and outlet quality x ... 

Condenser wind-down is the key to the column cooling rate at shutdown. The condenser cooling should be cut back so that column pressure is kept on control for as long as possible. The cutback may raise the condenser outlet cooling water temperature this temperatime must be kept below the boiling point, even if it means losing column pressure. 

Controlling heat input by a control valve in the reboiler condensate outlet line may adversely affect reboiler stability, especially in vacuum reboilers. This valve varies condensate level in the heating side of the reboiler. It therefore varies the point at which heating is first applied to the process side. This is equivalent to operating the process side at a fluctuating reboiler sump level. Further details of this and alternative control schemes are in Sec. 17.1.2. 

Typical examples are steam reboilers and refrigeration vapor reboilers. The control valve may be located either in the reboiler inlet line (Fig. 17.1c) or in the reboiler condensate outlet line (Fig. 17.16). 

The dynamic response of the vapor inlet control scheme is far superior to that of the condensate outlet scheme. Manipulating the inlet valve immediately changes the vapor flow. Reboiler pressure and heat transfer rate only lag a few seconds behind. On the other hand, the condensate outlet valve has no direct effect on vapor flow. Condensate flow determines condensate level, and this level changes slowly. Reboiler vapor rate and heat transfer tread on the heels of the level change. Because of this slow response, manipulating vapor flow is a far better means of control than manipulating condensate flow. 

The condensate outlet scheme permits reboiler (condensate-side) operation at a higher pressure because it eliminates the pressure drop in the inlet control valve. 

A smaller control valve is required with the condensate outlet scheme. 

The vapor inlet scheme may be troublesome when there is a small pressure difference between the reboiler heating medium and the condensate header (e.g., with steam reboilers using 15 to 35-psig steam). The problem is identical to that described in item 8 above, but it is caused by insiifficient AP rather them oversizing or clean siuface. In this case, the condensate outlet scheme of Fig. 11.Id is often preferred (234). The vapor inlet scheme shown in Fig. 17.1c can also be used and will provide better control, but at the expense of the additional pmnp. 

Figure 17.5a illustrates a flooded condenser with the control valve located at the condenser outlet. The control valve required is small... 

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-...

Figure 17.5/ shows a flooded condenser scheme similar to that of Fig. 17.5a, but with the control valve located at the condenser inlet. This method is inferior compared to Fig. 17.5a (77). It requires a larger control valve, is more difficult to understand, and it affects condensation at a lower temperature. The condenser outlet line must enter the reflux drum well below the liquid level. A pressure-equalizing line as in the method shown in Fig. 17.5a is also required.

Figure 18.7 Opening pressure-control valve cools condenser outlet.

Mercury vapor discharge from vents of reactors or storage tanks at normal atmospheric pressure is controlled readily by means of activated carbon. Standard units (208-L (55-gal) dmms) of activated carbon equipped with proper inlet and outlet nozzles can be attached to each vent. To minimize the load on the carbon-absorbing device, a small water-cooled condenser is placed between the vent and the absorber. 

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