Column Pressure Control

One common strategy for controlling column pressure is to manipulate a valve in a vent line from the reflux drum. If necessary, another line and automatic valve can be added to inject inerts, for example, nitrogen, before the vent valve. If the distillate is taken as a vapor stream, then the condenser may need to be run as a partial condenser with temperature-controlled cooling liquid on the condenser. 

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Chin states that a scheme having only a control valve in the hot gas bypass line manipulated by the column pressure controller will not work for the case of zero net... 

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. 

The VPC scheme is a different type of cascade control system. The primary control is the position of the valve. The secondary control is the column pressure controller is PI and is tuned fairly tightly so that it can prevent drops in pressure. Its setpoint is slowly changed by the VPC to drive water valve nearly wide open. A slow-acting, integral-only controller in the VPC. 

Column pressure control is frequently integrated with the condenser control system. This control is often regarded as the most important control in the column. It has been the author s experience that a column will not achieve stable operation imless steady pressure can be maintained. 

As previously described (Chap. 16), column pressure control is usually integrated with the condensation system. Pressure and condensation controls therefore need to be considered simultaneously. An ex-... 

Figure 17.7e shows column pressure control by adjusting column boilup. This method is complex, but it has worked smoothly in some instances (234). Either a flooded or a nonflooded reflux drum can be used in the latter case, reflux drum level can regulate the rate of condenstaion. Bottom flow is regulated by the bottom sump level. This method may be beneficial in some stripping columns receiving subcooled feeds, where feed temperature variations can affect column pressure to a larger extent than overhead condenser action. 

The vapor contains components that can condense out and are undesirable in the liquid in excessive quantities vaporized liquid losses in the gas incur a significant economic penalty Here excessive condensation will render the liquid product off-spec on lights insufficient condensation will cause too much liquid product to escape in the vapor stream, incurring an economic penalty. In this case, in addition to column pressure control, the rate of condensation must be controlled to obtain the desired vapor-liquid split. This case is perhaps the most common in the chemicals industry and is discussed in detail below. 

Controller Tuning. The reflux drum and column base are sized to provide 5 min of holdup when at 50% level. The base level control is proportional only with Kc = 2. The column pressure controller used Aspen default tuning of Xc = 20 and Ti= 12 min. 

Step 2 Start the heat up of the column and put aU controllers in automatic mode (middle tray temperature control, top column pressure control, and decanter temperature control). Step 3 When the bottom composition reaches 0.991 (mole fraction of acetic acid product), stop heating and collect bottom, aqueous, and organic products. The reason for using a stop criterion of 0.991 instead of the specification of 0.990 is to compensate for the small amounts of material on the column trays that drain into the bottom. 

If the drum is not flooded then it is better if the condensate enters the drum above the liquid level to avoid interaction between the dmm level controller and the column pressure controller. Otherwise a change in liquid level will affect the column pressure. A pressure equalising line between the column and the drum, although not essential, will keep the... 

The ideal solution is to locate control valves in both locations, as shown in Figure 12.47. That in the condensate line is used to control column pressure, while that in the bypass used to control drum pressure. If the condenser is below the drum there must be sufficient difference between the two pressures to overcome the maximum liquid head. If this is not the case then, on high pressure, the column pressure controller will saturate and the pressure will rise until it is sufficient to overcome the head. While not necessarily unsafe, it does mean that full control of pressure will be lost. 

Rather than rely on the process operator to maintain sufficient pressure difference, the drum pressure controller may be replaced by a differential pressure controller (dPC), as shown in Figure 12.48. The SP of this controller could be fixed and not adjustable by the operator, or a safe minimum limit configured. It might also include logic that disables column pressure control if the dPC is switched to manual. 

The two controllers will interact. If column pressure rises above its SP the controller will open the condensate valve so that more vapour passes through the condenser. But, if the column pressure rises, the measurement of the dPC will also increase and its controller will respond by opening the bypass. This will reduce the flow of vapour to the condenser - the opposite of what is needed. To break the interaction the dPC should be configured to use the SP of the column pressure controller, not its PV. 

Cooling-water temperature changes are usually seasonal, and will require no specific correction. If, for some reason, they are large and rapid, then it may be desirable to provide an enthalpy control system for the condenser. By measuring the temperature rise of the cooling water across the condenser, and multiplying it by the cooling-water flow rate, one has a measure of the heat transferred, This calculated value of qc can serve as the measured variable in an enthalpy control system. For column pressure control, the enthalpy control system can serve as the secondary loop in a cascade system. 

Column pressure control with flooded condenser... 

Condenser dynamics are radically improved over those achieved with once-through ctx>lant. Speed of response is greater and condenser dynamics do not change with load changes. Condensate-temperature and column-pressure control are easier. 

For tight pressure contol, we should use these models with caution. Most of the tight column pressure controls we have studied have closed-loop resonant frequencies in the range of 0.5-2 cpm. For the upper value one should make at least a rov h check of condenser and reboiler dynamics. It may be of interest that the only applications of tight pressure control we have found are in heat-recovery schemes where the vapor from one column serves as the heating medium for the reboiler of another column, and perhaps furnishes heat to other loads. If the vapor flow must be throttled to each load, constant up- and downstream pressures help good flow control. 

Partial signal flow diagram for column pressure control via manipulation of iner gas and vent valves... 

Column pressure control via flooded condenser drain-negligible inerts and reboiler steam flow or flow—ratio controlled... 

Column pressure control via flooded condenser, reboller steam not flow or flow ratio controlled, significant Inerts... 

This method of controlling pressure, although once popular, has fallen into some disfavor in recent years. This is particularly true for once-through coolant. Since its flow rate cannot be allowed to go too low—which would lead to fouling as well as excessive exit coolant temperatures, which, in turn, contribute to corrosion—it permits only limited control of pressure. Tempered coolant, which avoids these problems, is a better choice for column pressure control via coolant flow manipulation. 

In this analysis, column pressure control has been disregarded, as would be the case, for example, if the column overhead is vented to another vessel at atmospheric pressure. When pressure control must be considered, the flow rate of cooling water to the condenser will be a logical manipulated variable, and an energy balance around the condenser/reflux drum must be added to the model. The number of single-loop controllers would then be five. 

Figure 8.6 shows a scheme where column pressure is controlled by regulating the flow of the vapour product from the accumulator. The reflux is on flow control. A level controller is required to control the coolant flow in order to maintain accumulator liquid inventory. This method provides a smooth, rapidly responding column pressure control. 

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