Control differential pressure

When the free stream in an MB control system (Sec. 16.2) is the boilup rate, it is sometimes manipulated by the differential pressure across a column section or across the entire column (Fig. 19.12a). Column pressure drop is primarily a measure of column vapor load, although it is also influenced by the liquid load. Therefore, controlling differential pressure generally maintains a uniform vapor load in the column. 

Differential pressure control is popular in batch stills (44). Here 

Differential pressure measurement is generally one of the least reliable measurements in a column (Sec. 5.4). Low pressure drops (e.g., packed towers) and density variations (e.g., vacuum towers) make a reliable measurement particularly difficult to achieve. 

The suitable control range of differential pressure controllers may be narrow, especially with valve trays (Fig. 19.126). In the valvethrottling range of a valve tray, and below the weep point of a sieve tray, pressure drop may be insensitive to vapor loads. Differential pressure control may therefore be difficult to apply imder turned-down conditions. Packed towers are not prone to this limitation (44), because pressure drop tends to be sensitive to vapor load over the entire operating range (Fig. 14.3). 

The turndown problem can be overcome by cascading the differential pressure controller onto a heating medium flow controller. Under turned-down conditions, the differential pressure control is taken off cascade, and the heating medium is flow-controlled. The Fig. 19.12a scheme, for example, will then revert to scheme 16.46. 

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A differential pressure controller acts in split range on the inlet control valve and the bypass valves. The differential pressure governor is retained as the standby and backup system. 

A pressure limiting controller, in the event of excessively high absolute pressure in the regenerator, disables the differential pressure controller and limits the pressure to a preset maximum value. 

For column analysis and troubleshooting it is important to have pressure drop measured with a DP cell. The differential pressure can also be used to control column traffic. A good way to do this would be to let the differential pressure control the heating medium to the reboiler. The largest application for differential pressure control is with packed columns where it is desirable to run at 80 to 100% of flood for best efficiency. 

For liquid film seals, an overhead tank is normally used. The tank functions as part of the differential pressure control. The process gas is referenced to the top of the tank, and the tank s physical height becomes a manometric leg. The oil level is controlled in the tank. Figure 8-11 shows an alternate arrangement for the overhead seal tank, including the recommended operating levels and volumes. The material of the seal oil tank should be stainless, the same as that recommended for the accumulator. 

Normally, the reactor temperature and the stripper level controllers regulate he movement of the regenerated and spent catalyst slide valves, le algorithm of these controllers can drive the valves either fully Of [ or fully closed if the controller set-point is unobtainable. It is ext nely important that a positive and stable pressure differential be mail ined across both the regenerated and spent catalyst slide valves. r safety, a low differential pressure controller overrides the tempera re/level controllers should these valves open too much. The shutdov is usually set at 2 psi (14 Kp). 

Differential pressure control is often used on packed columns to ensure that the packing operates at the correct loading see Figure 5.22d (see p. 234). 

Condensate/main header differential pressure control system. 

Maintenance of a PSVE system is expected to cost 2% of the installed capital cost of the system per year. Operation and waste disposal costs are a function of concentration of contaminants and the airflow rate and will therefore vary widely (D14489S, p. 26). Once a system is installed, no utilities are generally required. If a valve and differential pressure control system are used, these could be run by solar-cell-powered batteries (D18119L, p. 384). 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Figure 8-53 shows a propylene-propane fractionator controlled at maximum boil-up by the differential pressure controller (DPC) across the trays. This loop is fast enough to reject upsets in the temperature... 

Monitoring differential pressure between the primary and secondary cooling systems would be one of the candidates for detecting the heat transfer tube rupture. Meanwhile, the pressure varies even at normal operation and therefore there is a concern of malfunction of CV isolation valves. In case of primary helium gas recovery flow rate, primary helium pressure control system only covers the rated operation since the system activates when the primary coolant pressure reaches about 3.95 MPa. On the other hand, the primary-secondary differential pressure control system covers start-up, shutdown and rated operations. The control system keeps supplying helium gas to the secondary cooling system during the scenario. Thus, monitoring the entire secondary helium gas supply would be an effective way to detect the tube rupture. 

Figure 5.27. Cont d. (c) Composition control. Top product take-off and boil-up controlled by feed, (d) Packed column, differential pressure control. Eckert (1964) discusses the control of packed columns, (e) Batch distillation, reflux flow controlled based on temperature to infer composition.

DPC - differential pressure controller DPT - differential pressure sensor/transmitter... 

If the differential pressure control loop were extremely fast, we could assume that the differential pressure across the valve remained constant, and the linearization exercise would be simplified enormously. But we shall examine the case where the differential pressure control loop is slow, and needs to be allowed for in calculating the response of flow to valve opening. 

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