Condensate removal Liquid level control

Figure 3.9. Steam heaters, (a) Flow of steam is controlled off the PF outlet temperature, and condensate is removed with a steam trap or under liquid level control. Subject to difficulties when condensation pressure is below atmospheric, (b) Temperature control on the condensate removal has the effect of varying the amount of flooding of the heat transfer surface and hence the rate of condensation. Because the flow of condensate through the valve is relatively slow, this mode of control is sluggish compared with (a). However, the liquid valve is cheaper than the vapor one. (c) Bypass of process fluid around the exchanger. The condensing pressure is maintained above atmospheric so that the trap can discharge freely, (d) Cascade control. The steam pressure responds quickly to upsets in steam supply conditions. The more sluggish PF temperature is used to adjust the pressure so as to maintain the proper rate of heat transfer.

If sufficient height is not available for a barometric leg, condensate may be collected in a small accumulator tank, the liquid level controlled, and condensate removed from the tank with a centrifugal pump. Such systems are called "low-level" hotwells and are illustrated in Figure 23-6. Each intercondenser must have its own accumulator, pump, and level control. Because this is costly, most jet systems are located high in the structure and use the barometric leg and hotwell system. 

The reactor effluent passed through porous stainless steel filters at the top of D-1 to a water-cooled condenser and then to the high pressure separator, F-4. The liquid from the high pressure separator was removed through a level control valve at the bottom of FA to a low-pressure receiver, F-7, which was surrounded by ice water. 

The liquid level in the flash drum is controlled by manipulating liquid product Lc. A temperature controller manipulates heat removal in the condenser to hold drum temperature constant. Compressor speed is controlled, which is assumed to fix the recycle gas flowrate FR. 

In the last three chapters, we have developed a number of conventional control structures dual-composition, single-end with RR, single-end with rellux-to-feed, tray temperature control, and so on. Structures with steam-to-feed ratios have also been demonstrated to reduce transient disturbances. Four out of the six control degrees of freedom (six available valves) are used to control the four variables of throughput, pressure, reflux-drum level, and base level. Throughput is normally controlled by the feed valve. In on-demand control structures, throughput is set by the flow rate of one of the product streams. Pressure is typically controlled by condenser heat removal. Base liquid level is normally controlled by bottoms flow rate. 

However, if condenser duty is being used to control pressure, reboiler heat input will also indirectly affect liquid level in the reflux drum. A change in vapor generation in the reboiler changes column pressure, which causes the pressure controller to change the condenser heat removal. Therefore, it is feasible to control reflux-drum liquid level with reboiler heat input. This configuration permits the use of the R/F stmcture. 

Liquid level in the stripper reflux drum is controlled by manipulating the heat removal in the condenser using a proportional Kq = 2 controller. 

Many evaporator calandrias condense steam at subatmospheric pressures. Consequently, condensate must be removed by pumping, either with mechanical pumps or pumping steam traps. Large condensate loads are usually best handled when liquid levels are controlled and condensate removed with the appropriate method. 

Control is relatively sluggish because liquid levels cannot be changed rapidly unless provisions are made to pump condensate to and from the condenser. This requires a separate condensate source. Generally, with flooded systems, condensate build-up occurs only at the rate at which it can be condensed, while condensate removal can be effected fairly rapidly. 

A conventional control structure for this process, which works for low to moderate activation energies, is shown in Figure 9. The flowrate Fob of gaseous fresh feed of B is manipulated to control system pressure. The flowrate Fqa of gaseous fi esh feed of A is ratioed to Fob and the ratio is reset by the composition controller, which maintains the composition of A in the circulating gas stream at yuA = 0.5. A bypass stream around the FEHE controls the furnace inlet temperature. Furnace firing controls reactor inlet temperature. Separator drum temperature is controlled by heat removal in the condenser (typically the cooling water valve is wide open to minimize drum temperature). Liquid product comes off on drum level control. 

Figure 11.4 shows the single reactive column operating in neat mode with two ternperamres controlled by manipulating the two fresh feeds. The reflux ratio is controlled. Vapor boilup is the production rate handle. Liquid levels are controlled by product removal rates. Column pressure is controlled condenser cooling water in all of the control stmcmres and is not shown in the figures. 

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

Our final example of a complex column is an azeotropic system in which we add a light entrainer to facilitate the separation of two components. The classical example of this type of system is the use of benzene or cyclohexane to break the ethanol-water azeotrope. As shown in Fig. 6.26, the vapor from the top of the column is condensed and fed into a decanter in which the two liquid phases separate. The aqueous phase is removed as product. The organic phase (the light entrainer) is refluxed back to the column. Some of the organic may also be added to the feed stream to alter the composition profiles in the column (if more entrainer is needed lower in the column). Note that the organic level in the decanter is not controlled. A small stream of fresh entrainer ivould be added to make up for any losses of entrainer over a long period... 

Another way of reducing the efficiency of the condenser is to reduce the effective surface area used for heat transfer. Figure 12.39 shows how, by placing the control valve under the condenser, liquid can accumulate in the condenser. This flooded condenser is less efficient because less heat transfer takes place in the submerged part of the tube bundle. Here sensible heat is removed in subcooling the liquid, in the exposed part of the bundle, heat of vaporisation is removed condensing the vapour. The pressure controller indirectly changes the level of liquid in the condenser. 

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