Accumulators, reflux

Overhead condensers sometimes need to be located in the stmcture. Usually, partial condensers need to be elevated above the reflux accumulator. Considerable stmcture cost reduction can be achieved if the process can use grade-mounted condensers. Mounting the exchangers at grade may require them to be designed with subcooling so that the reflux accumulator can be located above the condenser. This should be considered as part of the process design. 

To determine if steady state conditions exist, the temperatures and pressures in the column can be tabulated to assure that they are reasonably unchanging. Laboratoiy analyses are usually too slow and expensive for checking lined out conditions. Monitoring reflux accumulator boiloff is often an effective way of noting concentration changes. Simply let a sample of the accumulator liquid boil at atmospheric pressure in a bottle with a thermometer inserted. This method is limited to light hydrocarbons and is not accurate enough for precision fractionation. 

The reflux accumulator is a separator used to separate the acid gases from the condensed water. The water is accumulated and pumped back (n the top of the stripper as reflux. With the vapor and liquid rates know n. the accumulator can be sized using the procedures in Volume 1 for iwo phase separator.s. 

Horizontal or vertical tanks are used, depending on the size and duty. Where only a small hold-up volume is required this can be provided by extending the column base, or, for reflux accumulators, by extending the bottom header of the condenser. 

A proportional plus integral controller is used to control the level in the reflux accumulator of a distillation column by regulating the top product flowrate. At time t = 0, the desired value of the flow controller which is controlling the reflux is increased by 3 x 10-4 m3/s. If the integral action time of the level controller is half the value which would give a critically damped response and the proportional band is 50 per cent, obtain an expression for the resulting change in level. 

For this tower, the composition of the overhead vapor is the same as the overhead liquid product, made from the reflux accumulator. 

Subsequently the gas is precooled in exchanger E-6 and charged to a low temperature fractionator. This tower has a reboiler and a top refluxing system. At the top the conditions are 280psig and —75°F. Freon refrigerant at — 90°F is used in the condenser. The bottoms is recycled to the pyrolysis coil. The uncondensed vapor leaving the reflux accumulator constitutes the product of this plant. It is used to precool the feed to the fractionator in E-6 and then leaves this part of the plant for further purification. 

TABLE 18.1 Factors for Sizing Reflux Accumulators a. Factors F, and F2 on the Reflux Flow Rate... 

From equation 7.10, nf= 8. The feed flow and feed temperature control loops remove two of these and the unknown relationship specifying xf (which may be dependent upon the configuration of a plant upstream) reduces nf again by one—giving nf = 5. Hence, we require five control objectives to specify the system uniquely, i.e. to control it adequately. These will be to maintain xD (as a market requirement) and to control S, Pc, and the levels in the base of the column and the reflux accumulator (for operational feasibility). 

A proportional plus integral controller is used to control the level in the reflux accumulator of a distillation column by regulating the top product flowrate. At time t = 0 the desired value of the flow... 

More complex process with absorber, distillation column, pumps, reboilers, condensers, and reflux accumulators. Associated instrumentation is also complex. 

Effects of Column Holdup When the holdup of liquid on the trays and in the condenser and reflux accumulator is not negligible compared with the holdup in the pot, the distillate composition at constant reflux ratio changes with time at a different rate than when the column holdup is negligible because of two separate effects. 

Four alternative control schemes are commonly used for distillation column control, as shown in Figure 3.15 through Figure 3.18, respectively. Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. 

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