Residence time reflux drum

Rmin and the corresponding number of trays calculated ( 2N J. The shortcut models were replaced by rigorous RADFRAC units, where the reflux and distillate feed ratio were adjusted by means of design specifications, in order to meet the desired separation. The trays were sized using Aspen s facilities. Finally, the dimensions of the reflux drum and column sump were found based on a residence time of 5 min and aspect ratio H D = 2 1. Table 9.7 presents the results of distillation column sizing. 

We consider sieve trays with a diameter d = 0.13 m. Reflux drum and column sump are sized by assuming residence times of 5 min and 10 min, respectively. 

For example, the cost of a distillation column can be assembled from the cost of elements vertical cylindrical vessel, plus internals (trays or packing), reboiler, condenser, and reflux drum. The height of the shell can be determined from the number of trays and inter-stage height. The column diameter can be found by hydraulic calculations based on the flooding point. In this way, the volume of the cylindrical part can be easily evaluated. The volume of auxiliary vessels, as drum and reboiler, can be estimated from the residence time, typically of 10 minutes. 

The sizing of the vessels around the column is based on allowable residence times. For the reflux drum and reboiler sump the residence these are of 5 to 10 minutes. The choice between kettle and thermosiphon reboilers could be justified by the observation that the last gives less trouble in operation (Kister, 1992). 

When vapor is totally condensed, a cylindrical, horizontal reflux drum is commonly employed to receive the condensate. Equations (13-9) and (13-11) permit estimates of the drum diameter and length Ly by assuming an optimum LylD of four and the same liquid residence time suggested for a vertical drum. 

Unnecessarily large reflux drum residence times should be avoided. These do not only increase drum cost but also enhance liquid inventories (a distinct drawback when the liquid is hazardous) and incur an excessive composition lag that interferes with column control. 

Consider a simple dynamic system, the reactor/ column plant described in Table G.l, and assume that the column dynamics are fast compared to the reactor dynamics. Table G.3 indicates that the holdups in these two units are Hr = 2,400 lb-moles and Hr + 20 Hs + Hj) = 930 lb-moles. Because each unit has the same flow rate F, the mean residence times for the two units are in the ratio of 2,400/930, or approximately 2.5. The effect of chemical reaction normally is to make the reactor time constant somewhat smaller than its mean residence time (see Eq. 4-89) however, the portion of column holdup located directly in the recycle loop, that is, the reflux drum plus the stripping stages, is only about one-half the total column holdup. Thus, the actual ratio of the basic time constants for the two units is... 

Column diameter is calculated from the sizing relationships given in Chapter 3. A liquid height of 0.05 m is assumed on the stripping trays, giving a tray holdup of 650 mol on these trays. The holdup on the reactive trays is 1000 mol. The holdups in the column base and reflux dmm are sized to give a 5-min residence time when 50% frill, based on the total liquid entering. In the reflux drum this is the reflux flowrate, which is equal to the overhead... 

---