Reflux drum calculation

Calculate liquid densities, molar tray and condenser-reflux drum holdups, ana hquor and vapor enthalpies. Determine holdup and enthalpy derivatives with respect to time by forward difference approximations. 

Example 10 Calculation of Multicomponent Batch Distillation A charge of 45.4 kg mol (100 Ih-mol) of 25 mole percent heuzeue, 50 mole percent monochlorohenzene (MCB), and 25 mole percent orthodichloro-henzene (DCB) is to he distilled in a hatch still consisting of a rehoiler, a column containing 10 theoretical stages, a total condenser, a reflux drum, and a distillate accumulator. Condenser-reflux drum and tray holdups are 0.0056 and... 

As in Example BSTILL, a column containing four theoretical plates and reboiler is assumed, together with constant volume conditions in the reflux drum. The liquid behaviour is, however, non-ideal for this water-methanol system. The objective of this example is to show the need for iterative calculations required for bubble point calculations in non-ideal distillation systems, and how this can be achieved with the use of simulation languages. 

Next calculates the condenser outlet, or reflux drum temperature, that would result from the above water or air temperature (as discussed in Chap. 13). 

Raising the tower pressure also increases the reflux drum pressure, raising, in turn, the temperature at which the vapors condense. The rate of condensation is then calculated from the following ... 

This is, perhaps, an idea you remember from high school, but never quite understood. The phase rule corresponds to determining how many independent variables we can fix in a process, before all the other variables become dependent variables. In a reflux drum, we can fix the temperature and composition of the liquid in the drum. The temperature and composition are called independent variables. The pressure in the drum could now be calculated from the chart in Fig. 3.4. The pressure is a dependent variable. The phase rule for the reflux drum system states that we can select any two variables arbitrarily (temperature, pressure, or composition), but then the remaining variable is fixed. 

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. 

However, we see in this strategy that there is no flow controller anywhere in the recycle loop. The flows around the loop are set based upon level control in the reactor and reflux drum. Given what we said above, we expect to find that this control structure exhibits the snowball effect. By writing the various overall steady-state mass and component balances around the whole process and around the reactor and column. wre can calculate the flow of the recycle stream, at steady state, for any given fresh reactant feed flow and composition. The parameter values used in this specific numerical case are in Table 2.1. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

Follow the calculation procedure outlined in Table 6.6. First, calculate the reflux-drum volume from Equations 6.5.1 and 6.5.2 in Table 6.5. From Equation 6.5.2, take the average of the surge times. 

A dynamic model of a distillation column can be assembled from simpler units, as trays, heat exchangers (condenser, reboiler), reflux drum, valves and pumps (Fig. 4.5). Tray modelling has to answer two issues (1) accurate description of material and energy holdup, and (2) accurate pressure drop calculation. 

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. 

Separation of two liquid phases. If two liquid phases are to be separated, the reflux drum must provide sufficient settling time. The rate of settling can usually be calculated from Stoke s law. A more detailed discussion on settling and decanting is available in Ref. 319 and its cited literature. 

Calculating the height of the column is fairly easy if the number of trays is given. The typical distance between trays (tray spacing) is 0.61m (2ft). If there are Ni stages, the number of trays is N- 2 (one stage for the reflux drum and one for the... 

In preparation for exporting the steady-state flowsheet into Aspen Dynamics, aU equipment is sized. Column diameters are calculated by Aspen Tray Sizing. Reflux drums and column bases are sized to provide 5 min of holdup when 50% full, based on the total liquid entering the surge capacity. Pumps and control valves are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm. 

The usual sizing calculations are performed, the flowsheet is pressure checked, and the file is exported to Aspen Dynamics. Flow controllers are installed on the feed, distillate, and sidestream. Base level is controlled by manipulating bottoms flow rate. Reflux-drum level is controlled by manipulating reflux flow rate. 

Water from the reflux drum is recirculated into the overhead system just before the point at which water condensation begins. Where there are parallel condensing trains, the water must be uniformly distributed through spargers with a 10-psi to 20-psi pressure drop. Enough wash water must be recirculated to bring the overhead vapors to their calculated water dew-point temperature (see Appendix). Note that the vapor from the overhead of the crude lower is already at its hydrocarbon dew-point. 

Lower the liquid level in the drum to minimum. Next, block in the outlet from the drum and note the time for the drum to fill to its maximum safe level. From the vessel geometry, calculate the normal flow rate from the drum. This type of flow interruption can be tolerated in many distillation tower reflux drum services or when a product is being run down to the field. 

Check the pressure drop from the condenser outlet to the reflux drum inlet. Subtract the calculated static pressure loss (i.e., the height of liquid) from the measurement. If the result is more than one-half psi, consider enlarging the condenser outlet line. 

Exponential increase in tray pressure drop Calculated vs observed AP Fractionation drops as reilux rate is increased Reflux drum level jumps Lean-oil circulation rate reduced... 

---