Column distillation subcooled reflux

In azeotropic distillation, subcooled reflux can lower column ce iacity. 

In certain cases, the desired manipulated variable for a particular process cannot be directly adjusted. Distillation columns can be particularly sensitive to sharp changes in ambient conditions due to weather fronts or thundershowers, since both of these cases can cause significant increases in the reflux subcooling. Equating the heat lost by the condensing vapor to the heat required by the subcooled reflux results in the following equation ... 

D5. A distillation column is operating with a subcooled reflux. The vapor streams have an enthalpy of Hi = H2 = 17,500 Btu/lbmol, while the saturated liquid hi = 3100 Btu/lbmol. Enthalpy of the reflux stream is hg = 1500 Btu/lbmol. The external reflux ratio is set at Lq/D = 1.1. Calculate the internal reflux ratio inside the column, L1/V2. 

The feed flow rate is 2000 kmol/day. Feed is 48 mol% methanol and 52 mol% water and is a subcooled liquid. For every 4 moles of feed, 1 mole of vapor must condense inside the column. Distillate conposition is 92 mol% methanol. Reflux is a saturated liquid, and Lq/D = 1.0. Bottoms conposition is 8 mol% methanol. Boilup ratio is V/B = 0.5. Equilibrium data are given in Table 2-7. Assume that CMO is valid. Find the optimum feed plate location and the total number of equilibrium stages required. 

Condensers for vacuum towers often are operated at temperatures substantially below the dew point of the overhead vapor from the column. This is done to reduce the vapor pressure above the liquid distillate and minimize loss of distillate product in the noncondensable inerts that must be vented. The condenser must be operated above the solidification temperature of any components present in the entering vapor, which may require the use of tempered cooling water. If highly subcooled reflux is returned to the column, one additional theoretical stage should be added to the rectifying section to allow for reheating of the reflux liquid to its boiling temperature. 

The thermal quality of the solvent feed has no effect on the value of (S/F)mjn, but does affect the minimum reflux to some extent, especially as the (S/F) ratio increases. R nax occurs at higher values of the reflux ratio as the upper-feed quality decreases a subcooled upper feed provides additional refluxing capacity and less external reflux is required for the same separation. It is also sometimes advantageous to introduce the primary feed to the extractive distillation column as a vapor to help maintain a higher solvent concentration on the feed tray and the trays immediately below... 

To solve Equation 9.50, start by assuming a feed condition such that q can be fixed. Saturated liquid feed (i.e. q = 1) is normally assumed in an initial design as it tends to decrease the minimum reflux ratio relative to a vaporized feed. Liquid feeds are also preferred because the pressure at which the column operates can easily be increased if required by pumping the liquid to a higher pressure. Increasing the pressure of a vapor feed is much more expensive as it requires a compressor rather than a pump. Feeding a subcooled liquid or a superheated vapor brings inefficiency to the separation as the feed material must first return to saturated conditions before it can participate in the distillation process. 

A mixture containing 12 mol % water is to be separated by distillation into products with 99.5 and 0.5 mol % butanol. The accompanying flowsketch of a suitable process utilizes two columns with condensing-subcooling to 40°C. The 53% saturated solution is refluxed to the first column, and the 98% is fed to the second column. The overhead of the second column contains a small amount of butanol that is recycled to the condenser for recovery. The recycle material balance is shown with the sketch. 

The condenser is the stage where overhead vapors are condensed and liquid is returned to the top of the column as reflux. The condenser is partial if only part of the vapor is condensed and refluxed and the remainder leaves the condenser as vapor distillate. This type of condenser adds one equilibrium stage to the column trays because it holds a vapor phase and a liquid phase at equilibrium with each other. A total condenser is one where the entire overhead vapor is condensed (cooled to the bubble point temperature or subcooled to a lower temperature), part of the condensate is returned as reflux, and the remaining part is taken as liquid distillate. This type of condenser does not count as an equilibrium stage because no vapor-liquid separation takes place in it. The liquid distillate composition is identical to the composition of the vapor leaving the column top tray. 

Thus, in order to define the column operation uniquely, two specifications are required, as already concluded using the description rule (Section 17.1.3). These could be the reflux rate and distillate rate, Lg and D. Note that a subcooled condenser is assumed so that no phase equilibrium equation is written for stage 0 and no Foi variables exist. The column pressure profile is assumed fixed or determined independently from hydraulics calculations and is not included in the column variables. Also, the enthalpies and phase equilibrium coefficients are, in general, functions of the temperature, pressure, and composition (Chapter 1) and are therefore not considered as additional unknown variables. 

If reflux cannot be cut back (e.g., in an unrefluxed stripper, in azeotropic distillation, or when the packed section above the feed is close to its minimum wetting limit), boilup will need to be raised to compensate for the excess subcooling. Vapor and liquid traffic below the feed and reboiler duty will rise and effectively lower the column feed capacity. Premature flooding may result. If the lower capacity or higher reboiler duty cannot be tolerated, feed preheating (Fig. 12.5a)... 

In contrast, consider the response of schemes 16.4a,6, or e to a similar change. The drop in accumulator level will reduce distillate flow, while reflux flow rate will remain unaltered. The same quantity of reflux will enter the column, but at a lower temperature. It will be reheated upon entry by vapor condensation, ind this will increase liquid flow down the column. This is not the desired response. Eventually, the appropriate response will be established, but not until the control tray temperature drops and the temperature controller takes corrective action. In scheme 16.46, a subcooling disturbance disturbs at least the top part of the column. With the Fig. 16.4a and e schemes, it will distimb the entire column. 

Up to this point we have considered conplete distillation columns with at least two sections. Columns with only a stripping section or only an enriching section are also commonly used. These are illustrated in Figures 4-24A and B. When only a stripping section is used, the feed must be a subcooled or saturated liquid. No reflux is used. A very pure bottoms product can be obtained but the vapor distillate will not be pure. In the enriching or rectifying column, on the other hand, the feed is a superheated vapor or a... 

D30. A distillation column is separating methanol from water. The column has a total condenser that subcools the reflux so that 1 mole of vapor is condensed in the column for each 3 moles of reflux. Lq/D = 3. a liquid side stream is withdrawn from the second stage below the condenser. This side stream is vaporized to a saturated vapor and then mixed with the feed and input on stage 4. The side withdrawal rate is S = 500 kmol/h. The feed is a saturated vapor that is 48 mol% methanol. Feed rate is F = 1000 kmol/h. A total reboiler is used, which produces a saturated vapor boilup. We desire a distillate 92 mol% methanol and a bottoms 4 mol% methanol. Assume CMO. Equilibrium data are given in Table 2-7. Find ... 

D34. A distillation column with two equilibrium stages and a partial reboiler (three equilibrium contacts) is separating methanol and water. The column has a total condenser. Feed, a 45 mol% methanol mixture, enters the column on the second stage below the condenser. Feed rate is 150 mol/h. The feed is a subcooled liquid. To heat 2 moles of feed to the saturated liquid tenderature, 1 mole of vapor must condense at the feed stage. A distillate concentration of 80 mol% methanol is desired. Reflux is a saturated liquid, and CMO can be assumed. An external reflux ratio of L7D = 2.0 is used. Find the resulting bottoms concentration Xg. Data are in Table 1-1. 

E2. A distillation column is separating methanol and water. The column has open (direct) steam heating and a total of five stages. A liquid side stream is withdrawn from the second plate above the bottom of the column. The feed is 30 mol% methanol and is a subcooled liquid. One mole of vapor is condensed to heat 2 moles of feed to the saturated liquid tenperature on the feed plate. Feed rate is 1000 mol/h. A bottoms concentration of 1.5 mol% is desired. The steam used is pure saturated water vapor and the steam flow rate is adjusted so that Steam flow rate/Bottoms flow rate = 0.833. The side stream is removed as a saturated liquid. The side-stream flow rate is adjusted so that Side-stream flow rate/Bottoms flow rate = 0.4. A total condenser is used. Reflux... 

E4. A distillation column is separating methanol and water at a pressure of 1 atm. Feed rate of the mixture is 100 kmol/h. The feed is 25 mol% methanol and 75 mol% water. The feed is a subcooled liquid. To heat the feed to the boiling point, one mole of vapor condenses on the feed stage for every 10 moles of feed. We desire a distillate mole fraction that is 90 mol% methanol and a bottoms mole fraction that is 2.5 mol% methanol. We operate with a boilup ratio of 1.0. The column is operated with two reflux streams. The reflux to the first stage operates with a flow rate of Lq = 21.4 kmol/h. Sufficient reflux (L ) is returned to stage 2 for the column to operate. Both... 

Scheme J. This scheme directly adjusts the column material balance by manipulation of the distillate flow. The main advantage of this scheme is that it has the least interaction with the eneigy balance. In terms of a McCabe-Thiele diagram, this means that the slopes of the column operating lines can be held constant in spite of energy balance upsets. This independence ftom energy balance upsets is achieved by the scheme s ability to maintain a constant internal reflux even for variations in external reflux subcooling. When the temperature of the external reflux varies, the external reflux adjustment to maintain accumulator level offsets temporary internal reflux variations. If the accumulator level loop responds rapidly, the dis-tuibanoe will not propagate down the column, and the column s overall material balance remains undisturbed.

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