Columns with Stripping Vapor Feed

A material balance on the lighter component around a column section between any stage j in the stripping section and the bottom stage is written as 

Tj and Vj. The above equation, rearranged, is the stripping section operating line  

Applied to the bottom stage, j-1 = N,j = N+l,B = L, Y= = 0, and X = Xg. That is, the operating line intersects the X-axis at X = Xg. This intersection point represents passing streams below the column. With open steam, the operating line crosses the diagonal and ends at the X-axis. Equilibrium stage steps are constructed in the usual manner, between the operating line and the equilibrium curve. 

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The chemical driving force on a stage has inlet and outlet concentrations as boundary conditions. In the enriching section below the feed plate, a flowfrom liquid to vapor occurs while in the stripping section, the direction of flow is from vapor to liquid. For the column with specified inlet and outlet compositions, the entropy production rates are... 

Description The progressive Morphylane extractive distillation is a single-column extractive distillation configuration. The aromatics are removed from the vaporized feed material by the solvent in packing (2) until a residual content of 0.5-1% is reached. However, some of the nonaromatics are also dissolved. These are stripped off by aromatics vapors in packing (3). The solvent traces which go to the column head with the nonaromatics proportionate to the vapor pressure of the solvent are washed back with the nonaromatics reflux. 

The low volatility of the solvent also dictates that it be introduced in the column near the top, above the main feed, in order to maintain the required solvent concentration in as many trays as possible. The column thus contains three sections the top section between the overhead product and the solvent feed, the middle section between the solvent feed and the main feed, and the bottom section between the main feed and the bottoms product. The top section serves to reflux any solvent that might have risen with the vapor above the solvent feed. The objective is to minimize the solvent concentration in the overhead product. The middle section serves to absorb the component whose volatility has been lowered by the solvent and to minimize its concentration in the overhead. The lower section serves to strip out the component with the higher volatility and minimize its concentration in the bottoms. 

The column section is solved with the liquid feed Lq and the estimated vapor feed Vg, using Equations 12.32, 12.33, and 12.34. The flow rates ig and Vg can be used for V and L in Equation 12.32 to calculate the stripping factors. The calculated liquid product iy is flashed at the reboiler conditions to update Vg, and the calculations are repeated until convergence, that is, until stream Vg stabilizes. The results are as follows ... 

Figure 12.8 shows a model of a main column and a side-stripper. Vapor feed F goes to the bottom of the main column, which is steam-stripped with stream 51, using no reboiler. The column has a partial condenser with vapor distillate D. Side draw SD is taken from around the middle of the main column and steam-stripped in the side-stripper, with the overhead vapor, OH, returned to the draw tray in the main column. The side-stripper is steam-stripped with stream 52. 

Steam stripping is to be used to remove a solvent from contaminated soil. An enriching colunm will be used to recover the solvent from the stream. A vapor feed of 40 mol/hr with a composition of 20 mol% solvent and 80 mol% water enters an enriching column. The distillate stream is to have a flow rate of 5 mol/hr and a concentration of 90 mol% solvent. The internal reflux ratio is 0.875 and constant molar overflow (CMO) may be assumed. Graph the operating line to predict the number of equilibrium stages in this enriching column. 

The Lewis and Matheson approach in which the product flow rates di and, are selected as the independent variables is pursued no farther because of the problems of roundoff error encountered when this method is applied to complex columns with two or more feeds. The Lewis and Matheson method, however, has served the purpose of demonstrating the existence of the rectifying and stripping pinches for the case where the relative volatilities are constant and the total-flow rates of the vapor and liquid are constant within each section of the column. A proof of the existence of the pinches for the general case where the relative volatilities vary with temperature and the total-flow rates vary throughout each section of the column is presented in Sec. 11-5. 

By assuming that the vapor portion of the feed mixes only with the vapor streams in the column, and likewise for the liquid, a mass balance for the vapor streams around this section gives (subscripts S and R indicate stripping and... 

Papastathopoulou and Luyben, 1991). Basically, SSCs are convenient alternative schemes to sequences of simple columns when some limits on product purity are acceptable or when those limits correspond to a very low impurity content of light- and heavy-boiling components (Niesbach et al, 2013 Brambilla, 2014). The main idea comes from the composition profiles of the first column of the two sequences. In the direct sequence, the concentration of intermediate component B in the first column increases below the feed as that of the more volatile component A decreases (Brambilla, 2014). On the other hand, moving further down the column, the less volatile component increases in concentration. Therefore, the concentration of component B reaches a peak only to be remixed. At this highest point, one side stream is withdrawn from the vapor phase (Figure 9.3a) (Brambilla, 2014). The second column of the sequence is replaced by only one stripping section with limited separation capability between component B and less volatile component C. 

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... 

D18. We have a stripping column with two feeds separating acetone and ethanol at 1 atm Feed F is a saturated liquid and is fed into the top of column (no condenser). Flow rate of F is 100 kgmol/h, and this feed is 60 mol% acetone. Feed F2 is 40 mol% acetone, it is a two-phase feed that is 80% vapor, and flow rate is 80 kmol/h. We desire a bottoms mole fraction that is 0.04 mole fraction acetone. The column has a partial reboiler. Equilibrium data are in Problem 4.D7. 

D6. We are separating water from n-butanol in a stripping column. The feed [F = 100 kmol/h, z = 0.65(mole fraction water), a saturated vapor] is mixed with the vapor leaving the top of the column before the combined stream is sent to the total condenser and then to a liquid-liquid setder. The column has a partial reboiler and CMO is valid. The top layer from the liquid-liquid settler (x, = x = 0.573 mole fraction water) is sent as a saturated liquid reflux to the distillation column. The bottom layer (xp = = 0.975 mole fraction water) is the distillate product. 

D23.100 kmol/h of a saturated vapor feed that is 25 mol% nitromethane (NM) and 75 mol% water is to be separated in a system with two distillation columns and a liquid-liquid separator. The feed is sent to column W that produces a water product that is 0.01 mole fraction NM. The boilup ratio in column W is 4. The optimum feed stage is used. The vapor from column W is condensed and sent to the decanter. The water phase from the decanter (0.086 mole fraction NM) is refluxed to column W. The NM phase from the decanter (0.312 mole fraction water) is sent to stripping column NM. The nitromethane product from the bottom of stripping column NM is 0.02 mole fraction water. The boilup ratio in column NM = 3.0. Assume both columns operate at 1.0 atm pressure, that CMO is valid, that both condensers are total condensers, and that both reboilers are partial reboilers. Equilibrium data are in Table 8-3. Find ... 

G2. [Note This problem is quite extensive.] Biorefineries producing ethanol by fermentation have several distillation columns to separate the ethanol from the water. The first column, the beer still, is a stripping column that takes the dilute liquid fermenter product containing up to 15% solids and produces a clean vapor product that is sent to the main distillation column. The main column produces a distillate product between about 65 mole % and the ethanol azeotrope, and a bottoms product with very litde ethanol. The calculated diameter of the main distillation column is much greater at the top than elsewhere. To reduce the size and hence the cost of the main column, one can use a two-enthalpy feed system split the vapor feed into two parts and condense one part, then feed both parts to the main column at their optimum feed locations. This method reduces the vapor velocity in the top of the column, which reduces the calculated diameter however, a few additional stages may be required to obtain the desired purity. 

Figure 8.30. A temperature profile of column with steam stripping (a) and fraction of feed vaporized V/F as function of mass flow rate of steam st/F (b). 1, steam-heated reboUer ...

Because petroleum is a mixture with a wide interval of bubble temperatures of components and the required purity of products is not very great, reflux and vapor numbers in the sections are not large. Therefore, the heat brought in is used up mostly not for creation of vapor reflux, but for evaporation of those products that are withdrawn above the feed cross-section. Therefore, the split (direct, indirect, intermediate) has but a weak influence over energy expenditures, but it is very important to exclude multiple evaporation and condensation like in multicolumn sequences of simple columns. Columns with side strippings exclude multiple evaporation and condensation. 

A mixture of isobutane and n-butane is to be stripped (i.e., concentrate the isobutane). Find the number of ideal stages needed with the following given The relative volatility is two. A rate of 2200 kmole/hr of saturated liquid feed Xf =0.45) enters the column top. Also, vapor and liquid rates are constant in the column. Desired Xo and Xb are, respectively, 0.60 and 0.05. 

We will select 21 SCFM of air to strip each 1 gpm of water therefore, the air flow to the column will be 1,890 SCFM. Based on the inlet air having 80% relative humidity and the exit air leaving saturated with water vapor at HO F maximum temperature, the liquid effluent temperature will be 108 F by heat balance. The Henry s Law constants are 1,740 atm/mol fraction for the feed liquid and 840 atm/mol fraction for the effluent liquid. The stripping factors from Equation 5-19 at atmospheric pressure are 113 at the column bottom and 278 at the column top. To produce a 0.14 ppm by wt. ethyl benzene effluent will require an average of 6.95 transfer units. 

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