Distillation overall column

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

Hquid—Hquid-phase spHt the compositions of these two feed streams He oa either side of the azeotrope. Therefore, column 1 produces pure A as a bottoms product and the azeotrope as distillate, whereas column 2 produces pure B as a bottoms product and the azeotrope as distillate. The two distillate streams are fed to the decanter along with the process feed to give an overall decanter composition partway between the azeotropic composition and the process feed composition according to the lever rule. This arrangement is weU suited to purifying water—hydrocarbon mixtures, such as a C —C q hydrocarbon, benzene, toluene, xylene, etc water—alcohol mixtures, such as butanol, pentanol, etc as weU as other immiscible systems. 

FIG. 14-36 O Connell correlation for overall column efficiency for distillation, To convert centipoises to pascal-seconds, multiply fiy 10 , [O Connell, Trans, Am, Inst, Gfiem, Eng, 42, 741 (1946).]... 

Once packing heights are determined in other sections from HETP (distillation) or Koa (absorption), the height allowances for the internals (from Figure 1) can be added to determine the overall column height. Column diameter is determined in sections on capacity and pressure drop for the selected packing (random dumped or structured). 

Stirred tanks are modeled assuming that both phases are well mixed. Tray columns are usually modeled as well mixed on each tray so that the overall column is modeled as a series of two-phase, stirred tanks. (Distillation trays with tray efficiencies greater than 100% have some progressive flow within a tray.) When reaction is confined to a single, well-mixed phase, the flow regime for the other phase makes little difference but when the reacting phase approximates piston flow, the flow regime in the other phase must be considered. The important cases are where both phases approximate piston flow, either countercurrent or cocurrent. 

Efficiency (overall column efficiency) Ratio of the number of theoretical stages required to effect a distillation separation to the number of actual trays. 

Distillation stage calculations are usually performed with ideal stages, The number of ideal stages required for the separation is divided by the overall column efficiency (Sec, 7,1,1) to obtain the required number of trays. In packed towers, the number of stages in the column is multiplied by the HETP (Height Equivalent of a Theoretical Plate, see Sec. 9.1,2) to obtain the packed height. 

Figure 7.5 The O Connell correlation for overall column efficiency, (a) Plot for distillation (b) plot for absorbers. (Prom H. E, O Connell, Trane. AlChE, 42. p. 741, 1946, Reprinted courtesy of the American Institute of Chemical Engineers.)...

The basis and various parameters for the economic analysis are given in Table II. The overall column efficiency used was obtained from a plot of efficiency vs. the product of relative volatility and liquid viscosity (9), corrected to match predicted (10) data for the propane-propylene system. The value from the plot (9) was increased by a factor required to make the efficiency of the propane-propylene binary distillation equal to 100%. Costs were calculated by the Venture Analysis method (II), because this method yields the appropriate weighting factors for the fixed and operating costs in order to calculate the total costs. Results are expressed as annual costs, before taxes. The important process variables are discussed below. 

The calculated point efficiency must be converted to overall column efficiency, which will lower its value and make it closer to the O Connell prediction. The calculated value of Eog is slightly higher than obtained experimentally (Eog = 0.83-0.92) at the University of Delaware for bubblecap trays (Annual Progress Report of Research Committee, Tray Efficiencies in Distillation Columns, AIChE, New York, 1955). 

At total reflux, the column itself (without the backmixing) can only reach products that lie both on the same distillation cune (not residue curve) and on a straight line passing through the feed so as to satisfy the overall column species... 

The model of the distillation column used throughout the paper is developed by [10], composed by the mass, component mass and enthalpy balance equations used as basis to implement a SIMULINK model (figure 1) which describes the nonlinear column dynamics as a 2 inputs Q, Lvo/) and 2 output (Ad, Ab ). Implementations details for the overall column dynamics are given in [11]. 

FIGURE 12.60 O Connell method for estimating overall column efficiencies of distillation columns and absorbers. (H. E. O Connell, 1946. Trans AIChE 42 741.)... 

The distillate component flow rates may now be calculated from an overall column component material balance ... 

For a binary distillation column, convenient, compact representation of overall column composition dynamics in two equations has been suggested by Rippin and Lamb ... 

Generally speaking, the overall simulated result of a distillation tray column by using two-equation model and different Reynolds mass flux models is very close each other and checked with experimental measurements, but if detailed mass transfer and flow information on the trays are needed, the standard Reynolds mass flux model is the better choice. 

The feed split is simply the amount of feed that leaves as distillate versus the amount that leaves as bottoms. The other variable, fractionation, is the amount of separation that occurs per stage. The overall column fractionation depends on the number of stages, the energy input, and the difficulty of separation. A typical control scheme for this column is shown in Figure 8.3. 

We recommend the use of overall column efficiency factors. Overall column efficiency refers to the ratio of ideal (or theoretical) trays and actual physical trays. This is a single value that can range from 30-90%. If we consider the case of a distillation column having 20 physical trays and overall efficiency of 0.5, we would model it as a column with 10 ideal trays. With this approach, every tray remains in thermodynamic equUibrium and predictions away from the base operating scenario are reasonable. In the present work, we model the DA301, a reformate... 

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