Distillation overall column efficiency

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 ... 

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).]... 

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). 

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.)... 

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... 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

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. 

As with distillation, the correlation for overall tray efficiency for absorbers, given in Equation 10.7, should only be used to derive a first estimate of the actual number of trays. More elaborate and reliable methods are available, but these require much more information on tray type and geometry and physical properties. If the column is to be packed, then the height of the packing is determined from Equation 9.64. As with distillation, the height equivalent of a theoretical plate (HETP) can vary... 

As might be expected, the vapour phase may offer the controlling resistance to mass transfer in high pressure distillations. Values for tray efficiencies at elevated pressure are scarce [23, 24]. The prediction of tray efficiency may be approached in several ways. One way is to utilize field performance data taken for the same system in very similar equipment. Unfortunately such data are seldom available. When they are available, and can be judged as accurate and representative, they should be used as a basis for efficiency specification [25], Another way is to utilize laboratory-or pilot-plant efficiency data. For example a small laboratory-Oldershaw tray-column can be used with the same system. Of course, the results must be corrected for vapour-and liquid mixing effects to obtain overall tray efficiencies for large-scale design [26], Another approach is the use of empirical or fundamental mass-transfer models [27-30],... 

A process is defined similar to Example 17.2, with the same components, initial charge and composition, the same constant distillate rate and required composition, and the same column pressure. In this case the reflux ratio is maintained at twice the minimum value. The column has six actual trays plus the reboiler, the equivalent of seven actual trays. The overall tray efficiency is 65%, and the relative volatility of butane to pentane is assumed constant at 2.315. 

The point efficiency is a fundamental criterion of mass transfer, whereas the overall tray efficiency is the value that can be measured, especially for larger distillation columns. Large-column... 

A distillation unit consists of a partial reboiler, a bubble cap column, and a total condenser. The overall plate efficiency is 65%. The feed is a liquid mixture, at its bubble point, consisting of 50 mole% benzene in toluene. This liquid is fed to the optimum plate. The column is to produce a distillate containing 95 mole% benzene and a bottoms of 95 mole% toluene. Calculate for an operating pressure of 1 atm ... 

The overall efficiency lumps together everything that happens in the columns. What variables would we expect to affect column efficiency The hydrodynamic flow properties such as viscosity and gas flow rate would affect the flow regime, which affects efficiency. The mass transfer rate, which is affected by the diffusivity, will in turn affect efficiency. Overall efficiency is usually smaller as the separation becomes easier increases). The column size can also have an effect. Correlations for determining the overall efficiency will be discussed in Chapter 10. For now, we will consider that the overall efficiency is determined from operating experience with similar distillation columns. 

Distillation efficiency commonly is expressed as height equivalent to a theoretical stage (HETP). In a trayed column, the HETP value is the tray spacing divided by the fractional overall tray efficiency. A theoretical stage is a mass transfer stage from which the liquid and vapor streams leave in equilibrium. In a packed column, the liquid in the packed bed would be in equilibrium with the gas stream located one HETP value above it in the packed bed. Thus, the stepwise procedure McCabe and... 

The high-temperature process heat is normally recovered through steam generation however, to make the overall process efficient, lower temperature process heat must also be used. Therefore, processes normally pick up the heat after the high-pressure boiler and use it to reboil the distillation columns in the purification section, raise low-pressure steam, and heat boiler feedwater or... 

Ultimately, the importance of process control is seen through increased overall process efficiency, allowing the plant engineer to get the most from the process design. This is especially true of distillation control. Most distillation columns are inherently flexible, and a wide range of product yields and compositions can be obtained at varying levels of energy input. A key requirement of any control system is that it relates directly to the process objectives. A control system that does not meet the process objectives or produces results that conflict with the process objectives does not add value to the process. 

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

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