Pressure distillation design

A receiver for low pressure distillations designed by G. A. R. Kon (1930) is quite easy to make the joints D and C (Figure 61) are both standard B.19 joints the two taps at A andRare standard designs of three-way taps. For the part G a tube of about 3 cm diameter... 

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],... 

Optimizing the design of high-pressure distillation columns is not easy and often requires several solutions to solve a specific distillation challenge. 

Improving liquid flow patterns. A number of special tray designs have been developed to improve liquid velocity distribution on large-diameter trays. Their main applications are vacuum distillation. In pressure distillation, liquid flows are usually high and multipass trays are used, so that stagnant zones are seldom a problem. Some means of improving the liquid flow patterns are... 

Determine the relevant vapor-pressure data. Design calculations involving vapor-liquid equilibrium (VLE), such as distillation, absorption, or stripping, are usually based on vapor-liquid equilibrium ratios, or K values. For the tth species, K, is defined as K, = y, /x, where y, is the mole fraction of that species in the vapor phase and x, is its mole fraction in the liquid phase. Sometimes the design calculations are based on relative volatility c/u], which equals K,/Kj, the subscripts i and j referring to two different species. In general, K values depend on temperature and pressure and the compositions of both phases. 

One of the challenging aspects of distillation column control is the many limitations imposed on the operation of the column. There are hydraulic constraints, separation constraints, heat-transfer constraints, pressure constraints, and temperature constraints. We recommend the excellent books by Kister (1992 and 1990) on distillation design and operation. 

Obtain Property Data. These can include transport, physical, and thermochemica dots as needed for computations, Importantly, they include die necessary vapor-liquid equilibrium (VLE) data, measured or predicted for the ranges of composition, temperature, and pressure to be enconniered in the compulations. As will be noted later, the reliability of die VLE can seriously inflnence many distillation designs. 

BsW, H.Z., Distillation Design, McGraw-Hill, New York (1992). leva, M., Reconsider Packed-Tower Pressure-Drop Correlations, Eng. Prog., 88 (1), 65-72 (1992). 

The normal situation in distillation design is that the feed conditions are given flow rate F (mol/h), composition Zj (mole fraction component j, temperature and pressure Pp. The... 

The only complication in setting up the pressure-swing distillation simulation is the recycle stream of the high-pressure distillate back to the low-pressure column. One approach is to make a guess of the conditions of this second feed to the low-pressure column and then set up the both columns sequentially, starting with the low-pressure column with its two design specification xb = 0.005 and = 0.74) and then moving to... 

In conventional distillation design, tray holdup has no effect on steady-state compositions. In reactive distillation, tray holdup (or amount of catalyst) has a profound effect on conversion, product compositions, and column composition profiles. Therefore, in addition to the normal design parameters of reflux ratio, number of trays, feed tray location, and pressure, reactive distillation columns have the additional design parameter of tray holdup. 

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