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The gas flow capacity of conventional crossflow distillation trays is normally limited by the propensity of the gas to entrain liquid from the tray. Only a slight amount of such entrainment can be permitted without serious degradation of mass transfer efficiency. The tray-type contacting device described here is designed to entrain all of the liquid entering the tray, and then separate that liquid by a device located within the tray zone. As a result, considerably more gas flow can be accommodated in a distillation column sized for conventional crossflow trays. Complete distillation test data, taken at a semi-works scale, are presented along with a mechanistic model which represents the data and which can be used for extension to other distillation conditions.20916
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Keywords: distillation; gas-liquid systems; trays; retro-fitting; debottlenecking
Debottling distillation columns is an activity that has the promise of large capital savings but in the usual sense has some severe limitations. A recent paper on the subject suggests that currently available replacement devices,suitable for retro .fitting and for the purpose of increasing throughput capacity without loss of separating capability,offer capacity increases of the order of 25-30% when compared with modern designs of crossflow sieve- or valvetrays(Fair and Seibert, 1996)1 . In the present paper operating characteristics and performance results will be described for a new device called the the cocurrent tray. This device has been shown to have throughput capacities 50 –100% greater than those of conventional crossflow trays. Also included in the paper will be details of a model which can be used for further studies and to predict the hydraulic characteristics and mass transfer efficiency of the device.
The cocurrent tray operates on the principle that entering vapour and liquid are contacted such that all of the liquid is entrained upward with the vapour. A phase dispersion process takes place on each contacting stage. Integral with the stage is a vapour-liquid separator from which the vapour flows to the next stage above and the liquid flows to the next stage below. A sketch of the tray is shown in Figure 1.
The primary advantage of the cocurrent tray is its high throughput capacity. Since the phases flow cocurrently in the contacting zone, flooding by countercurrent contacting does not occur. Thus, the traditional limitations of ‘ultimate capacity,’ as proposed by Fractionation Research, Inc. (FRI) do not apply to this device. Entrained clear liquid is separated from the vapour within the device and thus does not produce the problems of vapour entrainment or froth handling found in conventional downcomers. On the other hand, the tray may not perform well at low vapour loadings because of the weeping and dumping, which, of course, can occur also in conventional crossflow sieve- or valve-trays.
Three series of tests of the cocurrent tray have been conducted at the larger-scale facilities of the Separations. Research Program at the University of Texas at Austin, each series with a different geometry. The . first two sets of test data have been reported previously (Fair and Seibert,1994)2; the results of the third set of tests will be emphasized here. The tests were carried out under distillation conditions using the cyclohexane/n-heptane test mixture at four pressures. Since similar studies have been made of other devices, using the same test mixture and equipment, a ready means of comparison is available. The purposes of this paper are threefold: to describe the device, to present representative performance test data, and to outline the methodology behind a mechanistic modeling effort to permit extrapolation of the test data to other situations.
DESCRIPTION OF TRAY OPERATION
Liquid from the downcomer (see Figure 1) is fed uniformly to the tray by means of perforated trough dispersers. Vapour to the tray flows upward through slotlike openings between the troughs. The open (slot) area is typically in the range of 15 to 25% of the tray active area. The high slot velocity atomizes the liquid and carries it upward to baffle-type entrainment collectors where a high proportion of the liquid is separated and sent to the downcomer feeding the tray below. The liquid flowing to the downcomer is free of entrained vapour and thus the downcomer backup comprises clear liquid only. Also, since the downcomer height is greater than the tray spacing, downcomer capacity is a consideration only at very high liquid loadings.