Tray, pressure distillation

The separation operation called distillation utihzes vapor and hquid phases at essentially the same temperature and pressure for the coexisting zones. Various lands of devices such as r andom or sti uctui ed packings and plates or tr ays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cyhndrical shell to form a column. Pacldngs are also generally contained in a cyhndrical shell between hold-down and support plates. A typical tray-type distillation column plus major external accessories is shown schematically in Fig. 13-1. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Tray columns are the best choice for high-pressure distillation, especially above 20 bar. An important factor in trays favour is that they have been used for many years in high-pressure distillation, while it is only in the last 10-15 years that packings have been used significantly in such duties, and especially for structured packings above about 15 bar. 

In high pressure distillation, tray operation is usually in the emulsion regime. In small diameter (less than 1.5 m) columns, or at low liquid loads, or the low end of the pressure range (towards 10 bar), however, the froth-and spray regimes can be found. 

The downcomer area required for trays not only increases with the liquid-flow-rate, but also with the difficulty in achieving separation between the vapour and the liquid phases. The volume required for the downcomer (downcomer residence time) increases at a lower surface tension and a smaller density difference between vapour and liquid. Because of the large downcomer area required to handle the high liquid flow rates typical of high-pressure distillations, a trayed column cross-sectional area may be 40% to 80% greater than the active tray area calculated from the vapour flow rates for such distillations. Thus, the downcomer area becomes a significant factor in the determination of the diameter of a tray column. 

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 notable feature of high-pressure distillation is the high efficiency that is usually obtained on trays. Figures close to 100% are not uncommon. However, the efficiency of trayed columns has been shown to increase only from atmospheric pressure up to a pressure of 11.5 bar. At higher operating pressures, the efficiency of the trays decreases with increasing pressure. There is an entrainment of vapour in the liquid phase which is carried back down the column. For example, for a C4-hydrocarbon separation the tray efficiency will be reduced by 16% as the pressure is raised from 11.5 bar to 27.6 bar. 

Practical guidance on how to get the best choice between trays and packings in high-pressure distillations is very difficult [33],... 

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by... 

Downcomer vapor underflow ("vapor entrainment" or "gas recycle1 ) is analogous to liquid entrainment. It reduces both tray capacity and efficiency (17,44,45). In low- and medium-pressure distillation systems, where gas density is significantly lower than liquid density, it takes only a small quantity of gas to generate volumes comparable to the liquid volumetric flow rate. The quantity of gas recycle is therefore small, and it has little effect on tray performance. At high pressures, the quantity of gas recycled is significant. An analysis of some FRI data (44) for iC4-nC4 distillation showed vapor entrainment increases from about 7 percent at 165 psia to about 50 to 60 percent at 400 psia on a molar basis. 

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... 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

A binary stream at the rate of 1000 kmol/h containing 35% mole of component 1 (the lighter component) is to be separated in a distillation column to produce 95% component 1 in the distillate and 90% component 2 in the bottoms. The column will have a partial reboiler and a partial condenser, and will operate at 1 atm. It is proposed to utilize an existing hot process stream in the plant as a heat source for the reboiler, which limits the reboiler duty to 58 x 10 kJ/h. Using either the Y-X diagram or the H-X diagram, determine the number of theoretical trays required, the optimum feed location, the product rates, and the condenser duty. Assume feed thermal conditions that result in a saturated liquid at the feed tray pressure. Use thermodynamic data from Problem 6.1. 

The specihcation functions, equation set 13.9, are often simply a constant. For instance, the mole fraction of component i in the vapor from the top stage (the distillate) can be specihed as = 0.95. Certain other parameters such as tray pressures and feed compositions are considered in most solution algorithms as hxed quantities, thus reducing the number of variables. 

The older method to raise the azeotrope containing 95 per cent weight to 99.9 per cent ethanol consists in absorbing the water on lime (CaO). This costly process has been abandoned. Azeotropic distillation in the presence of benzene is employed today. In principle, simple low pressure distillation should allow the separation of alcohol and water, because the azeotrope disappears below 2kPa. For ethanol concentrations between 95 and 100 per cent, however, the liquid am vapor phase compositions are substantially the same, implying extremely high reflux rates and a large number of trays. 

Seperation Unit. The elements of a separation process. Separation units of a separation process could be, for example, a gas centrifuge, a membrane module, a tray of distillation column or the evaporator of a multiple effect plant. It should be kept in mind, however, that in plate-and-frame modules as well as in modules of the spiral-wound-type usually every block or pressure-vessel contains more than one separation unit ... 

With the availability of economical and efficient packings, packed towers are finding increasing use in new distillation processes and for retrofitting existing trayed towers. They are particularly useful in applications where pressure drop must be low, as in low-pressure distillation, and where liquid holdup must be small, such as when distilling heat-sensitive materials whose exposure to high temperatures must be minimized. 

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