Range column tray

For more than two species and a reflux ratio set to 1.2 times the minimum, a rule of thumb is to compute the total number of trays required for a total reflux column to produce the separation desired and then double this number as a first guess (Douglas, 1988). The next decision is select the tray on which to feed the column. For multispecies columns, the placement is not obvious. Typically, one must search by placing it on any one of a range of trays using a tray-by-tray simulation, thereby discovering which tray location requires the least reflux to effect the desired separation. 

As a learning tool, the binary model is useful for qualitatively studying the characteristics of multistage separation. The model is used in this chapter to answer such questions as what effect the reflux ratio or product rate or number of trays has on separation or what is the minimum number of trays or minimum reflux ratio required to achieve a given separation, or over what ranges column performance specifications are feasible. 

At a given product rate, the reflux ratio may be varied quite independently by changing the vaporization rate in the reboiler and the condensation rate in the condenser, that is, by changing the condenser and reboiler duties. This represents internal circulation within the column and is thus independent of the product rate. Nevertheless, the two quantities are not entirely independent if one variable is fixed, the other may vary only within a certain feasible range. Outside of this range, the vapor or liquid phase may dry up on certain column trays, or one of the products may vanish. 

Perhaps the simplest of cross-flow column tray designs is the sieve tray or perforated tray. The tray is a flat metal plate and the vapor openings are holes drilled in the plate. The holes are usually round, ranging from 1/8- to 1/2-inch diameter. Sieve trays have no liquid seals to prevent liquid from flowing down the holes. Liquid flow down the holes is prevented only by the upward flow of the vapor. 

In the range of medium reaction velocities, mixed constructions are the ideal solution, for example special column trays with large holdup or reactor cascades with a column on the first stage only. 

The bottoms is essentially a binary methanol/water (23.5 mol% methanol), which is fed to a 32-stage column operating at atmospheric pressure. The number of trays in the second column was optimized by determining the total annual cost of columns over a range of tray numbers. Reboiler heat input and condenser heat removal are 8.89 and 9.53MW, respectively. The column diameter is 2.24 m. 

The area of the holes amounts to about 5-16% of the total area of the perforated plate. The distance between neighboured trays lies between 80 and 300 mm. It increases with the column s diameter and load range. The tray efficiency factor, i.e. the separation efficiency referred to one theoretical tray, ranges between 60 and 90%. Sieve tray columns are manufactured with diameters of up to about 6 m. 

A Hquid-phase isophorone process is depicted ia Figure 4 (83). A mixture of acetone, water, and potassium hydroxide (0.1%) are fed to a pressure column which operates at head conditions of 205°C and 3.5 MPa (- 500 psi). Acetone condensation reactions occur on the upper trays, high boiling products move down the column, and unreacted acetone is distilled overhead ia a water—acetone a2eotrope which is recycled to the column as reflux. In the lower section of the column, water and alkaH promote hydrolysis of reaction by-products to produce both isophorone and recyclable acetone. Acetone conversion is typically ia the range 6—10% and about 70% yield of isophorone is obtained. Condensation—hydrolysis technology (195—198), and other Hquid-phase production processes have been reported (199—205). 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

FIG. 23-38 Efficiency and capacity range of small-diameter extractors, 50 to 150 mm diameter. Acetone extracted from water with toluene as the disperse phase, V /V = 1.5. Code AC = agitated cell PPC = pulsed packed column PST = pulsed sieve tray RDC = rotating disk contactor PC = packed column MS = mixer-settler ST = sieve tray. (Stichlmair, Chem. Ing. Tech. 52(3), 253-255 [1980]). 

Superfractionation is an extension of distillation using smaller diameter columns and 100 or more trays to achieve reflux ratios exceeding 5 1. This equipment separates a narrow range aunponents such as of high-purity solvents, e.g., isoparaffins or individual aromatic compounds foi use. IS petrochemicals. 

The column is designed as an ammonia rectifier-stripper using fundamental design techniques. A 48-in. diameter column will handle at least 500 tons of refrigeration system load for the above temperature range, using 10 bubble cap trays with 32, 4-in. pressed steel caps per tray (slot area = 7.81 in. /cap riser area 4.83 in. /cap 3 ft 0 in. weir length). Tray... 

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

To operate the larger column at a reduced rate probably will not be too difficult if a bubble cap is specified. These columns have wide stable operating ranges. The only difficulties are that the tray efficiency may be low and the liquid holdup relatively large. This latter is especially undesirable if, as has been noted, the liquid is flammable, or if some undesired reaction takes place at the elevated temperatures within the column. 

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