Boil-up rate

Am. In.st. Chem. Eng. J., 29, 1017 (1983)], the steep concentration and temperature fronts can be extremely sensitive to small changes in reflux ratio, boil-up rate, product recoveiy and purity, and feed rate and composition. 

Still column is flooded by excessive boil up rates... 

Example 8-15 Batch Distillation, Vapor Boil-up Rate for Fixed Trays (used by permission of Treybal [129] clarification added by this author)... 

Figure 8-38A. Graphical integration for boil-up rate of batch distillation for Example 8-15. Used by permission, Treybal, R. E., Cftem. Eng. Oct. 5 (1970), p. 95.

Boil-Up Rate, pounds per hour per square foot... 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Repeat calculations, adjusting flow as necessary, until the assumed (weight fraction of vapor in reboiler exit) produces the proper boil-up rate. 

The still provides a constant vapour boil-up rate, which remains constant with respect to time. 

Molar holdup in still Molar holdup in reflux drum Vapor boil up rate Reflux rate Relative volatilities... 

V=10 -.VAPOUR BOIL-UP RATE R=lel0 STARTUP AT TOTAL REFLUX Pbar=l TOTAL PRESSURE Accur=le-2 rACCURACY FOR SUBROUTINE CONSTANT CINT=0.2, NOCI=5, TFIN=300 1 SIM INTERACT RESET GOTO 1 INITIAL INITIAL CONCENTRATIONS... 

Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analysers are available they can be incorporated in the control loop, but more complex control equipment will be needed. 

V is the boil-up rate. Design a 2x2 MIMO system with PI controllers and decouplers as in Fig. 10.14. 

The properties of a fractionating column which are important for isotope separation are (1) the throughput or boil-up rate which determines production (2) HETP (height equivalent per theoretical plate) which determines column length (3) the hold-up per plate which determines plant inventory and time to production (4) the pressure drop per plate which should be as small as possible. The choice of a particular column is invariably a compromise between these factors. The separation in a production column is of course less than it would be at total reflux (no product withdrawal). The concentration at any point in the enriching section can be calculated from the transport equation (see, e.g., London 1961)... 

A vacuum distillation plant operating at 7 kN/m2 pressure at the top has a boil-up rate of 0.125 kg/s of xylene. Calculate the pressure drop along a 150 mm bore vapour pipe used to connect the column to the condenser. The pipe length may be taken as equivalent to 6 m, e/d = 0.002 and p = 0.01 mN s/m2. 

Table 5.2 summarises the results for two cases (i) constant vapour boil-up rate, (ii) variable vapour boilup rate. The initial and final time optimal reflux ratio values are shown in Table 5.2 for both cases. The optimal reflux ratios between these two points follow according to Equation P.13 for each case. See details in the original reference (Robinson, 1969). 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

However, most of the batch distillation models (e.g. Mujtaba and co-workers Sorensen and Skogestad, 1996) relate the amount of distillate collected (Hamodei) with the vapour boil-up rate in the column (Vtomm), the internal reflux ratio (Rmodei) and the total operating time (tdig) by,... 

How much time would be required to obtain some specific product composition at some constant boil-up rate, or what boil-up rate would be required to obtain some specific product composition within some specified time under conditions of,... 

By continuously varying the reflux ratio during the course of the distillation, an essentially constant overhead concentration can be obtained. The boil-up rate is constant in this case, too, but as reflux ratio increases, the amount of liquid returned to the column increases. 

Because the boil-up rate associated with a specific distillation system should be known from past experience with the system, this calculation of total vapor load immediately produces a time required for the new separation. 

Conversely, in the design of a new system, the time available for the distillation can be divided into the vapor load to determine the boil-up rate required. 

As noted under the discussion of the coustant reflux ratio case, the boil-up rate of an existing distillation system should be well known, so use of that boil-up rate with the vapor quantity just calculated will yield the time required for the separation of the new system. Note that this value of time refere only to the distillation itself charging time, heat-up time, cooling time and clean-out time are not included. 

Determine the column diameter. The column diameter can be determined by taking the boil-up rate established for the separation of components having the smallest relative volatility (which, in this case, is given to be the dye-intermediate/coproduct separation) and using the design procedures outlined in Examples 8.2 and 8.4. 

The still pot is then heated so that the liquid boils gently, and a steady reflux is returned from the head of the column. The jacket temperature is adjusted to correspond to the vapor temperature at the head if a thermometer is used, or it is adjusted to adiabatic operation if a differential thermocouple is employed. The boil-up rate (throughput) is adjusted to a value which is appropriate for the column being used (Table 1-12) by regulating the amount of heat supplied to the still pot. The column is allowed to achieve equilibrium before any material is withdrawn. This is usually determined by the constancy of the vapor temperature or of the refractive index of the material at the column head and usually requires from one-half to several hours. The time necessary for establishing equilibrium is usually greatest for the columns with the highest number of theoretical plates. 

After most of one component has been withdrawn, an increase in vapor temperature will be noted, and the boil-up rate will begin to decrease. At this point, both the jacket heater and the still-pot... 

Several new problems are encountered when one conducts fractional distillations under reduced pressure. First, as the pressure is decreased, the boiling points of the constituents usually approach each other. This makes it necessary to use a column with a higher number of theoretical plates than would be required at atmospheric pressure. Second, as the pressure is decreased, the vapor velocity is increased. For example, at 25 mm, the vapor velocity for a given boil-up rate would be increased by a factor of 30 over that for atmospheric pressure. At still lower pressures, this ratio increases rapidly. Thus, to obtain comparable vapor-liquid equilibrium, the boil-up rate must be lower at reduced pressures (perhaps by a factor of the square root of the ratio of pressures). 

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