Distillate Vapor Flow Rate

There are cases where a vapor stream from the top of a distillation column is needed as the distillate. In that case, the distillate vapor flow rate can be manipulated to control the column pressure and the condenser can be controlled as a partial condenser to produce the reflux. The heat duty for a condenser is a function of the heat transfer surface area and the temperature difference between the cooling medium and the condensing vapors. One approach is to recirculate the cooling fluid and control the temperature of the coolant for a variable temperature difference between the condensing vapors and the coolant. Another approach can be to use 

In this chapter, four basic distillation control schemes were introduced along with a number of variations on each scheme. The concept was introduced that there can be more than one way to successfully control a distillation column. In some schemes, the separation power base is controlled by the ratio of steam/feed and then the distillate flow rate or reflux flow rate can be manipulated to control an MRT point in the distillation column. In other schemes, the separation power base is controlled by the ratio of reflux/feed, and the steam to the reboiler or the bottoms flow rate can be manipulated to control an MRT point in the column. Control of reflux drum level and column base level was presented as basic to all control schemes. Control of column pressure was considered to have an overriding effect on the stability of distillation column control. 

1 What control strategy would be recommended for a top-fed stripping column  

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Whether for a distillation, absorption, or stripping system the material balance should be established around the top, bottom, and feed sections of the column. Then, using these liquid and vapor rates at actual flowing conditions, determine the flooding and maximum operating points or conditions. Then, using Figures 9-21B, -21E, or -21F, establish pressure drop, or assume a pressure drop and back-calculate a vapor flow rate, and from this a column diam-... 

The number of columns is changing however, if extractive distillation is used. Therefore the fluid inventory in the process becomes a major safety parameter. The inventory depends on the size of the columns in the process. It was assumed that all the columns have been designed for the same superficial vapor velocity. Therefore the column area is directly proportional to the vapor flow rate. Also it was assumed that the liquid hold up is proportional to the column area. 

Vapor and liquid distillate product flow rates (lb - mol/h)... 

Table 5,14 gives a digital computer FORTRAN program for this three-component batch distillation dynamic simulation. The specific example is a column with 20 trays and relative volatilities of 9, 3, and 1. The vapor flow rate is constant at 100 mol/h. 

The classic hydraulic model (Sec. 6.2,1) oversimplifies tray action. There are five main flow regimes on distillation trays (12,99). ThesB regimes (Figs. 6.25 to 6.28) may all occur on the same tray under different liquid and vapor flow rates (Fig. 6.29). An excellent overview of the fundamentals and modeling of these flow regimes is presented by Lockett (12). 

In the diabatic distillation column, each element is small enough that equipartition of entropy production may be approximately achieved by adjusting the heat flows and thus the liquid and vapor flow rates. We assume that each element performs a specified duty of Ji0. The total cost function Ct for all N elements is... 

If the equilibrium ratios are functions of phase compositions as occurs in liquid extraction or extractive distillation, it is necessary to include more variables in the iterative process. It was later shown (3) that for liquid extraction problems with known stage temperatures, the minimum number of iteration variables for quadratic convergence is nm, the n vapor flow rates, and n(m — 1) of the phase compositions. The total number of variables is n(2m + 2) because the temperatures are known. The iteration sequence is completely different for this case as compared with the previous case with composition independent equilibrium ratios. 

Steam-jet ejectors are versatile and economic vacuum pumps and are frequently used, particularly in vacuum distillation. They can handle high vapor flow rates and, when several ejectors are used in series, can produce low pressures, down to about 0.1 mmHg (0.13 mbar). 

The evaluation of a separation sequenees requires information about the feasibility and the cost of the individual separation tasks. Determining the minimum energy demand of a separation has been foimd to be a good way of estimating the cost of a distillation system, because operation cost are dominated by the energy demand and investment cost are closely related to the vapor flow rate in the column [10], The minimum energy demand can be calculated using the RBM shortcut. The feasibility of the individual separation tasks can be checked with the application of PDB feasibility test. 

With negligible liquid holdup, negligible pressure drop, constant vapor and liquid flow rates from tray to tray, time-invariant vapor flow rate, and ideal thermodynamics, the batch distillation problem for product k may be stated as follows (Farhat et al., 1990) ... 

Pressnre control loops are nsed to maintain system pressure for distillation columns, reactors, and other process nnits. A pressnre control loop for maintaining overhead pressure in a column is shown in Fignre 15.31. The hnal control element is a control valve on the vent line, and the sensor is a pressnre sensor monnted on the top of the column. The output from the pressure controller goes directly to the control valve on the vent line. The objective of this loop is to maintain the column overhead pressnre at or near setpoint for changes in condenser duty and changes in vapor flow rate up the column. 

Many processes scale directly with the feed rate to the process, e.g., distillation columns and wastewater neutralization. For distillation columns, all the liquid and vapor flow rates within the column are directly proportional to the column feed rate if the product purities are maintained and the tray efficiency is constant. For wastewater neutralization, the amount of reagent necessary to maintain a neutral pFl for the effluent varies directly with the flow rate of the wastewater feed, as long as the titration curve of the wastewater remains constant. When the manipulated variable of a process is, in general, directly proportional to the feed rate, ratio control can significantly reduce the effect of feed rate disturbances on the process. 

The total interstage flow rate of heads or tails is a measure of the size of the separation plant. In a distillation plant, for example, the total volume of column internals is proportional to the total interplate vapor flow rate. In a gaseous diffusion plant, the total amount of power expended in pumping gas from one stage to the next is proportional to the total heads flow rate. 

The separation achieved by distillation in this example is considerably different from the separation achieved by absorption in Example 12.8. Although the overhead total exit vapor flow rates are approximately the same (530 Ibmole/hr) in this example and in Example 12.8, a reasonably sharp split between ethane and propane occurs for distillation, while the absorber allows appreciable quantities of both ethane and propane to appear in the overhead exit vapor and the bottoms exit liquid. If the absorbent rate in Example 12.8 is doubled, the recovery of propane in the bottoms exit liquid approaches 100%, but more than 50% of the ethane also appears in the bottoms exit liquid. 

The overall distillation plate efficiency, E , may be correlated in terms of the following variables liquid density, vapor density, liquid viscosity, vapor viscosity, liquid diffusivity, surface tension, pressure, temperature, pressure drop, liquid flow rate, vapor flow rate, bubble size, contact time. 

Side columns are used, for instance, in the most important distillation processes worldwide, the fractionation of air (see Fig. 11.2-18) and the distillation of cmde oil (Meyers 1996). The atmospheric tower of oil refineries consists of a main column and four stripping side columns (Fig. 11.2-12). In this tower the crude oil is split into six fractions which are processed further in several subsequent columns. Oil refineries also have some other interesting features. Steam is fed into the bottom of the main column and most of the side columns. This causes a stripping effect and reduces the temperatures in the columns (steam distillation). The overhead fractions of all side columns are fed into the main colunrn thus increasing the vapor flow there. So-called pump arounds effect a partial condensation of the vapor in the main column and, in turn, a reduction of the vapor flow rates in the upper sections. 

The major difference between the two cases is the distillate flow rate 42.15 lb mol/h in the first case and only 3.761bmoI/h in the second. The small vapor flow rate in the latter case corresponds to a volumetric flow rate of only 1.44ft /min. Considering the total volume of the 30-tray column (41.8 ft ) and the volume of vapor in thehalf-full reflux drum... 

Note that these tuning constants are different for the two design cases because of the different vapor distillate flow rates. Note also that the tuning constants are different for some of the different control stmctures because the effect of reboUer heat input (or the equivalent vapor flow rate to the condenser) on pressure is different depending on what manipulated variable is used to control pressure. Proportional level controllers are used with gains of 2. The default pressure controller tuning constants from Aspen Dynamics are used. 

It is important to remember that we have specified four variables (the impurity of toluene in the distillate, the impurity of toluene in the bottoms, the impurity of benzene in the sidestream, and the impurity of xylene in the sidestream). To achieve these four specifications, we have varied four variables (distillate flow rate, sidestream flow rate, vapor flow rate to the prefractionator, and reboiler duty). The one remaining design degree of freedom is the flow rate of liquid to the top of the prefractionator Lp (stream 2). 

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