Side Draw Flow Rate

When a side draw is used on a distillation column, there is another degree of freedom introduced into the control scheme. There are actually two material balance splits to keep in balance. One is the ratio of distillate/side draw, and the other is the ratio of side draw/bottoms. The separation power base can be set by the ratio of steam/feed, and then the distillate flow rate can be manipulated by a temperature controller for the MRT point above the side draw. The side draw flow rate can be manipulated by the second temperature controller for the MRT point below the side draw. 

A vapor side draw from a distillation column is not used very frequently, but, when it is, a large valve in the vapor side draw line can be used to control the side draw flow rate. However, the control is very sensitive when such a large valve is used. An alternative can be to use a fixed large valve position and a manipulated small valve position in parallel for control. 

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An increase in the overall volumetric flow rate of the liquid feed adds considerable liquid to the column system. Because the feed is primarily benzene, it is expected that the benzene side draw flow rate will increase. The flow rate overshoots and then assumes its new steady-state value. A similar-shaped curve exists for the distillate. This is expected due to the ratio control between the distillate stream and the reflux stream and the ratio control between the benzene side draw and the reflux stream. Throughout the overshoot in flow rates, the benzene purity in the side draw remains relatively constant. What is interesting to note is the response of the bottoms flow rate. Here, an inverse response is exhibited. The flow rate first decreases, then increases, overshoots, and finally assumes its new steady-state value. Why does the bottoms flow rate behave in such a maimer ... 

At this point, the inventories in the system have been placed under control and composition control can be considered. However, first the side stream material balance must be considered. The condenser level control refluxes any disturbances in vapour flow rate back down into the column as liquid. On the other hand, the base level controller sends any disturbances in liquid flow back up the column as vapour. To prevent a buildup of side stream material in the column, a route must be provided for the side stream material to escape. This can be accomplished by ratioing the liquid side draw flow to the reflux flow. 

In contrast to obstructive disease, restrictive disease markedly reduces TLC while preserving RV. The PEFR is generally reduced. Demonstrate these points by drawing a curve that is similar in shape to the normal curve but in which the flow rates are reduced. In addition, the left-hand side of the curve is shifted to the right, demonstrating a fall in TLC. 

A model of gravity induced film flow is developed which can predict the shape of the interface, namely the film thickness distribution around the perimeter of rectangular minichannel. The flow is considered in terms of two components namely in the corners and on the sides of the channel. It gives the simplified equation that describes the flow interface, which is written and solved numerically. The effect of the strong capillary action that draws the liquid to the comer of the channel is noted. By comparing the model with the experimental data, it is shown that it predicts well the experimental results on interface shape and flow rate redistrihution. 

If the draw is markedly less in rate than the internal liquid flow, it can be drawn under flow control from an internal or external sump, and arranged so that the tower internal reflux flows through the sump. It is a good idea, nonetheless, to have a seal baffle or a seal pan set up so that if the side draw rate is set higher than the internal reflux flow, the downcomer will not become unsealed. 

Two binary streams each containing component 1 (the more volatile) and component 2 are to be separated in a single column equipped with a total condenser and a reboiler. Feed Fl enters the column as saturated vapor and feed F2 enters the column as saturated liquid. A vapor side draw Sl and a liquid side draw S2 are taken from the column. Using the data below, determine the correct relative locations of the feeds and products, the distillate and bottoms flow rates, and the L/V ratio in each column section. The column uses a reflux ratio of 1.8. 

The liquid flow below the liquid draw tray is reduced by an amount equal to the side draw rate, and if the side draw is taken at tray j, Lj = Lj - Sf. Under the present simplifying assumptions, the vapor flow does not change at the liquid draw tray. Therefore, the L/V ratio below the liquid draw is less than the ratio above it, and the slope of the operating line below the draw is reduced. 

Feed or draw thermal condition parameter Side draw designation or molar flow rate Temperature... 

The McCabe-Thiele method will be used. The side draw is more concentrated in acetone than either feed and should therefore be located above the feeds. Calculate the internal liquid and vapor flow rates and the slope of the operating line between the condenser and the side draw ... 

One possible set of specifications would include the flow rates of three of the products and the column reflux ratio. The way the components are split among the products, the identity of the key components in each section are determined by the product rates, and the locations of the feed and products. In this example the products are specified at flow rates that correspond to the component flow rates in the feed. Thus, the distillate is specified at 12 kmol/h, the upper side draw at 48 kmol/h, and the lower side draw at 25 kmol/h. By overall material balance, the resulting bottoms flow rate is 15 kmol/h. 

An intermediate product such as the upper side draw, which is mostly propane, may contain impurities from components both lighter and heavier than the main component. The upper side draw is at the same time the bottom product of the top column section and the top product of the second section. The fractionation in the upper section determines to what extent ethane is stripped off from the propane product, and the fractionation in the second section determines to what extent butane is removed from the propane product. Again, in this situation since the number of stages in each section and the reflux ratio are all fixed, the fractionation is fixed. The propane recovery and purity depend mostly on its flow rate and on the flow rates of the adjacent products above and below it. If the propane product contains too much ethane, its flow rate should be cut back and the overhead rate increased. If the propane product contains too much butane, its flow rate should be cut back and the lower side draw rate increased. Table 9.14 summarizes the purities of components in different products at different flow rates. The recoveries can also be calculated from Table 9.14. The dependence of the other products compositions on their rates may be analyzed in a similar manner. 

The variations in the column flow profiles are caused by the liquid side draws. The liquid flow at the top of the column is the reflux rate. Each liquid draw removes a portion of the column liquid flow. With a reduced liquid flow, the vapor flow also decreases since the LIV ratio remains unchanged, assuming other factors are constant. The column flow profiles can be manipulated by redistributing the condenser duty among side coolers along the column. 

A column has two feeds, a partial condenser, a partial reboiler, a vapor distillate, a bottoms product, and a side draw. The feeds are of fixed flow rates, compositions, and thermal conditions. 

A distillation column with an attached side stripper is used to separate a feed stream F into products A, B, and C, as shown in the diagram. The main column has a total condenser and a partial reboiler, and the side stripper has a partial reboiler. The columns are existing units with fixed number of trays and feed and draw locations. The external feed, a product of an upstream unit, is of fixed flow rate, composition, and thermal conditions. The pressure in the columns is determined independently and is not available as a variable. Using basic modules representation, determine the degrees of freedom of this system. What variables would you specify to define the performance of these columns ... 

If the liquid side draw from tray j has a molar flow rate of and the component... 

Equation 12.24 is applicable to all column sections that are bounded by side products on both sides. The uppermost and lowest sections are bounded by the overhead or bottoms on one side and a side draw on the other. If a partial condenser is used, Equation 12.22 should be applied to the uppermost section. Expressed in terms of stream and component flow rates, the equation for the uppermost section takes the form... 

A distillation column is designed for the separation of a mixture of benzene, toluene, and biphenyl into a distillate (mostly benzene), a side draw (mostly toluene), and a bottoms product (mostly biphenyl). The column will operate at a pressure of about 175 kPa and a temperature ranging from about 90°C in the condenser to about 240°C in the reboiler. The three products define two column sections, and in each section a A -value for biphenyl, designated as the reference component, and relative volatilities for the other components are estimated based on the average temperature and pressure in the section. The feed stream component flow rates, relative volatilities, reference K-values, and product rates are given below ... 

When vapor flow up the column is small compared to the side draw rate, configuration 19.6c (with any of the Fig. 16.4 control schemes) can induce large swings in column vapor traffic above the side draw, and a severe interference with the pressure controller. This is analogous to the problem described with configuration 19.66. The vapor swings are most pronounced with schemes 16.46 and d, because the top section temperature is more sensitive to the feed than to the changes in the small vapor flow. 

Controlling the internal vapor flow to the section above the side draw Reboiler heat duty is measured and divided by the latent heat of the boiling mixture the measured side product flow is subtracted from the quotient to give the internal vapor rate in the section above the side draw. In a steam (or condensing vapor) reboiler, the internal vapor rate is computed as a constant times the measured steam rate less the measured side product flow, with the constant equal to the ratio of the latent heat of steam to that of the boiling mixture. An internal vapor controller (IVC) uses this computed internal vapor to manipulate product flow (Fig. 19.76). A limitation of this technique is that internal vapor is computed as a small difference between two large numbers and can therefore be in error. The error escalates as the internal vapor rate becomes a smaller fraction of the total vapor traffic below the side draw. 

The Fig. 19.86 system can be modified to become an improved version of the Fig. 19.8a system by cascading the composition controls onto the product ratio controls (68). If needed, an internal vapor flow control can be used to control the side-draw rate, and the bottom level controller cascaded onto the steam ratio control. 

If a product of intermediate composition is required, a vapor or liquid side stream may be withdrawn. This is commonly done in petroleum refineries and is illustrated in Figure 4-22A for a liquid side stream Three additional variables such as flow rate, S, type of side draw (liquid or vapor), and location or composition Xg or yg, must be specified. The operating equation for the middle section can be derived from mass balances around the top or bottom of the column. For the situation shown in Figure 4-22A. the middle operating equation is... 

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