Boilup requirement

This criterion is similar to using a reflux ratio of 1.2 times the minimum reflux ratio in a full distillation column. It provides a reasonable compromise between the number of trays and the vapor boilup required. The slope of the resulting operating line is the liquid-to-vapor ratio F/D ... 

Despite its lower vapor boilup requirements, no industrial installations of a two-column Petlyuk system have been reported. Two possible reasons for this, as noted by Agrawal and Fidkowski (1998), are (1) an unfavorable thermodynamic efficiency when the three feed... 

Overhead level control may be calculated simply by the method of Chapter 16, Section 3, but base level control by boilup is very difficult. It is normally used only when the average bottom-produa flow is very small. The characteristic time constant th should be at least 15 minutes and other design factors should be as indicated in Chapter 16, Section 7. In most cases base level control by boilup requires a dynamic analysis, and perhaps supplementary plant tests. If steam flow is measured with an orifice, a square root extractor should be used. 

The holdup on the reactive trays is 1000 mol. The vapor boilup required to achieve the desired 95 mol% purities of the two products at base case conditions is 28.91 mol/s, and the corresponding reflux flowrate is 33.55 mol/s. These purities correspond to a 95% conversion. 

TABLE 23 Coefficienls and Vapor Boilup Required for Constant uca and Obd Cases... 

The base case location of the fresh feed of reactant A is on tray 6 because there are five stripping trays. Figures 2.17-2.19 show the effect of moving the feed tray up into the reactive zone. The upper left graph in Figure 2.17 demonstrates that there is an initial decrease in the vapor boilup required to achieve the specified 95% conversion and impmities. Thus, the optimum feed tray location in this example is not at the very bottom of the reactive zone. 

These results are strikingly different than those obtained in the two-product system. There is an optimum number of stripping trays. Increasing Ns from 5 to 6 causes a decrease in energy consumption, as shown in the upper left graph in Figure 5.8. However, further increases result in higher vapor boilup requirements. 

As the lower left graph in Figure 16.4 shows, an increase in the number of reactors results in higher reactor cost, and reactor cost increases with an increase in reactor holdup. In contrast, the costs of the colunm, the heat exchangers (reboiler and condenser), and energy show an exponentially decaying curve as the reactor volume is increased. This is attributable to the dependence of these costs on the vapor boilup. The increase of the reactor holdups increases the conversion of the reactants. Because less reactant is fed back to the column from the external reactors, the separation is easier for columns. The result is a decrease in reflux and vapor boilup required to achieve the desired purities of product streams. Therefore, the costs related to the separation decrease as the reactor holdup increases. When the columns with different reactor numbers are compared, these separation related costs decrease as more reactors are used. 

Calculate boilup and check against required value. 

If the only disturbances were feed flow rate changes, we could simply ratio the reflux flow rate to the feed rate and control the composition of only one end of the column (or even one temperature in the column). However, changes in feed composition may require changes in reflux and vapor boilup for the same feed flow rate. 

Formulation of the mathematical model here adopts the usual assumptions of equimolar overflow, constant relative volatility, total condenser, and partial reboiler. Binary variables denote the existence of trays in the column, and their sum is the number of trays N. Continuous variables represent the liquid flow rates Li and compositions xj, vapor flow rates Vi and compositions yi, the reflux Ri and vapor boilup VBi, and the column diameter Di. The equations governing the model include material and component balances around each tray, thermodynamic relations between vapor and liquid phase compositions, and the column diameter calculation based on vapor flow rate. Additional logical constraints ensure that reflux and vapor boilup enter only on one tray and that the trays are arranged sequentially (so trays cannot be skipped). Also included are the product specifications. Under the assumptions made in this example, neither the temperature nor the pressure is an explicit variable, although they could easily be included if energy balances are required. A minimum and maximum number of trays can also be imposed on the problem. 

The higher heat transfer coefficients experienced by Hickman led to the concept of placing a peripheral reboiler and core condenser on either side of a rotating packed bed (50). This concept would be useful for distillation applications that need reflux and boilup. The internal exchangers as part of the rotor would decrease the required heat transfer surface area but would involve additional design and fabrication complexity. 

Equality constraints h(D°, D°) = 0 may include, for example, a ratio between the amounts of two products, etc. Inequality constraints g(u, D°) < 0 for the overall operation include Equations 7.14-7.18 (the first two of which are easily eliminated when m and H are specified) and possibly bounds on total batch time for individual mixtures, energy utilisation, etc. Any variables of D° and D° which are fixed are simply dropped from the decision variable list. Here, Strategy II was adopted for the multiple duty specification, requiring B0 to be fixed a priori. Similar considerations hold for V, the vapour boilup rate. The batch time is inversely proportional to V for a specified amount of distillate. Also alternatively, for a given batch time, the amount of product is directly proportional to V. This can be further explained through Equations 7.24-7.26) ... 

The control of the separation section is presented in Figure 10.11. Although the flowsheet seems complex, the control is rather simple. The separation must deliver recycle and product streams with the required purity acetic acid (from C-3), vinyl acetate (from C-5) and water (from C-6). Because the distillate streams are recycled within the separation section, their composition is less important. Therefore, columns C-3, C-5 and C-6 are operated at constant reflux, while boilup rates are used to control some temperatures in the lower sections of the column. For the absorption columns C-l and C-4, the flow rates of the absorbent (acetic acid) are kept constant The concentration of C02 in the recycle stream is controlled by changing the amount of gas sent to the C02 removal unit The additional level, temperature and pressure control loops are standard. 

If 17% benzene is unacceptable in the bottom product, reflux and reboil can be raised to achieve the required esparation in 10 stages. The slope of the rectifying section component balance line is increased, and that of the stripping section component balance line is lowered, ThiB is a trial-and-error calculation, which ende when 10 theoretical stages are accommodated betwean the componant balance line and the equilibrium curve, the top and bottom products are at their desired specifications, and the feed enters between stages 4 and 5. The slopes of the component balance lines will determine the new required reflux and boilup rate. The final result is shown in Fig. 2.10c. From this diagram,... 

Using these plots in preliminary computer runs with simple specifications such as product rate6 and reflux or boilup will also help to determine if more or fewer stages are needed or if the feed stage is properly located. These simple runs will also help establish initial profiles and even whether the separation is feasible at all. Once such problems are conquered, the column can be simulated at the desired conditions and with the required specifications. 

Global Newton Naphtali and Sandholm (42) Holland (8) High number of trays, Tew components All type mixtures including nonideal Requires good starting values Chemical and reactive systems Two of condenser duty, re boiler duly, reflux, and boilup plus all side product flows, one purity allowed... 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

The reason distillation is limited in its ability to reach the very low impurities level required is that the highly reactive DMSO decomposes at a finite rate as boilup heat is provided. 

Total reflux is similar to minimum reflux in that it is not usually a real condition. In total reflux, all of the overhead vapor is returned to the column as reflux, and all of the liquid is returned as boilup, so that there are no distillate and bottom flows out of the column. At steady-state, this means that the feed stream flowrate is also zero. Total reflux is used in actual columns during start up and also to test their efficiency. Total reflux is useful in a McCabe-Thiele analysis in order to find the minimum number of stages required for a given separation. 

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