Theoretical trays at operating reflux

F = intermediate feed Theoretical Trays at Operating Reflux... 

From the theoretical trays at operating reflux the actual trays for installation are determined ... 

Example 8-25 Scheibel-Montross Minimum Reflux, 80 Minimum Number of Trays Total Reflux — Constant Volatility, 80 Chou and Yaws Method, 81 Example 8-26 Distillation with Two Sidestream Feeds, 82 Theoretical Trays at Operating Reflux, 83 Example 8-27 Operating Reflux Ratio, 84 Estimating Multicomponent Recoveries,... 

Figure 8-28 presents the usual determination of optimum or near optimum theoretical trays at actual reflux based on performance. It is not necessarily the point of least cost for all operating costs, febrication costs or types of trays. A cost study should be made to determine the merits of moving to one side or other of the so-called optimum point From the Figure 8-28 ...

The method of Gilliland [23] (Figure S-24) is also used for multicomponent mixtures to determine theoretical trays at a particular operating reflux ratio, or at various ratios. 

Because a column cannot operate at total reflux and produce net product from the column, a reflux ratio of about 1.1 to 1.5 times the mmmMm reflux will usually give practical results. Be aware that as the reflux ratio comes down approaching the minimum, the number of theoretical and then corresponding actual trays must increase. 

Note that these values for theoretical trays do contain corrections in overall efficiency, and hence are not the actual trays for the binary distillation column. Efficiencies generally run 50-60% for systems of this type which will yield a column of actual trays almost twice the theoretical at the operating reflux. 

Point F on the figure represents conditions in the kettle or still with Xj, yj, or Xq, yo- Line DF represents slope of the operating line at minimum reflux. The step-wise development from point D cannot cross the intersection, F, where the slope intersects the equilibrium line, and leads to an infinite condition, as point F is approached. Thus, an infinite number of theoretical trays/stages is required, and... 

The minimum reflux ratio (L/D)min been determined to be 1.017. Using the Brown and Martin graph [9], evaluate the theoretical number of trays at an operating reflux of 1.5 times the minimum. The minimum number of stages was determined to be 22.1 including the reboiler. See Figure 8-49. 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

Numbers of Theoretical Trays and of Transfer Units with Two Values of kL/kG for a Distillation Process An equimolal mixture at its boiling point is to be separated into 95 and 5% contents of the lighter component in the top and bottoms products. The relative volatility is a- = 2, the minimum reflux is 1.714, and the operating reflux is 50% greater. The two values of kjJkQ to be examined are —1 and... 

A distillation column is designed to separate a 70-30 mole percent mixture to produce a distillate product with 97.5% mole component 1 and a bottoms product with 20% mole component 1. The column has a partial condenser and reboiler and operates at 1 atm pressure. Determine the required number of theoretical trays and the optimum feed location at a reflux ratio of 1.5. Calculate the recovery of component 1 in the distillate. Calculate the recovery at the same distillate purity and reflux ratio if the feed is introduced three trays below the optimum feed tray. Use thermodynamic data from Problem 6.1. 

The liquid from the expander is fed to the top of the demethanizer column as external reflux, and the valve outlet stream is fed to an intermediate tray. In this example, the column has 12 theoretical trays and a reboiler (a total of 13 theoretical stages). As shown in Figure 9.1, one stream goes to tray 1 and the other to tray 5, counting trays from the top of the column. Streams 3 and 4 are the overhead and bottoms products, respectively. The column and the reboiler operate at a constant pressure of 2900 kPa. The compositions, flow rates, and thermal conditions of the streams are given in Table 9.1. The process requirement is that the bottoms product should have a maximum of 0.001 mole fraction methane. 

Common operating practice usually involves setting the reflux at about 1.25Hn,in. Once the values of Nmin and are known, the Gilliland correlation between reflux ratio and number of theoretical stages allows the reflux ratio to be worked out for a column with a known number of trays (N) ... 

Total reflux. In distillation of a binary mixture A and B the feed conditions, distillate composition, and bottoms composition are usually specified and the number of theoretical trays are to be calculated. However, the number of theoretical trays needed depends upon the operating lines. To fix the operating lines, the reflux ratioR = LJD at... 

Provided the reflux is returned to the column at its bubble point, i.e. there is no subcooling by the condenser, Equations (12.18) and (12.22) can be plotted on the McCabe-Thiele diagram. These are shown in Figure 12.8 they lie between the vapour and liquid hues. By establishing a realistic reflux we have increased the number of theoretical trays required. We have saved energy but now require a taller column. Column design is therefore a trade-off between operating cost and cost of construction. 

If no additional capacity is needed, revamp of a trayed column with a smaller size IMTP packing increases the number of theoretical stages by up to 20%. These additional theoretical stages permit operation at a lower reflux ratio. This, in turn, reduces the condenser s refrigeration requirement. 

As the operating lines move farther away from the equilibrium curve with increased reflux ratio, the number of theoretical trays required to produce a given separation becomes less, until at total reflux the number of trays is the minimum... 

Let us now proceed in the opposite direction and progressively increase the reflux ratio. Both the logic of the preceding argument and Figure 7.25a indicate that this will lead to a decrease in the number of theoretical trays required. A limit is reached when no distillate is withdrawn and the entire overhead product is returned to the column as reflux. The operation is then said to be at total reflux. The reflux ratio becomes infinity, R = L/D = L/0 = < ,... 

Estimate the number of theoretical stages required at the operating reflux ratio. Applying a suitable tray efficiency, set the number of actual trays to be provided. Make a McCabe-Thiele analysis to check the number of theoretical stages and to locate the feed tray. 

The chemical equilibrium constant at 366 K [(Feq)366] and the relative volatilities (constant or temperature dependent) are specified for each case. Equimolal overflow is assumed in the distillation columns, which means that neither energy balances nor total balances are needed on the trays for steady-state calculations. Other assumptions are isothermal operation of the reactor, theoretical trays, saturated hquid feed and reflux, total condensers, and partial reboilers in the columns. Additional assumptions and specifications are the following ... 

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