Rectifying section

For constant molar overflow, Vn+1 = V= constant Ln-L = constant and equation (6-14) can be written as 

Equation (6-15) is the operating line for the rectifying section of the fractionator. 

The liquid entering the top stage is the external reflux rate, L0, and its ratio to the distillate rate, LQ/D, is the reflux ratio, R. For the case of a total condenser, with reflux returned to the column at its bubble point, L0 = L and R = LID, a constant in the rectifying section. Since V = L + D, 

Combining equations (6-15), (6-16), and (6-17) produces the most useful form of the operating line for the rectifying section  

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Decreases trays in the rectifying section but increases trays in the... 

The rectifying section contains three or four bubble cap (wine) plates in the top section of the stiU to produce distillates up to 160° proof. Whiskey stills are usually made of copper, especially in the rectifying section, which often yields a superior product. Additional copper surface in the upper section of the column may be provided by a demister, a flat disk of copper mesh. Stainless steel is also used in some stills. 

Distillers employ a somewhat unique process to make various products and have tailored approaches to control and reduce ethyl carbamate to their own particular process. Some of the methods used are the use of copper packing in the rectifying section of stills, increased frequency of cleaning stills and other equipment, and using a cool-down period in the cleaning procedure. Increased rectification also reduces ethyl carbamate. Keeping the system clean is critical to minimising ethyl carbamate. 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

In another alternative, shown in Fig. 13-3 7, the rectifying section may be operated at a pressure sufficiently higher than that of the stripping section such that heat can be transferred between any desired pairs of stages of the two sections. This technique, described by Mah et al. (op. cit.) and referred to as SRV (secondary reflux and vaporization) distillation, can result in a significant reduction in utihty requirements for the overhead condenser and bottoms reboiler. 

In the rectifying section, the equilibrium relationship for component at any stage n can be expressed in terms of component flow rate in the distillate d = Dxd and component absorption factor A = hjKiS... 

Equation (13-50) is used to calculate, from the previous stage, the (f/d) ratio on each stage in the rectifying section. The assumed temperature and phase-rate-profile assumptions conveniently fix all the A values for ideal solutions. The calculations are started by writing the equation for stage N ... 

The rate-based model gave a distillate with 0.023 mol % ethylbenzene and 0.0003 mol % styrene, and a bottoms product with essentially no methanol and 0.008 mol % toluene. Miirphree tray efficiencies for toluene, styrene, and ethylbenzene varied somewhat from tray to tray, but were confined mainly between 86 and 93 percent. Methanol tray efficiencies varied widely, mainly from 19 to 105 percent, with high values in the rectifying section and low values in the stripping section. Temperature differences between vapor and liquid phases leaving a tray were not larger than 5 F. 

One point is on the 45° line at Xq, and the slope is Lr/Vr, where Vr is the rectifying section vapor rate. Another point is on the Y axis above the origin at DXq/Vr. 

For columns in which there is a substantial flash of the feed liquid, or in which the feed is a vapor of a different composition than the internal vapor, a collector plate can be installed above the feed point. The purpose of this plate is to provide mixing of the vapor phase in the gas risers so that a more uniform vapor composition enters the rectifying section of the column. 

Because the feed tray is essentially non-effective it is suggested that an additional theoretical tray be added to allow for this. This can be conveniently solved by the nomographs [21] of Figures 8-16 and 17. If the minimum number of trays in the rectifying section are needed, the)t can be calculated by the Fenske equation substituting the limits of xpi for x jj and x i, and the stripping section can be calculated by difference. 

Equations for Stepwise Tray to Tray Calculations Rectifying Section Operating Line... 

When one of the components of the binary is water, and steam is used, the following equation is used for the operating stripping line (there is no rectifying section) ... 

Equation of operating line in rectifying section, light component 1591... 

Determine temperature of rectifying section pinch. Dxpi /Vr... 

The actual number of trays in the rectifying section (Nact)r be determined by ... 

Introduce components lighter than the light key which are not found in the bottoms in the same general manner as discussed for the rectifying section. 

Neglect the heavier than perchlor components in the rectifying section. 

Vr = Vapor flowrate up rectifying section of tower, lb mols/hr... 

Initial conditions or i or operating condition Pr = Pinch condition in rectifying section... 

Check the trays for a finishing tower operating under vacuum of 75 mm Hg at the top. The estimated pressure drop for the twenty trays should not exceed 50-60 mm Hg. Fifteen trays are in the rectifying section, five in the stripping section. 

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