Distillation overflow

Flow control of reflux is cascaded, if possible, from column overhead composition, and distillate overflows from die vapor—liquid disengagement space beneath the condenser. 

To avoid this problem, one may design a distillate overflow system that provides constant head for reflux. Consider, for example, the scheme of Figure... 

Liquid reaching the bottom of the column is partially vaporized in a heated r eboiler to provide boil-up, which is sent back up the column. The remainder of the bottom liquid is withdrawn as bottoms, or bottom product. Vapor reaching the top of the column is cooled and condensed to liquid in the over head conden.ser. Part of this liquid is returned to the column as r eflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product . In some cases only part of the vapor is condensed so that a vapor distillate can be withdrawn. 

A knowledge of the reflux ratio (obtained from the specified distillate and top vapor rates) permits the calculation of (f d) from which ( i/d) is obtained, etc. Equation (13-50) is applied to each stage in succession until the ratio 2/d in the overflow from the stage above the feed stage is obtained. The calculations are then switched to the stripping section. 

The HETP of a packed-tower section, valid for either distillation or dilute-gas absorption and stripping svstems in which constant molal overflow can be assumed and in which no chemical reactions occur, is related to the height of one overall gas-phase mass-transfer unit Hqc by the equation... 

At constant molal overflow The time required for the distillation only. 

An effort has been made to present the basic understanding of the method as it applies to systems involving unequal molal overflow, open steam distillation and single flash vaporization in Figures 8-42 and 8-43. 

The use of V-notches in a trough wall for overflow is more sensitive to leveling problems than the other designs, and for the same %- to Me-in. level tolerance produces a more severe non-uniform flow distribution. The quality of distribution from a V-notch is poor compared to the other types of trough distributor, but does have advantages in slurry systems [131]. It should not be used for critical distillation applications, but is good for heat transfer and where solids are in the system. 

Unequal Molal Flow, 63 Ponchon-Savarit Method, 63 Example 8-21 Ponchon Unequal Molal Overflow, 65 Multicomponent Distillation, 68 Minimum Reflux Ratio — Infinite Plates, 68 Example 8-22 Multicomponent Distillation by Yaw s Method, 70 Algebraic Plate-to-Plate Method,... 

In the titanium dioxide production plant where the chlorine process is employed, the wastewater from the kiln, the distillation column, bottom residue, and those from other parts of the plant first settle in a pond. The overflow from this pond is neutralized with ground calcium carbonate in a particular reactor, while the scrubber wastewater is neutralized with lime in another reactor. The two streams are sent to a settling pond before being discharged. 

The McCabe-Thiele Method is restricted in its application because it only applies to binary systems and involves the simplifying assumption of constant molar overflow. However, it is an important method to understand as it gives important conceptual insights into distillation that cannot be obtained in any other way. 

The Underwood Equation is based on the assumption that the relative volatilities and molar overflow are constant between the pinches. Given that the relative volatilities change throughout the column, which are the most appropriate values to use in the Underwood Equations The relative volatilities could be averaged according to Equations 9.47 or 9.49. However, it is generally better to use the ones based on the feed conditions rather than the average values based on the distillate and bottoms compositions. This is because the location of the pinches is often close to the feed. 

If the feed is partially vaporized, the vapor flow below the feed will be lower than the top of the column. For above ambient temperature separations, the cost of operating the distillation will be dominated by the heat load in the reboiler and the vapor flow in the bottom of the column. For below ambient temperature separations, the cost of operating the column will be dominated by the cost of operating the refrigerated condenser and hence the vapor flow in the top of the column. If constant molar overflow is assumed, the vapor flow in the bottom of the column V is related to the vapor flow in the top of the column by... 

This method is one of the most important concepts in chemical engineering and is an invaluable tool for the solution of distillation problems. The assumption of constant molar overflow is not limiting since in very few systems do the molar heats of vaporisation differ by more than 10 per cent. The method does have limitations, however, and should not be employed when the relative volatility is less than 1.3 or greater than 5, when the reflux ratio is less than 1.1 times the minimum, or when more than twenty-five theoretical trays are required(13). In these circumstances, the Ponchon-Savarit method described in Section 11.5 should be used. 

Even if you were only half awake when you read the preeeding chapter, you should have recognized that the equations developed in the examples eonstituted parts of mathematical models. This chapter is devoted to more complete examples. We will start with simple systems and progress to more realistic and complex processes. The most complex example will be a nonideal, nonequimolal-overflow, multicomponent distillation column with a veiy large number of equations needed for a rigorous description of the system. 

As a more realistic distillation example, let us now develop a mathematical model for a multicomponent, nonideal column with NC components, nonequimolal overflow, and inefficient trays. The assumptions that we will make are ... 

The digital simulation of a distillation column is fairly straightforward. The main complication is the large number of ODEs and algebraic equations that must be solved. We will illustrate the procedure first with the simplified binary distillation column for which we developed the equations in Chap. 3 (Sec. 3.11). Equimolal overflow, constant relative volatility, and theoretical plates have been assumed. There are two ODEs per tray (a total continuity equation and a light component continuity equation) and two algebraic equations per tray (a vapor-liquid phase equilibrium relationship and a liquid-hydraulic relationship). 

The model of a multicomponent batch distillation column was derived in Sec. 3.13. For a simulation example, let us consider a ternary mixture. Three products will be produced and two slop cuts may also be produced. Constant relative volatility, equimolal overflow, constant tray holdup, and ideal trays are assumed. 

AFTER TOTAL REFLUX STARTUP, DISTILLATE FLOW RATE IS FIXED ASSUMPTIONS CONSTANT RELATIVE VOLATILITIES (TERNARY) BQUIMOLAL OVERFLOW, IDEAL TRAYS... 

Example 12.6. Let us consider a much more complex system where the advantages of frequencynlomain solution will be apparent. Rippin and Lamb showed how a frequency-domain stepping technique could be used to find the frequency response of a binary, equimolal-overflow distillation column. The column has many trays and therefore the system is of very high order. 

Previous reports 13] emphasized the importance of sample handling, and indeed because of the very volatile nature of the compounds measured in this type of analysis, sample collection deserves special consideration. In general, narrow mouth glass vials with a total volume in excess of 50 ml are acceptable. The bottles need not be rinsed or cleaned with organic solvents, but simply cleaned with detergent and water, rinsed with distilled water, air dried, and dried in a 105°C oven for one hour. The vials are carefully filled with sample to overflowing (zero head space) and a Teflon faced silicone rubber septum is placed Teflon face down on the water sample surface. The septa may be cleaned in the same manner as the vials, but should not be heated more than one hour because the silicone layer slowly degrades at 105°C. 

Cold-Water Process. The cold-water bitumen separation process has been developed to the point of small-scale continuous pilot plants. The process uses a combination of cold water and solvent. The first step usually involves disintegration of the tar sand charge, which is mixed with water, diluent, and reagents. The diluent may be a petroleum distillate fraction such as kerosene and is added in a ca 1 1 weight ratio to the bitumen in the feed. The pH is maintained at 9-9.5 by addition of wetting agents and ca 0.77 kg of soda ash per ton of tar sand. The effluent is mixed with more water, and in a raked classifier the sand is settled from the bulk of the remaining mixture. The water and oil overflow the classifier and are passed to thickeners, where the oil is concentrated. Clay in the tar sand feed forms emulsions that are hard to break and are wasted with the underflow from the thickeners. 

The spent acid is distilled in order to separate acetic acid. The paste containing cyclonite and by-products that accumulate on the bottom of the retort is removed continuously, through a syphon overflow. The greater part of this mass (about 80%) is dissolved in acetic anhydride and returned to the reactor while the residue (about 20%) is mixed with ammonium nitrate to make cheap explosives. 

Since in an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains in the liquid phase, and since because of the similarity of the molar latent heats of nitric acid and water there is substantially constant molar liquid overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process. It is appropriate to represent the equilibrium situation as a pseudo-binary system for each magnesium nitrate concentration, and Figure 7 shows vapor-liquid equilibria on a nitric acid-water basis at a series of magnesium nitrate concentrations from zero to 0.25 mole fraction in the liquid phase. 

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