Distillation, differential

A simple application for batch distillation is what is known as differential distillation. It consists of a boiler containing a load of liquid. The liquid is progressively vaporized, with the vapor flowing to a condenser, producing a liquid distillate. There is no reflux, so the only stage with vapor-liquid equilibrium is the boiler. 

The model assumes perfectly mixed liquid in the boiler. As the distillation pro- 

With a single equilibrium stage and no reflux, the separation power in differential distillation is obviously limited. It is the equivalent of a batch flash operation. Consequently, practical applications would include the separation of wide-boiling mixtures, with low expectations on the purity of the products. 

For a single stage. Equation 17.8 may be applied where Xp is replaced with Y, the mole fraction of one of the components in the equilibrium vapor distillate  

The integral may be evaluated graphically, with X varying in small increments from initial conditions, Xq, to the final state, X. The corresponding Y values represent the equilibrium vapor composition. The amount of distillate and its cumulative composition are calculated by equations equivalent to Equations 17.9 and 17.10. 

Your objectives in studying this section are to be able to  

Understand the concept of differential distillation and how it differs from flash vaporization. 

Derive and apply Rayleigh equation to relate the compositions of the residue and composited distillate to the fraction of the feed differentially distilled. 

If during an infinite nuber of flash vaporizations of a liquid only an infinitesimal portion of the liquid were flashed each time, the net result would be equivalent to a differential distillation. 

In the simple differential distillation process, the vapor product is in equilibrium with the liquid in the reboiler at any given time but changes continuously in composition. The mathematical approach must therefore be differential. Assume that at any time during the course of the distillation there are L moles of liquid in the still of composition x mole fraction of A and that an amount dD mole of distillate is vaporized, of mole fraction y in equilibrium with the liquid. Then we have the following differential material balances  

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Distillation. Distillation separates volatile components from a waste stream by taking advantage of differences in vapor pressures or boiling points among volatile fractions and water. There are two general types of distillation, batch or differential distillation and continuous fractional or multistage distillation (see also Distillation). 

Differential Distillation Simple Batch, No Trays or Packing Binary Mixtures, No Reflux... 

The usual Raleigh Equation form [130] is for the conditions of a binary simple differential distillation (no trays or packing), no reflux, but with constant boilup. 

For a batch differential distillation where no reflux is used, there is only boilup of a mixture of the desired lighter component, which leaves the kettle, and a desired residual bottoms composition is left in the kettle. This type of distillation follows the Raleigh equation to express the material balance. However, while simple, not having tower packing or trays or reflux does not offer many industrial applications due to the low purities and low yields involved. Repeated charges of the distillate back to the kettle and redistilling w411 improve overhead purity. 

Differential Distillation—Simple Batch, Without Trays, Multicomponent Mixture... 

A mixture of hydrocarbons at 80 psia is to be differentially distilled until the mols of propane is reduced to 10 mols per 100 mols of bottom feed material. A kettle with bottom coil is to be used, and no trays. 

When the composition of the compounds in the still or bottoms changes significantly as the batch distillation progresses, an unsteady state condition will exist as for differential distillation (see discussion of this subject later). 

The results of the differential distillation end the same as the flash distillation, although the mechanism is somewhat different. This is a batch type operation distilling differentially. All sensible and latent heat are supplied separately from the steam or by superheat in the steam. Steam acts as an inert in the vapor phase, and quantity will vary as the distillation proceeds, while temperature and pressure are maintained. 

Batch Distillation, 45 Differential Distillation, 46 Simple Batch Distillation, 47 Fixed Number Theoretical Trays,... 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Differential condensation in which the liquid separates from the vapour from which it has condensed. This process is analogous to differential, or Rayleigh, distillation, and the condensation curve can be calculated using methods similar to those for determining the change in composition in differential distillation see Volume 2, Chapter 11. 

Multicomponent Differential Distillation 494 Dissolved Oxygen and BOD Profiles in a River 572... 

The simplest example of batch distillation is a single stage, differential distillation, starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. Since this vapour is richer in the more volatile component than the liquid, it follows that the liquid remaining becomes steadily weaker in this component, with the result that the composition of the product progressively alters. Thus, whilst the vapour formed over a short period is in equilibrium with the liquid, the total vapour formed is not in equilibrium with the residual liquid. At the end of the process the liquid which has not been vaporised is removed as the bottom product. The analysis of this process was first proposed by Rayleigh(24). 

Where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed, the process of steam distillation may be used. Steam is passed directly into the liquid in the still and the solubility of the steam in the liquid must be very low. Steam distillation is perhaps the most common example of differential distillation. 

Figure 10. Composite differential distillation profiles for the 2% Cloisite filled system a virgin state and after 504 hours of aging. Solid, dashed and lines with circle symbols correspond to virgin, N2 aged and air aged samples respectively.

In simple or differential distillation, the vapor evolved from the boiling mixture is immediately removed and condensed. Unless the boiling points of the two pure components differ widely, a reasonable degree of separation is not possible. This method may be used to remove low boiling solvents from aqueous solutions. 

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