Vapour-liquid equilibrium relative volatility

Plot the vapour-liquid equilibrium curve from data available at the column operating pressure. In terms of relative volatility ... 

One of the most successful methods for calculating the number of plates necessary for a given separation is due to Lewis and Matheson(38). This is based on the Lewis-Sorel method, described previously for binary mixtures. If the composition of the liquid on any plate is known, then the composition of the vapour in equilibrium is calculated from a knowledge of the vapour pressures or relative volatilities of the individual components. The composition of the liquid on the plate above is then found by using an operating equation, as for binary mixtures, although in this case there will be a separate equation for each component. 

Figure 6.2 shows vapour/liquid equilibrium (VLE) curves for an ideal mixture of solvents with various relative volatilities ranging from 10.0, at which a distillation separation is easy, to 1.5 below which an alternative technique may have to be sought for separation. In many cases the relative volatility is far from constant over the composition range. 

The last example given involves a case where the fference in relative volatilities of the components to be separated is reladvely small. If membranes are available with higher selectivities than the vapour-liquid equilibrium, pervaporation can be combined with distillation as shown in figure VI - 28. This approach is very attractive in case of debottlenecking of an existing distillation plant Most pervaporation applicadons can be found in the chemical process industry but they are also other areas such as the food and... 

Ionic hquids have stronger interactions with water than with alcohol. Therefore, the relative volatility of the alcohol is increased by addition of the ionic liquid. A higher ionic liquid concentration results in a higher relative volatility of the alcohol, eventually resulting in breaking of the azeotrope. " i4s,iso,154,156,158,159 general vapour-liquid equilibrium of... 

The composition of the vapour in equilibrium with a miscible liquid mixture at any temperature, e.g. on heating during distillation, will be enriched by the more volatile components. The composition of the liquid phase produced on partial condensation will be enriched by the less volatile components. Such fractionation can have implications for safety in tliat tlie flammability and relative toxicity of the mixtures can change significantly. 

Assuming the equilibrium to be expressed in terms of relative volatilities a, and theoretical plate behaviour, the relation between the vapour and liquid mole fraction compositions leaving the plate is given by... 

A batch still corresponding to a total separation capacity equivalent to eight theoretical plates (seven plates plus the still) is used to separate a hydrocarbon charge containing four (A, B, C, D) simple-hydrocarbon components. Both the liquid and vapour dynamics of the column plates are neglected. Equilibrium data for the system is represented by constant relative volatility values. Constant molar overflow conditions again apply, as in BSTILL. The problem was originally formulated by Robinson (1975). 

As pressure increases, the temperatures increase. Relative volatilities are reduced as temperatures rise, and this change increases the required reflux and/or the equilibrium stages. On the other hand, the contact-efficiency between vapour and liquid improves with increasing temperature. 

Until relatively recently, the fact that an experimental isotherm necessarily contained composite information concerning the adsorption of the two components of a binary solution was considered to be a major problem. For a rigorous interpretation it was felt necessary to process the data to obtain the so-called individual adsorption isotherm or separate adsorption isotherm of each component. However, this is not at all straightforward and requires the introduction of a number of assumptions relating to the structure of the adsorbed layer. The main problem is of course to know the composition of the adsorbed layer. One assumption often used in the case of volatile components is that introduced by Williams (1913) the solid will adsorb the same amount of each component from the vapour in equilibrium with the solution as from the solution itself. This of course implies that the adsorbed layer has the same composition at the liquid-solid and gas-solid interfaces and it requires numerous gravimetric measurements from the vapour... 

This tells us that the vapour is richer in the more volatile component (B) than the liquid with which it is in equilibrium. Using this relation we can now construct another curve, the lower solid line in Fig. 6.2, which tells us the composition of the vapour in equilibrium with liquids of various compositions. It is this difference in the compositions of liquid and vapour that enables us to separate components of a mixture by distillation. If we take a solution of composition xB(l) and allow it to boil, the initial composition of the vapour will be xB g), which is richer in component B. If the vapour is condensed and collected the process can be repeated and a liquid richer in component B, of composition x B(g), may be collected. When the original liquid is boiled, since relatively more of component B is lost to the vapour, the remaining liquid becomes richer in component A. 

Average relative volatility Phase equilibrium Molecular proportions Solubility of the liquid components Density of the vapour and reflux Viscosity of the vapour and reflux Surface tension of the vapour and reflux Specific heat of the components Heat of evaporation of the components Wetting properties... 

A relatively new class of compounds that has received attention over the last decade as providing environmentally benign reaction and separation media in chemical industry is ionic liquids. Ionic liquids are molten salts with melting points close to room temperature. Their most remarkable property is that their vapour pressure is negligibly small, so that ionic liquids are non-volatile, nonflammable and odorless. The precise description of the phase equilibrium of ionic liquids with pure and mixed solvents is important for process design. 

The dichloride is more difficult to produce in bulk but it can be tolerated as an impurity in the trichloride. Although the reaction equilibrium favours trichloride formation at low temperatures, a temperature of 1000°C is necessary for a reasonable reaction rate. A satisfactory technique is to boil titanium tetrachloride and allow the vapour, mixed with hydrogen, to pass over a tungsten fllament sheathed in silica and heated to 1000-1100°C. The trichloride product condenses on the walls of the reactor. The excess tetrachloride is condensed under reflux and the liquid dissolves the less volatile trichloride and carries it to the still. When the reaction has proceeded sufficiently, the excess tetrachloride is distilled away, leaving the trichloride in the form of a purple coloured crystalline deposit. This process is operated on a relatively small scale (about 0 5 tons per annum if run continuously), the reactor being of stainless-steel with a single glass condenser. 

The previous discussion has dealt with liquid injection. There are other more specialised injection techniques which are important for low-level and contaminant analysis such as adsorption of the sample from the vapour phase onto a solid substrate followed by thermal desorption of the adsorbed volatile onto a gas chromatographic column for analysis and head-space analysis. In head-space analysis the sample is allowed to come to thermal equilibrium at a controlled temperature in a sealed vial. The gaseous phase in the vial is sampled and analysed. This technique has two major advantages (i) only the volatiles in the sample are transferred to the column, thus reducing contamination and (ii) the components of interest are usually at relatively high concentration in the vapour as opposed to in the sample (which may be in any physical form, solid, liquid or paste). Quantification is complicated and is best done using standard additions where this is possible. 

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