Vapor–liquid equilibrium system

Vapor–liquid (reversible) equilibrium systems are used in unit operations, such as distillation, rectification and stripping, evaporation, and condensa- tion. (Note that gas–liquid equilibrium systems, which are relevant in unit operations dealing with scrubbing, cooling, etc. of a gaseous stream, have some similarity. However, these will be discussed separately, in the next section.) Data needed for process design are obtained by correlating the

compositions of both phases at equilibrium in certain conditions of temperature, absolute total pressure, and the partial pressure of noncondensable gases (inert, nonreactive) that may be present (assuming that such partial pressure does not exceed 70 to 80% of the total).

The pair of compositions for both phases can be obtained from a total reflux

test, where the vapors from a boiling liquid phase are totally condensed at the same absolute pressure and all the condensed liquid is returned to the

boiling liquid. As equilibrium is established, samples are withdrawn from both the boiling liquid and the refluxed condensed liquid, and completely analyzed for all components. If there are only two components, the plotting of the results is straightforward.

But since, in most cases, there are more than two components present in each phase, one has to decide from the start which two components are the

variables under study and which other components are to be considered as

parameters for the purpose of the present process design, together with the obvious physical parameters, such as the temperature, absolute pres- sure, and partial pressure of noncondensable gases. All the parameters have to be kept constant in each series of tests, to obtain a cross section for two variables.

One may see that the experimental program for a typical system with four to six components can become very complex unless one limits from the beginning the ranges of practical interest (see Chapter 5, Section 5.1). Once this chosen range is covered with a limited number of experimental “points,” the numerical results can be interpolated quite safely into a mathematical function by using one of the published theoretical correlation formulas. The correlated function then can be used for the process calculations of the

multiple stages equilibrium system in one of its forms: countercurrent, cocur- rent, or crosscurrent.

The process design can be done using the “theoretical stages” concept, and then translated into an equipment design, by relying on the correlation linking the “height of theoretical unit” with the operating conditions and the details of the chosen equipment. Such correlation has been published for several basic designs or may be obtained from the suppliers of more spe- cialized equipment.

When there are many condensable components from the beginning (as in petroleum processing, as an extreme case), one may have to “cut” the mixed feed by a coarse separation into two or three ranges (“heavies” or “lights”) and to treat each range as a separate problem with recycles at the starting point.

A special case is the concentration of a solution containing nonvolatile solutes by evaporation of water (or another solvent), leaving the nonvolatile solutes in the concentrated solution. The vapor phase contains only one component, but the concentrations of the solutes into the liquid phase increase gradually, decreasing the partial pressure of the water. In such case, the important data are the quantitative link between the absolute pressure and the boiling temperature of the solution and the concentrations of the solutes below their saturation limit. These data are essential, for example, for starting the design of energy-efficient, multiple-effect evaporators, which are a critical element in many processes (e.g., salts and sugars).

An equally important result of such tests can be any observation about the precipitation of certain solids from the liquid, and the form and behav- ior of such solids, in particular as to their incrustation inside equipment and pipes, or on heat exchangers surfaces (for their composition, see

Section 6.2.3); as well as the conditions of the release of any noncondens- able gases dissolved in the feed solution.

Another important field of process development is concerned with the separation between two volatile components that cannot be obtained directly due to the presence of an azeotrope or another particular feature of the equilibrium curve. (As a reminder, at the azeotropic point, the compositions of the liquid and of the vapors are identical.)

A well-known case is the system HCl-water (already mentioned in Chapter 4, Section 4.3.2) which is dominated by an azeotrope at 20 to 22% HCl (the exact number depends on the absolute pressure). Every ton of HCl generated “below the azeotrope” is accompanied by at least 4 to 5 tons of water at its maximum practical concentration and this feature may prevent or limit its use in other processes. “Breaking the azeotrope” means obtaining a more concentrated solution that can be handled at ambient temperature (say about 30 to 40% HCl) or even a 100% dry HCl gas, if needed.

Such a result is possible by using a cycle of CaCl2 brine in a close cycle, as the brine absorbs the water and releases the HCl, but this is a compli- cated process with many reflux streams. It is also an expensive process, both in the investment in the HCl-resistant equipment and in the energy consumption. Another commonly found complication can be the presence of nonvolatile components in the starting HCl solution, which can accu- mulate in the circulating brine. In such case, the starting solution should be completely evaporated upstream and the heat loads should be redis- tributed. This problem was at the time an open field for creative process design, aiming at a better use of the energy and the expensive heat exchang- ers, and of any possible synergetic utilization of sources of low-temperature waste heat.11–14 _{Different aqueous solutions were used, including MgCl}

2 and LiCl. It was also proposed to replace the expensive heat exchangers by direct contact heating with organic “heat carriers.” (See below and also Chapter 5, Section 5.1.5.)

Direct contact heating technology, with organic “heat carriers” (stable hydro- carbons, liquid, or vapors) — Certain processes need large heat exchangers made from expensive materials (resistant to corrosion, such as graphite, glass-lined, tantalum) to introduce heat into the process streams and evap- orate certain components, and similarly for removing heat in condensers. In other cases, heating of such solutions in a regular heat exchanger would precipitate solids and cause the rapid scaling of such heat exchangers.

One can resort to introducing very hot organic liquid or vapor “heat- carrier” in direct contact with the process stream to be heated. After heat transfer and equilibration, the organic liquid is separated, removed, washed, and reheated in a separate boiler made of cheaper materials. Although the heat carrier material would have a boiling point much higher than any of the components present, it can have a definite vapor pressure in the hotter parts of the equipment, which should be taken into account and included in the experimental program.