Distillation 1 Single-Stage Separation

When a mixture contains components with a broad range of volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce an effective separation. This is in essence a single-stage distillation process. However, by its very nature, a single-stage separation does not produce pure products hence further separation of both liquid and vapor streams is often required. 

Distillation is a countercurrent process in which equihbrium is maintained between Uquid and vapor, normally in a vertical column containing packing, thin wetted walls, or bubble cap plates. The single-stage separation factor for distillation is given by the vapor pressure isotope effect. Because mixtures of isotopic molecules show little or no nonideality in either liquid or vapor phase (Jancso et al. 1993), liquid vapor separation factors should be virtually independent of isotope enrichment (plate number), and the analysis of the distillation process is... 

Separation of mixtures of condensable and non-condensable components. If a fluid mixture contains both condensable and noncondensable components, then a partial condensation followed by a simple phase separator often can give a food separation. This is essentially a single-stage distillation operation. It is a special case that deserves attention in some detail later. 

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

The simplest continuous-distillation process is the adiabatic single-stage equihbrium-flash process pictured in Fig. 13-25. Feed temperature and the pressure drop across the valve are adjusted to vaporize the feed to the desired extent, while the drum provides disengaging space to allow the vapor to separate from the liquid. The expansion across the valve is at constant enthalpy, and this facd can be used to calculate To (or T to give a desired To). 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

A single-stage pipe still used for distilling whole crude is shown in Figure 1. The unit shown is designed to separate six products gas, overhead distillate, three sidestream distillates, and undistilled residue, or bottoms. Modem pipe stills usually have several sidestreams. 

Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

Single-stage flash distillation processes are used to make a coarse separation of the light components in a feed often as a preliminary step before a multicomponent distillation column, as in the distillation of crude oil. 

Low-temperature/high-pressure distillation. Rather than use a low-temperature single-stage condensation or refluxed condenser, a conventional distillation can be used. To carry out the separation under these circumstances will require a low-temperature condenser for the column, or operation at high pressure, or a combination of both. 

As this process consists of only a single stage, a complete separation is impossible unless the relative volatility is infinite. Application is restricted to conditions where a preliminary separation is to be followed by a more rigorous distillation, where high purities are not required, or where the mixture is very easily separated. 

Establish the separation capability of a single-stage (differential) batch still processing a mixture of two compounds having a relative volatility of 4.0. At the start of the batch separation, there are 600 mol of the more-volatile compound A and 400 mol of compound B in the kettle. When the remaining charge in the kettle is 80 percent B, how much total material has been boiled off, and what is the composition of the accumulated distillate ... 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

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