This invention relates to a method for increasing the efficiency of the separation of a solution into its volatile and non-volatile components in an evaporator.
It is often desired to separate a solution into its component parts. When the solution is one of an essentially non-volatile material in a volatile solvent, such separation can be carried out in a device commonly known as an evaporator. While there are a large number of evaporator types, all comprise two elements, a calandria and a vapor-liquid separator. Examples of common conventional evaporators are found in Perry's Chemical Engineers' Handbook, fifth edition (1973), at page 11-27. Although they are similar in design to stills and reboilers of distillation columns, evaporators differ in concept in that they do not conventionally comprise trays or packing to promote fractionating contact between vapor and liquid. Since the desired separation in an evaporator is between volatile and non-volatile components, for which the relative volatility of the latter to the former is essentially zero at conditions under which the evaporator is operated, vapor-liquid contact between such components would servo no distillative function.
Each of the calandria and vapor-liquid separator elements of an evaporator accomplishes a degree of separation between volatile and non-volatile components in the feed solution. In the calandria, the solution is heated to take advantage of the volatility differences between the components and effect a primary separation. The major portion of the non-volatile material is withdrawn from the evaporator calandria as a liquid product, which may be either a more highly concentrated solution or a slurry of the non-volatile in a minor portion of the solvent. The major portion of the volatile solvent is withdrawn from the calandria as a vapor stream.
The boiling action within the calandria of an evaporator produces small droplets of liquid, comprising non-volatile at a concentration approximating that in the liquid product, which droplets are carried along with the vapor flow from the calandria. Depending upon the particular circumstances of a given evaporator application, this entrainment of non-volatile into the vapor may represent the loss of a valuable non-volatile material or may result in contamination of the solvent vapor product. When it is considered that the concentration of the non-volatile component in the liquid product is generally many times that in the feed, it is realized that only a very small amount of entrainment of the liquid product has a significantly deleterious effect upon calandria vapor quality. For instance, if the concentration by weight of the non-volatile component in the liquid product is one thousand times its concentration in the evaporator feed, then an entrainment of only one part by weight liquid to one thousand parts vapor would completely negate any separation by the calandria.
In order to produce an evaporator vapor product relatively free of entrainment, the calandria vapor stream is in conventional practice passed to a vapor-liquid separator. Vapor-liquid separators are designed to accomplish removal of a high percentage of the entrained liquid droplets from the calandria vapor by taking advantage of such factors as the difference in density between the vapor and the liquid. Since the quantity of the entrainment is largely a function of vapor flow velocity, the calandria vapor may be passed upward at slow velocity through a large diameter vessel to allow the liquid droplets to settle against the vapor flow by the action of gravity. Cyclone separators and impingement baffles may also be used to coalesce the entrained mist into larger and more easily removed droplets. Wire mesh demister pads are commonly employed for this function, although when the non-volatile component of the liquid solution is a solid under evaporator conditions the pads have a tendency to become fouled and to lose their efficiency. Notwithstanding application of such established techniques of vapor-liquid separation, entrainment is often still a problem and generally the limiting factor in the performance of evaporators with respect to separation of a solution into its volatile and non-volatile components.