Typically in any process, such as evaporation, wherein a fluid material, such as a liquid, is separated into volatile and nonvolatile components, the volatile or vapor-phase components have entrained therein the nonvolatile material as fine liquid or solid droplets or particles. In such processes, the entrained particles are then carried over in the vapor phase, which is undesirable. Various techniques have been used in the past to prevent the carrying over of the entrained particles in the vapor phase.
Where the velocity as measured by an F factor is quite low, then the entrained particles, by their velocity of size or combination, drop out or are easily coalesced out of the vapor phase during the processing. However, to obtain proper F factors, often the equipment size must be altered to provide a low enough flow velocity, or other process and structural modifications undertaken to remove the carry over of the entrained particles in the vapor phase. The F factor referred to is the product of the velocity in feet per second multiplied by the square root of the vapor density in pounds per cubic foot. In a typical batch still design, F factors of 0.1 to 0.2 are usually employed, while in centrifugal-type vapor separators, F factors from about 2 to as high as 3.0 are employed in designing the apparatus.
In thin-film-type evaporation apparatuses and processes, a fluid feed material, such as a solution, emulsion, slurry, liquid, suspension or other mixture, is placed onto a heat-exchange surface in thin-film form to obtain a volatile component; that is, a vapor and a nonvolatile component, typically a liquid, solid like powder or salt, a slurry or a viscous material. In many processes, evaporation typically occurs under subatmospheric pressures, and, therefore, it is most desirable to maintain a low-pressure drop within the apparatus. Rotary-type thin-film evaporators, by themselves, are often good entrainment separators, since the rotation of the rotor blades tends to throw the resulting vapor against the interior chamber walls. However, particles entrained in the vapor are still contained in the volatile vapor phase typically under a (100) parts per million.
Entrained particles in the vapor phase of a thin-film evaporator are often removed through the use of a deentrainment type knitted or woven wire mesh-type packing or other packing material which is employed in the vapor outlet system, and prior to the condensation of the vapor.
In addition, in the past, some thin-film evaporators have employed rotating solid disc elements secured to the rotor shaft on which the rotor blades of the thin-film are secured. The disc elements rotating with the rotor shaft are secured in the vapor chamber of the apparatus between the product outlet and the vapor outlet. The disc elements have been composed of a solid plate extending across the cross-sectional area of the vapor chamber, and into close communication with the interior chamber wall. The disc elements have been used in pairs, each one spaced slightly apart from the other along the rotor shaft. In theory, the vapor phase containing the entrained particles passing around and over the rotating surface of the disc elements permits the entrained particles to be impinged on the surface of the disc elements, and then thrown by the centrifugal force of the discs to the inner interior wall of the vapor chamber. The pair of disc elements in essence increases the flow path of the vapor to serve as an entrainment technique to aid in the removal of entrained liquid of solid particles in the vapor phase.
The use of a pair of disc elements or stationary mesh-type material, while satisfactory in some processes and apparatuses, often creates a substantial and undesirable pressure drop; for example, for 4 millimeters of mercury, typically from 4 to 8 millimeters, in an evaporator where it is desired to operate below about 10 millimeters of mercury. In addition, the pair of rotating discs has been also unsatisfactory in that the entrained material thrown to the wall opposite the edges of the discs has a tendency to be reentrained in the vapor passing over the disc elements. Therefore, it is most desirable to provide in a thin-film-type rotor blade apparatus an effective, efficient and inexpensive means to remove entrained particles from the vapor phase without a correspondingly large pressure drop in relationship to the operating pressure of the apparatus and process.