In process engineering, compounds are frequently separated or mixed. At the same time, reactions may occur. Particular difficulties are presented by compounds which, in order to obtain a satisfactory quality, may only be treated in a narrow temperature range under certain pressure conditions. The treatment is particularly critical in the case of high viscosity compounds because the residual component becomes increasingly more difficult to evaporate with falling vapor pressure. In many cases, undesirable, barely separable compound concentrations occur even when components are mixed in. This may give rise to reductions in the quality of the product, particularly in the case of exothermic reactions.
There are a whole number of types of apparatus which are used for mixing, evaporating and for reactions.
A screw evaporator is known in which one or more heatable screws in a housing provide for the transport and, optionally, mixing of the liquid mixture, the component to be evaporated off optionally being run off in vacuo from the empty overlying space. The high viscosity liquid layer and the small evaporation surface complicate evaporation. The construction is complicated and unreliable in operation on account of the presence of rotating parts. The installation and operating costs are high.
In thin-layer evaporation, a liquid film from which one of the components evaporates is produced by mechanically rotating wiper elements on the heated wall of an elongated, cylindrical container. Evaporation of the residual component decreases continuously with falling vapor pressure unless a vacuum is applied. However, vacuum increases the danger of undesirable foaming. The rotating elements increase unreliability in operation. The investment and operating costs are high.
In falling-film evaporators, there are several parallel vertical tubes along whose inner wall liquid flows down in a thin layer. Gas flows through in the free inner space in the same or opposite direction. Because of the limited tube length, the residence time is often not long enough for residual evaporation. A certain quantity has to flow through in order to prevent local overheating by temporary breakage of the film which may lead to destruction of the heat-sensitive material. In addition, it is only possible to use liquids of low viscosity in which case the undesirable entrainment of liquid droplet cannot always be avoided.
In tubular coil evaporators a polymer solution (U.S. Pat. No. 3,834,441) is heated under excess pressure and is expanded through a nozzle into a helically wound tube. A liquid film is driven along the wall by the internally flowing gas stream to form a secondary stream. One disadvantage of this apparatus is that it is not possible to influence or modify an operation once it has started. There is no flexibility in the mode of operation of the apparatus. The specific output decreases towards the tube exit. When vacuum is applied, the output limit is rapidly reached by the critical mass flow density which determines when the film breaks. The apparatus may only be used for evaporating a particular component.
Accordingly, an object of the present invention is to provide an apparatus which, under a predetermined pressure and at a temperature which is controlled in sections over a wide working range, and allows either for intensive mixing by the measured addition of compounds to a liquid, optionally with simultaneous control of a certain reaction sequence, and/or for a thermal separation of one or more compounds, optionally with recovery of individual fractions. The substances must be carefully treated, even in the case of viscous liquids, with high selectivity in a static apparatus.