Equipment Design

Rising film evaporators, also known as Natural Circulation Evaporators, or Long-Tube Vertical (LTV) Evaporators, are the oldest and simplest of the Evaporators used today.

They have no moving parts, and have the advantage of very stable and reliable operation. This type of evaporator is well suited for the concentration of solutions that are not very close to the point of precipitation of solids, and are low in viscosity. Due to the low residence time of the product, thermosensitive products can be handled well by this machine.

However, to obtain natural circulation, fairly high steam-to-process temperature differences are needed, and this limits this Evaporator’s use in some multiple-effect arrangements.

The Falling Film Evaporator has gained increasing acceptance over the last few decades, and is now the most common type of evaporator. It takes its name from the fact that the liquid to be concentrated cascades down the inside of the heater tube walls as a film.

It usually employs a pump to circulate some process liquid to the top of the vertical heating element, to ensure wetting of the tube walls. In some applications, Falling Film Evaporator units can operate without this recirculation, however.  

SEP has extensive experience in designs that minimize the scale formation on the tube walls, and thus extend the operating cycle of the machine.

The Forced Circulation Evaporator/Crystallizer (FC) is the workhorse of the crystallization industry, as it provides a robust operation with predictable performance for materials that require large amounts of evaporation in order that crystallization may be achieved.

On rare occasions, FCs are used for non-crystallizing, but severely scaling evaporation applications, in an effort to prolong the unit’s operating cycle.

The FC provides mechanical (via a pump) circulation of the slurry through the heating element and back to the Crystallizer vapour head.

In some cases, such as in NaCl production, a Salt Leg in is provided on the FC Vapour head, where the crystals are collected and washed prior to their removal from the crystallizer, to optimize crystal size and purity. While, in general, a simple FC unit provides little latitude in crystal size control, poor design can reduce the crystal size obtained by the crystallizer.  

FC operation is usually hampered by the formation of crusts on its internal surfaces, which require stoppage of the unit to clean it. Proper selection of the operating parameters of the FC can, however, minimize this problem and substantially increase its on-stream time.

The DTB crystallizer is used when a larger particle size is desired. The crystallizer is a combination of a clarifier and a mixed tank in a single vessel. The mixed-tank part of the unit includes a slow-turning, high pumping efficiency internal circulator located concentrically in a tube (the Draft Tube) that helps direct the flow and improve the mixing of the slurry. The internal circulator provides the mixing of the slurry at a very low energy input, which is improves the growth rate of the crystals, and has low attrition for the crystals formed. The clarifier section provides the means to selectively remove and destroy smaller crystals in the mixed-slurry section, thus increasing the average product size.  

The destruction of the small particles is achieved either by heating or by diluting the brine containing these particles. After this has been done, the heated (or diluted) brine is returned to the crystallizer.

DTB units are also used successfully for crystallization from reaction, such as fertilizers made by the neutralization of an acid with a base, because of their high level of internal mixing and the ability to introduce the reactants into a zone of high turbulence.

A simplified variation of the DTB is the Draft Tube (DT) crystallizer, which is a unit that contains only the mixed-slurry feature of the DTB (no baffle, i.e. no clarification zone). The DT is used for some reaction of flash cooling operations, where the main objective is to provide a well-mixed volume.