DICTAMEN NÚMERO CHPM 12-15/011/2014/DPM DEL EXPEDIENTE Nº 47/12

This dependence of the results on the absolute size of certain types of equip- ment is explained in most cases by a “wall effect” and is related to the

nonhomogeneity caused by the inside flows near the walls or to significant

larger heat losses through the walls and the like. A larger wall effect can be quantified by a relatively larger ratio of the internal wall surface to the volume of the equipment. This effect can be more serious whenever the operation depends on a metastable dynamic balancing (“walking a tight

Figure 6.5 Liquid–liquid continuous settler.

mixed phase layer heavy liquid drops

passive interface

active interface feed in

heavy liquid phase light liquid phase

vent

nitrogen blanket option

apparent interface sight glass

rope”), in which a relatively short change can cause a collapse, for instance in a fluid bed solid gas contactor (see below).

6.4.3 Crystallizer

For example, it is well known that the average size of the crystals obtained from a continuous industrial crystallizer increases generally with the size of the equipment, up to a certain maximum related to the flow mechanism and residence time. Furthermore, this average crystal size distribution in the product can be very important as it determines critically the design and the daily operation of all the downstream operations involving the crystals, such as their filtration or centrifuging, washing, drying, screening, marketing, etc. The driving force for the crystallization is always a certain degree of supersaturation which is created purposely by a chemical reaction (addition or decomposition), or by a change of temperature (mostly by cooling), or by a change in concentration caused by the evaporation of water or of another solvent. Such supersaturation strives to decrease by precipitation on any existing crystal surface.

Figure 6.6 Liquid–liquid settler with sets of racks of inclined partitions.

passive interface active

interface

feed in

heavy liquid phase light liquid phase

vent

mixed phase

nitrogen blanket option

An additional important factor in the design and the daily operation of an industrial crystallizer is generally a size classification system between the larger crystals, which are ready to be removed as product, and the smaller crystals that should be left inside for some additional growing time. This size classification can be either on an internal or an external flow cycle, and it has to be done in unfavorable conditions, such as a concentrated, heavy and viscous “mother liquor.” In some decomposition crystallyzers, such as in the potash industry, there is a further complication as the feed is in the form of larger, lighter crystals, which have to be kept from mixing with the product until they are decomposed.

So, there are many types of crystallyzers in use, but their basic princi- ples are similar and relatively simple, and for each practical case the logical choice can be reduced a priori to two or three possibilities to be studied further in detail.

Some typical illustrations of the principle of a “draft tube” crystallizer are shown in Figures 6.7a and 6.7b, and of a dense slurry “Oslo-type” crys- tallizer for larger crystals in Figures 6.8a and 6.8b.

Coming back to the “wall effect” on the product’s size, the prevailing explanation from experts in this field is that the circulation flow of the slurry inside the crystallizer is slower near the walls. As a result, a relatively larger number of crystal nuclei does precipitate from the part of mother liquor that is passing in these regions than in the part remaining in the main cycle flow. There are also more “fines” generated in a smaller equipment by attrition with the higher RPMs of the impeller. This larger number of nuclei translates into a smaller average size of the crystal product for a fixed production tonnage. But this is only part of the story.

On the other hand, in most well-designed crystallizers, the amount of “fines destruction” is an effective (although somewhat expensive) operating tool for increasing the average crystal size by reducing the number of new crystals that are allowed to develop. Fines destruction is obtained on purpose and on demand by the dissolution of a part of the nuclei by using either local dilution or heating of the circulating “clear” mother liquor. In most processes, the amount of fines destruction can be enhanced in the test unit to balance the negative wall effect and to get larger and nicer crystals. The prediction of the final crystal size distribution in the final industrial unit from such small-scale tests does require a lot of experience (and perhaps some guessing) to balance between the contributions of the wall effect and the fines destruction. Large-scale piloting with “real” streams, of course, would be much preferable, but in most projects, such piloting is not practical until an advanced stage (if at all).

The final result of this dilemma is that any crystallizer installed in a first plant with a new process is generally oversized to allow more flexibility in the level of fines destruction and to be safer as regards the final crystal size distribution. As a matter of fact, many industrial crystallizers designed by reputable suppliers for new processes were finally operated at higher capac- ities, up to twice their nominal specification, after the plant’s experience was

(a)

(b)

Figure 6.7 Draft tube crystallizer.

feed product slurry vent external heater or cooler circuit feed crystals slurry product crystals slurry vent water waste solution

optimized and stabilized. (As the manager of one supplier said once, “You’ve got a good deal ... why should you complain?”)