Crystal-filter driers

An extension of the filter-driers are the reactor-filter-driers, as illustrated in Fig. 7.2-9 by an apparatus known under trademark name Nutrex" and developed by Rosenmund. Equipment of this kind is still more versatile and operation safer. In one vertical position the device can act as a filter, a granulator, and an apparatus for all operations with filter cakes (i.e. re-slurrying, smoothing, and squeezing). In the reverse position, it can operate as a reactor, extractor, evaporator, crystallizer, drier, etc. There are many other companies offering reactor-filter-driers, e.g. SEN, Giovanola, Schenk, and Cogeim. 

Generally, in MPPs one centrifuge or pressure/vacuum filter, one drier, and one fractionation system of reasonable capacity are installed for every two or three reactors/crystallizers. On average, about 2 to 5 m solvent storage capacity and the same amount for intermediates are needed per 1 m of reactor capacity. 

Sodium hydrosulfite is produced through the Formate process where sodium formate solution, sodium hydroxide, and liquid sulfur dioxide reacted in the presence of a recycled stream of methanol solvent. Other products are sodium sulfite, sodium bicarbonate, and carbon monoxide. In the reactor, sodium hydrosulfite is precipitated to form a slurry of sodium hydrosulfite in the solution of methanol, methyl formate, and other coproducts. The mixture is sent to a pressurized filter system to recover sodium hydrosulfite crystals that are dried in a steam-heated rotary drier before being packaged. Heat supply in this process is highly monitored in order not to decompose sodium hydrosulfite to sulfite. Purging is periodically carried out on the recycle stream, particularly those involving methanol, to avoid excessive buildup of impurities. Also, vaporized methanol from the drying process and liquors from the filtration process are recycled to the solvent recovery system to improve the efficiency of the plant. 

The chemist does not think much about how he or she manipulates the mechanics of a process, how chemicals are transported to the site, how they are moved from store to bench, how these often noxious chemicals are moved from their containers and weighed, how they are added to flasks, how the flasks are stirred and heated and cooled, how reflux and distillation are carried out, how and even why solvent recovery is done, how crystals are formed for the best filtration, how filtrations are done and products washed and taken from the filter to a drier and how the drier is operated, how dry powders are handled or offloaded, and how they are milled or micronized. In reality, the differences between the chemist s view of the preparation of a chemical and the chemical engineer s view are profound. In scale-up, the chemist... 

BR = Batch reactor FP = Filter press IE = Ion exchange VE = Vacuum evaporator CC = Cooled crystallizer FC = Filter centrifuge VD = Vibratory drier CW = Cooling water X = Xylose... 

A stream of a saturated brine suspension of crystals is continuously withdrawn from each evaporator-crystallizer, and the salt crystals separated on a continuous rotary filter with return of the brine to the evaporator. Much of the salt may be marketed in moist condition, or it may be passed through a drier moving countercurrently to heated air to give vacuum salt of typically 99.8-99.9% purity [21]. 

For the preparation of co-precipitated, e.g. mixed metal oxide, catalysts, the drying step cannot be carried out satisfactorily in the preparation robot. Usually a spray drying step is applied in the production of this kind of catalyst, because the liquid phase of the precipitate suspension still contains dissolved salts that are essential for the catalytic performance. Hence, the suspension must not be filtered off nor can be dried by evaporation due to crystallization reasons. Since there are no laboratory spray-dryers available for that sample size, another method had to be implemented and was found in the freeze drying of these materials [4]. With this method almost the same is done like in spray diying but on another time-scale. Where a spray drier evaporates the water very quickly and thereby prevents the crystallization of the still dissolved salts, Ihe freeze drier literally at first freezes the solution and no crystallization can occur while the water is sublimated. Hence, an identical product is obtained. 

Forced oxidation, which is a separate step after the actual desulfurization process, involves blowing air into the tank that holds calcium sulfite sludge, and results in the oxidation of the calcium sulfite (CaSOj) to calcium sulfate (CaS04). The caldiun sulfate formed by this reaction grows to a larger crystal size than calciiun sulfite. As a result, the calcium sulfate can be filtered or dewatered to a much drier and more stable material than the calcium sulfite sludge. 

The work-up could be also carried out as planned. Slow filtration through silica and charcoal removed the palladium as demonstrated in the lab. The product was crystallized nicely as a white powder. The yield of the first batch was lower than expected, probably due to some product hold-up on the filter and in the drier. The yield of the second batch was in the expected range. 

The mixture is then cooled to 10° and the crystals are removed by filtration on a sintered-glass crucible. The crystals are washed on the filter with dilute hydriodic acid (1 part concentrated acid to 2 parts water) and dried in vacuo over phosphorus(V) oxide in an Abderhalden drier, using boiling toluene to keep the temperature at 110°. Any unreduced germanium (IV) iodide is removed by sublimation in this process. The yield is about 9 g. (75 per cent based upon germanium(IV) iodide). Properties of gennanium(II) have been described previously. ... 

In most cases, crystals are recovered from ammonium sulfate slurry by recycling through the continuous or the automatic batch-type centrifuge, wherein the product is screened and spin-dried, washed with water and weak ammonia, and again spin-dried before being conveyed to the drier. In some plants, continuous top-feed filters -... 

Eventually, the crystal mass having still residual moisture of some 2% is unified with the filter cake of granular salt from the low-speed centrifuge. The mixed crystal mass is then dried in a fluidized bed drier to below 0.1% H2O. 

A continuous countercurrent hot-air turmel drier is to be designed to dry a filter-press cake of coarse crystals of an inorganic substance, insoluble in water. The filter-press cake will be placed on trays 1.0 m long by 0.9 m wide by 25 mm, 20 trays to-a truck, with 50 nun between trays. The tutmel drier will have a cross section 2 m high by 1 m wide. The trays have a reinforced screen bottom, so that drying takes place from both top and bottom of each tray. Production permits introducing one... 

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