Scraped crystallizer

In the group of suspension processes, there are the jacket cooled and the directly (e.g., by inert gases) cooled processes. The Amoco process, on the one hand, and the Maruzen or Chevron process, on the other hand, are, according to Ransley (1984), representatives for such processes. Other developments are scrapes crystallizers according to the Humble-Oil, the Krupp-Harpen (see Ritzer 1973), or the Hoechst AG (DE-PS 1969) process which are used for the production of p-nitrochlorobenzene. These scraped surface crystallizers are in fact solid layer processes, but in the handling of the products—crystals in suspension—they have to be treated like suspension processes. A number of processes that feature a combination of a scraped crystallizer and suspension techniques will be discussed. 

One type of suspension processes using a scraped crystallizer to create the feed suspension is the Phillips process (see McKay 1967). The crystallizer consists of a vessel with filters in the wall of the upper section. There is a heater at the bottom to remelt the crystals. Some of the melt is taken as product, some is used as reflux. The reflux serves mainly as washing liquid for the crystals which are forced downward by a reciprocating piston, or with pulses from a pulsator pump. The Phillips crystallizer column is shown in Figure 7.14. 

The soiution is aliowed to cool and the crystals of the P2P-bisulfite addition compound are then separated by vacuum filtration, washed with a little clean dH20 then washed with a couple hundred mLs of ether, DCM or benzene. The filter cake of MD-P2P-bisulfate is processed by scraping the crystals into a flask and then 300mL of either 20% sodium carbonate solution or 10% HCi soiution are added (HCI works best). The soiution is stirred for another 30 minutes during which time the MD-P2P-bisulfite complex will be busted up and the P2P will return to its happy oil form. The P2P is then taken up with ether, dried and removed of the solvent to give pure MD-P2P. Whaddya think of that ... 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

Insofar as they are used to purify other products, several processes used in the refinery fall under the classification of dewaxing processes however, such processes must also be classified as wax production processes (2). Most commercial dewaxing processes utilize solvent dilution, chilling to crystallize the wax, and filtration (28). The MEK process (methyl ethyl ketone—toluene solvent) is widely used. Wax crystals are formed by chilling through the walls of scraped surface chillers, and wax is separated from the resultant wax—oil—solvent slurry by using fliUy enclosed rotary vacuum filters. 

One of the early column crystallizers was that iatroduced for the separation of xylene isomers (see Xylene and Ethylbenzene). In this unit, shown schematically ia Eigure 25, -xylene crystals are formed ia a scraped-surface chiller above the column and fed to the column. The crystals move downward counter-currenfly to impure Hquid ia the upper portion of the column and melted -xylene ia the lower part of the column. Impure Hquor is withdrawn from an appropriate poiat near the top of the column of crystals while pure product, xylene, is removed from the bottom of the column. The pulse unit drives melt up the column as reflux and iato a product receiver. 

Scraped-Surface Crystallizer For relatively small-scale apph-cations a number of ciystallizer designs employing direct neat exchange between the shiny and a jacket or double wall containing a cooling medium have been developed. The heat-transfer surface is scraped or agitated in such a way that the deposits cannot build up. 

Diaminoanthraquinone [128-95-0] M 238.3, m 268°. Purified by thin-layer chromatography on silica gel using toluene/acetone (9 1) as eluent. The main band was scraped off and extracted with MeOH. The solvent was evaporated and the quinone was dried in a drying pistol [Land, McAlpine, Sinclair and Truscott J Chem Soc, Faraday Trans 1 72 2091 1976]. Crystd from EtOH in dark violet crystals. 

Any crystals which may form at the tip of the addition funnel are scraped off and allowed to drop into the reaction flask. 

The mixture was filtered, the ethyl acetate layer separated and washed with three 100 ml portions of water, dried over Na2S04, filtered and treated with 30 ml of sodium 2-ethyl-hexanoate in n-butanol (34 ml = 0.1 mol). The oil which settled out was scratched to induce crystallization. After stirring for 20 minutes the product, sodium 7-(a-bromoacet-amido)cephalosporanate, was scraped from the sides of the flask and collected. The filter cake was washed with several portions of acetone, air dried, and dried in vacuo over P Os. The yield was 22.5 g and decomposed at 193°C. 

The reaction mixture was removed from the vessel and distilled at a pressure of 30-60 mm, and a bath temperature of 30°C to 50°C until the methanol had all been removed. The extremely viscous tarry residue remaining in the still pot was given a very crude distillation, the distillate boiling at B2°C to 1 32°C/2 mm. In an attempt to purify this distillate by a more careful distillation, 5.3 g of a liquid distilling from 53°C to 150°C/5 mm was collected. At this point, much solid sublimate was noted not only in this distillate but in the condenser of the still. 7 g of the solid sublimate was scraped out of the condenser of the still. Recrystallization of the sublimate from ethyl acetate containing a small amount of petroleum ether gave beautiful crystals melting at 175°C to 177°C (5 g). Infrared analysis confirmed that this compound was hydroquinone (9% conversion). 

This type is similar in principle to the tank type, but the cooling surfaces are continually scraped or agitated to prevent the fouling by deposited crystals and to promote heat transfer. They are suitable for processing high-viscosity liquors. Scraped-surface... 

The entire dish was scraped with a razor. The crystals bunched together and dissolved into a caramel gum like substance. This substance was smeared across a 3X5 note card. 

The combined solvent fractions were poured into a glass baking dish and set aside for evaporation. After 4 days the dish was scraped clean and the crystals put to use. Success ... 

Make a small hole in the crystal slightly less than the length of the hand from the center of the crystal. Insert the screw from the front of the crystal and tighten it so that it projects from the concave side. Carefully scrape any paint from the edge of the hand where it will make contact with the screw. Replace the crystal in the watch. See Figure 10. 

The funnel should be preheated to prevent crystallization of the product in its pores. The filtration process can be accelerated by scraping the muddy cake from the funnel surface. 

At this point, you may have a cake of crystals in your Buchner funnel. The easiest way to handle this is to carefully lift the cake of crystals out of the funnel along with the filter paper, plop the whole thing onto a larger piece of filter paper, and let the whole thing dry overnight. If you are pressed for time, scrape the damp filter cake from the filter paper, but... 

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