Sulzer MWB process

One drawback of static crystallization is that crystal layer growth rates are very slow. In the Sulzer MWB process, growth rates are gready improved by allowing a film to dow down vertical tubes (83). 

In the Sulzer-MWB process the naphthalene fractions produced by the crystallization process are stored in tanks and fed alternately into the crystallizer. The crystallizer contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallizes out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallization process. The crystals that have been deposited on the tube walls are then partly melted for further purification. Following the removal of the drained liquid, the purified naphthalene is melted. Four to six crystallization stages are required to obtain refined naphthalene with a crystallization point of 80°C, depending on the quality of the feedstock The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

The Sulzer MWB process (Fischer, Jancic and Saxer, 1984) is a melt crystallizer that operates basically by crystallization on a cold surface, but with features which allow it to operate effectively as a multistage separation device. Consequently, it can be used to purify solid solution as well as eutectic systems. 

Figure 8.23. The Sulzer MWB process a) multistage flow diagram (C = crystal, L = liquor) b) plant layout (A = crystallizer, B = melt collection tank, C = residue melt storage tank, D = heat exchanger, E = pump)...

Figure 9.5 Flow diagram of the Sulzer-MWB process for the production of naphthalene by crystallization from the melt...

Figure 9.6 shows the Rutgerswerke naphthalene crystallization plant at Castrop-Rauxel/West Germany, which has a capacity of 60,000 tpa and applies a modified Sulzer-MWB process. 

The flowing melt improves the heat and mass transfer, reducing the boundary layer thickness and, therefore, the possibility of constitutional supercooling. Here, for instance, the Sulzer MWB process (falling film), nowadays called Sulzer falling film process (Figure 17.3), of Sulzer Chemtech Ltd needs to be mentioned. 

Figure 17.3 Flow diagram of a falling film crystallizer (Sulzer MWB process), (reproduced with permission from Sulzer Chemtech Ltd).

FIG. 20-16 Sulzer MWB-crystallization process, (a) Stepwise operation of the process, (b) System flow sheet. iSulzer Chemtech.)... 

Sulzer-MWB [Metalwerk A. G. Buchs] An obsolete process for recovering naphthalene from a coal tar fraction by multi-stage fractional crystallization from the melt. Formerly operated by Rutgerswerke at Castrop-Rauxel, Germany. 

The preferred route to higher punty naphthalene, either coal-tar or petroleum, is crystallization. This process has demonstrated significant energy cost savings and yield improvements. There are several commercial processes available Sulzer-MWB, Brodie type, Betz, and Recochem. 

Fig. 7-6. "[ vo-stage purification process according to the Sulzer-MWB crystallization process. 

The Sulzer-MWB and the Brodie crystallization processes are most commonly used to recover naphthalene by crystallization. Figure 9.5 shows the Sulzer-MWB flow sheet, which is operated in a modified form in plants with a naphthalene production of up to 60,000 tpa. 

Commercial Equipment and Applications The falliug-film ciystaUization process was invented by the MWB company in Switzerland. The process is now marketed by Sulzer Chemtech. Produc ts successfully processed in the falhug-film ciystaUizer are listed on Table 22-9. The falliug-film ciystaUizatiou process is available from the Chemtech Div. of Sulzer Canada Inc., 60 Worcester Rd., Rexdale, Outai-io N9W 5X2 Canada. 

The second group of the batch type of solid layer techniques are those with moving melts. Here again, three processes must be named the MWB-Sulzer, nowadays called Sulzer falling film (CH-PS 1967 U.S. 1985), the ICI-process (GB-PS 1964), and the BASF-process (DE-PS 1976), which is now distributed by the Kvaerner company. In all processes, the crystallization takes place on the inside of tubes, which are cooled from the outside. The melt coming from a feed tank is continuously circulated through the tubes until the crystal coat at the walls is thick enough, i.e., until... 

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