Crystalliser turbulence

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

The Standard-Messo turbulence crystalliser, Figure 15.18, is a draught-tube vacuum unit in which two liquor flow circuits are created by concentric pipes an outer ejector... 

Deposition of wax following crystallisation depends on the temperature differential between the bulk crude stream and the pipewall, and the Reynolds number. The larger the temperature differential, the greater the chance of wax deposition since molecular diffusion of dissolved wax to the pipewall intensifies as a result of the increase in wax concentration near the pipewall brought about by the temperature differential. With respect to Reynolds number, maximum wax accumulation is expected in the transition range (Reynolds number between 2000 and 4000). At low (laminar) flew, the net transport of wax to the wall 1s reduced by a relatively thick laminar layer adjacent to the pipewall and during turbulent flow, wax build-up 1s limited by erosion. 

The temperature and flow conditions within the heat exchanger will determine the location at which these various stages occur. For instance the supersaturation and crystallite formation may occur in the bulk fluid with the growing crystals moving towards the wall to form the deposit. The movement of foulant will under these circumstances, follow the processes described in Chapter 7 for particulate deposition. It is possible that due to the level of turbulence within the system, that some (or possibly many) of the crystallites formed are swept into re ons where the solution is not supersaturated. Under these conditions the particles will redissolve. On the other hand crystallisation may occur near or at the heat transfer surface. The presence of nucleation sites on a solid surface may encourage the formation of scale on the surface. Under these circumstances the process is largely governed by the mechanics of the crystallisation process. 

Rotating packed bed Excellent for mass-transfer limited processes. Uniform dispersion high turbulence thin films good controllability Some development work still proceeding Limited use outside odour removal/clean-up Crystallisation reactive stripping... 

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