Vertical decanter

Design Considerations Gravity decanters normally are specified as horizontal vessels with a length-to-diameter ratio greater than 2 (and often greater than 4) to maximize the phase boundary (cross-sectional area) between the two settled layers. This provides more effective utilization of the vessel volume compared to vertical decanters, although vertical decanters may be more practical for low-flow applications or when space requirements limit the footprint of the vessel. 

Feed >10% v/v, or contamination present, or ill-behaved, size as coalescence controlled. For vertical decanters, allow a total residence time that depends on density difference and interfacial surface tension. For a typical 0.5-m height of coalescent band (or a decanter of 0.7-m height), use an overflow total flow rate velocity of 1.5 (Ap/0.1)° L/s m, where the density difference is in units of Mg/m to determine the horizontal cross-sectional area. For horizontal configurations, use half of the vertical overflow velocity. Can add parallel plates or high- and low-energy combination coalescer promoters (see size increase, liquid-liquid. Section 16.11.9.2). 

The availability of inexpensive vertical decanters is very limited, such that in some circumstances one has to select a horizontal model even though a vertical one might have been preferred. The vertical decanter is designed and built primarily for high-temperature and/or pressurised operation. The vertical orientation lends itself to better and more reliable pressurisation, with only one end to be provided with rotational seals. The amount of engineering required for pressurisation and sealing makes the vertical decanter more expensive than the non-pressurised and open horizontal decanter. 

Figure 2.34. A vertical decanter (By courtesy of Alfa Laval).

Almost all of the vertical decanters installed have been designed to meet the Unfired Pressure Vessel, Explosion Proof Code requirements and chemical industry piping, vessel lubrication and instrumentation codes. Due to this, the cost of this design is higher than horizontal machines of equal process capacity. For those horizontal machines that meet the same environment and code requirements, the cost is comparable. 

There are a number of special variants of the basic horizontal or vertical decanter for simple clarification and dewatering. Special designs are available for three-phase separation, for thickening applications, with a filtration section, for leaching, for washing, and those for operating at extra high temperature and pressure. 

Other petrochemicals, especially terephthalic acid and PTA, and nylon, as well as polyethylene and polypropylene, require flameproof equipment, and use the vertical decanter, which can be pressurised, and will operate at higher temperatures. These also have polished flights, and chamfered flight tips, without hard surfacing. 

The vertical decanter, hanging from drive and bearings at the liquid discharge end of the bowl, was developed to meet the requirements of the chemical industry to overcome a temperature and pressure problems - it is much easier to pressurize the vertical design. 

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