Dense-Phase Systems

Dense phase systems are at cost disadvantage when multiple feed points are required, or when incremental compressed air for conveying is not available from existing sources. In the latter instance, the entire investment cost of an air compressor and associated auxiUaries must be allocated to the cost of the conveying system. 

Proprietary designs for rotary valve feeders (star valves) capable of continuous feeding of certain pelleted and granular materials into low velocity, dense phase systems, having system pressures up to 200 kPa (2 bars) have been developed. 

Figure 3. Classification of dense phase systems by vendors.

Figure 6. Trace air dense phase system (Semco).

In addition Paul has prepared a chart listing the characteristics of the dense phase conveying in comparison to dilute and medium dense phase systems as shown in Fig. 17. 

It is believed that the air velocities in a large-diameter dilute-phase system can be 50 to 100% higher than an equivalent well-designed dense-phase system. Hence, much greater wear problems are expected in the dilute-phase system, although significant advances have been made in the technology of wear-resistant materials and bends (Wypych and Arnold, 1993). Other features involved with dilute-phase transport systems include ... 

Before carbonization, the blend may undergo further treatment to enhance the ability of the blend to make better coke or to improve the process economics. For example, blends may contain as much as 10% w/w free moisture that will impede the flow properties and lower the bulk density. Thus, it may be beneficial to preheat (to ca. 200°C [390°F]) the feed before it enters the oven bulk density will be increased and the carbonization time will be reduced by this simple operation of water removal. The abrasion resistance of the coke may also be improved by this treatment. The preheating operation may be carried out by means of an entrained (dilute phase) system or by means of a fluidized-bed (dense-phase) system, but it is important that oxidation of the coal(s) be prevented to avoid adversely affecting the coking properties. 

In this section we will look at the distinguishing characteristics of dense and dilute phase transport and the types of equipment and systems used with each. The design of dilute phase systems is dealt with in detail and the approach to design of dense phase systems is summarized. 

The blow tank is automatically taken through repeated cycles of filling, pressurizing and discharging. Since one third of the cycle time is used for filling the blow tank, a system required to give a mean delivery rate of 20 t/h must be able to deliver a peak rate of over 30 t/h. Dense phase transport is thus a batch operation because of the high pressures involved, whereas dilute phase transport can be continuous because of the relatively low pressures and the use of rotary valves. The dense phase system can be made to operate in semi-continuous mode by the use of two blow tanks in parallel. 

Dilute and dense phase systems Dilute and dense phase conveying do not relate to any particular type of system. Any bulk particulate material can be conveyed in dilute phase. It is primarily the properties of the material that determine whether the material can be conveyed in dense phase, particularly in conventional conveying systems. 

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