Dilute-Phase Systems

As stated earlier, dilute-phase conveying is the commonly employed method for transporting a wide variety of suspended solids using air flowing axially along a pipeline. The method is mainly characterized by the low solids to air 

The number and type of bends used in the pipeline may constitute a critical design problem. The difficulties that may arise by the use of bends are 

Feeders for dilute-phase conveying systems (a) venture feeder and (b) star feeder. 

Dilute-phase pneumatic conveying systems (a) pressure system, (b) vacuum system, (c) combined system, and (d) closed-loop system. 

In terms of designing equipment for pneumatic conveying, there is another type of velocity, that is, the minimum conveying velocity, used to describe 

---

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 ... 

This term is restricted here to equipment in which finely divided solids in suspension interact with gases. Solids fluidized by liquids are called slurries. Three phase fluidized mixtures occur in some coal liquefaction and petroleum treating processes. In dense phase gas-solid fluidization, a fairly definite bed level is maintained in dilute phase systems the solid is entrained continuously through the reaction zone and is separated out in a subsequent zone. 

Wen, C. Y. and Galli, A. F. (1971). Dilute Phase Systems. In Fluidization. Ed. Davidson and Harrison. London Academic Press. 

CHAIRMAN SEGLIN I think one of the options he gave was the Westinghouse gasifier, the use of a type of backmixed dilute phase system for mixing the fresh coal into the hot bed. That s known technology. At least, it is published. 

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. 

Typical dilute phase systems are shown in Figures 8.5 and 8.6. Blowers are normally of the positive displacement type which may or may not have speed control in order to vary volume flow rate. Rotary airlocks enable solids to be fed at a controlled rate into the air stream against the air pressure. Screw feeders are frequently used to transfer solids. Cyclone separators (see Chapter 9) are used to recover the solids from the gas stream at the receiving end of the transport line. Filters of various types and with various methods of solids recovery are used to clean up the transport gas before discharge or recycle. 

For dilute-phase vertical transport the Institute of Gas Technology (1978) team on analysis of various investigators data on coal and coal-related materials recommends the Konno-Saito model (Table 4-2) for prediction of the pressure drop. The author has found generally that both the Konno-Saito and Yang models are about equally valid in analyzing dilute-phase systems. The Konno-Saito model as analyzed by the IGT team includes the acceleration effects and defines the solid velocity by expression 1 of Table 4-3. In the IGT analysis the acceleration effects as defined by Yang were also included in the calculations. The overall deviation between the measured and predicted values of the pressure drop for the IGT, Konno-Saito analysis was 30%, while it was 45% for the Yang model. 

Lasers are power tools in gas-solids flows they can be used in dilute-phase systems to measure the particle velocities. These laser Doppler velocimeters (LDVs) require... 

Figure 4.30 Stepped pipeline velocity profile for high pressure dilute phase system.

Care must be taken to imderstand that particles generally have to be kept in suspension as they flow. Some materials such as fly ash and cement, however, will move in what is termed the two-phase flow regime. Here one finds that a heavily loaded slow wave-like motion carries about half of the pipe volume with particles while a faster more dilute stream is carried above the dense lower layer. This type of conveying, since it is carried out at a lower than average velocity, can move a sizable capacity of material at much less energy consumption than the faster totally dilute phase system. This type of flow will not be possible with regular sand-like materials since their size and density make them less aerable. 

This case study demonstrates that dilute-phase systems are not straightforward , as well as the importance of designing not only the drop-out box (i.e. interface), but also the attached pneumatic conveying system, to suit the matetM and application. 

Quite often the root cause of a conveying problem is a lack of appreciation and understanding of the relevant fundamentals. This is particularly true even for dilute-phase systems, which are quite simple in appearance . Due to this apparent simplicity, many Signers, operators and even researchers can often overlook some of the unique and unusud phenomena resulting from a combination of product, mode of transport and component design or selection. 

Due to inaccurate and contradictory results, care should be exercised when applying the existing minimum transport models to the design or optimisation of dilute-phase systems. Some test data should be used to confirm optimal operating conditions and any deposition effects. 

---