Dilute-phase conveying

Dilute Phase Conveying. Dilute conveying systems, sometimes called disperse conveying or stream conveying, operate as positive pressure systems at pressures up to 100 kPa (14.5 psig), or as negative pressure systems (vacuum conveying) at pressures up to —50 kPa (—500 mbar). 

Pneumatic conveying systems and in particular dilute phase conveying systems are known to create a high stress on particulate solids leading to significant attrition. In contrast to fluidized beds, it is not the material loss which is the main problem. Depending on the application, problems may rather occur in a number of different areas. Attrition may, for example,... 

Despite the little experimental data, there are two models available in the literature. Adams etal. (1992) considered dense phase conveying. They tried to predict the amount of attrition as a function of conveying distance by coupling a Monte Carlo simulation of the pneumatic conveying process with data from single-particle abrasion tests. Salman et al. (1992) focused on dilute phase conveying. They coupled a theoretical model that predicts the particle trajectory with single particle impact tests (cf. Mills, 1992). 

Special care should be taken when selecting the mode of solids-gas flow. For example, flow separation and roping could occur even in very dilute-phase conveying systems (e.g., m < 1 for coal-fired boilers). Fluidized dense-phase also is possible for some systems and can offer many... 

It will be seen that the relationship obtained covers a very wide range of conveying conditions, although it is unfortunate, in this case, that the data could not be extended to cover dilute phase conveying also. This exercise was also carried out with a number of other materials and very similar results were obtained. In all cases, the ratio between vertical and horizontal pressure gradients was in a very narrow band, at... 

It is believed that this can be related to the differences in coefficient of restitution between the conveyed particles and the pipeline walls. On impact with the rubber, the particles will be decelerated, since the rubber will absorb much of the energy of impact. As a consequence, the particles will have to be re-accelerated back to their terminal velocity. The coefficient of restitution of the particles against the steel pipeline wall will be very much lower. This effect is clearly magnified by increase in velocity and explains why there is little difference between the two pipeline materials in low velocity dense phase conveying, but differ by 50% in high velocity dilute phase conveying. The results obtained with the barite were very similar. 

The third installation was constructed to conduct single particle attrition experiments under well defined stress conditions which are closely related to conditions in industrial conveying installations. For this purpose a pipe bend was chosen. The exact setup is given in Fig. 5. As it has already been discussed, the geometry of this setup was used for the simulations to determine the process function in dilute phase conveying. 

For dilute phase conveying numerical simulations with a commercial computational fluid dynamics code were carried out. The analysis of particle wall impact conditions in a pipe bend showed that they take place under low wall impact angles of 5-35° which results in low normal (5-25 m/s) and high tangential (33-44 m/s) impact velocity components. These findings lead to the conclusion that not only normal stresses caused by the impacts are important in dilute phase conveying but that sliding friction stresses play an important role as well. 

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... 

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

Figure 3.32 shows the four main types of dilute-phase conveying systems previously mentioned. The pressure system, also called positive-pressure, or push system operates at super-atmospheric pressure and is used for delivery to several outlets from one inlet (Figure 3.32a). Although most applications of these systems lie within the scope of dilute-phase conveying, under certain arrangements they can also operate as high-pressure, dense-phase...

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