Dense phase conveying

SOLID PHASE OR EXTRUSION FLOW DISCONTINUOUS DENSE PHASE 

High Pressure Airlock Waeschle, Fuller, Allied Flotronics Fuller Kinyon Pump Fuller 

Plug Forming Systems Warren Spring Labs, Takt-Shub (Buhler), Sturtevat External Air Injection Dynamic Air (Booster type system), Gericke (sonic nozzles) Internal Air By-pass - Fluidstat (Buhler), Turbuflow 

The Gatty system has air injected from a sub-pipe mounted inside the main conveying line. Since this system is without controls, it performs better for well-behaved plastic pellet systems. It is difficult to maintain a low velocity conveying condition with this system, Marcus et al. (1990). 

As mentioned earlier, there are two main mechanisms of low velocity dense phase flow. For materials that have good air retention, the material tends to be conveyed as a fluidised mass. 

1/1/= air velocity V = product velocity Qs = product mass flow Ql = air mass flow H = product load = Qs / QL 

At dilute phase conveying the air velocity wis much larger than the suspense velocity Vs and can be measured from approximately 18-35 ms .  

The ratio of product velocity v to air velocity w is smaller than 1. The product load H (Eq. (17.4)) which represents the ratio between the product mass flow Qj and the air mass flow is around 10 but in dependency of the material to be conveyed 

Depending on the characteristic of the vacuum generator (air-flow rate above vacuum level) dilute phase conveying leads to high throughput rates. However the high velocity may lead to wear or damage of the material to be conveyed. 

If in a horizontal or declined conveying line the air velocity tv is reduced under 20 m s the material starts to drop down in the lower half of the pipe and moves like a string. The formation of this string is strongly depending on the material. Plugs may occur and settle for a short time in the lower half of the pipe where 

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As shown in Fig. 14.2, the material conveying region is bounded by the air-only curve and the stationary-plug curve, where the air merely percolates through or flows above a packed bed of stationary particles. Dense-phase conveying occurs when v, the velocity of air, is below the so-called saltation velocity. 

There are several attractive features of dense phase conveying which warrant its use in some circumstances. In general, pipe wear is less and separation of the product is easier. KONRAD(79) has carried out a comprehensive review of the current state of the art. He points out that the technique is not yet well established and that there are considerable design difficulties, not least a satisfactory procedure for predicting pressure drop. The risk... 

Kencmnoton, J. M. Hydrotransporl 5, (BHRA Fluid Engineering, Hanover, Germany) (May 1978) Paper D7. Prediction of pressure gradient in dense phase conveying. 

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). 

Figure 24. Effects of conventional dense phase conveying and vibration assisted conveying on the particle size distribution of spray granules (the vertical bars indicate the confidence intervals. (Molerus et al., 1989.)...

There have been a number of advantages that have been listed for dense phase conveying by Paul (1990). These are quoted as... 

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. 

Vendors have used certain types of dense phase conveying operations. In the trade, these vendors and their unique modes of transport have been classified in Fig. 3. 

Destoop, T., Mathematical of Dense Phase Conveying Systems - A Revolutionary Move Towards Smart System Control, Woe. of1994 Bulk Solids Handling Conf, Rosemont, IL (1994)... 

Group A powders are the best candidates for dense-phase conveying and can achieve high solids/gas loadings. Note the dense-phase referred to here actually is fluidized dense-phase (Wypych, 1995a). 

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