Conveying, pneumatic

Pneumatic conveying systems are basically quite simple and are ideally suited for the transport of powdered and granular materials in factory, site and plant situations. The system requirements are a source of compressed gas, usually air, a feed device, a conveying pipeline and a receiver to disengage the conveyed material and carrier gas. The system is totally enclosed, and if it is required, the system can operate entirely without moving parts coming into contact with the conveyed material. 

low or negative pressures can be used to convey materials. For hygroscopic materials dry air can be used, for toxic materials a closed loop system can be used and for potentially explosive materials an inert gas such as nitrogen can be employed. A particular advantage is that materials can be fed into reception vessels maintained at a high pressure if required. 

With a suitable choice and arrangement of equipment, materials can be conveyed from a hopper or silo in one location to another location some distance away. Considerable flexibility in both plant layout and operation is possible, such that multiple point feeding can be made into a common line, and a single line can be discharged into a number of 

With vacuum systems, materials can be picked up from open storage or stockpiles, and they are ideal for clearing dust accumulations and spillages. Pipelines can run horizontally, as well as vertically up and down, and with bends in the pipeline any combination of orientations can be accommodated in a single pipeline run. Material flow rates can be controlled easily and monitored to continuously check input and output, and most systems can be arranged for completely automatic operation. 

Pneumatic conveying systems are particularly versatile. A very wide range of materials can be handled and they are totally enclosed by the system and pipeline. This means that potentially hazardous materials can be conveyed quite safely. There is minimal risk of dust generation and so these systems generally meet the requirements of any local Health and Safety legislation with little or no difficulty. 

In-plant slurry conveying systems are usually designed by the plant designer (or perhaps a consultant drafted into the team), while the 

As for slurry lines, the line routes should be reviewed at plant layout to minimize the route length and the number of bends. 

These several pressure drop relations hardly appear consistent, and the numerical results of Example 5.1 based on them are only roughly in agreement. 

Darby (1996) wrote a review article updating the methods for determining settling rates of particles in non-Newtonian fluids. 

From statements in the literature, it appears that existing slurry lines were designed on the basis of some direct pilot plant studies. 

Nonsettling slurries are formed with fine particles, plastics, or fibers. Although their essentially homogeneous nature would appear to make their flow behavior simpler than that of settling slurries, they often possess non-Newtonian characteristics which complicate their flow patterns. In Newtonian flow, the shear stress is proportional to the shear strain. 

The performance of pneumatic conveyors is sensitive to several characteristics of the solids, of which the most pertinent ones are 

The constants of such equations must be found experimentally over a range of conditions for each particular case, and related to the friction factor with which pressure drops and power requirements can be evaluated. The topic of nonsettlmg slurries is treated by Bain and Bonnington (1970) and Clift (1980). Friction factors of power-law systems are treated by Dodge and Metzner (1959) and of fiber suspensions by Bobkowitz and Gauvin (1967). 

Newtonian behavior of suspensions (a) viscosity as a function of shear rate, 0.4 wt% polyacrylamide in water at room shear stress as a function of shear rate for suspensions of TiOj at the indicated vol % in a 47.1 wt % sucrose solution whose Pa sec Denn, Process Fluid Mechanics, Prentice-Hall, Englewood Clijfs, NJ, 1980). 

---

Cross Correlation. Considerable research has been devoted to correlation techniques where a tracer is not used. In these methods, some characteristic pattern in the flow, either natural or induced, is computer-identified at some point or plane in the flow. It is detected again at a measurable time later at a position slightly downstream. The correlation signal can be electrical, optical, or acoustical. This technique is used commercially to measure paper pulp flow and pneumatically conveyed soHds. 

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

As with a plasma-arc furnace, various gases and pneumatically conveyed soHds have been added to the various types of a-c open-arc furnaces to decrease alloy loss, to stabilize the arc, and to decrease the noise level, but with mixed commercial success. 

Pipelines to transport soHds are called freight pipelines, of which three different types exist pneumatic pipelines, the use of which is known as pneumotransport or pneumatic conveying slurry pipelines, which may also be called hydrotransport or hydrauHc conveying and capsule pipelines. When air or inert gas is used to move the soHds in the pipeline, the system is called a pneumatic pipeline and often involves a wheeled vehicle inside the pipeline, propelled by air moving through the pipe (25). Slurry pipelines involve the transport of soHd particles suspended in water or another inert Hquid. HydrauHc capsule pipelines transport soHd material within cylindrical containers, using water flow through the pipeline for propulsion. 

Rotary V lve Feeders. Devices known as rotary valve feeders are commonly used for circular or square configured outlets. These are particularly useful when discharging materials to a pneumatic conveying system where a seal is required to prevent air flow through the hopper outlet. The discharge rate is set by the speed of rotation of the vanes or pockets of the valve. 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

Dryers. Drying, another type of evaporation technique, is suited for waste streams of very high soHds content. Several common types of dryers are vacuum rotary dryers, dmm dryers, tray and compartment dryers, and pneumatic conveying dryers. 

A continuously operating pneumatic conveying dryer is used for appHcations similar to the tray and compartment dryer. In this case, however, drying is performed in conjunction with grinding, as the soHds are conveyed and dried within the unit. 

Dried coal is pneumatically conveyed to feed bins, pressurized through lockhoppers, and fed to the refractory-lined fluidized-bed gasifier vessel by variable-speed screws. The gasifying agent, mixed oxygen and steam, is fed near the bottom of the gasifier. The bed operates at 2.5 MPa (25 bar) and... 

R. L. Brown and J. C. Richards, Principles of PowderMechanics, Pergamon Press, Oxford, 1970 D. Mills, Pneumatic Conveying Design Guide, Buttersworth, Boston, 1990. 

R. D. Marcus, G. E. Klinziag, and E. Rizk, Pneumatic Conveying of Solids. Chapman and Hall, New York, 1991. 

D. Mills, Pneumatic Conveying Design Guide, Butterworths, New York, 1990. 

H. A. Stoess, Pneumatic Conveying, Wiley-Interscience, New York, 1970. 

Plastics Pneumatic Conveying, G. Butters, ed. AppHed Science PubHshers, New Jersey, 1981. 

The cmde diatomite, which may contain up to 60% moisture, is first milled in a method that preserves the intricate stmcture of the diatomite. This material is fed to dryers operating at relatively low temperatures, where virtually all of the moisture is removed (see Drying). Coarse and gritty nondiatomaceous earth material is removed in separators and preliminary particle si2e separation is made in cyclones. For many producers, all of the manufacturing processes, with the exception of the calcination step, take place while the material is being pneumatically conveyed. The resultant material is termed natural product. This is the only type of diatomite made by some producers. 

Pneumatic-Conveying Devices See Sec. 21 for descriptions, ratings, and design factors on these devices. Use is primarily for transport purposes, and heat transfer is a very secondary consideration. 

Transport Transport units can be scaled up on the principles of pneumatic conveying. Mass and heat transfer can be predicted on both the shp velocity during acceleration and the shp velocity at full acceleration. The slip velocity is increased as the sohds concentration is increased. 

Powdered-product collection, as in pneumatic conveying the spray drying of milk, eggs, and soap and the manufacture of nigh-purity zinc oxide and carbon black... 

With hafQes Horizontal drum Double cone revolving around long axis Twin shell Ciihe Twin rotor Tiirhine Paddle mixer Sifter (tiirhosifter) Attrition mill Elevator loading Pneumatic conveying Vibrating... 

I Although these steps, when carefully selected, can aid mixing, caution must he exercised with pneumatic conveying and vibrating, as they may tend to separate materials. 

The process steps listed in Table I9-I can sometimes be used to promote mixing. However, they are primarily for funcI ions other than solids mixing. (Note precautions for pneumatic conveying and vibrating in Table I9-I.)... 

The capacity of a pneumatic-conveying system depends on (1) produc t bulk density (and particle size and shape to some extent), (2) energy content of the conveying air over the entire system, (3) diameter of conveying hne, and (4) eqmvalent length of conveying hue. 

Type of material / Type / Material particle characteristic Mechanical conveying Pneumatic conveying ... 

Hopper Trucks These trucks are used to transport by highway a wide variety of materials. Vehicle types range from the open-dumping kind to the closed type. Most common is the type that unloads by pressure differential into its own pneumatic-conveying system, which is temporarily connected to a storage silo. On this type of truck, the unloading of 18,100 kg (40,000 lb) of produces takes about 1 h, sometimes less. 

Pneumatic, conveying and storage Wet process materials handling, grinding, storage Particulates (dust) Wet materials, no dust and baghouse... 

Grinding and packing, air separator, grinding, pneumatic conveying, materials handling, packaging Particulates (dust) Local exhaust system and fabric filters... 

Coal is fed as a paste containing 25 wt % water, and sorbent is fed diy by a lock-hopper system with pneumatic conveying. The top size of each feedstock is 3 mm in). The latent heat lost evaporating the water fed with the paste is compensated by increased gas turbine power output resulting from the increased flue-gas mass flow rate. For the 80-MWe unit, there are six coal feed points (one per 4.5 m" [48 ft"]) and four sorbent feed points (one per 6.7 m" [72 ft"]), all entering beneath the tube bank along one wall. The bed depth is... 

NFPA 650 Standard for Pneumatic Conveying Systems for Handling Combustible Particulate Solids, 1998 edition. National Fire Protection Association, Quincy, MA. 

---