Blow tank

Types of Systems Generally, pneumatic conveyors are classified according to five basic types pressure, vacuum, combination pressure and vacuum, fluidizing, and the blow tank. 

The blow-tank principle can be used to feed regular pneumatic conveyors. Use of an Airslide or other fluidizing device at the bottom of the blow tank permits handhug non-free-flowing materials. This... 

Fig. 6-11. Schematic diagram of the kraft pulping process (6). 1, digester 2, blow tank 3, blow heat recovery 4, washers 5, screens 6, dryers 7, oxidation tower 8, foam tank 9, multiple effect evaporator 10, direct evaporator 11, recovery furnace 12, electrostatic precipitator 13, dissolver, 14, causticizer 15, mud filter 16, lime khn 17, slaker 18, sewer.

Venturi, product systems, or blow tanks will not be discussed here. 

Kraft pulping is a common process in the paper industry. Figure 8.4 shows a simplifled flowsheet of the process. In this process, wood chips are reacted (cooked) with white liquor in a digester. White liquor (which contains primarily NaOH, NaiS, Na2C03 and water) is employed to dissolve lignin from the wood chips. The cooked pulp and liquor are passed to a blow tank where the pulp is separated from the spent liquor weak black liquor which is fed to a recovery system for... 

At the downstream end of the pipeline it is necessary to disengage the solids front the gas and this is most usually carried out in cyclone separators (See Volume 2, Chapter 8). However, there may be a carry-over of tine particles which must be eliminated before the gas is vented, and gas filters or electrostatic precipitators may be used for this purpose (See again Volume 2, Chapter 8). At the upstream end, the particles must be introduced using some form of positive feeder, such as a rotary valve or a blow tank. 

With the advent of high-pressure feeders after World War II (e.g., blow tanks, screw pumps), many designers and users began to apply pneumatic conveying to much greater distances and/or capacities (e.g.,... 

Improved blow tank design to ensure an efficient and controlled discharge rate of material. 

Reliable valves for blow tank filling, venting and discharge. 

Some typical conventional blow tank designs are illustrated in Fig. 7. This type of feeder has been used successfully in industry to handle a wide range of products over relatively short distances (e.g., L = 100 to 200 m). However, the application of this type of blow tank to greater distances and/ or capacities (usually in conjunction with a second blow tank unit in series or parallel to ensure an essentially continuous mode of flow) resulted in a number of problems. 

Incomplete and/or inefficient discharge of material from each blow tank mainly due to rat-holing or funnel-flow effects promoted by... 

Figure 7. Conventional blow tank feeders (a) bottom-discharge arrangement (b) top-discharge arrangement.

Flow instabilities or even pipeline blockage during start-up procedures due to surge effects caused by the initial high pressure drop across the blow tank (and hence high instantaneous flow rate of material),... 

To avoid these problems, a number of significant developments have occurred in the area of blow tank design, such as ... 

These developments have resulted in a much more reliable and efficient blow tank feeding system, especially for long-distance applications, as shown in Fig. 8. Such systems now have been used successfully in many installations throughout Australia to meet the increasingly demanding requirements of conveying capacity and distance. 

Figure 8. Preferred tandem blow tank feeding system for long-distance pneumatic conveying.

It should be realized also that either option shown in Fig. 9 or 10 will produce a non-symmetrical flow pattern inside the blow tank (i.e., due to preferential feeding at the blow tank outlet) and hence, promote the possibility of arching, rat holing and/or formation of dead regions. For these reasons, the combined fluidizing-discharge-cone and cone-dosing valve system shown in Fig. 8 is preferred. 

Tandem 7 nf bottom-discharge cone-dosing blow tanks, similar to those shown in Fig. 8, were employed for this purpose and also to regulate the feed of material into the pipeline and alleviate the occurrence of imminent blockages (Wypych, 1995b). 

Blow tank technology was introduced extensively after World War II to not only improve plant efficiency but also to extend the future potential of pneumatic conveying (e.g., long-distance and/or large-throughput... 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

Kennedy, O. C., Wypych, P. W., and Arnold, P. C., The Effect of Blow Tank Air Injection on Pneumatic Conveying Performance, Pneumatech 3, Int. Conf. on Pneumatic Conveying Technol., Jersey, Channel Islands, UK (1987)... 

---