Tank-clearing process

In South Africa, owing to the diversity and small production volume of specific motor car models, it is not economically feasible to blow-mold HDPE fuel tanks. The only cost-effective plastic tank manufacturing process used in South Africa is rotational molding, using LLDPE with a density of 0.939 g/cm3. Some selected properties of LLDPE before and after petrol exposure are compared to HDPE in Table 16.2. It is clear that owing to weaker mechanical properties, LLDPE is far less suitable than HDPE for petrol containment, mainly because of its lower crystallinity. 

Recovery of solvent is possible by batch distillation of the mixture of liquids and solids yielding solvent of very good purity. Batch systems are preferred to continuous processes as the former can limit the quantity of solids to be removed and are therefore easier to handle. The simplest form of distillation consists of the heated vessel, condenser and one or more collection tanks. Clearly there is only one theoretical plate in this system and this is only suitable for the recovery of a single solvent or for initial separation of a mixture of solvents from residues. Further treatment would be required to separate the individual components of a mixture. 

Slurry storage is in equipment especially designed for the service. Designs and technical service are available from suppliers. Figure 7.44 shows a typical arrangement. The soda ash, unloaded pneumatically, meets a stream of water in the contactor at the top of the tank. Clear solution from behind a bafide in the tank supplies part of the stream to make the wetting of the particles more efficient. Solution transfer to the process is also from behind the baffle. A scrubber for the air used to unload the soda ash is also required. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Continuous clarifiers generally are employed with dilute suspensions, principally industrial process streams and domestic municipal wastes, and their primary purpose is to produce a relatively clear overflow. They are basically identical to thickeners in design and layout except that they employ a mechanism of hghter construction and a drive head with a lower torque capability. These differences are permitted in clarification applications because the thickened pulp produced is smaller in volume and appreciably lower in suspended solids concentration, owing in part to the large percentage of relatively fine (smaller than 10 Im) solids. The installed cost of a clarifier, therefore, is approximately 5 to 10 percent less than that of a thickener of equal tank size, as given in Fig. 18-94. 

The general charac teristics of the main types of reac tors—batch and continuous—are clear. Batch processes are suited to small production rates, to long reaction times, or to reactions where they may have superior selectivity, as in some polymerizations. They are conducted in tanks with stirring of the contents bv internal impellers, gas bubbles, or pumparound. Temperature controf is with internal surfaces or jackets, reflux condensers, or pumparound through an exchanger. 

Waste Reduction Processes can be redesigned to reduce the amount of waste generated. For example, transfer hnes between processes can be Blown clear pneumaticaUy to drive liquid into the Batch mix tank. 

The elements of the process are as follows. Acid and alkaline wastestreams are collected in a neutralization tank. Here sufficient lime is added to raise the pH to 6.0. At this point, no zinc hydroxide will precipitate but a portion of the iron, calcium sulfate, and other impurities will form a light precipitate. With a coagulant aid, the mixture is sent to a clarifier where a clear overflow containing the dissolved zinc is obtained. 

State clearly any further assumptions made and discuss their validity. Is an agitated tank the most suitable type of reactor for this process. 

The process of fuel injection first involves movement of fuel from the fuel tank by the fuel delivery pump. This low-pressure pump supplies fuel into the fuel delivery port of each plunger and barrel assembly. As the plunger moves past and clears the fuel delivery port, fuel flows into the barrel filling the chamber above the top of the plunger. Continuous rotation of the camshaft then forces the plunger in the opposite direction, closing off the flow of fuel into the barrel and trapping fuel within the barrel chamber. 

Start bubbling in C02. Mix slowly for 10 minutes or longer to produce a clear solution. Start C02 gas protection on the main mixing tank and continue for the balance of the process. 

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