Chemical processes

When we follow the sequence of processes in the chemical plant we can see that solid raw materials go to crushers, grinders and screens and then by conveyors to the reaction vessels. For liquid raw materials, the storage tank is the starting point, after which pumps, and perhaps proportioning devices, are required. The same is true of gases which must be propelled by means of fans, blowers and compressors; frequently they are to be cooled in coolers and washed. The devices used for the cleaning of a gas include mainly dust collectors, dry cyclones, filters and scrubbers.

Materials must be mixed, liquids as well as solids. This process in various types of mixers and agitators. At the other stages, solids are to be separated from liquids in filters, centrifuges or simply by settling the suspended solid in a settling tank or in continuous mechanical setting units generally known as thickeners. Solutions of solids are evaporated in evaporaters; they crystallize in crystallizers and they are dried in driers. Mixtures of liquids, on the other hand, are usually separated in distillation equipment.

Auxiliary equipment, such as vacuum pumps, heat exchangers and reaction vessels, especially kettles, are required in many of these operations, as well as numerous types of instruments for measurement and control.

Text 21

Devices for transporting and compressing gases and liquids

The transportation of a liquid or gas is a common operation. Devices used for this purpose are: fans, pumps and compressors.

FANS. A fan is a machine for applying power to a gaseous fluid to increase its energy content. This energy enables movement or flow of the gas against various degrees of resistance.

The function of a fan is to move air or gas through distribution systems and apparatus required for conditioning gas medium.

The fan consists of a rotating member called the wheel or impeller and a stationary member called the housing. The housing is provided with an intake opening (inlet) and with a discharge opening (outlet). The flow of air or gas is caused by the pressure differential created by the energy transmitted to the gas by the rotating wheel.

If no resistance to flow exists, as in the case of a fan in free space with no inlet and no outlet duct, the fan will provide the gas with velocity energy only, and no compression or rarefaction occurs. When either inlet or outlet duct is added, frictional resistance is imposed and partial compression occurs on the outlet side, whereas partial rarefaction occurs on the inlet side.

The extent of the resistance imposed at the discharge governs the quantity of gas delivered by the fan. Fans are volume blowers. A fan may have 6 or 8 flat steel blades or 60 very small curved blades. The latter belong to the multi-bladed type. Fans are generally belt-driven, usually from a special motor. The capacity of any fan is computed on the basis of free air, that is, air at atmospheric pressure and at 15º C.

Text 22

Gas Compressors

These machines are employed for bringing gases to a higher pressure level and for transporting them. The number of types of compressors is large: rotary compressors, piston compressors, diaphragm compressors and others.

Piston compressors have in principle the same construction as piston pumps. But the piston speeds and the number of strokes per unit of time are much higher. These compressors can be constructed for any capacity and delivery pressure.

The most usual type is that with lubricated cylinders. The presence of solvents in the gas does not present so much difficulties in this case as in the rapidly rotating vane compressor. But there are cases when no lubricating oil is admitted into the cylinders, e.g. in compressing oxygen. Some other lubricant must then be used or the cylinders must not be lubricated at all. In the latter case the cylinder liner or the piston ring must be made of graphite. The maximum speed is about 200 ft/min. and the maximum pressure is about 300 lb/min.

Diaphragm compressors is of almost the same construction, as the diaphragm pipe for liquids. The diaphragm can be flat or shaped as a tube or bellows and may be constructed of steel or rubber. The absence of the leaky stuffing box makes these compressors suitable for handling valuable or toxic gases. But it is expensive and vulnerable.

Text 23