Unit operations, liquid systems equipment

Separation of two liquid phases, immiscible or partially miscible liquids, is a common requirement in the process industries. For example, in the unit operation of liquid-liquid extraction the liquid contacting step must be followed by a separation stage (Chapter 11, Section 11.16). It is also frequently necessary to separate small quantities of entrained water from process streams. The simplest form of equipment used to separate liquid phases is the gravity settling tank, the decanter. Various proprietary equipment is also used to promote coalescence and improve separation in difficult systems, or where emulsions are likely to form. Centrifugal separators are also used. 

The most important aspect that must be considered when reviewing EDI and RO technologies is that these unit operations never remove the trace metals from the liquid stream [17,18]. Unlike ion-exchange resin, both EDI and RO employ reject streams to transport the now-concentrated contaminants out of the equipment and to another location for treatment or disposition, requiring yet more treatment equipments. This is much different from ion-exchange technology that physically removes the trace metals from the liquid stream. Even with their downside, RO and EDI may be able to play an important role in the waste treatment system if there are water reclamation targets that must be met. 

Relief valves are designed to respond automatically to sudden increases in pressure. A relief valve opens at a predetermined pressure. In a relief valve, a disc is held in place by a spring that will not open until system pressure exceeds its operating limits. Tremendous pressures can be generated in process units. When a system overpressurizes, safety valves respond to allow excess pressure to be vented to the flare header or atmosphere. This prevents damage to equipment and personnel. Relief valves are designed to open slowly, and thus are best for pressurized liquid service. They do not respond well in gas service, where quicker pressure reduction is needed. 

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