Unit operations, control evaporators

We have presented a very brief discussion of typical control schemes for a number of important unit operations furnaces, compressors, decanters, steam/power processes, liquid-liquid extractors, and evaporators. Each of these units requires the control of certain key variables if it is to do its job. A basic regulatory control system must be in place on each unit. 

CrystaDiantion is a commonly used industrial separation and purification technique- If the desired product is an evaporate (or filtrate) rather than a crystalliue phase, then the process emphasis is primarily that of separation rather than purification. In either case there is a strong irteraction and dependence between both degree of separation and purification and the particulate nature of the solid phase produced. Fundamental research on crystallization has focused mainly on understanding the variables that influence the structure and size of the crystalline phase, secogniaang that better knowledge and control of this aspect would permit improvement of the unit operation of crystallization, both as a separation and purification technique. 

The unit operations are as applicable to many physical processes as to chemical ones. For example, the process used to manufacture common salt consists of the following sequence of the unit operations transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening. No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the unit operations— transportation of fluids and solids, distillation, and various mechanical separations—are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves are conducted by controlling the flow of material and energy to and from the reaction zone. 

Steam heating coils are installed in open vessels (the unit operates at atmospheric pressure) for evaporation of the dilute liquor. The operation can be batch type (production of alum from dilute solution). The vapours should be removed by an exhaust hood, scmbbed and coimected to a chimney through a demister to control pollution. An ID fan is to be used for this. 

While the unit operation evaporation, that is, the mass transfer from the liquid phase to the vapor phase, still possesses a direct connection with vacuum techniques, the connection of today s single mass crystallization from solution with vacuum techniques is only indirect. The techniques of vacuum cooling and vacuum evaporation are only the mostly used means for inducing the crystallization process. The reason for the dominant position of vacuum crystallization over classical surface cooling crystallization is the considerably reduced inclination to form incrustations. Vacuum crystallization is used in the low vacuum field down to 1 mbar. There are also applications in the overpressure field, although with increasing pressure the number of applications is reduced. In vacuum crystallization, one can find all the classical process control options used in the more familiar vacuum evaporation processes. However, an important difference to evaporation is the fact that the separation process is not concluded with the crystallization step. The suspension formed still has to be separated into crystal mass and mother liquor. Crystallization is therefore always associated with a mechanical separation process. The better this separation, the greater the purity of the crystallized masses. 

However, the IC/5 offers all comfort functions available today measurement with up to eight sensors with AutoZero and AutoTune as virell as capability of simultaneous control of two evaporator sources. Moreover, it offers 24 material programs, with which 250 coatings in 50 processes can be programmed. To simplify operation and avoid errors, the unit also has a diskette drive. All types of crystal holders can be connected here. The thickness resolution is around 1 A, the rate resolution for rates between 0 and 99.9 A/s around 0.1 A/s and for rates between 100 and 999 A/s around 1 A/s. A particularly attractive option offered by the IC/5 is a microbalance... 

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

The dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

Actions shifting hazardous waste from one environmental medium to another. Many of the waste management, treatment and control practices used to date have simply collected pollutants and moved them from one environmental medium to another, as follows (a) collection of pollutants from air and water via pollution control devices, which are then legally disposed of in land disposal sites, and (b) transfer of volatile pollutants from surface impoundments, landfills, water treatment units, groundwater air stripping operations, to the air through evaporation. 

---