Upflow

Methane has also been used in aerobic bioreactors that are part of a pump-and-treat operation, and toluene and phenol have also been used as co-substrates at the pilot scale (29). Anaerobic reactors have also been developed for treating trichloroethylene. Eor example, Wu and co-workers (30) have developed a successful upflow anaerobic methanogenic bioreactor that converts trichloroethylene and several other halogenated compounds to ethylene. 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

Semicontinuous and continuous systems are, with few exceptions, practiced in columns. Most columnar systems are semicontinuous since flow of the stream being processed must be intermpted for regeneration. Columnar installations almost always involve the process stream flowing down through a resin bed. Those that are upflow use a flow rate that either partially fluidizes the bed, or forms a packed bed against an upper porous barrier or distributor for process streams. 

Propylene feed, fresh benzene feed, and recycle benzene are charged to the upflow reactor, which operates at 3—4 MPa (400—600 psig) and at 200—260°C. The SPA catalyst provides an essentially complete conversion of propylene [115-07-1] on a one-pass basis. A typical reactor effluent yield contains 94.8 wt % cumene and 3.1 wt % diisopropylbenzene [25321-09-9] (DIPB). The remaining 2.1% is primarily heavy aromatics. This high yield of cumene is achieved without transalkylation of DIPB and is unique to the SPA catalyst process. 

The use of fixed bed catalysts is described in several patents (33—37). Methods of operation include upflow, trickle bed, and even vapor phase. Typically, a large volume of solvent is used to moderate the temperature rise associated with the high heat of reaction for nitro group reduction. 

Chlorination can be carried out at 25°C or below. However, the reaction is exothermic and, as mills have used filtrate recycle, operating temperatures have unavoidably risen. Retention times are 30—60 minutes but decrease as temperature increases. In most mills the retention time cannot be changed because the tower is upflow in design. The normal pulp consistency is 3—4%, but the trend is toward higher (ca 10%) consistency or gas-phase chlorination. Target pH in the chlorination stage (also (7 ) is about 1.8. 

Towers. Towers are required to provide retention times from 30 minutes, as in the chlorination and hypochlorite stages, to as much as five hours in some chlorine dioxide stages. Upflow and downflow towers are common. Upflow towers are often used for medium consistency stages. These are particularly advantageous when a hydrostatic head must be maintained on a gaseous solution, as in chlorine dioxide stages. 

Design and operation of recirculation systems can be compHcated. Problems are avoided by using a sludge-blanket clarifier, in which feed enters below a blanket of accumulated and flocculated soflds which become fluidized in the zone-settling regime by the upflowing feed. Feed soflds are trapped in the blanket. The soflds content of the blanket continuously increases and part must be bled off in order to maintain the mass balance. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Eixed beds are operated in downflow mode and as such, provide some amount of soflds filtration however, influent soflds concentration must be kept low (less than 5 mg/L suspended soflds) to prevent rapid plugging of the bed. Entered soflds are periodically removed by backwashing. Upflow beds are more tolerant of soflds because they are fluidized and expanded by the wastewater entering at the bottom. In moving beds, the flow is countercurrent and makeup, fresh carbon is added continuously at the top of the unit while an equal amount of spent carbon is removed from the bottom. 

Anaerobic Filter. The anaerobic filter is similar to a trickling filter ia that a biofilm is generated on media. The bed is fully submerged and can be operated either upflow or downflow. For very high strength wastewaters, a recycle can be employed. 

Fig. 18. Anaerobic wastewater treatment processes (a) anaerobic filter reactor (b) anaerobic contact reactor (c) fluidized-bed reactor (d) upflow anaerobic...

FGG Gatalyst Goolers. Heat-removal systems have been used in commercial FCCUs since the early 1940s. The three basic designs are internal regenerator bed coils, external cods with ddute-phase upflow, and external cods with dense-phase downflow. 

External Dilute-Phase Upflow Cooler. The external ddute-phase upflow design (68) offers some control in the range of heat removal duties but generates relatively low heat-transfer coefficients [60—170 W/(m K)]- This design substantially increases the surface area requirement and thereby reduces the ultimate duty that can be achieved from a single bundle. In addition, poor mechanical rehabdity has been continuously experienced because of excessive erosion at the lower tube sheets as a result of the high catalyst fluxes and gas velocities imposed. 

Reversible Processes. Distillation is an example of a theoretically reversible separation process. In fractional distillation, heat is introduced at the bottom stiUpot to produce the column upflow in the form of vapor which is then condensed and turned back down as Hquid reflux or column downflow. This system is fed at some intermediate point, and product and waste are withdrawn at the ends. Except for losses through the column wall, etc, the heat energy spent at the bottom vaporizer can be recovered at the top condenser, but at a lower temperature. Ideally, the energy input of such a process is dependent only on the properties of feed, product, and waste. Among the diffusion separation methods discussed herein, the centrifuge process (pressure diffusion) constitutes a theoretically reversible separation process. 

Partially Reversible Processes. In a partially reversible type of process, exemplified by chemical exchange, the reflux system is generally derived from a chemical process and involves the consumption of chemicals needed to transfer the components from the upflow into the downflow at the top of the cascade, and to accomplish the reverse at the bottom. Therefore, although the separation process itself may be reversible, the entire process is not, if the reflux is not accompHshed reversibly. 

For the case of separating a binary mixture, the following conventions are used. The concentrations of the streams are specified by the mol fraction of the desired component. The purpose of the separation process is usually to obtain one component of the mixture in an enriched form. If both components are desired, the choice of the desired component is an arbitrary one. The upflowing stream from the separation stage is the one in which the desired component is enriched, and by virtue of this convention, a is defined as a quantity the value of which is greater than unity. However, for the processes considered here, a exceeds unity by only a very small fraction, and the relationship between the concentrations leaving the stage can be written, without appreciable error, in the form... 

Thus, the separative capacity of a stage is directiy proportional to the stage upflow as well as to the square of the separation effected. 

For the case under consideration, where the value of a — 1 is quite small, it follows that everywhere in the cascade, except possibly at the extreme ends, the stage upflow is many times greater than the product withdrawal rate. Thus L/ L — P) can be set equal to unity. Furthermore when the value of a — 1 is small, the stage enrichment can be approximated by the differential ratio dx/dn without appreciable error. The gradient equation for the... 

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