Steady-State Continuous Operation

Considering both the mass balance corresponding to a CSTR and the kinetic equation, a dynamic model was established and validated for steady-state continuous operation of the EMR (Fig. 10.6). Moreover, the model was analyzed to foresee the deviations of the steady-state that occur when short-term changes in operational conditions take place (Fig. 10.7). Higher Orange II loading rates resulting from a more concentrated influent or a variation of HRT caused an increase in dye concentration in the effluent, which was predicted accurately by the dynamic model. The validation was also observed when modifications in the peroxide addition rate occurred. 

A mode that is altogether different is the "continuous" operation, particularly the steady state continuous operation. If not otherwise indicated, we will assume that a continuous reactor is operating under steady state conditions. 

For several basic reasons, the extrusion process does not have the large number of possible process product interactions that the preceding molding methods presented. Due to this situation it can not fabricate the complex shapes and tighter tolerances obtained from molding. The process is a steady-state continuous production operation that can be brought to a condition of control. However it has its share of potential problems (Chapter 8, EXTRUSION). 

In contrast, the treatment of industrial steam generation plants is usually more difficult. There is a need to conform to a good working standard and to produce quality waterside conditions for a long period of time without serious upsets, as the systems are always very dynamic and operating conditions can continually vary. This is especially the case with those facilities whose manufacturing operations may employ some form of on-off cycle or up-down batching process, rather than a steady-state, continuous production stream. 

A batch reactor is never a steady-state process operator, unlike a continuous process in which a steady state is defined as the state of a process in which there is no change with time of any condition of the process. This includes the amount and average composition of the material within the process, so that in a continuous process, there can be no accumulation or depletion. Notwithstanding an unsteady-operation where composition changes with... 

The main conclusions to be drawn from this study are that the reactor design works well, and that steady state continuous flow operation requires excellent mixing of the gases and two liquid phases and high conversions. Improvements in the catalyst (ligand) are required to reduce leaching still further, but commercialisation will also require a different reactor design or more than one CSTR in series. 

The kinetics of the azo-dye conversion plays a fundamental role in the design and operation of bioreactors. Two methodologies can be followed to assess the parameters of reaction kinetics [51, 52] conversion rate estimated at the beginning of batch tests and conversion rate estimated during steady state continuous tests. 

Tests were carried out at 25°C and at initial pH 6.9. Cultures in the liquid medium were incubated in 50 mL Falcon tubes, continuously shaked at 220 rpm. Each culture contained a fresh Pseudomonas sp. 0X1 colony in 10 mL of medium. The airlift with 10 g of pumice was sterilized at 121°C for 30 min and then housed in a sterile room. One-day culture was transferred to the reactor and, after a batch phase, liquid medium with phenol as the only carbon source was continuously fed. The reactor volume V was fixed at 0.13 L. Aerobic conditions were established sparging technical air. Under these conditions microorganism started to grow immobilized on the solid s support. When immobilized biomass approached steady state, cyclic operation of the airlift was started by alternating aerobic/anaerobic conditions. 

Any product distribution that can be obtained in continuous steady-state flow operations can be gotten in a non-flow operation and vice versa. Figure 10.2 illustrates this. 

Batch reactors are frequently operated by chemists who are responsible for obtaining the (boring) prelirninary kinetic data on a process. Chemical engineers get involved when a continuous process is being considered because chemists do not understand anything beyond batch reactors. Steady-state continuous reactors are the subject of the next chapter. 

The larger reactor operates under a steady state, continuous flow conditions and was made of two 1 m cylindrical reactors of annular shape in order to use conventional Germicidal lamps (Figure 14). The system of tanks shown in the flow sheet was used to (i) feed the reactor with a constant flow rate and (ii) wash the system after each experimental run. The actual operating length (Zi) of each lamp (1.2 m long) was Im. Operation could be made with just one reactor or the two in series. 

Extraction columns usually are operated in a steady-state continuous-flow mode of operation with one liquid dispersed in the other. Mass transfer is then promoted by using various fixed or moving elements (various types of packings, trays, or agitators). These elements are... 

Continuous filters are most attractive when the process application is a steady-state continuous one, but the rate at which cake forms and the magnitude of production rate are sometimes overriding factors. A rotary vacuum filter, for example, is a dubious choice if a 3-mm (0.12-in) cake will not form under normal vacuum in less than 5 min and if less than 1.4 m /h (50 ft /h) of wet cake is produced. Upper production-rate limits to the practicality of batch units are harder to establish, but any operation above 5.7 m /h (200 ft /h) of wet cake should be considered for continuous filtration if it is at all feasible. Again, however, other factors such as the desire for flexibility or the need for high pressure may dictate batch equipment. 

Assuming that the annual production requirement has been established, one of the first problems faced in process design is to choose a process cycle so that material and energy balances on a time basis can be worked out and all of the necessary flow sheets prepared. Questions related to a 24-hr or 8 hr/day operation and production by sequential batch operation or on a steady-state continuous basis must be resolved. 

By contrast, in steady-state continuous processes the plant is dedicated to the process and this often gives economic justification for more technically-sophisticated means of operation and control. Techniques and mathematical models for deciding the design criteria for continuous processes are well established. This guide is not aimed at continuous processes, though many of the techniques and principles described are applicable to such processes. 

For a continuous stirred-tank reactor (CSTR) at steady state, the instantaneous MWDs and those of the accumulated polymer are the same, and therefore equation (3) can also be used to calculate the MWD of PP in a CSTR at steady state. For non-steady-state reactor operation (for instance, during grade transitions and reactor start-up or shutdown), the instantaneous MWD will vary in time. 

In batch operations, if the solids are of uniform size and density, the shiny produced below height Z of Eq. (11.22) will be of uniform solids concentration [101]. For mixed particles, the sluny can be quite nonuniform with respect to particle size and densi [79]. However, for steady-state continuous-flow operation, the effluent sluny for nondissolving solids must be identical in solids size and densi with the solids-feed mixture. The residence time of the different particles ne not be unifono, however, and their relative concentration in the vessel need not n essarily be identical widi that in the feed and effluent streams. 

Similarly, several authors have presented MSMPR methods for kinetics determination from continuous crystallizer operation (Chapter 3), which have become widely adopted. In an early study, Bransom etal. (1949) anticipated Randolph and Larson (1962) and derived a crystal population balance to analyse the CSD from the steady state continuous MSMPR crystallizer for growth and nucleation kinetics. Han (1968) proposed a method of kinetics determination from the moments of the CSD from a cascade of continuous crystallizers and assessed the effect of sample position. Timm and Larson (1968) suggested the use of the extra information present in transient response data to determine kinetics, followed by Sowul and Epstein (1981), Daudey and de Jong (1984) and Jager etal. (1991). Tavare (1986) applied the j-plane analysis to the precipitation of calcium oxalate, again assuming nucleation and growth only. 

Most high-tonnage commodity polymers are produced in continuous processes. The feed is metered continuously into the reactor and the effluent is removed continuously from the reactor. When polymerization reaches a steady state in operation, the rate of heat generated at any point in the system is usually constant. Continuous processes have advantages of easy operation and low costs, particularly suitable for large-volume production. The mass balances of reactants and products are in a general form of accumulation = flow in - flow out + production - consumption. For example, in the continuous free-radical polymerization, the mass balances for initiator, monomer and polymer, are... 

Besides performance, durability is another important concern of fuel cells. To evaluate fuel cell durability, two modes, steady-state (continuous running) and shutdown-restart cycling, are often used. In the steady-state mode, a fuel cell is operated continuously at a fixed current density to measure the voltage decay. Quite different durability data from a few hundred hours to nearly twenty thousand hours have been reported in the literature [2,28,47,55-58]. In principle, the durability is affected by many factors such as the chemical stability and conductivity stability of PEMs and the stability of catalyst layer. Oono and coworkers [56] studied the long-term cell degradation mechanism with a PA-doped PBI membrane (DL is unavailable from the literature). The H2/air fuel cell was continuously tested at 150°C at 0.2 A cm" without external humidification. They found that the fuel cell exhibited extremely... 

Safety. A large inventory of radioactive fission products is present in any reactor fuel where the reactor has been operated for times on the order of months. In steady state, radioactive decay heat amounts to about 5% of fission heat, and continues after a reactor is shut down. If cooling is not provided, decay heat can melt fuel rods, causing release of the contents. Protection against a loss-of-coolant accident (LOCA), eg, a primary coolant pipe break, is required. Power reactors have an emergency core cooling system (ECCS) that comes into play upon initiation of a LOCA. 

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