Stability of Operation

At the beginning of Sec. 10.2.b, it was stated that after the solution to the mass balance is used to decide the reactor operating conditions for optimum conversion (or selectivity), then the energy balance is utilized to determine the external conditions required to maintain the desired temperature. Thus, Eq. 10.2.b-l is solved together with the steady-state form of Eq. 10.2.a-5  

Note that QXT) is proportional to the heat removal rate by external heat exchange. 

To illustrate the procedure, consider an irreversible first-order reaction at constant volume, where 

This is then substituted into Eq. 10.4.a-2, which, when rearranged, becomes 

Equation 10.4.a-3 is a nonlinear algebraic equation to be solved for T, given values for all of the parameters. For the general case, similar manipulations would lead to 

Weight distribution of polymer in HCSTR parameter conversion, )c, number average degree of polymerization at zero conversion = 1000. From Gerrens [1976]. 

Vinyl acetate polymerization. Evolution of polydispersity with fractional conversion in three reactor types for typical parameter values. From Nagasubramanian and Graessley [1970]. 

The problem addressed in this section is the state arrived at by the reactor on a perturbation of the operating conditions. Do small perturbations necessarily lead to slight differences in operating conditions or can they lead to drastic changes  

Will the reactor necessarily return to its original steady state when the perturbation is removed The answers depend on whether the original steady state is stable or not. 

Considerable, but not complete, information on the stability of a steady state can be derived from the combined steady-state continuity and energy equations. Therefore, (10.2.2-1) is considered simultaneously with the steady-state form of (10.2.1-3)  

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Study the simple, open-loop (KC = 0) and closed-loop responses (KC = -1 to 5, TSET = TDIM, and 300 to 350 K) and the resulting yields of B. Confirm the oscillatory behaviour and find appropriate values of KC and TSET to give maximum stable and maximum oscillatory yield. For the open-loop response, show that the stability of operation of the CSTR is dependent on the operating variables by carrying out a series of simulations with varying Tq in the range 300 to 350 K. 

The purpose of a detector is to monitor the carrier gas as it emerges from the column and respond to changes in its composition as solutes are eluted. Ideally a detector should have the following characteristics rapid response to the presence of a solute a wide range of linear response high sensitivity stability of operation. 

The ideal HPLC detector should have the same characteristics as those required for GC detectors, i.e. rapid and reproducible response to solutes, a wide range of linear response, high sensitivity and stability of operation. No truly universal HPLC detector has yet been developed but the two most widely applicable types are those based on the absorption of UV or visible radiation by the solute species and those which monitor refractive index differences between solutes dissolved in the mobile phase and the pure mobile phase. Other detectors which are more selective in their response rely on such solute properties as fluorescence, electrical conductivity, diffusion currents (amperometric) and radioactivity. The characteristics of the various types of detector are summarized in Table 4.14. 

The complete design of any reactor must also take into account questions concerning the stability of operation, particularly in relation to heating effects. 

Stability of Operation... Space Required Good Good Poor... 

The liquid capacity is >30% total (volumetric capacity) If a spring valve is required, it should be a valve with liquid trim. Preferably, however, a modulating pilot-operated valve can be used. The modulating pilot ensures stability of operation whatever the phase and cannot be oversized as it adapts its flow to the need of the system. 

A large amount of effort has gone into the investigation and development of aerosol generation devices. Over the years, a wide variety of methods for the production of aerosols has emerged these methods depend on the technological requirements of the aerosol. For many scientific applications they include (1) control of the particle size distribution, (2) stability of operational performance for... 

It is therefore evident that only when the gas rate is less than the design value can stability of operation be insured. However, under this subcritical operation, excessive wall pressure, and therefore pressure drop, would be incurred. 

Chemical activity Apparent contact time 3 < T < 30 sec Stability of operation Homogeneous temperature in bed LJDt = 1-2 High activity excess reaction in dilute phase Low activity decrease of reaction yield Dilution with inert particles Multistage fluid bed... 

Fuel cells, due to their higher efficiency in the conversion of chemical into electrical energy vhth respect to thermo-mechanical cycles, are another major area of R D that has emerged in the last decade. Their effective use, ho vever, still requires an intense effort to develop ne v materials and catalysts. Many relevant contributions from catalysis (increase in efficiency of the chemical to electrical energy conversion and the stability of operations, reduce costs of electrocatalysts) are necessary to make a step for vard in the application of fuel cells out of niche areas. This objective also requires the development of efficient fuel cells fuelled directly vith non-toxic liquid chemicals (ethanol, in particular, but also other chemicals such as ethylene glycol are possible). Together vith improvement in other fuel cell components (membranes, in particular), ethanol direct fuel cells require the development of ne v more active and stable electrocatalysts. 

The operational lifetime of the ejectors must be in the region of at least a billion or more cycles. Food processing is usually a 24-hom-a-day, all-year-round operation. Operators cannot afford to shut down a machine regularly for even a few minutes to replace faulty ejectors. Under these circiunstances, machine rehability and stability of operation are critical. For certain large or heavy objects a solenoid valve may be used to control a pneumatically operated flap or plunger to deflect rejected items. Specialised ejectors have been developed for pulps or slurries to remove rejects by suction and are mounted above a flat belt, downstream from the inspection unit. 

FaciUtates automatic process control and stability of operation. 

Seven heat exchanger types were submitted in response to the DOE sohcitatiom (Details of the designs are preserrted in Avery and Wu (1994). For each type, measruemerrts were made at the ANL facility, of the heat trarrsfer coefficients, the pressure-drop in the fluids flow, the stability of operation, and other relevant behavior. 

Vol ter BV, Sal nikov IE (1981) Stability of operation regimes of chemical reactors. Khimija, Moscow (in Russian)... 

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