Reactant flow rates

Fig. 8.7 shows a second example (Cycle A2) of carbon dioxide removal by chemical absorption from a CCGT plant, but one in which the semi-closed concept is introduced— exhaust gas leaving the HRSG is partially recirculated. This reduces the flow rate of the gas to be treated in the removal plant, so that less steam is required in the stripper and the extra equipment to be installed is smaller and cheaper. This is also due to the better removal efficiency achievable—for equal reactants flow rate—when the volumetric fraction of CO2 in the exhaust gas is raised from the 4-6% value typical of open cycle gas turbines to about 12% achievable with semi-clo.sed operation. 

To simulate the process in the pilot reactor, the ratio of reactant flow rates should be the same as the ratio of total weight. With the feed rates in the correct proportion, the rate of heat release from the exothermic reaction, Q, should also be in the same proportion. However, R... 

Figure 2. Experimental trial used to Identify transfer function. In this experiment, the reactant flow rate was deliberately varied and the reactant temperature measured on-line in the pilot plant. This allowed us to identify the proper time series model.

Figure 3. First control trial. The temperature and reactant flow rate profile are shown in dimensionless units for the first pilot plant control trial. The PID algorithm and batch start-up control strategy were modified as a result of this trial.

Figure 5. Experiences at production scale. When (B) the Jacket coolant temperature (- -) dropped due to a plant modification, the reactant flow rate (- ) was increased automatically by the PID controller and maintained the reactor temperature (- in (A)) within 1 % of the setpoint.

Two experiments have been performed by varying the reactant flow rates and subsequently the residence time in the reactor. Details of each experiment are shown in Table 12.8. 

We carried out the reaction in a flow system under conditions such that the conversion level was high but well below equilibrium conversion. We used C.P. 1-butene from Matheson and passed it over 100-200 mesh Mobil silica-alumina catalyst [10% AljOj surface area, 393m g (BET)] the batch was heated 1 hr at 450°C in an air stream and kept in a closed container. Gas chromatographic analysis was used neither reactant impurity nor a thermal rate was found to be a complicating factor. The reaction was carried out at 120, 135, 150, and 165°C at several partial pressures, using N2 as diluent, up to 0.95 atm. The reactant flow rate was always 1.56 x 10" mole min A steady state was achieved in about 20 min, and the activity for a run was taken to be the average of three determinations made between 35 and 50 min. 

Kinetic data are frequently acquired in continuous reactors rather than batch reactors. These data permit one to determine whether a process has come to steady state and to examine activation and deactivation processes. These data are analyzed in a similar fashion to that discussed previously for the batch reactor, but now the process variables such as reactant flow rate (mean reactor residence time) are varied, and the composition will not be a function of time after the reactor has come to steady state. Steady-state reactors can be used to obtain rates in a differential mode by maintaining conversions small. In this configuration it is particularly straightforward to vary parameters individually to find rates. One must of course wait until the reactor has come to steady state after any changes in feed or process conditions. 

As sweep gas flow rate is increased, the performance of the reactor improves until the flow rate is about one thousand times the reactant flow rate. The concentration of all species, but most importantly formaldehyde decreases in the shell side of the reactor as this happens. This increases the driving force for permeation of all species. After increasing this flow rate to a certain point further increases in inert gas flow rate do not change the concentration gradient of any species along the reactor because the shell concentrations of all species is... 

Robben and co-workers have exploited these facts to measure mean and rms temperature fluctuations in a turbulent flat flame (2) and above a catalytic surface (8). By measuring the postflame temperature on a flat flame burner, as a function of reactant flow rate, a precise measurement of laminar flame speed was reported by Muller-Dethlefs and Weinberg (9). 

After simple algebraic manipulations, we are left with the following equations, where the variables i 2, FP2, Fu, FR 2 are unknown. Note that Eqs. (9.11) do not allow calculation of the reactor-outlet reactant flow rates FA2 and FB 2. 

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

Separation of the individual contributors can provide useful information about performance optimization for fuel cells, helping to optimize MEA components, including catalyst layers (e.g., catalyst loading, Nafion content, and PTFE content), gas diffusion layers, and membranes. It assists in the down-selection of catalysts, composite structure, and MEA fabrication methods. It also helps in selecting the most appropriate operating conditions, including humidification, temperature, back-pressure, and reactant flow rates. 

Here the reaction rate will be Vj jax = k(cAmin)" tid Vj = k(c )" respectively. The value of the reactant concentration for the mixing zone of the SPMR will be obtained as a result of its comparative mass balance. If we consider that the slip flow is not present (PMR case) or when it is present but the reactant flow rate is identical, then we can write ... 

The physical model of the reactor is a 350 mm high cylindrical vessel, with a diameter of 200 mm and an elliptical bottom. The operation volume is V = 12 10 m. The entrance of the reactants is placed near the middle of the reactor, more exactly at 130 mm from the bottom. The reactor s exit is positioned on the top of the vessel but below the liquid level. At the vessel centre is placed a mixer with three helicoidal paddles with d/D = 0.33. It operates with a rotation speed of 150 mirnf In order to establish the reactor flow model, this is filled with pure water which continuously flushes through the reactor at a flow rate of 6.6 10 5 m /s (similar to the reactants flow rate). At time t = 0, a unitary impulse of an NaCl solution with a Cq = 3.6 kg/m is introduced into the reactor input. The time evolution of the NaCl concentration at the exit flow of the reactor is measured by the conductivity. Table 3.5 gives the data that show the evolution of this concentration at the reactor exit. 

Form these two reactions the reactant fraction is given by f = I / G + 1) provided that the reactant flow rates are sufficiently large that product gases do not significantly accumulate inside the furnace. Under this restriction, the reactant fraction is f = 0.33 for a relative flow rate of G = 2. From the surface reaction we see that two moles of gas-phase product are produced for each mole of reactant, giving / = 1 and a normalized reactant yield of = /. = 0. 33. The parameters and b for the surface reaction probability are approximately 147 kJ/mol and 446, respectively. ... 

Shown in Figure 8.3 is the opposing reactants geometry in which the two reactants enter the membrane from its opposite sides. A reaction plane is formed inside the catalytically active membrane. This implies that the flow front of either reactant is fairly uniform due to the well-engineered microstructure of inorganic membranes. The reactants arrive at the reaction plane in a stoichiometric ratio. Thus undesirable side reactions are reduced. It is noted that as any of the reactant flow rate or concentration changes, the reaction plane will migrate to a new position inside the membrane so that mass balance is maintained. 

Because the pressure drop in the spherical reactor is very small, one could increase the reactant flow rate significantly and still maintain adequate pressure at the exit. In fact, Amoco uses a reactor with similar specifications to process 60,000 barrels of petroleum naphtha per day. 

Why is it that the operation (VII) does not accomplish the high rate of conversion which the formal derivation of (1) and (2) allows for the scheme VI The over-all reaction rate is limited by the rate of transport of intermediate product B from the generation zone to the sites for re-reaction, and this process is not taken into account in deriving (2). In the case of successive reaction zones, with a reactant flow rate F, the rate of transport of intermediate will be FeAs, and over-all conversion, therefore, will be limited to this rate. 

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