Residence-time parameter, critical reactors

The choice of the above three modes of catalyst placement relative to the membrane can significantly affect the reactor performance. From the analysis of catalytically active and passive (inert) membrane reactors [Sun and Khang, 1988], it appears that the critical parameter determining the choice is the reaction residence time. At low residence times, the difference between a catalytically active and a catalytically passive membrane is not significant. However, as the reaction residence time becomes high, the catalytically active membrane shows a higher reaction conversion. 

There are a number of drawbacks to using continuous processes. Resources are needed to develop the process the appropriate residence time to reach a level of suitable reaction completion must be determined under the desired conditions of temperature, flow rate, and any other critical parameters. The reaction system may have limited flexibility for running other reactions. Pressure drops occur when using tubular flow reactors, and these can be calculated [18]. Once the conditions have been developed, time is necessary to reach steady-state conditions. What happens to material produced while the conditions are approaching steady state Such material is not produced under the desired conditions and hence is atypical of the majority of the batch. Effective control equipment is mandatory for large-scale operations otherwise expensive material is at risk and may need to be reworked. 

Gas hold-up is a critical parameter in characterizing the hydrodynamic behavior and hence the performance of a bubble column reactor. It determines a) the reaction rate by controlling the gas-phase residence time and b) the mass-transfer rate by governing the gas-liquid interfacial area. It is mainly a function of the gas velocity and the liquid physical properties. 

The reaction is carried out in a 0.478 cm i.d. Inconel 625 reactor at 960 to 1,100 bar and 380° to 450°C with approximately one minute of residence time. Treatment at these conditions is often called supercritical water oxidation (SCWO) however, since the critical parameters of the PBX-9404 hydrolysis product mixture are unknown, the treatment may or may not be above the critical parameters for the hydrolysate. Therefore, the more general term hydrothermal treatment is used in this case. 

The length of the reduction section of the reactor is at least twice the length of the oxidation section but is of reduced diameter. The relative slag volumes and residence times are the critical parameters and are determined by the relative amounts of sulfides and oxide lead materials in the feed mix, particularly the amount of lead to be reduced. The lead pool maintained in the reduction zone is minimal and merely provides a channel for the lead to flow back to the oxidation section. 

The ratio of reaction and permeation rates is critical in designing an MR. Dimensionless numbers are important in parametric analysis of engineering problems. They allow comparison of two systems that are vastly different by combining the parameters of interest. Dimensionless numbers are used to simplify the meaning of the information in scaUng-up the reactor for real flow conditions and to determine the relative significance of the terms in the equations. The Damkohler number (Da) is the ratio of characteristic fluid motion or residence time to the reaction time, and the Peclet number (Pe) defines the ratio of transport rate by convection to diffusion or dispersion (Basile et al, 2008a Battersby et al., 2006 Moon and Park, 2000 Tosti et al., 2009). In the case of an MR, Da and Pe are defined in Equations [11.1] and [11.2]. 

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