Residence time apparent

Flow reactors usually operate more nearly at constant pressure, thus at variable volume with gases. An apparent residence time is defined as the ratio of the reactor volume to the inlet volumetric rate,... 

Both the reaction rate, r, and the volumetric flow rate, V , must be known in terms of the number of mols, n, of the reference component. The apparent residence time is popularly used to indicate the loading of a reactor. 

Since the rate of reaction r and the volumetric flow rate V at each position depend on T, P, and local molal flow rate n of the key component of the reacting mixture, finding the true residence time is an involved process requiring many data. The easily evaluated apparent residence time usually is taken as adequate for rating sizes of reactors and for making comparisons. 

BPSD/lb = (barrels of liquid feed at 60°F per stream day)/(lb catalyst), but it is advisable to write out such units in each case to avoid confusion with the standard meanings of the given acronyms. Since the apparent residence time is defined in terms of the actual inlet conditions rather than at standard T and P, it is not the reciprocal of GHSV or LHSV, although the units are the same. 

Another criterion sometimes used is to provide sufficient residence time in the downcomer to allow adequate disengagement of gas from the descending liquid, so that the liquid is relatively gas-free by the time it enters the tray below. Inadequate removal of gas from the liquid may choke the downcomer. Kister (loc. cit.) reviewed various published criteria for downcomer residence times and recommended those by Bolles (private communication, 1977) and Erbar and Maddox (Maddox, Process Engineer s Absorption Pocket Handbook, Gulf Publishing, Houston, 1985). Both sets of guidelines are similar and are summarized in Table 14-8. The residence times in Table 14-8 are apparent residence times, defined as the ratio of the total downcomer volume to the clear liquid flow in the downcomer. 

The apparent residence time is the ratio of the downcomer volume to the clear liquid flow in the downcomer, The downcomer volume is the tray spacing times the average downcomer cross-section area. The true residence time is the ratio of froth volume in the downcomer to the frothy liquid flow in the downcomer. The true residence time can alternatively be expressed as the ratio of the clear liquid volume in the downcomer to the clear liquid flow. The definition adopted here is that of the apparent downcomer residence time. The author found this definition to be better and to be consistent with riublished criteria for downcomer residence times (1]. 

Downcomer residence time. This check calculates the apparent residence time in the downcomer and ensures it is within the recommended guidelines in Sec. 6.2.9. 

Flow reactors usually operate at nearly constant pressure, and thus at variable density when there is a change of moles of gas or of temperature. An apparent resident time is the ratio of reactor volume and the inlet volumetric flow rate,... 

Equation (7-52) can be expressed in terms of volumetric concentration or in terms of conversions just as we did with the batch reactor. An apparent residence time based on feed conditions can be defined for a single-phase CSTR as follows ... 

An apparent residence time based on feed conditions can be defined for a single-phase PFR as follows ... 

All experimental results given were obtained at conditions of close to optimal feed rate, particle size and residence time for maximum liquid yield at the stated temperatures, as determined by over 200 bench scale runs and 90 pilot scale runs. These optimal conditions are those shown in Table I, that is, 480 to 520 C, 0.5 to 0.7 seconds apparent residence time and a maximum particle size of 1... 

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