True residence time

The ratio V,./Vq is of the volume of the reactor to the incoming volumetric rate and has the dimensions of time. It will he compared with the true residence time when the numher of mols changes as reaction goes on, or P and T also change. 

Pilot plant experiments represent an essential step in the investigation of a process toward formulating specifications for a commercial plant. A pilot plant uses the microkinetic data derived by laboratory tests and provides information about the macro kinetics of a process. Examples include the interaction of large conglomerates of molecules, macroscopic fluid elements, the effects of the macroscopic streams of materials and energy on the process, as well as the true residence time in the full-scale plant. 

In a continuous reaction process, the true residence time of the reaction partners in the reactor plays a major role. It is governed by the residence time distribution characteristic of the reactor, which gives information on backmixing (macromixing) of the throughput. The principal objectives of studies into the macrokinetics of a process are to estimate the coefficients of a mathematical model of the process and to validate the model for adequacy. For this purpose, a pilot plant should provide the following ... 

The true residence time is found by integration of the rate equation,... 

On the other hand, the true residence time must be found by integration,... 

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. 

The situation is more complicated if expansion or contraction of a volume element does occur and the volumetric flowrate is not constant throughout the reactor. The ratio VJv, where v is the volume flow into the reactor, no longer gives the true residence time or contact time. However, the ratio VJv may still be quoted but is called the space time and its reciprocal v/V, the space velocity. The space velocity is not in fact a velocity at all it has dimensions of (time) 1 and is therefore really a reactor volume displacement frequency. When a space velocity is quoted in the literature, its definition needs to be examined carefully sometimes a ratio Vi/ V, is used, where V/ is a liquid volume rate of flow of a reactant which is metered as a liquid but subsequently vaporised before feeding to the reactor. 

Figure 6.18 shows the effect of the recycle rate (R) on the residence time of a PFF system with recycling. Note that r is calculated based on the inlet flowrate that is the true residence time for the fermenter system. The actual r in the PFF is unimportant because it decreases with the increase of the recycle rate. When j3 -1, the bleeding rate is equal to the flow rate and the flow rate of filtrate stream L is zero, therefore, the recycle stream is not filtered. The residence time will be infinite if R is zero and decreases sharply as R is increased until it reaches the point the decrease is gradual. In this specific case, the optimum recycle ratio may be somewhere around 0.2. 

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]. 

Non-perfect bum out of organic components in the post combustion phase by reaching at least 850 for 2 sec. true residence time, and... 

Results obtained by Wirtz and Dague with a systemic residence time of 12 hours are shown in Figure 47, where their systemic residence time , also called hydraulic retention time (HRT), is defined as the quotient of the reactor volume and the input rate of raw waste water. Due to the 1 1 recycle scheme, the stream entering the reactor is twice as large as the input rate of raw waste water, so that the true residence time in the reactor is only 50 percent of the systemic residence time . 

For a constant-density system with no change in number of moles, with the true residence time tpfr ... 

It comprises the true width of the injected impulse and the true residence time and, therefore, it yields a realistic estimation of the sampling frequency rather than the classical parameters such as N, H, or a, which can be obtained only by injecting an infinitely short impulse, but have the advantage of stringent theoretical evaluation. 

The true residence-time behavior of a reactor can be described with the aid of a reactor cascade. The characteristic quantity for the behavior is the number of reactors N, and the distribution functions are given by Equations (2.2-40) and (2.2-41) ... 

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