Residence time distribution material characteristics

A practical method of predicting the molecular behavior within the flow system involves the RTD. A common experiment to test nonuniformities is the stimulus response experiment. A typical stimulus is a step-change in the concentration of some tracer material. The step-response is an instantaneous jump of a concentration to some new value, which is then maintained for an indefinite period. The tracer should be detectable and must not change or decompose as it passes through the mixer. Studies have shown that the flow characteristics of static mixers approach those of an ideal plug flow system. Figures 8-41 and 8-42, respectively, indicate the exit residence time distributions of the Kenics static mixer in comparison with other flow systems. 

Residence time distributions can be determined in practice by injecting a non-reactive tracer material into the input flow to the reactor and measuring the output response characteristics in a similar manner to that described previously in Section 2.1.1. 

Figure 9.8 Cross mixing and axial mixing as a function of the material feed characteristic and the residence time distribution...

Another scale-up variable that can be easily controlled is the length to diameter ratio L/D). Scale-up with an equivalent L/D ratio is beneficial in avoiding non-linear scale-up issues, especially at production scale. Residence time distribution (RTD) is a useful term for understanding scale-up of hot-melt extrusion processing. The RTD is used to attempt to quantify the average amount of time a material spends in the processor. The RTD depends on screw speed, screw element design, and material characteristics. The preference during scale-up is to maintain an equivalent ... 

For heat transfer applications the static mixer has an advantage over the empty tube in that it has a much more uniform residence time distribution characteristic, further reducing the possibility of damage to heat sensitive materials. The more uniform residence time characteristic has other advantages, particularly for the design of continuous flow tubular reactors. 

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

The Ballestra CSTR system, because of its back-mixing characteristics long residence time and residence time distribution, can expose the organic material to by-product formation by side rea dom.- (see 5.5.1.) This can prove disadvantageous for sensitive materials such as primary alcohols, alcohol ethoxylates and alpha-olefins. Thgefore> the Ballestra CSTR system is predominantly used for branched and linear alkylbenzenes and fatty acid methyl ester (FAMEl SUlplLQUation. 

In studying the mixing characteristics of chemical reactors, a sharp pulse of a nonreacting tracer is injected into the reactor at time t = 0. The concentration of material in the effluent from the reactor is measured as a function of time c(f). The residence time distribution (RTD) function for the reactor is defined as... 

---