Reactor Reynolds number

In laboratory fixed bed reactors, Reynolds numbers based on the pellet diameter can be as low as one, leading to a relatively thick stagnant fluid film around the pellet. The following correlations can be used to calculate the transfer coefficients [8] ... 

The Tc of CVD-derived Nb Ge films has been correlated with a parameter which involves a function of mole ratio, dilution, and reactor Reynolds number [52], Samples with high Tc (> 17 K) were produced over a wide range of flow conditions and mole ratios. 

Plans were underway to construct a new water and air test loop in addition to KAPL s Fluids Mechanics Laboratory that would be largely dedicated to the Space Power Program. These loops would provide the necessary flow capacity to match gas reactor Reynolds number and ensure dedicated testing could occur without impacting other programs. A concept design for the new test, facility was completed prior to termination of the project and details are provided in Reference 13-1. 

The value of tire heat transfer coefficient of die gas is dependent on die rate of flow of the gas, and on whether the gas is in streamline or turbulent flow. This factor depends on the flow rate of tire gas and on physical properties of the gas, namely the density and viscosity. In the application of models of chemical reactors in which gas-solid reactions are caiTied out, it is useful to define a dimensionless number criterion which can be used to determine the state of flow of the gas no matter what the physical dimensions of the reactor and its solid content. Such a criterion which is used is the Reynolds number of the gas. For example, the characteristic length in tire definition of this number when a gas is flowing along a mbe is the diameter of the tube. The value of the Reynolds number when the gas is in streamline, or linear flow, is less than about 2000, and above this number the gas is in mrbulent flow. For the flow... 

To achieve the goal set above, measurements for reaction rates must be made in a RR at the flow conditions, i.e., Reynolds number of the large unit and at several well-defined partial pressures and temperatures around the expected operation. Measurements at even higher flow rates than customary in a commercial reactor are also possible and should be made to check for flow effect. Each measurement is to be made at point... 

Mass velocities are still much smaller than in production reactors, and Reynolds numbers based on particle diameter are frequently much less than 100. Consequently flow is not similar to that in commercial reactors, and heat and mass transfer are much poorer. 

Figure 2.2.4 (Berty 1983) shows a tubular reactor that has a thermosiphon temperature control system. The reaction is conducted in the vertical stainless steel tube that can have various diameters, 1/2 in. being the preferred size. If used for fixed bed catalytic studies, it can be charged with a single string of catalytic particles just a bit smaller than the tube, e.g., 5/16 particles in a l/2 O.D. tube. With a smaller catalyst, a tube with an inside diameter of up to three to four particle diameters can be used. With such catalyst charges and a reasonably high Reynolds number— above 500, based on particle diameter—this reactor...

Peclet number independent of Reynolds number also means that turbulent diffusion or dispersion is directly proportional to the fluid velocity. In general, reactors that are simple in construction, (tubular reactors and adiabatic reactors) approach their ideal condition much better in commercial size then on laboratory scale. On small scale and corresponding low flows, they are handicapped by significant temperature and concentration gradients that are not even well defined. In contrast, recycle reactors and CSTRs come much closer to their ideal state in laboratory sizes than in large equipment. The energy requirement for recycle reaci ors grows with the square of the volume. This limits increases in size or applicable recycle ratios. 

The Reynolds number is 675, indicating that the fluid flow through the reactor is turbulent. 

The distribution of tracer molecule residence times in the reactor is the result of molecular diffusion and turbulent mixing if tlie Reynolds number exceeds a critical value. Additionally, a non-uniform velocity profile causes different portions of the tracer to move at different rates, and this results in a spreading of the measured response at the reactor outlet. The dispersion coefficient D (m /sec) represents this result in the tracer cloud. Therefore, a large D indicates a rapid spreading of the tracer curve, a small D indicates slow spreading, and D = 0 means no spreading (hence, plug flow). 

The region of stable trailing vortices was sustained to Reynolds numbers up to 5000 ( ). Other flow regimes were observed at higher Reynolds numbers but are probably not achieved in polystyrene reactors. 

A summary of the nine batch reactor emulsion polymerizations and fifteen tubular reactor emulsion polymerizations are presented in Tables III IV. Also, many tubular reactor pressure drop measurements were performed at different Reynolds numbers using distilled water to determined the laminar-turbulent transitional flow regime. 

The difference between the calculated and experimentally determined critical Reynolds number can be explained from the reactor configuration, which consisted of four coils connected by straight tubing section. The straight sections would lower the rela-... 

Figure 10. Monomer conversion rates as a function of emulsion Reynolds number for straight and helical tubular reactors...

Figure 11. Hypothetical conversion as a function of Reynolds number in helically coiled tubular reactors...

The maximum rate of polymerization has been confirmed to occur at the laminar-turbulent flow transition. The rate of polymerization was observed to be maximum at the transition for both straight reactors as well as for the helically-coiled reactor for which the transition is at a Reynolds number higher than that of the straight tube. The helically coiled tubular reactor is of industrial interest since it is much more compact and, consequently, the cost and the temperature control problems are more tractable. 

It is noted that the maximum value of rp in the helically coiled reactor is larger than the maximum observed in the straight tube reactor. The rp increases with increasing Reynolds number while the molecular weight (at a given conversion) decreases. 

The Reynolds number is constant when scaling in parallel, but it increases for the other forms of scaleup. When the large and small reactors both consist of a single tube. 

A factor of 2 scaleup at constant t increases both u and L by a factor of 2, but the pressure drop increases by a factor of 2 - = 6.73. A factor of 100 scaleup increases the pressure drop by a factor of 316,000 The external area of the reactor, IttRL, increases as S, apace with the heat generated by the reaction. The Reynolds number also increases as S and the inside heat transfer coefficient increases by 5 (see Chapter 5). There should be no problem with heat transfer if you can tolerate the pressure drop. 

Example 5.10 A liquid-phase, pilot-plant reactor uses a 12-ft tube with a 1.049-in i.d. The working fluid has a density of 860 kg/m, the residence time in the reactor is 10.2 s, and the Reynolds number is 8500. The pressure drop in the pilot plant has not been accurately measured, but is known to be less than 1 psi. The entering feed is preheated and premixed. The inlet temperature is 60°C and the outlet temperature is 64°C. Tempered water at 55°C is used for cooling. Management loves the product and wants you to design a plant that is a factor of 128 scaleup over the pilot plant. Propose scaleup alternatives and explore their thermal consequences. 

This section has based scaleups on pressure drops and temperature driving forces. Any consideration of mixing, and particularly the closeness of approach to piston flow, has been ignored. Scaleup factors for the extent of mixing in a tubular reactor are discussed in Chapters 8 and 9. If the flow is turbulent and if the Reynolds number increases upon scaleup (as is normal), and if the length-to-diameter ratio does not decrease upon scaleup, then the reactor will approach piston flow more closely upon scaleup. Substantiation for this statement can be found by applying the axial dispersion model discussed in Section 9.3. All the scaleups discussed in Examples 5.10-5.13 should be reasonable from a mixing viewpoint since the scaled-up reactors will approach piston flow more closely. 

Determine the reactor length, diameter, Reynolds number, and scaling factor for pressure drop for the scaleup with constant heat transfer in Example 5.12. 

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