Tube diameter tubular reactors

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases, and consequently closer to the ideal (Fig. 2). Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer... 

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...

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes. 

FIGURE 13.8 Temperature profiles using a simplified model of a tubular reactor with pure styrene feed = 135 C and = 20°C. The parameter is the tube diameter in meters. 

Tubular reactors are used for some polycondensations. Para-blocked phenols can be reacted with formalin to form linear oligomers. When the same reactor is used with ordinary phenol, plugging will occur if the tube diameter is above a critical size, even though the reaction stoichiometry is outside the region that causes gelation in a batch reactor. Polymer chains at the wall continue to receive formaldehyde by diffusion from the center of the tube and can crosslink. Local stoichiometry is not preserved when the reactants have different diffusion coefficients. See Section 2.8. 

The importance of the linear arrangement of mixer/funnel/tubular reactor is shown when processing in a set-up with a curved flow element (0.3 m long bent Teflon tube of 0.3 mm inner diameter) in between the funnel and tubular reactor [78]. If a straight tube of equal dimensions as given above is used, plugging occius after 30 s. Hence even short curved flow passages are detrimental for micro-chan-nel-based amidation studies. 

Low density polyethylene is made at high pressures in one of two types of continuous reactor. Autoclave reactors are large stirred pressure vessels, which rely on chilled incoming monomer to remove the heat of polymerization. Tubular reactors consist of long tubes with diameters of approximately 2.5 cm and lengths of up to 600 m. Tubular reactors have a very high surface-to-volume ratio, which permits external cooling to remove the heat of polymerization. 

It should be emphasized that for ideal tubular reactors, it is the total volume per unit of feed that determines the conversion level achieved. The ratio of the length of the tube to its diameter is irrelevant, provided that plug flow is maintained and that one uses the same flow rates and pressure-temperature profiles expressed in terms of reactor volume elements. 

Normally, the pressure drop in a tubular reactor is small compared with the operating pressure and in this case, the fluid can be treated as incompressible. Then the pressure drop in a tube of length L and diameter d is given by... 

In the RI model, all incident rays intersect at the center axis of the reactor tube, and Eq. 68 produces an infinite value of irradiance as r - 0. The DI model, on the other hand, proposes parallel layers of rays which are wider than the diameter of the tubular reactor and which traverse the reactor perpendicularly to its axis from all directions with equal probability. The calculated results of both models are far from reality, as found in industrial size photochemical reactors. Matsuura and Smith [107] proposed an intermediate model (PDI model, partially diffuse model, Figure 25b) in which parallel layers of rays are assumed, and the width of each is smaller than the diameter of the tubular reactor. These two-dimensional bands form by themselves radial arrangements, the center ray of each band intersecting the... 

On the other hand, the tubular reactor is a simple and inexpensive apparatus. It s small inner diameter requires a low thickness of the tube-wall to resist high pressure, and facilitates the removal or heating of the feed in order to operate the reactor under isothermal conditions. Solid catalyst can easily be placed in the tubular reactor. 

In Section 8.3.3, the safety performance of the tubular reactor was compared to the stirred tank reactor. This comparison can now be extended to a micro reactor. For this, we take a 10 m3 stirred tank vessel, a tubular reactor with 10 mm tube diameter length 1 m, and a micro reactor with 0.1 mm tube diameter and length 1 cm. We compare the following criteria that are important for the reactor safety ... 

A tubular reactor is to be designed in such a way that it can be stopped safely. The reaction mass is thermally instable and a decomposition reaction with a high energetic potential may be triggered if heat accumulation conditions occur. The time to maximum rate under adiabatic conditions of the decomposition is 24 hours at 200 °C. The activation energy of the decomposition is 100 kj mol-1. The operating temperature of the reactor is 120 °C. Determine the maximum diameter of the reactor tubes, resulting in a stable temperature profile, in case the reactor is suddenly stopped at 120 °C. 

Figure 1 shows a schematic view of the tubular reactor. Seamless tubing, of 1/4-inch OD, type 316 stainless steel, was used for the preheater and the reactor. The reactor itself was 187 feet long, wound into a helix of 1 ft. diameter. The reactor and preheater were immersed in separate drums, which were filled with water, and maintained at constant temperature. 

The flow patterns, composition profiles, and temperature profiles in a real tubular reactor can often be quite complex. Temperature and composition gradients can exist in both the axial and radial dimensions. Flow can be laminar or turbulent. Axial diffusion and conduction can occur. All of these potential complexities are eliminated when the plug flow assumption is made. A plug flow tubular reactor (PFR) assumes that the process fluid moves with a uniform velocity profile over the entire cross-sectional area of the reactor and no radial gradients exist. This assumption is fairly reasonable for adiabatic reactors. But for nonadiabatic reactors, radial temperature gradients are inherent features. If tube diameters are kept small, the plug flow assumption in more correct. Nevertheless the PFR can be used for many systems, and this idealized tubular reactor will be assumed in the examples considered in this book. We also assume that there is no axial conduction or diffusion. 

However, a small-diameter tube gives more pressure drop for a given flowrate through each tube and a given tube length. Of course, a larger number of parallel tubes that are shorter can be used to keep pressure drop at a reasonable level, but this increases the shell diameter of the reactor, which increases the cost. Mechanical problems also limit the minimum tube diameter. Typical tube diameter in cooled tubular reactors is 0.03 m. Typical tube diameter in a furnace-fired heated tubular reactor is 0.15 m. 

The next tubular reactor system explored is one with co-current flow of coolant. The same cooled reactor used in the previous section is used. It has 500 tubes packed with 39,260 kg of catalyst. Each tube is 0.1 m in diameter and 10 m in length. 

The combined stream is preheated to 122°C in a FEHE. A heater (HX3) is installed after the FEHE so that inlet temperature of the coolant stream in REACT2 can be adjusted to satisfy the energy balance when the exit temperature of the coolant stream is specified in this countercurrent tubular reactor. This temperature is 150°C, and the heat load in HX3 is 9.34 x 106 kcal/h. The stream is further preheated to 265°C in the tube side of reactor REACT2 by the heat transfer from the reactions that are occurring in the hot shell side of this vessel. There is no catalyst on the cold tube side, so the feed stream does not react but its temperature is increased. The stream is then fed to reactor REACT 1, which contains 48,000 kg of catalyst. This reactor is cooled by generating steam. The coolant temperature is 265°C (51 bar steam). This vessel contains 3750 tubes, 0.0375 m in diameter, and 12.2 m in length. The overall heat transfer coefficient between the process gas and the steam is 244 kcal h-1 m-2 °C 1. The heat transfer rate is 42 x 106 kcal/h. 

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