Tubular Reactors with Different Diameters

As described in Chapters 3, and 4, chemically reactive species in luminous chemical vapor deposition (LCVD) are created mainly by molecular dissociation. However, in the dilfused luminous gas phase that constitutes the major volume of the luminous gas phase in most practical configurations of LCVD, the actual creation of chemically reactive species occurs at the tip of glow that contacts with the freshly fed monomers. Thus, the size and shape of glow within a reactor is critically important however, these factors depend on the size and shape of the reactor. 

While there are large numbers of previously published articles on plasma polymerization (LCVD), little information is applicable to other systems having different configuration and reactor size. This is due to the highly system-dependent characteristics of plasma processes. The reactor geometry influences the location of the core of the luminous gas phase as well as the size of the total diffused luminous gas phase as described in Chapter 3. 

Three different size Pyrex glass tubes were used as reactor chambers, and their dimensions were as follows  

The monomer gas, perfluoropropene, was fed to one end of the reactor tube. The system pressure was monitored by Baratron absolute pressure gauge, which was placed between tube end and vacuum pump. The system pressure was recorded by a recorder but not controlled by a throttle valve. In this system, the pressure and the flow rate are coupled, and the same system pressure does not correspond to a unique flow rate. The flow rate to yield a system pressure increases slightly with the size of the tube. 

The plasma operating conditions performed in the study are summarized in Table 19.1. The monomer flow rates were determined by measuring the system pressure increase over the given time interval from the isolation of vacuum pump and then converted to flow rate (seem). Each reactor volume was measured by gas expansion method small size = 557 cm, medium size = 1278 cm, and large size = 2357 cm. (The measured reactor volume includes the volume of connecting tubes beyond the center part glass tube.) 

---

Figure 3.79 Dimensionless exit age distribution function using tubular reactors with different inner diameters [111] (by courtesy ofAIChE).

What if the first-order reaction were carried out in tubular reactors of different diameters, but with the space time, t, remaining constant The diameters would range from a diameter of 0.1 dm to a diameter of 1 m for V = p/p = 0.01 cm-/s, i/ = 0,1 ctn/s, and = ICF cmVs. How would your conversion change Is there a diameter that would maximize or minimize conversion in this range ... 

The use of cylinder tubular turbulent reactors with different diameter ratios d /dj, showed [3] that a decrease of d /d leads to quasi-plug flow mode formation at a lower consumption of reactant, introduced through the dj diameter axis branch pipe. In particular, upon the decrease of djd from 0.44 to 0.13, the axial volume flow rate, at which the quasi-plug flow mode starts to form, decreases by 60%. This allows us to use more concentrated reactant solutions in the chemical process. 

Clariant has installed a series of pilot units at the Department of Oxidation Catalysis, each equipped with reactor tubes of the different, industrially-realized diameters. The pilot reactors have been designed and built by DWE, Germany, one of the most experienced reactor construction companies in operation. The units are fully controlled by a Siemens S7 DCS system and equipped with mobile or multipoint thermocouples and on-Une gas analysis allowing a 24/7 operation, comparable to an industrial plant. Each reactor unit is connected to twin-switch condensers for product separation, mimicking an industrial environment. The oxidation of o-xylene is industrially processed in fixed-bed tubular reactors with up to 30,000 tubes. Reaction temperatures range from 300 to 450 C with o-xylene feed concentrations between 0.5 vol% and 1.8 vol% in air. The reaction is conducted at nearly... 

Table 5.2. Time constants of a 0.1 m/s gas and liquid flow in a 10 m tubular reactor with two different tube diameters, R...

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. 

Ethylene epoxidation reaction experiments over all studied catalysts were conducted in a differential flow reacfor, which was operafed at a constant pressure of 3.6 MPa and different reaction temperatures. The tubular reactor having 10-mm internal diameter was placed in a furnace equipped wifh a temperafure confroller. T)q)ically, 30 mg of a catalysf sample was placed inside the Pyrex tube reactor and secured with Pyrex glass wool plugs. The packed catalyst was initially pretreated... 

Another type of controlled radical polymerization employs a reversible termination with a nitroxide compound [21]. Rosenfeld et al. [22] reported details of the nitroxide-mediated radical polymerization of styrene and butyl acrylate at 140 °C in a 2.9 m tubular micro-reactor with an inner diameter of 900 gm. Whereas, for the low-heat-producing monomer, styrene, the differences between a batch process and the microtubular reaction were small, in the case of butyl acrylate the difference was high. This situation, which may have been due to the Trommsdorff effect in the batch reaction (Figure 14.11), indicated that the polymerization was no longer under control. By contrast, no such effect was observed in the tubular micro-reactor, and the degree of conversion remained quite low under the applied conditions. 

The catalytic filaments were introduced into the tubular reactor in the form of threads. A bundle of 100 filaments with a diameter of 7 pm each formedthreads of diameter of about 0.5 mm. The catalytic threads were placed in parallel into the tube to form a cylindrical catalytic bed of several centimeters length. This arrangement gives about 300 threads per cm within the tube cross section with a porosity of = 0.8. The specific surface per volume is in the order of 10 m m and, thus, about 50 times higher compared to washcoated tubes of the same inner diameter [8]. The performance comparison under identical experimental conditions with randomly packed beds with particles of silica and y-alumina of different shapes and sizes showed significantly broader residence time distribution compared to the structured filamentous packing with about five times lower pressure drop for the same hydraulic diameter and comparable gas flow rates. 

LDPE is being manufactured using either an autoclave reactor (AR as in ICI process) or a tubular reactor (TR process of IG Farbenindustrie, now BASF). The high-strength tubes are L = 0.5-2 km long with iimer/outer diameters ID/OD = 70/180 mm thus, L/OD = 3,000-11,000. Since the operating conditions in AR and TR are different (T = 180-330 and 140-340 °C and P = 100-250 and 200-350 MPa, respectively), the polymers have different properties. 

For the internal diffusion regimes the experimental runs in either continuous-fed tubular reactors or batch reactors are made with catalyst particles of different diameter (dp). In this case the constancy of the reagent conversion, changing dp, indicates the absence of internal diffusion. It is quite obvious that in such runs the value of dp must not change during the experiment. Ultrasonic irradiation of a slurry of catalytic particles in a liquid could reduce the value of dp due to a reciprocal abrasion of the solid. Therefore care must be taken to measure the value of the particle diameter before and after the runs. 

After the pre-mixing of butane in air, the gases enter the multi-tubular reactor cooled with a mixture of nitrite and nitrate of sodivun and potassium, where each tube has a 21 mm diameter and is 3.5 or 6 metres high. The catalyst, a VPO precvursor, is tabletted into different shapes and activated to transform the VOHPO4-0.5H2O in (V0)2P207. Nowadays the most modem generation of VPO catalysts are activated outside the reactors and are equilibrated so that the metal oxide loaded in the tubes is ready to work from the first instance of the reactants feed. It then ramps up to the... 

A plug flow or tubular flow reactor is tubular in shape with a high length/diameter (1/d) ratio. In an ideal case (as in the case of an ideal gas, this only approached reality) flow is orderly with no axial diffusion and no difference in velocity of any members in the tube. Thus, the time a particular material remains within the tube is the same as that for any other material. We can derive relationships for such an ideal situation for a first-order reaction. One that relates extent of conversion with mean residence time, t, for free radical polymerizations is ... 

The minicolumn - tubular or conical in shape - is usually made of glass, PTFE, Tygon , or PVC, and is 10-50 mm long and has an internal diameter of 1.5-3.0 mm. The location of these reactor(s) in an FI manifold depends on the type of material with which they are packed, which in turn is determined by their function in the overall process. Figure 8 shows the different possible locations of these units ... 

---