Double-pipe reactors

The flow diagram of the installed SCWO bench scale plant (Figure I) shows that feed, water and air are pumped and compressed separately, typically to 26-30 MPa. After preheating and mixing, the reactants are fed into the pipe reactor (PR) or double pipe reactor where oxidation takes place. After cooling and gas-liquid separation, the aqueous product and the off-gas are analysed. The whole SCWO bench scale apparatus is controlled automatically. 

TTie TWR used is a double pipe reactor, about 0.95 m in length, with an outer pressure bearing tube made of stainless steel (1.4989, o.d. 140 mm, i.d. 80 mm) and in inner porous tube of sintered stainless steel (1.4404, o.d. 66 mm, i.d. 60 mm, average pore width 35 pm). 

To overcome the problems of corrosion and plugging a double pipe reactor with porous inner wall has been installed. Waste and oxidant (air) are fed at the top of the reactor by... 

Fig. 2 Double pipe reactor with porous inner pipe...

Highly efficient destruction of toxic organic materials with high space-time yield No nitrogen oxides are formed. No expensive gas treatment required Heteroatoms are mineralised and leave the process with the aqueous phase Plugging of the tube reactor by dissolved or produced salts is a problem cr induced corrosion can be minimised by Ti-liners in the preheater and cooler Corrosion and plugging are reduced with the double pipe reactor... 

Properties of the reactive fluid within the inner tube are identified by the subscript Rx, and represents the kinetic rate law that converts reactant A to products. Only one independent variable is required to simulate reactor performance because axial coordinate z and average residence time for the reactive fluid trx are related by the average velocity of the reactive fluid j)rx. In comparison with the single-pipe reactor discussed earlier, the double-pipe reactor contains two additional design parameters that can be manipulated to control thermal runaway radius ratio k and average velocity ratio x/f, defined as follows ... 

This is advantageous for a well-behaved double-pipe reactor because the volumetric generation of thermal energy in the inner tube has a feasible escape route across the wall at / mside. whereas larger volumes of cooling fluid reduce the increase in Tcooi. 

Obviously, thermal runaway occurs in the previous example if the flow rate ratio is unity. However, it is possible to control a double-pipe reactor with = I by decreasing the radius ratio. This is illustrated in Table 4-4 for conditions described in the previous example. Thermal runaway occurs when k > /Ccriticai and the critical radius ratio lies somewhere between 0.10 and 0.15. 

The Runge-Kutta-Gill fourth-order correct numerical integration algorithm for coupled ODEs is useful to simulate this double-pipe reactor after temperature-and conversion-dependent kinetic rate laws are introduced for both fluids. The generalized procedure is as follows ... 

Figure 4-8 Effect of higher flow rate ratios on the conversion of an exothermic reactive fluid in a plug-flow reactor with endothermic cocurrent cooling in a concentric double-pipe configuration with radius ratio k =0.5. Both fluids enter the double-pipe reactor at 340 K.

Notice that the double-pipe reactor is well behaved when the outlet temperature of the cooling fluid is less than 337 K. On the other hand, if rcooi.iniet > 324.5 K,... 

Obviously, (a) represents a well-behaved double-pipe reactor, whereas (b) is in the regime of thermal runaway (see Figure 4-13). 

Stability analysis could prove to be useful for the identification of stable and unstable steady-state solutions. Obviously, the system will gravitate toward a stable steady-state operating point if there is a choice between stable and unstable steady states. If both steady-state solutions are stable, the actual path followed by the double-pipe reactor depends on the transient response prior to the achievement of steady state. Hill (1977, p. 509) and Churchill (1979a, p. 479 1979b, p. 915 1984 1985) describe multiple steady-state behavior in nonisothermal plug-flow tubular reactors. Hence, the classic phenomenon of multiple stationary (steady) states in perfect backmix CSTRs should be extended to differential reactors (i.e., PFRs). 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

The whole set-up for partial oxidation comprises a micro mixer for safe handling of explosive mixtures downstream (flame-arrestor effect), a micro heat exchanger for pre-heating reactant gases, the pressure vessel with the monolith reactor, a double-pipe heat exchanger for product gas cooling and a pneumatic pressure control valve to allow operation at elevated pressure [3]. 

The bulk of the styrene is to be heated to 85°C before being charged. This is done in a vertical double-pipe heat exchanger, which is directly above the reactor. To prevent polymerization from occurring in the heat exchanger or piping system, there are to be no obstructions between this heat exchanger and the reactors. 

The coolant is ducted into and away from the reactor by a concentric double pipe. The HTTR has demonstrated inherent and passive reactor safety. 

Figure 7.1. Typical membrane reactor configurations (a) reactor with plate-shaped membranes, (b) tubular-shaped membrane in double pipe configuration and (c) multichannel monolith.

Porous Vycor glass tube (in double pipe configuration), wall thickness 3 mm, length 600 mm, outer diameter 15 mm, mean pore diameter 45 A. Feed enters the reactor at shell side, permeate at tube side. ... 

Porous alumina tube externally coated with a MgO/PbO dense film (in double pipe configuration), tube thickness 2.5 mm, outer diameter 4 mm, mean pore diameter 50 nm, active film-coated length 30 mm. Feed enters the reactor at shell side, oxygen at tube side. Oxidative methane coupling, PbO/MgO catalyst in thin film form (see previous column). r-750X,Pr ed 1 bar. Conversion of methane <2%. Selectivity to Cj products > 97%. Omata et al. (1989). The methane conversion is not given. Reported results are calculated from permeability data. 

Nonporous silver membrane tube (99.99 wt.% Ag), (in double pipe configurationX thickness 100/im. Feed enters the reactor at shell side, oxygen at tube side. Oxidation of ammonia. Silver catalyst in membrane form (see previous column). Oxidation of ethanol to acetaldehyde. Silver catalyst in membrane form (see previous column). r- 250-380°C. The yield of nitrogen was 40%, the yield of nitrogen monoxide was 25%. r- 250-380°C. The yield of acetaldehyde was 83%. The yield with bulk powdered silver catalyst was 56%. Gryaznov, Vedernikov and Guryanova (1986)... 

Flow of Two Fluids. The major applications are in absorption, extraction, and distillation, with and without reaction. Other applications, also quite important, are for shell-and-tube or double-pipe heat exchangers, and noncatalytic fluid-solid reactors (blast furnace and ore-reduction processes). 

Phillips Par tide-forming process (Figure 5) In a double-loop reactor, constructed from wide-bore jaeketed pipe, the catalyst and growing polymer particles are suspended in a slurry and kept in rapid circulation to avoid polymer deposits on the reaetor walls. Due to its high surface-to-volume ratio, this reactor facilitates heat removal and allows short residence times. Typical reaction conditions are 100°C and 30-40 bar. Isobutane, a poor solvent for polyethylene, is used as a diluent and as a vehicle to introduce the catalyst into the reactor. The solid polymer is collected from a sedimentation leg and passed to a flash tank where the monomer and isobutane diluent are separated by evaporation and subsequently recondensed and recycled, while the polymer powder is fed into an extruder and formed into pellets. 

FIGURE 7SI Quenching (a) Heat transfer quenching by double pipe heat exchanger in an aerosol reactor, (b) Dilution quenching by the addition of a cold gas. 

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