Empty tube reactor

The pressure-drop and heat-transfer coefficients in empty tube reactors can be calculated using the methods for flow in pipes given in Volume 1. 

A reference to this method of operation was made earlier. There are several instances of industrial organic reactions that are bimolecular and exothermic. An important example is the production of chloromethanes. The temperature rise can be controlled by axially distributed addition of chlorine at several discrete points into a packed-bed, fluidized-bed, or empty tube reactor through which methane is passed (Doraiswamy et al., 1975). The membrane is an ideal choice for such reactions because now it can be allowed to permeate over the entire length of the membrane from the shell side into the inner tube or vice versa. 

Figure 3.12 Schematic representation of the different reactor types wall reactor (empty tube), reactor with a fixed bed (non-consolidated), and reactor that contains a structured element (consolidated).

In addition, the filament reactor can contain a membrane-separation function by grouping threads of filaments around an inner empty reactor core, that guides the permeate and may also increase permeation by reaction. Thus, the tube reactor constructed in such a way comprises two concentric zones, separated by a permeable Pd/Ag alloy membrane in the form of a tube. The reaction takes place in the filament zone. One product such as hydrogen is removed via the membrane and... 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

A plug-flow reactor (PFR) may be used for both liquid-phase and gas-phase reactions, and for both laboratory-scale investigations of kinetics and large-scale production. The reactor itself may consist of an empty tube or vessel, or it may contain packing or a fixed bed of particles (e.g., catalyst particles). The former is illustrated in Figure 2.4, in which concentration profiles are also shown with respect to position in the vessel. 

This is the primary chemical process in the refinery. The heavy gas oil stream is cracked into smaller hydrocarbons suitable for gasoHne. The empty tube furnace was first replaced with tubes fiUed with aluminosilicate catalyst pellets. Then it was found that the tubes could be replaced by a series of tanks with interstage heating to maintain the desired temperature. In all cases it was necessary to bum the coke out of the reactor by periodically shutting down and replacing the feed by air, a complicated and expensive process that lowers the capacity of the reactor. 

The flowsheet of a 1/2-ton (slurry) per day SYNTHOIL bench scale plant, currently in operation at the Energy Research and Development Administration laboratory in Bruceton, Pennsylvania, is shown in figure 1. The vertically placed reactor is made of two interconnected stainless steel tubings of 1.1-inch ID x 14.5-ft long each. The upper end of the first section is connected to the lower end of the second section with a 5/16-inch ID empty tubing. Thus, the plant may be operated with one or both sections of the reactor packed with catalyst while the fluids flow upwards through each. 

CSTR for most reactions. These conditions are best met for short residence times where velocity profiles in the tubes can be maintained in the turbulent flow regime. In an empty tube this requires high flow rates for packed columns the flow rates need not be as high. Noncatalytic reactions performed in PFRs include high-pressure polymerization of ethylene and naphtha conversion to ethylene. A gas-liquid noncatalytic PFR is used for adipinic nitrile production. A gas-solid PFR is a packed-bed reactor (Section IV). An example of a noncatalytic gas-solid PFR is the convertor for steel production. Catalytic PFRs are used for sulfur dioxide combustion and ammonia synthesis. 

In stirred tanks, the power input to agitate the tank will depend on the physical properties of the liquid. In tubular reactors, the axial dispersion in empty tubes may be estimated [e.g., Wen in Petho and Noble (eds.), Residence Time Distribution Theory in Chemical Engineering, Verlag Chemie, 1982] as... 

Reactor Dimensions. Borosilicate glass tube, 10-mm. i.d., with 6-mm. o.d. thermocouple well down the center. The catalyst was supported on a sintered-glass disk. The empty tube was tested for catalytic activity and found inactive towards thiophene at temperatures up to 550° C. 

As a sequel to the simple reactor model described above, two-zone cases for the bulk polymerization of styrene were also studied. Polymerizations in straight, empty tubes give rise to unfavorable temperature and velocity profiles which can lead to hydrodynamic or thermal instabilities. These instabilities may be avoided or postponed by manipulating the wall temperature. 

Varma, A., and R. Aris, Stirred pots and empty tubes, Chemical Reactor Theory A Review, L. Lapidus and N. R. Amundson, eds., Prentice-Hall, Englewood Cliffs. NJ (1977) Chapter 2. 

When a soluble catalyst is used, the multiphase mixture can be contacted in an open empty tube, and mixing of the various phases occurs through an interaction that is induced by local fluid dynamical phenomena. Alternatively, the tube may be filled with inert packing in an attempt to increase radial mixing for improved multiphase contacting and to promote heat transfer between the fluid and the reactor walls. The inert packing may be either randomly dumped into the tube or inserted in the form of the structured variety as illustrated in Figs. 2 and 4, respectively. 

We may first divide tubular reactors into those designed for homogeneous reactions, and therefore basically just an empty tube, and those designed for a heterogeneously catalyzed reaction, and hence to be packed with a catalyst. Both types can of course be operated adiabatically, and it was the simplest model of these that we discussed in the last chapter. If the temperature of the reactor is to be controlled this is through the wall, and the associated problems of heat transfer now arise. These include transfer at the wall and subsequent radial diffusion across the flowing reactants. In the empty tubular reactor there may be considerable variations in flow rate across the tube. For example, in the slow laminar flow the fluid... 

Heat Transfer to the Wall A number of investigations of heat-transfer coefficients at the wall in fluidized beds have been reported, and in all cases the values found for h, were considerably larger than those for an empty tube at the same fluid velocity. Presumably this is because the motion of solid particles near the wall tends to prevent the development of a slow-moving layer or film of gas, and the heat-carrying capacity of the particles themselves as they move between the center and the wall of the reactor is significant. 

Gas residence time 0.5 to 1.3 s gas velocity 3 to 10 m/s Re > 10, L/D > 100. To eliminate backmixing, Pe > 100. Liquid residence time 1 to 6 s liquid velocity 1 to 2 m/s Re > 10, L/D > 100. PFTR is smaller and less expensive than CSTR. PFTR is more efhcient/volume than CSTR if the reaction order is positive with simple kinetics. For fast reactions, nse small-diameter empty tube in turbulent flow. For slow reactions, use large-diameter empty tubes in laminar flow. If reaction is complex and a spread in RTD is harmful, consider adding motionless mixer (Section 16.11.6.10). Examples hydrolysis of corn starch to dextrose polymerization of styrene hydrolysis of chlorobenzene to phenol esterification of lactic acid. Gas-liquid see transfer line. Section 16.11.6.9, or bubble reactors. Section 16.11.6.11. Liquid-liquid see transfer line. Section 16.11.6.9, or bubble reactors. Section 16.11.6.11. 

Methyl Ester Hydrogenolysis. The flow sheet for the continuous methyl ester, catalyst slurry process is shown in Figure 1. The dry methyl ester, hydrogen, and catalyst slurry are fed cocurrently to a series of four vertical reactors operated at 250—300°C and 20,700 kPa (3000 psi). The reactors are unagitated, empty tubes, designed to provide adequate residence time, minimum backmixing, and a reasonable column height. Fresh catalyst powder is... 

The experiments were carried out in an all stainless steel microreactor system with four gas lines which was operated at pressures up to 100 bar. The gases were supplied by Linde with the following purities He 99.9999 %, N2 99.9999 %, H2 99.9999 %, the mixture of 25% N2 in H2 used as synthesis feed gas 99.9996 %. The feed gas was further purified by means of a purification unit described elsewhere [4]. The flows were regulated by electronic mass flow controllers. The reactor consisted of a glass-lined U-tube similar to the one described in ref. [13]. It was not possible to detect the desorption of N2, H2 or NH3 from the empty tube within the limits of detection. The U-tube was placed in a copper block to ensure isothermal operation. Gas analysis was performed using a mass spectrometer (Balzers GAM 445) which was calibrated for He, H2, N2 and NH3 by using a reference gas mixture. The calibration for H2O was carried out using a He stream saturated with H2O at room temperature. 

Sometimes the pressure drop in the reactor is sufficiently large to be necessary to account for it, instead of using an average value. For an empty tube the Fanning equation may be used in the usual Bernoulli equation ... 

In addition to use of the axial dispersion model to represent the longitudinal distribution of residence times in a reactor, the transverse or radial dispersion characteristics can also often be adequately modeled with a diifusionlike equation. This is most often done for empty tubes or packed beds the latter has been thoroughly covered in Chapter 11. Further aspects of the topic can be found in Levenspiel and Bischoff [1] and Wen and Fan [2]. 

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