Bubble column reactors, control

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

Bubbling column reactor BCR Also called "gas sparged reactor", it is little used in hydrogenations. Gas is fed, with partial recycling to increase turbulence, at the bottom of a virtually stationary L phase. Mixing is by far less efficient than in STR or JLR. BCR is preferred only when the overall reaction is slow it is an alternative for TBR (S 2.2.6) with better temperature control as a result of higher liquid holdup. 

If the values of local mean bubble diameter and local gas flux are available, a fluid dynamic model can estimate the required influence of mass transfer and reactions on the fluid dynamics of bubble columns. Fortunately, for most reactions, conversion and selectivity do not depend on details of the inherently unsteady fluid dynamics of bubble column reactors. Despite the complex, unsteady fluid dynamics, conversion and selectivity attain sufficiently constant steady state values in most industrial operations of bubble column reactors. Accurate knowledge of fluid dynamics, which controls the local as well as global mixing, is however, essential to predict reactor performance with a sufficient degree of accuracy. Based on this, Bauer and Eigenberger (1999) proposed a multiscale approach, which is shown schematically in Fig. 9.13. 

Gas hold-up is a critical parameter in characterizing the hydrodynamic behavior and hence the performance of a bubble column reactor. It determines a) the reaction rate by controlling the gas-phase residence time and b) the mass-transfer rate by governing the gas-liquid interfacial area. It is mainly a function of the gas velocity and the liquid physical properties. 

The temperature in a bubble-column reactor can be controlled by removing or adding heat through the column wall or a cooling coil. Based on studies with an electrically heated wall, the heat transfer coefficient varies with... 

Most problems in the design and performance prediction of bubble column reactors appear, because it is - up to now - not possible to control the fluid dynamics in such reactors. Especially the parcuneters of the turbulent flow in these reactors are of major importance. 

Van der Laan [82] reported attempts to model FT in a bubble column reactor. His model exhibited well-mixed liquid and two gas bubble regimes small bubbles that were well mixed and large bubbles that showed plug flow behavior (Figure 12.21). Van der Laan [82] also provided a summary of bubble column reactor models that others have utilized (Tables 12.1 and 12.2). He concluded that the FT slurry bubble column reactor is reaction controlled due to the low activity of the iron catalyst and the... 

Direct fluorination of toluene using elemental fluorine is not feasible since the heat release cannot be controlled with conventional reactors. So the process is deliberately slowed down. Hence, the direct fluorination needs hours in a laboratory bubble column. It can be completed within seconds or even milliseconds when using a miniature bubble column, operating close to the kinetic limit. The Bayer-Villiger oxidation of cyclohexanol to cyclohexanone with fluorine and aqueous formic acid (5% water) is done in miniature bubble column reactors at 60% conversion at 88% selectivity. 

Great efforts have been devoted to development of reactor to satisfy the requirements of different GTL processes. Several reactor types are currently used for FTS. For example, reactors for FTS include the multitubular fixed-bed, gas-solid fluidized-bed, and slurry bubble column reactors (Flussain et al., 2015). The differences between these reactors are largely related to different approaches to temperature control and the choice of catalyst. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Values of the ratio V(IVR given in Table 24.1 emphasize that most of the volume in a tower reactor (apart from a bubble column, data for which would be similar to a sparger-equipped tank) is occupied by the gas phase, and conversely for a tank reactor. This means that a, a in a tower and a, - a t in a tank. For mass transfer-controlled situations, a, is the more important quantity, and is much greater in a tower. For reaction-controlled situations, in which neither ai nor a is important, a sparger-equipped tank reactor, the cheapest arrangement, is sufficient. 

Bubble columns rely on nozzles, mixing plates, and impellers within the reactor to control the bubble size, which determines the interfacial area between gas and liquid phases. Clearly, the interfacial area can be varied over a wide range by suitable design of the mixer and flow pattern. 

Several measures can be taken to control the reactor temperature. For instance, cooling jackets or internal cooling coils can be used. External liquid recirculation through a heat exchanger is another possibility. An elegant solution is to operate at the boiling point of the liquid mixture. In bubble columns and stirred tanks thus near isothermal operation can be achieved by evaporation of one or more of the liquid components present. 

Contactors in which gas is dispersed into the liquid phase Plate columns (including control cycle reactors) Mechanically agitated reactors (principally stirred tanks) Bubble columns Packed bubble columns Sectionalized bubble columns Two-phase horizontal contactors Cocurrent pipeline reactors Coiled reactors Plunging jet reactors, ejectors Vortex reactors... 

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