Backmixing of the liquid phase

In packed-bed reactors, the catalyst is fully wetted, whereas the heat and mass transfer efficiency is higher than that observed in trickle-bed reactors. However, low operation efficiency may appear due to backmixing of the liquid phase. Moreover, high liquid-phase residence times can result in the occurrence of homogeneous side reactions. 

In spite of these advantages of slurry reactors, some technical difficulties are involved in the operation of these reactors. For example, separation of the catalyst and handling of the slurry is difficult the solids can produce erosion of the equipment and significant backmixing of the liquid phase does not allow operation in a plug-flow manner. 

The consequence of recycling is a high backmixing of the liquid phase and a large RTD corresponding to continuous stirred tank reactors. Compared to plug flow reactors (Bo > 100), the specific performance of recycle reactors is significantly... 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid-phase is important in order to decide if we have to use a plug- or backmixed-flow model. The liquid-phase can be considered well mixed if (Ramachandran and Chaudhari, 1980)... 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid phase is important in order to decide if we have to use a plug- or backmixed-flow model. For the specified Reynolds number, the Peclet number for the liquid phase using the Stiegel-Shah correlation (eq. (3.422)) is 0.15, much lower than in the trickle bed, which was expected as the backmixing in the liquid phase in packed bubble bed reactors is relatively high. The liquid phase can be considered to be well mixed if (Ramachandran, and Chaudhari, 1980) (eq. (3.423))... 

This case study on oxidation of sodium sulfide illustrates the design of a variety of gas-liquid reactors and compares their performances. Bubble column reactors are particularly attractive, as they offer advantages such as simplicity of construction and operation, but they suffer from such drawbacks as high pressure drop and backmixing in the liquid phase. To reduce the pressure drop, two modifications have been considered an external-loop air-lift reactor and a horizontal sparger reactor. Both result in substantial energy savings (because of low AP) under similar conditions of capacity and conversions in the gas and liquid phases. 

Following on the work of Teramoto, Hoffman, Sharma and Luss (28) have performed an analysis of the adiabatic gas-liquid reactor operating in continuous backmixed flow of the liquid phase for this consecutive (1,1) - (1,1) reaction. They used data relative to the system chlorine/n-decane with a selectivity ratio of k /kp = 1. 1 The boundary conditions were formulated in terms of overall material balances on the gas and liquid phases, so that for component A, the boundary condition at the film-bulk junction is given by... 

Unlike the previous analysis (38), the boundary conditions were properly treated as dependent variables found by taking account of film and bulk reaction balanced with the input and output flows and mass transfer fluxes across a tray. The approach (39) involved perfect backmixing of the liquid, but with provision of the possibility of axial mixing in the gas phase being represented by staging of backmixed zones. In practise though, the authors claimed that a single gas zone was sufficiently accurate. 

Juvekar and Sharma (50) only compared their results with those of Mashelkar (58) but found good agreement. However, unlike packed column, it is exceedingly difficult to accept the ideal flow pattern of Table 4 for many bubble column applications. Indeed, the backmixing in the liquid phase is unavoidable and it usually reduces the contactor performance. Many authors (7,59-64) have employed the axial dispersion model for the liquid phase -in some cases the gas phase too. One of the most sophisticated design procedure was described by Deckwer (63,64) who showed that... 

The backmixing of the solid phase and solid concentration profile become more important when the solid loading is high (e.g. DCL process) or when the solid particles are large with significant relative slip velocity with the liquid phase. If... 

The trickle flow and upflow (bubble flow) operations each have specific advantages. In trickle flow there is less backmixing in the liquid phase. Care should be t Jcen that the solid is completely wetted. When there is a risk of incomplete wetting, and possibly hot spots, upflow may be preferable. Generally, upflow is more suit for relatively low gas and liquid flow rates, which may be related to low reaction rates. Another advantage is the better heat transfer rate at the wall. 

Bubble columns. Tracers are used in bubble columns and gas-sparged slurry reactors mainly to determine the backmixing parameters of the liquid phase and/or gas-liquid or liquid-solid mass transfer parameters. They can be used for evaluation of holdup along the lines reviewed in the previous Section 6.2.1. However, there are simpler means of evaluating holdup in bubble columns, e.g. monitoring the difference in liquid level with gas and without gas flow. Numerous liquid phase tracer studies of backmixing have been conducted (132-149). Steady-state or continuous tracer inputs (132,134,140,142) as well as transient studies with pulse inputs (136,141,142,146) were used. Salts such as KC Jl or NaCil, sulfuric acid and dyes were employed as tracers. Electroconductivity detectors and spectrophotometers were used for tracer detection. The interpretation of results relied on the axial dispersion model. Various correlations for the dispersion... 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Assuming the liquid phases remain immiscible, the modelling approach for multicomponent systems remains the same, except that it is now necessary to write additional component balance equations for each of the solutes present, as for the multistage extraction cascade with backmixing in Section 3.2.2. Thus for component j, the component balance equations become... 

Concentration modulation experiments have been reported for applications to heterogeneous catalysis (48). The experimental implementation was accomplished by periodically flowing solutions with different (reactant) concentrations over the catalyst immobilized on the IRE. Fast concentration modulation in the liquid phase is limited by mass transport (diffusion and convection), and an appropriately designed cell is essential. The cell depicted in Fig. 12 has two tubes ending at the same inlet (65). This has the advantage that backmixing in the tubing upstream of the cell can be avoided. With this cell, concentration modulation periods of about 10 s were achieved (45,65). 

In slurry reactors, the liquid phase is completely backmixed, whereas backmixing in the gas and solid phases may not be complete. The gas-phase mixing depends on the design of the impeller and the nature of the bubbles, as well as the superficial gas velocity. The presence of gas reduces liquid-phase mixing however, an increase in gas flow increases the mixing. The mixing is also dependent upon the coalescence rate of the bubbles. 

No information is available in the published literature pertaining to the gas-liquid interfacial area, aL. It may be assumed that aL equals the disk area exposed to the air. The thickness of the liquid film on a vertically rotating disk partially immersed in a Newtonian liquid has been evaluated by Vijayraghvan and Gupta (1982). They also showed that the measured liquid holdup on the disk compares well with the values predicted from the flat-plate withdrawal theory. The gas-phase pressure drop is very low. The liquid and the gas phases are partially backmixed. The extent of backmixing is reduced by providing baffles. 

Finally, the rotating-disk reactor provides efficient gas-liquid mass transfer by constant renewal of the gas-liquid film on the rotating disk. The mass-transfer coefficient in such a reactor can be calculated using Eq. (6.49). The reactor provides a low pressure drop and partially backmixed gas and liquid phases. The extent of backmixing can be further reduced by the use of baffles. Once again, power consumption and mechanical difficulties may limit the size of such vessels. 

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