Airlift reactor

In some respects, airlift reactors (airlifts) can be regarded as modifications ofthe bubble column. Airlift reactors have separate channels for upward and downward fluid flows, whereas the bubble column has no such separate channels. Thus, fluid mixing in bubble columns is more random than in airlift reactors. There are two major types of airlift reactors, namely, the internal loop (IL) and the external loop (EL). 

The fundamental principles of the gas-to-liquid mass transfer were concisely presented. The basic mass transfer mechanisms described in the three major mass transfer models the film theory, the penetration theory, and the surface renewal theory are of help in explaining the mass transport process between the gas phase and the liquid phase. Using these theories, the controlling factors of the mass transfer process can be identified and manipulated to improve the performance of the unit operations utilizing the gas-to-liquid mass transfer process. The relevant unit operations, namely gas absorption column, three-phase fluidized bed reactor, airlift reactor, liquid-gas bubble reactor, and trickled bed reactor were reviewed in this entry. 

Eig. 23. Airlift reactors (a) spHt cylinder internal loop, (b) draft tube internal loop, and (c) external loop (94). 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

The height of airlift reactor s is typically about 10 times the diameter of the column (H = 10/)). 

Gas holdup and liquid circulation velocity are the most important parameters to determinate the conversion and selectivity of airlift reactors. Most of the reported works are focused on the global hydrodynamic behavior, while studies on the measurements of local parameters are much more limited [20]. In recent years, studies on the hydrodynamic behavior in ALRs have focused on local behaviors [20-23], such as the gas holdup, bubble size and bubble rise velocity. These studies give us a much better understanding on ALRs. 

The external-loop slurry airlift reactor was used in a pilot plant (3000 t/a) for one-step synthesis of dimethyl ether (DME) from syngas. Specially designed internals were used to intensify mass transfer and heat removal. This new technology is highly efficient and easy to scale-up to industrial. 

Reactor type Slurry bubble colunrn Slurry bubble column Slurry airlift reactor... 

Advances in multiphase reactors for fuel industry are discussed in this work. Downer reactors have some advantages over riser reactors, but suffer from some serious shortcomings. The coupled reactors can fully utilize the advantages of the riser and the downer. For fuel industry that involves gas-liquid-solid system, slurry bed reactors especially airlift reactors are preferred due to their performance of excellent heat control and ease of seale up. For high-pressure processes, the spherical reactor is promising due to its special characteristics. 

Spatial Profiles of Gas Holdup in a Novel Internal-loop Airlift Reactor... 

Internal-loop airlift reactors (ALRs) are widely used for their self-induced circulation, improved mixing, and excellent heat transfer [1], This work reports on the design of an ALR with a novel gas-liquid separator and novel gas distributor. In this ALR, the gas was sparged into the annulus. The special designed gas-liquid separator, at the head of the reactor, can almost completely separate the gas and liquid even at high gas velocities. 

In this model, energy balances are set up for the reactor and the separator tube separately, and two equations are obtained. The gas holdup can then be obtained from combining these two equations. Details can be found in Zhang et al. [7]. The comparison between the measured and calculated cross-sectional mean gas holdups is shown in Fig. 5. It can be seen that there is a satisfactory agreement between the experimental and calculated gas holdup in the different operating conditions. Therefore, it is reasonable to conclude that the energy balance model used in this work can describe the circulation flow behavior in the novle internal-loop airlift reactor proposed in this work. 

A specially built conductivity probe was used to investigate the gas holdup in a novel internal-loop airlift reactor. The gas holdup generally increases with increasing solid holdup due to increased flow resistance. A model based on energy balance was developed that can be used to predict the average gas holdup in this novel interal-loop airlift reactor. 

CFD simulation of hydrodynamics of gas-liquid flow in an oxidation airlift reactor... 

Airlift loop reactor (ALR), basically a specially structured bubble column, has been widely used in chemical industry, biotechnology and environmental protection, due to its high efficiency in mixing, mass transfer, heat transfer etc [1]. In these processes, multiple reactions are commonly involved, in addition to their complicated aspects of mixing, mass transfer, and heat transfer. The interaction of all these obviously affects selectivity of the desired products [2]. It is, therefore, essential to develop efficient computational flow models to reveal more about such a complicated process and to facilitate design and scale up tasks of the reactor. However, in the past decades, most involved studies were usually carried out in air-water system and the assumed reactor constructions were oversimplified which kept itself far away from the real industrial conditions [3] [4]. 

CFD modeling was conducted as a real industrial ALR taken as a background -a cyclohexane oxidation airlift loop reactor. The CFD software FLUENT6.0 was used to study two-phase flow in the reactor. 

A schematic diagram of cyclohexane oxidation airlift loop reactor is illustrated in Fig.l. This reactor consists of outer vessel (riser), coneentric draft-tube(downcomer) and gas... 

The Eulerian multiphase model is used to predict the dispersed gas-liquid flow in the airlift loop reactor. It involves a set of momentum and continuity equations for each phase. Model equation coupling is achieved through the pressure and interphase exchange coefBcdents [5],... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

Gavrilescu, M. and R.Z. Tudose, Residence time distribution of the liquid phase in a concentric-tube airlift reactor. Chemical Engineering and Processing, 1999. 38(3) p. 225-238. 

To remove urea and formaldehyde from synthetic wastewater, Campos and colleagues33 operated a coupled system consisting of a biofilm airlift suspension (BAS) reactor to carry out nitrification and an anoxic USB reactor to carry out the denitrification and urea hydrolysis (Figure 19.8). 

Recent reports on other forms of reactors are also available. Immobilized cell bioreactors, upflow sludge blanket reactors, draft-tube airlift reactor and other have been suggested. Each of these reactors has its own pros and cons and the advantages need to be evaluated with the whole process in mind. Further work is necessary in this area. Several process schemes have been considered and evaluated with the batch reactor design as the core BDS reactor ... 

Merchuk, J. C., Ladwa, N., Cameron, A., Bulmer, M., and Pickett, A., Concentric-Tube Airlift Reactors Effects of Geometrical Design on Performance, AIChE J., 40 1105 (1994)... 

New applications and novel reactor configurations or operational modes for three-phase systems are continually being reported. These include the operation of a three-phase fluidized bed in a circulatory mode (Liang et al., 1995), similar to the commonly applied gas-solid circulating fluidized bed the application of a three-phase fluidized bed electrode that can be used as a fuel cell (Tanaka et al., 1990) magnetically stabilized three-phase fluidized beds centrifugal three-phase reactors and airlift reactors. 

There are a wide variety of three-phase fluidized bioreactor designs possible. The conventional reactor, shown in Fig. 9, is fluidized by both gas and liquid entering at the bottom of the reactor and leaving at the top and is the most common type of three-phase fluidized bed bioreactor. This reactor may be configured to operate with little axial liquid mixing or in a well-mixed mode by adding a recycle stream. The airlift reactor or draft tube fluidized bed reactor, Fig. 10, is also frequently used. In this reactor, gas is injected at the bottom of a draft tube placed in the center of the... 

---