Gas-Liquid Mixing: Physical Considerations Department Editor: Scott Jenkins Chemical Engineering©

Gas-liquid reactors in the chemical process industries (CPI) have increasingly been designed to handle larger manufacturing scales. Since gas-liquid reactors can represent substantial capital and operating costs for the user, optimizing mixing and maximizing productivity are critical. The need for efficiency at larger scales places more importance on understanding the physical phenomena of mixing and more of a burden on equipment design.

Physical demands of mixing A number of complex physical phenomena must be considered to achieve optimal function of mixing equipment in cases where gaseous and liquid substances interact. For a gas-liquid reaction to occur, a low-density compressible gas must be dispersed into a much denser liquid with a reasonably long contact time.

Usually, significant turbulence must be induced into the liquid phase to aid mass transfer and reaction.

In addition, rapid movement of the liquid phase is often required at heat-transfer surfaces, which are often removed by some distance from mixing impellers. In some cases, the liquid phase can contain a significant level of solids, which must be kept suspended.

Gas-liquid reactors commonly consist of large pressure vessels with sophisticated internal components for gas feed and exhaust, liquid feed and outlet, heat-transfer and baffling, as well as agitation.

The two major categories of gas-liquid reactions are those with a “pure” feed gas, and those in which the gas contains a significant fraction of inert gases in addition to the reactant. For a gas-liquid reaction to take place, a molecule of gas must dissolve in the liquid phase and then meet a molecule of the reactant. A catalyst material is often present, in which case both reactants must meet on the active site of a catalyst.

The catalyst’s high specific surface area means that the reaction is usually limited by transport of the gas through the boundary layer around the gas bubbles and into the liquid phase (Figure 1).

The specific rate of mass transfer through the boundary layer is governed by the standard mass-transfer equation:

m = kLa (c* – c) (1)

where c is the actual concentration of dissolved gas and c* is the theoretical equilibrium concentration of dissolved gas.

The film mass transfer coefficient (kL) is mainly a function of the physical properties of the reactants, and is less sensitive to mixing conditions. The specific surface area (a) also depends on material properties, but can be significantly increased by changes to process design. These two factors are usually expressed together as a specific mass-transfer capacity (kLa), since it is difficult to measure either one directly.

The term (c* – c) in Equation (1) is sensitive to changes in process design in that influences can be designed to increase the theoretical equilibrium concentration of dissolved gas (c*).

According to Henry’s law, the value of c* is proportional to the partial pressure of the reactive gas. So the use of a reactant gas in a pure form raises process efficiency, but the presence of volatile solvents reduces it. Increasing reaction pressure can boost productivity, but higher pressures also mean higher operating and capital investment costs.

The target for the design engineer is to optimize kLa and operating pressure to achieve the required productivity at the lowest cost.

Gases with inert components
Many industrial gas-liquid processes involve reactant gases diluted with significant amounts of inert gases. This includes all processes using air (21% oxygen in nitrogen) or fluegas (carbon dioxide or sulfur dioxide in nitrogen), in which the feed gases and then discharged.
In cases where gases contain to the reaction stoichiometry is considerations, using air as an example:

• The mass transfer force (c* – c). about one-fifth

• 100% consumption of the oxygen from air is not possible, so reactors must operate with stoichiometric excess. Exhaust from industrial oxidation processes, for example, typically contain around 4–15% residual O2
• O2 partial pressure in the dispersed gas phase changes as the O2 is consumed, and this must be taken into account in reactor design
• Mass transfer cannot be increased by recirculating gas from the headspace, since the headspace gas concentration is depleted
• Since large amounts of inert gas are present, a stoichiometric excess of reactant is required; often high gas rates result, and flooding of impellers is possible
• Practically, the loss in mass transfer capacity generally requires larger reactors, and large reactors require particular attention to maintaining the homogeneity of the mixture

Impellers for high gas rates
Traditionally, for reactors that have high gas rates, combinations of impellers, such as flat-blade disc turbines (FBDTs), pitch-blade turbines (PBTs) and wide-foil impellers are used. More recently, impellers with concaveshapes are increasingly being used (Figure 2).

With these impellers, the gas feed can be dispersed into the liquid phase using a radially pumping primary disperser (PD). One or more secondary dispersers (SD) can also be used on the same shaft, but higher up in the liquid to provide a combination of axial blending of the liquid and redispersion of the gas (Figure 3).

The left diagram shows a compartmentalized flow pattern with FBDTs. The right side shows axially extended vortices with the SD. The two vortices created by the FBDT, one above and one below the impeller, “roll” over one another, while The PD/SD system generates significant material exchange in the axial direction. This reduces the required blend time.

When there are significant quantities of dispersed gas present, traditional impellers rapidly lose power, whereas the concaveblade impellers of the PD/SD type are much less affected.

Reference
1. Himmelsbach, W., Kellar, W. Lovallo, M., Grebe, T. and Houlton, D. Increase Productivity Through Better Gas-Liquid Mixing. Chem. Eng., October 2007, pp. 50–58.