Design Aspects of Bubble Column Reactor

Industrial bubble columns usually operate with a length-to-diameter ratio, or aspect ratio of at least 5. In biochemical applications this value usually varies between 2 and 5. The design and scale-up of bubble column reactors generally depend on the quantification of three main phenomena:

(i) heat and mass transfer characteristics

(ii) mixing characteristics

(iii) chemical kinetics of the reacting system.

In order to design bubble column reactors, the following hydrodynamic parameters are required: specific gas–liquid interfacial area, sauter mean bubble diameter, overall heat transfer coefficient between slurry and immersed heat transfer internals, mass transfer coefficients for all the species, gas holdups, physicochemical properties of the liquid medium. Gas holdup(ϵG) is one of the most important operating parameters because it not only governs phase fraction and gas-phase residence time but is also crucial for mass transfer between liquid and gas. Gas holdup depends chiefly on gas flow rate, but also to a great extent on the gas – liquid system involved. On one hand it defines the gas-phase volume present in the reactor and thus the gas-phase residence time. Like gas holdup, interfacial area(a) is also an important parameter for bubble column design. On the other hand, gas holdup(ϵG) along with the knowledge of mean bubble diameter helps in determining interfacial area(a). Generally, the interfacial area depends on the unit’s geometrical size, the operating parameters and the physical and chemical properties of the liquid. Knowing the bubble size and gas hold-up, it is possible to evaluate the specific interfacial area. The knowledge of overall mass transfer coefficient(kLa) and interfacial area(a) is needed in estimating the rates of mass transfer Interfacial mass transfer across the gas/liquid interface is also an important phenomenon in designing bubble column reactors, and it is measured in terms of the overall mass-transfer coefficient (kLa) (in units of s-1), where kL is the true mass-transfer coefficient and a is the effective interfacial area. In a bubble column reactor that contains coalescing liquid, for bubbles >1 mm, the gas/liquid interface is flexible and the mass transfer across the interface is a dynamic process associated with the dynamics of the interface. In a multiphase system, kL is estimated either from the knowledge of the surface renewal rates or the contact time. The intensive heat transfer rate which is also one of the most important characteristics in the operation of bubble columns. The heat transfer rate in gas–liquid bubble columns is reported to be generally 100 times greater than in single phase flow. This rate is influenced by a number of physical parameters and operating conditions:

(i) gas-holdup

(ii) superficial gas velocity

(iii) circulation velocity

(iv) the physical properties of the liquid,

to maintain catalyst activity, reaction integrity & product quality in column heat removal surfaces plays important role as processes are exothermic and endothermic All these factors are highly interactive, and control the performance of the bubble column. The proper design of a heat exchange system depends on knowledge of gas-to-liquid heat transfer. It is usually assumed that the gas output temperature is the same as the liquid output temperature. Therefore, it is important to understand and quantify all the design parameters for optimum operation and to minimize capital costs.