Process Intensification Opportunities in Multiphase Stirred Tank Reactors
Shankar B. Kausley, Manishkumar D. Yadav, Gaurav G. Dastane, Chandrakant R. Holkar, Aniruddha B. Pandit*
Manishkumar D. Yadav, Gaurav G. Dastane, Chandrakant R.
Holkar, Aniruddha B. Pandit, Institute of Chemical Technology, Mumbai; and Shankar B. Kausley, Tata Consultancy Services Ltd. In this article, an overview of process intensification in stirred tank reactors leads to many opportunities to enhance productivities. An overview is presented along with their industrial applications. Design aspects are also discuss.
The multiphase stirred tank reactors are widely used in industry due to their high heat and mass transfer coefficients, wide range of liquid phase residence times and capability of handling wide range of superficial dispersed phase gas velocities. The stirred tank reactors have application in the industry in terms of gas dispersion, solid suspension and gas-liquid-solid contacting. In the present article, an overview of process intensification in stirred tank reactor is presented along with their industrial applications. An overview of stirred tank reactors and their design aspects are also discussed.
Keywords: Stirred tank reactor, Gas-inducing impeller, Multi-stage stirred tank reactor, Fractal impellers, Hydrodynamic cavitation.
Prof. Aniruddha B. Pandit (J.C. Bose Fellow, FNA, FNAE, FNASc, FASc, MASc) is UGC Scientist ‘C’ (Professor’s Grade) in chemical engineering at Institute of Chemical Technology, Mumbai. He is actively involved in working with committees in the area of harnessing solar energy & with tribal population in extending the chemical engineering principles for drying of farm/ forest product & water disinfection for potable water. He has authored over 330 publications, 3 books and over 10 chapters and has 13 patents with over 19450 citations with h-index 76 and i10-index 269 (as per Google Scholar) and is on the Editorial board of several International Scientific Journals. *Corresponding author: Email: firstname.lastname@example.org Shankar B. Kausley is Scientist at Tata Consultancy Services Ltd. He has completed Ph.D. (Tech.) in chemical engineering from Institute of Chemical Technology, Mumbai. He has worked in the field of development of low-cost solutions for providing safe drinking water to Indian households and development of cost-effective solutions for reuse of industrial wastewater.
Manishkumar Yadav is pursuing Ph.D. (Tech.) in Chemical Engineering at Institute of Chemical Technology, Mumbai. His area of research interest is nanotechnology, process engineering and scale-up.
Gaurav G. Dastane is doing Ph.D. (Tech.) in chemical engineering at Institute of Chemical Technology, Mumbai. He is working on designing of cavitating devices. His work experience was in design validation of solid oxide fuel cells using CFD.
Chandrakant R. Holkar is doing Ph.D. (Tech.) in chemical engineering at Institute of Chemical Technology, Mumbai. He is working in the field of wastewater treatment and waste management.
The mixing of multiphase streams is one of the most common and important unit operations in chemical industries. Uniform mixing is important as it affects the rate of chemical reactions and quality (selectivity) of the product. There are two types of mixers used in the process, one requires it’s own power supply (using rotating mixing impellers) in addition to the power supplied by the flowing stream. The others require only the process stream as the energy source for mixing which are called motionless or ‘static’ mixers. Among the different ways to perform mixing, mechanically agitated stirred tank reactors are generally preferred considering the advantages which include high heat and mass transfer coefficients, good mixing capability, a wide range of liquid phase residence times, capability of handling wide range of superficial dispersed phase gas velocities. These benefits have led to the existence of stirred tank reactors in industry, from small, medium to large.
Mechanically agitated stirred tank reactors have the potential to provide an economic option for mixing of liquid-liquid, gas-liquid, solid-liquid, and gas-liquid-solid streams, provided these are designed according to the target application. The reactor should meet different performance related objectives such as homogenization, solid suspension, solid suspension in low viscosity liquids, gas dispersion, self-induction of gas and contacting gas/liquid/solid.
There is no ‘Universal’ reactor which can satisfy all the requirements. Additionally, mixing and blending operations, are inherently known to be very inefficient (η ~ 0.9 to 4%). Hence, there are enormous opportunities to enhance the performance of existing stirred tank reactors.
2. Design aspects of multiphase stirred tank reactor
In order to design a multiphase stirred tank reactor, different parameters of components affecting its performance needs to be considered. These parameters/components include i) reactor geometry (diameter and height), ii) impeller type iii) impeller diameter iv) impeller position v) impeller rotational speed vi) number of baffles and baffle width vii) number of nozzles and their size for liquid/gas injection and other reactor internals such as coil and sampling point. Another important parameter which needs to be considered is the energy required for the complete mixing of different streams. 2.1. Stirred tank reactor geometry
A typical stirred tank reactor is shown in Fig. 1. Generally, it’s a vertical cylindrical tank or vessel with dish end at the bottom. Some of the old designs have conical shape bottom, but these designs have inadequate mixing in the conical part. When solid is present, inadequate mixing in the conical part can even cause choking of the underflow outlet.
The impeller diameter to the tank diameter (D/T) is one of the important parameters and is typically in the range of 0.2 to 0.5. The reactor with lower D/T has lower cost and requires low power consumption. Low D/T impeller gives high shear and lower pumping rates. High D/T impeller system is costlier and has high operating cost. It produces low shear and high
pumping. The ratio of fluid height to the diameter of stirred tank (H/T) is another important parameter and is typically in the range of 0.5 -1.0. For stirred tank reactors with H/T ratio greater than 1, multiple impellers are required. The ratio of the distance of the impeller from the bottom to the tank diameter (C/T) is another parameter which is generally in the range of 0.1 to 0.3. Low C/T is required for mixing operation involving solid suspension. Baffles are flat plates attached to the interior of the tank and protrude inward to interrupt and prevent swirling of a fluid. The use of impeller creates sections of the solution that don’t move in the tank. In a cylindrical tank without baffles, impeller creates a vortex effect that causes the entire mixture to move only tangentially with no real axial mixing. The baffles prevent the vortex formation and cause the content in the tank to move from top to bottom. Generally, 2-4 baffles are used in the tank and placed at equidistance inside the tank. The baffle width to tank diameter ration (B/T) is in the range of 1/10 to 1/12.
2.2. Impeller design
Impeller is one of the most important components of stirred tank reactor. Impellers are divided into three main categories: radial flow, axial flow and mixed flow, depending upon the flow pattern developed by them in the liquid. The axial flow impellers include propellers and the axial flow hydrofoils of the Lightnin A 315. The radial flow impellers include Rushton turbine, paddle impeller and curved blade turbine. The mixed flow impellers include pitched blade turbines. The pitched blade turbines are available as upflow and downflow variants, both these impellers have greater axial component velocity compared to radial component of velocity (particularly when the agitator diameter to tank diameter ratio is less than 0.4).
The impellers can be further categorized based on whether they create shear field or bulk movement. The axial flow propeller, the hydrofoils and the mixed flow impellers (when D/T < 0.4) develop bulk axial patterns. The downflow type mixed flow impellers develop a mean flow directed towards the base of the vessel and is therefore useful for solid suspension in solid-liquid systems. These impellers are less efficient in threephase (gas-liquid-solid) systems both for gas dispersion and solid suspension. High shear impellers such as the radial flow Rushton turbine generate smaller bubbles with lower rise velocity. However, these impellers consume higher power. As a compromise for these two conflicting requirements of low power consumption and gas dispersion, the pitched blade turbine in the upflow mode has been suggested. The lat- ter has been shown to afford stable and efficient operation particularly in three-phase reactors[ 2]. For exam
1, ple, the upflow pitched blade turbine (PTU) yields 36% higher gas holdup than its downflow variant.
2.3. Energy requirement
The energy required for the mixing, is another important factor which needs to be considered during the design of multiphase stirred tank reactor, as the operating cost of the reactor depends on the energy consumption. The energy required is calculated from the power required to rotate the impeller, which is calculated according to equation (1).
P=N ρ N3 D5 (1)
Where P is the power required (W), N is the power
P number, ρ is the density of fluid (kg/m3), N is the speed of rotation (rev/sec) and D is the diameter of the impeller (m). The power number (N ) is the function of im
P peller type and geometry. It is a weak function of reactor geometry and strong function Reynold number, Re for Re<104. The power number (N ) for the given
P impeller geometry can be found from the correlation curves between Power number and Reynolds number.
The energy required for different operations/steps performed by different impeller is summarized in Table 1.
The power required to run the impeller depends upon the type of impeller and its geometric configuration of the tank-impeller combination.
P=φ(μ , ρL, D, T, g, N, impeller geometric param
L eters such as blade width, blade angle, thickness and other geometric details relating to impeller and vessel dimensions). Generally, dimensionless numbers are used to show the relationship.
The different dimensionless numbers used in above expression and that in general used in multiphase reactor design are given in Table 2. These dimensionless numbers help to understand the power consumption more clearly. For example, Froude number (NFr= N2D/g), is unimportant, for power number (N = P/ ρ
N³ D⁵) estimation, when reactor is designed in such a way that there is no vortex formation (i.e. by providing four baffles of width equal to 10% of stirred tank reactor diameter).
The power number (N ) for a given impeller geom
P etry can be found from the correlation curves between Power number and Reynolds number. The different parameters affecting the power number include number of blades, shape, size and alignment (flat/pitched) and flow (upflow and downflow). For e.g. the power number for pitched blade turbine (six blade) downflow impeller is 1.52. For impeller configuration with N > 10,000, the impeller power number (N ) remains
Re P constant[ 3].
Further, the torque requirement of the gear drive required for stirring can be calculated from the power consumption using following equation (3). The torque estimation is useful for selection of gear box, sizing of shaft and is especially a good criteria for flow velocity sensitive operations.
Torque (τ) = Power/2π N=N ρ N2 D⁵/ 2 π (3)
Since the reactor is supposed to perform multiple operations simultaneously and the total energy supplied is distributed for these various operations per- formed by the impeller depending upon its geometry (type of impeller) and its location, plenty of opportunities exist for optimization thru process intensification. 2.4. Scale-up criteria
Reactor design has been one of the most important tasks for any chemical engineer in process industry. Due to the advancement of computational power available as on today, science behind the reactor design has been understood and mathematically modelled in great detail. Still the plant scale design is preceded by pilot scale and lab scale data is used for the design validation mainly due to fear of failure at commercial (large) scale production. Scale up has been covered in great detail in many books and has received excellent reviews[ 6]. The overall conclusion based on
4– the science developed behind the scale-up is based on similarity. Generally, the geometric similarity and kinematic similarity are being considered as the acceptable scale-up criteria.
Geometric similarity: In case of stirred tank, various geometric dimensions can be considered during the design of bench scale or pilot scale plant. Reactor tank height to diameter ratio, impeller diameter to reactor diameter ratio are widely used.
Kinematic similarity: As mentioned in the case of geometric similarity, kinematic factor such as impeller tip velocity is kept constant for bench scale or pilot scale plant. It has been reported that kinematic similarity is a necessary condition but not sufficient.Since, the level of turbulence maintained at small scale is dif-
ficult to achieve at a large scale, hence, instead of tip velocity, power per unit volume is maintained constant.
Different aspects involved in scale up of homogeneous and heterogeneous reaction systems
Gas-Liquid System: Gas-Liquid (G/L) systems exist in a variety of processes used in chemical industry. Hydrogenation, chlorination, ozonation, oxidation, etc are few examples of G/L systems. In addition, many waste water treatment plants utilize large sized gas liquid contactors. Though the design of MAC/stirred tank remain same as discussed in pervious section, the gas - liquid mass transfer coefficient in the system is of paramount importance. Gas-liquid mass transfer coefficient (KLa) is generally rated as a function of power per unit volume and volumetric flow rate of gas. Where k a is the volumetric gas-liquid mass trans
L fer coefficient (s-1), P is the power requirement (W), V is the liquid volume (m3), VG is the superficial gas velocity, constants A, α and β are the regression coefficients which can be obtained from the experimental data.Various researchers have proposed aforementioned correlation with different constant and exponent values. More details can be found in work reported by Yawalkar et al.[ 7].
Gas-Liquid-Solid system: Three phase sparged reactors are widely used in process industry where solid particles are usually used as catalyst or reacting species. Three phase systems with requirement of excellent control over the heat transfer are generally carried out in stirred tank reactors. In addition, stirred tank reactors offer flexibility over the liquid phase residence time which is of utmost importance in process industry.
Design of three phase sparged reactors is based on various parameters; some important points are presented as below:
1. Gas phase holdup in the reactor is mainly governed by bubble diameter and terminal velocities of the bubbles.
2. Particle diameter has strong influence over the fractional gas hold-up.
3. For complete off bottom suspension of solids, the minimum superficial gas velocity must be calculated considering the particle density, concentration, diameter distribution and reactor diameter.
4. Settling velocity of solid particles is much different from terminal settling velocity which must be considered in design calculations. Correlation for solid-liquid mass transfer coefficient have been proposed by various authors
Sh = 2 + A(Re) (Sc) (5)
Where Sh is the Sherwood number, Re is the Reynolds number and Sc is the Schmidt number and constants A, α and β are the regression coefficients which can be obtained from the experimental data.
Details regarding design of three phase sparged reactor design can be found in review by Pandit and Joshi[ 8].
3. Applications of multiphase stirred tank reactor
The multiphase stirred tank reactor has applications in many chemical and allied industries, mining industry, wastewater treatment plants etc.
For example, hydrogenation of organics such as aniline to cyclohexylamine, benzene to cyclohexane etc. are few examples of industrially important reactions carried at a scale of more than 10,000 metric tonnes per annum. In hydrogenation reactions, about 20 – 50% excess hydrogen is supplied due to its low solubility. Hence, in these processes, the complete utilization of
hydrogen is one of the major challenges in design of stirred tank reactor. In conventional reactors it is recycled using external compressors. The stirred tank reactors with gas inducing impellers are found to recycle the hydrogen within the reactor and thus eliminate the recycle step (discussed more in section 4.1.2). This minimizes the capital and operating cost of the process.
Stirred tank reactors are also widely used in biochemical processes such as biomass production (e.g. single cell protein, Baker´s yeast, animal cells, microalgae), for metabolite formation (e.g. organic acids, ethanol, antibiotic, aromatic compounds, pigments), to transform substrates (e.g. steroids) or even for production of an active cell molecule (e.g. enzymes). Some of the important applications of stirred tank reactors in biochemical processes are presented in Table 3.
The different factors to be considered during construction of stirred tank reactor for biological applications include, sterility, aeration, mixing, temperature and ph control. The main reactor design challenge here is to reduce the power cost required for long fermentation periods and to perform uniform mixing in the reactor without any physical damage to the cells. Generally, multi-stage stirred tank reactors are found to be more suitable under these applications (discussed in detail in section 4.1.1).
4. Process intensification opportunities in multiphase stirred tank reactor
Multiphase stirred tank reactors need to be designed to perform the specific operations in the most energy efficient ways. The different steps involved are i) identification of the rate controlling step, ii) Estimation of the fraction impeller power dissipation used for the above iii) Selection/design of the impeller and operating conditions to maximize the performance.
4.1 Stirred tank reactor with modifications in impeller geometries and location
4.1.1 Multi-stage stirred tank reactors
The single stage stirred tank reactor has limitations in terms of liquid and gas phase backmixing. Further, with an increase in tank diameter the power consumption is ineffective in the wall region and gas dispersion becomes lower. These limitations are overcome by using multiple impellers and height to diameter ratio greater than one. The multistage contactors need thinner wall as compared to single stage contactor for the same contactor volume and thus can be used advantageously for high pressure operations. The ratio of the height of each compartment (Hs) to the column diam- eter varies in the range of 0.5 to 1.5. The compartments are usually separated by radial baffles to reduce extent of gas and liquid phase backmixing. Multi-stage stirred tank reactors are mainly used in different industrial applications such as fermentation, crystallization and polymerization.Multiple impeller systems are preferred over single impeller system, in bioreactors where shear sensitivity to micro-organisms is an important criterion for design. Multiple impellers offer lower shear as compared to single impeller systems due to an ability to operate at lower operating impeller speeds and allow the freedom of controlling the dispersed phase hold-up and the residence time over a wide range[ 18, 19].
4.1.2 Stirred tank reactor with gas-inducing impeller
The stirred tank reactor, when used for gas-liquid operation in a semi-batch operation (where gas is continuous phase and liquid is stationary phase) has limitations in terms of limited solubility of gas in liquid phase and hence the per pass conversion of gas is very low. Hence, it is necessary to recycle the unreacted gas back to the reactor, specifically for the case where gas may be highly toxic, expensive or may pose safety problem. Conventionally, the gas is recirculated using linking multiple tanks in series or by providing an
external loop with compressors. However, both these methods require external accessories and increase the fixed as well as operating expenditure.
Stirred tank reactor with gas-inducing impeller overcomes this limitation by re-circulating the gas in the reactor from the headspace of the reactor to the bulk liquid. A typical stirred tank reactor with gas-inducing impeller is shown in Fig. 3. The gas-inducing impeller consists of hollow shaft attached with hollow tube impeller. This reactor is a closed reactor with hollow impeller with hole at the top of the shaft and the tip of the impeller. When rotational speed of impeller increases, and reaches to its critical speed, the kinetic head at the tip of the impeller blade overcomes the static head of the liquid above it. At this rotation speed the gas from the headspace gets injected into the hollow shaft and then transferred to the liquid through hollow impeller. The gas-inducing impeller is operated above the gas-induction speed, so that the gas is distributed and dispersed in the liquid. Generally, a downward flow pitched blade impeller is provided above the gas-inducing impeller to uniformly disperse the gas induced in the bulk liquid. The stirred tank reactor with gas-inducing impeller is found to be useful in different processes such as alkylation, ethoxylation, froth-flotation, hydrogenation, chlorination, ammonolysis and oxidation[ 23].
4.1.3 Some recent advances: Stirred tank reactor with Fractal Impellers
The primary aim of any impeller is to reduce nonuniformities with minimum power consumption. Fractal impellers (FI) is one of the impeller fittings in the aforementioned criteria. Here, the impeller blades are placed in such a manner that instead of sweeping across the fluid, the blades just cut the fluid and hence, reducing the friction throughout the stirred tank. FI design is based on self-similar structure which assist in creating chaotic advection. FI help in achieving uniform distribution of energy throughput the reactor by occupying less than 0.4% of the total volume of the reactor.
In comparison to other impellers (radial or axial), the flow patterns developed by FI are majorly tangential flow in addition to small circulating eddies. Power number (N ) of FI is found to be 0.38 which is
P much lower in comparison to other conventional radial or axial flow impellers i.e rushton turbine (N =
p 6) and pitched bladed turbine (N = 1.84) respective
p ly. Kulkarni et al have studied FI extensively and found that for both solid suspension and gas dispersion FI outperformed the conventional impellers in terms of performance at lower power consumption[ 24].
Stirred tank reactor with hydrodynamic cavitation systems
Hydrodynamic cavitation is emerging as an effective process intensification tool in recent years. In this process, liquid is passed through a constriction such that the local pressure of the liquid drops and is lower than or equal to the vapour pressure. In such condition the liquid partially evaporates, forming cavities which grow as long as the pressure remains low, and eventually collapse downstream when the pressure is recovered. When the cavities collapse, a large amount of energy is released in the form of high temperature and pressure. If the cavity collapses asymmetrically, it also results in formation of a high velocity liquid micro jet, thus generating a large amount of shear[ 25].
Hydrodynamic cavitation can result into physical as well as chemical transformations. The high local temperature generated due to collapse of a cavity results into dissociation of water molecules and formation of highly reac- tive hydroxyl radicals. These radicals have very high oxidation potential and hence cavitation finds an application as an advanced oxidation process. Alternatively, the high velocity microjets also generate a lot of micro-turbulence and high shear, which results in physical breakage of any suspended material. The turbulence also ensures uniform mixing[ 26]. These characteristics of a hydrodynamic cavitation reactor make it a perfect candidate for use along with a stirred tank reactor. A schematic representation of the stirred tank reactor assembly with externally connected hydrodynamic cavitation reactor is as shown in Fig 5.
The mixture from the stirred tank reactor is pumped through the cavitating device and is sent back to the stirred tank, forming a closed loop. The cavitating devices typically used in such arrangement are venturi or orifice plates. Typical applications of such hybrid systems are in preparation of nano-emulsions and in gas-liquid operations.
Nano-Emulsification: A typical stirred tank reactor when used for emulsification process, results in a wide droplet size distribution. Also the droplet size is coarse and is typically in micron range. If the stirred tank reactor is used along with hydrodynamic cavitation reactor, the resultant emulsions have a much sharper size distribution and the droplet size of nano-scale can be obtained easily[ 27]. This is possible due to breakage of the droplets in the cavitation reactor to very fine size. The stirred tank is used as primary reactor where
coarse droplets are formed and the cavitation reactor can then be used to obtain nano-emulsions. The addition of cavitation reactor reduces the process time as droplets are reduced to nano-size within 20-30 passes through the cavitating device. Since the turbulence generated in the cavitation reactor ensures uniform mixing, the power requirement for the stirred tank reactor is also reduced.
Similar approach can also be used in crystallisation process to obtain nano-crystals, where cavitation can be used to reduce the crystal size by increasing the breakage rate.
Gas-Liquid Reactors: In stirred tank reactors with gas-liquid reaction systems, hydrodynamic cavitation reactor can be used as a process intensification tool to increase the efficiency. Cavitating device can break down the gas bubbles as the gas liquid mixture passes through it. This increases the available surface area for the reaction and accelerates the reaction. The microturbulence generated due to cavitation helps in overcoming mass transfer resistances. The high temperature and pressure conditions can also help in providing the activation energy for the reaction and reduce the overall energy requirement of the stirred tank reactor. In case of oxidation reactions, typically in waste water treatment, cavitation along with ozone can be used as an advanced oxidation process for achieving better efficiency.
4.1.4 Stirred tank reactor with draft tube cum heat exchanger
In case of gas-solid-liquid or solid-liquid reactions, the density of solid phase may be lower than the liquid phase resulting in floating of the solid phase. In case of light weight solid (reactant or catalyst), the nonuniform mixing of solids results in localized exothermic hot spots (popcorn -type bursting followed by gas emission).
In order to avoid such conditions, the draft tube stirred tank reactors (Fig. 6) are much efficient. Here axial flow impellers are generally used and solids are added near the eye of the impeller. The impeller sucks the fluid from the annulus into the draft tube and then back into the annulus. The low-density solid particles are transported downward into the draft tube and then into the annulus. This results in circulation and uniform mixing of lighter weight solid particles. The arrangement is also found to avoid impeller flooding.
Stirred tank reactor with draft tube cum heat exchanger are found to eliminate the refrigeration cost, reduces the overall operating cost at least by 50% and found to give stable and safe operation of the reactor. These are found to be useful in crystallization (draft tube crystallizer), fermentation (draft tube fermenter/ propeller loop reactor), esterification and transesterification (ion exchange resin catalyzed reactions) .
Multiphase stirred tank reactors are widely used in industry due to their high heat and mass transfer coefficients, wide range of liquid phase residence times and capability of handling wide range of superficial dispersed phase gas velocities.
The requirement of multiple objectives from the existing stirred tank reactors in the industry in terms of gas dispersion, solid suspension and gas-liquid-solid contacting etc, from the existing stirred tank reactor gives rise to design optimization and process intensification opportunities.
The different stages involved in the design of stirred tank reactor for specific operation includes, determination of rate limiting step, estimation of power dissipation required for this step and then selection/design of impeller and operating conditions to maximize the reactor performance.
The process intensification can be carried out in the existing stirred tank reactor by designing novel tank and impeller geometries or by providing the external accessories to enhance the performance in terms of increase in heat and mass transfer rates.
Process intensification based on design of reactorimpeller geometry include multi-stage stirred tank reactor (multi-impeller reactor), stirred tank with gas-inducing impeller and recent advances like stirred tank with fractal impellers. Stirred tank reactors with multiimpeller system are found to be effective in fermentation, crystallization and polymerization reactions. The stirred tank reactors with gas-inducing impeller are found to be effective in hydrogenation, alkylation and oxidation reactions. Use of fractal impellers have the potential to reduce the power consumption per unit volume of the liquid.
Process intensification using external accessories includes, combination of stirred tank reactor with cavitating device (flow constriction device in the form of venturi, orifice or valve) and stirred tank with draft tube cum heat exchanger. Stirred tank reactor with hydrodynamic cavitation are found to be effective for preparation of nano-emulsion, crystallization and wastewater treatment using advanced oxidation processes. The stirred tank reactor with draft tube is found to be effective in highly exothermic reactions.
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Fig. 1 Schematic of a typical stirred tank reactor geometry
Table 2 Different dimensionless numbers and their physical significance
Three phase sparged reactors are widely used in process industry where solid particles are usually used as catalyst or reacting species. Three phase systems with requirement of excellent control over the heat transfer are generally carried out in stirred tank reactors. In addition, stirred tank reactors offer flexibility over the liquid phase residence time which is of utmost importance in process industry. Table 3. Applications of stirred tank reactor in biochemical processes
Fig. 2. Multistage stirred tank reactor[ 19, 20] compartment of stirred tank reactor
Fig. 3 Schematic of stirred tank reactor with gas-inducing impeller Hydrodynamic cavitation is emerging as an effective process intensification tool in recent years. Hydrodynamic cavitation can result into physical as well as chemical transformations.
Fig. 4 Image of (a) fractal impeller and (b) stirred tank with fractal impeller[ 24]
Fig. 6 Stirred tank reactor with draft tube cum heat exchanger
Fig. 5 Stirred tank reactor with hydrodynamic cavitation system