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Comment PDF Processing & Handling

Photochemical Processes in Stirred Tank Reactors

By Werner Himmelsbach, Peter Rojan, Benjamin Multner, Wolfgang Last, André M. Braun, Alexander Peschl |

A general overview of industrial photochemistry and the design of large-scale stirred-tank photochemical reactors

Photochemical processes complement or may even substitute conventional (thermal) processes. The radiant energy initiates the reaction, for example by forming radicals, and in most cases, no thermal energy is needed to overcome the energy barrier according to Arrhenius’ law. Photochemical reactions are usually kept well below 100°C, hence leading to far fewer side reactions and formation of byproducts than thermal reactions. Except for photocatalyzed processes, no expensive catalysts with their elaborate handling requirements are needed.

In most cases, photochemical processes are performed in immersion-type tubular reactors in which turbulence is achieved and maintained by circulating the reaction system or by the production or the introduction of gases. Only recently have stirred-tank reactors — still the “workhorse” of the chemical industry — been adapted to the requirements of photochemical processes (Figure 1). This article describes the basics of industrial photochemistry and the present state-of-the-art for the design of stirred-tank photochemical reactors.

photochemical processes

Figure 1. An inside view of a stirred-tank photochemical reactor (see also the cover photo for an impression of the size)
Ekato/Peschl

Nomenclature

nomenclature

Photochemistry

For a conventional (thermal) chemical reaction, the rate, r, is given by the concentration cx of the reactants A and B, and the rate coefficient k, which is intrinsic to a specific reaction, as given by Equation (1):

 

1  (1)

 

Where k follows Arrhenius’ law (a list of all nomenclature is given in the box above):

2  (2)

 

At ambient temperature, k is too small for most reactions to get them started. To raise it to an economic rate, the temperature T must be increased. Therefore, most industrial reactions run at higher temperatures. High temperatures not only enhance the desired reaction, but also side reactions, leading to undesirable byproducts or to thermal decomposition of reactants, intermediates and products. Both entail costs for starting materials, waste treatment and an increased effort to purify the product.

Catalysts allow high conversion rates at lower temperatures as they reduce the activation energy, EA. But catalysts involve costs for their procurement or make up, handling, feeding, separation and recovery or disposal.

Process initiation by radiant energy allows for running reactions independently of temperature and in most cases, well below 100°C and without a catalyst. Photocatalysts for specific reactions are not used to decrease EA, but to impose a specific mechanism of chemical transformation.

The absorption of electromagnetic radiation of an appropriate wavelength promotes the reactant molecule from its ground state to an electronically excited state, from which it subsequently undergoes a mostly specific chemical transformation to a stable product or to a reactive intermediate capable of initiating a second thermal reaction. The most relevant industrial photochemical processes and their primary mechanisms are listed in Table 1 [1].

t1

A description of the immense diversity of the reaction mechanisms would be beyond the scope of this article. For detailed information, the interested reader is referred to the literature [1–9]. Representative for the predominant radical mechanisms is the photochemical halogenation: electronic excitation of ground-state molecules Cl2 or Br2 leads to a dissociative state generating the respective radicals Cl· or Br· that may add to C=C double bonds or abstract a hydrogen atom [1]. The latter is shown in Figure 2.

 

Figure 2.  In the reaction mechanism of chlorination, light is first absorbed by Cl2 to form Cl. radicals, which subsequently substitute a hydrogen in a hydrocarbon compound

Figure 2. In the reaction mechanism of chlorination, light is first absorbed by Cl2 to form Cl. radicals, which subsequently substitute a hydrogen in a hydrocarbon compound

Stirred-tank photochemical reactors

Large-scale industrial applications of photochemical reactions have been known for more than 100 years, when first attempts to up-scale photochemical chlorinations were rendered possible by the development of mercury vapor lamps [ 10]. Immersion-type photochemical reactors [1] were conceived to profit from the radiation emitted 360 deg radially from cylindrically shaped light sources. Such processes required modest knowledge in the domain of photochemistry, but proved to be a real challenge for engineers. Challenges include concerns primarily over the corrosion-resistant materials (for example, in the case of chlorinations), the new design of mechanically resistant quartz-metal interfaces and the design of new concepts ensuring high-turbulent conditions in the presence of fragile lamp installations. Particular attention had to be devoted to handling the exothermicity of chain reactions. The progress in the domains of physical and theoretical chemistry also fostered the understanding of the fundamentals of photophysics and photochemistry and opened a wide field of applications.

The present status of photochemical engineering provides technical and ecological answers for practically all up-coming projects and also allows a better economic analysis and control of costs of production and maintenance. Such viable answers need the cooperation of dedicated specialists in the domains of photochemical, materials, chemical and mechanical engineering.

A new concept of a stirred-tank photochemical reactor (Figure 3) has been developed mainly for multi-phase reactions (Table 1). It can be operated batch-wise, as fed-batch or continuously as a continuous stirred tank reactor (CSTR). The aspect ratio H/TD of the tanks is 1 to 1.5, reactor volumes range from 1 to 50 m3. Mixing provides the circulation of the substrate to the radiation source, the turbulence to disperse and dissolve a reactive gas and prevents solids from sedimentation. The glass or quartz wells housing the radiation sources are inserted through nozzles on the tank head and fixed to the wall or to the tank bottom. The upper end of the well on top of the tank nozzle has connecting devices for the electric power and the cooling medium of the radiation sources. The number of radiation sources is restricted by space constraints on the tank head; large tanks may contain up to 20 lamps. The heat of the reaction is removed by cooling through the tank wall with jacket or half pipes. If that is not sufficient, additional coils can be installed within the space between the wells and the wall. Cooling by evaporating solvents could be another alternative.

Figure 3.  This stirred-tank photochemical reactor incorporates eight light sources Ekato/Peschl

Figure 3. This stirred-tank photochemical reactor incorporates eight light sources
Ekato/Peschl

Photochemical process development usually starts with laboratory-size equipment, including commercially available reactors with which homogeneous liquid reaction systems, as well as heterogeneous gas-liquid, liquid-solid and gas-liquid-solid systems may be investigated (Figure 4). The objective of laboratory experiments is twofold: 1) choosing and optimizing the unit equipment and, in particular, the geometry of the photochemical reactor; and 2) investigating optimal conditions in terms of rate and specificity of the process.

Fig 3a

Figure 4. Laboratory-scale units can be used for the investigation and optimization of the specificity and rate of a photochemical process

Figure 4. Laboratory-scale units can be used for the investigation and optimization of the specificity and rate of a photochemical process

Absorption spectra of the photochemical reactant and the substrate mixture allow one to determine the wavelength(s) of excitation and to choose the corresponding radiation source. The selection of the latter and its power depends on the required range of excitation, the specificity of the reaction and the projected mass of production per unit of time.

The quantum yield is an additional design parameter to be considered when there are restrictions in the scaleup, for instance the geometrical parameters or the electrical power. The quantum yield represents the efficiency of the irradiation, and can be defined as the number of moles of reactant converted per number of moles of photons absorbed over the same time period and in the reference volume.

Additional parameters to consider are the residence time within the irradiated volume, the spectral range of excitation, temperature, substrate concentrations, solvent effects and reaction kinetics — also of subsequent thermal reactions and safety considerations.

The scaleup to and the design of the production plant on the basis of such laboratory testing involves, in most cases, further investigations in pilot reactors with typical volumes of 20 to 200 L. The engineer must consider and tune the following parameters, which are described in the following sections:

  • Adaptation of the photochemical reactor to the type and number of radiation source originally chosen and its/their electrical power calculated from the exitance necessary to produce the projected mass of product per unit of time
  • Agitation
    -Optimized residence time in the irradiated reactor volume for homogeneous reaction systems and high circulation rates of particles and slurries to thelight source to compensate for the short penetration depth of radiation
    -Optimized gas dispersion and gas-liquid mass transfer
    -Maintenance of suspension of solids
  • Heat transfer (reaction enthalpy and radiation sources) to maintain the process temperature
  • Mechanical integrity of the wells housing the radiation sources
  • Corrosion-resistant materials of construction
  • Safety aspects concerning radiation, radiation sources, corrosion and toxicity of chemicals

 

Irradiation and absorption

A new and reliable design of an agitated photochemical reactor with an elaborated concept of agitation and fixing of the wells with corrosion-resistant materials is, at present, the answer to an increasing demand for the processes summarized in Table 1. Figure 3 depicts the arrangement of a given number of light sources inserted into the required cooling and protection tubes (wells) that are located and secured among each other as well as connected to the reactor wall. Given the fact of a relatively limited chain reaction, as in the case of the chlorination of polymer granulates (for example, chlorinated polyvinyl chloride; C-PVC), and the rather low quantum efficiencies of most of the investigated photochemical reactions in heterogeneous media, a rather high incident radiant power density, I0, is required for a large production scale.

Up to the present, mercury (Hg) medium-pressure arcs are mostly used for the excitation within the ultraviolet and visible (UV/VIS) spectral domain of wavelengths >250 nm [1]. Because of their operating temperature of approximately 900°C, water-cooling and in some cases the implementation of insulating vacuum spaces are necessary.

Hg-arcs are contained in cylinders of borosilicate glass or quartz and emit over their entire circumference. They provide a broad line-spectrum with peaks of various intensities (Figure 5). Some light-emitting diodes (LEDs) emit quasi-monochromatic radiation (Figure 5) and may be used for photochemical processes, where high reaction specificity is required. The substitution of Hg-arcs exhibiting a radiant efficiency of approximately 30% by LED-arrays for large-scale applications needs further development, mainly for reasons of their implementation in wells, their electronic controls and the cooling of multiple and large arrays. Their unilateral emission and arrangement on a support does not allow for a quasi-homogeneous emission over the entire circumference of the well.

Figure 5. The emission spectrum of a medium-pressure mercury arc lamp is shown in blue. A light-emitting diode (red curve) emits quasi-monochromatically, here at 365 nm (ultraviolet)

Figure 5. The emission spectrum of a medium-pressure mercury arc lamp is shown in blue. A light-emitting diode (red curve) emits quasi-monochromatically, here at 365 nm (ultraviolet)

Commercially available medium-pressure Hg arcs are standardized in size, and have electric power in a range of 5 to 60 kW (Figure 6). Specific know-how is needed for the design of the electric power connection and supply, as well as for the cooling medium passing through the head of the well, which is bolted onto the tank nozzle.

Fig 5a

Fig 5b

Figure 6. Medium-pressure Hg-arcs are available with different electrical power ratings (top). The photo above shows the well head of a 25 kW, medium-pressure Hg-arc for the connections to the electric power supply, cooling medium and inert gas Peschl Ultraviolet

Even if the outer protection tube (made of glass or quartz) with its connection to the tank is a reliable and proven component, its breakage should be considered in the risk analysis. As the components of the reaction system are usually hazardous or corrosive (or both), such a rare event has to be detected by monitoring, and the leakage must automatically be collected in a containment. Those emergency procedures are fixed in detail through the HAZOP (hazard and operability study) of the reactor.

Relevant for the design of a photochemical reactor is the fact that the incident radial power, I0, is not constant throughout the reactor volume containing radiation-absorbing components. Its attenuation is due to chromophores, that is, functional groups of molecules absorbing radiation of given wavelengths [1, 2] and resulting in its exponential decrease with distance x from the source within the absorbing medium, as described by the Lambert-Beer law, Equation (3):

 

3 (3)

 

The drop of the relative radiant power, I/ I 0, with distance x from the entrance of the radiation is shown in Figure 6. The molecule-specific molar absorption coefficient, κ, depends on wavelength, λ. The concentration, c, of the light-absorbing substance is simplisticly assumed to be constant over the whole distance of the optical path. The spectral absorbance B(λ) is an additive property, which means for mixtures containing more than one absorbing compound, B(λ) is expressed as:

4 (4)

 

Usually, decadic molar absorption coefficients ε(λ) are determined in UV/VIS spectrophotometers and can be transformed into Naperian molar absorption coefficients by κ = 2.303 . ε.

Figure 7. This graph of the Lambert-Beer law shows the decrease in radiant power with distance x due to absorption (c2 > c1)

Figure 7. This graph of the Lambert-Beer law shows the decrease in radiant power with distance x due to absorption (c2 > c1)

Figure 7 demonstrates, in principle, one of the most important obstacles to be surmounted while up-scaling photochemical reactions: depending on B(λ) of the substrates and products, either their concentration or the reactor volume exposed to radiation could be severely limited in order to irradiate the total volume of the reactor. With lifetimes of electronically excited molecules of 10–6 to 10–12 s, moving these excited molecules by convective transport toward the non-irradiated volume of the reactor is impossible. Nevertheless, these constraints may be avoided or at least lessened, in particular in the scaleup of photochemical processes in stirred-tank reactors by the following measures:

  • Adapting the concentration(s) of the absorbing component(s) of the reaction system to the given optical path of the reactor
  • Increasing the irradiated volume with a larger diameter of the well, maintaining its optical path, adapting the radiant power density of the radiation source and optimizing the flux of the reaction system
  • Increasing the number of radiation sources, with the maximum being defined by the space on the tank head, maintaining their exitance
  • Adapting the agitator design for maximum and homogeneous pumping efficiency with frequent renewal of the fluid in the irradiated zones
  • Multiplying the number of reactors (numbering up versus scaling up)

Evidently, Lambert-Beer’s law cannot be applied to heterogeneous reaction systems, because the incident radiation will not only be absorbed, but also scattered and reflected by particles or bubbles. In heterogeneous media, the incident radiant power is attenuated with distance x, according to Equation (5):

 

5 (5)

 

Where E(λ) is the extinction coefficient, usually defined as the sum of the absorption Κ(λ) and scattering S(λ) coefficients:

 

6 (6)

 

For dispersions, Κ(λ) and S(λ) may be determined experimentally by spectrophotometric methods [11, 12].

 

Stirred-tank reactor

Mixing is important to move the reactants into, and to remove reaction intermediates and products of the overall reaction from, the irradiated volume. The number of contacts z of a fluid element with the irradiated zone correlates with the impeller pumping rate, q˙,and the volume of the tank, V, as given by Equation (7):

 

7 (7)

 

For scaleup with geometric similarity and constant specific impeller power per volume P/V, the pumping rate per volume is reduced with reactor size, represented by the tank diameter TD as length scale factor [13]

 

8 (8)

 

Consequently, by increasing the reactor volume, all of the fluid elements pass through the irradiated volume less frequently, and the photochemical reaction progresses more slowly. This decrease of the overall reaction rate can be partially compensated: the same impeller type with a larger impeller diameter D provides larger pumping rates even when operated at lower speed to comply with the same shaft power.

 

9 (9)

 

Another option to increase the irradiated volume would be the extension of the well’s circumference combined with an increase of radiant power to keep the exitance constant. Finally, the irradiated volume may also be increased by raising the number of radiation sources.

Particular attention must be paid to the hydraulic load on the wells created by intense or vigorous agitation. The oncoming flow generates pressure, p, on the quartz or borosilicate tubes that may be estimated by Equation (10) [14].

10 (10)

 

This pressure is not constant, it fluctuates substantially around its mean value due to the velocity fluctuations of the turbulent flow. Local velocities may be calculated by computational fluid dynamics (CFD), and the corresponding flow simulation shows their variation in the vicinity of the tubes (Figure 8). CFD may also be used to optimize the impeller arrangement to equalize the flow around the inserted wells avoiding stagnant zones.

Figure 8.  Shown here are horizontal sections of flow velocities at the well housing of radiation sources in a stirred-tank photochemical reactor

Figure 8. Shown here are horizontal sections of flow velocities at the well housing of radiation sources in a stirred-tank photochemical reactor Ekato

Additionally, the high energetic vortices from the turbulence spectrum, vortex shedding according to the Karman principle and blade passing frequencies of the impeller cause the lamp units to vibrate at its resonance frequency, if it coincides with the exciting frequency [13]. The vortex shedding frequency, f, may be evaluated from the inflow velocity v, the tube diameter d and the Strouhal number Sr [15]:

 

11 (11)

 

The Strouhal number assumes characteristic values for different geometric forms, such as cylinders. The blade passing frequency fB is calculated from the impeller speed, n, and the number of impeller blades, zB:

12 (12)

 

The concept used for the fixation and stabilization of the well’s housing ensures that the natural frequency of the tubes will be sufficiently higher than the exciting frequencies. Calculations by finite element analysis (FEA) and modal analysis must also take into account that the fluid-structure coupling leads to conditions where the liquid surroundings of the wells reduce the natural frequencies to approximately half [13]. The challenge linked to the construction of the fixation is threefold:

  1. The well holders must be designed in a way that gaps between holder and tube are excluded to avoid space for vibrations.
  2. Stress on the mounted quartz or glass tubes must be excluded, even if the tubes are not perfectly straight upon delivery and nozzles and holders are not perfectly aligned due to tolerances in the tank manufacturing process.
  3. The support must allow for the thermal dilatation of the tubes without stress. 

Agitator design

Beside the permanent renewal of the fluid within the irradiated zones, as described above, stirring of the reaction system must accomplish the following fundamental tasks:

Blending. In order to ensure equal physical and chemical reaction conditions in all areas of the tank, temperature and concentration gradients must be balanced. In the turbulent flow range, as is always the case in this type of reactor, the blend time number, that is, the product of stirrer speed, n, and blend time, θ, is constant [13]:

 

13 (13)

 

Identical mixing times and homogeneities on all operational scales (laboratory, pilot, production) are only achieved if n is kept constant for geometrically similar designs of the reactor unit. However, if n is kept constant, the required impeller power, P, would grow uneconomically high with P∝ D5, as seen by Equation (14):

 

14 (14)

 

Longer mixing times at lower speed would hence result, but can at least partially be compensated by a reasonable combination of type and number of impellers that would match with the impeding effects of the tank internals. The concept of the blend time fits exactly in with the model of the impeller pumping rate [see Equations (7–9)]: in order to achieve a defined degree of homogeneity, a defined number of re-circulations of the tank content is required.

Suspending. Educts, catalysts and products of photochemical processes are often solid components (for example, C-PVC, TiO2, oximes or polymer particles) that must be prevented from settling and be distributed as homogeneously as possible within the tank. The settling rate of the solids together with the solids concentration is relevant for the design of the agitator. The concentration of solids, their particle-size distribution, the liquid viscosity and the density difference between solids and liquid must therefore be known. Organic solids exhibit generally low densities (for instance, ~1,400 kg/m3 for PVC and 1,500–1,600 kg/m3 for C-PVC, depending on product specifications [16]) and their suspending should not be a critical issue.

Floating may at times be observed in the case of poorly wetting solids during concurrent gassing or de-gassing of a reaction byproduct, such as HCl in chlorinations (Figure 2). Since solid particles adhere to gas bubbles, they may rise to the liquid surface and develop with the foam into a stable layer. A surface impeller would break up the foam and minimize the layer by forcing the particles down into the turbulent reaction system.

Gas dispersion. A large fraction of the photochemical processes are based on the photochemical reactivity of bromine, chlorine, nitrosyl chloride, oxygen or sulfur dioxide that may be introduced into the reaction systems in liquid or in gaseous form. Gases must be dispersed as finely as possible to maximize the rate of dissolution in a given solvent or solvent mixture. The combined gassing system, as depicted in Figure 9, provides optimum mass transfer [17]: controlled by the tank pressure, the reactive gas may be injected and spread underneath the primary disperser. The fraction of the gas that is not immediately dissolved rises into the head-space of the reactor above the liquid level and is drawn back into the liquid by way of the upper self-inducing turbine. For slow photochemical reactions, the self-inducing turbine would suffice on its own to achieve sufficient transfer rates if the gas would be fed into the reactor from the top of the tank (Figure 3). Such an arrangement would reduce expensive piping of corrosion-resistant material within the reactor.

Figure 9. The combined gassing impellers provide maximum mass transfer with the primary disperser (Phasejet) below the self-inducing turbine (Gasjet) Ekato

Figure 9. The combined gassing impellers provide maximum mass transfer with the primary disperser (Phasejet) below the self-inducing turbine (Gasjet)
Ekato

Heat transfer. Heat from the exothermic processes, from the stirring and emitted by the radiation sources (despite cooling by transparent liquids circulating within the wells), must be removed from the reactor through the tank wall with a jacket or half-pipe coils. However, with low reaction temperature, the driving temperature gradient with the coolant is rather low. Installation of heat-exchangers within the reactor is possible, but limited by the space required for the lamps and by the additional costs for corrosion-resistant materials. Chilled water or cooling brine may be used to collect the residual heat of the process. The heat-transfer coefficient for the stirred side is calculated based on the known dimensionless numbers contained in Equation (15):

15 (15)

 

Where:

Nu = Nusselt number

Re = Reynolds Number

Pr = Prandtl number

Geo = factor considering the impeller type and the geometry of the heat-exchanging surface in case of cooling coils, as laid out in detail in Ref. 13.

 

Mechanical design and safety

Many substances involved in photochemical reactions (Table 1) are highly corrosive, which precludes the use of conventional stainless steels. The material of construction of the product-wetted parts of the agitator (impeller, shaft, mounting flange and mechanical seal) and lamp holders must be, for example, nickel-based alloys, such as Hastelloy, or titanium. Tanks may be made of, or cladded with, these materials, or made of glass-lined carbon steel.

The measures for a safe operation of the glass or quartz wells with the lamps have been described previously. Given the toxic and corrosive properties of some of these reaction systems and the emanating liquids, gases or aerosols, the sealing of the rotating agitator shaft is a fundamental safety element [18]. Even if pressure and temperature levels are moderate during most photochemical processes, only double-acting mechanical seals or hermetically sealed magnetic couplings are suitable options to ensure complete separation of the reactor content from the environment.

The double-acting seals hold a sealed chamber between two seal-ring pairs that is filled with a barrier liquid (Figure 10). Pressurization of the latter creates a secure separation between process space and the environment. The pressurized barrier liquid provides a hydrodynamic lubrication film between the surfaces of the rotating and the stationary rings, thus providing the necessary sealing function. As long as the pressure in the sealing chamber exceeds the pressure of the chemical process, the content of the tank cannot pass through the mechanical seal. In the rather unlikely case of a total failure of one of the pair of rings, the subsequent seal pressure drop would lead to immediate shutdown procedures. Until completion of such a procedure, the second seal pair will assume the seal function.

Figure 10. Shown here is a double-acting mechanical seal with pressure compensator, filling level monitor and central supply unit for multiple seals [13]

Figure 10. Shown here is a double-acting mechanical seal with pressure compensator, filling level monitor and central supply unit for multiple seals [13]

The pressure within the seal chamber can be held constant with pumps and pressure accumulators overlaid with gas. Alternatively, the pressure of the seal-liquid may be set to a defined value by a pressure compensator, as shown in Figure 10. The pressure compensator is equipped with a piston and exposed on one side to the reactor pressure. The piston area on the opposite side is reduced by the cross section of the piston rod, thus creating a higher pressure on the seal liquid when in a balanced state. The pressure ratio corresponds to the ratio of the two piston areas. With such a device, the seal pressure is always maintained at a higher level than that of the chemical process, without external energy, measurement or control devices.

The cylinder position can be transmitted to the process control system hence allowing for automatic refill of the barrier liquid in case of operational leakages in the range of the level switch position low/high (level switch low/level switch high; LSL/LSH). Monitoring the frequency of refills provides information about the state of wear of the seals. One refill unit can supply several agitators. The redundant switches (level switch low low/level switch high high; LSLL/LSHH) provide additional safety for this fundamental function.

 

Concluding remarks

Photochemical processes break customary rules of reactor engineering and design and have opened new or alternative pathways for the synthesis of new compounds and materials and their production on an industrial scale. New concepts of stirred-tank photochemical reactors, of the well fixation, as well as of new material combinations provide for their use in particular for processes starting in, or resulting in, a heterogeneous phase. The stirred-tank reactor, with its robust and flexible operation characteristics, also allows for batch, fed-batch and continuous operation.

Modern tools, such as numerical flow simulations (CFD), optimize the mixing process for multi-phase fluids, and finite element analysis (FEA) provides a reliable and economic design, even for critical materials, such as glass and highly corrosion-resistant alloys. Apart from new reactor designs, photochemical technology has made fast and profound progress in the domains of radiation sources and in situ photometric analysis. On a production scale, operational safety aspects — techniques of lamp fixation and sealing, electrical power control, monitoring of the process media and explosion-proof design — withstand any critical review.

References

1. Braun, A.M., Maurette, M.-T. and Oliveros, E., “Photochemical Technology,” Wiley, Chichester, U.K., 1991.

2. Turro, N.J., “Modern Molecular Photochemistry,” University Science Books, Mill Valley, Calif., 1991.

3. Gonzalez, M.C., Oliveros, E., Wörner, M. and Braun, A.M., Vacuum-ultraviolet Photolysis of Aqueous Reaction Systems, J. Photochem. Photobiol. C: Reviews, 5, pp. 225–246, 2004.

4. Herrmann, W.A., ed., Organic Peroxygen Chemistry, Top. Curr. Chem., 164, 1993.

5 Chen, M., Zhong, M., and Johnson, J.A., Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications, Chem. Rev., 116, pp. 10,167–10,211, 2016.

6. Braun, A.M. and Oliveros, E., Applications of Singlet Oxygen Reactions: Mechanistic and Kinetic Investigations, Pure Appl.Chem., 62, pp. 1,467–1,475, 1990.

7. Schmidt, R., Photosensitized Generation of Singlet Oxygen, Photochem. Photobiol., 82, pp. 1,161–1,177, 2006.

8. McKerrall, S., Singlet Oxygen in Organic Synthesis, www.scripps.edu/ baran/ images/grpmtgpdf/McKerrall_Mar_11.pdf.

9. Sangermano, M., Roppolo, I., and Chiappone, A., New Horizons in Cationic Photopolymerization, Polymers, 10, pp. 136–143, 2018.

10. Haberland, H.and Schaefer, W., Process of Making Penta- and Hexachlorethanes, U.S. Patent 1036224 A, Salzbergwerk Neu-Strassfurt und Teilnehmer, February 24, 1912.

11. Satu, M.L., Brand, R.J., Cassano, A.E., and Alfano, O.M., Experimental Method to Evaluate the Optical Properties of Aqueous Titanium Dioxide Suspensions, Ind. Eng. Chem. Res., 44, pp. 6,643–6,649, 2005.

12. Bohren,C.F. and Huffman, D.R., “Absorption and Scattering of Light by Small Particles,” Wiley, Weinheim, Germany, 2007.

13. Ekato, “The Handbook of Mixing Technology,” Ekato Holding GmbH, Schopfheim, Germany, 2012.

14. Idelchik, I.E., “Handbook of Hydraulic Resistance,” Jaico Publishing, Mumbai, India, 2008.

15. Stadtaus, M., H.Weiss, H., Himmelsbach, W. and Smith, J., Mechanical Design Aspects for High-Performance Agitated Reactors, Chem. Eng., April 2010, pp. 38–47.

16. Boedeker Plastics, Inc., Shiner, Tex., PVC (polyvinyl chloride) and C-PVC (chlorinated PVC) Specifications, www.boedeker.com/pvc_p.htm, accessed May 2018.

17. Himmelsbach, W. and others, Increase Productivity Through Better Gas-Liquid Mixing, Chem. Eng., October 2007, pp. 50–58.

18. Himmelsbach,W., and Krebs, R., Betriebssicherheit von Rührwerksanlagen,Chemie Ingenieur Technik, pp. 423–437, 2014.

Authors

HimmelsbachWerner Himmelsbach is senior consultant of EKATO RMTs R&D Dept. (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Phone +49-7622-29-227; Email: werner.himmelsbach@ekato.com). He has 35 years of experience in process design and development, plant design and maintenance, having previously worked for major international manufacturers of specialty chemicals and pharmaceuticals. Himmelsbach holds a M.S.Ch.E from the University of Karlsruhe, Germany.

RojanPeter Rojan is head of EKATO RMT’s Process Plant Solutions Sales Group (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Phone +49-7622-29-529; Email: peter.rojan@ekato.com). He has more than 20 years of experience with the international pharmaceutical and chemical industry and the sales of process plant equipment worldwide. Rojan previously worked for major engineering companies in the fields of mechanical and thermal separation processes. He holds a M.S.Ch.E from the University of Karlsruhe, Germany.

MultnerBenjamin Multner is head of the Mechanical Development of EKATO RMTs R&D Dept. (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Phone +49-7622-29-309; Email: benjamin.multner@ekato.com). He has 13 years of experience in mechanical development and design of process equipment. Multner holds a B.Eng. from the Cooperative State University of Loerrach (Germany).

Last-WolfgangWolfgang Last is head of EKATO RMTs R&D Dept. (Hohe Flum Strasse 37, D-79650 Schopfheim, Germany; Phone +49-7622-29-220; Email: wolfgang.last@ekato.com). He has 18 years of experience in process design, engineering, startup and production, having previously worked for major international manufacturers of specialty chemicals and solar silicon. Last obtained his Ph.D. in chemical engineering from the Technical University of Munich, Germany.

BraunAndré M. Braun is retired chair of the University of Karlsruhe (Germany) and head of consultant office Quantapplic (Weberstrasse 19, 55310 Mainz, Germany; Email: andre.braun@mac.com). After obtaining his Ph.D. in physical organic chemistry from the University of Basel, Switzerland, he focused on mechanistic and polymer photochemistry during his postdoctoral training at the California Institute of Technology (Pasadena, Calif.) and at Yale University (New Haven, Conn.). He worked for Ciba-Geigy AG (Basel, Switzerland) and was Privatdozent and head of a research group at the Ecole Polytechnique Fédérale de Lausanne (EPFL, Switzerland). He is a honorary doctor of the Universidad Nacional del Litoral (Santa Fe, Argentina) and a honorary professor of the Universidad Nacional de La Plata (Argentina).

PeschlAlexander Peschl is CEO of Peschl Ultraviolet GmbH, CEO of proQuarz GmbH and head of the consultant office Peschl UV-Consulting (Weberstrasse 19, D-55130 Mainz, Germany; Phone +49-6131-143-845-12; Email: a.peschl@peschl-ultraviolet.com). He has 19 years of experience building commercial-scale photoreactors, irradiation units, as well as special quartz glass. He worked for major manufacturers of fine chemicals and pharmaceuticals world-wide as a recognized specialist for applied photochemistry.

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ABB Ability™ technology to transform BASF rotating equipment into intelligent machinery and improve uptime and reliability
Detect and correct anomalies early in your batch processes
ABB at ACHEMA 2018
The Future of Project Execution
Digitalization: What does it deliver today… and tomorrow?

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