Static mixers: Effective means for intensifying mass transfer limited reactions
Introduction
In order to improve sustainability, the chemical process industry is rapidly moving toward cleaner synthesis, reduced environmental impact, improved energy efficiency, and the use of smaller and safer multifunctional process plants. Process intensification (PI) is one of the most effective approaches by which these objectives can be accomplished. It relies on the use of innovative approaches to achieve dramatic reductions in the size of the plant needed to achieve a certain production capacity, as well as achieving enhanced process performance. For example, it was reported that up to 72-fold reduction in reactor volume was achieved by applying process intensification to industrial situations while, simultaneously decreasing byproduct formation by about 75% [1]. Similarly, the substitution of a batch reactor handling viscous material with continuous flow through a static mixer resulted in a 15,000-fold reduction in residence time while simultaneously reducing the energy consumption by a factor of 18 [2].
One of the most effective PI approaches is based on matching the fluid dynamic conditions of the processing unit to the chemical/biological reaction requirements in order to enhance the reaction rate, improve selectivity, and minimize by-product formation. This approach is particularly effective in multiphase systems where the need to transfer material and/or energy across the interface between phases can often be the main factor affecting the overall performance of many industrial operations such as: absorption, distillation, liquid–liquid extraction, multiphase reactions, production of biodiesel, direct contact heat exchangers, stripping of VOC, aerobic wastewater treatment, ozone disinfection, and high-temperature catalytic oxidation. Unfortunately, our ability to design effective multiphase contactors, or even predict the performance of such units, is limited by inadequate understanding of the factors affecting bubble/drop breakage and coalescence and the absence of tools by which the performance of multiphase systems can be accurately predicted [3]. For example, it is estimated that less than 2% of the energy input to most present day contactors is utilized to form liquid–liquid and gas–liquid dispersions and maintain inter-phase contact [4].
The mass transfer limitations encountered in many multiphase reactions are usually overcome by increasing the energy input to the reactor (typically a mechanically agitated tank, a bubble column, or a gas-lift reactor) which results in the formation of more finely dispersed phase entities (bubbles and/or drops). This results in increasing the specific interfacial area of contact between the phases, a, and the mass transfer coefficient, K. The value of the latter depends on the interfacial mobility/stability, turbulence intensity, and the size of the bubbles/drops formed [5]. In most conventional liquid–liquid contactors, the use of this approach is, however, limited in the case of immiscible liquid systems because of the tendency to form difficult-to-separate dispersions with broad drop size distributions.
Most multiphase reactions involving immiscible liquids are traditionally conducted in mechanically agitated tanks (MAT), which exhibit complex hydrodynamic characteristics. The performance of these units is complicated by the fact that they have a very non-uniform spatial distribution of local energy dissipation rates with the liquids circulating in a non-controlled fashion between regions of high and low energy dissipation rates [2], [6], [7], [8]. This results in reducing the concentration driving force responsible for mass transfer and in the formation of broad drop size distributions. On the other hand, the conditions necessary for intensifying mass transfer limited reactions are approached when plug flow reactors equipped with screen-type static mixing elements are used. It is well known that, depending on the reaction order and the desired degree of conversion, the average concentration driving force responsible for mass transfer can be enhanced by an order of magnitude when a co-current plug flow pattern is adopted [9].
Recently, screens or grids were used to repetitively superimpose a very high uniformly distributed turbulence field on the nearly plug flow conditions encountered in high velocity pipe flows. This characteristic made them particularly effective in processing multiphase systems. For example, their ability to promote contact between immiscible liquids was found to be about 5-fold more energy efficient than mechanically agitated tanks equipped with Rushton-type impellers [10]. The very high turbulence intensities generated in the regions adjacent to the screens result not only in the formation of finely-dispersed phase entities (bubbles and/or drops) but also considerably enhance the value of the interphase mass transfer coefficient. The combined effect of these two factors resulted in inter-phase mass transfer coefficients as high as 13 s−1 being achieved in the case of liquid–liquid dispersions [11] and enables for 99% of equilibrium conditions to be achieved in less than 1 s. This can be mainly attributed to the impact that high-intensity microscale turbulence typically encountered in this mixer configuration can have on the dispersed phase mass transfer coefficient [5]. Furthermore, the use of multi-stage screen-type contactors was found to promote gas–liquid mass transfer in an energy efficient fashion (up to 4.2 kg Oxygen/kWh) or be used to achieve very high interphase mass transfer coefficients (as high as 4.2 s−1) at lower energy utilization efficiencies [12], [13].
To better illustrate the potential of using the aforementioned screen-type static mixers to intensify mass-transfer-limited multiphase reactions, the current study will consider the case of the desulfurization of diesel. The overall reaction rate of these multiphase processes is known to be relatively low because of mass transfer limitations across the interface [14], [15] and several means have been used in an attempt to overcome this limitation. These include the use of surface-active mass transfer agents/catalysts [16], increasing the mixing speed [17], and using ultrasound [18]. However, the use of a phase transfer agent contributes significantly to the cost of operation and may need to be separated at a later stage, while the difficulties associated with the recovery of the extra-fine emulsions typically generated at high rotational speeds limit the applicability of that approach. Although the use of ultrasound has proven to be very effective in achieving high sulfur removal efficiencies, the residence times remain very high (9–20 min) and the energy requirements associated with that approach can be very tasking for such long residence times. The objective of this work is therefore to show the effect of using tubular reactors equipped with screen-type static mixers on the diesel desulfurization reaction and compare the results obtained in this study with those obtained using conventional reactors typically utilized for such purposes.
Section snippets
Experimental
In this section, the diesel desulfurization reaction used to illustrate the benefits of applying process intensification to multiphase reactions is introduced, followed by a description of the experimental setup used and the range of parameters investigated. Finally, an analysis of the various methods by which the power consumption in continuously flowing reactors can be calculated is presented.
Results and discussion
In this section, the hydrodynamic performance of screen-type reactors, and their ability to focus energy dissipation rates within a very small fraction of the reactor's volume, is discussed first because of its relevance to mass transfer limited reactions. This is followed by a comparative evaluation of the desulfurization reactor performance as well as the energy required for different types of reactors.
Advantage of using screen-type static mixers to intensify multiphase reactions
Although CSTR are commonly used for large-scale multi-phase reactions, several devices have been used to accelerate the rate at which mass-transfer limited reactions can proceed. These include high shear continuous flow systems such as impinging-jet reactors, static mixers, single- and multi-stage rotor stators, orifice mixers, as well as flow-through ultrasonic reactors. Unfortunately, the extent to which liquid–liquid contacting can be promoted by these units is limited by their tendency to
Conclusions
The potential of using screen-type static mixers to intensify mass transfer limited multiphase-reactions was investigated using the case of diesel desulfurization by the inverse doctor treatment process developed by Imperial Oil. Based on the results shown above, it is possible to reach the following conclusions:
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Screen-type static mixers are capable of focusing turbulent energy dissipation within very narrow regions downstream of the screen, thereby generating local energy dissipation rates as
Acknowledgements
The financial support of Imperial Oil, the Environmental Science and Technology Alliance Canada, and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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