New Operating Modes of Production

Several new operating modes of production allow process intensification, although they have been mainly investigated at a laboratory and/or pilot stage.

The more relevant for practical applications were the following according to Charpentier [7]:

• reversed flow for reaction-regeneration,

• unsteady operations, cyclic processes,

• extreme conditions,

• pultrusion (a variation of the extrusion process),

• low-frequency vibrations to improve gas-liquid contact in bubble columns,

• high-temperature and high-pressure technologies,

• supercritical media and ionic liquids.

In reverse flow reactor operations, one or more process variables are intentionally and permanently (cyclic) perturbed according to design schedule [52, 53]. The technology was first introduced for removal of pollutants. However, it is well suited to optimize the use of the heat of reaction (for feed preheating) in the case of exothermic processes. The periodic flow reversal in such units allows the reaction heat to be retained within the reactor bed. After reversing the direction of the feed, this heat of reaction is used to pre-heat the cold reactant gases. Figure 3.9 shows the operating principle of the reverse flow reactor (RFR), exemplified for catalytic partial oxidation of methane. This dynamic operation creates process improvements that cannot be achieved by steady state operation. Expected benefits are energy savings, increased conversion, selectivity and productivity. Barriers are reaction kinetics (exothermic reactions, endothermic reactions as well as equilibrium reactions), energy storage, as well as reactor design operation and control. All these barriers stem from the dynamics of high flow reversal frequency.

Three way valve"

feed (CH4 + 02) Figure 3.9 Operating principle ofthe reverse flow reactor (RFR). The reactor consists of three zones - a catalyst bed between two beds of packing of heat-accumulating material.

Reverse-flow reactors have been used in three industrial processes: SO2 oxidation, total oxidation of hydrocarbons in off-gases and NOx reduction. The reverse-flow principle has also been applied in rotating monolith reactors used industrially for removal of undesired components from the gas streams and continuous heat regeneration. Studies have also been carried out on the use of reversed flow reactors for endothermic processes. Low-level contaminants or waste products such as volatile organic compounds can be efficiently removed in adiabatic fixed beds with periodic reversal by taking advantage of higher outlet temperatures generated in earlier cycles to accelerate exothermic reactions. Energy and cost savings are affected by this substitution of internal heat transfer for external exchange [54].

The RFR is a case of heat-integrated reactor concepts that have found fairly widespread application in PI, typically for reactions where the adiabatic temperature rise does not allow auto-thermal reactor operation. The RFR concept is also suited for extreme conditions of auto-thermal high-temperature millisecond contact-time catalysis. Liu etal. [55]havedemonstratedthatthermodynamiclimitations in catalytic partial oxidation of methane to synthesis gas as well as kinetic limitations in the oxidative dehydrogenation of ethane to ethylene at high-temperature conditions can be overcome via regenerative heat-integration in a reverse-flow reactor. Strong improvements in product yields in comparison to conventionally operated fixed bed reactors are obtained while maintaining the compactness of the short contact-time reactor and keeping the process independent of external heat sources. Overall, the application ofheat-integrated reactors to high-temperature catalysis is open to strongly intensified processes. The same group [56] has also demonstrated that RFR operation leads to strong improvements in synthesis gas yields over steady state (SS) operations for various catalysts, with particularly strong improvements for poorly performing catalysts. Furthermore, while the increased catalyst temperatures result in an accelerated deactivation of the unstable catalysts (Pt,Ir), heat integration leads to a complete compensation of this acceleration. RFR operation thus has an intrinsic "equalizing" effect on catalyst performance and thus offers a widely applicable reactor engineering approach to compensate for poor or degrading catalysts in high temperature partial oxidations.

Under most operating conditions a RFR eventually converges to a symmetric single-period operation so that the concentrations and temperature profiles after one flow reversal are a mirror image of those after the previous flow reversal. However, a cooled RFR may attain, under certain conditions, states with more complex periodicity, that is, states with period n > 1, nonsymmetric states and even complex quasi-periodic and chaotic states.

There are other possible unsteady (periodic) operation modes for a packed bed reactor that could lead to process intensification [57]. Indeed, there are several unsteady state strategies available to run a process unit such as a reactor. Pulses of different magnitude can be imposed on an input, or the input could be either changed progressively or varied according to an analytical function. However, not all unsteady state strategies are feasible in a commercial situation. Table 3.2 gives examples of the possibilities.

The pressure swing reactor (PSR) was developed from pressure swing adsorption (PSA) by simply adding a catalyst to the adsorbent. Consequently, PSR designs follow

Table 3.2 Examplesofperiodical operations in reactors to improve performances. Source: adapted from Aida and Silveston [57].

System

Manipulating variable

Performance enhancement

Water gas shift reaction in a

Pressure, composition and

Exceeded equilibrium

pressure swing reactor

flow

conversion

NOx reduction by NH3 in a

Flow and flow direction

Reduced ammonia

packed bed, catalytic reactor

consumption

SO2 absorption and oxidation

Flow

40% increase in SO2 removal

to sulfuric acid in a trickle bed

reactor

Synthesis of ammonia in a

Composition

100-fold increase in nitrogen

packed bed, catalytic reactor

conversion

Steam reforming of methane

Temperature

>95% H2 and less than 30ppm CO with a dolomite CO2 acceptor

those used for adsorption systems. Perhaps the most important incentive for exploring PSRs is that they can operate at lower temperatures for equilibrium-limited endothermic reactions than those usually employed. Dehydrogenation is a good example of such a reaction. Lower temperatures could reduce the importance of secondary reactions and would certainly lower capital and/or operating costs. In addition, for equilibrium-limited exothermic reactions with large activation energies, a PSR might increase reaction yield without requiring a reduction in temperature.

Temperature swing reactors (TSR) are an alternative to reverse flow reactors for the combustion of low concentration volatile organic compounds (VOCs). The most common case is the use of a rotating packed catalyst bed, where the upper part of the rotating disk is exposed to the cold stream containing the diluted VOC and acts as absorber for them. The lower part of the rotating disk is exposed to hot air. The adsorbed VOCs desorb and are combusted on the catalyst. The TSR is advantageous over RFR for very dilute VOC streams, but very careful design of the properties of the catalyst bed is necessary.

The concept of trapping a contaminant in low concentration by adsorption with periodic regeneration of the adsorbent-catalyst has been applied commercially by Toyota for NOx-trap catalysts used in converting NOx in diesel or lean burn engine emissions, for example, for reduction of NOx in the presence of O2 [63]. The catalyst acts as absorbent of NOx (in the form of surface nitrate-like species) in the presence of O2 (lean conditions), but a periodic switch of the air to fuel ratio to rich conditions (deficit of O2 with respect to stoichiometry for the complete oxidation of CO and hydrocarbons present in the car emissions to CO2) leads to regeneration by reducing trapped NOx to N2.

Process intensification is also possible by induced pulsing a liquid flow in trickle beds to improve liquid-solid contacting at low liquid mass velocities in the cocurrent downflow mode [64]. In a trickle bed reactor the liquid and gas phases flow

J. Liquid

Figure 3.10 Schematic diagram of a trickle bed reactor. Inset: schematic of the trickle flow.

J. Liquid

Figure 3.10 Schematic diagram of a trickle bed reactor. Inset: schematic of the trickle flow.

cocurrently downwards through a fixed bed of catalyst particles while the reaction takes place (Figure 3.10). The cocurrent upward flow operation provides better radial and axial mixing than the downward flow operation, thus facilitating better heat transfer between the liquid and solid phases. However, the downflow scheme is usually utilized due to better mechanical stability and less flooding, thus facilitating processing of higher flow rates and increased reactor capacity [65].

Trickle bed reactors operate in various flow regimes, ranging from gas-continuous to liquid-continuous patterns. They usually fall into two broad categories, referred to as low interaction regime (trickle flow regime) and high interaction regime (pulse, spray, bubble and dispersed bubble flow regimes). The low interaction regime is observed at low gas and liquid flow rates and is characterized by a weak gas-liquid interfacial activity and a gravity-driven liquid flow. The high interaction regime is characterized by a moderate to intense gas-liquid shear due to a moderate to high flow rate of one or both of the fluids.

In the trickle flow regime (inset in Figure 3.10) the liquid flows down the reactor on the surface of the packing in the form of rivulets and films while the gas phase travels in the remaining void space. The trickle flow regime can be further divided into two regions. At very low gas and liquid flow rates, the liquid flow is laminar and a fraction of the packing remains unwetted. This regime is called the partial wetting regime. If the liquid flow rate is increased, the partial wetting regime changes to the complete wetting trickling regime in which the packing is totally covered by a liquid film.

The pulse flow occurs at relatively high gas and liquid input flow rates. It refers to the formation of slugs that have a higher liquid content than the remainder of the bed. The pulsing behavior refers to gas and liquid slugs traversing the reactor alternately. It begins when the flow channels between packing are plugged by a slug of liquid, followed by blowing off the slug by the gas flow (Figure 3.11). Pulses always begin at the bottom of the bed, where the gas velocity is higher due to the lower pressure. As

the gas flow rate is increased, the incipient point of pulsing moves to the upper part of the reactor.

At present, trickle flow is the most common flow regime encountered in industrial applications. For process intensification, pulsing flow allows an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow [66, 67]. The operation of a trickle bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Axial dispersion is less than with trickle flow due to increased radial mixing and disappearance of stagnant liquid holdup. Wu et al. [68] have demonstrated that pulsing flow has a positive effect, particularly on selectivity, with respect to trickle flow. Also, such periodic operation with respect to liquid flow may help in getting process intensification for gas-limiting reactions or for petroleum applications where filtration and bed plugging are serious threats [69].

Forced dynamic (periodic, pulsing) operation of chemical reactors as a means for improving the reactor performance has been investigated since late 1960s. Through dynamic operation one can advantageously influence the kinetics of the adsorp-tion-reaction-desorption processes on the catalyst surface (solid-catalyzed gas-phase reactions), increase interfacial mass transfer rates (e.g., pulsing operation of trickle-bed reactors), shift the process beyond the equilibrium limitation or improve heat transfer (e.g., reversed-flow operation of fixed-bed catalytic reactors), or improve mixing characteristics of the system (e.g., variable-volume operation of the stirred tank reactors). The process rates have been improved by 50% or more in bench-scale experiments.

Another example of dynamic operations is the oscillatory baffled reactor (OBR). This technology consists of a cylindrical column containing equally spaced orifice baffles. Vortices are generated when fluid flow pass through the baffles, enabling significant radial motions, where events at the wall are of the same magnitude as those at the center. The generation and cessation of eddies creates uniform mixing in each baffled cell, collectively along the column. The degree of mixing is independent ofthe net flow, which makes it possible to realize a nearly plug-flow character (many CSTRs in series) in a flow system at long residence times. OBR offers enhanced mass and heat transfers over stirred tank reactors, the workhorse in fine chemical and pharmaceutical production. Using OBR it is possible to realize plug flow conditions even at low (laminar) flow rates, and thus to change batch to continuous production.

Table 3.3 Intensification effects of alternative energy forms. Source: adapted from Stankiewicz [21].

Energy source

Intensified element

Degree of possible intensification

Sustainability effect

Electric field

Interfacial area

500x

Energy

Heat transfer

10x

Microwaves

Reaction time

1250x

Energy, material

Distillation time

20x

efficiency

Light

Product yield/selectivity

Improved

Material efficiency,

selectivity

waste reduction, safety

Ultrasound

Reaction time

25x

Energy, material

Gas-liquid mass transfer

5x

efficiency

Liquid-solid mass transfer

20x

Supersonic

Gas-liquid mass transfer

10x

Energy, material

shockwave

coefficient

efficiency

Major benefits are significant energy/utility savings, higher yields and less side products/high product consistency. In addition, capital cost savings are achieved through much more compact designs.

Various other non-conventional reactor operational modes to improve performances have been proposed, based in particular on the use of alternative ways to either supply energy (microwave, light) or to induce high-energy microenvironments (e.g., cavitation effects by sonochemistry). The use of light, ultrasonic and microwave technologies to enhance the rates and improve the selectivities of catalytic reactions [21, 70, 71] is discussed in a more detail later. Table 3.3 summarizes the intensification effects possible by using alternative energy forms [21].

Three alternative energy forms have particularly attracted research interest: microwaves, light and ultrasound. Microwave frequencies ranges from 300 to about 300000 MHz. Polar molecules subjected to microwave irradiation exhibit dipole rotation, trying to align with the rapidly changing electric field ofthe microwave. The rotational motion of the molecule results in a transfer of energy. Additionally, in substances where free ions or ionic species are present, the energy is also transferred by the ionic motion in an oscillating microwave field. As a result of both these mechanisms the substance is heated directly and almost evenly. Heating with microwaves is therefore fundamentally different from conventional heating by conduction.

Microwaves accelerate chemical reactions, often by factors of hundreds, and in many cases significantly better product yields are reported. A relevant challenge to further amplify the effect of microwaves is to couple with microreactor technology. This coupling offers a great opportunity to increase process selectivity by an instantaneous heating of the reactants and a fast quenching of the reaction products. Fundamental challenges here include equipment materials and their interaction with microwave radiation, application of microwave energy to micro-volumes, modeling and optimization of microwave-driven process in microequipment. The influence of the molecular effects induced by the microwave field on transport and interfacial phenomena in multiphase separation systems, such as distillation, extraction or crystallization, presents another exciting and largely unexplored research area.

The use of light, either artificial or solar, in catalysis can lead in principle to high product selectivities, although examples are still limited. Another problem with the present types ofphotocatalytic reactors is that a significant portion ofemitted light is absorbed or dissipated before it reaches the catalytic site. This results in high energy demands and often makes photocatalytic reactors economically unattractive. An ideal photocatalytic reactor should be able to emit photons exactly where and when they are needed, that is, in the direct vicinity of the catalytic site and upon contact of the reacting molecules with that site.

Ultrasonics uses sound in the 0.1-100 nm wavelength range to enhance mixing and chemical reactions. Two levels of sonics are in common use: a low level with sufficient energy to enhance mixing but not sufficient to perform or directly assist the chemical reaction and a high level that imparts significantly more energy. The high level creates voids, acoustic cavitation, and this cavitation results in bubbles being formed. When the bubbles collapse they generate a micro-jet of fluid and a high-energy environment that can enhance the reaction rate.

Another interesting opportunity is magnetic-driven process intensification [72]. External inhomogeneous magnetic fields exert a magnetization body force. This force may be used in the case of electrically non-conducting and magnetically permeable fluids for hydrodynamic intensification of the chemical processes. Hence, in acting on paramagnetic or diamagnetic gases and liquids, this force can modify the direction as well as the magnitude of the gravitational force. An example of application is the possibility to influence two-phase flows through packed bed reactors by application ofexternal inhomogeneous magnetic fields. The effect could be used to intensify performances of mini-trickle-bed reactors for applications in fine and pharmaceuticals chemical processes. A positive-gradient in homogeneous magnetic fields promotes larger values of liquid holdup (and thus wetting efficiency in the trickle flow regime) and two-phase pressure drop [73].

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