Three-phase reaction zone

It is important that the electrolyte-containing pores and the gas-containing pores are intercormected for the production of a three-phase reaction zone. 

In hydrogen fuel cells, DSKs are used in their optimized form and are called Janus electrodes [35]. These electrodes consist of three layers. The middle layer is the working layer that contains the catalyst and the coarse, gas-fiUed pores. The outer layers are covering layers that contain fine electrolyte-fiUed pores that prevent the gas from escaping from the electrode. This construction is easier to handle because pressure variations do not strongly influence the stability of the three-phase reaction zone. 

To achieve a high efficiency and high performance, it is necessary to obtain large three-phase reaction zones. As previously described, the use of additives to obtain... 

A different interaction between the ionomer and the catalysts at the three-phase reaction zone. 

One additional function of catalyst support material is to enhance the platinum utilization in the electrode. This leads to an enlarged three phase reaction zone in the MEA and higher electrochemical active catalyst areas at the same catalyst loading of the electrode. For this reason, support materials with adequate surface area and porosity have to be chosen. Antolini [9] shows the benefits of using meso porous stractured carriers with pore sizes between 2 and 50 nm. Here, free pore volume is available for the electrolyte which enables an additional rise of three phase boundary zone and an increased interaction between catalyst and electrolyte [5, 6, 9-15]. 

The CL is where the electrochemical reactions occur, which makes it another key component inside the MEA of PEM fuel cells. The CL is a uniform layer with a thickness of 10-100 pm (usually <50 pm), composed of electrocatalyst powders, proton-conducting ionomer (e.g. Nafion ), and/or binder (e.g. PTPE). Almost all the important challenges in PEM fuel ceU development, such as high cost and low durability, arise from the CLs because they are complex, heterogeneous, contain expensive Pt-based catalysts, and have low stability. The reactions in PEM fuel cells have three phases, involving the reactant gases (e.g. H2 or O2), proton conductive ionomer (e.g. Nafion ), and electron conductor (e.g. carbon-supported Pt catalyst). Therefore, when designing a CL, it is desirable to extend and maximize the three-phase reaction zone to optimize fuel cell performance. 

Spray a certain amount (e.g. 0.5 mg cm ) of ionomer (e.g. Nafion) solution onto the CL surface to form ionic pathways and increase the three-phase reaction zone ... 

Aside from being sprayed on the GDL, the catalyst ink can also be applied to the membrane to make a Nafion-bonded hydrophilic CL. To efficiently extend the three-phase reaction zone and reduce the Pt loading, Wilson et al. [37,39,41] developed a thin-fihn electrode using Nafion ionomer as the bonder. Their preparation process uses a decal method, the details of which are as follows [39] ... 

For a Pt/C-based catalyst layer in PEM fuel cells, according to the three-phase boundary theory, Pt catalysts that are not in the three-phase reaction zone are useless in the PEM fuel cell reaction as they are not accessible for reactants, electrons, or protons these Pt catalysts are thus inactive. To compare different catalyst layer designs, the Pt utilization (Mpt(%)) can be calculated according to the following equation ... 

The combustion wave of HMX is divided into three zones crystallized solid phase (zone 1), solid and/or liquid condensed phase (zone 11), and gas phase (zone 111). A schematic representation of the heat transfer process in the combustion wave is shown in Fig. 5.5. In zone 1, the temperature increases from the initial value Tq to the decomposition temperature T without reaction. In zone 11, the temperature increases from T to the burning surface temperature Tj (interface of the condensed phase and the gas phase). In zone 111, the temperature increases rapidly from to the luminous flame temperature (that of the flame sheet shown in Fig. 5.4). Since the condensed-phase reaction zone is very thin (-0.1 mm), is approximately equal to T . 

The combustion wave of GAP copolymer is divided into three zones zone I is a non-reactive heat-conduction zone, zone II is a condensed-phase reaction zone. 

The combustion wave of GAP copolymer is divided into three zones zone I is a non-reactive heat conduction zone, zone II is a condensed phase reaction zone, and zone III is a gas phase reaction zone in which final combustion products are formed. Decomposition reaction occurs at Tu in zone II, and gasification reaction is complete at Ts in zone II. This reaction scheme is similar to that of HMX or TAGN shown in Fig. 5-5. 

The fluid-solid reactions are presented in this chapter while fluid-fluid (gas-liquid, liquid-liquid) and three phase reactions are presented in the next two chapters. Fluid-solid systems cover a major class of chemical reactions and encompass both liquid-solid and gas-solid systems. In either case, the fluid phase is a single homogeneous fluid. The solid phase acts as a catalyst and its arrangement in the main reactions zone is an important and complex task. 

In order to illustrate now the difficulties at the macroscopic level, let us consider an exothermal three phase reaction occuring in a trickle-bed reactor which processes a volatile liquid phase. Due to the heat of the reaction, a part of the liquid shall be vaporized and the reaction shall also occur - generally at a higher rate - on dry zones of the catalyst surface. The contacting patterns and the wetting of the catalyst shall therefore be strongly connected with the reaction process and the information obtained from cold hydrodynamic experiments shall not be very useful in this case, hydrodynamic and transport parameters must be measured during chemical operation of the reactor (5) ... 

Ceria has been used as the ceramic part (or as an addition) in nickel- or ruthenium-cermet anodes for hydrogen oxidation. " Beneficial effects have been reported and interpreted as most likely being due to the broadening of the three-phase boundary zone. However, one of the major drawbacks of using ceria in cells with YSZ-based electrolytes is its chemical reactivity with the YSZ electrolyte at high temperatures. Sintering of a doped ceria anode on a YSZ electrolyte at high temperatures (>1200°C) results in the formation of a reaction (diffusion) zone with limited oxide ion conductivity. ... 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

This electrochemical reaction contains the elementary step (4.1) and under conditions of backspillover can be considered to take place over the entire metal/gas interface including the tpb.1,15 18 This is usual referred to as extension of the electrochemical reaction zone over the entire metal/gas interface. But even under these conditions it must be noted that the elementary charge transfer step 4.1 is taking place at the three-phase-boundaries (tpb). 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

Slurry reactors. For three-phase systems the definition of conditions at which (catalyst) particles are in motion is important. Two limiting states with respect to particle behaviour can be distinguished (1) complete suspension, i.e. all particles just move, and (2) uniform suspension, i.e. the particles are evenly distributed over the whole reaction zone. The power required to reach the second state is much higher, while uniform suspension is not often necessary. Circulation of the liquid with the dissolved gas is usually sufficiently fast to provide reactants to the surface of catalyst particles if they are suspended at all. 

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

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