Chemical mass-transfer efficiency

Chemical reactions on solid surfaces can be realized in gas-solid and liquid-solid systems. In both cases the reaction takes place on the surface of the solid matrix, and therefore the molecules to be reacted need to get in contact with the reactive surface. Several transport regimes and interaction mechanisms define the mass transfer efficiency. They can be summarized as follows [6] ... 

Static extraction columns Spray column Baffle column Packed column (random and structured packing) Sieve tray column Deliver low to medium mass-transfer efficiency, simple construction (no internal moving parts), low capital cost, low operating and maintenance costs, best suited to systems with low to moderate interfacial tension, can handle high production rates Petrochemical Chemical Food... 

A chemical reaction is initiated by bringing the required reactants into intimate contact. Efficient execution of this process often defines the efficiency of the reaction and, therefore, requires considered examination when designing a reaction system. The physical transport of a component along a concentration gradient by molecular diffusion and turbulent convection is known as mass transfer. The transport of mass through an interface between two media or phases of the same medium is extremely important, since chemical reactions are normally coupled to the mass transfer efficiency. The mass transfer coefficient is used to quantify the efficiency of mixing in macrofluidic systems and is given by... 

In this chapter, fluid-fluid flow patterns and mass transfer in microstructured devices are discussed. The first part is a brief discussion of conventionai fluid-fluid reactors with their advantages and disadvantages. Further, the ciassi-flcation of fluid-fluid microstructured reactors is presented. In order to obtain generic understanding of hydrodynamics, mass transfer, and chemical reaction, dimensionless parameters and design criteria are proposed. The conventional mass transfer models such as penetration and film theory as well as empirical correlations are then discussed. Finally, literature data on mass transfer efficiency at different flow regimes and proposed empirical correlations as well as important hydrodynamic design parameters are presented. 

Mass Transfer Efficiency. The human kidney acts as a filter to remove metabolic waste products from the blood. A person s kidneys process about 200 quarts of blood daily to remove two quarts of waste and extra water, which are converted into urine and excreted. Without filtration, the waste would build to a toxic level and cause death. Patients with kidney failure may undergo dialysis, in which blood is withdrawn, cleaned, and returned to the body in a periodic, continuous, and time-consuming process that requires the patient to remain relatively stationary. Portable artificial kidneys, which the patient wears, filter the blood while the patient enjoys the freedom of mobility. Filtration systems may involve membranes with a strict pore size to separate molecules based on size or columns of particle-based adsorbents to separate molecules by chemical characteristics. Mass transfer efficiency refers to the quality and quantity of molecular transport. 

A trend in electrode preparation is to reduce the catalyst layer thickness to improve the mass transfer efficiency at the interface, such as the efficient movement of protons, electrons, and dissolved reactants in the reaction zone. In addition, a thinner electrode will be beneficial to reduce catalyst loading and increase mass power density. The deposition technique is an effective way to achieve a thinner electrode through depositing a nano-scale catalyst film on the substrate. Deposition methods include chemical vapor deposition, physical or thermal vapor deposition, sputtering deposition, electrochemical deposition, chemical deposition, as well as ion beam deposition. The following sections will focus on electrodes fabricated with these various deposition methods. 

Nevertheless, the computational methodology for mass transfer so far receives less attention. In many chemical engineering processes, the concentration profile is chiefly concerned as it is the basis for calculating the mass transfer efficiency. The conventional way to predict the concentration profile is using some simple but unreliable methods. The recent development of computation mass transfer (CMT) as described in this book provides a rigorous basis for dependable predictions of both concentration profile and the effectiveness of the mass transfer process as well as the interfacial effects on mass transfer efficiency. 

An industrial chemical reacdor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors. 

Engineering factors include (a) contaminant characteristics such as physical and chemical properties - concentration, particulate shape, size distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity (b) gas stream characteristics such as volume flow rate, dust loading, temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, corrosivity, and toxicity and (c) design and performance characteristics of the control system such as pressure drop, reliability, dependability, compliance with utility and maintenance requirements, and temperature limitations, as well as size, weight, and fractional efficiency curves for particulates and mass transfer or contaminant destruction capability for gases or vapors. 

The absorption of reactants (or desorption of products) in trickle-bed operation is a process step identical to that occurring in a packed-bed absorption process unaccompanied by chemical reaction in the liquid phase. The information on mass-transfer rates in such systems that is available in standard texts (N2, S6) is applicable to calculations regarding trickle beds. This information will not be reviewed in this paper, but it should be noted that it has been obtained almost exclusively for the more efficient types of packing material usually employed in absorption columns, such as rings, saddles, and spirals, and that there is an apparent lack of similar information for the particles of the shapes normally used in gas-liquid-particle operations, such as spheres and cylinders. 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

The diffusivity in gases is about 4 orders of magnitude higher than that in liquids, and in gas-liquid reactions the mass transfer resistance is almost exclusively on the liquid side. High solubility of the gas-phase component in the liquid or very fast chemical reaction at the interface can change that somewhat. The Sh-number does not change very much with reactor design, and the gas-liquid contact area determines the mass transfer rate, that is, bubble size and gas holdup will determine reactor efficiency. 

Airlift loop reactor (ALR), basically a specially structured bubble column, has been widely used in chemical industry, biotechnology and environmental protection, due to its high efficiency in mixing, mass transfer, heat transfer etc [1]. In these processes, multiple reactions are commonly involved, in addition to their complicated aspects of mixing, mass transfer, and heat transfer. The interaction of all these obviously affects selectivity of the desired products [2]. It is, therefore, essential to develop efficient computational flow models to reveal more about such a complicated process and to facilitate design and scale up tasks of the reactor. However, in the past decades, most involved studies were usually carried out in air-water system and the assumed reactor constructions were oversimplified which kept itself far away from the real industrial conditions [3] [4]. 

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