Chapter - 4 Convective Mass Transfer Essays - Thermodynamics

Chapter - 4 Convective Mass Transfer 4.1 Introduction 4.2 Convective Mass Transfer coefficient 4.3 Significant parameters in convective mass transfer 4. The application of dimensional analysis to Mass Transfer 4.4.1 Transfer into a stream flowing under forced convection 4.4.2 Transfer into a phase whose motion is due to natural convection 5. Analogies among mass, heat, and momentum transfer 4.5.1 Reynolds analogy 4.5.2 Chilton - Colburn analogy 6. Convective mass transfer correlations 4.6.1 For flow around flat plat 4.6.2 For flow around single sphere 4.6.3 For flow around single cylinder 4.6.4 For flow through pipes 7. Mass transfer between phases 8. Simultaneous heat and mass transfer 4.8.1 Condensation of vapour on cold surface 4.8.2 Wet bulb thermometer 4.1 Introduction Our discussion of mass transfer in the previous chapter was limited to molecular diffusion, which is a process resulting from a concentration gradient. In system involving liquids or gases, however, it is very difficult to eliminate convection from the overall mass-transfer process. Mass transfer by convection involves the transport of material between a boundary surface (such as solid or liquid surface) and a moving fluid or between two relatively immiscible, moving fluids. There are two different cases of convective mass transfer: 1. Mass transfer takes place only in a single phase either to or from a phase boundary, as in sublimation of naphthalene (solid form) into the moving air. 2. Mass transfer takes place in the two contacting phases as in extraction and absorption. In the first few section we will see equation governing convective mass transfer in a single fluid phase. 4.2 Convective Mass Transfer Coefficient In the study of convective heat transfer, the heat flux is connected to heat transfer coefficient as [pic] -------------------- (4.1) The analogous situation in mass transfer is handled by an equation of the form [pic] -------------------- (4.2) The molar flux N A is measured relative to a set of axes fixed in space. The driving force is the difference between the concentration at the phase boundary, CAS (a solid surface or a fluid interface) and the concentration at some arbitrarily defined point in the fluid medium, C A . The convective mass transfer coefficient kC is a function of geometry of the system and the velocity and properties of the fluid similar to the heat transfer coefficient, h. 4.3 Significant Parameters in Convective Mass Transfer Dimensionless parameters are often used to correlate convective transfer data. In momentum transfer Reynolds number and friction factor play a major role. In the correlation of convective heat transfer data, Prandtl and Nusselt numbers are important. Some of the same parameters, along with some newly defined dimensionless numbers, will be useful in the correlation of convective mass-transfer data. The molecular diffusivities of the three transport process (momentum, heat and mass) have been defined as: [pic] ----------------------------- (4.3) [pic] --------------------------- (4.4) and [pic]--------------------------- (4.5) It can be shown that each of the diffusivities has the dimensions of L2 / t, hence, a ratio of any of the two of these must be dimensionless. The ratio of the molecular diffusivity of momentum to the molecular diffusivity of heat (thermal diffusivity) is designated as the Prandtl Number [pic] ------------------------ (4.6) The analogous number in mass transfer is Schmidt number given as [pic] -------------- (4.7) The ratio of the molecular diffusivity of heat to the molecular diffusivity of mass is designated the Lewis Number, and is given by [pic] ------------- (4.8) Figure Lewis number is encountered in processes involving simultaneous convective transfer of mass and energy. Let us consider the mass transfer of solute A from a solid to a fluid flowing past the surface of the solid. The concentration and velocity profile is depicted in figure ( ). For such a case, the mass transfer between the solid surface and the fluid may be written as [pic] ---------------------- (4.1 a) Since the mass transfer at the surface is by molecular diffusion, the mass transfer may also described by [pic] ------------------------- (4.9) When the boundary concentration, CAs is constant, equation (4.9) may be written as [pic] ---------------------- (4.10) Equation (4.1a) and (4.10) may be equated, since they define the same flux of component A leaving the surface and entering the fluid [pic] --------------- (4.11) This relation may be rearranged into the following form: [pic] -------------------- (4.12) Multiplying both sides of equation(4.12) by a characteristic length, L we obtain the following dimensionless expression: [pic] ----------------- (4.13) The right hand side of equation (4.13) is the ratio of the concentration gradient at the surface to an overall or reference concentration gradient; accordingly, it may be considered as

TCE essays

TCE essays TCE is a nonflammable liquid it is colorless. It has a sweet odor and a sweet burning taste. TCE does not occur naturally in the environment. It is an artificial compound. It is produced synthetically from 1,2-dichloroethane. And stabilizers are added to the marketed product. It is used mainly as a solvent to remove greases from metal parts, it is also an ingredient in adhesives, typewriter correction fluid and spot and paint removers. 75-80% of TCE production is used for degreasing metals. Because of its good solvent properties it was formerly used in dry-cleaning facilities. It is also used to extract natural substances. An example would be in decaffeinated coffee or fruit juices. Skin contact with TCE can cause irritation and rashes on the skin. Drinking small amounts of TCE over a long period of time can cause liver and kidney damage, nervous system effects and impaired immune system function. Also during pregnancy it can impair fetal development. Drinking larger amounts can cause nausea, convulsions a coma and in some cases death. Breathing TCE for a long time can cause impaired heart function and coma or death. Breathing TCE for a short period of time can cause headaches, lung irritation, dizziness and difficulty concentrating. Some studies with mice have shown that high levels of TCE may cause liver or lung cancer. Also some studies of people exposed to TCE over long periods of time showed increased risk of cancer. These studies however, are inconclusive the cancer could have been caused by other things. You could be exposed by breathing air in from around the house contaminated by TCE vapors from the shower water. Also breathing in common household items such as, spot removers or correction fluid. Drinking, swimming or showering in water with TCE in it could have also exposed you. ...

Qualitative Analysis Essay Example | Topics and Well Written Essays - 1250 words

Qualitative Analysis - Essay Example However, most of these forms of answers sometimes provide additional information that may be vital for the research. Therefore, qualitative research is an essential tool in evidence based policing. If the offenders are involved in the interview or research, it should be noted that these characters tend to provide vague verbal expressions that have a high degree of uncertainty; thus, they may make the investigators to misinterpret such claims a factor that may mislead the investigation (Hayden, 2012). Moreover, such vagueness of expressions may lead to empirical difficulties to verify the effectiveness of the criminal profiles developed from such research work (Agarwal and Agarwal, 2008). Therefore, this essay aims at addressing the significance of effective and ineffective reporting of the qualitative criminal research findings. The article â€Å"The Evolutional View of the Types of Identity Thefts and Online Frauds in the Era of the Internet† by Shun-Yung Kevin Wang and Wilso n Huang is pegged on their research work on Online Frauds (Wang and Huang, 2011). From the article, it is apparent that these researchers have highly concentrated the negative effects of the internet thereby they might have disregarded other vital information that might have been provided concerning their topic research. However, it is important to analyze presented information that is not relevant to the topic of concern. Analysis of such information is vital for such information usually provides hints to other factors that contribute or are likely to contribute to the advancement of such crime (Greasley, 2008). Wang and Huang’s article aimed at addressing Types of Identity Thefts and Online Frauds in the Era of the Internet, a topic that might have just concentrated on factors that fuels such crime, but it is apparent that the reported they provided also contains the advancement of this criminal act that may be considered as positively reported negative evidence. For instan ce, they reported, â€Å"With the continuous advancement of Internet technology and personal computing devices in recent years, Internet crimes have risen to an alarming level (Wang and Huang, 2011).† This form of information is quite important since if the activities related to that crime are increasing, it would be upon the law reinforces to revolve of measures to take towards curbing; otherwise, reducing the same factors or activities. On the other hand, Wang and Huang seemed to have failed to record what most respondents tend to give at the end of the interview or following the structures of certain questions. In their report, they should have mentioned the solutions offered by their correspondents suggested to reduce or eradicate the internet related criminal activities (Faherty, 2008). For instance, they noted, â€Å"the integration of such diverse technological applications coupled with the rapid growth of online users make fraudulent activities likely to rise further , if no intervention is proposed and implemented (Wang and Huang, 2011).† In this part of their report, they failed to list other integrated services and applications that when coupled with the use of the internet and its high number of users will increase the crime immensely. Lack of reporting some negative evidences collected from qualitative criminal