Disperse system flow

Thus, the ratio of gas and liquid-phase flow rates Wg lu>i in tubular devices, as well as the dispersed system flow rate, exerts a substantial influence on the size of the dispersed inclusions. An increase of the volume-surface diameter of the dispersions, with an increase of the gas content (the growth of the Wg/wi ratio) can be compensated by the growth of the liquid-gas flow rate. Profiling of tubular turbulent device walls, to form diffuser-confusor transitions, is an effective way of reducing diffuser limitations for fast chemical reactions in the presence of an interphase boundary. 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

Detailed treatments of the rheology of various dispersed systems are available (71—73), as are reviews of the viscous and elastic behavior of dispersions (74,75), of the flow properties of concentrated suspensions (75—82), and of viscoelastic properties (83—85). References are also available that deal with blood red ceU suspensions (69,70,86). 

R. Roscoe, in J. J. Hermans, ed.. Flow Properties of Disperse Systems, North-HoUand Publishing Co., Amsterdam, the Netherlands, 1953, p. 1. 

The axial dispersion plug flow model is used to determine the performanee of a reaetor with non-ideal flow. Consider a steady state reaeting speeies A, under isothermal operation for a system at eonstant density Equation 8-121 reduees to a seeond order differential equation ... 

Fig. 3. A complete flow curve of plastic disperse system with a field of flow (with a very high viscosity tic) at stresses smaller than the yield stress...

In order to complete the discussion of methodical problems, we should mention two more methods of determining yield stress. Figure 6 shows that for plastic disperse systems with low-molecular dispersion medium, when a constant rate of deformation, Y = const., is given, the dependence x on time t passes through a maximum rm before a stationary value of shear stress ts is reached. We may assume that the value of the maximal shear stress xm is the maximum strength of the structure which must be destroyed so that the flow can occur. Here xm as well as ts do not depend or depend weakly on y, like Y. The difference between tm and xs takes into account the difference between maximum stress and yield stress. For filled polymer melts at low shear rates Tm Ts> i,e- fhese quantities can be identified with Y. 

Equations (299) and (300) depict the input-output relationships for the concentrations and the temperature in each phase for a given continuous steady-flow dispersed system. Therefore, (299) and (300) can be used in predicting the input-output relationships for a multistage multicomponent gas-liquid system with several continuous stirred vessels in series. 

The advantage of the two-phase micro flow contacting concept is easy phase separation, as the phases are never inter-mixed. However, in view of the normally facile separation of gases and liquids, this is not of major impact. A real large benefit stems from operating with gas and liquid layers of defined geometry with a knovm, defined interface, unlike most disperse systems having a size distribution of their bubbles in the continuous liquid. 

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion can become as high as 700 cm and the interfacial reaction in the dispersed system can be measured by a photodiode array spectrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After 200 ms, the dispersion system starts to separate, even in a rather viscous solvent like a dodecane. Therefore, rather fast interfacial reactions such as diffusion-rate-limiting reactions are preferable systems to be measured. 

A typical example is the protonation of tetraphenylporphirin (TPP) at the dodecane-acid solution interface. The interfacial protonation rate was measured by a two-phase stop flow method [6] and a CLM method [9]. In the former method, the stagnant layer of 1.4 jxm still existed under the highly dispersed system. In the CLM method, the liquid membrane phase of 50-100 /am thickness behaved as a stagnant layer where the TPP molecule has to migrate according to its self-diffusion rate. 

Rheology is the study of flow and deformation of materials under the influence of external forces. It involves the viscosity characteristics of powders, liquids, and semisolids. Rheological studies are also important in the industrial manufacture and applications of plastic materials, lubricating materials, coatings, inks, adhesives, and food products. Flow properties of pharmaceutical disperse systems can be of particular importance, especially for topical products. Such systems often exhibit rather complex rheological properties, and pharmaceutical scientists have conducted fundamental investigations in this area [58-64],... 

The work by group of Kozyuk [84—87] has illustrated the use of hydrodynamic cavitation for obtaining free disperse system in liquids, particularly in liquid hydrocarbons. It has been found that, there is substantial improvement in the quality of the obtained free dispersion, even in the absence of any catalyst. Also the geometry of a flow-constricting baffle body [84] effectively increases the degree of cavitation to substantially improve the quality of obtained free disperse system. 

Molerus O. Principles of Flow in Disperse Systems. New York Chapman and Hall, 1993. 

Explain carefully the dispersed plug-flow model for representing departure from ideal plug flow. What are the requirements and limitations of the tracer response technique for determining Dispersion Number from measurements of tracer concentration at only one location in the system Discuss the advantages of using two locations for tracer concentration measurements. 

In this group of disperse systems we will focus on particles, which could be solid, liquid or gaseous, dispersed in a liquid medium. The particle size may be a few nanometres up to a few micrometres. Above this size the chemical nature of the particles rapidly becomes unimportant and the hydrodynamic interactions, particle shape and geometry dominate the flow. This is also our starting point for particles within the colloidal domain although we will see that interparticle forces are of great importance. 

Taylor (T4, T6), in two other articles, used the dispersed plug-flow model for turbulent flow, and Aris s treatment also included this case. Taylor and Aris both conclude that an effective axial-dispersion coefficient Dzf can again be used and that this coefficient is now a function of the well known Fanning friction factor. Tichacek et al. (T8) also considered turbulent flow, and found that Dl was quite sensitive to variations in the velocity profile. Aris further used the method for dispersion in a two-phase system with transfer between phases (All), for dispersion in flow through a tube with stagnant pockets (AlO), and for flow with a pulsating velocity (A12). Hawthorn (H7) considered the temperature effect of viscosity on dispersion coefficients he found that they can be altered by a factor of two in laminar flow, but that there is little effect for fully developed turbulent flow. Elder (E4) has considered open-channel flow and diffusion of discrete particles. Bischoff and Levenspiel (B14) extended Aris s theory to include a linear rate process, and used the results to construct comprehensive correlations of dispersion coefficients. 

Plug Flow with Dispersion - Plug flow with dispersion is a concept that is often used to describe one-dimensional flow systems. It is somewhat more flexible in computational models because the mixing within the system is not dependent on reactor size, as with complete mix tanks in series. Plug flow with dispersion will be described in the second half of this chapter because special techniques are needed for the analysis. 

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