Unsteady-state phenomena

Propagation problems. These problems are concerned with predicting the subsequent behavior of a system from a knowledge of the initial state. For this reason they are often called the transient (time-varying) or unsteady-state phenomena. Chemical engineering examples include the transient state of chemical reactions (kinetics), the propagation of pressure waves in a fluid, transient behavior of an adsorption column, and the rate of approach to equilibrium of a packed distillation column. 

Unsteady state phenomena have been stated to be of greater importance for non-Newtonian than for Newtonian materials and therefore warrant experimental investigation. The prediction of pressure drop for two-phase flow of a gas and a non-Newtonian fluid seems to be in a well-perfected state but requires extension to situations in which the liquid flow is laminar. Apparently no information is yet available on the problems of mixing, entrainment, and other similar relationships which are of importance if such contactors are to be designed for chemical rather than mechanical purposes. 

In dealing with the problems mentioned above, consideration was given to those occurrences in the process which were largely unsteady-state phenomena. 

Number 3) above renders us into the uncomfortable realm of unsteady-state phenomena - we must realize that this is quite long-term as well as short-lived. 

In the discussion of the deactivation of active surfaces in Chapter 3, we made the point that the existence of deactivation changed an entire class of steady-state phenomena into unsteady-state phenomena. This is well illustrated by extending the analysis of diffusion and chemical reaction in heterogeneous catalysis to include deactivation. On a quantitative level a precise analysis can become intricate and difficult, so we will confine the presentation here to a more qualitative level (and it s tough enough at that). 

After all these comments on the presence of several transient events and unsteady-state phenomena involved in many instances of the emulsion life, how can it be useful to scrutinize the effect of physicochemical formulation on phase... 

The pressure drop reduction is said to occur due to the ball-bearing effect of the particulates. The magnitude of pressure drop reduction is a function of particle size and particle characteristics (e.g., density). However, some researchers argue that it is an unsteady-state phenomenon on that cannot be relied on to occur consistently. 

This phenomenon of increased conversion, yield and productivity through deliberate unsteady-state operation of a fermentor has been known for some time. Deliberate unsteady-state operation is associated with nonautonomous or externally forced systems. The unsteady-state operation of the system (periodic operation) is an intrinsic characteristic of this system in certain regions of the parameters. Moreover, this system shows not only periodic attractors but also chaotic attractors. This static and dynamic bifurcation and chaotic behavior is due to the nonlinear coupling of the system which causes all of these phenomena. And this in turn gives us the ability to achieve higher conversion, yield and productivity rates. 

A wide class of forced unsteady-state processes have already been realized on the commercial scale using specific dynamic phenomenon, that takes place during performance of an exothermic reaction in a fixed bed of catalyst. This phenomenon is referred to in the literature as wrong-way behavior of a fixed bed reactor [20]. Substantial differences in characteristic times of heat and mass transfer in a packed bed reactor result in a surprising rise of temperature inside the reactor after... 

The reactor model for the 2,4-D photolysis. The simplified kinetic expression represented by equation 6.69 has the same form as equation 6.73. However, during the 2,4-D photolysis the radiation absorption characteristics of the reacting medium change. This is a very distinct phenomenon because (i) the uranyl oxalate reaction is a photosensitized reaction and the radiation absorbing species is not consumed, and (ii) conversely, not only the 2,4-D absorption coefficient changes, but absorption by reaction products increases the total absorption coefficient above the initial value. This phenomenon produces an unavoidable coupling between the steady state radiation balance and the unsteady state mass balance. The total absorption coefficient can be obtained from equation 6.68. Then ... 

It was earlier reported [11] that using the same polymer and porous media, steady-state or unsteady-state flow can be obtained depending on the flow condition. Our further laboratory studies showed that this phenomenon is commonly observed with a wide variety of water soluble polymers. However, the critical flow parameters of different polymers can vary greatly. 

Although the knowledge on unsteady-state polymer flow is incomplete, a working hypothesis can already be given based on observed phenomenon in the laboratory. This description of unsteady-state polymer flow must be in good agreement with well-established experimental facts, which are as follows ... 

In general, the available methods are grouped into two groups steady- and unsteady-state methods, according toEqs (1.1) and (1.2). In most of the processes, diffusion is a three-dimensional phenomenon. However, many of the e5q)erimental methods used to analyze diffusion restrict it to a onedimensional process. Also, it is much easier to study tiieir matiiematical treatments in onedimension (which then may be generalized to a three-dimensional space). 

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