Time scales recirculation

Another classification of model is related to the time and space scales of interest. Ambient air quality standards are stated for measurement averaging periods varying from an hour to a year. However, for computational purposes, it is often necessary to use periods of less than an hour for a typical resolution-cell size in a model. Spatial scales of interest vary from a few tenths of a meter (e.g., for the area immediately adjacent to a roadway) up to hundreds of kilometers (e.g., in simulations that will elucidate urban-rural interactions). Large spatial scales are also warranted when multiday simulations are necessary for even a moderate-sized urban area. Under some climatologic conditions, recirculations can cause interaction of today s pollution with tomorrow s. Typical resolution specifications couple spatial scales with temporal sc es. Therefore, the full matrix of time scales and space scales is not needed, because of the dependence of time scales on space scales. Some typical categories by scale are as follows ... 

The Flow Reactor In flow reactor experiments designed for chemical kinetic interpretation, the objective is to achieve a plug-flow situation, where composition and temperature are uniform over the cross section of the reactor. This condition may be approximated both in the turbulent [442] and the laminar [233] flow regimes. In the turbulent flow regime, a high linear flow rate secures negligible recirculation flow. Each element of gas reacts as it moves, with the characteristic time scale for heat and mass transfer by... 

The experiments with the inert tracer may only show that the time, necessary for the fluid in the reactor to be well mixed, is much smaller than the average residence time. When a chemical reaction takes place, an additional time-scale, the time constant of the chemical reaction, appears. This time characterizes the reaction rate and can be defined as the time in which the reaction proceeds to a certain conversion, say 50%. For many practical heterogeneous catalytic reactions, the reaction time is so short that reactants entering the reactor may be converted without being mixed, for example, during the first cycle. For such fast reactions, of course, the reactor cannot be considered as gradient-free, whatever the recirculation ratio is. 

Depending on tlie time. scale of deactivation, the catalytic activity can be restored in different ways. For example, in fluid catalytic cracking, where the deactivation is very fast, a recirculating leacTor is used for continuous catalyst regeneration. However, if the deactivation is slow and constant conversion is desired 10 meet certain environmental regulations as in VOCoxidation, the temperature level can be used to compensate fur the loss of intrinsic catalytic activity. Under such additions, the deactivation effects are measured by the temperature increase required to maintain constant conversion. 

Note that the dispersion terms described in equation (6.18) are valid only in the limit of Fickian behavior. From the central limit theorem, this regime is reached when every particle has amply sampled each region (wakes, gaps, recirculation zones). The average time-scale to advect through a wake is (a(u)Yl, and the average time-scale to experience trapping within a recirculation zone is r/ (yad). Then, the Fickian limit is reached at time t r/ (yad) and (fl(M 1. 

The reactor time scale, T(reactor), which is the largest macro-mixing time scale, T(macro-mixing), and typically of the order of seconds, can be the mean residence time in the reactor, T(residence), but in a reactor equipped with an impeller, the recirculation time, T(recirculation), rather than the mean residence time, should be used. The recirculation time can be defined as the mean time between successive encounters of a given fluid element with the impeller. Typically Ty< T(recirculation) < T(residence). 

As is widely known, mixing in a bath is governed mainly by large-scale recirculation and turbulent motion. The former is characterized by the mean velocity components in the three directions, while the latter is characterized by the root-mean-square (rms) values of the three turbulence components and the Reynolds shear stresses. Desirable mixing condition would be realized when the two kinds of motions are produced together. Unfortunately, these motions on the mixing time in a bath subjected to surface flow control are poorly understood. This chapter discusses these effects with reference to experiments in which three types of boundary conditions are imposed on the surface of a water bath stirred by bottom gas injection. 

Figures 7.15 and 7.16 show the rms values of the axial and radial turbulence components and the Reynolds shear stress at z = 70 x 10 m in the CAS and disk models. The measured w rnis, v rms, and u V in the two models are approximately the same. Recalling that the mixing time in the disk model is much shorter than that in the CAS model, it can be concluded that the difference between the mixing times in the disk and CAS models is mainly due to the existence of a large-scale recirculation motion in the bath.

Milk-of-lime transfer pumps should be of the open impeller centrifugal type. Pumps having an iron body and impeller with bronze trim are suitable for this purpose. Rubber-lined pumps with rubber-covered impellers are also frequently used. Makeup tanks are usually provided ahead of centrifugal pumps to ensure a flooded suction at all times. Plating out of lime is minimized by the use of soft water in the makeup tank and slurry recirculation. Turbine pumps and eductors should be avoided in transferring milk of lime because of scaling problems. 

In conclusion, the following experiments on filtration-washing-deliquoring should be performed to produce data (viscosity of liquids, effective solid concentration, specific cake resistance, cake compressibility, etc.) that are necessary to evaluate times of individual steps of filtration at an industrial scale, i.e. to obtain the proper basis for scale-up of filtration processes measure the filtrate volume versus time make marks on your vacuum flask and take down the time when the filtrate level reaches the mark => no more experiments are needed for preliminary evaluations of filtration properties of slurries initially fines pass the filter medium => recirculate them to the slurry,... 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

In all probability, any serious prediction or evaluation of an actual recirculating cooling water, based on an index or model calculated from an analysis of the cooling water, is of dubious validity, as problems of scale and corrosion do not originate at the precise time of sampling and testing. They have already occurred. 

Deposition of scale does not occur instantaneously there is a time delay (scale induction time), which varies for each cooling system, based on its particular characteristics (including rate of recirculation, retention time, water chemistry, etc.). The induction time can be lengthened by the use of deposit control agents (DCAs). 

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