Mixing vessels

For this process, the wet soap is pumped into a mixing vessel (cmtcher) where the addition and mixing of minor ingredients may be achieved. 

T Diameter of mixing vessel or m ft 6c Time between coalescences s h... 

Refer to Fig. 15-41. The extrartor is an extension of the simple baffled mixing vessel into a multistage column. Although commercial apphcation has been made, data are scarce and are limited to towers of small diameter. The preferred proportions are Zt = 0.5df, d, > dj. 

Pickiug up the solids at the bottom of the tank depends upon the eddies and velocity fluctuations in the lower part of the tank and is a different criterion from the flow pattern required to keep particles suspended and moving in various velocity patterns throughout the remainder of the vessel This leads to the variables in the design equation and a relationship that is quite different when these same variables are studied in relation to complete uniformity throughout the mixing vessel. 

The film coefficient h is for the inner wall Dj is the inside diameter of the mixing vessel. The term L N p/ is the Reynolds number for mixing in which L is the diameter and Nr the speed of the agitator. Recommended values of the constants a, b, andm are given in Table 18-2. 

FIG. 23-7 Imp ulse and step inputs and responses. Typical, PFR and CSTR. (a) Experiment with impulse input of tracer, (h) Typical behavior area between ordinates at tg and ty equals the fraction of the tracer with residence time in that range, (c) Plug flow behavior all molecules have the same residence time, (d) Completely mixed vessel residence times range between zero and infinity, e) Experiment with step input of tracer initial concentration zero. (/) Typical behavior fraction with ages between and ty equals the difference between the ordinates, h — a. (g) Plug flow behavior zero response until t =t has elapsed, then constant concentration Cy. (h) Completely mixed behavior response begins at once, and ultimately reaches feed concentration. 

Baffle generally a flat plate attached to and perpendicular to the wall of a mixing vessel to alter the fluid circulation pattern and prevent excessive swirl of the fluid in the vessel. ... 

Side Entering describes a mixer design whieh has a horizontal shaft entering through the side of a mixing vessel. 

This form is partieularly appropriate when the gas is of low solubility in the liquid and "liquid film resistanee" eontrols the rate of transfer. More eomplex forms whieh use an overall mass transfer eoeffieient whieh ineludes the effeets of gas film resistanee must be used otherwise. Also, if ehemieal reaetions are involved, they are not rate limiting. The approaeh given here, however, illustrates the required ealeulation steps. The nature of the mixing or agitation primarily affeets the interfaeial area per unit volume, a. The liquid phase mass transfer eoeffieient, kL, is primarily a funetion of the physieal properties of the fluid. The interfaeial area is determined by the size of the gas bubbles formed and how long they remain in the mixing vessel. The size of the bubbles is normally expressed in terms of their Sauter mean diameter, dj, whieh is defined below. How long the bubbles remain is expressed in terms of gas hold-up, H, the fraetion of the total fluid volume (gas plus liquid) whieh is oeeupied by gas bubbles. 

To apply the mass transfer equation for design, the interfacial area, a, and mass transfer coefficient kL must be calculated. The interfacial area is dependent upon the bubble size and gas hold-up in the mixing vessel as given by ... 

The above two equations must be solved simultaneously and will require the solution of an equation of cubic form. These correlations are based on the gas phase being sparged into the mixing vessel. Gas dispersion from surface entrainment due to votexing, etc., is not included. The mixing power dissipation must be corrected... 

Application of the de minimis limitation to process streams must also be reviewed. Mixtures containing toxic chemicals can be added to a process or generated within a process. In both cases (assuming reporting thresholds are exceeded) a facility is required to consider and report releases from the process up to the point where the concentration of the chemical falls below the de minimis level. For example, a 10% solution of a listed chemical is mixed Into a formulated cleaning solution, resulting in a final concentration of less than 1%. Releases such as air emissions, from the mixing vessel must be counted, but releases from the finished formulation are not counted because the de minimis exemption applies... 

The following procedure may be adopted in the design of a mixing vessel for a given process. 

Sano, Y. and Usui, H., Interrelations among mixing time, power number and discharge flow rate number in baffled mixing vessels, J. Chem. Eng., Japan, 18 47-52, 1985. 

Equation 8-13 shows that in a perfeetly mixed vessel, the internal and exit eonditions are identieal. Figure 8-4 illustrates the internal age distribution eharaeteristies. 

The intensity funetion of a eompletely mixed vessel is a eonstant, A(6) = 1, a straight line parallel to the abseissa. 

The intensity funetion of two or more identieal, ideally mixed vessels in series starts from the origin and inereases monotonieally to an asymptotie value (A —> n), whieh is equal to the number of vessels. 

Wiped film stills in place of continuous still pots —Centrifugal extractors in place of extraction columns —Flash dryers in place of tray dryers —Continuous reactors in place of batch —Plug flow reactors in place of CFSTRs —Continuous in-line mixers in place of mixing vessels... 

We can now return to the problem of the feeding a salt and water to a mixing vessel that initially contains water, and from which the flow is governed by gravity without the... 

When prepanng mobile phase mixtures each individual component should be measured out separately and only then placed in the mixing vessel This prevents not only contamination of the solvent stock by vapors from the already partially filled mixing vessel (e g ammonia ) but also volumetnc errors caused by volume expansions or contractions on mixing... 

The CSD from the continuous MSMPR may thus be predicted by a combination of crystallization kinetics and crystallizer residence time (see Figure 3.5). This fact has been widely used in reverse as a means to determine crystallization kinetics - by analysis of the CSD from a well-mixed vessel of known mean residence time. Whether used for performance prediction or kinetics determination, these three quantities, (CSD, kinetics and residence time), are linked by the population balance. 

Misch-barkeit, /. miscibility, -behaiter, m. mixing vessel (tank, drum, etc.), mixer, -be-stand, m. (Agric.) mixed stand, mixed crop, -bleichen, n. mixed bleaching, -bottich, m. mixing tub or trough, -diinger, m. mixed fertilizer compost, -element, n. mixed element (a mixture of isotopes). 

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