Heat exchange – With means flexing – jarring or vibrating heat exchange surface
Reexamination Certificate
2001-07-25
2003-09-16
Bennett, Henry (Department: 3743)
Heat exchange
With means flexing, jarring or vibrating heat exchange surface
C165S104160, C122S00400R, C422S146000
Reexamination Certificate
active
06619383
ABSTRACT:
FIELDS OF THE INVENTION
The invention relates to a heat exchange between a gas and falling pulverulent matter.
BACKGROUND OF THE INVENTION
A common industrial operation entails recovery of heat from the combustion of a fuel or of “waste heat” from a chemical process. Such recovery often entails the cooling of a hot gas against water, the water being either heated or converted to steam. Conventional equipment for cooling a gas is often large in size, because coefficients of heat transfer from a gas to a metal surface, in general, are relatively small, e.g., only a few tens of watts/m
2
−C. Achieving a high coefficient of heat transfer entails acceptance of a high pressure drop in the gas to be cooled. In practice, a balance must be struck between the capital expense for providing a larger heat exchanger and the running cost of a smaller exchanger, requiring higher pressure drop necessary in smaller equipment for it to perform the desired heat exchange.
Often, gas to be cooled is dirty, and in some instances, the dirt has properties causing it to foul heat-transfer surfaces with which the gas comes into contact. A notorious example is the off-gas from an electrometallurgical procedure for making ferrosilicon. This gas, as it enters a waste-heat boiler, contains an exceedingly fine fume of silicon dioxide, which fouls boiler surface so rapidly that a practice is to subject the surface to a shower of ball bearings every few minutes, cleaning it of adhering fume particles; yet even with this expedient, a larger boiler surface must be provided than would be necessary for cooling a clean gas. In some instances, a gas to be cooled contains a corrosive chemical species (such as hydrogen chloride), harmful to metal surfaces and over time reducing their effectiveness for transferring heat. Another notorious example arises in the manufacture of a fine titanium dioxide powder by burning titanium tetrachloride. It is difficult to maintain a reasonably continuous operation of the enormous “trombone” heat-exchanger now used for cooling products of this combustion.
Heterogeneously catalyzed reactions, in general, are carried out either in fixed beds of a granular catalyst or in fluidized beds of a catalyst powder. In the latter, control of reaction temperature is relatively easy, since coefficients of heat transfer from a fluid bed to surfaces embedded therein are generally high, often in the hundreds of watts/m
2
−C. If, however, outcomes of a reaction are highly sensitive to axial gas dispersion (see Tshabalala and Squires,
AIChE Journal
, vol. 42, pp. 2941-2947, 1996), a fluid bed may not be a good choice. If a fixed bed must be specified, either a low coefficient of heat transfer from the reaction to surfaces within the bed must be accepted or a designer must adopt other expedients for controlling the bed temperature, such as employing a large gas recycle or injecting cold gas at intervals along the bed.
Herein, by the term “vibrated bed,” I mean a bed of powder in a chamber with a floor, this floor being vibrated vertically at a vibrational intensity sufficient to cause the powder to display the “coherent-condensed vibrated-bed state” (see Thomas, Mason, Liu, and Squires,
Powder Technology,
vol. 57, pp. 267-280, 1989). In this “state,” the powder becomes highly fluid. For example, application of only a small force is needed to move a stirring rod introduced into a vibrated bed from side to side. In general, intense vibration of a powder bed deeper than ~1 mm causes the powder to enter the coherent-condensed vibrated-bed state.
I now provide a definition of “vibrational intensity.” I take the “null position” of the aforementioned floor to be its elevation when at rest. When it is subjected to a vertical sinusoidal vibration, its vertical displacement &zgr; from its null position is given by &zgr;=&agr;
0
sin {overscore (&ohgr;)}t, where &agr;
0
=the maximum displacement (called “amplitude” in the terminology of vibrated-bed engineering art); {overscore ({dot over (&ohgr;)})}=2&pgr;ƒ; t=time; and ƒ=frequency. Vibrational intensity is the ratio of the floor's maximum acceleration to the acceleration of gravity, and is given by &agr;
0
{overscore (&ohgr;)}
2
/g. For coarse powders, the threshold vibrational intensity for creation of a vibrated bed is a little greater than 1.0; for fine powders, the theshold intensity can be considerably higher than 1.0. (See Thomas, Mason, Liu, and Squires, 1989.) In commercial practice, vibrational intensities greatly exceed these thresholds. Intensities as high as 15 are commonly used.
Industry employs vibrated beds extensively for drying particulate material. The beds are sometimes large, e.g., several meters in width and ten or more meters in length. Heat of drying is sometimes provided by indirect heat transfer across heat-exchange surface positioned within the drying bed. A vibrated bed presents coefficients of heat transfer comparable to those afforded by fluid beds (see Thomas, Mason, Sprung, Liu, and Squires,
Powder Technology,
vol. 99, pp. 293-301, 1998). Accordingly, the quantity of heat-exchange surface required for indirect heat transfer in a vibrated bed drier can be small. In other vibrated-bed driers, heat of drying is provided by direct heat transfer from a hot gas introduced into the bed from below (thereby creating an “aerated vibrated bed”).
Little power is required for vibrating a vibrated-bed drier if it is spring-mounted and vibrated at a natural frequency of its mount. An aerated vibrated bed for drying a relatively coarse pulverulent solid can often require far less power than a fluid bed for drying the same solid. The velocity of hot gas across the aerated vibrated bed can be small relative to the velocity necessary to fluidize the coarse solid, and so power required for gas compression can be far below that needed to supply hot fluidizing gas to a fluid-bed drier for the same solid. Power required for vibration can be as little as 10% of that which a fluid-bed drier requires for gas compression.
The high heat-transfer coefficients generally afforded by vibrated beds make them, in principle, attractive candidate devices for heat-exchange applications other than for drying particulate materials. As a practical matter, how to use a vibrated bed for recovery of heat from a hot gas, for example, is not obvious. Contemplating use of a vibrated bed for this application, I hoped to develop a heat exchanger in which hot gas would flow horizontally across the surface of a bed in which heat-exchange surface is embedded (see Sprung, Thomas, Liu, and Squires, in
Fluidization V,
edited by V. K. Ostergaard, Engineering Foundation, New York, 1986, pp. 409-416). With proper choice of particle size and vibration parameters, the surface of the bed would be dilute (i.e., surface powder would display the diffuse “coherent-expanded vibrated-bed state”—see Thomas, Mason, Liu, and Squires, 1989). I hoped for an effective exchange of heat from the hot gas to the diffuse surface of the bed; or, failing that, I hoped that obliging the gas to flow through constrictions created by vertical baffles extending from the ceiling nearly to the bed's surface would cause a sufficient quantity of powder to become entrained in the gas, thereby cooling the gas. Unfortunately, the coefficient for transfer of heat from horizontally flowing gas to a vibrated-bed surface was disappointingly small; and, as well, providing the vertical baffles did not sufficiently improve the rate of heat transfer at an acceptable pressure drop in the gas.
SUMMARY OF THE INVENTION
My invention overcomes the shortcomings of my aforementioned idea.
In the invention, a duct of substantially rectangular cross-section houses a vibrated bed of a pulverulent material and a superjacent space. The invention employs the vibratory motion that creates the vibrated bed to lift pulverulent matter from the bed in a continuous flow to substantially the elevation of the ceiling of the duct. The invention also employs the vibratory motio
Bennett Henry
Creighton Wray James
McKinnon Terrell
Narasimhan Meera P.
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