Gas and liquid contact apparatus – Fluid distribution – Valved
Reexamination Certificate
1999-07-09
2002-07-16
Chiesa, Richard L. (Department: 1724)
Gas and liquid contact apparatus
Fluid distribution
Valved
C261S117000, C261SDIG009
Reexamination Certificate
active
06419210
ABSTRACT:
BACKGROUND—FIELD OF THE INVENTION
This invention is intended to improve liquid-to-gas contacting in a reversed jet installation where the jet flow must be throttled for process control reasons.
Reversed jets (also known as counter-current sprays) are used where it is desired to intimately mix or contact a liquid with a continuously flowing gas stream. The velocity of the gas stream must be high enough to reverse the direction of the sprayed liquid within the confinement of the wall of the gas's conveying pipe or duct. Typical, but not limiting, uses for a reversed jet are for scrubbing particulate from a gas stream and/or for heating or cooling a gas stream.
BACKGROUND—DESCRIPTION OF PRIOR ART
FIG. 1
shows a reversed jet installation using a convenient process pipe elbow to mount the reversed jet's spray nozzle. A stream of gas
3
passes through a section of process pipe
6
. A liquid under a pressure
8
is sprayed as a jet
7
through a noble
4
into gas stream
3
in a direction opposite to that of gas stream
3
. The velocity of gas stream
3
must be at least great enough to cause an abrupt reversal of spray
7
and to convey any residual droplets. This velocity is often called the minimum flooding velocity. For a water spray and air the minimum flooding velocity is typically 30 ft./sec at the wall of pipe
6
. However, minimum flooding velocity can vary depending on the physical properties of the fluids and the operating conditions of the installation.
The liquid supply pressure at
8
can also vary but it is usually great enough to give spray
7
a velocity of at least 40 ft./sec. Typically, there is a relative velocity between the spray and the gas of at least 70 ft./sec. Spray
7
is usually a solid-cone type with a 15 to 20 degrees included angle. Liquid flow rate to nozzle
4
is determined by the nozzle's size and the difference between process pressure in pipe
6
and the liquid's supply pressure
8
. A valve
1
is used simply for on-or-off control of flow to nozzle
4
.
As drops of sprayed fluid
7
leave nozzle
4
, they face a high relative velocity with respect to counter-current gas stream
3
. This counter-current action continues with two factors acting to reduce the momentum of spray
7
. (1) The high relative velocity between a drop and the gas causes the drop to shatter to smaller droplets with a net gain in surface area and with each resulting droplet having a smaller mass. With net higher surface giving increased drag to lower their velocity and individually less mass than the original drop, the droplets rapidly lose momentum against counter-current gas stream
3
. (2) In addition, the momentum per unit cross sectional area of the cone-shaped spray reduces as spray
7
moves away from nozzle
4
. It is believed that a contact zone
2
forms at the point where the momentum per unit cross sectional area of counter current spray cone
7
equals the momentum per unit area of gas
3
. In contact zone
2
where droplets with high surface area are reversing their direction there is severe turbulence. This combination of small drops with their high interface surface area in a condition of high turbulence results in a rapid transfer of heat and mass between the liquid and gas phases.
Energy for shattering droplets in spray
7
and for maintaining a high degree of turbulence in contacting zone
2
is believed to be supplied by the kinetic energy of fluid jet
7
issuing from nozzle
4
(derived from D. Low U.S. Pat. No. 3,803,805 1974).
(1) In engineering units K.E.=W×V×V/2 g where:
W=pounds of fluid/sec.
V=fluid jet velocity in ft./sec.
g=32.2 feet/sec./sec.
K.E.=Kinetic energy of fluid jet in ft.-pounds/sec.
P.E.=Potential energy as represented by the difference between the static fluid supply pressure at
8
in FIGS.
1
,
2
,
3
and
7
and the static pressure in pipe
6
.
From the above relationship it is evident that kinetic energy per unit mass of sprayed fluid for contacting gas
3
with spray
7
varies as the square of the spray's velocity as it issues from spray nozzle
4
. Since ▪W▪ is also directly proportional to the spray's velocity, it is also evident that absolute or total kinetic energy in the spray varies as the cube of the spray's velocity.
As might be expected, a narrow-cone counter-current spray will penetrate farther into the gas stream than a wider cone spray. This is of particular importance in the elbow installation of
FIG. 1
where the spray must pass through the elbow area so that contact zone
2
will form in the horizontal leg of pipe
6
which is needed to confine contact zone
2
. It is also evident that a narrow angled spray cone is more suitable if the velocity of gas
3
is significantly higher than minimum flooding velocity.
The arrangements for a reversed jet as shown by
FIG. 1
is satisfactory for an installation where the flow from nozzle
4
is constant, such as in a scrubbing application where particulate is removed from a gas. However, if the installation, for example, requires cooling an incoming gas stream
3
that varies in temperature and enthalpy to a predetermined final temperature, then the flow rate of spray
7
will be called upon to vary. This variation is presently done by automating valve
1
in
FIG. 1
which then becomes FIG.
2
. An automatic controller
10
in
FIG. 2
now adjusts an automatic throttle valve
9
to vary liquid flow from nozzle
4
so as to maintain a predetermined temperature at a measuring point
11
. Instrument control lines
12
allow communication to and from controller
10
.
Using equation (1) for kinetic energy, FIG.,
2
A is a graphical representation of the kinetic energy in spray
7
as a function of flow when using the reversed jet configuration of FIG.
2
. Curve A shows the decrease in kinetic energy in spray
7
per unit mass of the spray as a function of flow from spray nozzle
4
. Curve B shows the total kinetic energy in spray
7
as a function of flow from spray nozzle
4
. This total kinetic energy is important in propelling the spray to contact zone
2
.
The arrangement in
FIG. 2
will give good contacting between liquid and gas phases if variations in the heat or cooling load of incoming gas
3
are small, for example, from a 100% load down to a 90% load. However, if load changes are large, then the energy available for good contacting between the phases may be inadequate. For example, suppose in
FIG. 2
during the start-up phase of an operation the cooling load of incoming gas
3
is only 20% or one fifth of its full load value. Then only one fifth as much liquid from nozzle
4
at one fifth the full-load jet velocity is needed to cool gas stream
3
to its final predetermined temperature at
11
. Since energy on a unit mass basis for contacting in zone
2
varies as the square of the jet's velocity, there is only one twenty-fifth or 4% as much energy per unit mass of sprayed liquid
7
as for the spray used for full cooling load. This low energy in the sprayed fluid may give insufficient contacting between gas
3
and spray
7
for the mixture to equilibrate before it reaches temperature measuring point
11
. In this example most of the spray fluid's potential energy at
8
in
FIG. 2
is wasted as pressure drop across automatic throttle valve
9
. The total kinetic energy of the spray reduces as the cube root of flow, and in this case it is only 0.8% of the energy at full spray flow.
In the above case of a reversed jet operating at 20% of its capacity, it is highly likely that the presumed jet would never form a contact zone
2
within the confining wall of pipe
6
in FIG.
2
. Instead, a feeble spray
7
would most likely be deflected toward a down-stream area
5
without good contacting with gas
3
.
This Invention
This invention is a method and the equipment needed to harness energy dissipated at throttle valve
9
in
FIG. 2
when spray nozzle
4
is called upon to deliver less than its maximum capacity for fluid flow. By this invention energy no
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