Static line mixer

Disclosed is a liquid mixer having a multiplicity of slotted orifice plates spaced apart along the flow path within a chamber. Liquid passes through and exits from the slots at a 30-60 degree angle to the exit face of the orifice plates, thereby inducing turbulence which causes good mixing. Preferably the slots are radially disposed in circular orifice plates fitted closely within a cylindrical chamber. The radial length L of the slots is preferably five times the slot width T, and the spacing S of the orifice plates is 4-8 times the width. Straight slots are simplest to make but curved slots are preferred. Radial slots in a circular disc are preferred but other orientations are useful. When used for dispersing small volumes of water into oil, water is injected transversely into the oil upstream of the orifice plates, to cause initial droplet formation; and, the oil-water fluid velocity through the slots is kept in the range of 80-1600 feet per second.

TECHNICAL FIELD 
The present invention relates to in-line fluid mixers which have no moving 
parts, particularly to mixers for introducing a fine dispersion of water 
into oil. 
BACKGROUND 
It is often desired to intermix two fluids intimately, on a continuous 
basis. Such is the case when there is a primary fluid flowing through a 
pipeline, and it is desired to evenly disperse the second fluid into the 
first. Such intermixing is particularly difficult when there is very 
little mutual solubility between the fluids, e.g., such as exists with 
common petroleum oil and water. 
Numerous devices to achieve intimate line mixing between two fluids have 
been known heretofore. Many line mixers are described in J. H. Perry, 
Chemical Engineers Handbook, Fourth Edition (1963), McGraw Hill Book Co., 
New York. One type is a jet mixer, wherein one of the liquids is pumped 
through a small nozzle or orifice into a flowing stream of other liquid. 
Such types of devices are generally only successful in liquids which have 
low interfacial tension, i.e., those that are miscible. Another type of 
mixer is one in which the liquids are flowed together simultaneously down 
a pipeline, and pass through a series of nozzles or orifice plates spaced 
apart along the pipeline. See for instance U.S. Pat. No. 3,856,270 to 
Hemker wherein a series of perforated plates having channels in their 
surfaces are placed in face to face contact within the fluid stream. 
Herbsman et al in U.S. Pat. No. 1,924,038 discloses an apparatus with a 
multiplicity of orifices and nozzles, to mix, divide, and induce a rotary 
motion in the fluid. Christenson et al in U.S. Pat. No. 2,802,648 
discloses a combination of jet mixer and orifice plate mixer. After the 
fluids are intermixed, they are caused to flow downstream through a series 
of perforated plates mounted along a shaft. 
Other mixers also are known, some of them quite elaborate, all with the 
goal of achieving good dispersions in a uniform manner. See for example 
U.S. Pat. No. 4,087,862 to Tsien and U.S. Pat. No. 3,582,365 to Lindsey. 
However, when mixing relatively crude or dirty materials it is a problem 
if a mixer is constructed of rather complicated passages, fragile 
passages, or very small passages. Such features create difficulty in 
obtaining uniform operating conditions, and can make the units difficult 
to maintain, and costly as well. 
The present invention is particularly concerned with introducing and 
dispersing as very fine uniform droplets a small quantity of water into a 
flowing stream of petroleum oil, as described in my U.S. Pat. No. 
4,335,737 for Apparatus and Method of Mixing Immiscible Fluids. In 
particular, the patented invention is aimed at dispersing small quantities 
of water in a fuel oil stream, becuase it has been found that doing such 
provides increased combustion efficiency and savings in energy costs. As 
is well known, the quantity of fuel which flows to a combustor can vary as 
a function of time. My related invention provides for the proper 
proportioning of the small quantity of water, according to the flow of 
fuel oil. But, to be effective, a line mixer, or emulsifier as it is 
called in my related application, must be capable of achieving good 
dispersion of the water at varying flow rates. In addition, the pressure 
drop through the mixer ought not to be so great as to necessitate 
exceptionally high pressures. Further, the mixer ought to be capable of 
operating with viscous liquids with substantial solid particulate content, 
as characterizes SAE No. 6 fuel oil. The prior art mixers are not well 
suited for this. 
Another problem with many types of mixers in the prior art is that they 
require rather involved engineering calculations when the size of the unit 
is being changed. That is, if successful results are achieved in one size 
of mixer, the complexities of fluid dynamics must be taken into account if 
a larger or smaller unit is desired. Simple proportioning, as is well 
known to those skilled in fluid dynamics, will not often achieve the same 
results. Thus, since there is a desire that line mixers have different 
total flow capacities, it is desired that the design of a mixer be such 
that it is readily made to different scales. 
DISCLOSURE OF THE INVENTION 
An objective of the invention is to provide a simple and reliable mixer 
having no moving parts, wherein the mixer is especially adapted for 
introducing small quantities of water into oil and obtaining an emulsion 
thereof. A further object is to provide a mixer which is not prone to 
malfunction when small quantities of particulate are present in the fluid 
streams. A further object is to provide a mixer design which may be 
readily altered to provide different volumetric capacities. 
According to the invention a mixer is comprised of a body having a chamber 
with a flow path for the flow therethrough of co-mingled fluids. Within 
the chamber are a multiplicity of spaced apart slotted orifice plates, 
perpendicular to the flow path. Preferably there are four circular flat 
plates and the slots are radially disposed in the plates. In each plate 
the slot passages are angled with respect to the longitudinal axis of the 
plate. Thus, the slots will discharge fluid at an angle, preferably 30-60 
degrees, to the exit surface of the plate. The slot passages may be 
straight or curved, but most importantly the discharge of the fluid at an 
angle to the exit of the orifice plate provides the good mixing action. 
Each slot will have substantial length L compared to the width T of the 
slot passage. However, the slots cannot be made too small in width 
elsewise they are prone to plugging by particulate. Therefore, for oil and 
water the slots are 0.030-0.065 inch in width and the length L is at least 
two times, and preferably five times, the width. Desirably, the total 
cross sectional flow area of all the slots in any orifice plate is equal 
to the flow area of the pipe delivering fluid to the mixer, to minimize 
pressure losses in the mixer. The through plate length D of the slot is 
less critical, provided it is sufficient (nominally at least twice the 
width T) to establish stream line flow through the slot and to achieve the 
desired slot exit flow conditions. 
For oil and water it is found that the flow velocity through the slots must 
be 80-1600 feet per second. Greater or lesser flow velocity results in 
poor emulsification, according to test data. The spacing between the 
adjacent orifice plates is important as well. Preferably the spacing S is 
4-8 times the slot passage width. If too close, excessive pressure drop in 
the mixer results and the desired turbulence at the slot exit region is 
not obtained. Any number of plates beyond one can be used, but the number 
ought to be minimized to that necessary to first reach the desired 
dispersion. With oil and water, for example, it has been found that four 
plates are needed to obtain a good emulsion; but more than that does not 
produce additional benefit insofar as the mixture is concerned. 
In a preferred embodiment the second fluid water is introduced into the 
first fluid oil upstream of the first orifice plate by means of an 
injection tube. The tube shape causes shearing of the water stream and 
high local turbulence. This creates an initial dispersion, thereby making 
the orifice plate action more effective. The desired injection mode is 
obtained by giving the water a velocity transverse to the oil velocity, 
and maintaining the water velocity at less than 7% of the oil velocity at 
that point. 
The mixer is especially advantageous because simple change in the number or 
length of the slots can alter the capacity of a particular unit. Thus it 
is easy, for instance, to maintain the fluid velocity at the slots in the 
range of 80-1600 feet per second which has been found critical for a good 
oil and water emulsion. The use of slots in the orifice plates, compared 
to circular orifices or other passages of less effectiveness, means that 
the minimum number of orifice plates can be used. Thus the pressure drop 
incurred by fluids passing through the unit is minimized. The mixer is 
easy to construct and service. 
These and other objects, features, and advantages of the invention will be 
understood further from the description which follows.

DESCRIPTION OF THE BEST MODE 
The invention is described in the terms of the introduction of a second 
fluid, water, into a pipeline stream flow of a first fluid, oil. This will 
illustrate the use of the invention in the apparatus described in U.S. 
Pat. No. 4,335,737, the disclosure of which is hereby incorporated by 
reference. Nevertheless, it will be understood that the invention will be 
useful for many other fluids and applications. 
FIG. 1 shows in longitudinal cross section the inventive mixer 20 as it 
appears installed in a pipeline. The mixer 20 is made of metal and is 
comprised of an inlet end 22 and exit end 24, connected by a hollow 
cylinder central member 26. The mixer is connected at its ends to the 
pipeline 28,30, through which oil flows. Captured within central member 
26, between the inlet and exit ends is an assembly 32 of spaced apart 
orifice plates 34. Each of the plates 34 is a disc having a multiplicity 
of angled slots 36, connecting the upstream disc face 37 with the 
downstream face 39, as described in more detail below. The discs 34 with 
cylindrical spacers 42, are mounted on a shaft 38, and they are retained 
on the shaft (which has threaded ends) by nuts 40. 
The inlet end 22 has a chamber 44, into which projects the second fluid 
injection tube 46. The end 47 of the tube 46 is closed. There are opposing 
discharge holes 48 along the length of the tube, where it projects into 
the chamber 44, as shown in the detail of FIG. 2. This enables fluid 
passing down the injection tube 46 to discharge into the chamber in a 
direction perpendicular to the longitudinal axis 49 of the mixer 20, to 
thereby provide a shearing action which causes initial disintegration of 
the water stream into droplets. The interior cavity of the inlet end 22 
narrows to passage 50, and then expands to the diameter of the interior 
chamber 52 of the central member, where the assembly 32 of orifice plates 
is positioned. The exit end 24 is configured similarly to the entrance 
end, but does not contain any projecting injection tube; it serves 
similarly to provide communication of the chamber 52 with the downstream 
pipe 30. It is seen that the interior passage 54 of the exit end narrows 
down to the nominal inside diameter of the pipe 30. Set screws 56 join the 
central member 26 to the inlet and outlet ends, and prevent them from 
separating. 0-ring seals 58 prevent leakage where the ends join the 
central member. 
FIG. 3 shows an axial section through the mixer, just downstream of the 
first orifice plate 60. It is seen that the orifice plate has a 
multiplicity of slots, radially disposed around its periphery. FIG. 4 is a 
more detailed fragment of the longitudinal cross section of the central 
portion of the mixer shown in FIG. 1, and when considered in conjunction 
with FIG. 3 will lead to an understanding of the particular nature and 
importance of the slots which characterize the orifice plates. In the disc 
60 shown in FIG. 3 the orifice plate has 16 equally spaced apart slots 36, 
which I have found to be most satisfactory. Each slot is characterized by 
a length L. The length is somewhat arbitrary and may vary, but usually it 
is made as long as possible without structurally weakening the orifice 
plate. FIG. 4 shows an end view of a slot 62 which lies along a radial 
which is normal to the plane of the paper. The slot has a width T and a 
through-plate length D, hereinafter characterized as depth. The slot 
D-length axis 51 is at an angle A to the longitudinal axis 49 of the 
mixer, which corresponds with the longitudinal axis of the disc. The 
orifice plate discs are spaced apart from each other a distance S, where S 
is the distance between the downstream side of a first disc, and the 
upstream face of the next disc. 
It should be appreciated that the fluid oil introduced from the entrance 
pipe 28 will flow at a first velocity through chamber 44, then increase in 
velocity through the passage 50, and then slow again as it enters the main 
chamber 52. The constriction 50 in flow area is for construction 
convenience of the particular design shown, and is not essential. When the 
oil flows past the water injection tube 46, water under a pressure greater 
than that of the oil in the chamber 44 is flowed through the holes 48, 
whereupon it first mixes with the oil, as large droplets. The holes 48 are 
sized so that the velocity of the water is relatively low. For instance 
four holes of 0.081 inch diameter are suited for 6-60 gallons per hour 
(gph) of water into 60-600 (gph) of oil flow. The nominal water velocity 
ranges between 1.5 to 15 feet per second (fps) and is about seven percent 
of the nominal velocity of the oil in the chamber 44. The water should 
have the foregoing low velocity as it exits from the holes 48, so that it 
is easily entrained by and first mixed with the oil, without flowing 
rapidly toward the periphery of the chamber 44. The holes 48 are placed 
perpendicular to the flow line of the oil, and the longitudinal axis 49 of 
the mixer, to promote a relative shear action of the oil on the water, and 
to avoid a blockage of the holes by any foreign particles which may be 
flowing along with the oil. 
The circular cross section of the tube 46 is designed to cause high 
turbulence immediately downstream of the tube. This desirably causes some 
initial mixing of the water within the oil, but for most applications this 
is entirely insufficient. When the mixture flows into central chamber 52, 
it is forced to flow through the slots 36 of the first orifice plate 34, 
60. Since the slots 36 are angled with respect to the longitudinal axis 
and the overall flow direction of the oil, the oil is turned to flow at an 
angle to the axis 49 and assumes a rotational swirling type motion, as it 
exits from the first orifice plate. It continues flowing axially 
downstream, where it encounters the second orifice plate, and thereafter 
the third and fourth orifice plate. At each the stream is divided and 
recombined, as it is forced to pass through the multiplicity of passages. 
Finally, the oil and water mixture, which will ordinarily be now found to 
be an emulsion through the action of the mixer, will exit through the 
passage 54 and proceed down the exit pipe 30. 
FIG. 5 shows how the oil water mixture flows through a typical orifice 
plate slot, and how the slot aids in mixing. Upstream of the plate the oil 
is flowing in a generally swirling pattern, as it approaches the entrance 
64 of a slot 66 in a disc 68. The mixture passes through the slot, and at 
the slot exit side 70 it is moving at an angle to the downstream face 72 
of the orifice, as represented by lines 74. At the exit face where the 
angle between the discharge flow lines 74 and the face 72 is acute, there 
is great turbulence generated, represented by curved lines 76. This 
turbulence is believed to operatively cause dispersion of the water within 
the oil, by breaking the water droplets into finer and finer particles, 
and ultimately causing what may be characterized as an emulsion. The 
invention is only effective if the angle A is appreciable. That is, slots 
which are normal to the exit surface (A=zero) are not effective in 
establishing the desired turbulent flow. Obviously, if A is made too great 
(approaching 90 degrees) then my device would not be functional, because 
the slot depth would be too great, the part would be very difficult to 
make, and there would be very few slots permitted in any given disc. 
Preferably, in the practice of my invention A is between 30-60 degrees; I 
have found that 45 degrees is most satisfactory. 
The slots may be placed in my orifice plates by conventional machining 
techniques, such as by sawing. Since I have identified the creation of the 
turbulent flow conditions at the exit of the slot to be important, the 
configuration of the disc fragment 79 shown in FIG. 6 would be an even 
better embodiment of the invention, but for the fact that it is more 
difficult to machine. As indicated in FIG. 6, the discharge end 78 of the 
slot 80 would be a curved passage, resulting in a nominal discharge angle 
A' for the stream flowline 82; the flowline being essentially tangent to 
the slot passage curve at the exit. As will be understood by those with 
skill in fluid dynamics, strong turbulence 84 will be created at the 
upstream side 81 of the curved passageway. Thus, the curved slot design 
can enable fewer orifice plates to be used when a certain dispersion is 
sought, thereby lowering the pressure drop through the entire mixer. 
From my work with heavy oil I have found that the disc spacing dimension S 
should be at least about twice the thickness of a 0.125 disc, and at least 
about 0.250 inch. If it is too small then excess pressure drop will be 
caused and the turbulent action at the slot exit may be inadequate to get 
good dispersion. If it is large the mixer will function acceptably but it 
becomes unnecessarily long. As will be appreciated with further discussion 
herein, the spacing S is also related to the width T of the slot, and it 
ought to be greater than 4 times the slot width, preferably 4-8 times. 
As shown in the Figures, preferably each orifice plate has slots angled to 
impart a circumferential velocity component to the fluid in the same 
sense. Alternate circumferential flow-reversing may be used but with 
greater pressure drop and somewhat decreased effectiveness when the 
spacing dimension S is small. The slot width T may vary. Preferably it is 
small, at about 0.060 inch or less. Relatively small slots of about 0.030 
inch width are usable, but only for fluid streams where there is an 
absence of particulates which may block the passage. 
Slots are particularly advantageous compared to other orifice shapes, such 
as circular holes. They provide relatively high ratio of peripheral edge 
area to cross sectional flow area, thereby increasing the region at which 
the turbulence takes place and decreasing the number of stages to achieve 
a desired dispersion. In addition the capacity of a unit, or of different 
units, may be varied easily by changing the slots' lengths L. The 
dimension L may be increased or decreased with assurance that satisfactory 
results will be achieved without fluid dynamic scaling problems of 
consequence. Since the phenomena occuring at the upstream side of the exit 
end of the slots has been identified as being important, it is desirable 
that the slot cross section aspect ratio, L/T, be maintained at a 
relatively high value, about 5:1 for efficient performance of a particular 
orifice plate. 
I have discovered important relationships for the slot area when dispersing 
water in heavy fuel oil. Referring again to FIG. 3, the slot flow area in 
aggregate for any disc is nominally equal to the number of slots 
multiplied by the length L and the slot width T. Preferably, the aggregate 
slot area of a disc will be about equal to the cross sectional flow area 
of the inlet pipe 28. With this relationship and with 300 SSU heavy oil, 
into which is introduced five volume percent water, a pressure drop of 
about 2 psig over each disc will result when the mixer is used to process 
540 gph of oil. For four plates in a mixer, this represents an acceptable 
pressure drop in the typical oil pipeline which feeds a combustor. 
For a mixer like that shown in FIG. 1, where each two inch diameter by 
0.125 inch thick disc has sixteen slots of about 0.5 inch length and width 
between 0.030-0.065 inch, and where the angle A is about 45 degrees, the 
velocity through the slots is critical, as illustrated by FIG. 7. The 
emulsion which results can be examined by means of a microscope. A 
satisfactory emulsion should have a bulk of the droplets at less than 
15.times.10.sup.-6 m, with the average around 7.times.10.sup.-6 m. As 
indicated in FIG. 7, for the apparatus with 0.032 inch wide slots, when 
the flow drops below 30 gph, or exceeds 600 gph, the fineness of the 
dispersion decreases unacceptably. In the first instance, the velocity 
through the device, and the slots in particular is probably too low to 
cause sufficient turbulence. At the higher flow rate, about 600 gph, it 
appears that different flow conditions are obtained, and the quality of 
the dispersion drops again. At the higher flow rate the pressure drop over 
all the four plates is about 8 psig. 
In contrast, the performance of the 0.062 inch slotted orifices is such 
that when the flow drops below about 60 gallons per hour, the dispersion 
becomes unacceptable. The upper limit was not able to be measured, but it 
is my conclusion from experiments that at a flow of 1200-1600 gph the 
quality of dispersion will again drop. At about 600 gph the pressure drop 
with the 0.062 inch slots is appreciably less at about 4-5 psig. This is 
understandable since the flow area of the totality of slots is 
approximately double that of the 0.032 inch wide slotted discs. From the 
foregoing it can be calculated that the fluid velocity through the slots 
is critical and should be at least 80 feet per second (fps) and less than 
1600 fps. 
My basic work was performed on mixers which contained four orifice plates 
through which the fluid passed sequentially. With lesser numbers, a 
dispersion inadequate for my purposes was created. However, lesser number 
of plates, even a single plate, may be satisfactory in other applications. 
For greater than four plates, up to eight, the improvement in dispersion 
for the water-oil mix did not warrant the increased pressure drop. 
However, in liquids where dispersion is more difficult, additional plates 
may be used beyond the four I found satisfactory. 
Other configurations of slotted orifice plates are within the scope of the 
invention, including but not limited to, the configurations shown in FIGS. 
8 and 9. In FIG. 8 it is seen that smaller shorter length slots 86 may be 
interspersed between the longer radial slots 88. (As in all the preferred 
orifice plates in my mixer, the slots have identical width. However, 
smaller slots or other holes may be placed on any orifice plate without 
adversely affecting the performance of my basic invention.) In FIG. 9, the 
slots 90 are arrayed parallel to a particular diameter of the orifice 
plate 92; this may simplify manufacture. And of course, there is no 
limitation on the exterior configuration of the orifice plates of my 
invention; they may be square, rectangular, etc. Other variations in the 
details of construction will be within the scope of the broader 
embodiments of the invention. While the mixer has been described above in 
terms of a body comprised of three separate elements, end 22, end 24, and 
central member 26, it should be evident that this configuration is but one 
which is convenient for construction and maintenance. Generally, the mixer 
is comprised of a body having an internal passage through which the second 
fluid flows, and wherein are placed the orifice plates. Similarly, the 
manner in which the orifice plates are spaced apart may be varied. For 
example, cylindrical spacers fitting the bore at chamber 52, at the outer 
diameter of the plates may be used; steps, projections, etc., in the bore 
chamber 52 also may be used. 
Generally, the in-line construction of the mixer is preferred. But the 
inlet and outlet need not be co-aligned; they may be offset or angled. 
Also, the injection tube may be located apart from the other body interior 
parts of the mixer. Or, in the case of two fluids which are presented 
already intermixed, but not fully dispersed, the mixer may be used without 
the injection tube at all. 
While the invention has been described in the foregoing preferred 
embodiment and alternatives, it should not be so limited, as it is capable 
of many modifications, and changes in construction may be made without 
departing from the spirit and scope of the invention.