Note: Descriptions are shown in the official language in which they were submitted.
CA 02224615 1999-11-04
1 FIELD OF THE INVENTION
2 The present invention relates to a nozzle assembly for producing and
3 atomizing a mixture of oil and steam, for injection into the chamber of a
4 reactor, such as a fluidized bed coker. The nozzle assembly comprises
mixing means, external of the reactor, for producing a bubbly flow mixture of
6 oil and steam, and an atomizing nozzle, internal of the reactor, for
converting
7 the mixture into a jet of fine liquid droplets. The invention also relates
to the
8 process involved in use of the assembly.
9 BACKGROUND OF THE INVENTION
Dry bitumen liquid is obtained from oil sands and is further processed
11 in a fluidized bed coker to produce low-boiling petroleum products. The
12 efficacy of this coking process depends upon effective and rapid heat
transfer
13 from the fluidized bed solids of the coker to the bitumen feed. This can be
14 enhanced by introducing the bitumen into the coker in the form of fine
liquid
droplets, thereby significantly increasing the surface area of the bitumen.
The
16 greater the surface area, the more effective and rapid the heat transfer to
the
17 bitumen. In addition, better heat transfer is achieved if the cross-
sectional
18 coverage area of the droplets is increased. The formation of a wide spray
of
19 evenly distributed fine bitumen droplets will increase the desired liquid
distillate products and decrease the undesired by-products, namely, gas make
21 and coke make.
2
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1 The bitumen feed is conventionally mixed with steam to produce a two
2 phase mixture and this mixture is injected into the fluid coker through
nozzles.
3 The nozzles induce some atomization of the bitumen so that a spray or jet of
4 bitumen droplets is injected into the coker. However, it is our belief that
the
commercial nozzles available only atomize a portion of the bitumen. We have
6 tested the prior art nozzles used in the present assignees' fluid coker
using
7 water and steam as the feed - the indication is that 70 - 90 % of the liquid
8 passed through the nozzle is in a non-atomized form. In addition, the
average
9 mean diameter of the atomized liquid droplets produced in the test were in
the
order of 400~,m for an air to liquid ratio (ALR) value of .008; this is larger
than
11 desirable for optimal coking of the bitumen to occur.
12 One aim of the present invention is to provide an improved nozzle that
13 atomizes most of the liquid processed into evenly distributed fine
droplets,
14 resulting in desirable droplet surface area of the bitumen when it is
subjected
to the coking process.
16 In service, nozzles used to atomize the bitumen feed are subject to
17 high wear rates and a high degree of plugging. Therefore it is desirable to
18 ensure that a nozzle not only atomizes the bitumen effectively but is also
19 erosion resistant in order to minimize replacement and repair costs. This
can
be achieved by designing a nozzle assembly with a minimum of internal parts.
21 In addition, the nozzle assembly should be "roddable" so that it may be
22 unplugged with a rod.
23 It is also desirable that a nozzle design be effective over a broad range
24 of gas/liquid ratios because the bitumen feed rate is variable.
3
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1 It is further desirable that the pressure drop required for achieving
2 satisfactory atomization is not excessive.
3 The literature teaches that several flow mechanisms can have an effect
4 on reducing liquid droplet size in two phase flow. More particularly it is
known:
6 ~ That longitudinal vibration of stretched liquid droplets (referred to as
7 "ligaments") can cause reduced diameter at the nodal points and
8 corresponding "necking", which can lead to droplet break-up and
9 formation of finer droplets (this is referred to as the "Rayleigh
instability");
11 ~ That longitudinal stretching or straining of a liquid droplet by an
12 accelerating carrier fluid flow can lead to droplet break-up.
13 Otherwise stated, situating droplets of one fluid in another fluid
14 undergoing acceleration can cause the droplets to undergo shear.
When the shear forces overcome surface tension forces, the
16 droplets will deform and can break up (this is referred to as the
17 "elongation effect");
18 ~ That mean and fluctuating shear stresses applied to larger droplets
19 can cause the droplets to rotate, then stretch and ultimately divide
into smaller droplets. Mean shear effects and stresses can be
21 induced in droplets by turbulent flow and fluctuations of the carrier
22 phase (the "Reynolds stress effect");
23 ~ That droplets can collide with each other or an impact surface and
24 break up if generated internal stress exceeds surface tension
effects; and
4
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1 ~ That droplets passing through a supersonic shock wave can break
2 up due to the effect of the sudden pressure rise.
3 It is the objective of the invention to provide a nozzle assembly, free of
4 moving parts, which can be "rodded" to unplug it and which is designed to
incorporate some or all of the previously mentioned flow mechanisms, to
6 combine in efficiently breaking up large liquid droplets in a two phase
flow.
7 It is to be understood that, while the nozzle was developed for service
8 with fluid cokers in the petroleum refining field, it is anticipated that it
will find
9 application in other fields where atomizing nozzles are used.
11 SUMMARY OF THE INVENTION
12 The present invention is concerned with producing a bubbly flow
13 stream of a mixture of heavy oil and steam and atomizing the mixture. The
14 mixture is a pumped flow or stream in which the oil is present as
relatively
large droplets, typically ranging in dimension between 6,OOO~,m (1/4") to the
16 internal diameter of the supply pipe in dimension. The objective is to
reduce
17 the average mean diameter of the droplets to a relatively fine size,
typically in
18 the order of 300~,m. It has been proven that the highest probability of
collision
19 of bitumen or heavy oil droplets with heated coke particles occurs when
both
bitumen droplets and heated particles have similar diameters; thus a droplet
21 size of 200 or 300~,m is desirable.
22 To achieve this, a nozzle is used having a longitudinal bore or flow
23 passageway of circular cross-section comprising in sequence: an inlet; a
first
24 contraction section of reducing diameter; a diffuser section of expanding
diameter; a second contraction section of reducing diameter; and an orifice
5
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1 outlet. The contraction sections accelerate the flow mixture and induce
2 droplet size reduction by elongation and shear stress flow mechanisms. The
3 second contraction section is designed to accelerate the mixture flow more
4 than the first contraction section. As a result, the fine droplets produced
by
the first contraction are further reduced in size in the second contraction.
The
6 diffuser section allows the mixture to decelerate and slow down before being
7 accelerated a second time.
8 Advantage is taken of the fact that, for a two-phase flow of liquid
9 droplets and gas, the speed of sound in the mixture is reduced in value to a
fraction of either the single-phase values of the gas or the liquid. The
reason
11 for this is that the square of the speed of sound in a media is the ratio
of the
12 adiabatic bulk modulus (liquid) or pressure (gas) divided by the density
times
13 a constant of the fluid properties which in the case of a gas (k = cplcv).
In the
14 case of the two phase mixture, the mixture assumes the pressure value of
the
gas and the density assumes the mixture density. For a two phase flow this
16 usually results in a relatively small numerator, the gas pressure, and a
17 relatively larger denominator, the mixture density. Hence a much lower
value,
18 for the speed of sound of the mixture, is obtained. In fact the speed of
sound
19 in the mixture is lower than either value for the individual gas or liquid
phases.
This means that, for two-phase nozzle flow, for quite a low mixture velocity,
21 the nozzle flow can be supersonic.
22 It has been shown by experiment that this assumption for the low value
23 of the speed of sound in a mixture of a liquid and a gas is only true if
the flow
24 pattern in the nozzle is of the bubbly flow type.
6
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1 The nozzle is therefore preferably designed in accordance with the
2 following criteria:
3 ~ the first contraction section should accelerate the mixture to
4 supersonic velocity - the possibility is then created for generating a
flow of relatively small droplets and generating a shock wave in the
6 diffuser due to the lower speed of sound value. If a shock wave is
7 created, it will assist in droplet size reduction;
8 ~ as previously stated, the second contraction section should
9 accelerate the mixture more than that achieved by the first section.
We successfully used a section that provided about twice the
11 acceleration. If the fine droplets produced by the first contraction
12 and diffuser sections are to be further reduced in size, it is
13 necessary to subject them to relatively increased elongation and
14 shear stress; and
~ the diffuser section should increase the diameter of the flow
16 passageway sufficiently so that the desired acceleration in the
17 second contraction section can be achieved. However the length of
18 the section needs to be limited to avoid excessive recombination of
19 droplets. We found a 3° section to be satisfactory while a 6°
section, total angle, was much less useful.
7
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1 By applying the configuration of tandem contraction sections separated
2 by a diffuser section and observing the aforementioned criteria in the
design
3 of the sections, we produced a nozzle which satisfactorily reduced
relatively
4 large water ligaments of 12,500~,m average mean diameter, present in an
air/water mixture, to relatively fine droplets having an average mean diameter
6 of 300~;m, produced in the form of a jet having substantially evenly
distributed
7 droplets.
8 In adapting the nozzle to service in a fluid coker, where it was to
9 process a steam and bitumen mixture, it was desirable to provide a more
active mixing or pre-conditioner means. The bitumen, a viscous heavy oil,
11 arrives at the coker in the form of a hot, pumped stream in a pipe. It is
12 necessary to convert the mixture into a bubbly flow condition. This was
13 accomplished using the following means:
14 ~ a straight pipe closed at one end by a valve and connected at the
other end with the nozzle;
16 ~ a pre-conditioning nozzle of reducing diameter positioned coaxially
17 in the pipe bore between its ends;
18 ~ an inlet upstream of the nozzle, for introducing the steam; and
19 ~ an inlet, close to and downstream of the nozzle, for introducing the
bitumen.
21 The steam jets out of the pre-conditioning nozzle and contacts the entering
22 slug of bitumen, breaking it up into ligaments. The fast-moving steam mixes
23 with the bitumen and further breaks up the ligaments, mainly by elongation
24 and shear stress, to partially reduce them in size, as the mixture moves
turbulently down the pipe. The pipe is not long - for most efficient use we
8
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1 suggest a length of 10 diameters between the pre-conditioner nozzle and the
2 atomizing nozzle entrance. However, due to practical constraints, this
3 distance may be longer. The combination of steam jetting, turbulence and
4 shear is intended to produce a stream in the condition of "bubbly flovW'.
The invention is characterized by the following advantages:
6 ~ use of the nozzles and active mixing means in connection with a
7 commercial fluid coker demonstrated improved upgraded oil yield;
8 ~ the nozzle has been shown to be capable of reducing the majority
9 of water droplets in a wateNair mixture from about 12,OOO~,m in the
feed to about 300pm in the jet product and the fine droplets are
11 evenly distributed in the product jet; and
12 ~ the nozzle is free of working parts and is roddable.
13 Broadly stated, in one aspect the invention comprises, in combination,
14 a reactor forming an internal reaction chamber and having a side wall
forming
an opening; and a nozzle assembly comprising a mixing means, external of
16 the reactor, and an atomizing nozzle, positioned within the reactor chamber
17 and connected with the mixing means through the opening; the mixing means
18 comprising piping connected with the atomizing nozzle and having an
internal
19 bore, the piping having first inlet means for introducing a stream of oil
under
pressure into the pipe bore upstream of the reactor opening and second inlet
21 means for introducing a stream of steam under pressure into the pipe bore
22 upstream of the first inlet means, a pre-conditioning nozzle, having a bore
of
23 reducing diameter, positioned transversely of the pipe bore, for
accelerating
24 the steam as it passes therethrough so that the accelerated steam contacts
the oil and produces a flow mixture of steam and oil; and the atomizing nozzle
9
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1 comprising a body forming a passageway extending therethrough, said
2 passageway having upstream and downstream ends and comprising the
3 following in sequence, an inlet for introducing the flow mixture, a first
4 contraction section of reducing diameter for accelerating the mixture to
supersonic velocity to reduce droplet size by elongation and shear stress, a
6 diffuser section of expanding diameter for allowing the mixture to
decelerate
7 and to enlarge the cross-sectional area of the passageway, a second
8 contraction section of reducing diameter for accelerating the mixture with
9 greater acceleration than in the first contraction section to further reduce
droplet size by elongation and shear stress and an orifice outlet for
11 discharging the mixture in the form of a jet of oil droplets distributed in
the
12 steam.
13 In another aspect, the invention comprises a method for atomizing
14 heavy oil to be injected into a reaction chamber of a reactor, comprising:
forming a bubbly flow mixture stream of oil and steam in mixing means
16 external of the reactor; and reducing the size of droplets by passing the
17 mixture stream through a nozzle passageway in the reaction chamber, said
18 passageway comprising, in sequence, an inlet section, a first contraction
19 section, a diffuser section, a second contraction section and an orifice
outlet,
so that the mixture stream is introduced into the passageway and accelerated
21 in the first contraction section to reduce droplet size by elongation and
shear
22 stress, then decelerated and cross-sectionally enlarged in the diffuser
section,
23 then accelerated a second time in the second contraction section, with
greater
24 acceleration than in the first contraction section, to further reduce
droplet size
CA 02224615 1999-11-04
1 by elongation and shear stress, and finally discharged through the outlet
2 section into the reactor chamber as a jet of oil droplets distributed in
steam.
3
4 DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional side view of the nozzle showing, in fanciful
6 form, the progression of liquid droplet diminution in the course of passage
7 through the nozzle, as understood by the inventors;
8 Figure 2 is a cross-sectional side view of the nozzle used in water/air
9 tests described below - the dimensions of the nozzle are set forth in Table
I;
Figure 3 is a schematic plan view showing the nozzle of Figure 2
11 attached to a pre-conditioning or mixing means to form a nozzle assembly;
12 Figure 4 is a cross-sectional side view of the nozzle used in connection
13 with a fluid coker vessel - the dimensions of the nozzle are set forth in
Table
14 II;
Figure 5 is a schematic plan view showing the nozzle of Figure 4
16 attached to a mixing means to form a nozzle assembly;
17 Figure 6 is a side view showing the nozzle assembly of Figure 5
18 connected to a fluid coker;
19 Figure 7 is a side view showing the pre-conditioning nozzle of Figures
5, 6 - the dimensions are set forth in Table II I;
11
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1 Figure 8 is a graph setting forth the Sauter mean diameter of the water
2 droplets produced by the nozzle of Figure 2, plotted against air/water
ratio;
3 Figures 9 and 10 are videographs showing the droplet size and pattern
4 of the jet produced in the water/air tests using a prior art nozzle at
different
view points downstream; and
6 Figures 11 and 12 are videographs showing the droplet size and
7 pattern of the jet produced in the waterlair tests using the nozzle of
Figure 4 at
8 different view points downstream.
9
DESCRIPTION OF THE PREFERRED EMBODIMENT
11 The invention will now be described in connection with a nozzle 1 used
12 to atomize water in air. The nozzle 1 is shown in Figure 3. Its dimensions
are
13 set forth in Table I below.
14 The nozzle assembly 3 comprised a tee mixer 2 threadably connected
with a nozzle 1. The tee mixer 2 comprised a .957" I. D. straight pipe 4. Air
16 under pressure from a compressor (not shown) was introduced axially into
the
17 bore 5 of the pipe 4. Water was introduced into the pipe bore 5 through a
tee
18 connection 6.
19 The nozzle 1 comprised a body 7 forming a longitudinal passageway 8
extending therethrough. The passageway 8 comprised, in sequence: an inlet
21 section; 9; a first contraction section 10; a diffuser section 11; a second
22 contraction section 12; and an orifice outlet section 13.
12
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1 The dimensions of nozzle 1 are set forth in Table I, now following:
2 Table I
a 1.938"
b 4. 823"
c 0.722"
d 2.384"
a 0.862"
f 0.616"
g 0.250"
h 0.512"
3
4 In the analytical design of the nozzle to achieve the desired results, the
first analysis is to determine the droplet diameter entering the first nozzle
6 contraction. This is obtained by considering theory for the break-up of
liquid
7 ligaments due to turbulent flow. The theory is applied to the constant
8 diameter pipe section from the point of the first mixing of the bitumen and
9 steam up to the entry to the nozzle. To apply the theory the turbulent
Reynolds stresses have to be calculated for the turbulent pipe flow and used
11 in an equation that predicts the break-up of the droplets and corresponding
12 new droplet diameters.
13
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1 The break-up of the bitumen droplets in the first contraction to smaller
2 droplets is also estimated by another equation. The smaller droplet diameter
3 produced by the accelerated flow in the contraction depends on the third
root
4 of the product of the surface tension coefficient of the bitumen times the
original diameter divided by the product of the bitumen density times the
6 estimated acceleration of the bitumen droplet. The acceleration of the
7 bitumen is obtained by considering the geometry of the contraction and two-
8 phase flow theory. For example, it can be shown that for a flow of bitumen
9 and steam at typical temperature and pressure for the fluidized bed coker
for
a flow rate of 1620 barrels per day and a steam flow rate of 103 Ib./hr., if
the
11 initial diameter of the bitumen droplet is 12,120~,m, over a length of 30
12 diameters the bitumen droplets reduce in size to 4,210~,m. On entering the
13 first contraction of the nozzle the acceleration of the mixture and
elongation of
14 the bitumen droplets causes the droplet diameter to reduce further to
618~,m.
For this part of the analysis a contraction ratio of 0.286 was assumed over a
16 length of 49.52~m (1.808"). To determine the final diameter of the droplet
17 from the complete nozzle, this analysis has to be repeated for the second
18 contraction.
19 During this analysis, the speed of sound for the mixture has to be
determined and the mixture Mach number also calculated for the first
21 contraction. The mixture Mach number is the mixture velocity divided by the
22 mixture speed of sound. If the mixture Mach number is equal to unity, then
23 the flow downstream of the contraction could be supersonic and an
additional
24 mechanism, to break-up the bitumen to smaller droplets, will be applied.
The
only differences between the supersonic nozzle flow in the proposed nozzle
14
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1 and the usual theory for single phase flow is that in this study the flow
media
2 is a two-phase mixture of bitumen droplets in steam flow and the nozzle is a
3 double nozzle with two contractions. Criteria has been developed to
4 determine the necessary and sufficient conditions for axi-symmetrical shock
waves to occur in the diffuser or outside of the nozzle in the turbulent jet
6 region. For a distribution of bitumen droplet sizes it is best to employ a
7 software program for this analysis.
8 In testing nozzle 1, the following conditions were observed:
9 water flow rate: 15 USGPM - 52 USGPM
air flow rate: 20 SCFM - 140 SCFM
11 These conditions were used to mimic the real life situation where liquid
12 feed rate varies. The tests were performed at atmospheric pressure at a
13 temperature of 15°C.
14 It was observed through a transparent window that the feed mixture
entering the nozzle 1 comprised water ligaments dispersed in the air. The
16 droplet size and distribution in the jet produced were determined in the
17 following manner. New methods for measurement and characterization were
18 developed to obtain data in the near-field of the spray from the nozzle tip
to
19 three feet downstream; laser interferometry and high-speed strobe imaging
methods were developed and employed successfully to characterize that
21 near-field spray. To minimize any bias of measurement, the orientation of
the
22 pre-conditioner and nozzle assembly was changed to a vertical one with the
23 spray projected downwards in a spray tower. The laser interterometry
24 provides a non-invasive technique for measuring velocities and droplet
sizes.
No probes are introduced into the flow and thus it retains all the droplet
sizes
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1 and flow field. Furthermore no calibration of the instrument is necessary.
The
2 instrument made use of an argon ion laser light source and the only
3 disadvantage of the system was that in very dense sprays or non-dispersed
4 sprays, the light burst may be too diffused and may not distinguish the true
signal from the background noise. In summary four main tests were made of
6 the design nozzles in the vertical flow test chamber as follows:
7 1. P. D.A. measurements;
8 2. high speed strobe light illuminated videography;
9 3. visual inspection using a SW laser light sheet source; and
4. spray flux determination using paternation sampling equipment.
11 Videograms are shown in Figures 9 and 10 of spray patterns that were
12 taped using a prior art commercial nozzle supplied with a fluidized coker
13 operated by the present assignees. The nozzle was operated at 50 USGPM,
14 air fed at 150 psig and the views were taken at 10" and 36" downstream,
respectively. The videograms indicate a dense liquid core. In this flow,
liquid
16 ligaments were present with little indication of atomization.
17 Videograms are shown in Figures 11 and 12 of spray patterns that
18 were taped using the nozzle of Figure 4 at distances of 10" and 36"
19 downstream. Much greater width and liquid break-up was observed.
The Sauter average mean diameter of the fine droplets produced by
21 the nozzle of Figure 4 was 250 - 350 ~,m. About 90% of the water processed
22 was in the form of fine droplets.
16
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1 The invention was also tested in a commercial coker using the nozzle
2 20 shown in Figures 4 and 5. The nozzle openings in the coker were
3 relatively small. The nozzle dimensions were modified to meet the limitation
4 in outside diameter, although the same criteria were used in the nozzle's
design as was the case for the nozzle of Figure 2. The dimensions of the
6 nozzle 20 are set forth in Table II, now following.
7 Table II
I 0.957"
____..-
1.925"
k 1.25"
I 0.612"
m 3.00"
n 0.880"
0 0.825"
p 0.250"
q 0.512"
8
9 To provide a better flow mixture for the nozzle 20, a pre-conditioning
assembly 21 having a contraction nozzle 22 mounted in the bore 23 of the
11 pipe 24, was provided. The pre-conditioning nozzle 22 was positioned
12 downstream of the steam inlet 25 and immediately upstream of the bitumen
13 inlet 26. The dimensions of the pre-conditioning nozzle 22 are set forth in
14 Table III, now following.
17
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1 Table III
r 0.936"
s 1.000"
t 0.250"
a 0.400"
2
3 A number of the nozzle assemblies 27 were used in a commercial fluid coker
4 and worked without problem to improve coker product yield.
It will be appreciated that the mixer/nozzle assembly has been
6 described in terms of the specific embodiment shown in the drawings.
7 However it is contemplated that other configurations of mixer piping will be
8 apparent to those of ordinary skill in the art. The scope of the invention
is
9 defined in the claims now following:
18
