CA1180036A - Process for the atomizing of a heavy liquid - Google Patents

Abstract

A B S T R A C T

INTERNAL MIX ATOMIZER AND PROCESS FOR THE
ATOMIZING OF A HEAVY LIQUID

An internal mix atomizer and process for the atomizing of a heavy liquid featuring prevention of spray collapse by use of critical spacing of atomizer features is disclosed.

Description

The invention relates to a process for the atomizing of a heavy liquid.
Many internal mix atomizers are designed so that critical two phase flow occurs at the exits of the atomization ports or orifices. Thus, -the pressure at the exits of the ports is higher than ambient pressure. Accordingly, the spray jets will expand ou-tside the atomizer to a much larger diameter than that of the ports. To a point, an increase in either liquid flow or atomizing fluid flow or both will result in a higher pressure in the in-ternal mixing chamber of an internal mix atomizer and through the exits of the ports, and a larger jet spray will result.
In the case where the distance between two neighbouring atomization ports is too small, the expanded jets can get too close together and collapse of the jets will result. Again, a se-t of sprays may collapse because the number of ports is too great.
This first type of collapse phenomenon is to be distinguished from collapse of the jet sprays when the jet sprays come in contact at larger distances, e.g., 30 centimetres or so, downstream from the atomizer. External factors influencing the above-mentioned second type of collapse of the jet sprays include the crossflow of other fluids or presence of other bodies close to the atomizer. Accord-ingly, an atomizer design that minimizes or eliminates the first type oE collapse of the spray jets wou]d have great utility.
An object oE the invention is to provlde a process for the akomlzing o~ a heavy liquid, wherein the above clescribed first type of collapse Oe formed s~pra~ jets is minimized or eliminated.
rrhe invention provides a process for the atomization of a heavy liquid, comprisin~ pass:ing a heavy liquicl ancl an atomizing -- 1 ~

fluid to an internal mixing chamber of an internal mix atomizer having a plurality o-E interior passages each communicating, at one end thereof, with the internal mixing chamber, and each terminating at the other end -thereof, as an exit port on the exterior of the atomizer, the passages being located in the atomizer so that the exit ports are positioned in relation to each other in such a manner that for each pair of adjacent exit ports the ratio ri () ~ rj (O) -- -ti wherein:
ri () and rj (O) are the jet radii directly after passage through exit ports i and j, respectively, and tij is the distance between the centre of exit port i and the centre of exit port j, is less than about 0.8.
The heavy liquid may be diluted and/or may be at elevated temperature and pressure. Further the heavy liquid may be combustible, wherein said heavy combustible liquid is combusted utilizing the concepts described herein. The invention is particu-larly useful where liquid throughputs are high, say in the order of magnitude of 2250 to 3650 kg/hr.

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Any sui-table heavy liquid may be utilizecl. In general, suitable liquids should have a kinematic viscosity, when supplied to the atomizer, equal to or less than about 20 mm2/s. Such a value may be arrived at by elevation of the temperature of the liquid or by dilution with a lighter liquid, or both as will be recognized by those skilled in the art. Useful liquids, for example, for combusting as a fuel, are those having a carbon/
hydrogen weight ratio equal to or greater than 10 and a kinematic viscosity (at 21C) grea-ter than 300 mm2/s.
If a diluent is employed, any suitable volume, e.g., from 0 percent to 50 percent by volume, preferably 0 percent to 25 percent by volume, may be used. Again, any suitable or con-ventional diluent may be used. In the case of heavy residues, for example, gas oil may be used. Similarly, any suitable atomiz-ing medium may be used. Particularly preferred, in the case of heavy liquids, such as residual oils or pyrolysis pitch, are steam or methane. As will be understood by those skilled in the art, the atomizing medium may be utilized at a wide variety of pressures and temperatures. For example, steam at 315C and ~100 kPa may be employed for some of the heavier liquids. As indicated, to assist flowability, the temperature of the heavy liquid may also be elevated by heating, e.g., up to 260C.
With these considerations in mind, it has been Eound, that for a ~ivcn hcavy liquid fLow, atomizing ~luid Elow, and geome-tric conEigura-tlon of exit ports oE potentially unequal size, ) 0 3 ~

a critical equivalent pitch circle diameter exists for which no s~ray collapse will take place. The determining parameters for t~e spray collapse involved are:
- The hea~ liquid ~low f`or each port j, m - The atomizing ~luid f'low for each port j, m - The momenturrL flow ~or each port ,j, - The nurnber of exit port~, n - The equivalent pitch circle diameter, D
For these puposes, tne equivalent pitch circle diameter is defined as the ~uantity 4A/P where A is the enclosl~?d area formed by join-ing the centresl of adjacent exit ports with straight linesegments and P is the perimeter of the enclosed area A.
With an approxima-te approach it is possible to determine the critical relation between the size of the individual spray jets, the equivalent pitch circle diameter, and the number of atomization ports. In order to prevent spray collapse of the first type mentioned in the above, it is necessary that the fraction o~ the distance between adjacent exit ports that is occupied by the associated spray jets directly after expansion be smaller 0 than a cri-tical value a, or~ in equivalent terms, ri ()~ rj (0) < ~ for all pairs of adjacent exi-t ports tij (i ~ j) where ri(0) and rj(O)are the jet radii. directl~ after ex- 5 ~ansion ofexit ports i and j, respectively, and tij is the distance between the centre o~ exit port i and the ce~tre o:~
exit port j. The critical value of ~, is smal:Ler than 1, gener-ally l~ss than about o.8. The jet radius a~ter expanslon ~or exit hole j can be found when accepting the foLlowin~ as-~umptions - The spray je-ts e~pand instan-taneousl.y do~s-tre~n of the exi-t por-ts.
- Droplet and gns velocitie~ are indentica:L af-l;er expansioll.
~ The atomizinæfluid -tempernture or density wil.L be (letermine(lb~
satllrated conditions a-t ambient pressure. No con;lensa-tion of 003~

the i'l~licl due -to the expansion will occur. Heat transfer between the droplets and -thevapour under these conditions iS SQ fast that -this assumption is reasonable for hot liquids.
I~ cold heavy liquids are used, this assumption is not valid.
Thevapour veloci-ty after expansion is now given by:
u j = u j = gJ

( mOj mvi ) The jet radius a~ter expansion is given by:

rj(O)= ~ 1/2 \ ~r p u .
0 \~ v VJ
(Pv = o.6 kg/m at a~bient pressure) Accordingly, the critical relation can be rewritten in terms of the liquid ~low, atomiæing fluid ratio and momentum flow for adjacent ports and the dis-tance between ports:

~ /2 ~ + ~ /

< ~( ir PV) 1/2ti;
where ~ = constant less than about 0.8.
F'or the case of a circular array of equally spaced exit po~ts o~
equal ~iæe the equivalent p:itch circle d:iameter is in ~act the pitch circle diameter D arld the pit;ch diaaleter and the number oE' atomi~;a-tion por-ts are rela-ted by -the expression -t = D sin (-l80ln) where t is the clis-tance between adjacent ports. For this case, the criticnl p-i-tch circle diameter, a~ a E'unct;on of` number o~ ports, liquid f`low and ato~iY,irlg -E`luid rat,io can now be ~iven by:

~ 3l~0~6 mO [ v ( + v ) ~ ~r P g ~ 1/2 ~ sin (180/n) where ~ = less than about o.8.
Accordingly, design o~ the atomi~er of` the invention so that the pitch circle diameter is less than the mentioned value allows operation without f'ear of the type of jet spray collapse described. In the case where the atomizer is employed as a component of a burner, the utilization of the invention, say with diluted pyrolysis pitch, excess air7 e.g., about 8 -to 15 percent by volume, and steam, results in an e~fective burn with low particulates emissions.
In order to describe the invention more fully, re~erence is made to the accompanying drawings wherein Figure 1 illustrates a cross section o~ an embodiment of the invention, and Figure 2 represents a top view o~ the same embodiment.
More partict~arly, the Figures show an internal mix aJtomi7.e~
according to the invention comprising a member 1 provided with a mixing chamber 2 communicating with a plurality of passages 3, prefer-abIy cyIinarically shaped.Chamber 2 is adupted by opening ~ and threads 5 for communication and connection with a liquid/~luid supply source (not shown). Passages are positioned in member 1 in accordance with the principles described herein, that is, they are spaced so that the relationship discussed, supra, is observed and the critical pitch diameter with respect to the exit ports 6 of passages 3 is observed. Means may be provided, such as slots 7, for anchorin~ the atomiæer in place in, for exarnple, a burner or ~ uid coNtactor.
3n In operatlon, a liquid and a fl.uid employed ~or a-tomatization of' the ].iquid are introduced under pressture into mixing chamber 2 via a source, such as a supply tube or tubes tnot shown).

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The liquid an~l f]uid may or may not be mixed prior to ent.ry into mixing chamber 2. The mixed fluids are forced through the passages 3, and through the exit ports 6 where they expand because o~ reduction in pressure. Exit ports 6 are pre~erably circular in shape, and are preferably, as shown, a-t an angle to the exterior surface of member 1. Preferably, the atomizer according to the invention is of substantially cylindrical shape, although other shapes are permissible. ~n general, the exit ports are spaced around the periphery of the atomizer at a location somewhat disposed from the liquid supply~fluid supply opening.
For example, if the atomizer is generally cylindrical, as illustrated in the embodiment of the drawings, the exit ports may be spaced, in accordance with the relationship described herein, in the side of a frustoconical section whose smaller base is the end of the atomizer opposite the liquid-fluid supply opening of the atomizer, the side of the frustoconical section terminating at the sides of the atomi~er.
As indicated, although not shown in the drawings, at izer 1 is eminently adapted for inclusion with a suitable burner or contacting structure.
In order to illustrate the invention, the following procedure was carried out utilizing an internal mix atomizer according to the in~ention. The fo:Llowing input and design data were used:
number of exit ports, n 7 pitch circle diameter, D-~-56 mm total atomization angle - o5 dJ(0~/tn ~ 0.73 In thi~ procedure the lnternal mlx a-tomizer of the ;nvention was empLcyed ln a burner, the liquid being atomized was pyrol~sis pltch, -the diluent was gas oil (20 percent by weight) and the .~luiclizing medill~n ~as steam a-t about Ill~c~o kPa. The m~ss ratio o:~ s-team to pitch/gas oil WtlS O . 1~ . The followlng -table, Table 1, shows -the compositlon and properties o.~ the pitch, gas oil mi~ture:

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T A B L _ Composition of ~est Mixture 80~w Pyrolysis Pitch 20%w Pyrolysis I,igh-t Gas Oil Carbon 92.8 ~Ow Hydrogen 6.93 ~Ow SuLphur 0.13 ~Ow ~itrogen 130-260 ppmw Gxygen 0.14 ~Ow Total 100 /w Conradson Carbon .7-.~esidue: 24 %w Viscosity at 100C 55 mm /s Higher Heating Value 40,472 kJ/kg Specific Gravity at 70C I.10 The mixture was supplied at a rate of about 3175 kg/hr., and was fired at 132C, which corresponds to a 17 mm /s viscosity.
The ~urn was carried out with the atomizer mounted in a fron-t f'ired boiler utilizing 8 to 15 percent by volume excess air, steam being premixed wlth the pitch/gas oil mixture. q'he atomlzer produced a good f'lame with 7 independe~t flame ~in~ers.
The f`:Lames were short in comparison with firebox depth.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the atomization of a heavy liquid, com-prising passing a heavy liquid and an atomizing fluid to an inter-nal mixing chamber of an internal mix atomizer having a plurality of interior passages each communicating, at one end thereof, with the internal mixing chamber, and each terminating at the other end thereof, as an exit port on the exterior of the atomizer, the passages being located in the atomizer so that the exit ports are positioned in relation to each other in such a manner that for each pair of adjacent exit ports the ratio wherein:
ri (O) and rj (O) are the jet radii directly after passage through exit ports i and j, respectively, and tij is the distance between the centre of exit port i and the centre of exit port j, is less than about 0.8.

2, The process as claimed in claim 1, wherein the heavy liquid is passed at elevated temperature to the internal mixing chamber.

3. The process as claimed in claim 1, wherein the heavy liquid contains up to about 50 percent by volume of a diluent.

4. The process as claimed in claim 3, wherein the heavy liquid contains up to about 25 percent by volume of a diluent.