EP0058437A1

EP0058437A1 - Internal mix atomizer and process for the atomizing of a heavy liquid

Info

Publication number: EP0058437A1
Application number: EP82200034A
Authority: EP
Inventors: Hendrikus Johannes Antonius Hasenack
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1981-02-10
Filing date: 1982-01-13
Publication date: 1982-08-25
Also published as: CA1180036A; JPS57150465A

Abstract

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 an internal mix atomizer and 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 outside 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 internal 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 set 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 are coming 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. Accordingly, an atomizer design that minimizes or eliminates the first type of collapse of the spray jets would have great utility.
An object of the invention is to provide such an improved internal mix atomizer.
The internal mix atomizer according to the invention thereto comprises a member having an internal mixing chamber adapted to receive a liquid and fluid supply, 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 in the surface of the member, the passages being located in the member 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
r_i(0) and r_j(0) are the jet radii directly after expansion of exit ports i and j, respectively, and t_ij is the distance between the centre of exit port i and the centre of exit port j, is less than about 0.8.
This inventive internal mix atomizer is particularly suited for elimination of spray collapse of the first type described in the above at high liquid/fluid throughput. In particular the internal mix atomizer is eminently suited for atomizing heavy liquids, such as heavy residues, for combustion in burners for a wide variety of purposes.
Another object of the invention is to provide a process for the atomizing of a heavy liquid, wherein the above described first type of collapse of formed spray jets is minimized or eliminated.
The process for the atomizing of a heavy liquid according to the invention thereto comprises passing a heavy liquid at a maximum kinematic viscosity of about 20 mm²/s and an atomizing fluid to an internal 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(0) and r_j(0) are the jet radii directly after expansion of exit ports i and j, respectively, and t_ij 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 particularly useful where liquid throughputs are high, say in the order of magnitude of 2250 to 3650 kg/hr.
Any suitable heavy liquid may he utilized. 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 21⁰C) greater 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 conventional diluent may be used. In the case of heavy residues, for example, gas oil may be used. Similarly, any suitable atomizing medium maybe 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 315°C and 4100 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 260°C.
With these considerations in mind, it has been found, that for a given heavy liquid flow, atomizing fluid flow, and geometric configuration of exit ports of potentially unequal size, a critical equivalent pitch circle diameter exists for which no spray collapse will take place. The determining parameters for the spray collapse involved are:

- The heavy liquid flow for each port j, m .
- The atomizing fluid flow for each port j, m .
- The momentum flow for each port j,g_j
- The number of exit ports, n
- The equivalent pitch circle diameter, D_e

With an approximate 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 of the distance between adjacent exit ports that is occupied by the associated spray jets directly after expansion be smaller than a critical value a, or, in equivalent terms,
r_i (0)+ r_j (0) < a for all pairs of adjacent exit ports t.. ij (i ≠ j) where r_i(0) and r.(0)are the jet radii directly after expansion of exit ports i and j, respectively, and t_ij is the distance between the centre of exit port i and the centre of exit port j. The critical value of a, is smaller than 1, generally less than about 0.8. The jet radius after expansion for exit hole j can be found when accepting the following assumptions:

- The spray jets expand instantaneously downstream of the exit ports.
- Droplet and gas velocities are indentical after expansion.
- The atomizingfluid temperature or density will be determined by saturated conditions at ambient pressure. No condensation of the fluid due to the expansion will occur. Heat transfer between the droplets and the vapour under these conditions is so fast that this assumption is reasonable for hot liquids. If cold heavy liquids are used, this assumption is not valid. The vapour velocity after expansion is now given by:

The jet radius after expansion is given by:
(p _v = 0.6 kg/m³ at ambient pressure)
Accordingly, the critical relation can be rewritten in terms of the liquid flow, atomizing fluid ratio and momentum flow for adjacent ports and the distance between ports:
< α( π P_v) ^1/2t_ij where a = constant less than about 0.8.
For the case of a circular array of equally spaced exit ports of equal size the equivalent pitch circle diameter is in fact the pitch circle diameter D and the pitch diameter and the number of atomization ports are related by the expression t = D sin (180°/n) where t is the distance between adjacent ports. For this case, the critical.pitch circle diameter, as a function of number of ports, liquid flow and atomizing fluid ratio can now be given by:
where a = less than about 0.8. Accordingly, design of the atomizer of the invention so that the - pitch circle diameter is less than the mentioned value allows operation without fear 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 air, e.g., about 8 to 15 percent by volume, and steam, results in an effective burn with low particulates emissions.
In order to describe the invention more fully, reference is made to the accompanying drawings wherein Figure 1 illustrates a cross section of an embodiment of the invention, and Figure 2 represents a top view of the same embodiment.
More particularly, the Figures show an internal mix atomizer according to the invention comprising a member 1 provided with a mixing chamber 2 communicating with a plurality of passages 3, preferably cylindrically shaped:Chamber 2 is adapted by opening 4 and threads 5 for communication and connection with a liquid/fluid 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 anchoring the atomizer in place in, for example, a burner or liquid contactor.
In operation, a liquid and a fluid employed for atomatization of the liquid are introduced under pressure into mixing chamber 2 via a source, such as a supply tube or tubes (not shown).
The liquid and fluid- may or may not be mixed prior to entry into mixing chamber 2. The mixed fluids are forced through the passages 3, and through the exit ports 6 where they expand because of reduction in pressure. Exit ports 6 are preferably circular in shape, and are preferably, as shown, at 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. In 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 atomizer.
As indicated, although not shown in the drawings, atomizer 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 invention. The following input and design data were used:

number of exit ports, n―7
pitch. circle diameter, D―56 mm
total atomization angle―85°
d_j(0)/t_n = 0.73

The mixture was supplied at a rate of about 3175 kg/hr., and was fired at 132°C, which corresponds to a 17 mm²/s viscosity. The burn was carried out with the atomizer mounted in a front fired boiler utilizing 8 to 15 percent by volume excess air, steam being premixed with the pitch/gas oil mixture.-The atomizer produced a good flame with 7 independent flame fingers. The flames were short in comparison with firebox depth.

Claims

1. An internal mix atomizer comprising a member having an internal mixing chamber adapted to receive a liquid and fluid supply, 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 in the surface of the member, the passages being located in the member 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

r_i(0) and r_j(0) are the jet radii directly after expansion of exit ports i and j, respectively, and t_ij is the distance between the centre of exit port i and the centre of exit port j, is less than about 0.8.

2. A process for the atomization of a heavy liquid, comprising passing a heavy liquid at a maximum kinematic viscosity of about 20 mm²/s. and an atomizing fluid to an internal 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 r.(0) and r_j(0) are the jet radii directly after expansion of exit ports i and j, respectively, and t_ij is the distance between the centre of exit port i and the centre of exit port j, is less than about 0.8.

3. The process as claimed in claim 2,wherein the heavy liquid is passed at elevated temperature and pressure and the atomizing fluid is passed under pressure to the internal mixing chamber.

4. The process as claimed in claim 2,wherein the heavy liquid is combustible, and the process further comprising providing air in stoichiometric excess, in the vicinity of the exit ports of the internal mix atomizer to produce a combustible atomized mixture and combusting said mixture.

5. The process as claimed in claim 4,wherein the heavy combustible liquid is passed at elevated temperature to the internal mixing chamber.

6. The process as claimed in any one of the claims 2-5, wherein the heavy liquid contains up to about 50 percent by volume of a diluent.

7. The process as claimed in any one of the claims 2-5, wherein the heavy liquid contains up to about 25 percent by volume ofa diluent.