WO2011041337A2

WO2011041337A2 - Systems and methods for reducing entrainment background

Info

Publication number: WO2011041337A2
Application number: PCT/US2010/050611
Authority: WO
Inventors: Haitham Naeem Saleh Yousef
Original assignee: Invista Technologies S. A. R. L.
Priority date: 2009-10-02
Filing date: 2010-09-29
Publication date: 2011-04-07
Also published as: CN102574094A; WO2011041337A3; SA110310733B1

Abstract

Disclosed is an apparatus to reduce particle entrainment during reaction, the apparatus being an oxidation reactor configured to oxidize an aromatic feedstock to produce an aromatic carboxylic acid, the reactor comprising at least one outlet vent through which off-gas can exit the reactor. Also disclosed is a system for producing an aromatic carboxylic acid, the system comprising an oxidation reactor including at least one outlet vent through which off-gas can exit the reactor; and an overheads system including at least one condenser through which at least part of the off-gas can condense and water can pass to generate steam.

Description

SYSTEMS A D METHODS FOR REDUCING ENTRAPMENT

BACKGROUND

CROSS-REFERENCE TO RELATED APPLICATION This application claims benefit of priority from U.S. Provisional Application

No. 61/248,173 filed October 2, 2009.

The production of aromatic carboxylic acids, such as terephthalic acid (TA), typically involves the liquid-phase oxidation of an aromatic feedstock compound, such as paraxylene, using molecular oxygen in a solvent, usually in the presence of a catalyst that incorporates a promoter. In general, the solvent, molecular oxygen, feedstock, and catalyst are continuously fed into an oxidation reactor at an elevated temperature and pressure. Feedstock oxidation and other reactions within the reactor produce a high-pressure gaseous stream or "off-gas" that typically comprises nitrogen, unreacted oxygen, carbon dioxide, carbon monoxide and, where bromine is used as a promoter, methyl bromide. Because the oxidation reaction is exothermic, the solvent is often allowed to vaporize to control the reaction temperature. Therefore, the off-gas may further comprise vaporized solvent.

The off-gas from the reactor contains a significant amount of energy, which can be recovered to reduce the total energy consumed during production and at least partially offset the cost of obtaining the high temperatures and pressures required by the oxidation reactor. Such recovery often involves passing the off-gas through one or more condensers of an overheads vapor handling system, or "overheads system," to transfer heat from the off-gas to water that also passes through the condensers to generate steam, or to another heat transfer medium. In addition, the residual, non-condensable off-gas can be cleaned to remove toxic, noxious, or flammable components, for example by high-pressure combustion or scrubbing.

In addition to the aforementioned gaseous components, the off-gas from the reactor often contains various particles, which may include liquid droplets of solvent or water as well as solid particles of the aromatic carboxylic acid. Such particles can reduce the efficiency of the condensers and other components of the overheads system. It is therefore desirable to minimize the entrainment of such particles into the overheads system. Although various impingement devices have been developed for reducing particle entrainment, such devices often become clogged such that their use is disadvantageous.

BRIEF DESCRIPTION OF THE FIGURES

The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. In the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a schematic view of an embodiment of a system for producing an aromatic carboxylic acid.

FIG. 2 is a partial side view of a first example embodiment of an oxidation reactor that can be used in the system of FIG. 1 , the reactor having a single outlet vent.

FIG. 3 is a partial side view of a second example embodiment of an oxidation reactor that can be used in the system of FIG. 1 , the reactor having multiple outlet vents.

FIG. 4 is a schematic plan view of an oxidation reactor illustrating a first example vent configuration.

FIG. 5 is a schematic plan view of an oxidation reactor illustrating a second example vent configuration.

FIG. 6 is a schematic plan view of an oxidation reactor illustrating a third example vent configuration.

FIG. 7 is a schematic plan view of an oxidation reactor illustrating a fourth example vent configuration.

FIG. 8 is a graph that plots particle entrainment as a function of particle diameter for various vent configurations. FIG. 9 is a graph that plots particle entrainment as a function of particle diameter for a single vent configuration at various radial positions.

FIG. 10 is a graph that plots particle entrainment as a function of particle diameter for a double vent configuration at various radial positions. FIG. 11 is a graph that plots particle entrainment as a function of particle diameter for a triple vent configuration at various radial positions.

FIG. 12 is a graph that plots particle entrainment as a function of particle diameter for a quadruple vent configuration at various radial positions.

FIG. 13 is a graph that plots particle entrainment as a function of particle diameter for a double vent configuration at various radial positions for a small reactor.

FIG. 14 is a graph that plots entrainment as a function of the number of outlet vents for an equivalent radial position.

FIG. 15 is a graph that plots entrainment as a function of radial location for various vent configurations.

FIG. 16 is a graph that plots entrainment as a function of vent diameter for single and double vent configurations.

FIG. 17 is a graph that plots entrainment as a function of radial location for a double vent configuration and different reactor sizes.

DETAILED DESCRIPTION

As described above, it is generally desirable to reduce particle entrainment to avoid adversely affecting overheads system efficiency. As described below, entrainment can be reduced by controlling the position and/or number of reactor outlet vents. In some embodiments, entrainment is reduced by positioning a single outlet vent a preferred distance from the center of a reactor headpiece. In other embodiments, entrainment is reduced by providing the headpiece with multiple outlet vents.

Various embodiments of systems and methods are described in the following disclosure. Although particular embodiments are described, the disclosed systems and methods are not limited to those particular embodiments. Instead, the described embodiments are mere example implementations of the disclosed systems and methods.

Referring now to FIG. 1 , illustrated is part of an example system 10 for producing an aromatic carboxylic acid by the exothermic, liquid-phase oxidation of an aromatic feedstock compound. In FIG. 1 , many components of the system 10 are not illustrated as being beyond the scope of the present disclosure. The system 10 comprises a reaction system that generally includes an oxidation reactor 14 in which the aromatic feedstock can be oxidized to produce the aromatic carboxylic acid. More particularly, an aromatic carboxylic acid can be produced within the reactor 14 through the high pressure, exothermic oxidation of a suitable aromatic feedstock compound in a liquid-phase reaction using an oxidant, an oxidation solvent, a catalyst, and a promoter.

The aromatic feedstock compound used can be any aromatic compound that has oxidizable substituents that can be oxidized to a carboxylic acid group. For example, the oxidizable substituent can be an alkyl group such as a methyl, ethyl, or isopropyl group. The substituent can also be a partially-oxidized alkyl group, such as an alcohol group, aldehyde group, or ketone group. The aromatic portion of the aromatic feedstock compound can be a benzene nucleus or it can be bi- or polycyclic, for example a naphthalene nucleus. The number of oxidizable substituents on the aromatic portion of the aromatic feedstock compound can be equal to the number of sites available on the aromatic portion of the aromatic feedstock compound, but is generally fewer, for example approximately 1 to 4 or 2 or 3. Examples of suitable aromatic feedstock compounds include toluene, ethylbenzene, orthoxylene, metaxylene, paraxylene, 1-formyI-4-methylbenzene, 1-hydroxymethyl-4-methylbenzene, 1 ,2,4- trimethylbenzene, 1-formyl-2,4-dimethylbenzene, 1 ,2,4,5-tetramethylbenzene, alkyl, hydroxymethyl, formyl, and acyl substituted naphthalene compounds such as 2,6- and 2,7-dimethylnaphthalene, 2-acyl-6-methylnaphthalene, 2-formyl-6- methylnaphthalene, 2-methyl-6-ethylnaphthalene, 2,6-diethylnaphthalene, and the like. For the purposes of this disclosure, it is assumed that the aromatic feedback compound is paraxylene (PX), which is used to produce terephthalic acid (TA).

Molecular oxygen can be used as the oxidant. In some embodiments, air, which may be oxygen enriched or depleted, is the preferred source of molecular oxygen. The oxygen-containing gas fed to the oxidation reactor 14 can comprise an exhaust gas-vapor mixture. To avoid the formation of explosive gas/vapor mixtures, the flow rate of oxygen-containing gas fed to the oxidation reactor 14 is controlled to maintain an oxygen concentration in the headpiece in the range from approximately 0.5 to 8 volume percent oxygen (measured on a solvent-free basis). For example, a feed rate of the oxygen-containing gas sufficient to provide oxygen in the amount of from approximately 1.5 to approximately 2.8 moles per methyl group will provide approximately 0.5 to 8 volume percent of oxygen (measured on a solvent-free basis) in the overhead gas-vapor mixture.

The oxidation solvent can be a low molecular weight aliphatic monocarboxylic acid having approximately 2 to 6 carbon atoms, or mixtures thereof with water. In some embodiments, the solvent comprises acetic acid or a mixture of acetic acid and water.

Suitable catalysts include those heavy metals having an atomic number of approximately 21 to 82, such as a mixture of cobalt and manganese, and suitable promoters include bromine compounds such as hydrogen bromide, molecular bromine, and sodium bromide. The catalyst/promoter employed in producing crude terephthalic acid can, for example, comprise cobalt, manganese, and bromine components, and can additionally comprise accelerators. The ratio of cobalt (calculated as elemental cobalt) in the cobalt component of the catalyst-to- paraxylene in the liquid-phase oxidation can be in the range of about approximately 0.2 to 10 milligram atoms (mga) per gram mole of paraxylene. The ratio of manganese (calculated as elemental manganese) in the manganese component of the catalyst-to-cobalt (calculated as elemental cobalt) in the cobalt component of the catalyst in the liquid-phase oxidation is suitably in the range of about approximately 0.2 to 10 mga per mga of cobalt. The weight ratio of bromine (calculated as elemental bromine) in the bromine component of the catalyst-to- total cobalt and manganese (calculated as elemental cobalt and elemental manganese) in the cobalt and manganese components of the catalyst in the liquid-phase oxidation can be in the range of approximately 0.2 to 1.5 mga per mga of total cobalt and manganese. Each of the cobalt and manganese components can be provided in any of its known ionic or combined forms that provide soluble forms of cobalt, manganese, and bromine in the solvent in the oxidation reactor 14. For example, when the solvent is an acetic acid medium, cobalt and/or manganese carbonate, acetate tetrahydrate, and/or bromide can be employed. To achieve 0.2:1.0 to 1.5:1.0 bromine-to-total catalyst metals as a molar ratio, suitable sources of bromine include elemental bromine (Br₂), ionic bromine (for example HBr, NaBr, KBr, NhUBr, etc.), or organic bromides that are known to provide bromide ions at the operating temperature of the oxidation (e.g., benzylbromide, mono- and di- bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylene-di-bromide, etc.). The total bromine, including molecular bromine, organic bromine, and ionic bromide, is used to determine the elemental bromine-to-total catalyst metals ratio. The bromine ion released from the organic bromides at the oxidation operating conditions can be readily determined by known analytical means. For the oxidation of paraxylene to terephthalic acid, the minimum pressure at which the oxidation reactor 14 is maintained is typically that pressure that will maintain a substantial liquid phase of the paraxylene and the solvent. When the solvent is an acetic acid-water mixture, suitable reaction gauge pressures in the oxidation reactor 14 can be in the range of approximately 0 kg/cm² to 35 kg/cm², and typically are in the range of approximately 10 kg/cm² to 20 kg/cm². The temperature range within the oxidation reactor can be, for example, approximately 120°C to 250°C. By way of example, the solvent residence time in the reactor 14 is approximately 20 to 150 minutes, and preferably approximately 30 to 120 minutes. The oxidation reactor 14 typically comprises a vessel that is constructed of or lined with a corrosion-resistant material, such as titanium or glass. Because the oxidation reaction is conducted at an elevated pressure, the reactor 14 is constructed to withstand the high pressures used for the oxidation reaction. In addition, the contents of the oxidation reactor 14 are agitated to optimize the oxidation reaction and maintain the solid reaction product in suspension. Agitation comprises specific fluid mixing configurations and the oxidation reactor 14 can be equipped with one or more mechanical agitators (not shown). Crude terephthalic acid, which is the solid reaction product produced by the oxidation reaction, leaves the reactor 14 along an outlet line 16 in the form of an oxidation reaction slurry that comprises a mixture of crude terephthalic acid, water, acetic acid, catalyst metals, oxidation reaction intermediates, and reaction byproducts. The slurry is then processed using a variety of procedures and equipment (not shown) to produce purified terephthalic acid. As mentioned above, significant amounts of heat are produced during the liquid-phase oxidation within the oxidation reactor 14. In view of the considerable amount of energy that is comprised by the reaction off-gas, steps are taken to recover at least a portion of that energy and use it within the plant. For instance, the off-gas is used to raise various pressures of steam that can, for example, be used to drive and/or heat other components of the system 10. In the embodiment of FIG. 1 , the off-gas exits the headpiece of the oxidation reactor 4 through one or more outlet vents 17 and passes into a gas line 18 that delivers the off-gas to a series of condensers 20, 22, and 24, which condense the off-gas and raise steam. In some embodiments, the first condenser 20 produces a first pressure of steam (e.g., at approximately 145°C and 4.5 bar), the second condenser 22 produces a second pressure of steam (e.g., at approximately 130°C and 3 bar), and the third condenser 24 produces a third pressure of steam (e.g., at approximately 100°C and 1 bar).

The steam from the condensers 20, 22, and 24 can flow to various other components (not shown) of the system 10 through steam lines 26, 28, and 30. Example components that may receive steam include reboilers, driers, turbines, heat exchangers, and the like. When the off-gas passes through the condensers 20, 22, and 24, organic materials within the off-gas condense and that condensate can be returned to the oxidation reactor 14 after processing. As a consequence of the heat transfer that occurs within the condensers 20, 22, and 24, the remaining off-gas is relatively cold, but is still at relatively high pressure and can also be used for energy recovery purposes. Therefore, after passing through the condensers 20, 22, and 24, the off-gas exits through a gas line 32 to be delivered to another system component (not shown). One example of such a component is an expander. The off-gas can be cleaned to remove toxic, noxious, or flammable components, for example by high-pressure combustion or scrubbing.

FIGs. 2 and 3 illustrate two example oxidation reactors that can be used in the system 10 of FIG. 1. Beginning with FIG. 2, shown is a single vent oxidation reactor 40 comprising a vessel 42 that is capped by a headpiece 44. Extending from the headpiece 44 is a single outlet vent 46 that is generally parallel with the vessel but offset from the center of the headpiece. The vent 46 can be coupled to a gas line that leads to the condensers (e.g., gas line 18 in FIG. 1 ) or can form part of such a gas line. Shown in FIG. 3 is a multiple vent oxidation reactor 50, which also comprises a vessel 52 that is capped by a headpiece 54. The reactor 50, however, comprises two outlet vents 56 and 58 that together join a single line 60 downstream of the reactor. Although only two outlet vents are illustrated in FIG. 3, it will be clear from the disclosure that follows that the reactor 50 could alternatively comprise more than two outlets that join together downstream of the reactor. The two-vent arrangement shown in FIG. 3 is provided to illustrate the concept of a multiple vent reactor.

FIGs. 4-7 illustrate in schematic plan view various vent configurations that can be employed for an oxidation reactor. Beginning with FIG. 4, illustrated is a vent configuration similar to that shown in FIG. 2. Therefore, a headpiece 70 comprises a single outlet vent 72. The center of the vent 72 is offset from the center of the headpiece 70 by a center-to-center distance C2C. The C2C distance can be considered to comprise a percentage of the headpiece radius R in which the headpiece center is at 0% R and the outer edge of the headpiece is at 100% R. FIG. 5 illustrates a vent configuration similar to that shown in FIG. 3.

Therefore, a headpiece 80 comprises two outlet vents 82. The center of each vent 82 is offset from the center of the headpiece 80 by a center-to-center distance C2C. Therefore, each vent 82 is equidistant to the headpiece center. In addition, the centers of the vents 82 are separated from each other by a vent-to- vent distance V2V.

FIG. 6 illustrates a further vent configuration. In the embodiment of that figure, a headpiece 90 comprises three outlet vents 92 that are arranged in an equilateral triangle configuration. The center of each vent 92 is offset from the center of the headpiece 90 by a center-to-center distance C2C. Therefore, each vent 92 is equidistant to the headpiece center. In addition, the centers of the vents 92 are separated from each other by a vent-to-vent distance V2V.

FIG. 7 illustrates yet another vent configuration. In FIG. 7, a headpiece 100 comprises four outlet vents 102 that are arranged in a square configuration. The center of each vent 102 is offset from the center of the headpiece 100 by a center-to-center distance C2C. Therefore, each vent 102 is equidistant to the headpiece center. In addition, the centers of the vents 102 are separated from each other by a vent-to-vent distance V2V.

The effectiveness of the above-described vent configurations in terms of entrainment reduction was evaluated by modeling using computational fluid dynamics. Specifically, computer-based fluid dynamics simulations were performed on various vent configurations. Table 1 identifies some of the variables that were used in the simulations.

TABLE 1

As indicated in Table 1 , single, double ("straight line"), triple ("equilateral triangle"), and quadruple ("square") vent configurations were tested. The radial position of the vent or vents was varied for each configuration, as indicated by the C2C distance and the percentage of the headpiece radius ("%Radius"). The vent diameters were selected such that the total vent area for each configuration was the same irrespective of the number of vents. For instance, the two vents in the double vent configuration each had a vent area that was half that of the vent in the single vent configuration. The original or initial configuration (a single vent configuration) is identified in the table in bold. To model the effect of different vent configurations, the flow characteristics of equivalent particles are calculated using established computational fluid dynamics techniques. Other modeling techniques can also be used. In this analysis, fourteen (14) particle size groups were used to represent entrainment particles, which could be liquid and/or solid particles. In particular, particles of 1 , 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1 , 0.05, 0.03, 0.01 , and 0.005 millimeters (mm) in diameter were used. Each second, 250 particles from each particle size group were assumed to be released from the surface of a liquid within the oxidation reactor. A simulation duration of 120 seconds was used, which resulted in a total of 420,000 particles (14*250*120) being released and tracked. At the end of each simulation, the results were post-processed and the number of particles in each size group that exited through the vent or vents (i.e., that entrained) were identified and used to evaluate performance. The results were then presented in two ways: (1 ) the percentage of entrained particles in every size group as a function of the number of vents, the vent radial position, and the reactor size, and (2) the percentage of entrained particles' volume as a function of the number of vents, the percentage of the headpiece radius, and the reactor size.

The results of the simulations are described below in relation to the graphs of FIGs. 8-17. Beginning with FIG. 8, plotted is particle entrainment as a function of particle diameter for each of the single, double, triple, and quadruple vent configurations. As indicated in FIG. 8, the percentage of particles that were entrained decreased as the number of vents increased. For example, approximately 37% of the 0.01 mm diameter particles were entrained in the single vent configuration, while less than 5% of those particles were entrained in the quadruple vent configuration.

Referring next to FIG. 9, illustrated is a graph that plots particle entrainment as a function of particle diameter for the single vent configuration at various vent positions. As can be appreciated from FIG. 9, for a single vent the percentage of particles that were entrained decreased as the radial position (i.e., distance of the vent center from the headpiece center) decreased. For example, nearly 40% of the 0.01 mm diameter particles were entrained when the vent center was separated from the headpiece center by approximately 81% of the headpiece radius, while approximately 27% of those particles were entrained when the vent center was separated from the headpiece center by approximately 18% of the headpiece radius.

FIG. 10 plots particle entrainment as a function of particle diameter for the double vent configuration with the vents being positioned at various radial distances from the headpiece center. As indicated in FIG. 10, the lowest percentage of particle entrainment occurred when the vent centers were separated from the headpiece center by approximately 40% of the headpiece radius (one of four positions evaluated). More generally, the results indicate that the optimal position for the vents in the double vent configuration is at some position between the 20% and 58% of headpiece radius range, whether it be at, below, or above the 40% position. With reference to FIG. 11 , plotted is particle entrainment as a function of particle diameter for a triple vent configuration at various vent positions. In this case, the lowest percentage of particle entrainment occurred when the vent centers were separated from the headpiece center by approximately 41 % of the headpiece radius (one of four positions evaluated). More generally, the results indicate that the optimal position for the vents in the triple vent configuration is at some position between the 23% and 58% of headpiece radius range.

FIG. 12 plots particle entrainment as a function of particle diameter for a quadruple vent configuration at various vent positions. In this case, the lowest percentage of particle entrainment occurred when the vent centers were separated from the headpiece center by approximately 58% of the headpiece radius (one of four positions evaluated). More generally, the results indicate that the optimal position for the vents in the quadruple vent configuration is at some position between the 29% and 71% of headpiece radius range. Turning to FIG. 13, plotted is particle entrainment as a function of particle diameter for a double vent configuration at various vent positions for a small reactor (8.05 meters (m) in diameter). In this case, the lowest percentage of particle entrainment occurred when the vent centers were separated from the headpiece center by approximately 36% of the headpiece radius (one of four positions evaluated). More generally, the results indicate that the optimal position for the vents in the double vent configuration in a small reactor is at some position between the 25% and 58% of headpiece radius range. Comparison with FIG. 10 shows a similar trend to the results for a larger reactor, but a reduced sensitivity to the reduction of entrainment by changing the radial position of a double vent for a smaller reactor. FIG. 14 plots volume entrainment as a function of the number of outlet vents. As can be appreciated from FIG. 14, increasing the number of vents can result in a significant reduction in the volume fraction entrained in headpiece, thereby reducing the carryover of particles into the reactor vent. Regarding FIG. 15, plotted is volume entrainment as a function of vent location radius for each of the single, double, triple, and quadruple vent configurations. As can be appreciated from that figure, a clear optimal vent location (identified by the minimum point of each curve) exists for multiple vent configurations. FIG. 16 compares the volume entrainment as a function of vent radius for single and double vent configurations. As indicated in FIG. 16, entrainment in terms of volume percentage for the double vent configuration was approximately 70% less than that for the single vent configuration when all vents were positioned the same distance from the headpiece center i.e., 2.9 (m). Finally, FIG. 17 plots entrainment as a function of vent location for a double vent configuration and different reactor sizes, i.e., an original reactor (9.92 m in diameter) and a smaller reactor (8.05 m in diameter). As can be seen from FIG. 17, there is little difference between entrainment levels between the two reactor sizes, which suggests that the relationships identified above are universally applicable.

In view of the above-described results, it will be appreciated that, in some embodiments, it is desirable to adjust the position of the outlet vent in a single vent configuration so that the vent center is separated from the headpiece center equal to or less than approximately 18% of the headpiece radius to reduce entrainment. In addition, it will be appreciated that, in some embodiments, it is desirable to increase the number of outlet vents to reduce entrainment. The simulations suggest that optimal results may be achieved when a relatively large number of outlet vents, such as four or more outlet vents, are used.

Claims

CLAIMS Claimed are:

1. An oxidation reactor configured to oxidize an aromatic feedstock to produce an aromatic carboxylic acid, the reactor comprising:

at least one outlet vent through which off-gas can exit the reactor.

2. The reactor of claim 1 , wherein the reactor comprises headpiece from which extends a single outlet vent, the vent being positioned relatively close to a center of the headpiece.

3. The reactor of claim 2, wherein a center of the vent is separated from the headpiece center by no more than approximately 29% of the headpiece radius.

4. The reactor of claim 2, wherein a center of the vent is separated from the headpiece center by no more than approximately 18% of the headpiece radius.

5. The reactor of claim 1 , wherein the reactor comprises multiple outlet vents.

6. The reactor of claim 5, further comprising a headpiece from which the vents extend, the vents being equidistant from a center of the headpiece.

7. The reactor of claim 1 , wherein the reactor comprises two outlet vents.

8. The reactor of claim 7, wherein centers of the vents are separated from a center of the headpiece by approximately 20% and 58% of the headpiece radius.

9. The reactor of claim 1 , wherein the reactor comprises three outlet vents.

10. The reactor of claim 9, wherein the vents are arranged in an equilateral triangle configuration.

11. The reactor of claim 9, wherein centers of the vents are separated from a center of the headpiece by approximately 23% and 58% of the headpiece radius.

12. The reactor of claim 1 , wherein the reactor comprises four outlet vents.

13. The reactor of claim 12, wherein the vents are arranged in a square configuration.

14. The reactor of claim 12, wherein centers of the vents are separated from a center of the headpiece by approximately 29% and 71 % of the headpiece radius.

15. A system for producing an aromatic carboxylic acid, the system comprising:

an oxidation reactor including at least one outlet vent through which off- gas can exit the reactor; and an overheads system including at least one condenser through which at least part of the off-gas can condense and water can pass to generate steam.

16. The system of claim 15, wherein the reactor comprises a headpiece from which extends a single outlet vent, the vent being positioned relatively close to a center of headpiece.

17. The system of claim 16, wherein a center of the vent is separated from the headpiece center by no more than approximately 29% of the headpiece radius.

18. The system of claim 16, wherein a center of the vent is separated from the headpiece center by no more than approximately 18% of the headpiece radius.

19. The system of claim 15, wherein the reactor comprises multiple outlet vents.

20. The system claim 19, further comprising a headpiece from which the vents extend, the vents being equidistant from a center of the headpiece.

21. The system of claim 15, wherein the reactor comprises two outlet vents.

22. The system of claim 21 , wherein centers of the vents are separated from a center of the headpiece by approximately 20% and 58% of the headpiece radius.

23. The system of claim 15, wherein the reactor comprises three outlet vents.

24. The system of claim 23, wherein the vents are arranged in an equilateral triangle configuration.

25. The system of claim 23, wherein centers of the vents are separated from a center of the headpiece by approximately 23% and 58% of the headpiece radius.

26. The system of claim 15, wherein the reactor comprises four outlet vents.

27. The system of claim 26, wherein the vents are arranged in a square configuration.

28. The system of claim 26, wherein centers of the vents are separated from a center of the headpiece by approximately 29% and 71% of the headpiece radius.