Patent Application
20080075655
A gas mixing device which comprises eductor means having a first gas inlet and a second gas inlet is described in one embodiment of the present invention. The eductor means are preferably a venturi-type eductor and the eductor means will have a first opening and a second opening both at opposite ends to each other and in fluid communication. The gas mixing device is also in fluid communication with a reactor where the gaseous feed streams that are mixed in the gas mixing device will react. Preferably this is a catalytic partial oxidation reactor. The first gas inlet and the second gas inlet can be employed for feeding the various gaseous feedstreams that will be used in the reactor for mixing.
In an alternative embodiment, the gas mixing device may be two gas mixing devices in fluid communication with each other to ensure the adequate mixing of the input gases prior to their being fed to a catalytic partial oxidation reactor. This will significantly reduce the flammability risk by minimizing the potential of oxygen enriched areas being present. Other advantages of this arrangement of two gas mixing devices is that they can be employed in tuning the various reactant gas streams such that the proper desired ratio of carbon monoxide to hydrogen is achieved. This tuning of the ratio of carbon monoxide to hydrogen is accomplished by the addition of an inert gas into the second stage device. The gas mixing device may be used in a process for the catalytic partial oxidation of hydrocarbons to produce hydrogen and carbon monoxide which comprises feeding to the gas mixing device a first feedstream of a hydrocarbon-containing gas, which is typically a C1 to C4 alkane, and a second feedstream of an oxygen-containing gas, typically air. The two feedstreams are mixed in the gas mixing device and fed from the gas mixing device to the catalytic partial oxidation reactor.
In a further alternative embodiment of the present invention, an improved process for the catalytic partial oxidation of hydrocarbons wherein feedstreams of hydrocarbon-containing gases and oxygen-containing gases are fed to a catalytic partial oxidation reactor to produce hydrogen and carbon monoxide, the improvement comprising mixing the hydrocarbon-containing gas stream and the oxygen-containing gas stream in a gas mixing device prior to their being fed to the catalytic partial oxidation reactor is disclosed.
The eductor means may be selected from those eductor types that will provide the appropriate motive force to the gas feedstreams being mixed. For example, a jet eductor will lift, entrain and pump out a low pressure fluid utilizing a high pressure motive fluid. Other examples of eductors include gas/steam motive eductors such as those available Penberthy under their GL Series, liquid motive eductors, such as those available from Penberthy under their LL Series and steam heating and mixing eductors such as those available from Penberthy under their ELL Series and CTE Series. Typically these eductors are constructed from materials such as cast-iron, steel, stainless steel, bronze, plastics and other alloy materials.
The catalytic partial oxidation reactor will contain a reduced metal catalyst. The metal catalyst employed in the present invention consists of a ceramic monolith support structure composed of ceria coated on zirconia substrate and coated or impregnated with a transition metal or combinations thereof. The monolith support is generally a ceramic foam-like structure formed from a single structural unit wherein the passages are disposed in either an irregular or regular pattern with spacing between adjacent passages. The single structural unit is used in place of conventional particulate or granular catalysts, which are less desirable. Examples of such irregularly patterned monolith supports include filters used for molten metals. Examples of regularly patterned supports include monolith honeycomb structures used for purifying exhausts from motor vehicles and used in various chemical processes. Preferred are the ceramic foam structures having irregular passages. Both types of monolith supports are well known and readily available commercially.
The catalyst element consists of a ceramic foam monolith composed substantially of zirconia, coated with about 15 to 20 wt. % ceria providing surface area for the metal impregnation, and contains 0.5 to about 5 wt. % noble metal, which is preferably rhodium in metallic form, and most preferably about 2 wt. % rhodium. Optionally, a transition metal, such as nickel at 2 to 4 wt. % may be used by itself or in combination with the rhodium. The reactor can also contain several ceramic foam disks including those with catalyst impregnated on them and with blanks to fill the void space. The blanks may be made of alumina, zirconia, cordierite or mixtures thereof. The blanks may be spaced and sized according to their effect on flow distributions and the desired final mixing step.
Optionally, the reduced metal catalyst consists of a transition metal selected from the group consisting of nickel, cobalt, iron, platinum, palladium, iridium, rhenium, ruthenium, rhodium, osmium and combinations thereof all supported on or in a ceria-coated zirconia monolith support.
The gas feedstreams that are mixed in the gas mixing device are fed to the reactor at pressures of between about 1 to about 30 atmospheres. They also may be fed at a standard gas hourly space velocity of about 50,000 to about 500,000 per hour and typically at temperatures greater than 100° C.
Turning to the figures,
In order to illustrate the workability of the present invention, computer flow dynamic (CFD) simulations were conducted. CFD modeling was used to evaluate the geometry and size of mixing devices, for example, mixing tees, static mixers and eductors, in the creation of a fully mixed flow out of two or more individual flows.
The modeling was done through the numerical solution of Navier-Stokes equation using finite volume or finite element schemes for the meshed domain. This way the velocity value and direction was calculated and species concentration was balanced for each cell (or node) of the mesh. Although CFD modeling can be used to perform various evaluations, the following information is essential for the monolith pre-mixing design: velocity fields in the computational domain and concentration profiles of various species in the mixing device and the pipework following the mixing device.
Velocity fields provide insight in the flow patterns including presence of any back flow, channeling, stagnation zone(s), dispersion and deviation from ideal flows. These fields can be generated for the whole domain where fluid motion takes place. The velocity field can be represented by velocity contours as well as velocity vectors. These graphs are used in designing the mixing device of the present invention. The longitudinal velocity profiles and vectors can identify the areas of turbulence, flow short-circuiting and stagnation. Given this information the changes in the equipment selection are introduced to optimize the turbulence of the fluid flow inside the mixing device, especially in the mixing areas. The knowledge of the flowfield inside the mixing device also allows to compare the local gas velocities with the sonic velocity at the identical conditions to assure the long-term integrity of the mixing device. Similar comparison with the flame velocity can aid in the assessment of local flammability.
Yet another important component of CFD modeling is species transport. The uniform distribution of the species in the outgoing flow is the ultimate measure of the mixing efficiency for a particular mixing device. The convective (as well as molecular) and turbulent diffusion of species is essential when two or more components are mixed. The typical output of species modeling is generated in the form of concentration profiles. Cross sectional and longitudinal concentration profiles help to assure the adequate mixing of components inside the equipment and introduce changes into the equipment design and size should the mixing be inadequate.
CFD simulations can be run in an unsteady state mode when the assessment of transient processes (including reactor start-up and shutdown) is required. Such analysis allows for modeling flammable pocket formation in real time (should such formation take place). It also facilitates in the determination of the optimal flow arrangement for the mixing device.
Three generic configurations were considered: a mixing tee, static mixer, and an eductor. As the gases that are employed in the catalytic partial oxidation process mix inside these three fixtures, a certain fraction of the gas can attain a flammable concentration. When a mixing tee is used, the pocket of flammable gas propagates co-currently with the flow direction. Should a spark occur somewhere in the system, such arrangement can lead to flame propagation. When a static mixer is used, the pocket of flammable gas repeatedly hits the internal vanes of the static mixer perpendicular to the flow. This condition can create local overheating of the gas where it impinges on the vanes and can serve as ignition sources and cause the flame to propagate. The above observed phenomena were not noticed when an eductor is utilized for the gas mixing means. The use of an eductor leads to the smallest of the three non-propagating flammable regions during mixing of the gases, thus lessening the hazards associated with synthesis gas production. Furthermore, the area of highest velocity of gas inside the eductor is contained by the thickest walls which provide for additional safety if an ignition source emerges. The highest velocity is also a critical parameter in ensuring that the gas velocity is greater than the flame velocity.
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.
