BACKGROUND OF THE INVENTION Autothermal reformers ("ATR") are used to convert natural gas, steam and oxygen into synthesis gas ("syngas") using a combination of partial oxidation and reforming. In gas- to-liquids ("GTL") applications utilizing the Fischer-Tropsch process for the production of hydrocarbons, the preferred synthesis gas feed has an H2 : CO ratio of between about 2: 1 and about 2.2 : 1.

Commercial ATR systems currently in use for generating syngas for Fischer-Tropsch synthesis utilize °2 rather than air. Commercial ATRs employ a flame or ignition means and allow for the homogeneous partial oxidation reaction of natural gas, steam and air in a zone free of any catalytic material. The partial oxidation ("POX") reaction creates hot gases which are typically in excess of 2200°F and which then flow into a catalyst bed and undergo endothermic reforming while cooling. Relatively high, greater than about 0.6, steam to natural gas ratios must be employed in existing commercial ATRs in order to avoid soot formation within the high temperature region. Additionally, ignition means, such as burner nozzles and related mechanical equipments in existing commercial ATRs are complex and have limited operating life due to the stresses associated with high temperature operations.

Feed mixtures for existing commercial ATRs typically consist of air, steam and natural gas in ratios which result in an approximate 2.05 to 2.3 H2 : CO ratio. Such ATR feed gas ratios are typically in the following ranges: Air/Natural Gas (A/NG) 2.5-3. 2; Steam/Natural Gas (S/NG) 0.6 to 2.0.

There are several factors that determine the specific ATR feed ratios appropriate for a particular application of the resulting syngas. Such factors include, but are not limited to, the composition of the natural gas, desired syngas compositions, and amount of molecular H2 added to the ATR feed mixture for hydrodesulfurization. The primary constituent of typical field natural gas is methane (>50 volume %) and the concentration of heavier hydrocarbon constituents, typically C2 to Clo hydrocarbons can range from about 1% to about 15%. Other

non-hydrocarbon constituents, for example argon, nitrogen, CO2, and H2S, may also be present.

Existing commercial ATRs employ mixing of the Natural Gas, air and steam feed constituents. The NG and air are conveyed to the ATR separately and the steam may be fed into the ATR separately or alternatively, may be mixed with either the NG or air prior to feeding into the ATR.

In order to achieve the desired synthesis gas composition, existing commercial ATR operations generally occur at elevated temperatures in the range of 1600°F to in excess of 2200°F. The design of any commercial ATR involves balancing several process variables including pressure, reactor volume and compression costs. In commercial ATRs utilizing an ignition means or flame, as the pressure increases the extent of methane conversion to CO diminishes. Moreover, higher pressures result in a higher volumetric heat release in the partial oxidation zone with the corresponding thermal, mechanical and soot formation issues.

In the startup of a commercial ATR system, initial ATR feed is typically an inert material, such as steam, nitrogen and possibly natural gas, with initial operation at temperatures less than 400°F. As the ATR temperature is increased, the ATR feed gas composition is transitioned to a mixture of steam and natural gas prior to the introduction of air or oxygen. Upon the introduction of air or oxygen and the transition to the ATR feed gas composition appropriate to producing a synthesis gas suitable, for example, for a Fischer- Tropsch process, the ATR feed gas mixture becomes flammable. A primary safety concern involves the introduction of flammable mixtures into process volumes downstream of the ATR. In ATR systems utilizing an ignition means or flame, the flammable ATR feed mixture undergoes partial oxidation in a specific volume within the reactor designed to handle the flow rates and temperatures associated with the combustion reaction. The ignition means or flame of commercial ATR ensures combustion of the flammable oxygen and natural gas mixture within the ATR and prevents the flammable mixture from exiting the ATR. In flameless ATR systems, however, there is a concern that all or part of a flammable feed mixture might not undergo POX reactions within the ATR and may flow into downstream components. Such failure to undergo POX might occur, for example, because of insufficient ATR catalyst activity. It is not desirable to permit the unreacted flammable feed mixture to exit the ATR because downstream equipment is not necessarily constructed to withstand the high temperatures/pressures generated in the POX reaction. To size and construct the downstream equipment to safely incur such temperatures and pressures would be prohibitively expensive.

There exists a need for an ignition-less syngas production process which prevents introduction of flammable mixtures to process components downstream of the ATR. There further remains a need for a method to determine ATR catalyst activity.

SUMMARY OF THE INVENTION The invention provides processes for the safe start-up and operation of a commercially sized ignition-less ATR system employing flammable mixtures of air and natural gas. The start-up procedure is applicable to all partial oxidation and ATR systems which utilize flammable feed gas mixtures and rely on the intrinsic activity of the catalyst and do not employ a flame, or ignition devise, such as a burner, glow plug or other type of device for initiation of the oxidation reaction. The inventive process further avoids unwanted side reactions which may occur during the start-up process.

In the start-up process, the ATR is initially heated with natural gas or an inert to a temperature between about 230° and about 300°F. Once the ATR is above the boiling point of water at the operating pressure, steam and natural gas can be used for continuous heating.

The ATR feed gas is heated to between about 600° and about 1000°F prior to the introduction of an oxidant. Once the partial oxidation is established in the flame or high temperature zone, air is introduced incrementally up to the desired flow rate as the temperature of the entire system approaches the desired operating value.

The process introduces air during start-up into a ignition-less ATR system in such a way as to avoid the possibility of a deflagration event associated with the ignition of a flammable mixture upstream or downstream of the catalyst volume. Upon heating the ATR to an appropriate temperature using NG, steam and/or inert mixtures (between about 600° to about 1000°F), air is introduced at sufficiently low flow rates so as to ensure that the feed mixture is outside of the flammability envelope. Under these conditions, the initiation of the partial oxidation reaction occurs under the conditions in which the feed gas is non-flammable and incapable of sustaining a flame. Using the inventive method, the failure of the catalyst to initiate the partial oxidation reaction will not result in any dangerous or unsafe operating conditions due to the passage of the unreacted feed gas through the catalyst bed and into downstream process volumes.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph depicting the flammability envelope for a natural gas and air mixture.

Fig. 2 depicts the flammability envelope of a typical natural gas and air mixture and further shows composition lines representing specific ATR feed mixtures.

Fig. 3 is a graph which depicts composition transients associated with the startup process of the invention.

Fig. 4 is a graph depicting the ATR feed gas composition changes during one embodiment of the start-up process Fig. 5 is a graph depicting the temperature changes in two locations within the ATR during one embodiment of the start-up process.

Fig. 6 is a cross-sectional diagram of an autothermal reactor useful in connection with embodiments of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION The terms"inert material"or"inert mixing and shielding material"refer generally to a material that does not initiate any significant oxidation, reforming, or otherwise serve as an active catalytic surface with respect to soot or carbon formation. The inert mixing-and- shielding material may carry out several functions including, but not limited to, promoting mixing of the feed gases, shielding the feed nozzle from the heat of reaction, and as a flame arrester.

The term"Cx", where x is a number greater than zero, refers to a hydrocarbon compound having predominantly a carbon number of x. As used herein, the term Cx may be modified by reference to a particular species of hydrocarbons, such as, for example, C5 olefins. In such instance, the term means an olefin stream comprised predominantly of pentenes but which may have impurity amounts, i. e. less than about 10%, of olefins having other carbon numbers such as hexene, heptene, propene, or butene. Similarly, the term"Cx+" refers to a stream wherein the hydrocarbons are predominantly those having a hydrocarbon number of x or greater but which may also contain impurity levels of hydrocarbons having a carbon number of less than x. For example, the term Cls+ means hydrocarbons having a carbon number of 15 or greater but which may contain impurity levels of hydrocarbons having carbon numbers of less than 15. The term"Cx-Cy", where x and y are numbers greater than zero, refers to a mixture of hydrocarbon compounds wherein the predominant component hydrocarbons, collectively about 90% or greater by weight, have carbon numbers between x and y inclusive. For example, the term C5-Cg hydrocarbons means a mixture of hydrocarbon compounds which is predominantly comprised of hydrocarbons having carbon numbers between 5 and 9 inclusive, but may also include impurity level quantities of hydrocarbons having other carbon numbers.

Embodiments of the invention are well-suited for synthesis gas generation of a nominal 2 to 2.1 H2 : CO ratio involving the use of an oxygen-containing gas (preferably air or

oxygen-enriched air) in which there is significant amount (i. e, greater than about 25%) of <BR> <BR> diluent (e. g. , N2). The feed gas components may be introduced into a mixing volume or device, such as a mixing tube or suitable device which allows the feed gas constituents to become completely mixed within a very short time frame and without a significant pressure drop. The velocity and temperature within the mixing volume are maintained at appropriate levels to prevent pre-ignition of the combustible mixture, which would result in a flame and high combustion temperatures. It is preferable to maintain the gas velocity of the oxygen containing mixture above the burning velocity or burn back velocity of the mixture. This minimum acceptable velocity depends upon the composition of the flame mixture as well as the temperature and pressure. The methodologies for determining the burning velocity account for the maximum temperature, pressure and residence time attainable during all phases of operation in order to prevent flash back or pre-ignition. The methodologies also include the impact of the feed composition since pre-ignition occurs more readily with <BR> <BR> increasing oxygen and C2+ content. For example, high levels of C6 (i. e. , greater than about<BR> 0.1 vol%) may require a lower mixing temperature (i. e. , about 700° F) compared to natural gas sources that have only trace amounts of propane as the heaviest hydrocarbon component.

The oxygen-containing air is introduced only at about 700-2000° F ; while natural gas is usually about 700-1050° F.

In one embodiment of the invention, a process is provided in which syngas is produced in a zoned autothermal reformer. Referring to Figure 6, an autothermal reformer system 100 is shown. The system 100 includes a number of zones: a mixing zone 110, an inert disengaging zone 120, a catalytic expansion 140, an active catalyst or reaction zone 160 an inert exit zone 180 and an exit zone 190. The expansion zone 140 is supplied with a packed bed of an inert mixing-and-shielding material 141. The active catalyst zone 160 contains an active reformer catalyst 161.

A light hydrocarbon feed gas enters the reformer 101 through conduit 105 and steam feed enters through conduit 106. The light hydrocarbon feed gas predominately contains hydrocarbon gases having a carbon number of 4 and less, and may include, for example, natural gas or similar feed mixtures. The feed gas and steam feed are fed into a mixing zone 110 through conduit 107. An oxygen-containing gas, such as air, is introduced into the mixing zone 110 through a conduit 108. The three feed components, feed gas, steam feed, and oxygen-containing gas, are referred to collectively as the feed gas mixture, and are mixed within the mixing zone 110 wherein any of a number of commercially available and known means for mixing such components may be utilized. By way of example but not limitation,

the mixing means in mixing zone 110 may include those disclosed in the following U. S. patents: 3,871, 838 ; 4,477, 262; 4,166, 834 ; 4,865, 820; and 4,136, 015. The disclosures of each of these patents is incorporated herein by reference. Conventional methods employing jets or nozzles at sufficiently high turbulent Reynolds numbers which are known to those skilled in the art could also be used.

The mixing of the three feed components should occur in a reasonably short residence time, i. e. less than about 300 milliseconds, so as to avoid ignition of the flammable feed gas mixture. Additionally the pressure drop across the mixing zone should be kept as low, i. e. less than about 25 psig, in order to minimize the pressure losses and power requirements for maintaining elevated pressure. The maximum allowable residence time in the mixing zone depends upon the temperature, pressure and composition of the feed gas mixture. In gas to liquid ("GTL") operations utilizing a steam to carbon ratio of less than about 0.5 : 1 and an oxygen to carbon molar ratio in the range of about 0.45 : 1 to about 0.7 : 1 ("typical GTL feed composition"), the preferred residence time in the mixing zone is less than about 200 milliseconds. Longer time periods may be employed when operating at lower air to carbon molar ratios, temperature and/or pressure. The use of shorter residence times in the mixing zone 110 may provide greater flexibility in the design of disengaging zone 120 and expansion zone 140.

The minimum velocity of the feed gas mixture in the mixing zone 110 is generally higher than the laminar flame velocity to prevent the undesirable propagation of the oxidation reaction back towards the inlet mixing zone 110. The flame velocity can be estimated according to the methods disclosed in Glassman, Irvin, 1996, Conibustioll, Third edition, Academic Press; Zabetakis, Michael G. , Bureau of Mines, 1967, Safety with Cryogenic Fluids, Plenum Press, New York; Lewis Bernard and von Elbe, Guenther, 1987, Combustion Flames and Explosions of Gases, Third Edition, Academic Press, and such methods are known to those skilled in the art.

Under typical GTL conditions (between about 2.3 : 1 and about 3: 1 air to carbon molar ratio and less than about 0.5 : 1 steam to carbon molar ratio at about 200 psig and about 900°F feed gas temperature), the minimum velocity of the feed gas mixture is between about 30 and about 40 ft/sec. In a commercial operation, the velocity of a feed component mixture exiting a mixing zone is generally in the range of about 70ft/sec to about 300 ft/sec. The feed gas mixture exits the mixing zone 110 and is conveyed to the disengaging zone 120.

The disengaging zone 120 separates the mixing zone 110 from the expansion zone 140 and is also referred to herein as the"process volume. "In the disengaging zone 120, the

velocity of the feed gas mixture is reduced to no more than about 20 ft/sec. When operating at elevated temperatures, i. e. greater than about 900°F, or pressure, greater than about 200 psig, and using a typical GTL feed composition, shorter total residence times in the mixing zone 110, i. e. less than about 200 milliseconds, and disengaging zone 120, i. e. less than about 200 milliseconds, are generally used. The disengaging zone 120 provides the appropriate volume to dissipate any radial velocity gradients which may arise in the mixing zone 110.

The disengaging zone 120 can be completely or partially filled with a solid inert material 121, such as MgO, provided that the inlet portion of the disengaging zone 120 contains inert material 121. In one embodiment of the invention, the process volume is only partially filled in order to minimize the pressure drop which occurs with high velocity gas flow through a packed bed. The maximum depth (or volume) of the inert material 121 in the disengaging zone 120 is determined by the maximum allowable residence time for the feed component mixture in which the ignition time delay is not exceeded. Those skilled in the art would understand how to calculate the volume of inert material 121 so as to meet these criteria.

Expansion zone 140 generally provides additional mixing due to the relatively high particle Reynolds number and associated turbulence encountered by the feed gas mixture in the expansion zone 140. As shown in Fig. 6, disengaging zone 120 and expansion zone 140 are formed by the flaring of the reformer 101 by an expansion angle 115. Expansion angle + 115 is chosen so as to ensure that minimum back mixing occurs as the velocity of the feed gas mixture decreases. One of ordinary skill in the art would understand how to calculate expansion angle 115 to meet such objective.

When operating with a partially filled disengaging zone 120, the expansion angle 115 should preferably be set at relatively low values, i. e. less than about 30°. The actual limit of the expansion angle 115 depends upon the ignition time delay and depth of the inert material 121 relative to the onset of gas expansion.

In some embodiments of the invention, the depth of the inert material 121 in disengaging zone 120 is at least about 3 inches. Such minimum depth provides a thermal shield to prevent the transfer of heat from the active catalyst zone 160 to the mixing zone 110.

In another embodiment of the invention, the disengaging zone 120 is completely filled with inert material 121 with an excess of inert material 121 extending into a lower portion of the mixing zone 110. An excess of inert material 121 ensures that the disengaging zone 120 remains completely filled with solids even in the event there is settling of the inert

material 121 or other packing materials in the zones underlying, and/or downstream of, the <BR> <BR> disengaging zone 120 i. e. , zones 140,160, 180 and 190. The amount of excess inert material 121 required to offset such settling typically ranges from about 1 % to about 4% of the sum of the volumes of the inert disengaging 120, catalyst expansion zone 140 and active catalyst zone 160. Where the inert disengaging zone 120 is completely filled, the expansion angle 115 can be set at relatively high values, i. e. up to about 60°.

The process of the invention does not utilize an extant ignition source, such as a flame, to initiate and propagate the partial oxidation reaction which occurs in catalyst zone 160. The feed gas mixture passes through the mixing zone 110 and inert disengaging zone 120 within a time interval smaller than that associated with the ignition time delay, which is between about 200 and about 2000 milliseconds depending upon the feed gas mixture composition, pressure and temperature wherein the time delay generally increases with any of : (1) decreasing pressure and temperature; (2) increasing steam to carbon ratio; or (3) decreasing air to carbon ratio. The ignition time delay defines the maximum allowable combined residence time for the feed gas mixture through the mixing zone 110 and the inert disengaging zone 120.

In some embodiments of the invention, temperature in the active catalyst zone 160 is about 1800°F or less, which indicates that there is not a flame. The"active catalyst volume" consists of those volumes containing active catalyst and include the active catalyst zone 160, and, optionally, a lower section 142 of the catalyst expansion zone 140. The active catalyst volume is the volume wherein partial oxidation and reforming occur. In most cases, the temperature at the inlet, or uppermost portion, of the active catalyst volume is generally between about 60oF and about 200°F higher than the adiabatic equilibrium temperature present throughout the active catalyst zone 160.

As previously discussed, the gas velocities through the mixing zone 110 should preferably be sufficiently high to prevent flash back. The burning velocity of the feed gas mixture defines the minimum acceptable velocity. Typically these values are on the order of 30 ft/sec under typical operating conditions used to make a 2: 1 H2 to CO molar ratio synthesis gas. One skilled in the art would understand how to calculate the burning velocity of the feed gas mixture.

The superficial gas velocity based on a solid free cross sectional area is above the burning velocity of the feed gas mixture, which is about 30 ft/sec. Preferably the superficial gas velocity is between about 70 and about 200 ft/sec as the feed gas mixture enters the inlet portion of the disengaging zone 120. In some embodiments of the invention, the cross

sectional area for flow increases as the feed gas mixture progresses through the disengaging zone 120 in order to further reduce the superficial gas velocity to a value of less than about 30 ftlsec. In some embodiments of the invention, a depth of about 12 inches of inert material 121 is employed in the disengaging zone 120. Greater depths or larger volumes may be employed in embodiments in which additional mixing of the feed gas components prior to contacting the active catalyst is desired. The size and shape of the inert material 121 is preferably selected to minimize the pressure drop and provide an effective barrier to radiant heat transfer between the active catalyst volume and the mixing zone 11 The disengaging zone 120 provides the appropriate volume to dissipate any radial velocity gradients which may arise in the mixing zone 110. Larger particles, i. e. , greater than about 25 mm, are<BR> preferred for minimizing pressure drop. Smaller particles, i. e. , less than about 100 mm, are preferred to achieve better mixing. The total residence time for the feed gas mixture through the mixing zone 110 and inert solids 121 is less than the corresponding ignition time delay for the feed gas mixture composition and process conditions. The maximum volume of the inert material 121 is therefore, determined by the value of the ignition time delay.

In some embodiments of the invention, the maximum volume of inert solids 121 is established by the feed gas mixture residence time through the mixing zone 110 and the inert volume of the disengaging zone 120 under the desired turn down conditions for feed gas mixture throughput. The feed gas mixture residence time through the inert mixing zone 110 and inert disengaging zone 120 should preferably be less than the ignition time delay.

In some embodiments of the invention, the transition from inert solids to catalyst occurs within the disengaging zone 120. Parameters useful in determining the location of the active catalyst include the residence time of the gas feed mixture and the desired pressure drop. If the catalyst is located too deep in the bed relative to the flow rate, the residence time may exceed the ignition time delay and partial oxidation can occur prior to the feed gas mixture contacting the catalyst. This is an undesirable situation since excessive temperatures, i. e. , greater than about 2100°F, could result causing soot formation and/or damage to the mechanical integrity of the inert solids and system components. If the catalyst is located too high in the disengaging zone 120, the gas velocity at the time of contact with the catalyst may result in an excessive pressure drop. The combination of partial oxidation and reforming results in an expansion of the gas by a factor between about 2 to about 4. Consequently the catalyst should be located at a position which provides a sufficiently short residence time to prevent homogeneous ignition while allowing sufficient reduction in the feed gas mixture velocity to avoid excessive pressure drop due to the increase in the gas velocities.

In one embodiment of the invention, the gas velocity of the feed gas mixture at the point of contact with the catalyst is between about 5 ft/sec and about 10 ft/sec and the particle size of the catalyst ranges between about 15 mm and about 35 mm. Higher feed mixture velocities may be employed in some embodiments but may result in an increase in the pressure drop. Larger particle sizes may be employed in some embodiments and will allow the use of higher contact velocities. Lower velocities can be employed in some embodiments with the effect of reducing the extent of mixing and possibility of decreasing the radial uniformity of the feed gas mixture velocities to unacceptable levels.

In some cases it may be desirable to utilize inert solids through the entire disengaging zone 120 volume. This is the preferred embodiment when higher pressure drops are acceptable or desirable in order to provide additional mixing of the feed gas components beyond that achieved in the mixing zone 110.

As the feed gas mixture contacts the catalyst volume in the catalyst expansion zone 140 or the catalyst zone 160, both partial oxidation and reforming occur simultaneously. The rate of partial oxidation is more rapid than reforming. Therefore, within the initial, or inlet, catalyst volume a greater extent of partial oxidation occurs. The temperature rise within an inlet portion of the catalyst volume is only slightly higher than that of the adiabatic equilibrium temperature in the bulk of the catalyst volume. The presence of the partial oxidation reaction within the inlet catalyst volume increases the temperature from about 60° to about 200°F above the adiabatic equilibrium temperature. The temperature rise due to the partial oxidation reaction occurs within the inlet portion of the catalyst volume and is approximately proportional to the volumetric flow of the feed gas.

The means for mixing the feed gas components in mixing zone 110 may be any of known conventional method that employs jets or nozzles at sufficiently high turbulent Reynolds numbers.

The gas expansion due to partial oxidation and reforming results in higher gas velocities. In some embodiments the inlet portion of the catalysts placed within the lower sections of the expansion zone in order to mitigate the higher pressure drops associated with increasing gas velocity. The preferred gas velocities after partial oxidation and at least a small fraction of the reforming (approximately 10-30% approach to equilibrium) are from about 3 to about 10 ft/sec to minimize the pressure drop through the reactor. Using 25 mm particles the pressure drop ranges from about 15 to about 20 psig. The location and velocity of the feed gas at the point of contact with the catalyst is based on maintaining the feed gas residence time at valves less than the ignition time.

The catalyst zone 160 consists of sufficient catalyst to allow equilibrium conversion of the feed gas to synthesis gas having an H2 : CO ratio of about 2: 1. For most cases the amount of catalyst required to achieve equilibrium conversion at representative conditions for a Fischer Tropsch synthesis gas corresponds to a gas hourly space velocity of about 10,000 hr-1 or higher, depending upon the activity of the catalyst. The other critical factor defining the volume of active catalyst is the minimum acceptable amount required to catalyze the partial oxidation at startup. This depends upon startup conditions. For a typical set of startup conditions (T feed: 800°F, P 60 psig, Air/NG 0.7, Steam/NG 1) the catalyst volume should be that corresponding to about 2500 GHSV. The criteria which defines the larger minimum catalyst volume should be employed in establishing the total amount of catalyst.

The inert mixing-and-shielding material 141 generally has the following characteristics: (1) ability to withstand the temperatures to which the inert mixing-and- shielding material 141 is exposed (up to between about 2000° and about 2200°F), (2) catalytically inert, and (3) non-reactive to high steam partial pressures. The inert mixing-and- shielding material 141 may take any shape or size consistent with an acceptable pressure drop for the specific application. Acceptable inert mixing-and-shielding materials 141, include, for example, alumina, zirconium oxide, magnesium oxide, and other refractory type oxides. Such inert materials may also comprise nickel. For example, a commercially available Ni/MgO based catalyst (available from Johnson Mathey, Inc. Taylor, Michigan under the designation catalyst #734 is useful in the present invention.

In general, when the feed gas mixture contacts the catalyst bed in catalyst zone 160 the temperature may increase from a range of about 700° to about 1100°F up to a temperature of about 1900° to about 2100°F. The excess heat generated at the interface of the catalyst may radiate upwards and cause the temperatures to rise in the mixing zone, in some applications. The mixing zone 110 may undesirably become a combustion zone due to the extreme exothermic heat generated during the reaction. It is usually desirable to avoid these excessively high temperatures in the mixing zone 110, and the inert material located between the mixing zone 110 and the active catalyst zone assists in avoiding such a temperature problem.

In some applications, the inert mixing and shielding material 141 may serve as a flame arrester during upset conditions in the reactor. The flame arrester may comprise a pipe that functions like a filter element to stop a flame from proceeding backwards in a gas stream of combustion gas. One further advantage of the inert mixing and shielding material in the

application of the invention is that it may provide additional mixing of gas to provide a more homogenous gas stream that is provided into the catalyst bed for reaction.

The commercial practice of employing homogenous combustion (solid, i. e. soot, free) combustion zones often leads to high temperatures. Additionally, the contacting of a pre-mixed feed gas with an active oxidation catalyst may lead to high temperatures causing homogenous soot formation in the gas phase.

As one aspect of the present invention, it has been discovered that the addition of MgO based particles as the inert mixing and shielding material, at the inlet of the reactor, can serve to minimize soot formation and allow stable operations at equilibrium conversions.

The MgO may be any commercial grade of MgO, but sometimes also may contain a small percentage of another metal or alloy, such as Nickel (Ni). One suitable MgO source is obtained from Johnson Matthey, Billingham, Cleveland, UK under the catalyst designation #734.

The inert mixing-and-shielding material 141 in the inlet of the ATR 101 has been seen to improve process efficiency, in part, by not producing soot and achieving equilibrium conversion of a natural gas/oxygen/nitrogen/steam mixture to synthesis gas.

Relatively high inlet velocities (greater than about 40 ft/sec) are desirable to prevent flashback through the transfer volume between the reactors active catalyst zone and the points where the constituent feeds are mixed. The minimum velocity to prevent flashback is usually referred to as the"flashback velocity"for the combustible mixture, and it depends upon the mixture composition and the temperature, pressure, surface/volume ratio as well as other geometrical considerations.

Figure 6 presents a preferred embodiment for gas to liquid operations involving the use of air as the oxygen source with typical natural gas feeds. This preferred embodiment allows the gas to expand in both the inert disengaging zone and the catalyst expansion zone in order to minimize the overall pressure drop across the reactor system. Other embodiments can be employed when other design objectives are desired. For example, if utilizing a higher oxygen content such as oxygen-enriched air the residence time between the mixing zone and the active catalyst may have to be reduced. In such embodiments, the invention may utilize a design in which the inert disengaging zone 120 is eliminated and gas expansion occurs solely in the catalyst expansion zone 140. In alternative embodiments, the catalyst expansion zone 140 may be eliminated with sufficient gas expansion occurring in the inert engaging zone 120. The extent of gas expansion depends upon the desired pressure drop across the reactor and the ignition time delay. The expansion angles 115 and @ 168 are determined by both

the desired pressure drop while expansion angle 115 depends upon the required pre-pox residence time for avoiding pre-ignition.

In a flameless ATR, steam and natural gas is premixed and the resulting partial oxidation reaction occurs in a catalyst bed in conjunction with the reforming reaction. The absence of an ignition means or flame simplifies the mechanical design of the system and allows operations at significantly lower steam to natural gas ratios, less than about 0.4, compared to existing commercial ATR designs.

In other embodiments of the invention, the steam may be mixed with either the NG or the air prior to entering the ATR. In a preferred embodiment, a portion of the steam is mixed with both the NG and air feed constituents.

During start-up of the ATR with excess steam it is preferred to add the majority of the steam (greater than 50%), and most preferably greater than 75%, with the air. As the steam is decreased and the feed composition approaches its final operating value, greater than about 60% of the steam may be fed with the NG flow.

During start-up of an ATR, the feed gas composition flowing through the reactor system undergo a transition from inert or non-flammable to a flammable mixture. The term "flammable mixture"herein means a gas composition which possesses sufficient oxidant and fuel to allow a flame to initiate and propagate throughout the gas mixture.

In accordance with some embodiments of this invention, a flameless ATR reactor is pre-heated using a nonflammable feed gas mixture, such as natural gas ("NG"), to a temperature sufficient to initiate catalytic POX. Upon reaching a temperature in which condensation does not occur, steam may be introduced in conjunction with the natural gas.

The preferred pre-heat temperature can vary with the type of catalyst and the extent of activity with a specific catalyst. In commercial operations it is anticipated that the catalyst activity will be less than that possible under ideal conditions due to aging and other operating conditions that partially or wholly deactivate the catalyst. In producing synthesis gas for GTL operations, it is preferable to pre-heat the feed gas (es) to the highest possible value compatible with the feed gas composition and process equipment metallurgical constraints.

In most applications, the feed gas pre-heat temperature can be as high as 950°F.

Natural gas compositions typically encountered in production fields may be used in the inventive processes. With NG compositions which contain high quantities (>5 vol%) of C2+ constituents the maximum allowable pre-heat temperatures may be lowered.

With certain POX catalysts such as rhodium, the maximum pre-heat temperature may be as low as 400°F. However, in the case of aged Ni-based catalysts, higher pre-heat temperatures (>650°F) are preferable.

After heating the ATR reactor to a temperature at which flameless catalytic, i. e., heterogeneous, POX is initiated, air is introduced at sufficiently low rates to maintain the feed composition outside of the flammability envelope. In this embodiment of the inventive process, the air to natural gas ratio is incrementally increased from an initial value well outside the flammability envelope. Fig. 1 presents the flammability limits for a typical natural gas and air mixture determined in accordance with the data and computational procedures described in the U. S. Bureau of Mines Bulletin #627. Fig. 1 depicts the volume % of NG vs. the volume % steam with volume % air being 100%- (volume % NG + volume % steam). The solid lines identified as"Upper NTP"and"Lower NTP"represent the upper and lower flammability limits for a typical NG, which contains about 85% to 95% methane and about 5% to 15% C2H6 in a mixture of steam and air at normal temperature and pressure.

The upper and lower flammability limits intersect at the point defined as the minimum 02 level required to sustain a propagating flame within the mixture. At higher temperatures and pressures, the flammability limits expand to encompass a broader range of NG values as represented by the composition range included in the flammability envelope at conditions representing ATR startup conditions, 750°F and 60 psig.

Fig. 2 shows the flammability limits as well as composition lines representing ATR feed mixtures containing an air/NG ratio of 1.0 to 2.8 and steam/NG ratios varying from 2.0 to 0.25. As shown in Fig. 2, ATR feeds containing an air/NG ratio of 2.8 lie within the flammability envelope while use of an air/NG ratio of 1.0 leads to mixtures which are well outside of the flammability envelope regardless of the steam/NG ratio.

In one embodiment of the startup method, a feed composition which is outside of the flammability envelope is used. The use of air/NG ratios which are well outside the flammability envelope allows the catalyst to initiate the POX reaction without the risk of introducing a flammable mixture to process volumes downstream of the ATR. In the event that the ATR catalyst has insufficient activity to initiate partial oxidation, the downstream process volumes will fill with an air/NG mixture which cannot propagate a flame and lies outside of the flammability envelope.

In this embodiment of the inventive process, the final feed gas composition is generally between about 5% and about 10% steam, between about 20% and about 30% NG, and < about 2% H2 of the NG flow (or less than about 0.6% of the total flow) with air. The

final feed gas composition is reached by initially introducing a feed gas with an air/NG and steam/NG ratio well above the upper flammability limit. Upon introduction of these non- flammable mixtures, the onset of pre-reforming can be observed through a decrease in the catalyst bed and downstream process temperatures and/or analysis of the ATR exist gas composition.

Fig. 3 depicts composition transients associated with the startup process. The two dashed lines show representative startup scenarios initially starting with feed gas mixtures well outside of the flammability envelope but employing different steam to natural gas ratios, designated as High Steam and Low Steam. The required changes in composition to reach the final feed gas composition, are represented by the sequential change in feed ratios as indicated by the solid arrows along the dashed lines. The arrows depict the composition changes associated with the feed gas as the air/NG and steam/NG ratios transition from initial startup to final feed gas, i. e. operating ATR feed gas composition. The total amount of gas flow through the startup process may vary or may be held constant during this transition from non-flammable to flammable feed gas composition.

In one embodiment of the invention, a relatively constant gas throughput at the value appropriate to maintain the desired pressure drop and appropriate gas mixing under high turbulent Reynolds number flow, > about 100,000, is employed. As the air concentration in the feed gas approaches a value corresponding to the flammability limit, the gas velocity should be sufficient to ensure that the feed gas residence time prior to contacting the catalyst is less than the time required for auto-ignition. That is, the feed gas should reach the catalyst zone of the ATR prior to the onset of auto-ignition. As shown in Fig. 3, the use of a high steam ratio allows the feed gas to transition into the flammable region at modest air and NG levels with approximately 53% steam. Under such conditions, the onset of partial oxidation should be observed well before the feed gas composition transitions into the flammable region. A temperature decrease in the ATR or a change in the ATR exit gas composition may be used to confirm the onset of pre-reforming.

When the ATR does not employ a flame, burner system or related ignition means, the initiation of the partial oxidation depends upon the activity of the catalyst. That is, the catalyst should preferably have sufficient activity to ensure that POX occurs under thermally stable conditions and in the same reactor volume in which catalytic reforming will occur, i. e., the catalyst zone of the ATR.

The high steam levels in the High Steam embodiment minimize soot formation associated with CO disproportionation over the temperature regime from about 1200°F to

about 1400°F. Upon passage into the flammable zone, the ATR feed gas composition is transitioned to that containing an approximate 2. 8 air/NG ratio and a 0.25 steam/NG ratio.

This transition occurs by incrementally increasing the air flow or decreasing the steam flow or both. The NG flow may be maintained at a relatively constant rate. However minor adjustments may be necessary, as indicated in Fig. 3 if the total ATR feed gas velocity is to be maintained constant.

The Low Steam embodiment curve depicts changes in the ATR feed gas composition starting in the very fuel rich non-flammable region well above the upper flammability limit and incrementally transitioning to the final, operating ATR feed gas composition. In one embodiment of the invention, during startup under conditions of constant ATR feed gas velocity, the steam and NG flows may be decreased incrementally as the air flow is increased. The onset of the partial oxidation reaction should be observed well before the feed gas mixture transitions into the flammable region. For example, at 30% steam, 35% NG and 35% air, the ATR feed gas mixture contains sufficient 02, approximately 7.3%, to cause a measurable temperature increase within the catalytic bed and a significant change in the gas composition. The compositions presented in Fig. 3 do not show the small amount of H2 which is present in the feed, typically <2 volume % of the NG flow. The H2 feed level is typically introduced at a fixed ratio with respect to NG. Consequently, it can be treated as part of the NG flow and its presence at low levels has no impact on the startup process.

The ATR feed gas velocity can vary during the startup, especially when the composition is outside of the flammability envelope. However, as the ATR feed gas mixture transitions into the flammable region, the ATR feed gas velocity should be sufficient to prevent auto-ignition prior to contact with the catalyst bed. Thus, the ATR feed gas velocity may be adjusted according to process volumes preceding the ATR catalyst bed. In some embodiments, the final operating pressure for the flameless ATR is in the range from about 100 to about 400 psig. The pressure at startup may be less, in the range of between about 30 and about 100 psig.

The High Steam startup curve shown in Fig. 3 crosses the flammable region at a relatively high steam concentration, approximately 50 volume% in the ATR feed gas. The incremental composition changes associated with the High Steam startup involve decreasing the steam rate while increasing the air feed rate and maintaining a relatively constant NG rate.

The High Steam embodiment of the invention is particularly useful in commercial systems in which there is a desire to reduce the amount of soot generated.

The High Steam embodiment is generally a less complex process because the major flow changes are associated with only two feed components, i. e. the NG and H2 flows may be held constant throughout the startup procedure. At the point where the ATR feed gas composition enters the flammable region, the ATR feed gas velocity is maintained at the appropriate level to ensure that the inlet residence time is shorter than the auto-ignition time.

In another embodiment of the invention, a process to determine if there is sufficient catalyst activity for the initiation of partial oxidation is provided. This embodiment utilizes detection of reforming of the heavier hydrocarbons in the natural gas prior to introduction of the air as an indication of catalyst activity. In adiabatic reactors the reforming of >1 volume % of the NG flow containing the C2+ will result in a measurable decrease in the reactor temperature due to the endothermic nature of the reforming reaction. Catalyst beds which possess sufficient activity towards reforming of the C2+ constituents will generally possess sufficient activity to initiate partial oxidation. Alternatively or additionally, analysis of the effluent gas indicating conversion of C2+ constituents may be used for detecting catalyst activity.

When air is initially introduced into the ATR, the preferred steam and NG levels are preferably sufficiently high so that the composition is well above the upper flammability limit. In the event that the catalyst does not possess sufficient activity for initiating partial oxidation, the passage of this feed gas to downstream process volumes will not result in the accumulation of a flammable mixture. Consequently the risk of a deflagration event is essentially eliminated.

This method of startup can be applied under conditions of constant or varying total feed gas flow rate. The method employs gas velocities corresponding to inlet feed gas residence times in excess of the ignition or auto-ignition time delay. At the lower pressures and oxygen/NG ratios utilized during the initial phases of startup, lower total, gas velocities can be employed.

Although the transition into the flammable mixture can occur anywhere outside the flammability envelope, it is preferred to utilize mixtures which are fuel rich and transition through the upper flammability limit.

Example A flameless ATR system operating at 60 psig and gas feed temperatures of approximately 790° to 870°F was started up using the low steam ratio method described herein. Figs. 4 and 5 illustrate the ATR feed gas ratios and ATR temperatures during the startup procedure. Referring to the time axis presented with each of Figs. 4 and 5, between the time period of Tl to T2 wherein T2 = Tl + 30 minutes. The system was at a steady state temperature between approximately 790° to 890°F using a mixture of steam and natural gas and an air to natural gas ratio of zero, as indicated by the S/NG and A/NG ratio in Fig. 4 wherein S = steam and A = air. At approximately T2 + 5 min. , natural gas was introduced at a S/NG ratio of approximately 0.7, as shown in Fig. 4. The natural gas consisted of about 93% methane, about 3-5% C2+ constituents, and about 2%-4% trace non-hydrocarbon inerts.

The ATR catalyst possessed sufficient activity to conduct reforming conversion on the C2+ constituents within the NG. The onset of pre-reforming is indicated by the decrease in temperature, about a 20° to 40°F temperature drop, across the ATR over the time period of T2 + 10 min. to T2 + 20 min. as shown in Fig. 5.

The reforming activity observed with the decrease of the gas feed temperature is a positive indication that at least some of the catalyst surface area exists in a metallic state capable of promoting partial oxidation. Shortly before T2 + 30 min. (=Tl + lhr.) air was introduced at a relatively low level as shown in Fig. 4. The onset of partial oxidation was immediately observed by the temperature rise observed throughout the ATR volume. At air feed levels which were well below that necessary to develop a flammable mixture, i. e., air/NG <1.2, a temperature rise was observed throughout the reactor. As shown in Figs. 4 and 5, as the air concentration in the ATR feed was increased there was a corresponding increase in the reactor temperature. The observed temperature rise identifies the onset of partial oxidation at ATR feed gas compositions which are well outside the flammability limit.

What is claimed is: