Patent Application
20060039847
The present invention relates to a process for manufacturing ammonia from nitrogen and hydrogen.
Ammonia synthesis from hydrogen and nitrogen is an industrially important process. Many researches have studied this process searching for improvements. Improvements have been made, particularly with respect to catalysts, but the basic process, known as the Haber process, has remained unchanged since its development almost one hundred years ago.
The Haber process is based on the reaction:
This reaction is reversible, meaning both the forward and reverse reactions proceed at appreciable rates under typical reaction conditions. Lowering the temperature shifts the equilibrium in favor ammonia production, but lowering the temperature also lowers the reaction rate. Catalysts accelerate the reaction, but even with catalysts the reaction rate places a lower limit on the practical temperature range. Increasing the pressure also shift the equilibrium in favor of ammonia production, but increasing the pressure increases both pumping costs and equipment costs.
A typical industrial process involves a compromise in both temperature and pressure. For example, a typical process is carried out at about 500° C. and 250 atm over an iron catalyst. At these conditions, the reaction equilibrium limits the single pass conversion to about 20%. The overall conversion is increased by separating the product from the reactor exhaust and recirculating the unconverted reagents.
A typical separation process involves cooling the exhaust to condense ammonia and then physically separating the liquid ammonia. Other separation processes have been mentioned. U.S. Pat. No. 4,180,553 proposes recovering hydrogen by membrane separation. U.S. Pat. No. 1,219,663 describes separating ammonia by adsorption onto carbon or absorption into a liquid. U.S. Pat. No. 4,537,760 describes an adsorptive separation process in which a hot stream of ammonia-containing gases is used to regenerate the adsorbant followed by condensation of ammonia from the gases.
There remains a long felt need for an improved ammonia synthesis process.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. The primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the invention relates to a process for synthesizing ammonia from hydrogen and nitrogen in which an adsorbant is provided to adsorb ammonia as it is being produced. Adsorption of the product drives the ammonia synthesis reaction and allows the process to be carried out at pressures where the gas phase equilibrium conversion is too low for a conventional system to be practical.
The invention is applicable to both small and large scale ammonia synthesis. A small scale ammonia synthesis plant can be a stationary or vehicle-mounted plant used to supply reducing agent for selective catalytic reduction of NOx in diesel exhaust. Ammonia can be desorbed from the adsorbant, extracted from the adsorbant, or stored and transported in its adsorbed state.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In an exemplary process according to the invention, one or more of the three vessel 103-105 is supplied with hydrogen from the hydrogen source 101 and nitrogen from the nitrogen source 102, generally in a 3:1 molar ratio. The hydrogen and nitrogen react in the vessel to produce ammonia. The ammonia adsorbs onto the adsorbant, reducing the concentration of ammonia in the gas and driving the reaction forward.
The reaction is driven forward in accordance with Le Chatelier's principle. Le Chatelier's principle states that when a system at equilibrium is subject to a change, the system will respond to relieve the effect of that change. In this case, the system is the gas phase and the change is removal of ammonia from the gas phase by capturing ammonia on the adsorbant. This tends to reduce the gas phase ammonia concentration. The system responds by producing more ammonia, which tends to maintain the ammonia concentration. The overall effect is that system produces more ammonia than it would in the absence of the adsorbant.
Hydrogen and nitrogen are supplied in an amount that is generally limited by the capacity of the adsorbant. The hydrogen and nitrogen can be supplied at once at the beginning of the reaction, can be supplied continuously as the reaction progresses, or anywhere in between. Where inert contaminants reach substantial concentrations, it may be desirable to vent a small amount of gas from the reactor to keep the concentration of the inerts from becoming too high.
The reaction can be carried out at comparatively low pressures. In one embodiment, the reaction vessel is at pressure of about 50 bar or less, in another embodiment, about 25 bar or less, in a further embodiment, about 10 bar or less, and in a still further embodiment, at about atmospheric pressure. At low pressures, the gas phase equilibrium of the reaction of hydrogen and nitrogen to form ammonia is relatively low. According to the invention, however, ammonia is selectively adsorbed from the gas onto the adsorbant. This reduces the availability of ammonia for the reverse reaction and allows the net conversion of ammonia to proceed beyond that which would result in gas phase equilibrium in the absence of an adsorbant. In one embodiment, the single pass conversion is at least about two times the gas phase equilibrium conversion.
Preferably, the net conversion of reagents fed to the vessel reaches at least about 35%, more preferably at least about 50%, still more preferably at least about 75%, and most preferably at least about 90%. These are single pass conversion rates, which are calculated based on feed rates, whereby reagents removed from the vessel and then recirculated back into the vessel would be counted twice. According to the invention, such recirculation is generally not necessary. If the reagents are not fed in stoichiometric proportions, the conversion rate is based on the limiting reagent. The ammonia substantially remains in an adsorbed state unless and until the adsorbant is regenerated. The net adsorption of ammonia onto the adsorbant over the course of the ammonia synthesis is comparable to the net rate of ammonia generation.
In certain applications, the final product is the ammonia in an adsorbed state. This is particularly useful when the ammonia is used to supply a reducing agent for selective catalytic reduction (SCR) of NOx in exhaust. SCR reduces ammonia even in a lean atmosphere. While this is a promising approach to meeting NOx emission-limiting regulations for cars and trucks, distribution and supply of ammonia is a major obstacle. The invention provides a way of overcoming this obstacle. The ammonia can be produced in small plants, which can either be vehicle-mounted or stationary. Stationary plants can be small and low cost, installed for example at fuel stations. The adsorbed state provides a safe an efficient way of storing and transporting ammonia. The ammonia can be desorbed as needed, by heating the adsorbant, for example. If the ammonia decomposes into hydrogen and nitrogen when it is heated and desorbed, this is not necessarily an issue in that SCR may proceed effectively with hydrogen in place of ammonia.
In other applications however, it is desirable to recover the ammonia from the adsorbant and regenerate the adsorbant without having the ammonia decompose into hydrogen and nitrogen. Ammonia can be recovered and the adsorbant regeneration by desorbing the ammonia or extracting it. Extraction can be, for example, with water. Desorption can proceed by any suitable method, including reducing the ammonia partial pressure and/or increasing the temperature. Ammonia partial pressure can be lowered by lowering the total pressure and/or using an inert carrier to dilute the ammonia in the gas phase. An inert carrier could be N2, which may be flowed over the adsorbant to reduce the ammonia partial pressure and carry away desorbed ammonia. In one embodiment, a N2 carrier is used together with heating. Desorbed ammonia can be captured by any suitable method, including for example adsorption on a solid adsorbant, adsorption by a liquid (extraction), or condensation. Desorbed ammonia can be stored as a compressed gas or liquid. A liquid can be an aqueous solution.
Where ammonia is desorbed by increasing the adsorbant temperature, there is a concern that the ammonia synthesis reaction may reverse. Generally, the reaction will begin to reverse in the absence of a catalyst at temperatures above 450° C. Preferably, desorption takes place at temperatures of about 450° C. or less. There are two general approaches to synthesizing and adsorbing ammonia according to the invention and desorbing the ammonia at an elevated temperature without the synthesis reaction being reversed. One approach is to create a temperature gradient within the reaction vessel during ammonia synthesis whereby the ammonia is adsorbed at a temperature below that at which it is produced. The other is to provide a catalyst which contacts the ammonia during synthesis but not during regeneration. These two approaches can be combined.
The first position, with the slots aligned, is for ammonia generation. Reaction takes place on the catalyst 122 and drives the concentration of ammonia in the gas phase towards an equilibrium at the temperature of the catalyst 122 and the prevailing pressure. Adsorption of ammonia on the adsorbant 125 steadily removes ammonia from the gas phase. Thus, ammonia is simultaneously produced on the catalyst 122 and adsorbed on the adsorbant 125.
Ammonia migrates from the catalyst 122 to the adsorbant 125 without pumping. A fan can be provided to increase the circulation rate, but is generally unnecessary. The flow of gases entering the reactor 120 can also be used to induce circulation, however, there is essentially no pressure gradient between the catalyst 122 and the adsorbant 125. Strictly speaking, small pressure gradients exist due to such phenomena as the adsorption of ammonia, the flow of reagents into the vessel, temperature gradients, and the change in number of moles due to reaction, however, the pressure gradients created by these phenomena are negligible in comparison to the vessel pressure, which is preferably essentially uniform at any given time during the course of the ammonia synthesis in the reactor 120.
In this example, there is generally a significant temperature gradient between the catalyst 122 and the adsorbant 125. This gradient can be used to advantage. A gradient naturally arises because the ammonia synthesis reaction is exothermic. Adsorption is also generally exothermic, but usually much less so than is the reaction. Furthermore, the catalyst and any support is generally less massive than the adsorbant and any support. Preferably, a temperature gradient of at least about 75° C. develops between the catalyst 122 and the adsorbant 125 during ammonia synthesis, more preferably at least about 100° C., and still more preferably at least about 125° C.
The temperature gradient can be further enhanced by heating the catalyst and/or cooling the adsorbent. Generally, heating the catalyst is unnecessary and undesirable, except perhaps to initiate the synthesis process. Ample heat is generated by the reaction. Higher temperatures give higher reaction rates, but also a lower maximum gas phase ammonia concentration. Excessively high catalyst temperatures may also result in catalyst degradation. The catalyst may need to be cooled, but preferably the catalyst loses sufficient heat to the adsorbant to maintain the catalyst temperature within an acceptable range without any additional cooling.
Where the adsorbant is cooled during synthesis, it is generally cooled by heat exchange with a cooling fluid. One or more passages can be provided through the adsorbant for thermal contacting with a cooling agent. The reactor 120 has a central passage 126 adapted to either heat or cool the adsorbant 125. The adsorbant 125, can be cooled for example by passing ambient air through the central passage. This same passage can be used later to heat the adsorbant. For example, the adsorbant can be heated by passing hot exhaust through the central passage. An adsorbant can also be provided with an electrical resistance heater.
Optionally, the vessel is first charged with nitrogen. Optionally, the reaction zone is pre-heated, for example with electrical wires. The hydrogen entering the vessel encounters the reaction zone 132 and reacts with nitrogen to produce ammonia and heat. Whether or not the reaction zone 132 is pre-heated, it tends to experience a greater reaction rate due to the higher concentration of reagents and becomes hotter than the rest of the adsorber/catalyst 133. The higher reaction rate means greater heat release, further increasing the reaction rate and further enhancing the temperature difference. Optionally, the catalyst is provided only in the reaction zone 132. Optionally, insulation or a physical break is provided to reduce the conduction of heat between the reaction zone 132 and the rest of the adsorber/catalyst 133.
The ammonia produced in the reaction zone 132 diffuses through the adsorber/catalyst 133 and is adsorbed until the adsorbant becomes saturated. Once synthesis is complete, the reaction zone 132 can be allowed to cool. The adsorber/catalyst 133 can then be heated to desorb ammonia without reaching the temperatures prevailing the reaction zone 132 during ammonia synthesis.
During regeneration, nitrogen is fed through feed tube 146, an exhaust valve 147 is open, and there is essentially no recirculation of gas between the catalyst 143 and the adsorbant 142. The nitrogen serves both to reduces the partial pressure of ammonia in the reactor and to carry the ammonia away from the catalyst 143. The nitrogen can further be used to heat the reactor. During regeneration, the reactor temperature is generally raised, for example to a temperature from about 400 to about 450° C. The reactor produces a dilute ammonia stream, for example, having an ammonia concentration from about 5 to about 15%. A stream at this concentration can be used directly for SCR of NOx.
The ammonia synthesis reaction takes place over the catalyst 152. Hydrogen is supplied by a hydrogen source 151 and nitrogen from a nitrogen source 152. The reagents are supplied stoichimetrically, or with a slight excess of hydrogen. Excess nitrogen and/or accumulated inerts can be vented through port 156. Preferably, a substantial gradient in the hydrogen concentration develops over the length of the reactor.
The reactor 150 can operate with a substantial temperature gradient between the adsorbant bricks 160 and the catalyst 152. The ammonia synthesis reaction heats the catalyst 152. The bricks 160 can be relatively cool when they enter the reactor 150. The bricks 160 draw heat from the reactor 150 and may advantageously cool the catalyst 152, however, the bricks can perform this function without rising to the catalyst temperature. If the ammonia is adsorbed by the bricks at a low enough temperature, the ammonia can be recovered by heating the bricks without any substantial breakdown of ammonia. Optionally, energy to heat the bricks during regeneration can be drawn from the catalyst 152, whereby the energy requirements for the entire process can be kept minimal.
The reaction vessels of the present invention all contain an adsorbant. The adsorbant is generally distributed throughout the bulk of the vessel, whereby even when a catalyst is used the majority of the ammonia production takes place in close proximity to the adsorbant, for example, which about one meter, more preferably within about 10 cm. Likewise, the majority of the void volume of the vessel is generally in close proximity to the adsorbant.
The hydrogen source 101 can be any suitable hydrogen source. Suitable hydrogen sources include tanks, electrolysis devices, and fuel reformers. A fuel reformer can be a catalytic reformer, a steam reformer, an autothermal reformer, or a plasma reformer and is optionally mounted on a vehicle. A reformer converts fuel, such as diesel, gasoline, propane, methane, or natural gas into synthesis gas (syn gas). Relatively pure hydrogen can be extracted from syn gas by any suitable method, for example, temperature or pressure swing adsorption.
The nitrogen source 102 can be any suitable source of relatively pure nitrogen. The nitrogen source is often a membrane separator, although pressure and temperature swing adsorption systems can also be used to obtain relatively pure nitrogen from air. Typically, a membrane separator will admit argon, which can accumulate in the reaction vessel. Such accumulation is limited by venting a small amount of gas with a high argon concentration.
Preferably, a reactor according to the present invention operates at or below a pressure at which nitrogen is recovered from air or at which hydrogen is recovered from syn gas, whereby the reagent can be supplied to the reactor without further compression.
Any suitable adsorbant material can be used. Examples of adsorbants are molecular sieves, such as zeolites, alumina, silica, and activated carbon. Further examples are oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Be or alkali metals such as K or Ce. Still further examples include metal phosphates, such as phosphates of titanium and zirconium.
Molecular sieves are materials having a crystalline structure that defines internal cavities and interconnecting pores of regular size. Zeolites are the most common example. Zeolites have crystalline structures generally based on atoms tetrahedrally bonded to each other with oxygen bridges. The atoms are most commonly aluminum and silicon (giving aluminosilicates), but P, Ga, Ge, B, Be, and other atoms can also make up the tetrahedral framework. The properties of a zeolite may be modified by ion exchange, for example with a rare earth metal or chromium. While the selection of an adsorbant depends on such factors as the desired adsorption temperature and desorption method, preferred zeolites for ammonia storage generally include faujasites, rare earth zeolites, and Chabazite. Faujasites include X and Y-type zeolites. Rare earth zeolites are zeolites that have been extensively (i.e., at least about 50%) or fully ion exchanged with a rare earth metal, such as lanthanum. A Chabazite is preferably a calcium Chabazite. A Chabazite can be cation exchanged with lithium to improve adsorption capacity.
The adsorbant is typically combined with a binder and either formed into a self-supporting structure or applied as a coating over an inert substrate. A binder can be, for example, a clay, a silicate, or a cement. Generally, the adsorbant is most effective when a minimum of binder is used. Preferably, the adsorbant bed contains from about 3 to about 20% binder, more preferably from about 3 to about 12%, most preferably from about 3 to about 8%. A preferred composition for small adsorbant pellets that can be used to form monoliths, larger pellets, or a porous coatings over an inert substrate such as screening, is molecular sieve crystals with about 8% or less portland cement as a binder. This composition can provide structural integrity and high utilization of the molecular sieve's adsorption capacity.
A catalyst is not necessarily required and in some cases, the adsorbant acts as a catalyst, particularly where the adsorbant is a zeolite. Where a catalyst is used, it can be any suitable catalyst for the reaction of hydrogen and nitrogen to produce ammonia. Typical catalysts include iron, which may be promoted with oxides of one or more of aluminum, magnesium, calcium, and potassium. Other catalysts include Group VIII metal compounds such as ruthenium, optionally combined an alkali metal and a Ba compound, molybdenum oxycarbonitride, and nickel-molybdenum. Combinations of a Group VIII metals and Group VIB metals in the nitride form can also be employed. Still further catalyst are described in the literature and are available commercially. The catalyst may be combined with a binder.
For mobile applications, adsorbants and catalyst are either formed into a self-supporting structures or applied as a coating over inert substrates. Mobile applications create restrictions on weight, dimensions, and durability for adsorbant and catalyst beds. For example, an adsorption or catalyst bed for a vehicle exhaust systems must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.
Beds that have an adsorbant function tend to be large in comparison to beds that have only a catalytic function. To limit the total mass and thermal inertia, an adsorbant bed preferably comprises a high loading of adsorbant per unit bed mass. Preferably, an adsorbant bed comprises at least about 40% adsorbant by weight. The weight of an adsorbant bed includes any inert substrate and any binders, but does not include any housing. Preferably an adsorbant bed comprises at least about 20% adsorbant by volume.
Adsorbant and catalyst beds are optionally provided with mechanisms for heating and/or cooling. For example, a bed can be permeated with heat-exchange passages in fluid isolation from the passages provided for adsorption or catalysis. A hot or cold fluid is circulated through the heat-exchange passages to heat or cool the bed. A cooling fluid could be, for example, engine coolant or ambient air. A heating fluid could be, for example, hot exhaust or a fluid that draws heat from hot exhaust or a heat-producing device such as an ammonia synthesis reactor, a fuel reformer, or an adsorber. Another option is electrical resistance heating. Where a bed includes a metal substrate, the metal substrate can be used as an electrical resistance heater. A bed can also be permeated by wires for electrical resistance heating.
An adsorbant or catalyst bed can have any suitable structure. Examples of suitable structures may include monoliths, packed beds, and layer screening. A packed bed for a mobile application is preferably formed into a cohesive mass by sintering the particles or adhering them with a binder. When the bed has an adsorbant function, preferably any thick walls, large particles, or thick coatings have a macro-porous structure facilitating access to micro-pores where adsorption occurs. A macro-porous structure can be developed by forming the walls, particles, or coatings from small particles of adsorbant sintered together or held together with a binder.
Preferably an NH3 adsorption bed has a large capacity for adsorbing a NH3 species at a typical adsorption temperature NH3 partial pressure. Preferably, the adsorbant can adsorb at least about 3% NH3 by weight adsorbant at a typical adsorption temperature and 1 atm NH3 partial pressure, more preferably at least about 5% by weight adsorbant, and still more preferably at least about 7% by weight adsorbant. The weight of adsorbant does not include the weight of any binders or inert substrates.
The invention has been shown and described with respect to certain aspects, examples, and embodiments. While a particular feature of the invention may have been disclosed with respect to only one of several aspects, examples, or embodiments, the feature may be combined with one or more other features of the other aspects, examples, or embodiments as may be advantageous for any given or particular application.
