Patent Application
20070282020
These and other aspects, features, and advantages of the present invention will become apparent upon further consideration of the following detailed drawing figures, in which:
The present invention is a process and a system for creating methanol from synthesis gas. The synthesis gas is a stream containing H2, CO, N2 and CO2, generated from a N2 containing oxygen stream, such as air. The synthesis gas stream is reacted through a methanol loop system to create methanol. A specific embodiment of the present invention combines the Starchem system for creating synthesis gas, utilizing an air stream with the conventional methanol loop system for reacting synthesis gas to create a methanol stream, while avoiding prior art concerns of efficiency and methanol yield resulting from the presence of nitrogen in the system. Another specific embodiment of the present invention is a methanol production process such as described herein, which can be mounted and operated on a seagoing vessel, as well as on land.
In one aspect, the invention provides a process for producing methanol, comprising the steps of providing a synthesis gas stream comprising H2, CO, and CO2, and N2, wherein the synthesis gas stream comprises at least about 6 mole % N2 and wherein the synthesis gas stream comprises H2, CO, and CO2 in a ratio of (H2−CO2)/(CO2+CO), which is less than about 1.80. The synthesis gas stream is then combined with a hydrogen-rich gas stream to form a makeup gas stream, which is then combined with a recycle gas stream to produce a reactor feed stream. The reactor feed stream is introduced into a reactor system containing a methanol conversion catalyst, wherein a portion of the reactor feed stream is converted to methanol. A reacted gas stream is withdrawn from the reactor system and the reacted gas stream is separated into a crude methanol product stream and a gas stream. The gas stream is split into a recycle gas stream and a purge gas stream. The recycle gas stream is mixed with the makeup gas stream to form the reactor feed stream. The purge gas stream is then separated into a fuel gas stream comprising CH4 and N2 and a hydrogen-rich stream comprising H2, and the hydrogen-rich stream is mixed into the synthesis gas stream.
In another aspect of the invention, a similar process for converting a natural gas to methanol is provided. The process includes providing an air stream having an oxygen content less than about 22% and enhancing the oxygen content of the air stream to between about 28% and 94% oxygen, thereby creating an enhanced oxygen stream. A natural gas stream comprising methane is then provided and the natural gas stream is partially oxidized in an autothermal reformer using the enhanced oxygen stream to create a synthesis gas stream comprising H2, CO, and CO2 and N2. The synthesis gas stream is then combined with a hydrogen-rich gas stream to form a makeup gas stream, wherein the synthesis gas stream composition remains substantially unchanged after exiting the autothermal reformer until combined with the hydrogen-rich gas stream. The makeup gas stream is combined with a recycle gas stream to produce a reactor feed stream, which is introduced into a reactor system containing a methanol conversion catalyst; wherein a portion of the reactor feed stream is converted to methanol. A reacted gas stream is then withdrawn from the reactor system and separated into a crude methanol product stream and a gas stream. The gas stream is split into a recycle gas stream and a purge gas stream. The recycle gas stream is combined with the makeup gas stream to form the reactor feed stream. The purge gas stream is separated into a fuel gas stream comprising CH4 and N2 and a hydrogen-rich stream comprising H2, and the hydrogen-rich stream is mixed into the synthesis gas stream.
In yet another aspect of the invention, a process is provided for producing methanol that comprises the steps of providing an air stream having an oxygen content less than about 22% and enhancing the oxygen content of the air stream to between about 28% and 94% oxygen, thereby creating an enhanced oxygen stream. A natural gas stream is provided that comprises methane. The natural gas is partially oxidized in an autothermal reformer using the enhanced oxygen stream to create a synthesis gas stream comprising H2, CO, and CO2, and N2. At least about 90% of the synthesis gas stream is then combined with a hydrogen-rich gas stream to form a makeup gas stream, which is combined with a recycle gas stream to produce a reactor feed stream. The reactor feed stream is introduced into a reactor system containing a methanol conversion catalyst, wherein a portion of the reactor feed stream is converted to methanol. A reacted gas stream is withdrawn from the reactor system, which is separated into a crude methanol product stream and a gas stream. The gas stream is split into a recycle gas stream and a purge gas stream. The recycle gas stream is then mixed with the makeup gas stream to form the reactor feed stream. The purge gas stream is separated into a fuel gas stream comprising CH4 and N2 and a hydrogen-rich stream comprising H2, and the hydrogen-rich stream is mixed into the synthesis gas stream.
In a further aspect of the invention, a process is provided for producing methanol, which comprises the steps of providing a synthesis gas stream comprising H2, CO, and CO2 and N2, wherein the synthesis gas stream comprises at least about 16 mole % N2 and wherein the synthesis gas stream comprises H2, CO, and CO2 in a ratio of (H2−CO2)/(CO2+CO) of about 1.73. The synthesis gas stream is combined with a hydrogen-rich gas stream to form a makeup gas stream, which is combined with a recycle gas stream to produce a reactor feed stream. The reactor feed stream is then introduced into a reactor system containing a methanol conversion catalyst, wherein a portion of the reactor feed stream is converted to methanol. A reacted gas stream is withdrawn from the reactor system and separated into a crude methanol product stream and a gas stream. The gas stream is then split into a recycle gas stream and a purge gas stream. The recycle gas stream is mixed with the makeup gas stream to form the reactor feed stream. The purge gas stream is separated into a fuel gas stream comprising CH4 and N2 and a hydrogen-rich stream comprising H2, and the hydrogen-rich stream is mixed into the synthesis gas stream.
In the present invention, the Starchem system for creating synthesis gas from the oxidation of natural gas using an air or other oxygen-containing stream is combined with a traditional methanol loop system. The present invention overcomes prior art concerns of using nitrogen containing synthesis gas in the traditional methanol loop system.
Starchem systems such as those disclosed in U.S. Pat. No. 5,472,986 and U.S. Pat. No. 7,019,039 B1 (“Starchem Patents”) include processes for creating synthesis gas using an air stream, or other oxygen and nitrogen containing stream, separating the air stream into high oxygen and low oxygen content streams, and utilizing the high oxygen content stream in a partial oxidation reaction with natural gas to create a synthesis gas containing H2, CO and CO2. The Starchem Patents are incorporated herein by reference in their entirety for all purposes.
In a specific embodiment of the invention, the synthesis gas stream 212 contains at least about 8 mole % nitrogen. In another specific embodiment of the invention, the synthesis gas stream 212 contains at least about 20 mole % nitrogen. In a more specific embodiment of the invention, the synthesis gas stream 212 contains at least about 35 mole % nitrogen.
The makeup gas stream 216 is further combined with a recycle gas stream 218 to produce a reactor feed stream 220. Feed stream 220 is introduced into reactor vessel 222, which contains a methanol conversion catalyst.
A portion of the reactor feed stream 220 is then converted into methanol. The reacted gas stream containing methanol 224 is withdrawn from the reactor system and separated into a crude liquid methanol product stream 228 and gas stream 230. The gas stream 230 is then split into two streams, the first stream being the recycle gas stream 218, which is combined with the makeup gas stream 216 to form the reactor feed stream 220. The second portion is a purge gas stream 236. The purge gas stream 236 is separated into two streams, the fuel gas stream 234, which contains CH4, CO, and CO2 and N2, and a hydrogen-rich gas stream 214. The hydrogen-rich gas stream 214 is then mixed into the synthesis gas stream 212 to form the makeup gas stream 216.
A natural gas stream containing methane 210 is partially oxidized in an ATR using an enhanced oxygen stream 206 to create synthesis gas stream 212. In one embodiment of the invention, the partial oxidizing step creates a synthesis gas stream 212 containing H2, CO, and CO2 in a stoichiometric number of (H2−CO2)/(CO2+CO), which is less than about 1.80; i.e., it is deficient in hydrogen. In a specific embodiment of the invention, the synthesis gas stream 212 is created that has a stoichiometric number of about 1.77. In another specific embodiment of the invention, a synthesis gas 212 is created that has a stoichiometric number of about 1.73. In yet another specific embodiment of the invention, a synthesis gas stream is formed that has a number in the range of about 1.34 and about 1.80.
In a further embodiment of the invention, when hydrogen is extracted from the purge gas stream 236 to form a hydrogen rich stream 214 that is combined with hydrogen poor synthesis gas 212 and recycle gas 218, a reactor feed gas 220 is produced in the reactor 222 with a stoichiometric number between about 2.05 and about 10.
A portion of the reactor feed stream is converted into methanol. The reacted gas 224, which contains methanol, is withdrawn from the reactor vessel 222 and separated into crude liquid methanol product stream 228 and gas stream 230. The gas stream 230 is then split into two streams, the first stream being recycle gas stream 218, which is combined with the makeup gas stream 216 to form the reactor feed gas stream 220. The second portion is a purge gas stream 236. The purge gas stream 236 is separated into two streams, the fuel gas stream 234, which contains CH4, CO, and CO2 and N2, and a hydrogen-rich gas stream 214. The hydrogen-rich stream 214 is then mixed into the synthesis gas stream.
Using the synthesis gas production method disclosed in
As discussed above, a method has been previously disclosed wherein methanol is produced by contacting a synthesis gas formed from an oxygen-enriched gas stream for the partial oxidation of natural gas using a methanol loop reactor system. There are key distinctions, however, between this prior art system and the invention described and claimed herein. In the prior art system, a portion of the synthesis gas is not subjected to the methanol production synthesis loop. Rather, the synthesis gas is split into two distinct streams on leaving the ATR. The first stream is diverted to a water gas shift reactor, converting CO to H2, and then through a pressure swing absorber to extract the H2. The H2 that has been distracted joins the second stream and the combined flow is a hydrogen enhanced syn gas with a Ns equal to about 1.85. Since the extraction of the H2 requires the use of additional equipment, one skilled in the art will recognize that the elimination of this extraction step results in a methanol production process that is more efficient and more economical. Additionally, the total methanol yield from the AUS process is lowered by the additional purge required as a result of the additional process equipment.
In a specific embodiment of the present invention, the syn gas comprises about 19 mole % nitrogen and the methanol loop is operated with a high level of H2. The reactor feed gas, therefore, has a stoichiometric number of about 3.3. The excess H2 reacts to maintain the CO at a very low level, i.e., about 3.8 mol %. Thus, there is very little CO in the purge gas. This enables the mols of CO in the fuel gas to be maintained at a value less than 10% of the CO entering the system in the synthesis gas. Similarly, the H2 is separated from the purge stream leaving behind in the fuel gas an amount of H2 that is less than 3.5% of the H2 entering the system in the synthesis gas. The sum of the mols of reactants (H2+CO) leaving the system in the fuel gas is about 5% of that entering the system as syn gas. The CO2 that has not reacted with H2 in the methanol loop will leave the system in the fuel gas. This amounts to about 15% of the CO2 entering the system in the synthesis gas. The retention of H2 in the methanol loop, reacting the CO to low values and rejecting carbon from the system as CO2, will always result in a MUG stoichiometric number greater than 2.05.
A prime benefit of the use of air separation systems is that they allow an alternative to cryogenic oxygen, which significantly reduces the costs associated with the preparation of the synthesis gases. The presence of cryogenic oxygen along with large quantities of methanol creates an extremely hazardous and potentially explosive situation, particularly when there is limited space available, such as on a sea going vessel. The present invention avoids this hazard and allows marine application. All of the embodiments of the invention may be practiced on a seagoing vessel in accordance with the invention. The process can be implemented and completely contained on a ship, barge, or other seagoing vessel. As such, the invention can be brought to natural gas production areas, such as those at offshore production facilities, to convert the abundant natural gas into methanol. The methanol can then be periodically transferred from its seagoing vessel to tankers for transportation to the market. The nominal capacity of the methods of the present invention is approximately 5000 to 15000 metric tons per day (“MTPD”).
The Ns the MUG stream can be controlled by the methanol loop design parameters. When the loop pressure is held constant by a back pressure controller in the fuel gas stream or varying the power to the MUG compressor, the inerts will leave the system in the fuel gas stream. In addition, there will be some H2 and CO and a major amount of CO2 leaving in the fuel gas.
In one embodiment of the invention, the MUG comprises H2, CO and CO2 in a ratio of (H2−CO2)/(CO2+CO), which is at least about 2.05. In another embodiment of the invention, the MUG comprises H2, CO, and CO2 in a ratio of (H2−CO2)/(CO2+CO), which is at least about 2.4. In yet another embodiment of the invention, the MUG comprises H2, CO, and CO2 in a ratio of (H2−CO2)/(CO2+CO), which is at least about 3.6.
Although the invention has been described with reference to its various embodiments, from this description, those skilled in the art may appreciate changes and modifications thereto, which do not depart from the scope and spirit of the invention as described herein and claimed hereafter. The following Examples illustrate certain embodiments of the invention, in comparison to the AUS Process. The Examples illustrate specific embodiments of the invention, and is not meant to limit the scope of the invention in any way.
The impact of the amount of nitrogen in the methanol loop in the methods of the invention on the amount of methanol produced per pound of natural gas was calculated. With reference to Table 1, below, the calculation was performed assuming a concentration of 42% oxygen, 21% oxygen and 70% oxygen in the air stream shown in
The results show the level of oxygen enrichment has an insignificant effect on the stoichiometric number of the effluent from the ATR. It is less than 1.77. Table 1 demonstrates that the process shown in
The methanol made per pound of natural gas increased 15% when the oxygen concentration increased from 21% to 42%. However, the increase in methanol was only 2.5% when the oxygen increased from 42% to 70%.
With reference to
Parameters for the study were from the Starchem Plantship analysis performed by Lurgi. The parameters dictated the natural gas and air flow rates, steam/carbon, pressures, cooling water temperatures, etc. A water quench to 500° F. out of the ATR was used when flow was into path A. Quench is necessary to avoid metal dusting and provide steam for the water gas shift. For calculations with zero flow into path A, also known as the H2 extraction stream, no quench was used.
Two sessions were performed. The first involved varying the percentage of flow into path A in 10% increments. It was found that any amount of flow into path A resulted in a decreasing methanol yield as the flow increased. The second session involved increasing the efficiency of H2 separation in path A from 70% to 85%.
The results from the second session are shown in
The total amount of inerts that must be purged from the process is the same in both the AUS Process and the methods of the invention describe herein. The purge from path A, however, always has a higher concentration of reactants (H2+CO) than the purge from the methanol loop. Thus, more reactants that could be converted to methanol are lost when there is flow into path A. When path A is utilized, methanol is produced at a rate of 997,948 lb/hr. As the amount of synthesis gas diverted to path A is increased, however, the amount of methanol produced decreases. When path A diverts 30% of the synthesis gas, methanol is produced at a rate of only 942,306 lb/hr., which is an at least 5% decrease.
The method of control of the invention described and claimed herein renders the AUS Process split of synthesis gas into two streams totally unnecessary and, in fact, harmful to the methanol yield. One stream in AUS Process has the hydrogen content enhanced by water gas shift and rejects nitrogen and CO2. In the present methods of the invention, the water gas shift occurs in the methanol reactor and the N2 and CO2 rejection are performed at a much lower cost and at the same time, more of the reactants are preserved to make additional methanol.
Although the invention has been described with reference to its various embodiments, from this description, those skilled in the art will understand that certain changes and modifications to the various embodiments of the invention, which do not depart from the scope and spirit of the invention, are nevertheless within the scope of the invention.
This application claims priority to U.S. Provisional Application No. 60/809,260 filed May 30, 2006, which is hereby incorporated in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 60809260 | May 2006 | US |
