Optimizing methanol plant operations through minimal capital investments
In the constant search for improvement, Methanex has developed a state-of-the-art project to improve plant operation at its methanol production facility in Punta Arenas, Chile (FIG. 1).
In the constant search for improvement, Methanex has developed a state-of-the-art project to improve plant operation at its methanol production facility in Punta Arenas, Chile (FIG. 1). The Chile 4 Standalone Operation project not only provided a solution to an operational constraint, but also enabled the plant to function independently and provide a low-cost solution to increase process efficiency and site reliability.
FIG. 1. Aerial photo of Methanex’s methanol facility in Punta Arenas, Chile.
This article provides the development process performed by Methanex, starting from the business context through an explanation of the original design and the proposed (now implemented) new design—and, finally, depicting the benefits derived from the project’s implementation.
Business case
Over the past 5 yr, global methanol demand has increased by 5%/yr. In addition to traditional uses of methanol (e.g., acetic acid and formaldehyde), other important products are being produced, including dimethyl ether (DME) and olefins. Methanol is also used as a feedstock for blended motor fuels.
One of the most widely used raw materials to produce methanol is natural gas. A natural gas-based methanol plant contains four primary processing steps: desulfurization (natural cleaning and conditioning), reforming (conversion of natural gas into synthesis gas), methanol synthesis (production of crude methanol from the synthesis gas) and distillation (purification of crude methanol).
Reforming is usually the most capital-intensive step and the most relevant in terms of the overall process efficiency of a methanol plant. A natural gas-based methanol plant has different technologies available for the reforming process, with the most prevalent technologies including steam reforming (conventional steam reforming), combined reforming (a conventional steam reformer followed by an autothermal or secondary reformer), and an autothermal-only reformer.
A critical factor for all methanol-producing facilities involves achieving a certain design value for the synthesis gas stoichiometric ratio. This ratio measures the hydrogen-to-carbon oxide molecule fraction, specifically (H2 – CO2)/(CO + CO2) in the synthesis gas feeding the methanol synthesis step. Normally, a ratio of at least 2 is required for the formation of methanol in the synthesis unit, with the best process efficiency achieved with the figure as close to 2 as practical.
The Chile 4 Standalone Operation project constitutes a development that Methanex has implemented on a plant located in southern Chile at Cabo Negro, Punta Arenas. This development considers the design and implementation of an integrated autothermal-only reforming plant. The plant—which originally required a feedstock provided by nearby conventional steam reforming facilities (an imported hydrogen-rich stream) and fresh natural gas—is now capable of operating in a standalone condition; specifically, this plant has become independent of any feedstock from the nearby conventional reforming facility, and the design change applied is unique among plants of this kind.
Chile 4 original design
Chile 4 is a 2,400-metric-tpd, oxygen-based methanol plant at Cabo Negro, Punta Arenas, Chile. The plant has four main processing units, including:
- Natural gas preparation and reforming: Natural gas compression, feed gas hydrogenation and desulfurization, saturation and reforming (via an oxygen-fed autothermal reformer)
- Methanol synthesis: Makeup and recycle compression, methanol reactors and outlet gas cooling
- Methanol distillation: Degassing, topping column and refining distillation columns
- Air separation unit: A 1,600-metric-tpd (99.5% purity) oxygen plant.
In the reforming unit, synthesis gas is produced from natural gas and purge gas from the adjacent Chile 1–3 methanol plants (hydrogen-rich stream). The availability of external purge gas as additional process feed allows the application of a single autothermal reformer-only reforming step to produce the synthesis gas (FIG. 2).
FIG. 2. Integrated methanol production in Chile 4 (original design).
Chile 4 plant modification
This plant incorporates design features that result in a very efficient plant in terms of natural gas consumption, but it does require additional feedstock imported from a nearby conventional steam reforming plant (i.e., Chile 1). This extra feedstock is a portion of the synthesis loop purge gas, which is a hydrogen-rich stream. This additional hydrogen allows the Chile 4 plant to achieve a stoichiometric ratio higher than (and close to) a value of 2. This characteristic of the Chile 4 plant made it unable to operate in an autonomous manner.
However, the modification in FIG. 3 was applied, which enabled the Chile 4 plant to operate in a standalone condition. This was achieved by purifying two streams from its own process, namely:
FIG. 3. Chile 4 standalone methanol production (adapted) block flow diagram.
- The purge gas stream coming from the synthesis loop unit of Chile 4
- A portion of the reformed gas stream coming from the exit of the reforming unit of the Chile 4 plant.
The solution (added to the Chile 4 flowsheet/plant), which allowed standalone operation, incorporated a pressure swing adsorption (PSA) package. The PSA separates hydrogen from carbon compounds in the purge gas and reformed gas streams. This high-purity hydrogen stream produced from the PSA unit can then be fed to the front-end reforming section of the Chile 4 plant, creating a synthesis gas that complies with the stoichiometric design requirement of the synthesis loop and eliminates the need for imported (hydrogen-rich) purge gas from the Chile 1 plant.
The most interesting aspect of this project is related to the modification of an existing plant that was not designed to operate with a PSA unit. Although some plants operate using their own purge gas, Chile 4 purge gas had insufficient hydrogen to reach the required stoichiometric ratio for methanol synthesis. Therefore, an extra supply of “impure hydrogen” was required to feed the PSA and to bridge the shortfall of hydrogen required to achieve the necessary stoichiometry ratio of feed gas to the methanol synthesis unit. The synthesis gas stream (i.e., feed or makeup gas to the methanol synthesis unit) became the solution to provide the additional hydrogen required for the methanol synthesis unit. Feeding a certain percentage of the synthesis gas stream, in addition to the synthesis loop purge stream, to the PSA unit enabled the production of sufficient extra hydrogen to meet the design stoichiometry target (> 2) in the feed stream to the methanol synthesis unit.
The project also managed to recover compounds rejected by the PSA after the hydrogen-rich stream was returned to the process. The rejected stream (noted as tail gas in FIG. 3) is a carbon-rich stream and is returned as a fuel source to the existing heating furnace. As such, no stream was wasted in this design, and returning the tail gas as a fuel source allowed the plant to maintain a high overall process efficiency level. When operating in standalone mode, the efficiency level achieved on the Chile 4 plant was comparable and competitive with the efficiency level of most of Methanex’s remaining plants around the world.
Startup modification of the redesigned plant
In addition to the installation and connections required to install a PSA package, a minor modification of the Chile 4 plant’s startup was required. The autothermal reformer unit was designed to start up and operate with a feed gas mixture that has a certain minimum level of hydrogen in its feed stream. This hydrogen level helps ensure ignition of the oxygen, which is also fed to the autothermal reformer. The original design concept required a methanol synthesis purge stream from the nearby conventional reforming plants feeding the Chile 4 plant upstream of the autothermal reformer. This imported hydrogen-rich purge gas stream facilitated not only the startup, but also allowed the stoichiometric ratio in the feed to the methanol synthesis unit to be met.
Operating in standalone mode, this import purge stream from the nearby plant is not available. Therefore, the startup modification resulted in allowing a small amount of natural gas feed to be admitted to the autothermal reformer. Over an unlit autothermal reformer (i.e., before oxygen is admitted), the natural gas will partially reform at temperature and produce a reformed gas product that contains hydrogen. A portion of this reformed gas can be recycled upstream to the front end of the plant by means of a new eductor unit, using steam or feed gas as a motive force. This startup configuration is different from the original design; therefore, installation of new piping and a new eductor was required to perform this standalone startup step.
Takeaway
The Chile 4 plant became a standalone autothermal reforming plant by transforming the original integrated design with only few modifications—i.e., a plant originally dependent on a hydrogen-rich feed from a nearby steam methane reforming facility can now operate independently through the minimal investment modification described in this article.
The original design parameters required an import hydrogen-rich purge stream for startup and operation. Methanex personnel managed to transform the plant to start up and operate in a standalone condition, with minimal investment and with different process units from other methanol plants designed to operate autonomously with only an autothermal reforming unit. The benefits of this type of plant compare favorably vs. a combined-step or two-step reforming plant, with the potential for lower investment cost and higher natural gas/methanol efficiency.
In addition, the new design allows for the flexibility to operate in different modes—i.e., the ability to switch between the original and new designs depending on operational or business situations. Therefore, the Chile 4 plant can work with an integrated operation (such as receiving hydrogen-rich gas from another plant to achieve the most efficient operation) or in standalone operation, enabling the plant to optimize operations when the nearby plant is offline or unavailable. Methanex will continue to monitor and evaluate the performance of this plant to measure its benefits and challenges vs. existing methanol plants around the world. HP
The Author
Soto, J. - Methanex Corp., Punta Arenas, Chile
Jorge Soto is the Director of Projects and Business Development at Methanex Chile, and has more than 30 yr of engineering, project and operational experience in methanol plants. He earned a chemical engineering degree from the University of Magallanes in Chile.
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