Anne K. Rhodes
Refining/Petrochemical Editor
Terra International Inc., Sioux City, Iowa, has started up a 400 ton/day (3,200 b/d) methanol plant at its Woodward, Okla., manufacturing complex.
The methanol plant is integrated with the company's existing ammonia facility there (see cover photo).
Terra says it is operating the only integrated methanol/ammonia facility in the U.S. The decision to integrate the two systems stemmed from a strong methanol demand, projected to increase, and low capital requirements because the two processes can share common components.
The unique design of Terra's ammonia plant facilitated the methanol tie in. There are, in fact, only two such plants, designed by Fluor Corp. The other plant is in Trinidad.
Typically, the CO2-removal sections of ammonia plants operate at 380-400 psi. The Fluor-designed process, however, operates at about 1,800 psi. This high pressure eliminated the need to add an expensive compression section. In fact, the $15.5 million required for the methanol integration project was only about one third the capital cost of a new methanol plant.
This is the first venture by Terra, a nitrogen fertilizer producer, into the methanol industry. The company plans no further methanol capacity additions at this time.
DEMAND
Global methanol supply and demand currently are in balance at about 22.4 million metric tons. U.S. demand, however, exceeds supply.
Because methanol is a feedstock for gasoline additive methyl tertiary butyl ether (MTBE), methanol demand for MTBE production will exceed that for formaldehyde use when the MTBE market goes into full swing late this year, predicts Terra.
The company says that, by 1996, MTBE will account for more than 32% of domestic methanol demand (Fig. 1).
Terra has positioned itself as a regional methanol supplier, serving users in Arizona, Arkansas, New Mexico, Oklahoma, Texas, Iowa, Kansas, and Missouri. Most of the company's market is within a 500-mile radius of the plant.
Currently, about one third of Terra's methanol product is used to produce MTBE. This methanol is shipped by rail to two refineries in the region.
Another third of the production is sold to formaldehyde manufacturers. And the final third is used for such purposes as gas dehydration, industrial distribution, and window washing fluid production.
CONSTRUCTION
Terra's integrated methanol production process is designed by Haldor Topsoe (Fig. 2). The Woodward complex is the first plant to produce commercial methanol using this technology. (A small plant in Egypt uses the process to purify the ammonia stream.)
With the reforming and compression sections already in place in the Woodward ammonia plant, only distillation and reaction units had to be constructed to integrate the methanol process.
Terra decided to build the methanol plant in November 1992. Construction was completed in April 1994, and first methanol was shipped in May.
Terra acted as its own general contractor, doing most of the project and construction management in house. Haldor Topsoe undertook some of the process design. Detail design was performed by the Benham Group, Tulsa.
The critical path in the project, requiring about 1 year to fabricate, was the methanol reactor. The other major piece of equipment installed was a 180 ft methanol distillation tower containing 83 trays.
Terra added four single-floor methanol tanks, with secondary containment, as part of the new project. These tanks have lined berms, leak-detection systems, and cathodic protection.
PROCESS
Methane from natural gas is the source of fuel and feedstock for production of both ammonia and methanol. Terra secures its natural gas supply from ample local and regional sources.
In the process scheme, natural gas enters the plant at 600 psig (Fig. 2). The feed gas is preheated, desulfurized in zinc oxide beds, and mixed with ammonia-methanol process condensate. The condensate is then vaporized, and the steam-gas mixture is preheated.
Additional steam is added and the mixture is heated further in the convection section. The feed then passes through catalyst-filled tubes in the primary reformer radiant section.
Hydrocarbons react with the water to produce carbon monoxide, carbon dioxide, and hydrogen. The effluent then passes to the secondary reformer.
Air is compressed, preheated, and mixed with steam at the top of the secondary reformer. This provides controlled combustion in the space above the nickel catalyst, allowing the chemicals to react more completely. The air also supplies the nitogen required for ammonia production.
Adjusting the air flow rate changes the hydrogen-to-nitrogen ratio, which enables Terra to vary the product mix. After passing through the secondary reformer, the effluent is cooled before entering the high-temperature shift converter.
The shift reactors convert carbon monoxide to carbon dioxide, which then is removed by physical absorption.
When ammonia alone was produced, the cooled stream was processed through the high and low-temperature shift converters in series, with intermediate cooling. Now that methanol production is integrated into the complex a high concentration of carbon monoxide in the methanol reactor is needed to produce methanol efficiently.
In the new integrated process, about 50% of the process flow bypasses the high-temperature shift converter, and 100% bypasses the low-temperature converter.
Excess reaction steam from the reformer is condensed and removed from the process synthesis gas in a series of recoiler exchangers in the methanol and ammonia distillation sections. This residual water is recycled to the primary reformer.
When ammonia alone was produced, raw synthesis gas from the low-temperature converter was cooled, dried, and compressed. The compressed gas entered a packed absorber, where carbon dioxide was removed by countercurrent contacting with cold, lean propylene carbonate. The CO2-rich propylene carbonate was regenerated by flashing to atmospheric pressure in several stages, followed by air stripping.
When ammonia and methanol are coproduced, the air stripper is bypassed to produce the carbon dioxide concentration necessary for methanol production. In this instance, the synthesis gas from the CO2-removal unit is preheated, then sent to the methanol reactor. As the gas passes through catalyst-filled tubes, methanol is produced from the reaction of carbon oxides and hydrogen.
Because the methanol synthesis reaction is exothermic, 450 psi steam is produced on the shell side of the reactor to control the heat of reaction.
The methanol converter effluent is cooled, and the crude methanol is separated from the process gas stream. The ammonia synthesis gas stream must be free of all traces of methanol because carbon oxides, including methanol, poison the synthesis catalyst.
The process gas flows to a methanator, where the remaining carbon oxides form methane and water. The stream is then compressed and fed to the ammonia synthesis loop.
The crude methanol is flashed to remove dissolved gases. The flash gases supplement the fuel to the steam generator. Also used as supplementary fuel are any remaining low-boiling impurities, which are removed in a stabilizer distillation column.
Methanol from the stabilizer column is sent to a methanol concentrator Column, which removes higher-boiling impurities (mostly alcohols and water). The condensate from this column is combined with natural gas and used as process steam to the reformer. Any methanol or higher alcohols reform into synthesis gas components for ammonia and methanol production.
The finished methanol is pumped to one of two methanol holding tanks for quality testing. Off-spec product is transferred to a crude methanol tank and blended with feed for redistillation. On-spec product is pumped to a larger storage tank or loaded into trucks or rail cars.
START-UP
The ammonia plant has had a few problems since start-up. The plant was coming off of a 386-day ammonia plant run-a long run for any ammonia plant-when the methanol section was brought on stream. The subsequent problems, says Terra, are unrelated to the methanol tie in.
Terra expects the problems to be ironed out and the plant fully ramped up some time this summer.
The plant began making U.S. Grade AA methanol within a day or two of start-up. Table I shows the specifications for this chemical grade of methanol.
Energy requirements for the plant will decrease, relative to operation before the integration. Methanol emissions also will decrease, because the process integration eliminates emissions of byproduct methanol previously generated as a result of low-temperature shift conversion. The only methanol emitted by the integrated process will be fugitive emissions.
Terra has installed a new distributed control system (DCS) in the plant. The methanol plant already is on the new DCS system, and Terra will be putting the ammonia plant on over the next several months.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
