Improved Synthesis of DMC via Oxidative Carbonylation of DME/Methanol Mixtures
Amoco Corporation, Chicago, IL and Mega-Carbon Company, St. Charles, IL
Enichem has commercialized a continuous solution/slurry phase process for preparation of dimethyl carbonate (DMC) via the copper (II)-catalyzed oxidative carbonylation of methanol. As of 1993 the capacity of this plant was 22 million pounds/year with worlwide demand approximately half of this amount. Recently in Japan Ube industries has completed construction of a semicommercial plant with a capacity of 11-22 million pounds per year.
DMC has very strong growth potential as a phosgene replacement in some applications and as a high oxygen high octane fuel addiitive (RM/2=105). As a phosgene replacement there is a strong environmental incentive to use DMC since it would replace a very toxic compound with a relatively nontoxic one and eliminate environmental concerns resulting from hydrogen chloride production and recycle. DMC has the potential to replace the two largest applications in the phosgene market, toluene diisocyantate and poycarbonate resin which constitute 46% and 14% of U.S. the market, repectively. Replacement of one-half of the 2 billion/year market would represent an additional demand for DMC of 555 million pounds/year (in phosgene equivalents). Economic advantages for using DMC instead of phosgene in such processes has been estimated to be as high as 5 cents/pound not including savings resulting from capital equipment and systems required to address corossionand safety concerns associated with phosgene use.
DMC also has very strong potential to replace part of the growing worldwide MTBE market which is expected to reach 66 billion pounds/year by the end of the century. As a gasoline blending agent DMC has an oxygen content of 53% and a blending octane value of 105, and these high values dictate a somewhat higher overall value for DMC in comparison to MTBE.
The key to entering this market and the phosgene replacement market lies in the development of an efficient low cost DMC process based on inexpesive starting materials. Its current cost of $1.40/lb (non-contract) is prohibitively expensive. There are inherent problems in the Enichem process which limit per-pass methanol conversion to about 20% as the result of coproduction of water. This coproduction also results in catalyst degradation/deactivation and hardware corrosion. Production reates of 0.1 LHSV are reported for this system. Similar problems also exist in gas phase processes such as that developed by Dow Chemical which utilize a copper (II) catalyst supported on carbon. Catalyst modifications have reportedly solved some the deactivation problems but methanol conversion is still limited to about 25%.
The synthesis of dimethyl carbonate (DMC) by oxidative carbonylation of methanol is typically carried out in one of two ways. ENIChem uses a liquid phase system with the reaction conducted in a slurry reactor, containing copper chloride "catalyst" suspended in liquid methanol. Carbon monoxide and methanol are fed into the reactor bottom and oxygen added at a carefully controlled rate along the reactor to avoid explosion. CuCl2 functions as a red-ox agent with the oxidation and
reduction proceeding simultaneously in the same reactor. CuCl2 is removed from the product stream outside of the reactor. Dow Chemical has developed a vapor phase process based on supported copper catalyst with activated carbon used as the support. In either case, the reaction is highly exothermic (76 kcal/mole DMC) and the coproduction of water reduces the reaction rate. The amount of water in the reactor and the rate of water removal are critical limits on the rate of DMC production.
Objective of the Proposal
A new catalyst system and reactant mixture has been uncovered which has the potential to solve these conversion and deactivation problems and significantly lower the cost of DMC production. This system is highly active in the gas phase oxidative carbonylation of mixtures of dimethyl ether and methanol to DMC. Water produced via the oxidative carbonylation of methanol is consumed in the hydrolysis of dimethyl ether (DME) to methanol:
2 CH3OH + 1/2 O2 + CO -----> CH3O(CO)OCH3 + H2O
CH3OCH3 + H2O -----> 2 CH3OH
Net: CH3OCH3 + 1/2 O2 + CO -----> CH3O(CO)OCH3
This approach allows for in situ dehydration of the reaction medium. Initial studies with several related catalyst systems have revealed high conversions of the CH3O moiety of up to 42-53%. This is a clear indication that reduced water in the reaction medium can improve conversion, and this should provide a significant cost advantage in processing. Additional cost saving should result from the utilization of DME as starting material which is cheaper to produce than methanol on a BTU-equivalent basis. Lower water levels should also result in less catalyst deactivation and corrosion and less production of trace chlorine-containing volatile products which would otherwise have to be removed for environmental reasons.
These in addition to other significant process improvements could reduce the cost of DMC in large scale production facilities to 20-30 cents/lb which would make it competitive with MTBE. Significant additional R&D is required in order to define reaction conditions and catalyst parameters for optimum conversion, catalyst productivity, and catalyst lifetime. Since the findings reported here are initial, there is a strong potential for defining major catalyst and process improvements in future R&D involving the in situ dehydration approach.
In addition to the gas phase catalyst evaluations described here a new type of reactor concept is also proposed which will address many of the deficiencies of the basic designs described above. The concept is prompted by earlier work in the late 1960s by Monsanto on continuous propylene hydroformylation in a gas-sparged reactor. The basic idea is to suspend the catalyst in a high boiling solvent such as dioctylphthalate and then feed an excess of gas so that the reaction products are stripped from the reactor leaving the catalyst inside suspended in the solvent. This eliminates the problem of external separation of the catalyst from the reaction products and also deals with the reaction exothermicity by conducting the reaction in a well mixed reactor with some of the reaction heat used to vaporize the products such as DMC. Outside of the reactor, a chiller is installed on the gas product stream to function as a partial condenser so that the condensible materials (DMC, methanol, and dimethyl ether) are removed with the unreacted CO and nitrogen recycled back to the reactor inlet. Additionally the coproduced water will be rapidly removed in the stripping gas so that the CuCl2 catalyst will function at high efficiency. The Monsanto studies used propylene with an excess of hydrogen and carbon monoxide with a soluble rhodium catalyst and the system performed very well. We believe this concept can be translated to the oxidative carbonylation of methanol and result in many of the same benefits. A simple process flow diagram is shown in Figure 2 and illustrates the basic idea of this new reactor concept. We believe that the feasibility of this concept for DMC synthesis is relatively high and would be very interested in moving forward to test its feasibility.
Not only does the gas-sparged reactor lead to improved performance in the laboratory, but it will also scale quite easily to commercial production and consists of a relatively simple design with no elaborate reactor internals or catalyst disengagement sections. A particularly convenient aspect of this reactor concept is that fresh catalyst can be added and spent catalyst withdrawn while the reactor is operating, thus eliminating the need for reactor shutdown to change catalyst. This addition-withdrawal scheme is, of course, similar to the idea used in coal liquefaction for the H-Coal ebbulated reactor and many of those details have already been worked out on loading and discharge ports.
Since the idea needs to first proceed through a process feasibility stage, we would like to move rapidly into that phase without spending large amounts of money and special design. We propose to use air as the oxygen source, rather than molecular oxygen, since it is easier to handle and less hazardous. It will be mixed with carbon monoxide and sparged into the bottom of a tubular reactor with dimethyl ether and methanol fed separately with metering pumps. A small sintered metal
al frit will be positioned in the bottom of the reactor to disperse the gas and agitate the reactor contents. We expect the mixing in the reactor to simulate a single continuous stirred tank reactor (CSTR). This will distribute the reaction heat so that there are no "hot spots" leading to CO oxidation to carbon dioxide or unexpected catalyst deactivation.
Scope of Work
Amoco intends to conduct this program in cooperation with Mega-Carbon Company, a small business in St. Charles, IL. They have a team of R&D members that are highly experienced in catalyst design, preparation and reactor testing and have run many systems similar to what we are proposing. Some of the experimental work will be conducted at their facilities since they have experimental space available which lends itself well to a rapid construction and test schedule. Mega-Carbon Company is also commercially involved with fabrication of high surface area carbon shapes for absorbant and catalyst applications. This combination of manufacturing capability and catayst expertise is a key to the design and preparation of superior catalysts for oxidative carbonylation of DME/methanol mixtures and to the testing of a potentially superior reactor design(s).
Amoco shall be The Principle Contractor in the proposed project and Mega-Carbon shall be a Secondary Contractor. Amoco and Mega-Carbon shall supply all necessary personal, facilities, materials, and services to execute the following tasks of this project:
Task 1. Project Management Plan (to be completed)
Task 2. Experimental Studies (condensed except for Task 2.6)
2.1 Refurbishing of gas phase oxidative carbonylation reactor (CEU-16) for improved temperature control and feed vaporization (Amoco).
2.2 Synthesis and evaluation in gas phase pilot plant of non-chloride based supported copper (II) catalysts for improved performance over extended periods of operation (Amoco).
2.3 Determination of optimum feed composition for maximum combinations of reactant conversion/ DMC selectivity (Amoco).
2.4 Determination of physical and chemical properties of process stream mixtures for commercial process design (Amoco).
2.5 Design and construction of microclave liquid phase screening reactors for rapid initial catalyst evaluation (Mega-Carbon/Amoco).
2.6 Design and construction of gas-liquid phase sparge reactor for DMC synthesis via oxidative carbonylation of DME/methanol mixtures (Mega-Carbon/Amoco).
2.6.1 Construct the reactor (~ 300 cc) from simple stainless steel heavy wall pipe fitted with unions.
2.6.2 Attach feed system to reactor consisting of gas metering of CO and air, with a metering pump used to feed a blend of DME and methanol.
2.6.3 Install product recovery system using high pressure double ended vessel as the partial condenser chilled with a mixture of dry ice and acetone. Back pressure regulator can be used to maintain reactor at 10-15 atm and vent the unreacted nitrogen and CO through it.
2.6.4 Instrument the reactor with temperature controls and high limit temperature alarms to automatically shut the unit down in case of temperature runaway. To prevent overpressure, the reactor will be fitted both with preset relief valves and rupture discs handle spontaneous pressure excursions.
2.6.5 Establish product sampling and analysis system with online GC analysis of reactor off gas.
2.7 Synthesis of new carbon-based catalysts for DMC via oxidative carbonylation (Mega-Carbon).
2.8 Rapid screening of new catalyst designs in Microclave reactors (Mega-Carbon).
2.9 Evaluation of highly active and/or selective catalysts in gas-liquid phase sparge reactor (Mega-Carbon).