Mobil Research & Development Corp. has developed a new process that converts propylene and water to diisopropyl ether (DIPE), a high-octane gasoline blending stock.
Michael J. McNally, manager, motor gasoline group, at Mobil's Paulsboro, N.J., research facility described the process to refiners at the National Petroleum Refiners Association Annual Meeting, Mar. 22-24, in New Orleans.
Coauthors of the paper were Patrick J. Costello, B. Ellen Johnson, Charles M. Sorensen, and Stephen S. Wise, of Mobil R&D in Paulsboro; and Lawrence K. Low of Mobil Oil Corp., Princeton, N.J.
In cases where fuel oxygen is in short supply, propylene utilization for DIPE production can be an attractive option for oxygenate production. DIPE's low vapor pressure blending value will also help the refiner meet new requirements for lower fuel volatility.
The process is particularly suited to operations where ZSM-5 catalyst technology is used to maximize light olefins production from fluid catalytic cracking (FCC), for the purpose of octane enhancement. This is because the alkylation unit can be fully loaded with the more desirable C4 olefins.
BACKGROUND
Regulatory demands for minimum fuel oxygen content will soon require that ethers and alcohols be used as fuel blending stocks.
Because of the availability of refinery isobutylene, methyl tertiary butyl ether (MTGE) will be the most widely used ether.
Ethyl tertiary butyl ether (ETBE) could become important, but it would compete for the same isobutylene as MTBE.
Tertiary amyl methyl ether (TAME) may also become more important as C5 olefins are reduced to meet more stringent Rvp regulations and possible future controls on gasoline light olefins content.
Although the tax incentive makes ethanol an attractive fuel oxygen source, most refiners find ethanol addition difficult, if not impossible, because of the incompatibility of alcohols with most pipeline distribution systems.
Historically, propylene has not been utilized as a fuel ether source except where DIPE, a by-product of isopropyl alcohol (IPA), found its way into gasoline. Although it is an acceptable fuel oxygen source, its availability as a by-product from current IPA manufacturing sources is limited.
Early developers of aviation gasoline recognized the potential of DIPE as a high-octane gasoline component, but the concurrent development of alkylation technology sidelined further interest in propylene etherification. 1
The Mobil DIPE process maximizes DIPE while minimizing IPA, so that the ether is ready for gasoline blending.
DIPE PROCESS
Primary process elements of the DIPE process are shown in Fig. 1.
A propane/propylene (P/P) splitter can also be included in the feed purification section to increase the concentration of propylene in the P/P feed and obtain maximum DIPE production by recycle of unconverted olefin. Splitter design depends critically on individual refineries because P/P purity varies widely with FCC operation.
The DIPE reaction takes place in a fixed-bed catalytic reactor via a series of reaction steps.
IPA is a reaction intermediate, and is recycled virtually to extinction within the process. The recovery section produces fuel-grade DIPE, which meets the following specifications:
- IPA, < 2.0 wt %
- Water, < 0.1 wt %
- Ketones, < 0.3 wt %
- Organic acids, < 20 ppm
A primary advantage of the process is that DIPE production is completely refinery-based and does not depend on an outside supply of alcohol. Propylene feedstock is readily available, and the amount can be controlled by FCC severity throughput, and the use of ZSM-5 additives. 2
In refineries with existing C3 alkylation, FCC operation can be varied to reload the alkylation plant with butylenes and/or amylenes only, and all propylene can be routed to DIPE manufacture.
For a 75,000 b/d FCC unit running at high severity with ZSM-5, over 10,000 b/d DIPE can be produced. DIPE can also be combined with refinery MTBE manufacture to produce maximum amounts of ether while maintaining alkylation capacity.
This combination, or combinations with TAME, can provide a refiner with ether self-sufficiency.
ECONOMICS
Table 1 shows the economics for a 10,000 b/d DIPE unit. The production cost, 98 cents/gal, compares favorably with published MTBE manufacturing costs, by either refinery-based or butane dehydrogenation routes.
Capital costs in Table 1 are based on an instantaneous 1992 investment of $118 million, including 35% for off sites and a 15% return on investment (ROI).
The availability of low-pressure steam could reduce process costs significantly. On the other hand, larger units may require investment for refinery gas plant debottlenecking to process increased light olefins resulting from higher FCC severity and ZSM-5 addition.
BLENDING PROPERTIES
The physical properties and blending characteristics of DIPE, as compared to MTBE and TAME, are shown in Tables 2 and 3.
Blending octane values were measured in a variety of gasoline compositions. RON ranges from 107 to 110, MON, from 97 to 103, and (R+M)/2, from 102 to 106. A weak inverse correlation of blending octane with base olefin content has been observed at both 2.0 and 2.7 wt % fuel oxygen.
MTBE shows 6-7 RON higher than DIPE, but similar MON. Limited data for TAME indicate similar octane blending values to DIPE.
Samples of DIPE from the Mobil process contain small amounts Of C6 and C9 olefin oligomer. These olefins reduce product blending octane by about 0.1 octane number/1% oligomer.
DIPE blends provide about 1% butane bonus over MTBE, in attaining Rvp specifications.
DRIVEABILITY
Like MTBE, DIPE provides a large reduction in ASTM D86 distillation temperatures when blended into hydrocarbon-only base gasoline. DIPE at 18% (2.7 wt % oxygen) has its greatest effects on the mid-range of the distillation curve.
Front-end distillation temperatures are not reduced as much as with 15% MTBE because of DIPE's higher boiling point and lower Rvp (Fig. 2). However, the overall improvements in ASTM Driveability Index (DI = 1.5T10 + 3T50 + T90) are very similar for the two ethers.
Several driveability test programs have consistently shown DIPE to be similar to MTBE in terms of distillation effects and cold-start and warm-up driveability.
EMISSIONS BENEFITS
Mass exhaust emissions' results for a DIPE fuel blend are similar to those found by the Auto/Oil Air Quality Improvement Research Program for MTBE.
In a limited emissions test program, an 18% DIPE blend showed an increase in the level of propylene over the base fuel. This was expected because of the chemical structure of this ether.
A 15% MTBE blend and an 18% ETBE blend both showed increases in C4 hydrocarbons. For almost all of the aldehydes measured in the exhaust, the DIPE fuel had emissions levels that were between the levels for the MTBE and ETBE blends. Propionaldehyde did not appear in the exhaust from the DIPE fuel.
Reactivity calculations of the exhaust showed no significant difference between the DIPE and MTBE blends.
COMPATIBILITIES
Extensive testing indicates that gasoline additives in current use are compatible with DIPE. In standard laboratory side-effects tests, no differences were seen between MTBE and DIPE at concentrations up to 2.7 wt % fuel oxygen.
In addition, DIPE was found to have no effect on the performance of the deposit control gasoline additive package.
DIPE/gasoline blends are also compatible with all materials used in vehicle fuel systems. In volume swell testing on 18 elastomers and plastics being used in current and/or future automotive fuel systems, fuels containing DIPE and ETBE had lower or equivalent volume swell compared to fuels containing MTBE.
OTHER EFFECTS
No unusual effects of DIPE/gasoline blends on overall vehicle durability have been found in mileage accumulation tests.
Likewise, DIPE does not increase the tendency of gasoline to form gum or peroxides in laboratory storage stability tests. Standard gasoline antioxidants inhibit gum formation and maintain acceptable oxidation induction periods.
DIPE is virtually nontoxic and has not caused adverse systemic effects or tissue toxicity.
REFERENCES
- Buc, H.E., et al., "A New High-Octane Blending Agent," J. of SAE, Vol. 39, September 1936, p. 339.
- Dwyer, F.G., Schipper, P.H., and Gorra, F., "Octane Enhancement in FCC via ZSM-5," 1987 National Petroleum Refiners Association Annual Meeting, Paper AM-87-63, March 1987, San Antonio.
- Barker, D.A., et al., "The Development and Proposed Implementation of the ASTM Driveability Index for Motor Gasoline," SAE 881668, 1988 Society of Automotive Engineering International Fuels and Lubricants Meeting and Exposition, Oct. 10-13, 1988, Portland.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
