Rising demand for methyl tertiary butyl ether (MTBE) has spawned interest in finding a cost-effective means of increasing production from existing units. A commercial trial of an improved MTBE catalyst was conducted recently at Lyondell Petrochemical Co.'s Channelview, Tex., plant.
The results were reported at the World Conference on Refinery processing and Reformulated Gasolines Mar. 22-24 in San Antonio. Authors of the paper were Richard Chavez, Lyondell, Robert Olsen, Rohm & Haas Co.; and Michael Ladisch, Purdue University. Lyondell's adiabatic, single-reactor system exhibited 94-97.6% isobutylene conversion. This rate is 4-5% greater than would have been achieved with Rohm & Haas' first-generation MTBE catalyst under similar process conditions.
CATALYST
The original catalyst used in MTBE production was Rohm & Haas' Amberlyst 15 Wet, according to the report. But a new catalyst with increased conversion, called Amberlyst 35 Wet, enhanced oxygenate production in the Lyondell trial.
The new catalyst changes the activity coefficients of at least one of the components of the MTBE reaction, resulting in higher equilibrium conversion relative to its first-generation counterpart.
Key catalyst properties are:
- Particle size, 0.4-1.25 mm
- Apparent density, 0.82 g/ml
- Surface area, 44 sq m/g
- Moisture content, 56%
- Concentration of acid sites, 1.9 meq/ml (5.4 meq/g)
- Porosity, 0.35 cc/g
- Average pore diameter, 300 .
Suggested operating conditions are:
- Maximum temperature, 284 F. (140 C.)
- Minimum bed depth, 24 in. (0.61 m)
- Liquid hourly space velocity (LHSV), 1-5 hr-1.
TRIAL
Lyondell's No. 2 MTBE unit processes feed from the planes fluid catalytic cracking unit. The recycle reactor is computer controlled.
Typical operating conditions include:
- 10-15% isobutylene
- 1.0-1.1 methanol-to-isobutylene ratio
- 100-115 F. inlet temperature
- 3-5 hr-1 LHSV.
Start-up was carried out over 1 1/2 days. The catalyst was loaded into the reactor in a wet state, then backwashed. The reactor was flushed with butylene feed containing 12-14% isobutylene at less than 100 F.
Methanol was mixed with the butylene feed to produce a methanol-to-isobutylene ratio of about 1.0. The feed was passed through the reactor for about 17 hr before the inlet temperature and flow rate were increased.
At this point, the remaining water in the reactor formed tertiary butyl alcohol (TBA), and this continued for some time. The temperature was adjusted and MTBE formation began, causing a further increased in the reactor temperature profile. About 31 hr after start-up, MTBE meeting commercial specifications was produced. Isobutylene conversion at 38 hr was 92-93%. The feed to the debutanizer vas about 95 wt % MTBE, with about 2% TBA.
As process conditions were optimized, isobutylene conversion increased over the following 4 weeks to 94-97%.
The figure shows that isobutylene conversion increases with an increase in methanol-to-isobutylene ratio. In addition, it was possible to attain high isobutylene conversion with substoichiometric methanol-to-isobutylene ratios upstream of the recycle addition. This was evident during 8 days when the ratio was less than 1, yet conversion remained greater than 95%. It was also noted that the total flow rate did not affect conversion significantly. Isobutylene conversion remained about 95% over an LHSV range of 3.2-4.6. After more than 11 months on stream, the unit had no measurable decline in catalyst activity, according to the report (Table).
Selectivity to MTBE is greater than 99% and no significant concentrations of C8 dimers or dimethyl ether are formed. Maximum isobutylene conversion remains greater than 97%.
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