CHEMICAL ENGINEERING

FALL 1999

CASE STUDY PROJECT

Production of MTBE from Methanol

Based on the Vertical Integration of Design in Chemical Engineering case studies developed by Ronald J. Gatehouse and George J. Selembo, Jr. of the Pennsylvania State University

Consider the case study of a large petrochemical company, who’s Petroleum Production and Refinery Division has been combusting the bottom residue from its crude oil vacuum distillation columns as supplemental fuel and landfilling any balance for many years now. Due to a recent change in environmental regulations, however, this option will no longer be possible. Therefore, the division of the Engineering Department will be investigating the possibility of designing a plant to convert this crude oil vacuum residue into MTBE (methyl-t-butyl ether). MTBE is an oxygenated hydrocarbon which is extremely valuable as a gasoline additive under the new air pollution control regulations. The first step of the process involves using partial oxidation of the residue to form carbon monoxide and hydrogen. These intermediate products are then used to make methanol which is later converted to MTBE upon the addition of isobutylene.

There are five basic reactions that occur in the production of MTBE from crude vacuum residue. The first step is the partial oxidation of the carbon and hydrogen in the vacuum residue stream using pure oxygen:

CnHm (l) + n/2 O2 (g)  n CO (g) + m/2 H2 (g) (1)

The next reaction is the water gas shift reaction, which is used to adjust the H2 to CO ratio for the optimal production of methanol:

CO (g) + H2O (g)  CO2 (g) + H2 (g) (2)

Methanol is then produced from carbon monoxide and hydrogen by the reaction:

CO (g) + 2 H2 (g)  CH3OH (g) (3)

The formation of dimethyl ether and water from carbon monoxide and hydrogen also occurs as a side reaction when producing methanol:

2 CO (g) + 4 H2 (g)  C2H6O (g) + H2O (g) (4)

The final reaction involves the production of MTBE from methanol and isobutylene:

CH3OH (l) + C4H8 (l)  C5H12O (l) (5)

Your particular assignment will focus on the final phase of production, which involves the conversion of methanol and isobutylene to MTBE. In order to do this the methanol product must be fed to an MTBE reactor, where methanol and isobutylene are reacted to form MTBE with an 85.3% single pass conversion of methanol. The methanol is fed in a 8% molar excess in order to facilitate the removal of MTBE in the MTBE separation column. The isobutylene feed to the MTBE reactor is contained in a stream of mixed hydrocarbons; in which the isobutylene makes up 30 mol% of the hydrocarbon stream. The mixed hydrocarbons can be assumed to have an average molecular weight of 60 (excluding isobutylene). The product stream from the MTBE reactor is then separated in a pressurized distillation column where 99% of the MTBE is recovered in the bottoms stream. The exiting MTBE stream also contains 0.5 mol% of the mixed hydrocarbons that enter the column. The overhead stream is then further cooled and fed into a liquid-liquid extraction unit, where water is used to recover 96% of all the methanol fed to the extractor. The water used to extract the methanol is fed in a 4:1 molar ratio of water to all entering methanol. In addition, the exiting water (extract stream) from the liquid-liquid extractor contains 5% of the hydrocarbons, 1% of the MTBE, and 2% of the isobutylene fed to the extractor. Three percent of the water is absorbed into the organic stream. The water-methanol extract stream is cooled and fed to a flash drum where all remaining organics are removed in the vapor product, and the water and methanol leave as liquid. The vapor leaving the flash drum also contains 2.5% of the entering water and 3.5% of the entering methanol. Finally, the methanol and water are separated in an atmospheric distillation column; the overhead, which contains 99% of the methanol fed is combined with the fresh feed methanol to the MTBE reactor. The water removed from the distillation column is cooled and recycled to the liquid-liquid extraction unit as the feed solvent stream. A small stream of water is added to this recycle stream in order to make up the water lost in the liquid-liquid extraction unit and the flash drum.

Assignment

Calculate the composition and flowrate of every stream for the process of converting methanol and isobutylene to MTBE. Assume that the plant is designed to produce 90,000 metric tons of MTBE per year based on operating the plant 24 hours per day, 329 days per year.