ALKYLATION OF MTBE RAFFINATE INCREASES OCTANE, UNLOADS ALKY UNIT

Nov. 18, 1991
A methyl tertiary butyl ether (MTBE) unit and a sulfuric acid alkylation unit work well together, producing higher alkylate octanes and reducing the load on the alkylation unit, according to Randy Peterson, senior process engineer at Stratco Inc., Leawood, Kan. Depending on the levels of feed contaminants, the acid consumption per volume of alkylate can decrease as a result of the reduced load, Peterson reported at Stratco's European Alkylation Seminar in Amsterdam last May.

A methyl tertiary butyl ether (MTBE) unit and a sulfuric acid alkylation unit work well together, producing higher alkylate octanes and reducing the load on the alkylation unit, according to Randy Peterson, senior process engineer at Stratco Inc., Leawood, Kan.

Depending on the levels of feed contaminants, the acid consumption per volume of alkylate can decrease as a result of the reduced load, Peterson reported at Stratco's European Alkylation Seminar in Amsterdam last May.

Additional propylenes, butylenes, or amylenes can also be alkylated to take advantage of the resulting excess capacity.

The flow diagram of a typical MTBE unit shown in Fig. 1 shows the source of the C4 raffinate available for alkylation.

MTBE AND ALKYLATION

MTBE is an oxygenate produced by the reaction of equimolar amounts of isobutylene and methanol over a fixed-bed ion exchange resin catalyst. The blending octane numbers of MTBE are typically 118 RON and 100 MON, or 109 (R + M)/2.

However, the actual blending octane is a function of the concentration of MTBE and the composition and octane of the gasoline into which it is blended.

Over the past 10 years, at least 20 U.S. refineries have added MTBE units upstream of their alkylation units. Table 1 lists these refineries, and the type of alkylation units they employ.

Table 2 lists MTBE units upstream of alkylation units in the construction or planning stages.

An MTBE unit typically consumes at least 90% of the isobutylene in the olefin feed to the alkylation unit. In an H2SO4 alkylation unit, this results in alkylate octanes approximately 0.75-1.0 octane numbers higher, for a butylene feed. If the unit is an HF design, the octane usually decreases.

Typical alkylate octane values for the different butylene isomers are shown in Table 3. Note that the MTBE unit removes the lowest octane isomer from the butylene feed to an H2SO4 alkylation unit. For an HF unit, however, isobutylene is the second highest octane isomer.

ACID CONSUMPTION

The acid consumption for a Stratco H2SO4 alkylation unit is dependent on operating conditions, feed contaminants, and quantity and type of olefin feed, says Peterson.

When an MTBE unit is added, the quantity of olefins to the alkylation unit decreases (unless additional olefins are alkylated). This lowers the space velocity and improves operating conditions. Therefore, the only parameter that adversely affects acid consumption is the level of contaminants in the MTBE feed.

The MTBE unit produces oxygenate contaminants which, if not recovered, will increase acid consumption in an alkylation unit. These oxygenates are MTBE, methanol, and dimethyl ether (DME). The level of oxygenates in the alkylation unit feed depends on the type of oxygenate recovery system and its operation.

A water washing column can reduce the level of oxygenates in the MTBE raffinate to 400 ppm (wt) or less. Molecular sieve driers downstream, or in place of, the water wash system can reduce the level of oxygenates to below 10 ppm (wt).

The decision to add molecular sieve driers is purely an economic one. The refiner must compare the H2SO4 regeneration cost with the capital and operating costs of the oxygenate-recovery system.

ADVANTAGES, DISADVANTAGES

Stratco sums up the advantages and disadvantages of adding an MTBE unit up-stream of an existing H2SO4 alkylation unit as follows:

ADVANTAGES

  • The refiner produces gasoline that burns more completely. Cleaner-burning gasoline helps to reduce smog and other types of air pollution.

  • The refinery's gasoline pool octane numbers are increased.

  • The refiner can capitalize on the market demand for a premium product while increasing the operating flexibility of the refinery.

  • In H2SO4 alkylation units, the lowest octane isomer-isobutylene-is removed from the butylene stream and replaced with a high octane component-MTBE.

  • Removal of isobutylene from the olefin stream reduces throughput in the H2SO4 alkylation unit and decreases the space velocity. This results in a significant increase in octane and better overall unit performance.

  • Additional alkylation capacity provided by the MTBE unit can be used for processing propylene, butylene, or amylenes.

  • A smaller quantity of a less-expensive reactant (methanol) is required to make MTBE. Isobutylene alkylate requires 0.63 bbl isobutane per bbl alkylate, whereas MTBE requires only 0.34 bbl methanol per bbl MTBE.

  • Overall alkylation isobutane consumption is reduced because of the conversion of isobutylene to MTBE.

  • Catalytic reforming severity can be reduced, which is especially important when reformers are near their limit. This helps lower aromatic and benzene content in the gasoline pool.

  • A high-octane blending component makes it easier to blend gasoline more closely to octane specifications and avoid "octane giveaway."

DISADVANTAGES

  • Less MTBE (1.26 bbl) than alkylate (1.77 bbl) is produced from a barrel of isobutylene.

  • Less butane can be blended into the gasoline pool with MTBE because its vapor pressure is higher than that of alkylate.

  • Contaminants from the MTBE unit, such as methanol, DME, and MTBE, can increase the alkylation unit's acid consumption per volume of alkylate. However, a properly operating oxygenate recovery system reduces these adverse effects to a minimum.

  • HF alkylation units typically produce a lower-octane alkylate from MTBE raffinate because the isobutylene removed in the MTBE unit is a good feedstock for an HF unit.

  • HF alkylation units are more severely affected by oxygenate contaminants in the feed. These contaminants increase production of acid-soluble oils and constant boiling mixtures. The production of these wastes can be a major problem because of the fixed size of the monel HF regeneration column. The quantity of waste requiring disposal also increases.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.