New etherification process commercialized in Finland

The phase-out of leaded fuels in Europe, Asia, and parts of Africa and Latin America is increasing demand for octane and octane-bearing fuel components such as ethers.

Early solutions to the problem of increasing octane while reducing tailpipe emissions involved use of methyl tertiary butyl ether (MTBE).

According to Neste, using both tertiary amyl methyl ether (TAME) and MTBE can give refiners increased blending flexibility for volatility control. But the economics associated with TAME production often make TAME units difficult to justify.

Neste Oy and Neste Engineering, Porvoo, Finland, have commercialized a process that produces more oxygenates by etherifying isohexenes and isoheptenes in light gasoline from the fluid catalytic cracking (FCC) unit. Neste engineering associate Matti Koskinen discussed Neste's experience in a presentation at the European Refining Technology Conference, Oct. 28-30, 1996, in London.

Coauthors of the paper were Harri Järvelin, Petri Lindqvist, and Esa Tamminen.

The NExTAME process

Neste was the first European refiner to introduce gasoline that meets U.S. reformulated gasoline requirements, said Koskinen. Adding an ethers unit at its Porvoo refinery was a logical next step for the company.

Fig. 1 [38840 bytes] shows a simplified flow diagram of Neste's NExTAME process. The process produces ethers from C5, C6, and C7 tertiary olefins, using a much wider cut of FCC gasoline than conventional etherification units.

NExTAME has been described in detail in other papers.^{1 2} The process centers around a distillation column and side reactors. A side stream containing reactive isoamylenes and heavier tertiary olefins is circulated through the side reactors, where additional ethers are formed.

Almost all of the unreacted methanol from the prereactors is consumed in the side reactors, says Koskinen. This enables the process to operate without a methanol recovery section.

The Porvoo unit

Neste commercialized the NExTAME process in a 3,100 b/d unit at its 160,000 b/d Porvoo refinery. The unit has been operating since May 1995. It produces 2,100 b/d TAME, 900 b/d tertiary hexyl methyl ether (THxME), and 100 b/d tertiary heptyl methyl ether (THpME).

Diolefins in the feed are selectively hydrogenated to improve product quality and reduce fouling of the etherification catalyst. Neste's selective hydrogenation process is called NExSELECT.

Fig. 2 [19493 bytes] tracks the performance of this unit since start-up. The unit has exceeded the design specification for diolefins concentration of <500 ppm(wt). according to koskinen, the remaining unreacted diolefins are stable and form no gums.

The conversion of ethers also has exceeded design values (Fig. 3 [20870 bytes]). And the methanol concentration in the bottoms product is typically less than 50 ppm.

According to Koskinen, the Porvoo refinery has seen an overall octane gain of about 2 RON and 3 MON. TAME and heavier ethers now comprise 6% of Finland's total gasoline pool.

Process improvements

Since starting up the Porvoo unit, Neste has made further improvements to the NExTAME scheme. "This is accomplished by recycling the side draw from the distillation column back to the etherification reactors and eliminating the side reactors from the main separation tower," said Koskinen.

Koskinen cautioned, "The total ethers production potential present in a modern petroleum refinery cannot be fully realized without MTBE synthesis, or its ethanol-based counterpart, ETBE."

Neste has developed a process called NExETHERS for production of ethers from C4-C7 olefins. A flow diagram of the NExETHERS process is shown in Fig. 4 [41312 bytes].

The process uses azeotropic phenomena and integration of methanol recovery and raffinate purification to boost yields without requiring additional process equipment. The new process eliminates the methanol recovery section required with conventional reactive-distillation technologies.

Instead of a conventional alcohol-recovery section, methanol recycle and C4 purification of light oxygenates are combined in a single tower-the C4 column. Neste says this rerouting of the side-draw mixture boosts overall MTBE yields to 99%.

"The distinctive, novel features of the new technology are related to how the overhead stream from the main fractionator is processed," said Koskinen. "Most of the unreacted methanol and iso-olefins present in the reactor effluent are recycled back to the reactor from the main distillation column."

References

1. Järvelin, H., Tamminen, E., Ewy, G., and Ragsdale, R., "NExTAME Process-Operating experiences from a commercial unit and some economical considerations," National Petroleum Refiners Association annual meeting, Paper AM-96-70, Mar. 17-19, 1996, San Antonio.

2. Järvelin, H., Lindqvist, P., Ignatius, J., and Aittamaa, J., "Development of the next generation TAME-process," AIChE annual meeting, Nov. 13-18, 1994, San Francisco.

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