The government of Thailand has granted development incentives to seven petrochemical projects, one of them involving a division of Exxon Corp.

The total package of investment comes to nearly 18 billion baht (US$720 million) and will add to the already lengthy list of new petrochemical facilities being constructed in Southeast Asia.Exxon Chemical Co. gets a portion through its 35 percent shareholding in Aromatics (Thailand) Co. Other interests are held by Thai Oil Co. (40 percent) and the Petroleum Authority of Thailand, or PTT, (25 percent) both state-run entities.

It involves a plant costing 5 billion baht that will produce 232,000 metric tons a year of benzene, 52,000 tons of toluene and 355,000 tons of xylenes, according to a release from the government's Board of Investment.

This project is part of Thailand's second national petrochemical complex development, which is expected to begin operations in 1995.

The plant will be sited at Sriracha in the province of Chonburi, southeast of Bangkok, the capital.

The biggest chunk of the new projects is being undertaken by Thai Petrochemical Industry Co., with investment totaling 11 billion baht. The company is largely owned by textile magnate Boonnam Boonamsap.

These endeavors include 100,000 tons of caprolactam and ammonium sulfate (a by-product) and 20,000 tons of propylene oxide. Thais will hold 80 percent and 85 percent respectively of the plants, with the rest owned by various overseas interests.

The other main petrochemical project blessed by official privileges will produce 20,000 tons a year of isopropyl alcohol. It will be 51 percent Thai- owned with the rest in Japanese hands.

In recent weeks, Exxon has announced plans to go ahead with a 300-million- ba ht solvents plant in Thailand with local partners, and a Japanese group led by Mitsui & Co. is bidding for an ethylene facility with capacity of 350,000 tons a year.

Even with all the new capacity, supplies of ethylene and propylene may still be tight.

PTT, the sole supplier of feedstock to both petrochemical complexes, gets its natural gas from one of its subsidiaries and from the Unocal (Thailand) Ltd. unit of Unocal Corp. of Los Angeles, which gather from offshore fields.

The only onshore gas site is owned by the Esso Korat Exploration & Production Co. unit of Exxon, but its output isn't considered suitable for petrochemical applications.

Estimates from National Petrochemical Corp. indicate that domestic demand for ethylene this year will be 238,000 tons, increasing to 961,000 by the year 2000. Supply will cover demand this year, but fall short by 1995 and miss the target by 296,000 tons at the turn of the century.

The position for propylene is similar but less drastic. Demand is forecast this year at 43,000 tons, rising to 340,000 by the year 2000. Supply will exceed demand through 1995, but fall short by 65,000 tons in 2000.

Keeping the plants running may also be a headache, according to a recent investigation by the Nation newspaper of Bangkok.

It cited four substantial power failures in the first three months of the year at National Petrochemical. They ranged from one of 30 hours, caused by equipment problems in the complex to more than 117 hours for a variety of reasons including a PTT shutdown.