The “thrombotic disease” in MTO units urgently requires a radical cure

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The need for a radical cure for the \"thrombosis\" problem in MTO plants / Author/Source: China Chemical Industry News Date: 03-06-2019 Clicks: 7 To date, 28 coal (methanol) to olefins (MTO) plants have been built in China, with a total production capacity of 13.29 million tons. However, the water systems of these MTO units commonly suffer from operational issues; catalysts and waxes frequently clog the heat exchange equipment as well as the trays in the wash towers, severely affecting the units’ operation under conditions of full capacity and optimal performance. Although companies have tried various technical measures, the results have been minimal. An effective cure is urgently needed for the “thrombotic disease” in MTO units. 　　The MTO unit uses crude methanol or purified methanol as raw materials, and through catalytic reactions it produces low-carbon olefin product gas. This product gas is washed and cooled in a quench water system; after the catalyst and water are removed, it is sent to an olefin separation unit where products such as ethylene, propylene, and tetrahydrocarbons are purified. The MTO process produces light olefin products, while generating 56% water as a by-product, as well as oily substances. 　　An Xuebo, an assistant engineer at China Coal Shaanxi Yulin Energy Chemical Co., Ltd., explained that the oil substances present in the water system of the MTO unit account for 0.3% of the total amount of product gas. Due to their low freezing point, these oils tend to condense in the quench water and washing water, and they easily adhere to the pipe walls, causing blockages in the trays of the washing towers as well as in the heat exchange equipment. This leads to blockages in the alkali washing tower system, the pipelines for discharging waste alkali, and the pipelines for discharging butter. Among them, the blockage of the alkali scrubber tower has a significant impact on the stable operation of the product gas compressor system, and it constitutes the main bottleneck restricting the safe, stable, and long-term operation of the entire plant. 　　According to the reporter’s investigation, to address the waxing problem in the MTO water system, the company has adopted various technical measures, such as installing oil separation tanks in the water system to remove oils, adding oil removers before the wastewater enters the stripping tower, setting up oil-water separation devices within the water circulation process, using demulsifiers, performing offline cleaning of heat exchangers and air coolers, carrying out online steaming in the tower, injecting extractants for washing on an online basis, cleaning the tray surfaces online, and adding scale-inhibiting and dispersing agents. However, none of these measures have yielded satisfactory results. 　　Installing oil traps, oil removers, and oil-water separation devices can reduce to some extent the amount of oil that enters the wastewater stripping tower, but it is not possible to prevent oil from condensing within the wastewater stripping system and thus clogging the equipment pipelines. Oil-water separation can help reduce the accumulation of waxy substances in the water system, but it does not address the waxing problem associated with MTO processes. Offline cleaning of heat exchangers and air coolers, as well as online cleaning of trays, can help reduce oil accumulation and remove waxy substances, but this increases maintenance costs. Moreover, frequent load reduction measures also have a significant impact on the long-term operation of the facility. The addition of demulsifiers can facilitate oil-water separation in the water system, but this affects the stripping efficiency of the subsequent wastewater stripping towers. Online injection of extractants for cleaning can help mitigate waxing in the water system, but the cost of these extractants is considerable. Although the addition of dispersants can help prevent catalyst clogging, the dispersed waxy substances will still precipitate in heat exchangers, trays, and pipelines at low temperatures. 　　Lan Tianhua, an engineer at Fude (Changzhou) Energy and Chemical Development Co., Ltd., told reporters that none of these improvements can solve the problem fundamentally; they can only alleviate blockages. Moreover, they are costly and require significant human and material resources, addressing only the symptoms rather than the root cause. 　　Gu Lanfang, an engineer at Shenhua Baotou Coal Chemical Co., Ltd., also said that the existing technical measures cannot fundamentally solve the problem of high solid content in the water system. He believes that to address the \"thrombosis\" problem in MTO units, further research is needed, focusing on aspects such as the chemical reaction properties, mechanical strength, and hydrothermal stability of the catalysts themselves, in order to find an effective solution to this issue.