3 million tons of propylene oxide capacity is running on the road of green transformation

Propylene oxide profile

Propylene Oxide (PO) is the third largest propylene derivative after polypropylene and acrylonitrile, and is also an important basic organic chemical raw material. Mainly used in the production of polyether polyols, propylene glycol, propylene glycol ether, propylene carbonate, isopropanolamine, 1,4-butanediol and propylene alcohol, etc. It is the main raw material of non-ionic surfactants, oil field demulsifiers, pesticide emulsifiers, etc. It has a wide range of applications in automotive, construction, food, tobacco, medicine and cosmetics industries.

Production plants are mainly concentrated in Asia, Western Europe, and North America. The main production enterprises are as follows. In recent years, new production capacity has mainly come from China. Due to the maturity of the market and the limitation of local propylene resources, there are basically no new production plants in North America and Western Europe.

Our country's current propylene oxide production capacity is 3.297 million tons/year, of which Shandong's enterprise capacity accounts for more than 40%, leading the country, with a certain market control power and has the voice over, with the gradual rise of new production capacity in other regions, This pattern will be improved, the specific distribution is shown in the table below.

Due to the shutdown of some domestic production capacity due to environmental protection, and the low start-up rate of some equipment using new technologies and unstable production, the growth rate of propylene oxide production in our country has slowed down in the past two years. With the expansion of production capacity, our country's self-sufficiency rate of propylene oxide in 2017 exceeded 90%, import dependence was less than 10%, and supply and demand gradually tended to be weakly balanced.

In the future, our country will release nearly 3 million tons of production capacity, mainly concentrated in Jiangsu, Fujian and other regions. The concentrated launch of new production capacity is bound to fill the gap between supply and demand. By 2021, it may face the situation of overcapacity.

Propylene oxide production process

At present, the production process of propylene oxide mainly includes the chlorine alcohol method, the co-oxidation method and the direct oxidation method, of which the chlorine alcohol method is the most used, accounting for more than 50%. However, due to the environmental problems caused by this process, the co-oxidation method and the direct oxidation method are mostly used in the future.

Chlorohydrin method

The chlorohydrin method is a traditional process for synthesizing propylene oxide, which began in the 1930s and has been industrialized for more than 80 years. The main raw materials are propylene, chlorine gas, quicklime and water. First, chlorohydrin reacts with propylene through chlorine gas and water to generate the intermediate chloropropanol, and then uses lime water to saponify with it to obtain propylene oxide. The reaction product is sent to the initial distillation column and the rectifying column for product separation to obtain the product propylene oxide and the by-product dichlorohydrin.

• Advantages : The process is relatively short, the process is mature, the requirements for the purity of raw propylene are not high, and the investment is low.
Disadvantages
• Disadvantages : A large amount of chlorine is required, which is seriously corrosive to equipment, and a large amount of wastewater containing calcium chloride is discharged. The waste residue and exhaust gas pollute the environment and are difficult to treat. It is only suitable for construction in large chlor-alkali plants along the coast.

Due to the environmental problems of the chlorine alcohol process, our country has stopped the approval of new propylene oxide plants using the chlorine alcohol process, and enterprises in various countries have also improved the process.

Dow combines a chlorohydrin process unit with a chlor-alkali unit, uses sodium hydroxide instead of lime water for saponification, and the chlorine and alkali obtained by electrolysis are used as propylene oxide chlor-alkali raw materials. The sodium chloride generated by saponification is treated and electrolyzed to generate chlorine gas and sodium hydroxide. This process not only avoids wastewater pollution, but also realizes a closed-circuit cycle of sodium chloride.

Rumbs Using tert-butyl alcohol as a circulating carrier, first reacting tert-butyl hypochlorite with water and chlorine in sodium hydroxide solution to obtain tert-butyl hypochlorite, tert-butyl hypochlorite reacts with propylene and water to form tert-butyl alcohol, chloropropanol, and finally chloropropanol and sodium hydroxide are saponified to obtain propylene oxide. However, this process is more complicated and costly.

In terms of saponification wastewater treatment , Lin Haibo et al. proposed to introduce carbon dioxide into the saponification wastewater to the pH of the wastewater system slightly below 7, convert calcium hydroxide into calcium carbonate, and then add an appropriate amount of sodium carbonate to convert calcium chloride into calcium carbonate in the wastewater system, and filter and separate to obtain solid calcium carbonate and light brine containing organic matter. This method, while treating wastewater, obtains precipitated calcium carbonate powder materials, salts and water, which can be used in other industries. It is currently a more advanced saponification wastewater treatment method.

Co-oxidation

Ethylbenzene (PO/SM)

The process mainly uses the peroxide of ethylbenzene and propylene for epoxidation to generate propylene oxide, and at the same time generates by-product phenylethanol.

• Advantages : No chlorine is used as raw material, the reaction is relatively smooth, and the pollution is small.
• Disadvantages : long process flow, strict explosion-proof requirements, large investment, harsh operating conditions and a large proportion of combined products.

Isobutane method (PO/TBA)

The process was developed by Alco Corporation of the United States, mainly using propylene and isobutane as raw materials, including isobutane oxidation, propylene epoxidation, product separation, refining, catalyst recovery and preparation, and treatment of tert-butanol.

• Advantages : Overcome the shortcomings of the three wastes of the chlorine alcohol process, such as serious pollution, large corrosion and the demand for chlorine resources.
• Disadvantages : Complex process, long process, high equipment requirements, and high investment costs.

Cumene Hydrogen Peroxide Process (CHP)

Japan's Sumitomo Chemical Company has developed the cumene hydrogen peroxide process, which mainly includes the preparation of cumene hydrogen peroxide, propylene epoxidation, and hydrogenation.

• Advantages : This process has no by-products, is not limited by co-products, enhances the flexibility of PO production, and has no pollution to the environment, low investment, and low cost. It is an environmentally friendly clean production process.

Direct oxidation

Direct Hydrogen Peroxide Oxidation (HPPO)

• Advantages : simple process flow, small footprint, high product yield, no other co-products, less waste, environmentally friendly clean production process.
• Disadvantages : Short industrialization time, process to be improved, difficult storage and transportation of hydrogen peroxide solution.

Direct Oxidation of Oxygen

This process is the direct oxidation of propylene by oxygen under the action of a special catalyst to prepare propylene oxide, which has the advantages of not generating by-products and not requiring other high-priced reagents. However, the current conversion and yield are very low, and there is still a long way to meet the industrial production conditions.