Development Trends of China’s Light Hydrocarbon Comprehensive Utilization

By Li Zhenyu, Wang Hongqiu, Huang Gesheng, Li Dingjie and Ren Wenpo, PetroChina Petrochemical Research Institute

In recent years, due to the high price of crude oil in the international market, the cost of olefins produced using naphtha as raw material, like ethylene and propylene, has climbed constantly, increasing the production cost of downstream chemical products. Seeking low cost raw materials and making good use of existing resources are important ways to improve the competitiveness of enterprises. Therefore, the comprehensive utilization of light hydrocarbons has attracted more and more attention.
   Light hydrocarbons are mainly byproducts of natural gas processing, activities in oil fields and gas fields, petrochemical production, refineries, and oil and gas processing plants. The main component of natural gas (containing a small amount of C2) is C1. The main components of liquefied petroleum gas (LPG) are C3 and C4. C1, C2, C3 and C4, all gaseous under normal temperature and pressure, are called the gaseous light hydrocarbons. C5-C16 hydrocarbons are liquid under normal temperature and pressure, and are called liquid light hydrocarbons. At present, light hydrocarbons are used as raw materials for ethylene cracking units both at home and abroad. In addition, light hydrocarbons are separated to produce all kinds of single-component alkanes or olefins that can be further used as raw materials for producing products with high added value.

Chemical utilization of shale gas

Shale gas, which is an important unconventional natural gas resource, is an important field of natural gas industrialized exploration under the present economic and technical conditions. Globally, the recoverable shale gas is estimated at 187 trillion cubic meters, of which China holds the lion’s share, with 36 trillion cubic meters, accounting for around 20% of the world total and 24 trillion cubic meters higher than the holdings of the United States which rank second. It is expected that by 2035, the output of unconventional natural gas in the United States, China and Canada will all account for over 50% of their natural gas output.
   According to the data from HIS, the production cost of ethylene using ethane as raw material is around 18 cents/lb (US$396.8/t), while the production cost of ethylene using naphtha as raw material is around 54 cents/lb (US$1 025.1/t). For the petrochemical and plastic downstream producers in North America, the acquisition of low-cost natural gas raw material has greatly enhanced the export competitiveness of ethylene, polyethylene and other derivative products.
   China has very rich shale gas resources, but the domestic exploration and development of shale gas resources is still in its initial stages. According to the 12th Five-Year Plan for the Development of Shale Gas (2011-2015), by 2015, China’s proven geological reserve of shale gas will reach 600 billion cubic meters, the recoverable reserves will be 200 billion cubic meters and the output of shale gas will reach 6.5 billion cubic meters. By 2020, China plans to produce 60 billion-100 billion cubic meters of shale gas annually.

Comprehensive utilization of LNG

1. Chemical utilization of ethane

Liquefied natural gas (LNG) mainly contains methane, but also contains a certain proportion of ethane (6%-8%) and propane (less than 2%). If separated from LNG, ethane will be a high-quality raw material for ethylene production. Ethane is recovered from LNG by either of two processes. One is the solution for recovery of ethylene mixtures: methane (being transported to users through pipelines) is separated through a methane tower added to a receiving station; the discharge from the bottom of tower is a mixture composed mainly of ethane but containing small amounts of propane and butane. Ethane, propane and butane can all be used as raw materials for ethylene production. The alternative technique is the solution for recovery of ethane: methane (being transported to users through pipelines) is first separated in the methane tower; the methane tower bottom mixture enters an ethane tower for separation; the discharge from top of the tower is ethane (being used as raw material for ethylene production); and the bottom of the tower yields LPG.

2. Comprehensive utilization of cold energy

In recent years, the utilization technology of LNG cold energy has begun to develop. The utilization of LNG cold energy can be divided into direct utilization and indirect utilization. Direct utilization includes LNG cold energy power generation, air liquefaction separation, preparation of liquid carbon dioxide and solid carbon dioxide, warehouse refrigeration, light hydrocarbon separation and cutting, desalination of seawater, etc. Indirect utilization includes cryo-comminution of wastes, freezing food, LNG-based cold storage, etc. Among these, LNG cold energy is most widely used in air separation, principally in Korea, Australia and Taiwan province. The air separation unit of CNOOC (China National Offshore Oil Corporation) LNG project in Putian of Fujian province went on stream in 2009. CNOOC also has a project for LNG cold energy cryo-comminution of waste tires (20 thousand t/a) under construction. PetroChina and Hangzhou Hangyang Co., Ltd have jointly constructed an air separation unit near PetroChina’s LNG receiving station at Rudong of Jiangsu province. The first phase of the project (60t/h LNG) is expected to be completed and put into operation in 2013, to efficiently utilize the LNG cold energy in the receiving station.

3. Transportation fuel

As a transportation fuel, LNG has the advantages of high energy density, convenient implementation, cleanliness, freedom from sulfur and good economic efficiency. Therefore, the development of LNG for transportation fuels is attracting more attention. At present, LNG cars have already been demonstrated in many provinces and cities of China or are already included in local development plans. The regions with rapid development of LNG cars include gas rich regions like Xinjiang, and economically developed regions with large market demand such as southeast coastal regions. In addition, LNG ships have great potential in inland river shipping in China. Compared with existing inland cargo vessels using diesel fuel, LNG fueled vessels have advantages in economic efficiency and safety. Therefore, wherever merited by local cargo traffic, LNG filling stations can be set up in the inland and coastal regions to provide highly efficient LNG fuel for ships.

Comprehensive utilization of LPG

LPG is mainly obtained as a byproduct of petroleum refinement, the exploitation of natural gas and crude oil or ethylene production via steam cracking. The composition of LPG from these three sources differs greatly due to differences in the processing of raw materials and in the technology applied. For instance, LPG produced from fluid catalytic cracking (FCC) units mainly contains C3, C4 saturated hydrocarbons and olefins. In addition to containing n-alkanes, C4 fraction also contains a substantial amount of isoalkanes and olefins.
   The formulation of civil/commercial fuel and the manufacture of petrochemicals are the main sectors consuming LPG. In 2011, the consumption of LPG in the civil/commercial fuel sector fell 3 percentage points to 48% of total consumption, while its use as raw material for petrochemicals climbed 3 percentage points to 28%. At present, the main directions of LPG deep processing are aromatization, propane dehydrogenation to propylene, MTBE production and alkylation gasoline production. With the butadiene supply gap growing, oxidative dehydrogenation of butylene to butadiene has also attracted more attention.

1. Production of BTX using aromatization technology

Aromatizing LPG to aromatic hydrocarbons is a value-added use of LPG, eases the tight supply of BTX (benzene-toluene-xylene mixture). The technology was developed on the basis of the ZSM-5 molecular sieve catalytic technology. In recent years, aromatization technology for light hydrocarbons like LPG has progressed remarkably. The industrialized technologies mainly include the GTA process developed by Sinopec Luoyang Petrochemical Engineering Co., Ltd and the nano-forming process developed by Dalian University of Technology. In order to improve the economic efficiency of the process, PetroChina Petrochemical Research Institute cooperated with Dalian University of Technology to conduct the complete conversion of the butylene in LPG through aromatization and alkylation reactions to produce high octane gasoline components in a fixed bed reactor equipped with a nano HZSM-5 zeolite molecular sieve catalyst at a lower temperature (360-410?C). At the same time, propane and butane as matching products have been used as qualified raw materials for ethylene production.

2. Propane dehydrogenation to propylene

With the demand and value of propylene growing constant, along with a great expansion in the scale of production units, technology for the catalytic dehydrogenation of propane has become attractive. At the end of 2011, there were 20 PDH (propane dehydrogenation) units in China with a total capacity of 6.54 million t/a, accounting for around 6.5% of the world’s total. Many companies have planned to construct units. If these units are put into operation on schedule, PDH capacity will reach around 15.0 million t/a by 2015.
   The existing PDH units all use high purity propane with low sulfur content from wet oilfield associated gas as raw material, the purity of propane is over 97%, and the volume fraction of gaseous sulfur impurities is below 100μl/l. China’s wet oilfield associated gas resources are scarce, while the sulfur content of LPG – a byproduct of refining is higher, and the quality of this propane cannot meet the raw material requirements of the PDH process. Therefore, the operators of PDH units in China must import high purity liquefied propane extracted from foreign oilfield associated gas.

3. Production of MTBE using C4 components

In addition to its use as gasoline additive, MTBE (methyl tert-butyl ether) can also be used to produce high-purity isobutylene through cracking, as raw material for MMA (methyl methacrylate) and butyl rubber production. MTBE is a good reaction solvent and reagent. For example, it is used as alkylation solvent for isopentene, methyl ester and phenol. MTBE can be used to produce tert-butylamine, trimethylacetic acid, tert-butanol and tert-butoxyacetic acid so as to provide high-quality raw materials for the production of other fine chemicals.
   With the constant growth of demand, the production technology of MTBE has improved continually. Existing MTBE units generally use the process of isobutylene and methanol synthesis.

4. Production of alkylated gasoline using C4 components

Alkylated gasoline features a high octane number, a good antiknock property, no olefins or aromatic hydrocarbons, low sulfur content and low vapor pressure, and it is an ideal blending component. Isobutane in LPG can be separated, which is done mainly for the production of alkylated gasoline. At present, producers of alkylated gasoline mainly use the traditional sulfuric acid method or the hydrofluoric acid method alkylation process. Among the 212 alkylation units in commercial operation, 111 units, with an average capacity of 8 807 barrels/day, use the latter technology, and their combined capacity accounts for around 47% of the world’s total alkylation capacity. Ninety units, with an average capacity of 9 590 barrels/day, use the sulfuric acid alkylation technology, and their combine capacity accounts for 42% of the world total alkylation capacity.
   The hydrofluoric acid method is highly toxic, and its development has slowed down in recent years. However, because the production of alkylated gasoline has attracted increasing attention and new underlying technology has been developed, sulfuric acid alkylation technology has also improved continually.

5. Butylene oxidative dehydrogenation to butadiene

With the light weight of ethylene raw materials and diversification of the ethylene production mode in the world, the proportion of butadiene directly produced by extraction of C4 byproduct of ethylene has declined, while the proportion of butadiene produced using methods like butylene and butane dehydrogenation has gradually increased. Butane and butylene in LPG can be transformed into butadiene. The catalytic dehydrogenation of butane is done by one of two processes – the one-step or the two-step. Due to the long process and complicated operation, the two-step process is far less commonly used than the one-step process. At present, the butylene oxidative dehydrogenation process has become an effective way to resolve the imbalance of butadiene supply and demand.
   With the development of sectors such as electronic and electrical appliances, automobile, construction, infrastructure, energy saving and environmental protection, the demand growth for butadiene derivative products like polybutadiene rubber, solution-polymerized styrene butadiene rubber, and ABS (acrylonitrile-butadiene-styrene) as well as its products will increase the demand for butadiene. China started its research in the oxidative dehydrogenation of butylene to butadiene in the 1960s. PetroChina Jinzhou Petrochemical Co., Ltd has developed complete fluidized bed technology for the oxidative dehydrogenation of butylene to butadiene using a catalyst from Lanzhou Institute of Chemical Physics. The Design Institute of Sinopec Qilu Petrochemical Co., Ltd has developed proprietary technology using a B-02 catalyst developed by Sinopec Beijing Yanhua Petrochemical Co., Ltd and a two-stage adiabatic fixed bed reaction process developed by East China University of Science and Technology. The 100 000 t/a butadiene project of Zibo Qixiang Tengda Chemical Co., Ltd is China’s only operational butylene dehydrogenation to butadiene unit, and the n-butylene raw material mainly comes from LPG supplied by Sinopec Qilu Petrochemical Co., Ltd and its surrounding oil refining enterprises. According to incomplete statistics, China has four LPG to butadiene projects with a total capacity of 450 thousand t/a. Two are under construction and the other two are still in the planning stage.

Conclusion

In recent years, China's national economy has maintained rapid development. The rapid growth of infrastructure and consumption has promoted rapid growth of the demand for chemical products such as oil products, synthetic rubbers and synthetic resins. An imbalance between the supply and demand for related raw materials is increasingly prominent, and China's dependence on imported petroleum has reached as high as 58%. Full use of the C2 in shale gas, C2, C3 and C4 hydrocarbons in oilfield associated gas, C2 hydrocarbons in the imported LNG, and hydrocarbons in LPG of the refineries and ethylene plants as chemical raw materials, can reduce China's dependence on imported petroleum, alleviate the supply shortage of olefin raw material, optimize the ethylene raw material structure and reduce the production cost of ethylene.