Process technology for producing synthetic oil from natural gas

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Process technology for producing synthetic oil from natural gas: Synthetic oil derived from natural gas is considered a clean fuel; its advantages lie in the absence of impurities such as sulfur, nitrogen, nickel, and aromatic compounds, which are undesirable components. It meets the strict requirements of modern engines as well as the increasingly stringent environmental regulations, thus opening up a new pathway for the production of clean energy. Over the next 15 years, the production capacity of GTL plants is expected to increase to 45–67.5 million tons per year. According to statistics, there are already 10 GTL plants under construction or planned worldwide, with capacities ranging from 220,000 to 4.5 million tons per year. With oil prices remaining high at present, it is expected that more GTL plants will be built worldwide. Therefore, both from the perspective of production technology and economic benefits, there are ample reasons to believe that the 21st century will witness a peak in the development of synthetic oil produced from natural gas. GTL production process 　　GTL production technology can be divided into two main categories: direct conversion and indirect conversion. Direct conversion processes do not require syngas production facilities, but the high stability of methane molecules during this process poses significant technical challenges. Several direct conversion processes that have been developed to date are not economically viable, and thus have not been commercialized yet. Indirect conversion mainly involves the production of syngas, which is then used to synthesize synthetic oil via the Fischer-Tropsch process. Compared with the former method, the indirect process has lower operational costs, and it has become the recognized synthetic route. Its main process flow consists of four parts: syngas production, F-T synthesis, synthetic oil treatment, and reaction water treatment. A brief introduction to each of these main process units is as follows: 1. Syngas Production In the GTL process, the capital investment for syngas production accounts for approximately 60% of the total investment, and its production costs make up around 60% of the total production costs. Therefore, reducing the capital investment and production costs associated with syngas production plays a decisive role in improving the economic efficiency of GTL. 　　The main methods for syngas production currently include steam methane reforming (SMR), partial oxidation (POX), and autothermal reforming (ATR). 　　The production of syngas via natural gas steam reforming is an endothermic reaction that requires a large amount of heat during the process, resulting in high energy consumption. This reaction is slow, and the production facilities need to be large-scale with high investment costs. The H/CO ratio of the syngas produced by this method is too high, generally exceeding 3/1 (when no CO2 is recovered), making it unsuitable as a feed gas for synthetic oil production. 　　The catalytic partial oxidation of natural gas with pure oxygen is a mild exothermic reaction ； The reaction can be carried out at very high space velocities; the H2/CO ratio of the resulting syngas is generally below 2/1, with the reaction temperature ranging from 1,200 to 1,500°C. 　　The autothermal conversion method is a new syngas production process that combines steam conversion and partial oxidation in a single step. It offers advantages such as low reaction temperature, low oxygen consumption, an H/CO ratio of 2/1, and a composition suitable for the production of synthetic oils. 　　Among them, in the POX and ATR methods, the production of pure oxygen requires expensive investment in air separation equipment as well as high costs for oxygen production. To this end, Syntroleum improved the ATR method by using air instead of oxygen, thereby avoiding the need for pure oxygen production. Additionally, since the system is self-heat-balanced, a heat transfer system is not required, which makes the ATR reactor simpler and more compact. The water-to-carbon ratio is also lower than that of the traditional ATR method, resulting in a significant improvement in its economic efficiency. By selecting the appropriate water-to-carbon ratio, oxygen-to-carbon ratio, and reaction temperature, an ideal raw gas for synthetic oil can be obtained; however, this method has a low effective utilization rate of natural gas due to the inability to recycle the exhaust gases. Air Products has developed a new ion-exchange membrane for syngas production (referred to as ITMSyn-gas); using this technology at temperatures above 700°C eliminates the need for oxygen production plants and other processes involved in syngas production. Its feature is a high oxygen flow rate, and it can be used in slurry reactors to synthesize liquid hydrocarbons; it is reported that this technology can reduce costs by 25%. 2 FT Synthesis Process This is a method of converting syngas into liquid hydrocarbons using catalysts. It was invented in 1923 by German scientists Frans Fischer and Hans Tosch; it is commonly referred to as FT synthesis. Petroleum companies around the world have invested substantial resources in developing catalysts and processes for this synthesis, and significant progress has been made. Below, we will focus on the relevant processes employed by companies such as Exxon, Shell, Sasol, Syn-troleum, Rentech, and Intevep. 2.1 Exxon’s AGC-21 process Exxon’s Advanced Gas Conversion for 21st Century technology, abbreviated as AGC-21 technology. Over the past 20-plus years, Exxon has invested $300 million in developing AGC-21 technology, holding 400 U.S. patents and 500 international patents related to this technology. A 3-year pilot plant test with a capacity of 200 barrels per day was carried out at the Baton Rouge refinery from 1990 to 1993; currently, Exxon claims to be able to design GTL plants with a daily production capacity of up to 50,000 barrels. The process involves the reaction of natural gas, oxygen, and water vapor in a nickel-based catalyst reactor to produce syngas with an H/CO ratio of approximately 2:1. Subsequently, under the action of a highly active cobalt-based catalytic system, an FT synthesis reaction takes place in a slurry bed reactor to yield an alkane mixture with a wide range of molecular weights. Finally, this mixture is subjected to fixed-bed hydroisomerization to be converted into liquid hydrocarbon products. 2.2 Shell’s SMDS process Shell’s Middle Distillate Synthesis technology, abbreviated as the SMDS process, is considered to be the most successful example of GTL plants in the world today. The industrial plant for this process was successfully put into use in Malaysia in May 1993, with a total investment of 850 million dollars. Its production capacity is 12,500 barrels per day, and the capital cost per barrel per day is 68,000 yuan. The process involves using a shell-and-tube gasification technique to react natural gas, oxygen, and water vapor in a gasifier; the resulting syngas undergoes an F-T reaction in a tubular fixed-bed reactor equipped with cobalt-based catalysts to produce heavy paraffin. This paraffin is then subjected to hydrocracking and distillation to yield various liquid hydrocarbon products for sale. The facility has now been modified to enable the production of diesel, naphtha, high-quality paraffin, and other products. 2.3 Sasol’s SSPD process The South African company Sasol uses coal as a raw material and the PT synthesis process to produce various oils. Since 1955, the company has operated three production facilities: Sasol I, Sasol II, and Sasol III. The total cost of building these facilities was approximately 6 billion dollars. These facilities are used for the large-scale production of synthetic oils and related products, with an annual output of 7.1 million tons of oils and related chemicals. In 2001, sales amounted to $5.399 billion, with operating profits reaching $1.4 billion. The SasolSlurry Phase Distillate technology is abbreviated as SSPD process. It consists of three stages: the first stage is the conversion of natural gas into syngas, the second stage is the synthesis of paraffinic hydrocarbons in a suspended-bed reactor, and the third stage is the distillation of the intermediate fractions. Currently, Sasol has transferred its technology to South Africa’s Mossgas company, which has built a plant with a capacity of 1.24 million da to convert offshore natural gas into synthetic oil; it is currently the largest GTL plant in the world to utilize FT technology. 2.4 Syntroleum Process Synthroleum Company was established in 1984 and has built two GTL demonstration plants. Syntroleum’s syngas production utilizes its own ATR process, in which air is used in place of oxygen for autothermal conversion to produce nitrogen-containing syngas, thereby achieving an H2/CO ratio that is nearly ideal for the desired reactions. The syngas is then passed through the fluidized bed reactor in a single pass at a high space velocity without a recirculation loop, and liquid hydrocarbons with a certain chain length are directly synthesized under conditions of 2.1–3.5 MPa and 190–232°C. This approach prevents the aggregation of N, reduces the need for a hydrocracking step, and also results in lower operating pressures. The Syntroleum process features a simple reactor structure, easy start-up and shutdown, and low investment, which helps to reduce costs. This technology is suitable for small-scale operations processing 5,000 barrels per day, or even as low as 2,500 barrels per day; the investment cost ranges from $12,000 to $27,000 per barrel per day. 2.5 Rentech 　　Rentech has been engaged in this process since 1981; it uses suspension reactors and iron catalysts to convert natural gas into liquid hydrocarbons. In 1992, the company built a pilot plant with a capacity of 250 barrels per day. Currently, Rentech is planning to convert a methanol plant with a capacity of 75,000 tons per year in Commerce, Colorado, USA, into a GTL plant with a production capacity of 1,000 barrels per day, producing diesel, kerosene, and paraffin. 2.6 Intevep Intevep has been developing the FT process since 1991; it uses a unique fluidized bed reactor that combines the advantages of both slurry reactors and tubular fixed-bed reactors, ensuring uniform mixing of reactants without any backmixing. The catalyst particle size used is larger than that in a slurry reactor, allowing the catalyst to be easily separated from the product paraffin through the free space above the fluidized bed layer, thus facilitating online catalyst loading and unloading. Since the catalyst particles are kept within the fluidized bed, a catalyst recovery system can be eliminated. At the same time, the syngas passes through the reactor once, with no gas recovery system in place. Intevep expects to commercialize its technology within the next two years. 3 Product separation and hydrogenation Synthetic oil processing mainly involves subjecting paraffins and other synthetic oil products to hydrogenation, followed by product distillation, in order to obtain the products required by the market. The processing of synthetic oils follows basically the same procedures as those used for conventional oils; it is a highly mature process. Currently, Chevron holds a strong competitive advantage in providing hydrotreatment technologies specifically tailored for the GTL sector. 4. Treatment of reaction water Water is a by-product of FT synthesis reactions, and it also contains various oxygen-containing hydrocarbons such as acids, alcohols, ketones, and aldehydes. These oxygen-containing hydrocarbons must be removed before the reaction water can be reused or discharged, in order to meet the requirements for use or disposal. The typical treatment method is to use a simple distillation system to remove most of the oxygen-containing compounds from the top of the tower (with the exception of acids), while the acids remain in the liquid at the bottom of the tower. Due to their small quantity, the oxidized compounds at the top of the tower have no value for recovery and are generally sent to a furnace for burning. The bottom liquid from the tower, along with other wastewater from the facility, is sent to a biochemical wastewater treatment plant for treatment.