In the early 1950s, the discovery of large oil fields in the Middle East made the development and application of indirect liquefaction technology fall into a low tide, but South Africa was an exception. Because of its apartheid policy, South Africa was embargoed by oil from all countries in the world, which prompted South Africa to make up its mind to fundamentally solve the energy supply problem. Considering the poor quality of coal in South Africa, it is not suitable for direct liquefaction. After repeated argumentation and scheme comparison, indirect liquefaction of coal was finally chosen to produce petroleum and petroleum products. Sasol No.1 Plant was put into operation on 1955, mainly producing fuel and chemicals. The energy crisis in 1970s prompted Sasol to build two larger coal-based Fischer-Tropsch devices, with the design goal of producing fuel. When the 1980 and 1982 plants were completed and put into operation, the price of crude oil had exceeded $30/barrel. At this time, the comprehensive production capacity of the three factories in Sasol has been about 7.6 million tons/year. Due to the large-scale production of Sasol, it remains profitable despite the fluctuation of crude oil price. South Africa not only broke the oil embargo, but also became the first country in the world to industrialize the Fischer-Tropsch synthesis technology of coal liquefaction. 1992 and 1993 built two Fischer-Tropsch synthesis plants with natural gas as raw materials, namely MOSS GAS 100000 tons/year in South Africa and Shell's 500000 tons/year plant in Pintulu, Malaysia. Main reactions of Fischer-Tropsch synthesis:
The generated alkane: NCO+(2n+1) H2 = CNH2N+2+NH2O.
Olefin: nCO+(2n)H2 = CnH2n+nH2O.
There are also some side effects, such as:
Methane generation: CO+3H2 = CH4+H2O.
Methanol is generated: CO+2H2 = CH3OH.
Ethanol output: 2CO+4H2 = C2H5OH+ H2O.
Coking reaction: The fixed-bed reactor with 2CO = C+CO2 was first developed by Ruhrchemir and Lurge, which is called Arge reactor for short. 1955 the first commercial Arge reactor was built and put into operation in south Africa. The reactor is 3m in diameter and consists of 2052 tubes, with an inner diameter of 5cm, a length of 12m and a volume of 40m3. Boiling water is outside the tube, and the reaction heat in the tube is taken away by the evaporation of water to produce steam. The tube is filled with an extruded iron catalyst. The operating conditions of the reactor are 225℃ and 2.6MPa. Liquid wax, which accounts for about 50% of the product, flows down the catalyst bed. Based on the pilot test results of Sasol, a large reactor with working pressure of 4.5 MPa was put into operation at 1987. The size of tubes and reactors is basically the same as that of large reactors.
Usually, the radial temperature difference of multi-tube fixed bed reactor is about 2 ~ 4°c, and the axial temperature difference is15 ~ 20 C. In order to prevent catalyst deactivation and carbon deposition, the maximum reaction temperature must not be exceeded, because carbon deposition will lead to catalyst damage and reactor tube blockage, and even the catalyst needs to be replaced. The operating temperature of fixed bed iron catalyst should not exceed 260℃, because too high temperature will cause carbon deposition and block the reactor. In order to produce wax, the general operating temperature is about 230℃, and the maximum design capacity of the reactor is 1500 barrels per day.
The advantages of fixed bed reactor are: simple operation; Because the liquid product flows downwards along the laminar flow of the catalyst bed, the catalyst and the liquid product are easy to separate, which is suitable for Fischer-Tropsch wax production. A small amount of H2S remaining due to the unstable operation of the syngas purification unit can be absorbed by the upper part of the catalyst bed, while other parts of the bed are not affected. Fixed bed reactors also have many disadvantages: reactors are expensive to manufacture. The high pressure drop caused by the high gas velocity flowing through the catalyst bed and the required tail gas circulation increase the gas compression cost. Diffusion control of Fischer-Tropsch synthesis requires the use of small catalyst particles, which leads to high bed pressure drop. Because the pressure drop on the tube side can reach up to 0.7 MPa, the stress on the reactor tube bundle is quite large. Large-diameter reactors require very large tube thickness, which makes the reactor expensive to scale up. In addition, the pipeline containing catalyst can not withstand too much operating temperature change. According to the required product composition, iron-based catalysts need to be replaced regularly; Therefore, a special detachable grid is needed, which makes the reactor design very complicated. Refilling catalyst is also a boring and time-consuming job, which requires a lot of maintenance work, resulting in long downtime; This also interferes with the normal operation of the factory. Germans studied the three-phase bubbling bed reactor in the 1940s and 1950s, but it was not commercialized. Sasol's R&D department began to study slurry bed reactors in the mid-1970s. 1990 made a breakthrough in research and development, and a simple and efficient wax separation device successfully passed the test. 100 barrels/day pilot plant was officially started in 1990. In May 1993, Sasso realized the start-up of the slurry bed reactor with ID=5m, 20m height and 2500 barrels per day.
Sasol's slurry reactor can use iron catalyst to produce wax, fuel and solvent. The pressure is 2.0 MPa and the temperature is higher than 200℃. The reactor is filled with foaming liquid reaction products (mainly Fischer-Tropsch wax) and catalyst particles suspended therein. The core and innovation of Sasol slurry bed technology is its patented process of separating wax products from catalysts. This technology avoids the expensive steps of stopping and replacing the catalyst in the traditional reactor. The slurry bed reactor can run continuously for two years with only one maintenance. The design of the reactor is simple. Another patented technology of Sasol slurry bed technology is to effectively separate the "slurry" entrained in the outlet gas of the reactor.
In order to separate synthetic wax from catalyst, a typical slurry bed reactor generally has 2 ~ 3 layers of filters, each layer of filter consists of several filter units, and each filter unit consists of 3 ~ 4 filter rods. Under normal operation, the synthetic wax is discharged through the filter rod, and the catalyst is blocked by the filter rod and remains in the reactor. When the filter rod is blocked by fine catalyst particles, it can be cleaned by backwashing. Under normal working conditions, part of the filter unit is draining wax, part is backwashing, and the third part is standby. In addition, in order to take away the reaction heat, two or three layers of heat exchange coils are arranged in the reactor, and boiler feed water enters the tubes, so that the reaction heat in the tubes is taken away through the evaporation of water to generate steam. The reaction temperature is controlled by adjusting the pressure of the steam drum. In addition, the lower part of the reactor is provided with a syngas distributor, and the upper part is provided with a dust collector and a demister. The operation process is as follows: the synthesis gas is evenly distributed on the cross section of the reactor through the gas distributor, and when it flows upward through the slurry bed composed of catalyst and synthetic wax, FT synthesis reaction occurs under the action of catalyst. The generated light hydrocarbon, water, CO2 and unreacted gas are discharged from the gas phase outlet at the upper part of the reactor, and the generated wax is filtered by the built-in filter and discharged from the reactor. When the filter is blocked and the pressure difference between the inside and outside of the reactor is too large, start the standby filter, cut off the wax discharge valve blocking the filter, and then open the backwash valve for flushing until the pressure difference disappears. In order to keep the catalyst activity in the reactor, the reactor is also equipped with a fresh catalyst/wax inlet and a catalyst/wax outlet. Fresh catalyst can be added regularly and quantitatively as needed, and old catalyst can be discharged at the same time.
Compared with the fixed bed, the slurry bed reactor is much simpler, which eliminates most of the disadvantages of the latter. The pressure drop of slurry bed is much lower than that of fixed bed, so the gas compression cost is much lower than that of fixed bed. Online addition and removal of catalyst can be easily realized. The total amount of catalyst required by slurry bed is much lower than that of fixed bed under the same conditions, and the catalyst consumption per unit product is also reduced by 70%. Due to full mixing, the isothermal performance of slurry bed reactor is better than that of fixed bed, so it can run at higher temperature without worrying about catalyst deactivation, carbon deposition and crushing. At a higher average conversion rate, the selectivity of products can also be controlled, which makes the slurry bed reactor especially suitable for high activity catalysts. The production capacity of Sasol's existing slurry bed reactor is 2,500 barrels per day. In 2003, commercial reactors with ID=9.6m and 65,438+07,000 barrels per day were designed for Qatar and Nigeria. Sasso believes that it is also feasible to design a reactor with a capacity of 22,300 barrels per day using Co catalyst, which has great advantages in economic scale. About 1955, Sasol enlarged the circulating fluidized bed reactor (CFB) developed by Kellogg Company of the United States by 500 times in the first factory (Sasol Phase I). The enlarged reactor has an inner diameter of 2.3 meters and a height of 46 meters, and its production capacity is 1500 barrels per day. Since then, many difficulties have been overcome and the design and catalyst formula have been revised many times. This reactor, which was later named to synthesize alcohol, has been running successfully for 30 years. Later, by increasing the pressure and size, Sasso tripled the treatment capacity of the reactor. 1980, eight synthetic alcohol reactors with ID=3.6m were built in Sasol Phase II and Sasol Phase III respectively, with a capacity of 6500 barrels per day. High density iron-based catalyst is used. The pressure of circulating fluidized bed is lower than that of fixed bed, so its gas compression cost is lower. Due to the rapid circulation and backmixing caused by high gas velocity, the reaction section of circulating fluidized bed is almost isothermal, and the temperature difference of catalyst bed is generally less than 2℃. In circulating fluidized bed, the temperature fluctuation range in the circulating loop is about 30℃, and an important feature of circulating fluidized bed is that new catalyst can be added or old catalyst can be taken out.
Circulating fluidized bed also has some disadvantages: complex operation; Fresh circulating materials enter the bottom of the reactor at 200°C and 2.5 MPa, and part of the catalyst flowing down from the standpipe and slide valve is taken away. In the catalyst deposition area, the catalyst and gas are separated. The gas leaves the cyclone separator, and the catalyst is separated from the gas and returned to the separator due to the decrease of linear velocity. It is difficult to separate fine catalyst particles from tail gas. Generally, cyclone separator is used to realize this separation, and the efficiency is generally higher than 99.9%. However, due to the high mass flow rate through the separator, even 0. 1% catalyst is abundant. Therefore, these reactors are usually equipped with an oil scrubber downstream of the separator to remove these fine particles. This increases the equipment cost and reduces the thermal efficiency of the system. In addition, the wear caused by iron carbide particles at very high linear speed requires the use of ceramic lining to protect the reactor wall, which also increases the reactor cost and downtime. Generally, the synthetic alcohol reactor operates at 2.5 MPa and 340℃. In view of the limitations and defects of circulating fluidized bed reactor, Sasol successfully developed a fixed fluidized bed reactor, which was named Sasol Advanced Synthetic Alcohol Reactor (SAS).
The fixed fluidized bed reactor consists of the following parts: a container containing a gas distributor; Catalyst fluidized bed; Cooling pipes in the bed; And a cyclone separator for separating entrained catalyst from the gas product.
Fixed fluidized bed operation is relatively simple. The gas enters from the bottom of the reactor through the distributor and passes through the fluidized bed. The catalyst particles in the bed are in a turbulent state, but generally remain motionless. Compared with industrial circulating fluidized bed, they have similar selectivity and higher conversion rate. Therefore, the fixed fluidized bed has been further developed in Sasol, and a demonstration device with an inner diameter of 1 m was started in 1983. A commercial device with an inner diameter of 5 meters was put into use in 1989, all of which met the design requirements. 1in June, 1995, the commercial demonstration device of SAS reactor with a diameter of 8 meters was successfully started. 1996 sasso decided to replace the 16 synthetic alcohol circulating fluidized bed reactors in sasso no 2 and sasso no 3 plants with eight SAS reactors. Among them, there are 4 SAS reactors with a diameter of 8 meters, each with a production capacity of 1 1000 barrels per day; The other four reactors with a diameter of10.7m have a production capacity of 20,000 barrels per day. This work was completed in 1999. In 2000, Sasso added a ninth SAS reactor. The operating conditions of the fixed fluidized bed reactor are generally 2.0~4.0 MPa and about 340℃, and the iron catalyst used is generally similar to that of the circulating fluidized bed.
Under the same production scale, the manufacturing cost of fixed fluidized bed is lower than that of circulating fluidized bed, because it is small in size and does not need expensive supporting structure. Because SAS reactor can be placed on the skirt, the cost of its supporting structure is only 5% of that of circulating fluidized bed. Because of the low linear velocity of gas, the wear is basically eliminated, and there is no need for regular inspection and maintenance. The pressure of SAS reactor is reduced and the compression cost is low. Carbon deposition is no longer a problem. The dosage of SAS catalyst is about 50% of synthetic alcohol. As the reaction heat increases with the increase of reaction pressure, the increase of coil cooling area makes the operating pressure as high as 40 bar, which greatly improves the production capacity of the reactor.