Production of butadiene by the ACN method

The ACN method refers to the acetonitrile method, in which acetonitrile is used as the solvent for extractive distillation, and the water wash tower is primarily used for recovering the acetonitrile solvent. This method was first developed successfully by the American company Shell, and industrial production of it began in 1956. It uses 10% aqueous ACN as a solvent and consists of process units such as extraction, flash evaporation, compression, high-pressure desorption, low-pressure desorption, and solvent recovery. In 1977, Shell added condensers and wash towers during the renovation, and combined and compressed the vapor streams from flashing and low-pressure desorption. Approximately 8% of this vapor was condensed and sent to the wash tower to have the solvent removed, while the vapor at the top of the tower was returned to the feed distillation tower, thereby removing the C5 hydrocarbons from the C4 hydrocarbons. The remaining gas is sent partly to the high-pressure desorption tower, and the other part is sent to the bottom of the extractive distillation tower as a reboiling gas to provide heat energy, thereby eliminating the need for a reboiler and reducing steam consumption. About 1% of the solvent at the bottom of the wash tower is sent to the solvent recovery and purification system to ensure the quality of the recycled solvent. According to this method, raw materials with a high content of alkynes require hydrogenation treatment, or precision distillation and two-stage extraction must be employed to obtain butadiene of high purity. This method is represented by the Italian Sir process and the Japanese JSR process. The Italian Sir process uses ACN with 5% water content as the solvent, and employs a 5-tower process (ammonia scrubbing tower, first extraction distillation tower, second extraction distillation tower, light-end removal tower, and heavy-end removal tower). A ammonia wash tower is added before the first extractive distillation tower to remove 0.04%~0.08% of aldehyde compounds from the feedstock. Alkynes are taken off from the side stream of plate 75 in the second extractive distillation column and sent to the contact condenser. The material at the bottom of the degassing tower and that at the bottom of the contact condenser are combined, and their thermal energy is recovered for use in the feed evaporator. This process not only enables a butadiene yield of 96%–98%, but also allows for the separation of butadiene from alkynes, with the purity of the butadiene product reaching over 99.5%. The feature of this technology is its simple process, with solvent desorption taking place in the lower section of the extractive distillation column ； The first extraction distillation column uses two-point feeding, which helps to improve the concentration distribution of the liquid phase inside the column, reduces the liquid phase load in the upper section of the column, and lowers energy consumption ； A heat exchanger is installed at the lower part of the first extractive distillation column, serving as an intermediate reboiler; this allows for the full utilization of the thermal energy at the column bottom to improve the efficiency of separating hydrocarbons from the solvent ； The technique of removing acetylenes via a side stream at the 75th tray of the second extractive distillation column is employed to achieve almost complete separation of butadiene from acetylenes. The Japanese JRS process uses ACN with 10% water content as the solvent, and employs two-stage extractive distillation; the first extractive distillation column is composed of two columns connected in series. This process underwent two major modifications in 1980 and 1988. The modification carried out in 1980 involved the use of heat coupling technology: all the butadiene-rich vapor from the top of the second extractive distillation column was sent directly to the middle section of the degassing column without being condensed first. At the same time, a portion of the liquid stream flowing downward in the degassing column was drawn off from the middle trays and sent to the second extractive distillation column as reflux. As a result, no condenser was needed at the top of the second extractive distillation column; all of this heat was applied to the degassing column, reducing the thermal load on its bottom reboiler by about 40% compared to before the implementation of heat coupling, thereby achieving significant energy savings. The modifications made in 1988 were primarily aimed at addressing the issue of thermal energy recovery in the system; intermediate condensers were installed in the concentration tower and the light-end removal tower, and the concentration tower was connected in series in the upper and lower sections near the feed plate. This approach allowed for a significant reduction in the load on the upper tower without affecting its operating conditions. The tower is divided into upper and lower sections; the operating pressure in the lower section is increased, which raises the temperature inside the tower, allowing the intermediate condenser to recover high-quality thermal energy. Furthermore, the thermal energy from the wastewater at the bottom of the solvent recovery tower can be used to preheat the feed pipeline of this tower; in addition, some heat energy can also be recovered from the alkynes extracted from the side stream by the stripping tower. As a result, this process has the lowest energy consumption among similar processes. The advantage of producing butadiene using the ACN method is its low boiling point, low temperatures for extraction and stripping operations, and easy prevention of butadiene’s self-polymerization ； Stirring can be carried out at high pressure, eliminating the need for a butadiene gas compressor and reducing capital costs ； Low viscosity, high tray efficiency, and fewer actual trays required ； Low toxicity; minimal corrosivity to carbon steel under operating conditions ； Azeotropes are formed with n-butane, butadiene dimer, etc., respectively, which complicates the solvent purification process and increases operating costs ； The vapor pressure is high, resulting in significant loss of solvents being emitted with the exhaust gases ； There are many wash towers used for solvent recovery, resulting in a relatively long process flow.

With the same ACN process but different patents, the function of the wash tower varies. The second floor provides information on some of ACN’s butadiene processes, but Shell’s processes have been improved somewhat.

In the acetonitrile process, the butadiene washing tower is primarily used to wash the crude butadiene coming from the top of the second extraction column (which contains some acetonitrile). However, there are also butadiene production plants using the acetonitrile process that do not have such a washing tower; in these cases, the gaseous crude butadiene coming from the top of the second extraction column goes directly into the distillation system, where distillation is carried out by first removing the heavier components and then the lighter ones

The acetonitrile method for butadiene is basically the same. The so-called small extract gas-phase inlet/exit weight reduction is merely a means of thermal integration.