Brown process flow

Attention. . . There’s very little material on the forum that specifically pertains to the Brown process~! ~! ~! ~!

The energy-saving measures taken by the U.S.-based Brown Company mainly involve reducing the consumption of fuel natural gas. Specifically, this is achieved by decreasing the load on the primary reformer (increasing the outlet CH4 content from the original 10% to around 30%), while increasing the load on the secondary reformer and introducing excess air into it (which generates a large amount of reaction heat to provide the necessary heat for the conversion of residual CH4). As a result, the temperature in the primary reformer decreases, leading to a reduction in fuel natural gas consumption. Meanwhile, cryogenic purification is used to remove excess nitrogen, and a gas turbine drives the air compressor; the energy consumption per ton of ammonia is 28.4 GJ. In China, a low water-carbon ratio energy-saving technology has been applied to the primary converter of the imported Brown plant. For ammonia synthesis, a process featuring three towers and three waste heat boiler circuits is employed; this utilizes waste heat to generate high-pressure steam, thereby further reducing energy consumption.

Cryogenic purification is the core technology of the Brown process. The cryogenic purification unit consists of two sets of plate-fin heat exchangers, one oil-braked turboexpander, and a distillation column equipped with a top-mounted reboiler-condenser. In the cryogenic purification unit, the boiling point differences among the various components in the raw syngas are utilized to cause them to condense at low temperatures and then undergo separation and rectification. As a result, excess nitrogen, residual methane, and approximately half of the CO and Ar present in the raw syngas are removed as waste gases. This process ensures that the hydrogen-to-nitrogen ratio in the syngas becomes 3:1. Part of the waste gas is heated and used as regeneration gas for the molecular sieve, while the remainder serves as fuel gas for the primary reformer. After cryogenic treatment, the H2 and N2 content in the raw gas reaches 99.75%, which **improves the quality of the raw gas** compared to other processes. The process is as follows: The gas at 4.4°C exiting from the molecular sieve dryer (H2:N2 ≈ 2:1) is cooled to -129°C in a cryogenic purification unit by exchanging heat with purified syngas and waste gas. It then undergoes expansion via an expansion turbine and a throttle valve. Due to isentropic expansion and pressure reduction, the gas converts its potential energy into the cooling capacity required by the purifier, thereby cooling it further to -132°C. The expanded gas is subsequently cooled to -175°C in a heat exchanger before entering the purified gas distillation column. The power extracted from the expander is varied as needed to control the liquid level at the bottom of the distillation column. The liquid coming out from the bottom of the distillation column is depressurized through a throttle valve and then sent to the shell side of the top cooler of the distillation column as a cooling medium. This cools the materials at the top of the distillation column and provides reflux liquid for the column. The bottom product of the distillation column mainly consists of excess nitrogen from the secondary reformer, methane in the gas, about 60% hydrogen, and about 50% remaining carbon monoxide. The partially vaporized liquid leaving the condenser is heat-exchanged with the purifier feed, then reheated and vaporized, and finally exits as waste gas. Part of the waste gas serves as regenerant gas for the dryer, and then it, along with the remaining waste gas, goes to the reformer to be used as fuel. The purified syngas is heated to 2 °C after heat exchange for ammonia synthesis. The hydrogen-nitrogen ratio of the purified gas is precisely controlled at 3:1 by an online hydrogen analyzer. As a flexible link between the front-end units (gas generation and decarburization) and the subsequent synthesis units, the cryogenic purification unit can absorb any fluctuations—such as an increase in nitrogen content caused by excessive air supply to the secondary furnace at the front end of the system, or an increase in methane content due to higher CO2 levels at the decarburization outlet. These fluctuations can be mitigated by adjusting the operating conditions of the expander within the cryogenic purification unit to increase its cooling capacity. This ensures that the composition of the gas exiting the purification unit remains stable, without affecting the normal operation of the synthesis units.