Ammonia Synthesis at Low Pressure

The overall goal of this procedure is to produce ammonia from air, water and wind-generated energy in a low-pressure process with negligible carbon dioxide emissions. This method can help address key issues in ammonia production such as reaction efficiency, product distribution and carbon dioxide emissions. The process uses a solid ammonia-selected absorbent rather than a condenser.

Continually removing ammonia from the system decreases the occurrence of the reverse chemical reaction. The main advantage of this process is that it can performed at lower pressure which is safer and less expensive. Cory Marquart and I operate a plant for the conventional process that is 30, 000 times smaller than typical plants.

This gives a standard for comparison with the absorbent process. Mike Reese and Cory Marquart are operating the conventional small-scale plant in Morris, Minnesota. Demonstrating the absorption-based process will be Mahdi Malmali, a post-doctoral fellow affiliated with our laboratory.

Prior to beginning the process, start up the ammonia generation plant and ramp the catalytic reactor to 440 degrees Celsius. Set the condenser temperature to minus 18 degrees Celsius. Next, turn on the air dryer, the air compressor and the nitrogen generator.

Turn on the nitrogen gas booster to fill the nitrogen supply tanks. Turn on the chiller, water deionization unit, electrolyzer and the hydrogen gas booster. Verify that the chiller is operational and cooling liquid is flowing through.

Next set the nitrogen inlet regulator to 300 PSI. Open the nitrogen valve to begin pressurizing the process skid. Close the nitrogen bypass valve when the skid reaches 300 PSI.

Then, set the hydrogen inlet regulator to 1, 200 PSI and open the hydrogen valve. Once the skid reaches 1, 200 PSI, close the hydrogen valve. Start the skid to begin cycling the one-to-three nitrogen-to-hydrogen gas mixture through the system.

After the process has finished, store the condensed ammonia at 150 PSI. To prepare the reactor, use a mortar and pestle to grind the catalyst particles to less than one millimeter in size. Pack the ground catalyst into 0.25-inch tubing.

To prepare the absorber, pack 80 grams of calcium chloride or an equivalent amount of another ammonia selective absorbent into a 30-centimeter column with an inner diameter of 2.3 centimeters. Plug the ends of the column with sufficient packing support material to immobilize the absorbent. Install the absorber and the reactor in the experimental apparatus.

Heat the reactor to 450 degrees Celsius using the recommended activation temperature procedure while flowing hydrogen through the reactor at 500 SCCM to finish the catalyst activation. Keep the activated catalyst under a blanket of ultra-pure nitrogen gas when the system is idle. To begin the ammonia generation process, close the outlet valve and ramp the reactor to 400 degrees Celsius and the absorber to 180 degrees Celsius.

Flush the system with ultra-pure nitrogen gas by repeatedly pressurizing and venting the system. Then, pressurize the system with a one-to-three mixture of nitrogen and hydrogen gases. Stop the nitrogen and hydrogen mass flow controllers.

Close the reactor inlet valve. Open the reactor outlet valve and start the recirculation pump. Monitor the reactor and absorber temperatures as the reaction starts and adjust them as necessary to compensate for exothermic processes.

Run the process for five hours or until the absorber starts to break through as indicated by a decrease in the change in system pressure over time. Open the nitrogen inlet valve and the ammonia outlet valve and reduce the system to atmospheric pressure. Flow nitrogen through the system at 100 SCCM while heating the absorber for five hours to release the ammonia.

The high pressures used in the conventional method are necessary to favor the forward reaction at high temperatures. In the low-pressure separation method, the ammonia is more completely separated before it could undergo the reverse reaction. A significant increase in the rate of conversion was observed relative to the conventional method.

The initial reaction rates of the separation method were consistent with published values and separate chemical kinetics experiments. Temperature variation was observed in the initial reaction rates of the reaction absorption method. The overall reaction rate showed much less temperature dependence indicating that the catalytic reaction rate is not a major limiting factor for the conversion rate in the separation method.

The absorbent efficiently removed the ammonia from the system, but the capacity of the column decreased after each cycle. Investigation into prolonging the life of the absorbent column is underway. The absorption kinetics were not found to be a significantly limiting factor for ammonia production.

The recirculation flow rate was found to correlate with the reaction rate indicating that it is the primary controlling step for the process. This process can make ammonia from air, water and wind. The air and water are sources of nitrogen and hydrogen by pressure-swing absorption and electrolysis respectively.

The energy for the process comes from the wind. The Haber-Bosch process is a century-old reaction that feeds a large fraction of the world's population. Trying to improve it has high risk and potentially high reward.

Our original idea for this work was to make family farms in rural Minnesota more self-sufficient for their nitrogen fertilizer. The promise of a fuel for energy-short cities by storing wind-based energy as liquid ammonia was an unexpected gain. The key to the absorption-based process is to develop a stable reusable absorbent that efficiently traps ammonia as it is produced.

By replacing the conventional condenser with an absorbent bed, we are closer to a cheaper, safer ammonia generation process suitable for small-scale manufacturing.