(674d) Simultaneous Reaction and Separation

Chemical engineering rests on two foundations: reaction engineering and unit operations. These foundations, run sequentially, are the key to the low-cost production of commodity chemicals in large, continuously operating, dedicated chemical plants. For example, for the manufacture of ammonia, methane is burnt in air to produce a mixture of hydrogen, nitrogen, and carbon monoxide. This mixture is treated with the water gas shift reaction to produce more hydrogen and carbon dioxide. After the CO₂ is carefully separated, the gases are reacted in the Haber process at 180 bar and 400°C to produce ammonia. Because the kinetics is slow, conversion is only 20 percent so a huge separation and recycle are required. These steps are sequential.

On this centennial of the AIChE, we ask whether this basic sequence of reaction, separation, and recycle will continue in the next century. Our pause is fueled by the limitations of fossil fuels and by the need to use renewable energy sources, like wind. For example, we wonder how we can make ammonia on a farm. We would use electricity from a windmill to make hydrogen by electrolysis of water, and to make nitrogen by the membrane separation of air. We will then feed the hydrogen and nitrogen to a Haber process reactor. Now our reactor's feed will not depend on fossil fuels and will not need the complex separations currently necessary to remove carbon oxides.

Moreover, we show in this paper that we can increase conversion in the Haber process reactor from 20 percent to 90 percent by carrying out reaction and adsorption simultaneously. Thus we are relaxing the past mantra of reaction-then-separation; we are carrying out the two steps simultaneously, as reaction-and-adsorption. Such possible chemical processes can help us to explore if the chemical industry of the future will continue to be a few large plants or whether it will involve many much smaller facilities. Such processes explore whether chemical manufacture will remain centralized or whether it may become at least partly distributed.