The White Gold
So now we have ammonia. But what can we do with it? One option is to convert it to urea. Urea is an interesting substance. It is an organic chemical, which is also produced in the human body to dispose of nitrogen. It is also the chemical which proved wrong the common belief in the 18th century, that organic substances cannot be created from inorganic raw materials. In 1828, Friedrich Wöhler was the first to synthesize urea from inorganic compounds. At the end of the 19th century, a method was discovered to synthesize urea from ammonia and carbon dioxide. This process is still the basis for modern urea production.
Although urea is still used as fertilizer on a large scale, it is also a raw material for many other industries. Chemicals made from urea include urea formaldehyde, which is used to glue the wooden chips together in chipboard, and melamine, which is used as a finish on chipboard panels. Urea is also used in toothpaste and chewing gum (I bet you didn't know that!) and some pharmaceuticals.
Urea is synthesized from ammonia and carbon dioxide. Since carbon dioxide is a waste material from ammonia plants and carbon dioxide is expensive to transport, it is convenient to build ammonia and urea plants on the same location. The two raw materials react in a high pressure reactor. The reaction takes place in two stages in the same reactor. First, an intermediate is formed, called ammonium carbamate. If the mixture remains in the reactor long enough, the second reaction takes place: ammonium carbamate splits into water and urea. Since Murphy Law is always present, the ammonium carbamate is only partly converted, so at the top of the reactor we have a liquid containing urea, ammonium carbamate, water and ammonia. Ammonia? Yes! By adding more ammonia to the reactor then theoretically necessary, more ammonia carbamate is converted to urea (If you have some degree in chemical engineering you'll understand. For all others: believe me, it's true).
The next step is to separate the urea and water from the remaining ammonia and ammonium carbamate. There are several ways to accomplish this separation. Depending on age and make of a urea plant you will find different techniques. The one described below is somewhat older, but still accurate for a lot of urea plants.
After the liquid mixture leaves the reactor, we reduce the pressure. If we keep the temperature high enough during this "decompression", the ammonium carbamate will fall apart in ammonia and carbon dioxide. Both ammonia and carbon dioxide are gaseous at this temperature and pressure, so these gasses will "evaporate" from the liquid mixture. This separation process is called decomposition (because the ammonium carbamate decomposes into ammonia and carbon dioxide) and is executed in vessels called decomposers. Usually the pressure is reduced in a few steps in different decomposers. After the last decomposer, all we have left is a mixture of urea and water or, since urea readily dissolves in water, a urea solution.
We'll leave the urea solution for what it is and digress to the gases that leave the decomposers. Since it is a waste of raw materials (not to mention an environmental disaster!) to vent these gases into the atmosphere, they are recovered. In the decomposition section, we have seen that ammonium carbamate decomposes into ammonia and carbon dioxide, due to the low pressure and the high temperature. If we lower the temperature, the reverse reaction takes place: ammonia and carbon dioxide react back to ammonium carbamate. Simultaneously, the ammonium carbamate is absorbed into water. The resulting solution is recycled back to the reactor, where it will be (partly) converted to urea.
OK, back to the urea solution. We only want the urea, so we have to get the water out. Two methods are available: We can evaporate the water or we can crystallize the urea. Evaporation is the most common practice: the solution is heated and the water boils out of the solution. the final result is a urea melt, which still contains a little water (0.5 - 1 %). This method has one major drawback: impurities are not removed (worse: due to the heating process, extra impurities are created), and the end product only contains 98 to 99 % urea.
The other method is crystallization. In that case we do not heat the solution, but we create a vacuum above the solution. The vacuum "pulls" out the water, and the solution is concentrated until there is more urea then can be dissolved in the water: The urea starts to crystallize. If the "slurry" (the mixture of crystals and solution) contains enough crystals, it is sent to centrifuges, where the solution is separated from the crystals. The crystals are dried and the result is a very pure product. In crystalline form the urea purity can be higher then 99.8 %.
Now we come to the final step. Most customers prefer granules instead of a hot urea melt or crystals. Again several techniques are available to achieve this, but the most common process is (still) a process called "prilling". This process takes place in a prill tower, which has some visual resemblance to a water tower (see picture). Before urea can be prilled it has to liquefied. The liquid urea is led to one or more "prillheads" in the top of the tower. These prillheads form droplets, which fall down a long, empty shaft. From the bottom of the tower, huge amounts of air are blown through the same shaft to cool the droplets so much that they will solidify while they are falling and reach the bottom of the tower as granules. These granules (with an average diameter of approximately 1.5 mm) are stored and are sold as bulk (transported in barges or silo trucks) or in bags of 25 to 1000 kg.