Regarding the sulphur manufacturing process, prilling molten sulphur has been done in the past. However, due to some unfortunate events, prilling sulfur was discontinued in the industry. Recent developments have changed this perception, and the possibility of prilling sulfur is being looked at once again as a possible high throughput alternative to other sulphur finishing processes. The goal here is to educate the reader on the current state of sulphur forming technologies and where prilling fits in as an alternative.
The Discovery of Prilling
Back to Where it All Began
The inventor of the prilling method was a plumber from the UK named William Watts. In 1782, he discovered that by letting molten lead flow through a small orifice and fall from an elevated height, that on the way down, the lead drops would solidify and form spherical pellets. These pellets would be similar in size and more spherical than most of the current production methods at the time.
The basic principle behind the prilling process has not changed much since William Watts discovered it in 1782. The main differences are that today a prilling machine is used where large amounts of liquid jets are produced by forcing the melt through a nozzle which then breaks up into small droplets. This sulfur solidification unit or sulfur forming unit provides better control of the jet break-up and in turn better quality and size of the product. Additionally, air can flow through the tower and dust can be collected by filter installations or with the use of scrubbers making operation safer.
The main markets for prilling are the fertilizer market and plastics finishing. Prilling is the preferred finishing method when high throughput and stable operation is required. Prilling also offers flexible operation and narrow particle size distribution (PSD) compared to other finishing methods. The marketplace for prilling is expanding as the technology matures and new innovations are realized.
Prilling of Sulfur
In the world of sulfur, converting molten sulfur into large quantities of solid particles has always been a challenge. Crushing a solid block of sulphur into smaller chunks was utilized as the main method of obtaining transportable sulfur pieces. Due to considerable dust formations while handling solid sulfur pieces in this form, the industry started looking for new technologies to form their product.
One of the adapted technologies was air prilling, which was already utilized by the fertilizer industry. The finished product from these prilling facilities were large quantities of small yet perfectly spherical pellets with a polished surface and a relatively narrow PSD, leading to a free-flowing product. At this point, when prilling was the most dominant method of producing solid sulfur, the well-known SUDIC specification (Sulphur Development Institute of Canada) was also made, leading to a market standard for all sulphur technologies.
Nevertheless, the industry developed new ways of production, such as sulfur granulation process, (steel belt) sulphur pastillation and wet prilled sulphur, which took over the role of being the most dominant method of particle engineering in the sulfur industry.
Two Main Concerns
This shift in production method was the result of two main concerns. Firstly, the prilling towers designed in the ‘60s operated on an open-air design. This means that all cooling air used for solidifying the prills is immediately ejected into the environment. All the dust particles formed during the prilling process, but also any vapors from the sulfur melt, is taken with the air stream into the environment. This led to environmental problems in the downwind areas of the prilling plant.
Secondly, a disastrous fire incident in the Middle East led to the conclusion that dry prilling in itself was to be considered a high-risk technology. The low ignition energy, combined with the potential buildup of static charge in the solidifying prills, can lead to dangerous sparks. The combination of these two aspects, coupled with a hot climate, led to the disastrous incident in the Middle East.
Safer and More Sustainable Future
In the last few years, prilling has been under development to handle a wide variety of products. The addition of an air treatment section was applied in the late 80s, where either a dry filter or wet scrubber is utilized to drastically cut down on (dust) emissions from prilling towers. However, as the environmental impact of all industrial plants on their surrounding became more apparent, the laws became more and more stringent on the prilling production. This resulted in a surge of new research on prilling, with a closed-loop process as one of the main achievements.
Closing the Loop
For the closed-loop process, the used cooling medium (in this case air) is first led to an air treatment section, and afterwards to a heat exchanger. The resulting cleaned and cooled air can be reused in the tower. The main benefit of closing the loop is the fact that emissions are reduced to zero. Next to that, the cumulated heat removed from the cooling medium can be reused somewhere else in the plant, leading to a higher degree of heat integration of the total plant.
Safely Handling Sulfur
Prilling is already widely applied in other industries, where the same hazards with regard to fire and dust explosions exists, mainly near the air treatment section. When applying the closed-loop principle, all of the cooling medium is recycled, removing the necessity of having to use ambient air as a cooling medium. Practically any gas can be chosen because the waste of the cooling medium will be very limited. Currently, the first prilling towers with the inert nitrogen gas as a cooling medium have started to emerge, leading to an intrinsically safe method of prilling with no emissions and low fire hazards.
These prilling towers form a safe and high capacity production method of converting melt into a preferable finishing product. These innovations are showing promise in the field of sulphur as well. It is already empirically proven that sulfur prills can be formed and that they have a range of benefits over other sulphur solidification technologies. The main challenge now is to alter the closed-loop prilling system as is used in the world of plastics and fertilizers prilling, towards a system that can safely handle sulfur.