First Of Its Kind 5,000 tpa furfural plant using SupraYield® technology.
SupraYield® – Award Winning Technology
SupraYield® is a registered trademark for a patented process invented by Dr. K. Zeitsch, developed by International Furan Technology (Pty) Ltd (a member of the DalinYebo Group) and now owned by Proserpine Corporative Sugar Milling Association Limited (PSM Biorefinery – NB: December 2011: purchased by Sucrogen, a subsidiary of Wilmar Sugar Mills). DalinYebo owns the exclusive and global marketing rights.
Development of the SupraYield® Process
The SupraYield® process was developed by Dr. Karl Zeitsch in collaboration with the DalinYebo team at the end of the 1990s. After years of intensive study and experimentation into the chemistry and thermodynamics of the pentosan to furfural reaction. Dr. Zeitsch had noted that the TAPPI analytical procedure (T223 cm-84) for the determination of pentosan in wood relied on achieving a 100% yield in the conversion from pentosan to furfural. He realized that if he could understand and explain the reason for the difference between this perfect yield in the analytical process and the roughly 50% yields achieved in industry he would have the key to designing a significantly better process.
The underlying factor that made this perfect yield attainable in the TAPPI procedure was discovered by Dr. Zeitsch to be the fact that the loss reactions involving furfural could only occur in the hot liquid phase (For a better understanding on the characteristics of furfural, please refer to the article on the furfural chemistry). Once the furfural was in the vapor phase, or once the liquid had been cooled, the loss reactions were halted. In the TAPPI procedure the boil over of water and furfural is so high that the furfural is removed from the liquid phase as fast as it is produced, and then the vapors are rapidly condensed and sub-cooled. On the other hand, in all the industrial processes then existent the furfural was generated in the liquid trapped inside the feedstock particles and it had to diffuse to the surface of the particle where it could be picked up by the stripping steam. The diffusion and stripping steps are slow, thus subjecting the furfural molecules to an extended exposure to possible loss reactions in the hot liquid phase.
The quantities of acid used and the degree of boiling required in the TAPPI process make it totally uneconomic to scale up to an industrial level. Dr. Zeitsch postulated that if he could make use of the very high relative volatility of furfural in water by getting the solution inside the particles to boil he could very rapidly get the furfural to flash into the vapor phase and protect it from the loss reactions. Unfortunately, as a further complication in the industrial processes, the dissolved pentose in the water causes a boiling point elevation of a few degrees, thus making it impossible to boil the solution by injecting saturated steam. Even if superheated steam is introduced into the digester it is rapidly cooled to the saturation temperature and cannot induce boiling. External heating of the digester would cause local hot spots resulting in rapid polymerization and fouling, so this route was also not a viable means of inducing the necessary boiling.
Dr. Zeitsch’s genius was to force the liquid inside the particles to boil by gradually reducing the pressure in the digester. As long as the pressure in the digester could be held slightly below the vapor pressure of the liquid, the solution would continue to boil. As the vapor is flashed from the liquid phase the solution cools and its vapor pressure decreases, making it necessary to keep lowering the digester pressure in a carefully controlled fashion.
One disadvantage of this process is that as the pressure in the digester is reduced, and the temperature decreases, a point is reached where the pentosan to furfural reaction effectively stops. The way that Dr. Zeitsch overcame this was to initially heat the contents of the digester to a high enough temperature such that the furfural would all have been formed and removed by the stage that the temperature was low enough to be a problem. If sufficiently high pressure steam is not available to do this in one step the digester can be re-pressurized and gradually depressurized as before, until no further furfural is removed in the vapor stream.
At very high temperatures and pressures it is found that no acid needs to be added to catalyze the reaction, but this is not of much interest in an industrial context because it would be too expensive to build the digesters and boiler for these pressures. Hydrochloric acid cannot be used because of high corrosion and nitric acid leads to massive losses due to nitration. The only two acids of industrial interest are sulphuric and phosphoric. Phosphoric acid does give better yields than sulphuric acid, but it cannot be justified currently on economic grounds.
The steam consumption in the SupraYield® process is significantly less than in any other process because the removal of the furfural from the particles in the digester is achieved by lowering the pressure to induce boiling rather than by using large quantities of stripping steam. A fortunate side effect of this is that the concentration of the furfural in the product stream from the digesters is much higher than in the traditional processes. This leads to further steam savings in the distillation plant and more importantly, to significantly less effluent. Not only is there less volume of effluent, but the less volatile compounds like acetic acid that are stripped out of the residue by the steam in the traditional plants are carried over to a lesser extent in the SupraYield® process and they remain in the residue where they can be burnt.
The high temperatures used in the SupraYield® process result in very rapid reaction times, and the innovative method of removing the furfural from the solution by boiling it also reduces the time required. The resulting short batch times for this ingenious process result in fewer digesters being required than in the historical processes, leading to capital cost savings. The higher concentration of furfural in the product stream not only leads to the above mentioned steam savings in distillation, but of course also leads to smaller and cheaper distillation columns.
Novel Technology – Proven Designs
During the design phases, we took great care in maintaining all the proven design aspects of a furfural plant and only changed those that were necessary. E.g.: Spherical digesters similar to the ones in use in existing plants are used in Australia and India. The key physical difference is the pressure rating of the digester, i.e. the thickness of the shell and the digester weight. The Feedstock/Residue handling and furfural distillation technology will be the same as used in other plants.
The first commercial SupraYield® plant in Australia was scaled-up from the our pilot plant in South Africa. PSM Biorefinery summarised the risk behind this approach in the their Development Application as follows: “Considering that this process relies on the physical properties of the components, and that these are not affected by the scale of the operation, there are no issues due to scaling up the SupraYield® process from pilot plant to full size”.
In November 2009, Proserpine Sugar announced the first production of furfural from bagasse: “The plant which is highly automated worked well”.