The key components of the Green Fuel plant were sourced and imported from Brazil, a country considered to have the world’s first sustainable bio-fuels economy and over 30 years of experience in ethanol production. However, 60% of the plant was manufactured in Zimbabwe by our local engineering team under close supervision by experts from Brazil, ensuring local skills development.
Our modern plant uses the latest technology to produce high-grade anhydrous ethanol. A high pressure “Caldema” boiler increases energy recovery allowing the ethanol plant to be energetically self-sufficient and produce excess electricity.
Ethanol is produced in stages – cane preparation, milling, fermentation of sugars, distillation and dehydration.
The sugarcane arrives on rigid in-field trucks to the mill. The cane is billeted by the mechanical harvesters in the field and therefore arrives already partially prepared for milling. The cane is fresh as it reaches the factory less than 2 hours after it has been harvested (no double handling and contamination). The cane truck goes through a weighbridge and the sugarcane is then offloaded onto a feeder table.
As the cane is billeted by a mechanical harvested, cane preparation is minimised. Only a shredder is needed and we aim for a preparation index of 87% before milling.
The milling section consists of one 64 inch mill on an electric drive and four 54inch steam driven mills and we have plans to add more mills in the near future. The shredded cane from the cane preparation section is fed to the first mill where it is squeezed between the rollers at a very high compression. The juice (sugars and moisture) is extracted and the bagasse is fed to the next consecutive mill and so on.
Hot imbibition water is added in a countercurrent manner, this is essentially to try and extract the sugars by dissolution. The clean imbibition water is pumped into the fourth mill first, which has the lowest concentration of sugar. Once the juice has been extracted and collected from this mill, the water and juice are then pumped to mill number three and so on.
The boiler uses the bagasse from the mills as fuel. The boiler produces steam at 45 bar and de-superheated steam @ 450 degrees Celsius. The steam is used to drive the mill turbines and the Turbo Alternator (TA) which produces the ‘green’ sustainable electricity required to run the factory. The exhaust steam from these turbines is then used in the distillery process. Any additional steam requirements by the evaporator come from let-down stations. The exhaust steam is at approximately 1.8 bar @ 130 degrees Celsius. The high-pressure caldema boiler is very efficient and uses the latest technology.
Surplus bagasse can then go into animal feed, paper, chipboard and energy briquettes or electricity generation for re-sale.
The juice is then pumped from the mixed juice tank in the distillery to the evaporator.
The evaporator serves three purposes, firstly to condense the exhaust steam to return to the boiler, secondly to concentrate the mixed juice from a sugar percentage of approximately 14 Brix to a sugar concentration of approximately 22 Brix for feeding to the fermenters (referred to as mash or syrup) and thirdly to produce process steam V1 for use in the distillery.
Once concentrated, the mash or syrup it is then fed to a buffer tank.
From the buffer tank, most of the mash is then fed to the fermenters along with a dose of yeast.
This occurs when the yeast ferments the sugars in the mash producing ethanol and carbon dioxide, along with small amounts of other intermediary products. This alcohol rich solution is referred to as beer. The fermentation is done in a batch process in order to give the yeast the longest possible time period in which to ferment the sugars. The cycle takes 48hrs to complete, yielding beer with approximately 10% Alc. The beer is then heated before it is sent to the Hydrous (IA) Section of the distillery.
Hydrous (IA) Section
From the fermenters, the beer is then fed to the hydrous section. It is firstly fed into the stripper column which removes the solids from the solution. The byproduct of this process is called Vinasse which is then pumped into the dams on the farm to be utilized as fertilizer for the sugarcane.
From the stripper, the intermediary solution, which is higher in alcohol than the beer and without yeast and other solids, is sent to the rectifying column. The rectifying column removes most of the water from the ethanol.
Ethanol forms an azeotrope when mixed with approximately 4% water, which means the mixture of 4% water and 96% ethanol has a lower boiling point than either of the pure products. This, in turn, means that it is not possible to reach anhydrous ethanol by standard distillation methods. Accordingly, this IA product (i.e. hydrous ethanol) is then sent to a recycle tank in the tank farm. This tank acts as a buffer between the hydrous and anhydrous distillation sections.
Anhydrous
The IA is then pumped from the recycle tank to the anhydrous section of the distillery where cyclohexane is added to it. The ternary mixture of water and cyclohexane has a boiling point of approximately 62 degrees Celsius, which is lower than the boiling point of the anhydrous ethanol. This means that the anhydrous or absolute ethanol product comes out from the bottom of the anhydrous column and is pumped to the storage tanks. The ternary mixture is then separated and sent to the recovery column to remove the water from the cyclohexane and recycle the cyclohexane back to the anhydrous column.
For the ethanol to be used as a vehicle fuel, it is important that water is removed, and this is achieved through distillation. The distillation is a series of columns where the beer is turned into alcohol. The hydrous section first – turned into 96% alcohol/ethanol – this is called hydrous ethanol and is known as industrial grade or drinking-strenght alcohol. This has 4% water content which is not good for blending with fuel so we do oen more step to take the 4% of water out. This results in Anhydrous ethanol which has a water conent of less than 0.4% and a purity level of 99.6% – this is ideal for fuel blending.
When ready for dispatch, the anhydrous ethanol is pumped from the storage tanks to the loading bay where it is denatured with petrol and loaded into tankers. A corrosion inhibitor is also added to the mixture. The quality of the ethanol is checked, the volumes and quality are recorded and the tanker is dispatched.
