Decanter centrifuge and post treatment operation

A decanter centrifuge applies a gravitational force of several thousand G, which reduces the settling time of the particles. It is also favoured to maintain a large G-force, which will result in an improved separation.

The rate at which sedimentation occurs is an important characteristic of the decanter centrifuge separation process. The sedimentation rate is influenced by the particle size, the shapes of the particles, their differential densities and the viscosity between the particles and the liquid. This process characteristic can be improved by utilising flocculating agents. The sedimentation rate is also dependent on the separating factor of the decanter centrifuge, which takes into account the centrifugal force and the gravity in the centrifugal force field.
The exterior bowl and the scroll conveyor rotate at different high speeds. This differential speed between the two is accountable for the sedimentation throughout the decanter centrifuge cylinder. A high differential speed results in a smaller residence time of the cake settlement, so it is necessary to keep the cake thickness to a minimum to avoid impairing the discharge quality. Keeping the cake thickness to a minimum also aids in the improvement of the cake dewatering process. For this reason it is necessary to obtain an optimal differential speed to balance the cake thickness and quality.

The characteristic above all affect the clarity of the liquid output which is dependent on the volumetric throughout rate, where a higher flow rate will result in a poor liquid clarity. Another characteristic that influences the clarity of the liquid output is the differential speed. A low differential speed results in a better clarity therefore aiding in the separation process. The G-Force also plays a role in the clarity of the liquid discharge. Higher G-force results in an increase in the separation of the solid particles from the liquid and yields a better clarity.

The production of a waste stream is small in comparison to the overall process output; however can still pose a number of significant problems. Firstly, the volume of waste in the process reduces the available volume to be used for the process. Direct disposal into the environment of especially oil wastes can be detrimental to the surroundings if a treatment is not applied. The post-treatment system applied to the waste product should depend on the specific treated product required. The objectives of post-treatment can range from achieving a product that can be safely disposed, recycled into the refining process or requires an adequate water phase to be re-used in the process.

The objectives of post-treatment vary between different industries where in order to perform an efficient and economical process; the decanter centrifuge must be tailored to the task at hand. In the food manufacturing industry, decanter centrifuges are utilised in oil extraction machines. An oil extraction machine can process up to fifteen metric tonnes per hour of organic wastes and are found either within the process plant or outdoors if designed for the climate. The waste material enters the inlet chute and is softened into a sludge which is then steam heated. This mixture then enters a three-phase decanter centrifuge, also known as a tricanter centrifuge.

A tricanter centrifuge operates on a similar principle to decanter centrifuges, but instead separates three phases, consisting of a suspended solids phase and two immiscible liquids. Sedimentation of the suspended solids occurs as normal where they accumulate on the wall of the bowl and are conveyed out of the centrifuge. The two liquid phases are separated using a dual discharge system where the lighter liquid phase such as oil, is separated over a ring dam via gravity, and water, which is commonly the heavier liquid phase, is discharged using a stationary impeller under pressure. Each of the three components, solid, oil and water, are distributed to different storage tanks.


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