Increasing the profit of a decaffeination plant by solvent exctraction system
Increasing the profit of a decaffeination plant by solvent exctraction system
In Italia il consumo di caffé decaffeinato ha fatto registrare negli ultimi anni un trend positivo di crescita spinto dalla domanda di prodotti salutistici, attestandosi al 7% del mercato totale di caffè.
Producers must therefore build decaffeination plants in order to meet market demands.
Given the growth prospects and processing volumes, it is advisable to integrate production lines with caffeine extraction plants designed to recover caffeine from decaffeination wastewater.
The crude caffeine thus obtained can be resold to the food and pharmaceutical industries for high value-added preparations, increasing revenues and reducing overall plant operating costs.
Among extraction technologies, the one best suited for caffeine recovery is solvent extraction (methylene chloride).
These plants are fed with wastewater from the decaffeination process and consist of a liquid-liquid extraction column: a small-diameter impeller (80 mm) and a cell assembly, i.e., the volume enclosed between two perforated plates.
Inside the central compartment of the column, within which the impeller rotates, the rotating impeller draws in the liquid present in the cell from both below and above.
The two streams mix as they leave the impeller.
By modifying the dimensional parameters of the cell and the rotational speed, it is possible to adjust:
- the degree of mixing within the cell
- he shear effect that reduces the size of the droplets of the dispersed phase within the other phase
The efficiency of the plant is also influenced by the selection of the hole diameter of the perforated plates; this varies according to certain characteristics of the substances forming the mixture to be separated:
- the difference in specific gravities
- the viscosity values
- the rheological behavior of the components.
During the extraction process, several phases of the same mixture can be identified inside the column:
- Continuous phase, the one that “fills” the volume of the column between the two phases.
It is usually the stream with the higher specific gravity.
- Dispersed phase, the one dispersed by the impellers into the bulk, forming droplets of variable size that are statistically smaller than the diameter of the holes in the discs enclosing each individual stage (see adjacent photo).
To design an efficient caffeine recovery plant based on solvent extraction, certain parameters must be available:
- Specific Flow Rate, expressed as the quantity flowing through the (free) cross-section of a column per unit of time (kg/m²/h).
- Theoretical Stage, a volume expressed in real terms representing the minimum value sufficient to carry out a physical or physico-chemical operation with an efficiency equal to 1 (m³, liters, etc.).
Since this is a mass transfer process, the path length also plays an important role. The specific flow rate of the substances determines the efficiency results. - Distribution Coefficient. This is the parameter that characterizes the distribution of the solute between the two phases. It is the value used during the design phase to determine the number of stages in the column. In practical conditions, the value of the Distribution Coefficient varies with relative concentrations; therefore, each compartment is characterized by the value corresponding to its average equilibrium condition.
Before starting plant construction, it is highly advisable during the design phase to simulate the column operation in the laboratory in order to determine the Distribution Coefficient and the extraction efficiency.