New Continuous Precipitation / Crystallizations Systems

Figure 1 shows a Continuous Precipitation System designed to take advantage of continuous operation to process a batch of material, yet allowing enough flexibility for the same equipment to be applicable in a number of purification schemes. The Traditional Tank Precipitation flowsheet is based on a "batch" tank in order to facilitate the precipitation. Unlike a normal batch system, Feed and Precipitant addition are continuous along with removal of the resulting slurry out the bottom of the vessel. Unlike a batch system in which you would be concerned about tank volume and mixing time, this continuous system is designed around the length of time a particle of solution remains in the precipitation vessel. This residence time is the volume of the vessel divided by the flowrate through the vessel, and must be long enough for the precipitation to take place. Another key aspect of this continuous system is an efficient solid-liquid separator that can operate continuously with the rest of the system. This separator could be in the form of a continuous centrifuge or a continuous cake filtration device. After the solids and liquids are separated, the solvent can be removed or partially recycled back to the Precipitation Tank if more solute removal is possible. As with any continuous system, the mass of material being added to the system must be equal to that leaving the system, thus some solvent purge from the system is always required.

The bottom part of Figure 1 shows a variation of this precipitation system. In this version the Precipitation Tank is replaced with a static mixer to facilitate contact between the Feed and the Precipitant Addition. This system is more compact and efficient if the slurry being formed can flow though the mixer without plugging it. The residence time of liquid in the mixer has the same constraints as that of the Precipitation Tank.

Figure 2 shows a similar continuous system, this time concentrating on crystallization instead of precipitation. As mentioned earlier, crystallization uses evaporation and/or cooling to produce the supersaturation needed to remove the solute from solution. In the case of the continuous system outlined here only cooling was considered. The other methods, isothermal evaporation and adiabatic cooling, would require extra equipment to facilitate heat input and vapor handling that may prove too complicated and expensive for a system designed for flexible and economical operation on a small scale. Based on the same ideas as the continuous precipitation system, the crystallization system uses a cooling jacket on the Crystallization Tank to create a supersaturated solution. Again the residence time of the solution flowing through the tank is critical, and can be adjusted by the flows in and out of the tank. The rate of cooling here essentially replaces the Precipitant Addition in the previous example. Again, an effective solid-liquid separation is need and a recycle is possible to maximize the removal of solute from the solution. One alternative to this system seen in Figure 2 is the addition of an external cooler in place of a cooling jacket. This external cooler can add more controlled and uniform cooling, as long as crystallization does not occur in the cooler itself facilitating its fouling and/or plugging.

Steadfast Equipment supplies two basic components for the Disposable Rotary Drum Filter, the Filter itself which is a single use disposable unit, and the Drive and Holder assembly, which is reusable.