Centrifugal approach

Steve Drewery, managing director of Cinc UK, highlights the use of a new centrifuge designed to speed up the liquid/liquid separation of pharmaceutical products

Engineers faced with the task of designing and installing a new process involving two liquid extractions at a well established UK pharmaceutical company successfully incorporated a series of liquid/liquid separators to enhance and simplify their existing batch process which used columns.The technology, introduced by US company Cinc, is a patented new approach to real-time liquid processing using an annular centrifugal contactor device that can be used as both a separator and a contactor. As it performs both functions, the centrifuge is a valuable tool in numerous industrial applications requiring the separation, extraction, reaction or washing of two liquid phases. Its unique design provides mixing and separation in a single, compact unit.

Figure 1 shows a cutaway view of the centrifuge housing and rotor and details the significant design features, including the liquid flowpath.

mixed phases

Conventional methods of two-phase liquid/liquid separation using gravity are slow, inefficient and offer poor process control. The Cinc centrifuge is a decanter style for liquid processing. It is designed to separate or, indeed, mix two immiscible liquids, and can be used as an alternative to disk stack centrifuges, separation columns, gravity-based decanters, coalescent or DAF type separators, mixer/settler or extractor systems.

Two immiscible liquids of different densities are fed to the separate inlets and are rapidly mixed in the annular space between the spinning rotor and stationary housing. The mixed phases are directed toward the centre of the rotor bottom by radial vanes in the housing base. As the liquids enter the rotor, they are accelerated towards the wall. The self-pumping rotor is divided into four vertical chambers which are dynamically balanced by the pumped liquids.

The mixed phases are rapidly accelerated to rotor speed once trapped in a quadrant, and separation begins as the liquids are displaced upward by continued pumping. The separating zone extends from the diverter disk to the lighter phase weir, which provides a transit time for the liquid/liquid interface to form and sharpen. The interface should be positioned half way between the lighter phase weir and the heavier phase underflow at the top of the separating zone by selecting the proper heavy phase weir ring.

Performance is optimised despite changes in flow rate or liquid ratios as the interface position can shift a significant distance without loss of separation quality. Equilibration of pressure between the centrifuge housing, discharge pipes, and receiver tanks ensures trouble-free operation over a wide range of process conditions.

The technology delivers a clean liquid phase resulting in increased yield, less waste generation, less organic solvent use and improved product quality. The speed of separations can be increased resulting in lower cycle times and more efficient use of plant and equipment. The small centrifuge volume improves control by reducing the total quantity of liquid in process.

The design also allows multiple units to be connected in series without intermediate pumps for processes that require multi-stage counter-current extraction or washing. A counter-current process with two or more centrifuges will improve the efficiency of a batch process without increasing the volume of solvent or wash water.

The technology offered the perfect solution for the pharma company's new installation and it commenced with successful testing using a 1.9l/m V2 centrifuge in the company's laboratory. The trials enabled the process to be designed using V10 models (113.5l/min) in a counter-current mode. The system was installed in 20 linear feet of space, and replaced six hours of interrupted, manual separations with a process that takes one hour per batch.

CIP benefits

Another benefit was a CIP option, which uses a hollow through-shaft that starts below the bottom plate of the housing and extends into the upper rotor assembly. It is equipped with a series of high-pressure spray nozzles for each quadrant. These nozzles provide coverage of the internal wall of the rotor, the aqueous underflow, and the upper rotor assembly.

A rotary union that is permanently attached to the tail shaft provides the inlet for the desired cleaning solution and allows the process to be fully automated. The steps for cleaning are quite simple. The feed to the centrifuge is halted and the rotor stopped, which drains the holdup volume into the annulus.

Next, draining the liquid from the centrifuge exposes all the internal rotor surfaces to the cleaning solution spray. Cleaning solution is then pumped to the centrifuge via the rotary union until the unit is clean. After cleaning, the process is reversed and the centrifuge is put back in service.

no interruptions

The total operation is performed in minutes without disassembly and when multiple units are set up in parallel to handle a continuous process, sequential cleaning can be used to avoid flow interruptions.

A new design for the bottom seal and bearing have been incorporated into a single replaceable exchange cartridge assembly, which allows the bottom seal and bearing to be replaced in minutes, providing minimal down time and low-cost maintenance. The units were supplied with explosion proof motors, vibration sensors and electropolish on all wet surfaces.