Coflux^® batch reactors

Introduction
Stirred tank reactors are used for many different types of unit operation in the batch process industries. This includes chemical synthesis, polymerisation, crystallisation, fermentation, liquid/liquid extraction, mixing, dissolution, distillation and evaporation etc. Frequently, the same vessel will be used for multiple unit operations with different products. Apart from the benefit of versatility, batch reactors are also ideal for handling difficult materials such as slurries or products with a tendency to foul.

Despite many years of development, considerable scope remains for improving batch reactor performance. Three notable areas in this respect are energy efficiency, temperature control, and process analytics. Over the past 8 years, a UK company (Ashe Morris Technology) have been developing a heating/cooling jacket with a variable geometry heat transfer surface. Referred to as Constant Flux (or Coflux^®), this new design transforms the functional capabilities of batch reactors.

Constant flux (Coflux^®) control
Heat transfer fluid is circulated through the heating/cooling jacket. Two control valves are used to regulate the temperature of the heat transfer fluid by injecting fresh hot or cold fluid as required. Some systems use heat exchangers rather than direct injection.

Coflux^® jackets
One obvious benefit of the variable area jacket is the ability to adjust the jacket height to suit the fill level in the vessel. This is particularly useful for operations like crystallisation, evaporation or distillation.

This type of jacket is more energy efficient since it only uses the minimum amount of heat transfer fluid. This reduces unnecessary pumping energy and reduces cross mixing between the hot and cold (heat transfer fluid) wells on large systems.

Sluggishness and erratic temperature control response is eliminated on large reactors. The jacket heating/cooling power can be altered by varying the heat transfer area. This allows the user to employ maximum ‘gain’ with zero or negligible overshoot. The replacement rate of heat transfer fluid in the jacket is also up to 50 times quicker than a traditional single jacket design. This improves the reaction time on large systems. The use of area as a control parameter also allows the user to maintain a constant jacket temperature. This eliminates the problem of transient hot/cold spots in the jacket.

Perhaps the most compelling benefit of the variable area jacket lies in its heat measuring capabilities. In traditional reactors, accurate heat balance measurement is severely hampered by transient oscillations in jacket temperature (a necessary component of good temperature control). The variable area valve eliminates the need for these oscillations (without impairing temperature control). The jacket area can also be reduced to increase measurement sensitivity for monitoring weak exotherms or endotherms.

Coflux^® Heat balance measurement is an invaluable process monitoring tool. The rate of heat liberation or absorption (power) has a direct relationship to the rate of process change. The total heat liberated or absorbed (enthalpy) reveals the extent of process change. Other parameters can also be derived such as heat transfer coefficient and specific heat. The heat transfer coefficient allows the operator to observe physical changes in the process such as the formation of deposits on the heat transfer surface or changes in viscosity. From the heat transfer coefficient, wall temperatures can also be estimated. This is particularly useful as it allows the operator to employ more extreme jacket temperatures without causing thermal damage to the product.

Heat measurement (calorimetry) is a Process Analytical Technology (PAT) tool which can monitor chemical, physical or biological processes. It can monitor rate of product formation in a chemical reaction, process end points, reactant accumulation, rates of nucleation and crystallisation, product yield, cell population changes, dissolution rates etc. It represents a solution for multiple tasks. Coflux^® calorimetry, which is based on heat balance measurement, is uniquely simple to use. The operator only has to turn on the reactor heating/cooling system to monitor the process.

Conclusion
Increased global competition combined with ever more stringent environmental regulations are placing chemical and pharmaceutical manufacturers under increasing pressure to cut cost and reduce waste. Tackling the sub optimal performance of batch reactors will be a vital part of this process.

The US Food and Drugs Administration have stated that “Continuous improvement and knowledge management tools” are key components to achieving better understanding and control of manufacturing processes. For batch reactors ,this presents a challenge to find robust and flexible Process Analytical tools. Whether for improved process understanding or feedback control, Coflux^® calorimetry provides a simple analytical tool that can monitor most unit operations performed in batch reactors.

For many processes, yield and quality are closely linked to temperature control. Coflux^® jackets set new standards in temperature control. Not only do they eliminate sluggish response and instability in bulk temperature control but they also allow the user to monitor and control wall temperatures and minimize hot or cold spots.

Batch reactors can consume large quantities of energy. The Coflux jacket improves energy efficiency by reducing flow and cross mixing of heat transfer fluid.

The heating/cooling capacity of conventional reactors is constrained by the need to prevent thermal damage at the heat transfer surface. The ability to estimate wall temperatures (as the Coflux^® system can from the power and temperature data) and variable height jacket allow Coflux reactors to be operated safely with more extreme jacket temperatures. This allows the user to operate much closer to the maximum theoretical heating or cooling power of the reactor.