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Environment

Better Control of Batch Reactors

New heat-transfer technology takes temperature control of batch reactors to unprecedented level

by Michael Freemantle
November 21, 2005 | A version of this story appeared in Volume 83, Issue 47

Evaluation
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Credit: Photo by Michael Freemantle
Morris tests a constant-flux temperature control demonstration unit.
Credit: Photo by Michael Freemantle
Morris tests a constant-flux temperature control demonstration unit.

A new heat-transfer technology enables better control of temperature in continuously stirred tank reactors than is possible with existing methods. Known as constant flux or Coflux, the technology promises many benefits for the fine and specialty chemicals and pharmaceutical industries, including better yields and selectivities and reduced waste from processes involving complex chemistries. It is applicable to all scales of production, from the bench to industrial scale. It can also be used for process monitoring.

The technology was developed and patented by Ashe Morris, a company based in Hertfordshire, England, that was founded in 2000 by chemical engineer Robert Ashe and electrical engineer David Morris.

Batch reactors have been and continue to be the workhorses of the pharmaceutical and fine and specialty chemicals industries. For more than 100 years, cooling or heating of continuously stirred tank reactors has relied on regulating the temperature of heat-transfer fluid in the reactor's jacket by adjusting its mass flow rate or its delivery temperature, Ashe says. Coflux controls temperature by varying the area available for heat exchange.

The principle is similar to heating or cooling the contents of a flask by lowering or raising it in a water bath. As the flask is lowered and then raised, the effective heat- transfer area increases and then decreases.

The technology employs a bank of multiple but independently controlled heat-transfer copper pipes instead of a single large jacket. The pipes are fixed to flat copper bands wrapped around the vessel and are connected independently to vertical feed and return pipes. Lowering or raising a piston in the feed or return pipe varies the heat-transfer area.

The technology is interesting because it is noninvasive; the reactor internals remain the same as for a traditional process, says Nilay Shah, professor of process systems engineering at Imperial College London and a nonexecutive director of Ashe Morris.

In traditional jacketed reactors, the temperature of the contents cannot be adjusted rapidly because of thermal inertia, Shah notes. The thermal inertia of the Coflux system, on the other hand, is much lower, he says. That and the fact that the heat-transfer area is varied mechanically enable the system to respond faster and to achieve better temperature control, he explains.

By measuring the flow rate of the heat-transfer fluid and the temperatures of the heat-transfer fluid in the feed and return pipes, the Coflux reactor can also be used as a precision calorimeter. As a process proceeds, the heat evolved or absorbed can be continually calculated online, and the rate and progress of changes can be monitored in real time.

Online calorimetry is an important monitoring tool for almost any chemical, physical, or biological process. Batch processes can be inherently inefficient because they are controlled according to fixed recipes that often are not optimized and do not take into account conditions that can vary from batch to batch, Morris explains. Constant-flux technology with online calorimetry enables process efficiency to be optimized.

Advance
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Credit: Courtesy of Ashe Morris
A batch reactor with constant-flux temperature control is jacketed by a bank of independently controlled copper pipes. A temperature control device (TC) moves a piston in the heat-transfer-fluid (blue) return pipe to control process temperature (Tp). As the piston rises, more and more of the copper pipes are brought into play, and the heat-transfer area increases. When the piston lowers, the area decreases.
Credit: Courtesy of Ashe Morris
A batch reactor with constant-flux temperature control is jacketed by a bank of independently controlled copper pipes. A temperature control device (TC) moves a piston in the heat-transfer-fluid (blue) return pipe to control process temperature (Tp). As the piston rises, more and more of the copper pipes are brought into play, and the heat-transfer area increases. When the piston lowers, the area decreases.

By allowing operators to control the accumulation of reactants as they are fed into the vessel and to detect process end points, the technology can improve capital productivity and reduce the cost of materials, Ashe says. The operator can also identify problems as they arise and take preventative action when necessary, he adds. For example, hazardous accumulations and runaway reactions can be avoided by early detection of overheating and rapid correction by fast cooling.

Ashe started developing the idea of constant-flux heat transfer in 1990 when he was investigating ways of measuring heat changes in large systems. I came to the conclusion that a fundamental reappraisal of classical temperature control methods was needed, he says. His solution was variable-area heat-transfer surfaces made from small, incremental surfaces. Calculations suggested that incredible improvements in heat measurements are possible at any scale if area is used as a control parameter. Other benefits emerged; for example, better temperature control.

After about 10 years' work on the technology, Ashe joined forces with Morris, an expert in control systems and plant design. Together they filed patents and developed a prototype design for the constant-flux reactor. In 2001, Richard Barker, an engineer with commercial experience, joined the company as managing director.

Construction of the prototype began in 2002, Barker says. In June 2003, we carried out tests on a 10-L prototype and showed that heating and cooling responses were very fast and that we could control the process temperature to better than 0.1 oC.

He points to specific benefits of the design. For one, a higher heat-transfer-fluid temperature can be used without causing the process temperature to overshoot. The result is faster and more stable temperature control than is achieved in standard reactors.

The design also prevents thermal damage to heat-sensitive products in the splash zone above the liquid surface, Barker adds. That's because the heat-transfer area can be limited to the liquid-covered surfaces inside the vessel.

Constant-flux reactors are also more energy efficient than traditional reactors, Barker notes. Calculations show that the rate of energy wastage in cooling a 4,500-L Coflux system is less than 0.1 kW, compared with 1050 kW for a traditional 4,500-L reactor.

Several companies have tested the reaction calorimeter in the field.

Last year, the specialty chemical company Clariant carried out the first field trials at its plant in Horsforth, in England. Clariant monitored power and enthalpy data for a series of test reactions. The company used the data to monitor reaction rates and rates of powder dissolution and to track the progress of processes.

The results were encouraging, according to James Wilson, Clariant's R&D manager at the site. They provided insights into the mechanics of the chemical reactions, he notes. Coflux could be utilized to optimize synthesis processes within Clariant, he says.

Ashe
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Credit: Photo by Michael Freemantle
Credit: Photo by Michael Freemantle

Earlier this year, AstraZeneca became the first pharmaceutical company to undertake field trials of the evaluation unit. The company commissioned tests at its site in Macclesfield, England, to see if the technology could be used as a scalable process-monitoring tool in its primary manufacturing processes. More than 35 tests were carried out to evaluate the accuracy, sensitivity, and usability of the technology.

Barker
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Credit: Photo by Michael Freemantle
Credit: Photo by Michael Freemantle

Coflux met our performance criteria and has proved that it can be a versatile and simple-to-use process-monitoring tool, with potential applications across a range of manufacturing process steps, says Katie Murray, associate principal scientist at AstraZeneca.

Her colleague, Ian McConvey, a process-engineering team leader, comments that Coflux offers practical advantages over more complex tools for process analytical technology because the output data are in the form of fundamental units: joules and watts.

Recently, Contract Chemicals, a manufacturer of fine organic chemicals in Prescot, England, also tested the evaluation unit.

Meanwhile, Syrris, a company in Royston, England, that produces flow reactor products for drug discovery and development, has licensed Coflux to develop and manufacture laboratory-scale reaction calorimeters based on the technology. The calorimeters are supplied to laboratory equipment supplier Radleys, in Saffron Walden, England, for use with its Lara Controlled Laboratory Reactors.

Ashe Morris has also licensed the technology to Powder Systems Ltd. (PSL), a Liverpool-based company that designs and supplies process solutions for the chemical, pharmaceutical, and bioprocess industries. PSL will develop and manufacture Coflux-based reactors at lab- to production-scale capacities, to be marketed as ChemFlux reactors. Andrew Daniel, PSL's ChemFlux sales manager, says Coflux technology will reduce batch-to-batch variability, rejects, and failures.

Online monitoring also fits with the Food & Drug Administration's Process Analytical Technology (PAT) initiative, Daniel adds.

FDA launched the PAT initiative in 2002 to encourage innovation in pharmaceutical manufacturing and better understanding and control of manufacturing processes. The goal is to develop processes that consistently yield a predefined quality at the end of the manufacturing process (C&EN, Feb. 21, page 201).

The idea behind PAT is to continuously analyze process parameters to optimize production and achieve reproducible product specification, Daniel explains. Coflux technology fits because it can track the progress and end point of unit operations that display thermal activity.

A growing body of evidence indicates that the technology will make a significant impact on chemical and pharmaceutical process economics, Barker says. With its impact also on process design, operation, and safety, constant-flux reactor technology is a fundamental breakthrough in heat-transfer technology.

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