Alternative to the microreactor
1 Aug 2008
Microreactors have been A focus of GROWING interest for chemical and pharmaceutical manufacturers, attracted by the prospect of faster reactions, higher yields and reduced use of solvents. These performance advantages are, in part, linked to the heat transfer characteristics of microreactors due to the high surface/volume ratios associated with the use of small flow channels.
Despite their excellent heat transfer capabilities, microreactors have some significant drawbacks, however. The constricted flow channels of micro- reactors cause relatively high pressure drops — creating fabrication problems for the reactor and pump, especially where corrosive materials are handled — and are vulnerable to blockage.
UK-based AM Technology claims to have developed a class of continuous reactor that offers the performance capabilities of microreactors, but which have lower pressure drops, a reduced tendency to block and are simple to dismantle and clean.
According to Robert Ashe, the chemical engineer co-founder of AMT — along with electrical instrument engineer David Morris — despite the many advantages, there is currently over-enthusiasm about the potential of continuous reactors as a replacement for batch technology.
"The idea that continuous is better than batch is a wrong starting point. Batch reactors do a very good job and can store material for regulatory approvals," said Ashe. "By comparison, continuous reactors take a long time to set up and present more risk, as a problem in any stage of the reaction holds up the entire process. They also require expensive pumps to deal with corrosive substances at very high pressures."
The challenge facing industry, therefore, is to develop chemical reactors that offer the capabilities of microreactors but without the inherent drawbacks. This means designing reactors with larger flow channels, according to Ashe, who highlights the importance of basic heat transfer theory (see panel, p18) in understanding how larger flow channels can be used without sacrificing performance.
While high thermal differences improve the cooling power of a reactor, it can then suffer from poor temperature control as the process heat output in a reactor is not uniform. If a high thermal difference is used to control the peak heat load in one part of the reactor, cold zones will be created elsewhere within the reactor. If a lower thermal difference is used, the reactor will suffer a hot spot.
AM Technology was set up in 2000 to develop continuous processes for the fine chemical and pharmaceutical industries, using technology developed over recent years with the support of Imperial College London. This has led to development of a reactor that is broken up into a series of discrete stages, so that each stage can be optimised to provide the ideal conditions for that particular phase of the reaction.
The new Coflore reactors can be built and used as microreactors. Although it has taken several years to perfect the design of the modules, the underlying principle is well known and has been used in other areas of chemical engineering, such as multi-stage distillation columns, said Ashe.
The modular design allows the user to adapt the reactor to suit a range of applications and the individual modules can be dismantled so that all the process surfaces can be accessed for cleaning.
"Used in this way, they will always outperform conventional microreactors in terms of cooling power for a given pressure drop," said Ashe. "We believe, however, that the most useful role for these reactors will be to provide microreactor performance on systems with larger flow channels."
There are two basic types of Coflore reactor. The Variable Channel Reactor (VCR) is designed for fast reactions and the Agitated Cell Reactor (ACR) is for slow ones. The ACR unit was launched at the end of April, while the VCR is scheduled to be launched this October.
The VCR is a modified plate exchanger with configurable channel depths. The figure (p17) shows the typical product flow path through the reactor.
The flow channel is broken up into a series of stages. The thickness of each stage can be varied to limit or increase the heat output within the given stage.
When sized correctly, these variable depth channels create a uniform heat output throughout the reactor, even though the rate of heat output per unit volume of product is changing.
With a uniform heat output, high thermal differences can be used without creating cold zones. Each stage also has an independent cooling supply, which can be adjusted to fine-tune localised temperatures within the reactor.
The ability to increase the thermal difference allows the user to employ larger flow channels without sacrificing heat transfer performance. Larger flow channels have higher flow capacity, less pressure drop and have a reduced propensity to block.
The ACR is a multi-stage, continuously stirred tank reactor. As with the VCR, the capacity of the reaction stages can be adapted to suit the reaction profile.
Product flows through the cells in the reaction block via interconnecting channels in a zig-zag fashion from bottom to top. A cooling plate at the back of the reactor gives each cell a fixed cooling area, while cell volume is modified by varying the size of an insert within each cell.
Correct sizing of the inserts provides uniform heat output throughout the cells. This allows end users to employ high thermal differences without creating cold zones.
The ACR employs a mechanical system to maintain good mixing — slow reactions can present mixing problems. The inserts in the reactor are loose and the entire reactor is mounted on an agitating platform, which causes the inserts to vibrate and mix the product.
Calculating heat transfer
Heat flow (q) across a heat transfer surface is determined as follows:
q = U.A.(Tp - Tj)
Where q = cooling capacity at a given point in the heat exchanger (W)
A = heat transfer area at the given point within the heat exchanger (m2)
U = overall heat transfer coefficient (W.m-2.K-1)
(Tp - Tj) = thermal difference between the process and cooling jacket (°C)
As the heat transfer coefficient (U) is a relatively fixed value there are only two practical options for increasing the cooling power, heat transfer area and thermal difference. Microreactors use higher heat transfer areas. An alternative (or additional) option is to increase the thermal difference between the jacket and the vessel.
Despite their excellent heat transfer capabilities, microreactors have some significant drawbacks, however. The constricted flow channels of micro- reactors cause relatively high pressure drops — creating fabrication problems for the reactor and pump, especially where corrosive materials are handled — and are vulnerable to blockage.
UK-based AM Technology claims to have developed a class of continuous reactor that offers the performance capabilities of microreactors, but which have lower pressure drops, a reduced tendency to block and are simple to dismantle and clean.
According to Robert Ashe, the chemical engineer co-founder of AMT — along with electrical instrument engineer David Morris — despite the many advantages, there is currently over-enthusiasm about the potential of continuous reactors as a replacement for batch technology.
"The idea that continuous is better than batch is a wrong starting point. Batch reactors do a very good job and can store material for regulatory approvals," said Ashe. "By comparison, continuous reactors take a long time to set up and present more risk, as a problem in any stage of the reaction holds up the entire process. They also require expensive pumps to deal with corrosive substances at very high pressures."
The challenge facing industry, therefore, is to develop chemical reactors that offer the capabilities of microreactors but without the inherent drawbacks. This means designing reactors with larger flow channels, according to Ashe, who highlights the importance of basic heat transfer theory (see panel, p18) in understanding how larger flow channels can be used without sacrificing performance.
While high thermal differences improve the cooling power of a reactor, it can then suffer from poor temperature control as the process heat output in a reactor is not uniform. If a high thermal difference is used to control the peak heat load in one part of the reactor, cold zones will be created elsewhere within the reactor. If a lower thermal difference is used, the reactor will suffer a hot spot.
AM Technology was set up in 2000 to develop continuous processes for the fine chemical and pharmaceutical industries, using technology developed over recent years with the support of Imperial College London. This has led to development of a reactor that is broken up into a series of discrete stages, so that each stage can be optimised to provide the ideal conditions for that particular phase of the reaction.
The new Coflore reactors can be built and used as microreactors. Although it has taken several years to perfect the design of the modules, the underlying principle is well known and has been used in other areas of chemical engineering, such as multi-stage distillation columns, said Ashe.
The modular design allows the user to adapt the reactor to suit a range of applications and the individual modules can be dismantled so that all the process surfaces can be accessed for cleaning.
"Used in this way, they will always outperform conventional microreactors in terms of cooling power for a given pressure drop," said Ashe. "We believe, however, that the most useful role for these reactors will be to provide microreactor performance on systems with larger flow channels."
There are two basic types of Coflore reactor. The Variable Channel Reactor (VCR) is designed for fast reactions and the Agitated Cell Reactor (ACR) is for slow ones. The ACR unit was launched at the end of April, while the VCR is scheduled to be launched this October.
The VCR is a modified plate exchanger with configurable channel depths. The figure (p17) shows the typical product flow path through the reactor.
The flow channel is broken up into a series of stages. The thickness of each stage can be varied to limit or increase the heat output within the given stage.
When sized correctly, these variable depth channels create a uniform heat output throughout the reactor, even though the rate of heat output per unit volume of product is changing.
With a uniform heat output, high thermal differences can be used without creating cold zones. Each stage also has an independent cooling supply, which can be adjusted to fine-tune localised temperatures within the reactor.
The ability to increase the thermal difference allows the user to employ larger flow channels without sacrificing heat transfer performance. Larger flow channels have higher flow capacity, less pressure drop and have a reduced propensity to block.
The ACR is a multi-stage, continuously stirred tank reactor. As with the VCR, the capacity of the reaction stages can be adapted to suit the reaction profile.
Product flows through the cells in the reaction block via interconnecting channels in a zig-zag fashion from bottom to top. A cooling plate at the back of the reactor gives each cell a fixed cooling area, while cell volume is modified by varying the size of an insert within each cell.
Correct sizing of the inserts provides uniform heat output throughout the cells. This allows end users to employ high thermal differences without creating cold zones.
The ACR employs a mechanical system to maintain good mixing — slow reactions can present mixing problems. The inserts in the reactor are loose and the entire reactor is mounted on an agitating platform, which causes the inserts to vibrate and mix the product.
Calculating heat transfer
Heat flow (q) across a heat transfer surface is determined as follows:
q = U.A.(Tp - Tj)
Where q = cooling capacity at a given point in the heat exchanger (W)
A = heat transfer area at the given point within the heat exchanger (m2)
U = overall heat transfer coefficient (W.m-2.K-1)
(Tp - Tj) = thermal difference between the process and cooling jacket (°C)
As the heat transfer coefficient (U) is a relatively fixed value there are only two practical options for increasing the cooling power, heat transfer area and thermal difference. Microreactors use higher heat transfer areas. An alternative (or additional) option is to increase the thermal difference between the jacket and the vessel.
