GB2545924A

GB2545924A - Improvements in or relating to temperature control systems

Info

Publication number: GB2545924A
Application number: GB1523153.3A
Authority: GB
Inventors: Morris David
Original assignee: Autichem Ltd
Current assignee: Autichem Ltd
Priority date: 2015-12-31
Filing date: 2015-12-31
Publication date: 2017-07-05
Also published as: GB201523153D0

Abstract

A continuous reactor and continuous reaction process comprises a reactor tube 1 with a jacket 2 containing heat transfer fluid 3, which is used to heat or cool process material in the reactor tube. Internal mixing of the heat transfer fluid creates turbulent flow. This can be done by providing flow interrupting elements, such as loose spheres 4 or baffles (7, fig 2), in the jacket, or by momentarily reversing the direction of flow of the heat transfer fluid. The spheres can have a high conductivity and may be metal spheres having a copper exterior. Flow reversal may be conducted by use of synchronised valves (8, 9). Actuators (programmed pumps that inject the heat transfer fluid) may be used to pulse the flow of the heat transfer fluid in the reverse direction. The process may be a chemical reaction or a physical reaction, such as crystallisation. A controlling loop may use analytical instrumentation to monitor the system. The reactor may comprise multiple stages, and the rate of change in heat may be measured at each stage.

Description

IMPROVEMENTS IN OR RELATING TO TEMPERATURE CONTROL SYSTEMS

The present invention relates to temperature control systems and is particularly concerned with the control of the temperature of reactants passing through a continuous flow reactor such as tubular reactors provided with a shell. Reactions with which the invention is concerned may be chemical reactions or physical reactions involving a change of state, for example crystallisation. Equally they may be physiochemical reactions.

Reactor vessels may be designed for continuous flow of the reactants or they may be batch reactors. Most reaction vessels whether they be continuous flow reactors or batch reactors are provided with temperature control means such as external jackets through which temperature control fluid flows. Temperature control is important as it provides a means for controlling the provision or extraction of energy to or from the reaction which in turn controls when the reaction takes place and the speed of reaction. With continuous flow reactors this can determine where within the reactor and to what extent the reaction takes place. Variations in the energy transfer profile along the reactor can lead to undesirable variations in the product. If for example, the reaction is a crystallisation reaction temperature variations can result in an undesirable broad particle size distribution of the crystals.

The design of heat transfer systems employed on traditional batch reactors consist of what can be considered as a tank within a tank. The process vessel is, as far as is reasonably practical, surrounded by a slow flowing volume of heat transfer fluid. This design is preferred by many vessel manufacturers as it is low cost and simple to fabricate. Although this design has been in use for many years it has a number of recognised drawbacks, which include, poor distribution of the heat transfer fluid, uneven heating and cooling, hot and cold spots and sluggish temperature control resulting in oscillations in the process temperature leading to undesirable variations in the product of the reaction. These inherent design flaws can have a direct impact on a process. In processes such as crystallization these issues can lead to secondary nucleation and result in an end product with a wide particle size distribution of the crystals. In bioprocesses employing enzymes, these issues can result in heat denaturisation of the enzymes.

Continuous reactors are usually tubular reactors which are provided with a surrounding jacket through which temperature control fluid flows usually in parallel with the direction of flow of the reactants within the reactor. It is however difficult to optimise the temperature control to ensure the desired degree of control along the length of the reactor and undesirable hot and cold spots can be formed along the length of the reactor. For example the injection of a cold temperature control fluid can cause an undesirable cold spot at the point of injection. Furthermore, variations in the laminar flow of the heat transfer fluid within the jacket can lead to temperature variations in the reaction material.

The present invention is aimed at overcoming these difficulties particularly in continuous flow reactors.

The invention is concerned with processes that require heating or cooling, and in which a jacket is provided in which heat transfer fluid can be used to heat or cool the process material contained in a reaction vessel.

According to the present invention internal mixing of the heat transfer fluid is provided within the heat transfer jacket in order to create turbulent flow of the heat transfer fluid. In a preferred embodiment the turbulence in the heat transfer fluid is created by the provision of flow interrupting elements within the jacket. The choice of the interrupting elements may depend upon the size and shape of the reactor. For smaller scale reactors such as laboratory reactors the interrupting elements may be loose spheres which pack out the jacket and create turbulent flow conditions, as the heat transfer fluid is pumped through the jacket. Larger scale reactors with larger volume heat transfer jackets may require baffles to produce sufficient turbulence. Other interrupting elements can however be used such as other loose shapes, prefabricated baffles, flow disrupters or turbulator assemblies.

The flow interruption elements that are provided can be any suitable size or shape. They may be baffles integral with the internal walls of the heat transfer jacket but we prefer to use loose elements within the jacket. In particular we have found that if the jacket is packed with loose elements around which the heat transfer fluid flows it is possible to achieve more uniform control of the energy of the system and also control of the temperature of the reactants.

It is also important that the flow interruption elements have a high thermal conductivity as this can help to reduce the formation of temperature differentials within the heat transfer fluid in the jacket. Metal spheres or metal baffles have been found to be particularly useful, in particular metal spheres having a copper exterior are especially useful in small reactors.

Turbulence in the heat transfer fluid may also be accomplished by reversing the direction of flow of the fluid within the jacket or by pulsing the flow in a single direction. Reversing may be achieved by the provision of synchronised pumps at both ends of the heat transfer jacket.

Pulsing may be achieved by suitable programming of the pump system that injects the fluid into the jacket. The reversal may be periodic and occasional. We have found that particularly good control may be achieved if one uses the reversal of flow in combination with the flow interrupters such as the baffles or the spheres having a metal exterior. The reversal of the heat transfer fluid may momentarily reverse the direction of flow of the heat transfer fluid. In the preferred embodiment the direction of heat transfer fluid flow remains overall in the forwards direction through the jacket, but the momentarily reversal of flow creates an oscillation in the flow and turbulence/mixing is subsequently created as the fluid passes through the flow restrictions created by the interrupting elements. In the preferred embodiment of the invention, fast activated valves, pistons or diaphragms may be used to momentarily pulse the flow in the reverse direction.

Accordingly a further embodiment of the invention provides a continuous reaction process wherein a process fluid flows through a reactor provided with a jacket through which heat transfer fluid passes wherein the direction of flow of the heat transfer fluid is periodically reversed.

The invention therefore provides an improvement in the performance of the traditional jacket design of continuous reactors. Mixing of the heat transfer fluid whilst it is resident in the vessel jacket helps to eliminate hot and cold spots and also helps to create turbulence to reduce sluggish temperature control and avoid the consequences thereof in the product of the reaction. The provision of interruption elements in the jacket itself has the additional benefit that it will reduce the overall fluid volume required to achieve a desired heating or cooling effect. The mixing of the heat transfer fluid also has a twofold effect on the jacket performance. Firstly the interruption elements create turbulent flow conditions, yielding better mixing of the heat transfer fluid and a more even temperature distribution. Secondly, the interruption elements also help create higher flow velocities through the jacket, helping to eliminate any stagnant areas and create well mixed flow channels so that the flow of the fluid is close to plug flow.

In the preferred embodiment of the invention, baffles or loose spheres are used as interruption elements, but similar results can be achieved with the use of other loose shapes, prefabricated baffles, flow disrupters or turbulator assemblies. Fabricating the baffles or spheres from solid copper, copper coated material or other material of high thermal conductivity further enhances the performance as the high thermal conductivity of the material will quickly and efficiently distribute the heat throughout the jacket. As indicated mixing of the heat transfer fluid may be further improved by the momentary reversal of the heat transfer fluid flow direction.

Flow reactors are traditionally operated as open loop steady state systems where the process is switched on and manually adjusted until an acceptable output is achieved. Residence times of the reactants within the reactors are usually held at an ideal constant. Whilst this technique works well at small scales, at industrial scales where plant disturbances are more prevalent and of more significance, process variabilities are often encountered.

This invention can also overcome these problems and in a further embodiment of the invention the reactor system can be operated as a continuous plug flow reactor that combines CSTR and tubular technology to improve heat transfer and mixing control. This can be achieved by closing the controlling loop using analytical instrumentation to monitor the system and using the analysis to cause changes in the operating conditions to achieve the desired temperature profile and energy balance along the reactor. The analysis required will depend upon the nature of the reaction although analytical techniques that may be used included particle size analysis, Fourier Transform Infra Red Spectroscopy, Ultra Violet Resonance Spectroscopy, Raman Spectroscopy or measurement of the heat of reaction. For example, the reactor may consist of multiple stages with the rate of change in heat measured at each stage; allowing for an assessment of the reaction rate and the stability of the reaction along the reactor.

To be able to operate such a control methodology effectively the placement of the analytical instrument or sensor is critical. In traditional long continuous flow reactors determining the exact location of a process event is almost impossible and consequently it is not possible to position instrumentation such that it can reliably measure and control the process. With the present invention however control of the reaction will be improved by the internal flow distribution and mixing systems, this invention provides an even temperature profile which reduces or eliminates the formation of hot and cold spots and controls the energy flow. Internal flow distribution will be controlled by the reactor’s heat transfer system, which will ensure temperature is uniformly distributed across the heat exchanger wall; thereby supplying each molecule in the reaction material with the desired and controlled amount of energy. By custom designing a multi-stage system it is possible to determine the exact region of the reactor where an event will take place allowing instrumentation to be strategically positioned to allow reliable closed loop control of residence time. With reactions such as crystallisation this allows improved control and identification of the onset of crystal nucleation and provides control of the speed of crystallisation and crystal size.

The present invention is illustrated by reference to the accompanying Figures in which Figure 1 shows a continuous flow reactor according to the invention.

Figure 2 is a longitudinal cross section of a flow reactor of the invention in which the flow of heat transfer fluid can be reversed.

Figure 1 shows a continuous flow reactor comprising a tubular vessel (1) provided with a heat transfer jacket containing heat transfer fluid (2), which can be used to heat or cool the process material contained in the tubular vessel (1). Internal mixing of the heat transfer fluid (3) is provided within the heat transfer jacket (2). Loose spheres (4) are used to pack out the jacket and create turbulent flow conditions.

Figure 2 shows how the mixing of the heat transfer fluid can be further improved by the momentary reversal of the heat transfer fluid flow direction. In Figure 2 the reactor (1) is provided with an inlet (5) for the heat transfer fluid and an outlet (6). The reactor is also provided with a temperature control jacket (2) in which are provided baffles (7) to cause turbulent flow of the heat transfer fluid. Control valves (8) and (9) are provided to allow periodic reversal of the direction of flow of the heat transfer fluid. The direction of heat transfer fluid flow remains overall in the forwards direction (A) shown in Figure 2 through the jacket (2), but the momentary reversal of flow in direction (B) of Figure 2 creates an oscillation in the flow and turbulence/mixing is created as the flow passes through the flow restrictions created by the baffles (7). The two heat transfer control valves (8) and (9) are synchronized to operate simultaneously. In the preferred embodiment of the invention, fast acting actuators are used to momentarily pulse the flow of the fluid in the reverse direction.

The heat transfer system of the present invention may be used with any tubular reactors. We have however found that it is particularly useful when used with a continuous tubular reactor of the type described in our copending Application Number GB1523157.4.

In copending Application Number GB1523157.4 we describe a continuous reactor in which the contents of the reactor are mixed by means of what we have called a flip-flop agitator system. A Flip-Flop agitator system acts as both an agitator and a baffle. The agitator system is rotated in one direction to accelerate the process fluid radially within the reaction vessel. The agitator system is then quickly stopped before being driven in the reverse direction, which creates turbulence and mixing. In a preferred embodiment, as described in copending Application Number GB1523157.4 through holes are formed into the body of the agitator, which permit and encourage radial flow of the reaction materials through the agitator body. This arrangement provides a mixing system for the contents of the reactor that is much closer to plug flow conditions and is scalable from small laboratory systems to much larger industrial scales.

In a further improvement described in copending Application Number GB1523157.4 to provide further enhanced mixing and turbulence free moving agitation elements, are housed within the main agitator body and through holes are also provided to push and pull the reaction fluid tangentially to the axial plane of the agitator. Centrifugal and gravitational forces may be used to drive the free moving elements.

The Flip-Flop agitator action operates as follows: • A flip is a partial, a full or multiple full rotation of the agitator in a single direction. • A flop is a partial, a full or a multiple full rotation of the agitator in the opposite direction to a flip. • A stop is when the agitator is stationary for any period of time between 1 nanosecond and 1 hour.

The mixing action is described by any of the following combinations of movement: • A flip, followed by a stop, followed by a flip. • A flop, followed by a stop, followed by a flop. • A flip, followed by a stop, followed by a flop. • A flip followed by an immediate flop.

We have found that the use of the heat transfer system of this invention together with a tubular reactor employing a flip-flop agitator system such as that described in copending Application Number GB1523157.4 provides a particularly effective reaction system with improved control of temperature and energy utilisation leading to the associated improvement in the quality of the product of the reaction. The reactors may be used for any physical or chemical reactions and could also be used in separations.

The invention is illustrated by reference to the following Example.

Example

For comparison a thermal image has been taken of a 300mm long x 50 mm diameter glass conventional reactor jacket that is used to cool a flow reactor, as illustrated in Figure 3.

The process fluid is cooled by the flow of heat transfer fluid in the reactor jacket and the thermal image shows the cooling profile and performance of the cooling jacket as shown in Figure 5 in which we have highlighted the reactor tube walls (10) and (11) shown in the section to clearly show their location and that the process fluid flows inside the reaction vessel as indicated by arrow (12) and the heat transfer fluid enters the cooling jacket tangentially to the flow and leaves the jacket tangentially to the flow as indicated by arrows (13) and (14).

The reactor jacket exhibits poor plug flow and mixing characteristics. Channelling of the heat transfer fluid within the jacket is clearly evident, resulting in an approximate 10°C differential temperature existing around a large part of the jacket circumference.

At any point on the heat transfer surface the ability to add or remove heat from the process is determined in simple terms by the heat transfer fluid power gain or loss (qhtf), which is determined from the mass flow rate, specific heat capacity of the fluid and temperature change of the heat transfer fluid according to the following formulation: E1 qhtf = m.Cp.AT Where: ΔΤ is the change in temperature of heat transfer fluid at any point in the jacket (°C)

Cpis the Specific Heat Capacity of the heat transfer fluid at a given temperature (J.kg-1.K-1) m is the mass flow of heat transfer fluid at any point in the jacket (kg.s-1)

In areas of the jacket what there is stagnant or very low mass flow of heat transfer fluid it can be seen that the lack of mass flow will have a significant effect on the performance of the heat exchanger. A repeat of the experiment was undertaken using the same flow reactor, but on this occasion the reactor jacket was packed with lose copper coated spheres according to this invention and as illustrated in the Figure 4.

In this case as shown in Figure 6, where the numerals indicate the same features as in Figure 5, the heat transfer fluid enters the cooling jacket tangentially to the and the heat transfer fluid leaves the jacket tangentially to the flow. The process fluid flows inside the reaction vessel and change in the bands seen in the thermal image of the heat transfer fluid flowing in the jacket demonstrates that better mixing is achieved and that channelling of the Heat Transfer fluid has been greatly reduced resulting in a much more uniform temperature within the process fluid. A flow characteristic that is much closer to plug flow conditions is observed.

Claims

1. A continuous reactor comprising a reactor tube and a jacket in which heat transfer fluid flows to heat or cool the process material contained in the reactor tube wherein internal mixing of the heat transfer fluid is provided within the heat transfer jacket in order to create turbulent flow of the heat transfer fluid.

2. A continuous reactor according to Claim 1 in which the turbulence in the heat transfer fluid is created by the provision of flow interrupting elements within the jacket.

3. A reactor according to Claim 2 in which the interrupting elements are loose spheres within the jacket.

4. A reactor according to Claim 2 or Claim 3 in which the interrupting elements are baffles.

5. A reactor according to any of the preceding claims in which the flow interruption elements have a high thermal conductivity.

6. A reactor according to Claim 5 in which the interrupting elements are small spheres.

7. A reactor according to Claim 6 in which the spheres are metal spheres having a copper exterior.

8. A reactor according to any of the preceding claims in which turbulence in the heat transfer fluid is accomplished by reversing the direction of flow of the fluid within the jacket.

9. A reactor according to Claim 8 in which the direction of flow of the heat transfer fluid remains overall in the forwards direction through the jacket and the reversal of the direction of flow is momentary and creates an oscillation in the flow and turbulence/mixing is subsequently effected as the fluid passes through the flow restrictions created by the interrupting elements.

10. A reactor according to Claim 8 or Claim 9 in which actuators are used to momentarily pulse the flow of the heat transfer fluid in the reverse direction.

11. A reactor according to any of the preceding claims employing a controlling loop using analytical instrumentation to monitor the system and using the analysis to cause changes in the operating conditions to achieve the desired temperature profile and energy balance along the reactor.

12. A reactor according to Claim 11 comprising multiple stages and the rate of change in heat is measured at each stage; allowing for an assessment of the reaction rate and the stability of the reaction along the reactor.

13. A continuous reaction process wherein a process fluid flows through a reactor provided with a jacket through which heat transfer fluid passes wherein the direction of flow of the heat transfer fluid is periodically reversed.

14. A continuous reaction process wherein reactants flow through a reactor tube provided with a jacket which heats or cools the process material contained in the reactor tube wherein internal mixing of the heat transfer fluid is provided within the heat transfer jacket in order to create turbulent flow of the heat transfer fluid.

15. A continuous reaction process according to Claim 14 in which the turbulence in the heat transfer fluid is created by the provision of flow interrupting elements within the jacket.

16. A process according to Claim 15 in which the interrupting elements are loose spheres within the jacket.

17. A process according to Claim 15 or Claim 16 in which the interrupting elements are baffles.

18. A process according to any of the preceding claims in which the flow interruption elements have a high thermal conductivity.

19. A process according to Claim 18 in which the interrupting elements are small spheres.

20. A process according to Claim 19 in which the spheres are metal spheres having a copper exterior.

21. A process according to any of Claims 13 to 20 in which the direction of flow of the heat transfer fluid is momentarily reversed within the jacket.

22. A process according to any of Claims 13 to 21 comprising a chemical reaction.

23. A process according to any of Claims 13 to 21 comprising a physical reaction.

24. A process according to Claim 23 comprising crystallisation.