Patent Application
20200261867
The present invention relates to an improved mixer for flowing materials.
During passage through the system, a chemical or physical change may take place which is termed a process operation and the flowing fluid is a process material. Such a process operation includes but is not limited to mixing, chemical reaction, enzymatic reaction, cell growth, crystallisation and polymerisation. The process operation may also be an extraction. The process material is free flowing and may be homogenous liquid or a mixture of phases such as immiscible liquids, gas liquid mixtures, liquids with particulate solids such as slurries and suspensions, supercritical fluids or a combination of these.
The terms mixer and mixing are used to include simple mixing of materials and also to mixing of fluids which involves a chemical or physical change such as chemical reactions, enzymatic reactions, cell growth, polymerisation or physical change such as crystallisation. The term fluid includes liquids, gasses, slurries, suspensions and mixtures thereof. The fluid is a flowing material.
Historically, these process operations and others have predominantly been performed in batch reactors. These are large stirred batch tanks which may have heating/cooling surfaces and they process one tank volume at a time. The continuous system of this invention involves material continuously flowing along a tube and allows one to process multiple tube volumes without interruption and therefore has a higher output per unit volume than batch equipment. This allows the use of a physically smaller apparatus which is more energy efficient and inherently safer than batch equipment. Reduced size also contributes to better performance as the mixing distances are shorter and better heat transfer as the ratio of heat transfer area to volume is increased. This improved performance contributes to improved product yield and improved product purity subject to chemistry.
GB2507487 describes a mixer comprising a tube body which holds a fluid and is rotated in reversing arcs. Internal static and dynamic mixing elements work together to promote mixing of the fluid as the tube rotates. The mixers are supported by a central shaft. The use of a central shaft however is not preferred. Furthermore it has a low velocity and therefore makes an insignificant contribution to mixing. Additionally it obstructs the mixing pattern of fluid spilling off the mixing blades. It also makes assembly more difficult to mount mixing blades which rotate at the same velocity and direction as the tube.
The present invention therefore provides a mixer comprising a sealed tube provided with inlets and outlets for the process fluid and said tube is rotatable in arcs around the longitudinal axis of the tube containing a mixing element comprising one or more blades mounted at each end on a blade carrier and said blade carrier is supported within the tube in a manner that allows the one or more blades to rotate in the same direction and angular velocity (in degrees per second) as the tube rotates in arcs.
The present invention also provides a system as described above wherein the mixing element rotates in the same direction and angular velocity as the tube as the tube rotates in arcs although the mixing element is free to rotate at a different angular velocity to the tube during the transition phase between one arc of rotation of the tube and the next.
The present invention therefore provides a mixer comprising a sealed tube provided with inlets and outlets for the process fluid and said tube is rotatable in arcs around the longitudinal axis of the tube containing a mixing element comprising one or more blades mounted at each end on a blade carrier and said blade carrier is supported within the tube in a manner that allows the one or more blades to rotate in the same direction and angular velocity (in degrees per second) as the tube rotates in arcs.
The present invention also provides a system as described above wherein the mixing element rotates in the same direction and angular velocity as the tube as the tube rotates in arcs although the mixing element is free to rotate at a different angular velocity to the tube during the transition phase between one arc of rotation of the tube and the next.
The tube is preferably horizontal or substantially horizontal.
The blades are fixed relative to the blade carrier. Blade carriers are located at each end of the tube and supported by the tube or end flange and are preferably mounted so that the blades are not touching the tube walls. The centre of rotation of the mixing element is within the inner third of the tube diameter and more preferably at the centre of the tube. The supports for the blade carriers may be fixed such that they rotate at the same angular velocity and direction as the tube. This is referred to here as the tube drive stroke. The supports for the blade carriers may also allow the blade carriers to rotate at a different angular velocity and/or direction to the tube and said difference can be caused by the drag effect of the process material. This is referred to here as the fluid drive stroke. The whole cycle of each rotating arc may be the tube drive stroke. It is preferred that a combination of the tube drive stroke and the fluid drive stroke is used. To achieve a combination of the two, the mixing element needs to be free to rotate but the degree of free rotation is limited by one and preferably two travel stops. These stops are fixed to the tube or end flange and limit the degree of free rotation of the mixing element. The travel stops push the mixing element during the tube drive stroke.
The shape of the tube is preferably round but other shapes may be used. The tube is preferably horizontal and of a length that is preferably at least twice the diameter of the tube and more preferably greater than 3 times the diameter and even more preferably greater than 5 times the diameter. The length of the tube may be selected according to the operation to be performed within the tube however the preferred length is between 500 mm and 2 metres. Other lengths can be used according to need. The preferred tube diameter is between 50 mm and 1 metre. Other tube diameters can be used according to need.
The long axis of the tube is the axial plane and axial mixing means that fluid elements within the tube change position in relation to other fluid elements within the axial plane. The radial plane is at right angles to the axial plane and radial mixing means that fluid elements change position in relation to other fluid elements within the radial plane. The desired mixing regime gives a high ratio of radial to axial mixing. This ensures a narrower residence time distribution of process material within the reactor which in turn leads to improved residence time control for improved product yield and quality. Different process fluid phases may travel at different speeds through the system and in some cases in different directions.
The mixer of this invention is designed to give orderly flow for any given phase. Orderly flow means that any two fluid particles of a given phase entering the system at time zero discharge from the tube at substantially the same time. Orderly flow is a key factor in controlling residence time. The mixer may be used with high viscosity fluids but preferably with fluids with a viscosity of less than 100 centipoise.
The present invention provides a novel tubular mixer design in which there is no central shaft supporting the mixer blades between the blade carriers and which employs tubular systems rotating in arcs.
The tube rotates in arcs and the rotation may be driven by various means including compressed air, a motor with drive gears or other means. The preferred method is a motor which drives a belt or chain to turn the tube. It is preferred that a drive control system is used which includes sensors to detect when the tube has reached the end of an arc of rotation at which point a signal is sent to the drive system to reverse the direction of rotation. The maximum arc of rotation (Θ1) of the tube is 3600 but an angle of 1800 or less is preferred and an angle of 1500 or less is more preferred.
Fluid flow through the system is driven by external fluid feed such as by pumps, pressure transfer or gravity assisted flow. It is preferred that the system is run full with process material.
The mixing blade or blades are mounted on the blade carrier at a position between the centre of the tube and the tube wall. It is preferred that the blade or blades occupy the outer 70% of the radius of the tube and more preferably the outer 50% and even more preferably the outer 30%. It is preferred that the blade has no contact with the tube wall. It is preferably mounted at a distance of not more than 25 mm from the inner surface of the wall of the tube and more preferably not more than 15 mm from the wall. There is no central shaft to support the mixer blades between the two blade carriers.
It is preferred that the blade is straight in the axial plane. It is preferred that the blade is not straight in the radial plane and is bent or curved. This improves rigidity and also creates a bias in the fluid pumping to promote a slow rotation of fluid in the tube.
The tube rotates through an arc. In cases where low shear and long mixing times are applicable, the rotation speed may be up to 10 minutes or longer to complete a single arc. Where high shear or fast mixing time is required, the time to complete an arc may be less than 1 second. The drive mechanism for the rotation of the tube can be gears, a drive belt, a drive chain or pistons.
This system delivers a residence time distribution of the process material in the tube equivalent to 3 or more continuously stirred tank reactors in series and more preferably 5 or more.
It is preferred that tube travel stops are fitted to the external surface of the tube to prevent the tube from over rotating. Over rotation is undesirable and can lead to decoupling of the process and heat transfer connections to the tube. The stops may be based on sensors that signal the drive system to stop rotation in a given direction. They may also be mechanical stops whereby a solid projection on the tube makes contact with a solid projection which is fixed to a stationary surface which is not part of the tube. A combination of sensors and mechanical stops is preferred.
Mixing element travel stops are also used to restrict the free rotational movement of the blade carriers. The angle of separation (Θ2) of the travel stops varies according to the amount of fluid drive travel required of the mixer blades. The value of the fluid drive travel in degrees is Θ2×N per arc of rotation where N is the number of blades used. The minimum value of Θ2 is the blade width such that the mixers blades (11) are stationary relative to the tube (3) at all times. A value which is 100 or greater is preferred.
The tube may be provided with an external heating or cooling jacket for controlling the temperature of the process material within the tube. It is preferred that when used the external heating/cooling jacket is a spirally wound channel around the external surface of the tube. Such a channel carries heat transfer fluid and preferably has a cross sectional area of 2000 mm2 or less and more preferably 200 mm2 or less. The channel may be in the form of a welded half pipe wrapped around the tube or a pipe wrapped around the tube. It is preferred that said pipe is of a material with good thermal conductivity. Copper is the preferred material for the heat transfer fluid pipe. It is also preferred that the heat transfer channel comprises two or more spiral sections wrapped around the body of the tube each with their own feed and discharge pipes. Alternatively electrical heating may be used.
It is preferred that the mixer system of this invention is assembled by fabricating the blade carrier and blades separately. It is preferred that each blade is made from a single piece of material. Any number of blades can be used but the preferred number is 6 or less more preferably 2 to 4. The blades may be provided with holes or slots to improve mixing. The blades may also have cut outs at the edge. The blades may also use flexible or hinged surfaces which change geometry according to the direction of rotation.
One or more of the blades may be of a different weight to create an unbalanced system but it is preferred that the blades are of similar weight so that the mixer system is balanced so that the blades are driven only by the action of the travel stops and the fluid movement.
The system may be provided with one or more radial baffles along the internal length of the tube to restrict axial mixing. A system with 3 or more baffles is preferred and a system with 4 or more baffles is more preferred. The diameter of the baffles can be varied but a clearance between the baffles and the tube walls of 20 mm or less is preferred, a clearance of 10 mm or less being more preferred. The baffles may be fitted in different ways but the preferred method is to use slots in the baffle through which the mixer blades are fed. The baffles may be fixed in place in the axial plane using an interference fit or spacer bars may be used. They may also be welded or screwed in place.
The components of the system of this invention may be made with different materials to achieve the right combination of mechanical strength and chemical resistance required for the particular process operation. Materials that may be used include steel, stainless steel, alloys, exotic metals, plastic, ceramic materials and glass. Combinations of these materials where one material provides mechanical strength and a different material provides the chemical resistance may be used. Examples include steel coated with tantalum, glass, ceramic or plastic.
An emergency relief system may be used and the preferred system is a bursting disc mounted at the centre of the end flange.
The system of this invention is particularly useful as a continuous chemical reactor. It is also useful as an extractor.
The invention is illustrated by reference to the accompanying Figures.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1715735.5 | Sep 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/076428 | 9/28/2018 | WO | 00 |
