GB2599890A

GB2599890A - Improved loop reactor

Info

Publication number: GB2599890A
Application number: GB2012872.4A
Authority: GB
Inventors: Ashe Robert
Original assignee: Individual
Current assignee: Individual
Priority date: 2020-08-18
Filing date: 2020-08-18
Publication date: 2022-04-20
Also published as: GB202012872D0; WO2022038124A1

Abstract

A loop reactor comprising a head tank 1, a heat exchange column, 5 and conduits 3 and 4. The heat exchange column is provided with a tangential stirrer 6 and a means is provided for pumping fluid through the head tank, conduits and heat exchange column in a recycling loop. Preferably the flow through the heat transfer column is orderly flow. Preferably the head tank is provided with a pitched blade stirrer 2 in order to drive flow through the loop. Also disclosed is a process for crystallisation via pumping a fluid around a loop reactor comprising a head tank 1, heat exchange column 5, and conduits 3 and 4. The heat exchange column is provided with a tangential stirrer 6 and a means is provided for pumping fluid through the head tank, conduits and heat exchange column in a recycling loop.

Description

IMPROVED LOOP REACTOR

The present invention is a method and equipment serving as a loop reactor for processing industrial fluids. It operates in batch or fed batch mode. In batch mode, the inventory of process material remains substantially constant during the process cycle. In fed batch mode, additional material may be added after the start and vapour may be removed by evaporation but fluid for downstream processing is not removed until the end of the process cycle. It may also be used in continuous mode as for example the embodiment with 3 or more systems connected and operating in series as described herein.

The reactor of this invention may be used for handling fragile materials, viscous fluids and temperature sensitive fluids and their operations. Crystallisation is the preferred use and in particular, the production of crystals derived through organic synthesis or of biological origin. Common product areas include pharmaceuticals, foods, fine chemicals, and other natural products. Crystal growth is regulated by controlling the supersaturation conditions using cooling, addition of anti-solvent, evaporation or a combination of these. Good mixing is needed for uniform saturation conditions and uniform distribution of particles. Shear at the heat transfer surfaces is also required to promote good heat transfer, reduced temperature gradients and reduced fouling at these surfaces. However, excessive mixing can damage the product.

Crystallisation is initiated by nucleation using different techniques. Crystal growth relies on maintaining the desired super saturation conditions within the metastable zone. Different methods are used to achieve this as material comes out of solution. These include cooling, anti-solvent addition, evaporation or a combination of these.

Ideal crystallisation products have good filtration, washing and drying characteristics. These give reduced losses, higher purity and uniform dryness in the final product. This relies on crystals of the right size and size distribution.

The desired conditions for crystal growth are controlled and uniform super saturation conditions in the bulk fluid and at the heat transfer surfaces combined with uniform spatial distribution of particles. These conditions need to be maintained with minimum frequency and speed of particle collisions with surfaces or other particles.

Historically, batch crystallisation has been performed in standard batch reactors referred to here as SBR's. These are stirred vessels with heat transfer jackets where the ratio of working height to diameter is generally between 0.8 and 1.5. SBR's with anchor stirrers generate high shear at the heat transfer surfaces giving good heat transfer and low saturation gradients at the surfaces. Low axial and radial flow with anchor stirrers can however lead to nonuniform saturation gradients and different crystal growth rates. Pitched blade stirrers give good axial and radial flow but low shear at the heat transfer surfaces. This leads to high saturation gradients at the heat transfer surface, poor heat transfer and increased surface fouling. Attempting to address the limitations of either stirrer type through higher mixing speeds leads to crystal damage. Many crystals are fragile and susceptible to damage from collisions with other particles or surfaces.

Extensive prior art exists for loop reactors and the various designs incorporate different features to address specific needs of process types. For example, the Buss loop reactor cited in patent WO 2019/060034 Al uses the loop principle to achieve high gas liquid mass transfer rates. The loop principle is used in patent W02008/114052 to pass fluid through an ultrasonic flow cell chamber with the addition of antisolvent. External loop systems also exist in the form of forced loop crystallisers such as in US patent 3503803A. These however are continuous systems based on the mixed-suspension, mixed-product removal principle whereby undersized particles are separated and recycled. Under these conditions, particle damage and residence time control are less critical. Under batch or fed batch conditions, residence time control, low particle attrition and control of supersaturation conditions in the bulk fluid and at the heat exchange surface are necessary for good product quality.

The method and equipment of this invention is a batch or fed batch reactor with one or more external loops. As with any batch system this can be operated in continuous mode subject to having multiple stages. In continuous mode this is differentiated from prior art in that it maintains orderly flow by using multiple stages.

The method and equipment of this invention gives uniform saturation conditions in the bulk fluid, uniform particle distribution and low saturation gradients at the heat transfer surface. It achieves this with minimal particle damage, better heat transfer using reduced mixing power and reduced fouling at the heat transfer surface. Systems of this type are easier to model and control.

Accordingly, the present invention provides a loop reactor comprising a head tank which is connected to a heat transfer column by conduits and the heat transfer column contains a tangential stirrer and means is provided to pump fluid through the head tank, heat transfer column and conduits in a recycling loop.

This invention addresses these problems and handles free flowing liquids which may also contain solids and or gases. The equipment is sealed below the maximum liquid level to give leak free containment. Materials of construction are specified to suit and may include steel, alloys, exotic metals, glass, plastic, ceramic or coated materials such as glass or plastic lining. The preferred volume of the reactors of this invention is 0.1-20 m° of working capacity and 0.1-10 m° more preferred.

Axial flow refers to the plane in the direction of fluid flow through the loop. Radial flow refers to flow at 900 to the axial plane. Tangential flow refers to rotational flow in the radial plane.

Pitched blade stirrers used here are primarily for pumping fluid in the axial plane but also contribute to radial and tangential flow they may also provide a small degree of circulation in the head tank which delivers fluid to the reactor. Tangential stirrers generate tangential flow but also contribute to localised radial flow. The blade heights of tangential stirrers are greater than 50% of the height of the heat transfer surface and more preferably 100%.

The reactor of this invention is particularly useful for crystallisation and in a further embodiment the invention provides a process for crystallisation comprising passing a process fluid from a head tank around a loop wherein crystal growth is driven by controlling the saturation conditions. Heat is added to or removed from the fluid as it passes through a heat transfer column which forms part of the loop where it is brought into contact with the heat transfer surface by means of a tangential stirrer. The system is filled at the start of a cycle and crystals are taken off at the end of the cycle.

The use of the reactor of the present invention has been found to enable crystallisation to be effected with substantial reductions in mixing energy and stirrer tip speeds relative to the fluid.

The invention is illustrated by reference to the accompanying Figures in which Figure 1 shows a loop reactor of the invention cut away to show the internal stirrers. Figure 2 shows the tangential stirrer that is used in the heat transfer column of Figure 1. Figure 3 shows a reactor of the invention comprising three loops.

Figure 1 shows the loop reactor with cutaway views showing internal stirrers. The head tank 1 has a pitched blade stirrer 2 for pumping the fluid through the loop. The head tank may be different shapes according to need or a vertical section of pipe. The blade of stirrer 2 is positioned in the vertical plane between the conduits 3 and 4 on the head tank. The direction of pumping can be up but pumping down in the head tank is preferred. A pitched blade stirrer 2 in the head tank is preferred to inline pumps or centrifugal pumps mounted within conduits 3 or 4. This allows the use of a stirrer with a diameter which exceeds the conduit diameters to give reduced shear at the blade tips and therefore less particle damage. It is preferred that the stirrer blade diameter is greater than twice the diameters of conduits 3 and 4.

Conduits 3 and 4 connect the head tank 1 to the heat exchange column 5. The heat exchange column 5 has an external jacket 7 with connections for the passage of heat transfer fluid. The temperature of the process fluid as it passes through the heat exchange column is controlled by the heat transfer jacket 7 which lies between the upper conduit 3 and the lower conduit 4 connection points to the heat transfer column. Conventional methods may be employed for temperature control using a temperature measuring element in the process fluid, a controller, and a control device regulating either the flow or temperature of the heat transfer fluid or both. Additional heating or cooling jackets may be used on the head tank 1 and the conduits 3 and 4 as required.

The tangential stirrer 6 sweeps the surface area covered by the jacket 7. Figure 2 shows a detail of the tangential stirrer 6 with blades 8. The movement of the stirrer 6 and its blades 8 generates high shear at the heat transfer surface at low tip speeds. The blades may be supported in different ways on the shaft. The tangential stirrer may have single or multiple blades. It is preferred that the blades have substantially continuous and uniform distance between the blade edges and the internal surface of the column 7 over the full length of the column. The blades can be straight, curved or angled and may include scrapers. The blades may also have slots, holes or a spiral twist. The blade widths are sized according to need but it is preferred that they do not cover the full diameter of the column to allow some fluid to spill back around the inner edge of the blade to effect radial mixing. It is preferred that clearance between the blades and column wall (Cma") is less than 50 mm and more preferably less than 20 mm so that shear is generated at the heat transfer surface at low rotational speeds of the stirrer. The tangential stirrer 6 may have stabilizer rings to prevent lateral movement. The following is an illustration of the factors and conditions that need to be considered when employing the present invention.

The preferred minimum heat transfer area (HTAmin) for a given working volume (V) is equal to or greater than that calculated using the relationship below: HTA,nin = 4.V0.711 Where HTAmin = the minimum heat transfer area for a given size (m2) V = working volume of the whole system (m3) Values lower than HTAmin can be used but high values are preferred to give a heat transfer area to working volume comparable to an industrial SBR where the ratio of working height to diameter falls in the range of 0.8 to 1.5 The preferred diameter of the pitched blade stirrer 2 for a given working volume (V) is equal to or greater than that calculated using the relationship below: Dmin = 1200.V0333 Where Dmin = blade diameter (mm) V = working volume of the whole system (m3) comprising the heat tank, associated conduits and the heat transfer column Specific numbers cannot be stated for minimum flow Wmin of the fluid through the loop as these are subject to system size and operating conditions. However, Wmin is preferably equal or greater than any of 4 parameters: 1 The turnover time does not exceed 90 seconds 2 It provides sufficient axial velocity to prevent fluids of different density from back flowing in the heat transfer column.

3 The axial velocity of the fluid in the heat transfer column is greater than the settling velocity of solids in the fluid and more preferably greater than twice the settling velocity 4 The volumetric flow of the fluid is such that the temperature of the fluid leaving the heat transfer column is not lower than the limit for supersaturated conditions at any given time.

It is preferred that the turnover time does not exceed 2 minutes and more preferred 1 minute and even more preferred 30 seconds. The turnover time is the time required for one volume of fluid in the system to pass through the loop.

It is preferred that the maximum axial velocity in any part of the loop is not greater than 20 times the lowest axial velocity in any part of the loop and more preferably not greater than 10 35 times The preferred fill volume is at or above the top of the upper loop conduit 3.

It is preferred that the maximum vertical height Hmax of a single jacketed section of the heat exchange column 5 does not exceed 3 metres to accommodate headroom constraints in buildings. To meet the desired value of HTAmin, without exceeding Hmax multiple loops can be used as shown in figure 3. It is preferred that these are directly connected to the head tank or common header as shown but a manifold arrangement can be used.

Horizontal or sloping heat transfer columns may be used subject to the flow and return conduits 3 and 4 connecting to the head tank above and below the pitched blade stirrer, respectively.

It is preferred that the pressure drop in the loop with a turnover time of 1 minute is not greater than 0.2 bar and more preferably not greater than 0.1 bar. Low pressure drops across the pitch blade stirrer blade reduce the differential speed between the pitched blade stirrer and the fluid to reduce particle damage.

It is preferred that the maximum fluid velocity U. in any part of the loop does not exceed 2 metres per second and more preferably does not exceed 1 metre per second during the crystal growth phase. Higher velocities can be applied during phases where the product is not vulnerable to mechanical damage.

Variable speed drive units are preferred for the pitched blade and tangential stirrers.

Orderly flow is preferred in the heat transfer column and conduits. This means unidirectional flow in the axial plane with substantially no back mixing. It is preferred that the fluid passes into the heat transfer column through the lowest conduit 4 and leaves the column through conduit 3.

It is preferred that the shear generated at the heat transfer surface by the tangential stirrer is as high as possible depending on the nature of the fluid in the heat transfer column is not less than 80 s-1.

Means to provide back mixing within the head tank can be used to allow for variations in fill level. Different methods can be used such as an angled baffle in the head tank to cause up flow, a second stirrer blade above the upper conduit or a baffle or turn up on the return conduit directing fluid up. Some excess speed in the pitched blade stirrer may also be used to create a small degree of rotational flow in the head tank.

Low specific mixing powers are achieved by default on small volume systems. This product is primarily used for small and large volumes. It is preferred that the mixing power applied by the pitched blade stirrer does not exceed 0.25 Watts per litre.

Specific dimensions of the loop cannot be given since these values are size dependent. It is preferred that the method and system is capable of meeting the combined parameters P, which are: a turnover time of less than 2 minutes, orderly flow in the heat transfer column and conduits and the head tank generated by the pitched blade stirrer not exceeding 0.2 bar. These 3 parameters apply irrespective of system size and dictate the diameters of the conduits 3 and 4, and the heat exchange column 7.

In a multi loop system, cross mixing between different parts of the loop can be improved by varying the length or using a flow restrictor in one of the loops.

Reduced head losses in the loop can also be achieved by having tangential connections of conduits 3 and 4 to either the head tank or the heat transfer columns or both.

Example

A loop crystalliser has a working volume of 2.5 m3. The value of HTAmin is calculated as 7.67 m2 and Dinin is 523 mm. A 0.6 m diameter head tank 1 is suitable. Using three loops with jacket heights of 1.8 m each gives a total heat transfer area of 10.18 m2 which is greater than HTArnin. The ratio of agitator diameter to tank diameter can be higher than an SBR because fluid flows in one direction. The volumetric flow through the loop is 0.083 m3/s. The limiting case for sizing diameters in the loop is the conduits. A common conduit which is an extension of the head tank serves the three individual loop conduits 3 and 4 for each column. The common conduit serves the individual loop conduits. This has a diameter of 350 mm, giving an axial velocity of 0.87 m/s. The individual conduits 3 and 4 have a diameter of 200, giving an axial velocity of 0.88 m/s. Both velocities are below Un,ax. The operating speed of the radial stirrer is' 50 rpm. The tangential stirrer has four blades 8, an operating speed of 20 rpm and a wall clearance of 10 mm. The capabilities of this design are well differentiated from the SBR in all aspects of performance.

The system of this invention has the following benefits: More uniform saturation conditions in the axial and radial planes make it simpler to model and control.

Axial flow from bottom to top of the heat transfer column prevents denser cold fluids and solids accumulating in the lower sections of the reactor which results in uniform saturation gradients in the vertical plane with uniform crystal dispersion. This gives better control of saturation conditions and reduced frequency of particle collisions and the particles are well dispersed.

An external loop reduces friction and requires less stirrer power. This combined with smaller and lower speed stirrers reduces the relative tip velocity of the blades within the fluid by an order of magnitude or more compared to an SBR with a pitched blade. This leads to reduced particle damage.

The tangential stirrer generates high shear at the heat transfer surface giving reduced surface fouling, improved heat transfer rates and reduced saturation gradients in the axial plane.

The heat transfer surface is fully submerged maximising heat transfer area and eliminating heating or cooling dry surfaces.

Fluid travels substantially in the same axial direction which reduces the speed and frequency of inter particle collisions.

Claims

CLAIMS1 A loop reactor comprising a head tank which is connected to a heat transfer column by conduits and the heat transfer column has a tangential stirrer and means is provided to pump fluid through the head tank, heat transfer column and conduits in a recycling loop.
2. A loop reactor according to Claim 1 wherein the flow of the fluid through the heat transfer column and conduits is orderly flow.
3. A loop reactor according to Claim 2 wherein fluid passes into the heat transfer column through the lower conduit and returns to the head tank through the higher conduit.
4 A loop reactor according to Claim 2 which uses a pitched blade stirrer located in the head tank to generate axial flow through the loop which is located between the lower and higher conduits which connect the head tank to the heat transfer column.
5. A loop reactor according to Claim 4 wherein the mixing power applied by the pitched blade stirrer does not exceed 0.25 Watts per litre.
6. A loop reactor according to any of the preceding claims wherein the shear generated by the tangential stirrer at the heat transfer surface is not less than 80 s-1.
7. A loop reactor according to any of the above claims wherein Winn of the fluid meets the 4 combined parameters defined herein.
8. A loop reactor according to Claim 4 where the diameter of the pitched blade stirrer is greater than the diameter of the conduits 3 and 4.
9 A loop reactor according to any of Claims 1 to 8 which operates continuously using 3 or more stages with each stage comprising of a head tank 1, a heat transfer column 5 and conduits 3 and 4 connecting the head tank to the heat transfer column.
10. The use of a loop reactor according to any of the previous claims as a batch or fed batch crystalliser.
11. A process for crystallisation comprising a pumping fluid around a loop reactor comprising a head tank which is connected to a heat transfer column by conduits and the heat transfer column has a tangential stirrer and means is provided to pump fluid through the head tank, the heat transfer column and conduits in a recycling loop.
12. A process for crystallisation according to Claim 11 wherein the fluid flow through the heat transfer column and conduits and orderly flow.
13. A process for crystallisation according to Claim 11 or Claim 12 where fluid passes into the heat transfer column through the lower conduit and returns to the head tank through the higher conduit.
14. A process for crystallisation according to any of Claims 11 to 13 which uses a pitched blade stirrer located in the head tank to generate axial flow through the loop which is located between the lower and higher conduits which connect the head tank to the heat transfer column.
15. A process for crystallisation according to any of Claims 11 to 14 wherein the mixing power applied by the pitched blade stirrer does not exceed 0.25 Watts per litre.
16. A process for crystallisation according to any of Claims 11 to 15 wherein the shear generated by the tangential stirrer at the heat transfer surface is not less than 80 s-1.
17.A process for crystallisation according to any of Claims 11 to 16 wherein Wmin of the fluid meets the 4 combined parameters as defined herein.
18. A process for crystallisation according to any of Claims 11 to 17 where the diameter of the pitched blade stirrer is greater than the diameter of the conduits 3 and 4.
19. A process for crystallisation according to any of Claims 11 to 18 as a batch or fed batch crystalliser.
20. A process for crystallisation according to any of Claims 11 to 19 which operates continuously using 3 or more stages with each stage comprising of a head tank 1, a heat transfer column 5 and conduits 3 and 4 connecting the head tank to the heat transfer column.