WO2014167506A1

WO2014167506A1 - Flow reactor with pinched pipe sections for mixing and heat transfer

Info

Publication number: WO2014167506A1
Application number: PCT/IB2014/060552
Authority: WO
Inventors: Amol Arvind Kulkarni; Vivek Vinayak Ranade
Original assignee: Council Of Scientific & Industrial Research
Priority date: 2013-04-10
Filing date: 2014-04-09
Publication date: 2014-10-16

Abstract

A pinched pipe flow reactor to prevent back-mixing and to enhance heat transfer is disclosed herein. The reactor comprises a pipe having at least two converging / pinched section [102] connected to each other by a non-pinched section [101], characterised in that axis of the each converging / pinch section is between 0° to 90° and arranged in different perpendicular planes to the pinch. A method of enhancing mixing, mass transfer and heat transfer comprising the pinched pipe flow reactor is further disclosed.

Description

FLOW REACTOR WITH PINCHED PIPE SECTIONS FOR MIXING AND HEAT TRANSFER

FIELD OF THE INVENTION

The present invention relates to a flow reactor comprising of plurality of fluidic components which helps retain agility and re-configurability of the continuous chemical processes with improved mixing and heat transfer characteristics. Particularly, the invention relates to a modification of a pipe/tube composed of varied permutations and combinations of a plurality of extent of pinch. The components are connected to each other using connectors that facilitate the connection of at least two similar or different pinched tubes/pipes. Further, a method to enhance mixing, mass transfer and heat transfer is also disclosed herein.

BACKGROUND OF THE INVENTION

Typically the enhancement in the inline mixing and heat transfer in straight pipes is achieved either by inserting static mixers or modifying the internal surface by creating roughness. However many times the static mixers or other alternatives are not available for all pipe sizes and they need to be inserted in the pipes. Also, the static mixers are available only in specific materials of construction and thereby limit the applicability of pipes for specific reactions having incompatible material. During the usage either for reactions or for heat transfer, the static mixer elements get blocked are become difficult to remove from the pipes. Thus, in addition to the cost of static mixers, the pipes also need to be discarded in case of problems. These shortcomings in the current practices to enhance mixing and heat transfer in pipes indicate the need for a more flexible and efficient alternative.

Pipes are being used as continuous flow reactors for over decades. However the nature of the reactor has largely been like a simple tubular reactor with either straight (US7018591, US20030055300), tubes connected using 180° bends (US5779994, US3773470, US3148037), the helical, lamellar or spiral configuration, all with constant flow area. A few configurations with inserts have also been used (US20100040190). However these configurations do not bring out a significant impact on the enhancement of mixing or reactor performance.

Further, US4763727 discloses a panel heat exchanger that has a plate made of a heat- conducting material and at least one pipe connected with it, this pipe having a meandering shape, and through which a heat exchange medium flows. The plate is provided with slots the width of which is adapted to the diameter of the pipe. The pipe is inserted into the slots approximately flush with the plate and is held by deformations extending transversely to its longitudinal axis at the slot edges in a form-fitting way. As mentioned in the descriptions, the deformations introduced in US4763727 create continuous contact surfaces between the pipe and the carrier plate that ensure a good heat transfer.

US6920917 intends to provide an inexpensive double-pipe heat exchanger having high performance. It comprises an inner pipe and an outer pipe which constitute a double pipe without adding a heat-transfer facilitating material such as an inner fin. In the double-pipe heat exchanger having the inner pipe and the outer pipe, the outer pipe is dented from its outside toward its inside, thereby forming a plurality of projections which are dented toward the inner pipe. Examples of shapes of the projection are substantially conical shape, substantially truncated shape, substantially spherical surface shape, substantially cylindrical shape, substantially elliptic cylindrical shape and the like. The heat transfer performance is not deteriorated because a distance between the inner pipe and the outer pipe is substantially equally maintained by the projections of the outer pipe disposed around the inner pipe.

Despite the above, there exists a need for an efficient system for carrying out processes in a simple, quick and reconfigurable manner. Further, there is a need in the art to provide a system devoid of inserts in the pipes/tubes, and yet retain the efficiency of the system. Also, a modular system, avoiding a tube-in-tube approach is preferred.

Tube-in-tube is a typical heat exchanger. In tube-in-tube system the projections from outer tube side help to increase the turbulence thereby enhancing the heat transfer rates. We are focusing on its application as rector where the pinching helps mixing inside the tubes without affecting the heat transfer

OBJECT OF THE INVENTION

Main object of the present invention is to provide a flow reactor that enhances mixing and heat transfer.

Another object of the present invention is to pro vide a method for enhancing mixing and heat SUMMARY OF THE INVENTION

Accordingly, the present invention provides a flow reactor, comprising a pipe (100) including at least a first pinched section (102·,) and a second pinched section (102₂), wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

each of the first and second pinched sections comprises a set of pinch portions (202) defining a pinch axis (208 ) and a set of expansion portions (207) defining an expansion axis (209); the pinch axis (208) being substantially perpendicular to the expansion axis (209);

an angle between the pinch axis of the first pinched section (102i) and the pinch axis of the second pinched section (102₂) is between 0º and 90°; and

a perimeter of the first pinched section (1020, the non-pinched section (101) and the second pinched section (102?.) are substantially same.

Additionally, the present invention provides a heat transfer method or a method of mixing, comprising allowing a fluid to pass through a flow reactor comprising a pipe ( 100) having at least a first pinched section (102 j) and a second pinched section (102?), wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

each of the first and second pinched sections comprises a set of pinch portions (202) defining a pinch axis (208) and a set of expansion portions (207) defining an expansion axis (209); the pinch axis (208) is substantially perpendicular to the expansion axis (209);

an angle between the pinch axis of the first pinched section (102 ]) and the pinch axis of the second pinched section (102₂) is between 0° and 90°; and

a perimeter of the first pinched section (102 ]), the non-pinched section (101) and the second pinched section (102₂) are substantially same.

To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings. Brief Description of Figures:

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Fig, 1 illustrates an overview of the pinched pipe in accordance with an embodiment of the present invention;

Fig. 2 illustrates an isometric view of the pinched tube in accordance with a first embodiment of the present invention;

Fig. 3 illustrates an isometric view of the pinched tube in accordance with a second embodiment of the present invention;

Fig. 4A shows the front view of the pinched section 102 and Fig. 4B shows a top view of the pinched section 102;

Fig. 5 A shows a cross sectional front view of the pinched section 102 and Fig. 5B sows a cross sectional top view of the pinched section;

Fig. 6A shows the front view of a pipe having three pinched sections 102 joined together by non-pinched sections in accordance with an embodiment of the present invention wherein the angle between the pinch axis of the first pinch section and the pinch axis of the second pinch section is 90° and similarly, the angle between the pinch axis of the second pinch section and the pinch axis of the third pinch section is 90" and Fig. 6B shows a top view of the same;

Fig, 7 shows the front cross sectional view of the pipe having first and second pinched sections 102 joined by a non-pinched section, wherein the angle between the pinch axis of the first pinch section and the pinch axis of the second pinch section is 90°;

Fig, 8 shows cross sectional view of the pipe as illustrated in Fig. 7;

Fig. 9 illustrates a front view of a pipe with sequence of pinched sections [105] having pinch axis oriented at about 45° with respect to the previous pinch axis;

Fig 10 illustrates cross-section of a pipe shown in Fig. 9 showing the expansion regions [207 A, 207B, 207C and 207D] from successive pinch sections having 45° tilt.

Fig. 11A illustrates a serpentine configuration of the pinched pipe having sequence of segments [103] with unidirectional pinch and Fig. 1 IB illustrates a serpentine configuration of the pinched pipe having sequence of alternately pinched sections having axis perpendicular to the previous pinch;

Fig, 12 illustrates tracer response curves for pinch pipe-with variation of pinched cross- section; and

Fig. 13 illustrates enhancement in the heat transfer coefficient estimated as the ratio of heat transfer coefficient with the pinched tube to that of the straight tube at identical Reynolds number as shown in Fig. 12.

Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details thai will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

Detailed Description:

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. It will be understood by those skil led in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

Reference throughout this specification to "an aspect", "another aspect" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase "in an embodiment^5*, "in another embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises... a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.

For the purposes of this document, Pipe and tube are the same.

For the purposes of this document, "pinching" is defined as: decreasing the diameter or cross sectional area of a pipe or tube by any mechanical means, while retaining the perimeter of the pipe or tube.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

Referring to Fig. 1 , the present invention provides a tube or a pipe comprising a plurality of pinched sections (102) joined by non-pinched sections (101), wherein the pinched sections and the non-pinched sections are alternative placed. More particularly, referring to Fig. 2 and Fig. 3, wherein Fig. 2 provides an isometric view of a pipe (100) in accordance with a first embodiment and Fig. 3 provides an isometric view of a pipe (100) in accordance with a second embodiment, it can be noticed that the present invention provides a flow reactor, comprising a pipe (100) including at least a first pinched section ( 102 j ) and a second pinched section (102₂), wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

an angle between the pinch axis (208) of the first pinched section (1020 and the pinch axis (208) of the second pinched section (102?) is between 0° and 90°; and

a perimeter of the first pinched section (102₃), the non-pinched section (101) and the second pinched section (102₂) are substantially same.

In accordance with an embodiment of the present invention, the first pinched section (102₁), the non-pinched section (101) and the second pinched section (102₂) is repetitively arranged to obtain the tube (100) of any desired length. In accordance with another embodiment of the present invention, two non-pinched sections (102i and 102 <) have equal length.

In accordance with yet another embodiment of the present invention, two non-pinched sections have (102i and 102₂) un-equal lengths.

In accordance with still another embodiment of the present invention, a length of the pinched section (102i or 102₂) is equal to a length of the non-pinched section (101).

In accordance with a further embodiment of the present invention, a length of the first pinched section (102) or 102₂.) is not equal to a length of the non-pinched section (101).

In accordance with a furthermore embodiment of the present invention, a length of each of the first pinched section(102i), the second pinched section (102₂) and the non-pinched section (101) is greater than a diameter of the pipe.

In accordance with another embodiment of the present invention, the first pinched section (102i) and the non-pinched section (101) are formed together. In accordance with yet another embodiment of the present invention, the first pinched section(102i), the non-pinched section (101) and the second pinched section (102₂) are formed together.

In accordance with still another embodiment of the present invention, at least one of the first pinched section(102₁), the non-pinched section (101) or the second pinched section ( 102₂) is formed separately and connected to the remaining sections using a connector.

In accordance with a further embodiment of the present invention, the cross section of the pinched section ( 1021 or 102_?.) is circular or elliptical. (QUERY: HOW CAN IT BE CIRCULAR?)

In accordance with a furthermore embodiment of the present invention, a shape of the pinch portion (202) is selected from a straight shape, a curved shape or a combination thereof. In another embodiment of the present invention, an inwardly projecting length of the set of pinch portions (202) forming part of the first pinch section (102]) is equal to an inwardly projecting length of the set of pinch portions (202) forming part of the second pinch section (102₂). In yet another embodiment of the present invention, an outwardly projecting length of the set of expansion portions (207) forming part of the first pinch section ( 102 j ) is equal to an outwardly projecting length of the set of expansion portions (207) forming part of the second pinch section ( 102₂). In still another embodimeni of the present invention, an inwardly projecting length of the set of pinch portions (202) forming part of the first pinch section (1020 is not equal to an inwardly projecting length of the set of pinch portions (202) forming part of the second pinch section { 102;:). In a further embodiment of the present invention, an outwardly projecting length of the set of expansion portions (207) forming part of the first pinch section (102s) is not equal to an outwardly projecting length of the set of expansion portions (207) forming part of the second pinch section (102?). In a furthermore embodiment of the present invention, the internal cross section of the first pinched section (1020 is not equal to the internal cross section of the non-pinched section (101). In still another embodiment of the present invention, the internal cross section of the second pinched section (102₂) is not equal to the internal cross section of the non-pinched section (101).

Additionally, the present invention provides a heat transfer method, comprising allowing a fluid to pass through a flow reactor comprising a pipe (100) having at least a first pinched section (1020 ^and a second pinched section { 102 ··}. wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

an angle between the pinch axis (208) of the first pinched section (1020 and the pinch axis (208) of the second pinched section (102₂) is between 0° and 90°; and

a perimeter of the first pinched section(1020, the non-pinched section (101) and the second pinched section (102₂) are substantially same.

Referring to Fig. 4, the expanded view of a pinch section and more particular an expanded front view is shown in Fig. 4A and an expanded top view is shown in Fig. 4B. Since the pinch potions and the expansion portions are visible in the front and the top view respectively, the expansion [207] caused by converging / pinched section [202] can be said to be yielded in a direction perpendicular to pinch direction.

Referring to Fig, 5A and Fig. 5B, which are the cross section front view and the cross sectional top view, the inner construction of the pinch portion and the expansion portions are demonstrated. It can be said that the cross section of said pinch portion (202) is circular or ellipse and can be caused by a pinch (201 ). The inner diameter at the pinched portion 202 is less than the pipe diameter (204). On the other hand, the cross section (205) as well as the perimeter (206) of the expanded portion is greater than the pipe diameter 204. In yet another embodiment of the present invention, shape of said pinched portion is selected from a straight shape, a curved shape or combination thereof connected to the pipe on either sides of pipe. In yet another embodiment of the present invention, connections between two pinched sections or a pinched and non-pinched section of the pipe is achieved by chemical, physical or physiochemical means.

In yet another embodiment of the present invention, said pipes are made up of metallic or non-metallic materials.

It can be said that the present invention provides a pinched flow reactor to enhance mixing and heat transfer via modification of flow area without affecting the perimeter of the pipe comprising at least one pinched section having same or different extent of pinch followed by a diverging section connected to a straight section, charecterised in that axis of the each pinch section is between 0° to 90° and arranged in different perpendicular planes to the pinch.

The pinched section may have a straight shape, a curved shape or combinations thereof connected to the pipe on either sides. The extent of pinch therefore determines the decrease in diameter and cross sectional of the pipe. The pinched pipes of different sizes or extent of pinch may be arranged together in any numbers and any arrangement in periodic or aperiodic sequences. Further, the entire sequence of simil ar or different extent of pinch and the distance between subsequent pinch points can be made from a single pipe or multiple pipe sections connected to each other using connectors that facilitate the connection with two or more similar or different components in periodic or aperiodic sequences. The connections between two pinched pipes or between two pinched sections of a tube may be achieved by chemical, physical or physico-chemical means.

The pinched section may be arranged in periodic or aperiodic manner. The subsequent pinched sections are joined by a single pipe or connected by connectors facilitating the connection with two or more components in periodic or aperiodic sequences. The pinched sections may be joined together by physical, chemical or physiochemical means. The pipes are made up of metallic or non-metallic materials. The pinched section may have a straight shape, a curved shape or combinations thereof connected to a pinched section on either sides, The pinched section may have a straight shape, a curved shape or combinations thereof connected to a pinched section on one side and to the pipe on the other side.

The pinched pipes of the invention may be of configurations selected from straight, spiral, helical coils and such like or combinations thereof.

The pinched tube/pipe comprises of a non-pinched section (of specific diameter), a converging / pinched section [102] yielding reduction in the flow area (at constant perimeter) followed by a non-pinched section [101].

Referring to Fig. 6A, which demonstrate front view of a pipe 100, it can be seen that the same comprises of three pinched sections 1021 , 102? and 102₃, which are separated from each other by non-pinched sections 101. The three pinched section are unidirectional i.e. the pinch axis of the first pinched section, the second pinched section and the third pinched section are parallel to each other (i.e. 0°). Fig. 6B shows the top view showing the corresponding expansion portions. Referring to Fig. 7, which demonstrate cross sectional front view of a pipe 100, it can be seen that the same comprises of two pinched sections 102_\ and 102₂, which are separated from each other by non-pinched sections 101 . The two pinched section are non-unidirectional i.e. the pinch axis of the first pinched section is not parallel to each other and in accordance with this particular embodiment, make an angle of 90°.

Fig. 8 shows the cross sectional view of the pipe 100 as shown in Fig. 7 specifically showing the expansion portions 207A and 207B. It can be said that the expansion portion 207B corresponds to the first pinched section 102i while the expansion portion 207B corresponds to the second pinched section 102₂.

In yet another alternative construction, as shown in Fig, 9, the pipe 100 comprises of a first, a second, a third and a forth pinch sections which are joined by non-pinched sections 101. in this embodiment, the pinch axis rotates gradually and by 45° with respect to a previous pinch axis. Fig. 10 shows the cross sectional view of the aforesaid pipe.

A few examples are provided to illustrates the working of the present invention. It should be however, noted that the examples provided hereinafter are only by way of illustration and not meant to restrict the scope of the claims in any manner. The scope of the claims is intended to be restricted only on the basis of the claims and their equivalents.

EXAMPLE 1: Extent of dispersion

Residence time distribution for a tracer pulse was estimated numerically by solving the transport equations through computational fluid dynamics. This method is used for exploring the nature of mixing in the device. The residence time distribution (RTD) which actually indicates the extent of dispersion in the system were measured for the complete device and also the sequence of middle converging section of the micro mixer alone. The simulated RTD curves for the computational domain are illustrated in Fig. 12.

Example 2: Flow synthesis

Reaction of bromobenzene in acetic acid (bromobenzene to acetic acid volume ratio was kept at 1 :5) with nitrating mixture (60:40 v/v, 68% HN0₃ & H₂S0₄ respectively) was carried out using only the coil made of straight tube and the coil made using the pinched pipe. The pinched pipe having an outer diameter D, the distance between two successive pinch sections was maintained at 10D. In both the cases, simple T-joint was used for bringing the two reactants together. For the case of normal helical coil, the conversion at the outlet at 80 ^bC and a residence time of 10 minutes was 76%, while with a helical coil made out of pinched tube with successive perpendicular pinch at a distance of 10D yielded 84.6% conversion.

Example 3: Flow synthesis

For the case of Example 3 and the conditions therein, when the distance between two successive pinch was maintained at 5D and all the pinch in the same direction, the conversion at the outlet was 92.7%.

Example 4: Flow synthesis For the case of Example 3 and the conditions therein, when the distance between two successive pinch was maintained at 51) and the successive pinch being applied perpendicular to the previous, the conversion at the outlet was 95.3%.

Example 5:

The enhancement in the heat transfer coefficient estimated as the ratio of heat transfer coefficient with the pinched tube to that of the straight tube at identical Reynolds number is shown in Fig. 13. In the figure R/a is the ratio of distance between two pinch positions to the tube diameter. L/T) is the ratio of length of tube to the tube diameter. The data is compared with the enhancement ratio for twisted tape at identical L/D, where it continuously decreases with increasing Reynolds Number.

ADVANTAGES OF INVENTION

® Retains agility and re-configurability of the continuous chemical processes, with improved mixing and heat transfer ability.

® Affords modularity to construction of tube/pipe.

* A voids inserts and so aids in ease of cleaning.

While specific language has been used to describe the disclosure, any limitations arising on account of the same are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implerneni the inventive concept as taught herein.

The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions any flow diagram need not be implemented in the order shown; nor do ail of the acts necessarily need to be performed. Also, those acts that are not dependent on oilier acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.

Claims

1. A flow reactor, comprising a pipe (100) including at least a first and a second pinched sections (102), wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

each of the first and second pinched sections comprises a set of pinch portions (202) defining a pinch axis and a set of expansion portions (207) defining an expansion axis;

the pinch axis is substantially perpendicular to the expansion axis;

an angle between the pinch axis of the first pinched section and the pinch axis of the second pinched section is between 0° and 90°; and

a perimeter of the first pinched section, the non-pinched section and the second pinched section are substantially same.

2. The flow reactor as claimed in claim 1 , wherein the first pinched section, the non- pinched section and the second pinched section is repetitively arranged.

3. The flow reactor as claimed in claim 2, wherein two non-pinched sections have equal length.

4. The flow reactor as claimed in claim 2, wherein two non-pinched sections have unequal lengths.

5. The flow reactor as claimed in claim 1, wherein a length of the first pinched section is equal to a length of the second pinched section.

6. The flow reactor as claimed in claim 1 , wherein a length of the first pinched section is not equal to a length of the second pinched section.

7. The flow reactor as claimed in claim 1, wherein a length of each of the first pinched section, the second pinched section and the non-pinched section is greater than a diameter of the pipe.

8. The flow reactor as claimed in claim 1 , wherein the first pinched section and the non- pinched section are formed together.

9. The flow reactor as claimed in claim 1, wherein the first pinched section, the non- pinched section and the second pinched section are formed together.

10. The flow reactor as claimed in claim 1, wherein at least one of the first pinched section, the non-pinched section or the second pinched section is formed separately and connected to the remaining sections using a connector.

11. The flow reactor as claimed in claim 1 , wherein the cross section of the pinched section is circular or elliptical. (NOTE: HOW CAN IT BE CIRCULAR?)

12. The flow reactor as claimed in claim 1 , wherein a shape of the pinch portion is selected from a straight shape, a curved shape or a combination thereof.

13. The flow reactor as claimed in claim 1 , wherein an inwardly projecting length of the set of pinch portions forming part of the first pinch section is equal to an inwardly projecting length of the set of pinch portions forming part of the second pinch section.

14. The flow reactor as claimed in claim 1 , wherein an outwardly projecting length of the set of expansion portions forming part of the first pinch section is equal to an outwardly projecting length of the set of expansion portions forming part of the second pinch section.

15. The flow reactor as claimed in claim 1, wherein an inwardly projecting length of the set of pinch portions forming part of the first pinch section is not equal to an inwardly projecting length of the set of pinch portions forming part of the second pinch section.

16. The flow reactor as claimed in claim 1, wherein an outwardly projecting length of the set of expansion portions forming part of the first pinch section is not equal to an outwardly projecting length of the set of expansion portions forming part of the second pinch section.

17. The flow reactor as claimed in claim 1, wherein the internal cross section of the first pinched section is not equal to the internal cross section of the non-pinched section.

18. The flow reactor as claimed in claim 1, wherein the internal cross section of the second pinched section is not equal to the internal cross section of the non-pinched section.

19. A heat transfer method or a method of mixing, comprising allowing a fluid to pass through a flow reactor comprising a pipe (100) having at least a first and a second pinched sections (102), wherein the first and the second pinched sections are spaced apart from each other and include there between a non-pinched section (101); characterized in that:

the pinch axis is substantially perpendicular to the expansion axis;