FR2780772A1 - Plate heat exchanger for use in a crystallization process - Google Patents

Abstract

The exchanger contains a number of elements (6) exchanging heat between the first and second fluids, parallel to the plates. The elements are placed next to each other both parallel and perpendicular to the plates over their whole length and at their ends are distributors and guides so the fluids circulate in contra-flow in each element and follow the whole length of each element in succession Plate heat exchanger with first and second fluids circulating through it, with a considerable length, in particular for an installation for crystallizing a substance contained in a liquid (aqueous) solution comprising first and second fluids. The heat exchange elements are parallelepipeds with square or rectangular bases. The distributor and guides includes collectors fixed to the ends of the heat exchange elements so the two fluids flow from the end of one element to the adjacent one. At least some collectors have walls dividing them into two compartments to distribute the first and second fluid respectively. The distributor and guides also have pipes connecting distribution and reception collectors, one for each fluid. Crystallization process for a chemical contained in solution, for example aqueous, in the form of purified and calibrated crystals, using such a plate heat exchanger where a first fluid containing non-calibrated and impure crystals is circulated to be heated by exchange with a second fluid at a dissolution temperature to produce a homogenous solution containing the impurities. This solution is introduced into a reactor to separate the impurities out and produce a second purified solution, which is circulated through the heat exchanger as the second fluid. At the outlet for the second fluid, water and purified and calibrated crystals are recovered. The fluids flow through the exchanger at a rate of between 0.5 and 1 m/s and a pressure of the order of 200 bars. The channels in the exchanger have a length of between 20 and 300m. The first solution is heated to a temperature of the order of 300 degrees C before passing to the reactor. The chemical being crystallized is paraxylene, dimethyl naphthalene, an aromatic carboxylic acid or an anhydride such as terephthalic acid or isophthalic acid, trimellitic acid, trimellitic anhydride, dicarboxylic naphthalene acid, diphenylcarboxylic acid, pyromellitic acid or pyromellitic anhydride and oxybisbenzoic acid.

Description

The invention relates to a plate heat exchanger which can be

  used in particular in a crystallization plant and a process for

  crystallization using the heat exchanger.

  In the chemical industry, for the production of substances in the form of crystals or calibrated particles, it may be necessary to crystallize the substance contained in an aqueous solution. he

  this is particularly so in the case of the production of terephthalic acid.

  as (TA) and purified terephthalic acid (PTA).

  After a chemical preparation, an aqueous solution is obtained

  containing crystals of the substance in suspension. These crystals

  close impurities and their dimensions lie, for a large part

  tie of them, apart from the desired crystal sizes in the finished product.

  It is therefore necessary to carry out the purification and crystallization of the substance, so as to obtain calibrated crystals or particles of

  perfectly determined dimensions and containing only residual contents

very weak dual impurities.

  For this, in particular in the case of terephthalic acid, the starting solution is brought to a high temperature for dissolving the crystals

  of the substance, the homogeneous solution obtained at high temperature is introduced.

  erasing in a chemical reactor carrying out the separation of impurities and

  cools the purified homogeneous solution, so as to obtain a crystallization

  tion of the substance in the form of perfectly calcified crystals or particles

  brées. The homogeneous solution at a temperature of the order of 300 C under one. pressure of 200 bar, which has been purified in the chemical reactor, undergoes several expansion and cooling in successive tanks. In the last container, the substance is obtained in the purified and crystallized state, with a determined crystal size, suspended in water. It is enough then

  to ensure the separation of crystals and water.

  The starting solution containing the unpurified crystals is heated by successive passages in heat exchangers, in which the heating of the solution is ensured by the vapor recovered in each

  expansion tanks of the purified solution. At the end of the last exchange

  geur, a homogeneous solution is obtained in which the crystals contain

  mant the impurities are dissolved, this solution being at a high temperature

ture, of the order of 300 C.

  Such a purification and crystallization installation, which requires the use of successive expansion tanks, for example four tanks, heat exchangers associated with the expansion tanks and

  connecting pipes between the different elements of the installation, is com-

  flexible and costly in terms of the investments necessary for its construction

operation costs.

  It would therefore be desirable to have a method and a device for dissolving the crystals and bringing them to temperature

  of the solution before entering the chemical reactor as well as cooling

  smoothing of the solution at the outlet of the chemical reactor and crystallization of the substance in the form of crystals of a required size, by continuous circulation of the starting solution and of the solution recovered at the outlet of the

  chemical reactor, in a single heat exchanger.

  However, the crystallization process of the substance in the form of calibrated particles requires a very slow circulation speed of the purified solution in the heat exchanger, this speed possibly being of the order of 0.5 to 1 m / s and a very long path of the solution

  and purified solution inside the heat exchanger.

  Due to the very low speed of circulation of the solution in

  heat exchanger, there are significant risks of deposition of

  rates formed or present in the solution and clogging of the canals

The heat exchanger.

  Plate heat exchangers are known which consist of stacks of sheet metal plates, for example steel

  resistant to corrosion, between which are circulated channels

  tion of at least two heat exchange fluids. The sheets are formed so as to have corrugations along sinuous paths which lengthen the path of the exchange fluids inside the plate exchanger and thus promote heat exchanges. In addition, these sinuous paths make it possible to obtain a turbulent flow of the fluids in the channels of the plate heat exchanger, even at a very low speed.

  circulation of these fluids, and therefore promote heat exchange.

  Plate exchangers can therefore appear to be suitable for the crystallization process of substances contained in an aqueous solution. However, it is necessary to provide exchange paths between very long fluids, of the order of 20 to 300 m, so

  than conventional plate heat exchangers have

  so many such exchange path lengths can have dimensions

  sions considerable and in particular a length of the same order as the

  length of the exchange route. Such lengths are not designed

  due to the cost of manufacturing very long plate heat exchangers, their excessive bulk in chemical production plants and technical difficulties which may arise during

  of their use and maintenance.

  In areas other than crystallization, it can also be

  necessary to have plate heat exchangers having a par-

  very long exchange fluid circulation courses, without

  the overall size of the plate heat exchanger is not excessive.

  The object of the invention is therefore to propose a plate heat exchanger having a circulation path with mutual heat exchange,

  a first and at least one second fluid, of great length, in particular

  bind for a crystallization installation of a substance contained in a liquid, for example aqueous solution constituting one of the fluids, this heat exchanger having a reasonable total bulk and

  economic operating conditions.

  For this purpose, the plate heat exchanger according to the invention comprises a plurality of heat exchange elements between the first and the second fluids each consisting of a stack of corrugated plates between which are provided spaces for circulation of the first and second fluid, in a general direction corresponding to a direction

  longitudinal of the plates, the elements being arranged so as to

  center with respect to each other in a first transverse direction

  dirty parallel to the plates and in a second transverse direction per-

  pendular to the plates, so as to be juxtaposed along their entire length

  and comprising at their longitudinal ends means for distributing

  bution and guidance of the first and second fluids, such as the first and the

  second fluids flow against the current in each of the elements and follow

  the entire length of each of the elements, successively. The invention also relates to a crystallization process

  of a substance contained in a solution, by passage through a sample

plate heat gor.

  In order to clearly understand the invention, a description will now be given, by way of example, of a crystallization installation comprising a heat exchanger according to the invention, the heat exchanger and its process.

  of use for the crystallization of a solution.

  FIG. 1 is a schematic view of an installation for crystallization

  tion comprising a plate heat exchanger according to the invention.

  Figure 2 is an exploded perspective view of the plate heat exchanger

according to the invention.

  Figure 3 is a side view along 3 of the plate heat exchanger.

  Figure 4 is an end view along 4 of the plate heat exchanger

shown in figure 2.

  Figure 5 is a perspective view of an end portion of a

  exchange element of the plate heat exchanger.

  Figure 6 is an end view along 6 of Figure 5 of the element

  plate heat exchanger.

  FIG. 7 is a schematic view of the front part of a heat exchanger

  plate ger according to the invention showing the distribution and circulation of

  first and second exchange fluids in the plate heat exchanger.

  In Figure 1, there is shown schematically an installation

  tion for carrying out the purification and crystallization in the form of calibrated crystals of a substance such as terephthalic acid contained

in an aqueous solution.

  The installation, generally designated by the reference 1, comprises

  carries a chemical reactor 2 making it possible to extract the impurities of a chemical substance in a homogeneous solution at high temperature and a

  plate heat exchanger 3 according to the invention.

  The installation 1 is intended to carry out the purification and the calibration of the crystals of a substance contained in an aqueous solution, following a chemical manufacturing process. The substance is in the form

  crystals in suspension in the starting solution, these crystals contain

  mant of impurities and having sizes generally falling below

  outside the size range required for the crystals of the finished product.

  The starting solution is introduced, at a temperature which may be of the order of 100 C, under a high pressure of the order of 200 bars, into the

  inlet pipe la of a first exchange fluid in the plate heat exchanger

  ques 3. The starting solution circulates in the channels reserved for the first exchange fluid of the plate exchanger 3, along the entire length of these channels, as indicated by the arrow in the upper part of

The plate heat exchanger 3.

  The solution recovered at the outlet of the chemical reactor 2 is introduced into the circulation channels of the second exchange fluid of the plate exchanger 3, via the inlet line ld of the second exchange fluid in

  the plate heat exchanger 3. The solution from the chemical reactor 2 circulates

  flows in the plate heat exchanger, along the entire length of the channels

  served to the second exchange fluid, as represented by the arrow in the

  lower part of the plate heat exchanger 3.

  By heat exchange between the fluids, the starting solution is brought to a temperature of the order of 300 C, under a pressure of 200 bars, the crystals containing the impurities of the starting solution are gradually dissolved in the solution, if although at the outlet lc of the first exchange fluid from the plate exchanger 3, a homogeneous solution is obtained at a temperature of the order of 300 C and which is introduced into the chemical reactor 2. Inside of chemical reactor 2, the homogeneous solution is

  purified, so that the impurities contained in the crystals of

  part are separated from the solution.

  The purified homogeneous solution recovered at 300 at 200 bars at the outlet of the chemical reactor 2 is introduced into the line 1 d entering the

  second exchange fluid in the plate exchanger 3.

  By heat exchange with the starting solution which circulates at

  very current of the purified solution in the plate exchanger 3, the homogeneous purified solution is cooled gradually, so that pure crystals suspended in the solution are formed in this solution

  aqueous whose chemical content gradually decreases.

  When the solution purified by the outlet pipe of the second exchange fluid 1b from the plate exchanger 3 is recovered, this solution is in

  made up of water containing suspended crystals perfect-

  calibrated. The circulation at very low speed of the purified solution inside the plate exchanger 3 made it possible to achieve crystallization under conditions such that the crystals formed were all of the required size.

  for the finished product or fall within a very narrow size range.

  In addition, the flow of the purified homogeneous solution inside

  of the heat exchanger is a turbulent flow despite the low vi-

  circulation flow in the plate heat exchanger, in particular because the fluid circulation channels are delimited by sinuous undulations of the plates of the exchanger. This avoids any deposit of crystals

  inside the channels of the plate heat exchanger.

  The plate heat exchanger provides very good heat exchange efficiency, lowering the temperature of the solution between

  the inlet of the first exchange fluid of the plate exchanger 3 and the outlet

  tie 1b of the second exchange fluid of the plate exchanger being less than C. To carry out the desired heat exchanges in the plate heat exchanger 3, it is necessary to have a long circulation path for the fluids exchange for example of the order of 20 to 300 m. The heat exchanger

  plate heat 3, shown in particular in Figure 2, allows

  read such a long course of fluids without requiring an over-

  dimensioning of the heat exchanger.

  The heat exchanger 3, shown in FIG. 2, has a very thick outer casing 4 in which the bundle of the plate heat exchanger is placed.

  The bundle 5 of the plate heat exchanger consists of elements

  exchange elements 6 each produced in the form of a stack of plates

  ques 7 made of stainless steel sheets having a very long length

  less than their width. For example, it is possible to use sheets having a width of the order of 0.5 m and a length of the order of 15 m. The elements 6 of the bundle 5 of the heat exchanger have substantially the shape

  of parallelepipeds having a square cross section.

  The exchange bundle 5 comprises sixteen exchange elements 6 which are arranged relative to each other in an adjacent manner, in

  a first transverse direction parallel to the plates (transverse direction

  horizontal horizontal in Figure 2) and in a second transverse direction

  dirty perpendicular to the plates (vertical direction in figure 2).

  The sixteen exchange elements 6 are juxtaposed along their entire length, inside the bundle 5 and separated from each other by

spaces 8.

  The exchange elements 6 juxtaposed constitute four rows ver-

  tical, respectively designated by the letters a, b, c and d, and four ran-

  horizontal ages designated respectively by the letters a ', b', c 'and d' (fi-

gure 7).

  Each exchange element is arranged in a vertical row or column and in a horizontal row which define its position in a square mesh network. We will designate any exchange element of the set of exchange elements by the reference 6 followed by the indication

  of the column and the horizontal row in which this element is located

  is lying. For example, the element located in the angle of the beam in the first

  column a and in the bottom horizontal row a 'will be denoted by

6aa '.

  Referring to Figures 2 and 3, as well as Figures 5 and 6, we see

  that each of the elements 6, of generally parallelepiped shape, comprises

  carries an outer casing 9 consisting of sheets assembled together in the form of a parallelepipedic box in which are mounted and fixed, in parallel arrangements, the plates 7 of the element

  for exchanging the plate heat exchanger made of sheets, the length of which

  length corresponds to the length of an exchange element and to the length to-

  beam tale. The sheets 7 have undulations having a path

  sinuous in a generally longitudinal direction.

  As can be seen in FIGS. 5 and 6, each heat exchange element 6 has, in its end portions, a central partition

  parallel to two sides of the parallelepiped housing 9.

  The sheets 7 are fixed along their entire length to the side walls.

  rales of the housing 9 and, at their ends, on the central partition 10. The sheets

  7 are thus maintained in parallel arrangements with one another, in

  denier that fluid circulation channels 11 are formed between two

  successive and superimposed sheets 7.

  As can be seen in FIG. 5, the housing 9 of the exchange elements 6 comprises, fixed in the extension of the upper wall and of the lower wall of the housing, two thick plates 12 and 12 ′ ensuring the maintenance in superposition with a spacing 8 of housings of two

  adjacent exchange elements. The thick plates 12 and 12 'are equally

  ment overflowing on the lateral sides of the housing 9, so as to ensure

  a space between the housings of the heat exchange elements available

  sés in vertical rows or adjacent columns.

  The channels 11 for circulation of the exchange fluids of each of the exchange elements 6 open, at each of the longitudinal ends of the exchange element, into a fluid distribution chamber of a

  collector, by a flat end face 13 or 13 ′ of the stack of plates

  ques 7 and the housing 9. The end portions of the plates 7 have edges directed angularly and symmetrically with respect to the longitudinal direction of the plates, so that the planar faces 13 and 13 ′ of

  The stack forms between them a dihedral angle greater than 90.

  The manifold chambers into which the circulation channels 11 open are divided into two compartments, at the level of each of the exchange elements, by an extension towards the outside of the partition.

  median 10 of the stack. One of the compartments, in which a pre-

  mier set of channels 11 opens, by the face 13 of the stack, as-

  on the distribution or recovery of a first exchange fluid and the other compartment, into which a second set of circulation channels 11 opens, through the face 13 ′ of the stack, ensures the distribution or the recovery a second exchange fluid. The inclination of the faces 13 and 13 '

  eliminates or reduces areas of fluid stagnation.

  As can be seen in FIG. 6, the end of one channel out of two in the transverse direction perpendicular to the plates 7 is closed off, along the face 13 and along the face 13 ′ of the stack, the channels closed along the face 13 being inserted between two channels closed along the face

  13 'and vice versa. In this way, successive channels 11 in the direction

  transverse tion perpendicular to the plates 7 are traversed alternative-

  ment by the first and by the second fluids, the longitudinal ends of each of the channels 11 opening into two compartments of manifolds for distributing the same fluid being either both closed or all

two open.

  In this way, a circulation of a first fluid is ensured

  the entire length of the exchange element, in a first direction, inside a first set of channels 11 of the exchange element and the circulation in a second direction, along the length of the element

  of exchange, of a second exchange fluid, in a second set of

  channels 11 each inserted between two channels of the first set. The channels 11 each delimited between two plates 7 of the plate exchanger are separated into two parts by the partition 10 only in the vicinity of their ends where distribution or recovery of the exchange fluids is ensured. The exchange fluids then circulate, along the length of the exchange element, across the width of the channels 11, between the walls

  sides of the housing 9 of the exchange element.

  We will now describe all the collectors allowing

  to ensure the circulation of two exchange fluids in the heat exchanger

  their with plates 3, so that inside each of the exchange elements 6, the first and second exchange fluids flow in opposite directions along the length of the exchange element and that each exchange fluids pass successively, along their entire length,

each of the exchange elements 6.

  The different collectors of the heat exchanger will be described in particular with reference to Figures 2, 3 and 4. The collectors are constituted by sheet metal envelopes of substantially semi-cylindrical shape arranged in the direction of the vertical rows or columns of elements d exchange, closed at their ends by

  additional closing walls fixed in a sensitive contour

  slightly rectangular or square on the walls or the middle partition of

  housings of the exchange elements, at the longitudinal ends of these elements

  ment. A first and a second manifold 14, 14 ′ ensure that

  the inlet and outlet of the first and second exchange fluids in the

  heat exchanger, by an end face of the heat exchanger bundle which will be designated as the inlet face of

  the heat exchanger and which is located on the right in Figures 2 and 3.

  The first manifold 14, by which the entry of the first exchange fluid and the outlet of the second exchange fluid is ensured, is fixed along the contour of the end of the housing of the exchange element 6aa 'arranged in the lower left corner of the heat exchanger harness. The collector has an open square contour which is fixed along the contour of the end of the housing of the exchange element 6aa 'and an internal volume which is separated into two half-compartments by the extension towards the outside.

  of the partition 10 of the end part of the exchange element 6aa '.

  In a first compartment of the collector 14 there opens a con

  pick 15 for feeding the first exchange fluid into the bundle of

the heat exchanger.

  In the second compartment of the collector 14 there opens a con

  pick 16 for the outlet of the second exchange fluid. Lines 15 and 16 work

  pour the outer casing 4 of the heat exchanger in the same direction parallel to the axial direction of the casing 4. The pipe 16 has two bent parts, so as to be offset towards the part

  axial center of the heat exchanger, in relation to line 15.

  A second manifold 14 'having a shape identical to the shape of the manifold 14 is fixed to the end part of the housing of the exchange element 6da' situated on the side of the entry face of the bundle 5 of

  the heat exchanger. The collector 14 'has two compartments.

  In a first compartment of the manifold 14 ', there is a pipe 15' for the outlet of the first exchange fluid and in the second compartment of the manifold 14 'there is a pipe 16' for the inlet of the second exchange fluid. As well as lines 15 and 16, lines 15 'and 16'

  pass through the outer casing 4 of the heat exchanger in a direction

axial tion.

  In FIG. 4, the arrangement of the connection flanges is shown.

  lines 15, 16, 15 'and 16' relative to the convex bottom of the casing 4 of the heat exchanger. The four connection flanges of the inlet and outlet pipes for the exchange fluids are located on the same side of the casing 4 and in neighboring areas, which facilitates the

  construction of the fluid circuit of the crystallization installation.

  The heat exchanger further comprises six collectors each delimiting an outlet or inlet compartment for one of the exchange fluids added along an end face of an exchange element, at the level of

  the inlet face of the heat exchanger.

  Four collectors are arranged at the ends of exchange elements of the upper horizontal row of. These collectors will be designated by the references giving the position of the exchange elements with which they are associated, these collectors 17ad ', 17bd', 17cd 'and 17dd' being associated respectively with the exchange elements 6ad ', 6bd', 6cd 'and 6d of

  the horizontal top row of. These collectors delimit comparisons

  distribution or recovery of the first exchange fluid.

  Similarly, two collectors 17ba 'and 17ca' are arranged in the ran-

  horizontal bottom row a ', at entry compartments or

  outlet of the second fluid in the exchange elements 6ca 'and 6ba'.

  Four collectors 18a, 18b, 18c and 18d comprising an open contour of rectangular shape are each fixed on the end parts of the entry face of two superposed exchange elements in a row a,

  b, c or d vertical exchange elements.

  The collectors 18a, 18b, 18c and 18d, which are each separated into two compartments by the partitions 10 of the exchange elements ensure the transmission of the first and second exchange fluids between the parts

  end of the side of the inlet face of the two super exchange elements

  poses according to which they are placed.

  Similarly, on the face of the exchange bundle 5 opposite the inlet face are placed two rows of collectors 18'a, 18'b, 18'c and 18'd and 18 "a, 18" b, 18 "c and 18" d, analogous to the collectors 18a, 18b, 18c and 18d, and each ensuring the junction of the end portions on the side of the face of

  exit of the beam, of two exchange elements superimposed in the direction

  vertical tion in a column a, b, c or d of exchange elements.

  Finally, collectors in the form of a cylinder portion are arranged so as to cover two distribution compartments of the same fluid with two adjacent exchange elements, to put them in communication,

  on the entry side of the heat exchanger bundle.

  A first of these collectors 19 located in the horizontal row

  lower than exchange elements connects two compartments

  distribution elements of the first exchange fluid. Two other collectors 19 'are arranged in the upper row of exchange elements and connect two distribution compartments of the second

exchange fluid.

  Finally, two lines 20 connect, in the row

  horizontal upper of exchange elements, respectively, the collect-

  17ad 'and 17bd' teurs and 17cd 'and 17dd' manifolds for the distribution of the

mier exchange fluid.

  A pipe 20 'connects, in the lower horizontal row of exchange elements to' the collector 17ba 'and the collector 17ca'

  distribution of the second exchange fluid.

  In order to explain the operation of the heat exchanger

  and the circulation path of the first and second fluids in the sample

  heat gor, we will refer to Figure 7 in which there is shown schematically the inlet face of the bundle 5 of the heat exchanger.

  On the entrance face, we drew, in the form of rectangles juxtapo-

* sés, the end faces of the inlet and outlet compartments of the

  mier or second fluids. In the rectangles representing the end faces ensuring the entry or exit of the first fluid in an exchange element, the figure I has been indicated and in the rectangles representing the end faces ensuring the entry or exit of the second exchange fluid, the figure II has been indicated. It has also been indicated, inside each of the rectangles, symbolically, the direction of circulation of the fluid in the exchange element. The circulation in the forward direction, that is to say in the direction going from the entry face towards the exit face of the exchange beam, is indicated by a circle and the circulation in the return direction, it that is to say in the direction going from the exit face to the entry face of the exchange beam, is indicated by a circle inside which is drawn a cross. The rectangles with a circle therefore represent faces

  end of inlet compartments of the first or second fluid following

  vant the indication carried in the rectangle and the rectangles comprising a circle in which a cross is drawn therefore represent end faces of the first or second fluid outlet compartments

exchange.

  The two compartments of the manifold 14 of the exchange element 6aa 'located in the lower left corner in Figure 7 and the two compartments of the manifold 14' of the exchange element 6ad 'located in the lower right corner of the entry face of the exchange beam ensure respectively the entry of the first fluid and the exit of the second fluid and the entry of the second

  fluid and the output of the first fluid.

  Also shown in Figure 7, schematically, the connecting pipes 20 and 20 'and the collectors 19 and 19' ensuring the transfer of fluids between exchange elements located in the same

horizontal row.

  We will now describe the circulation of fluids in the beam

  plate heat exchanger, with particular reference to the figures

  res 2 and 7. The first exchange fluid is brought into the compartment reserved for the first fluid of the manifold 14 disposed at the end of the exchange element 6aa '. The first exchange fluid is distributed in the circulation spaces 11 which are reserved for it between the plates 7 and then circulates inside the exchange element 6aa 'in the forward direction. At the outlet of the exchange element 6aa ', the exchange fluid is transferred by the manifold 18'a into the end part on the exit face side of the reserved compartment

  to the first fluid of the exchange element 6ab 'superimposed on the element 6aa ;.

  The first exchange fluid then circulates in the return direction inside the

The element 6ab '.

  Due to the presence of the collectors 18a and 18 "a, the first fluid

  then circulates successively, in the outward and return directions, in the

  upper elements 6ac 'and 6ad' of the exchange element column a. The first fluid is then transferred from the collector 17ad 'to the collector 17bd'

through a pipe 20.

  The first fluid then flows successively through all of the elements.

  superimposed elements of the exchange element column b, being returned from one exchange element to another by the collectors 18, 18 'and 18 "located

  on the entry and exit faces of the exchange beam.

  25. When it has come out of the exchange element 6ba ', the first

  mier fluid is transferred by the collector 19 into the inlet compartment of the exchange element 6ca ', to flow successively in one direction and in the other in each of the exchange elements of column c

exchange items.

  At the outlet of the element 6cd ', the first fluid is transferred via a line 20 into the inlet compartment of the first fluid of the element

  6dd 'delimited by the collector 17dd'.

  The exchange fluid then passes, successively in one direction and in the other, the exchange elements of the column d of exchange elements to reach the outlet compartment of the manifold 14 '

communicating with line 15 '.

  We could similarly explain from Figure 2 and Figure 7 the circulation of the second exchange fluid, from the inlet pipe 16 'to the outlet pipe 16 showing that the second exchange fluid , like the first exchange fluid, successively traverses each of the exchange elements in alternating directions of circulation, the circulation of a fluid in an exchange element being carried out against the current with respect to

  to the circulation of the other exchange fluid.

  It is clearly seen, in FIG. 7, that a circulation of the first and second exchange fluids is obtained in countercurrent throughout the bundle of the steam generator, following a circulation path whose length is equal to the length of an exchange element multiplied by the total number of exchange elements, i.e. by sixteen, in the case of

  embodiment which has been described.

  It is thus possible to produce compact installations and relatively structured

  simple to perform physical or chemical reactions

  by heat exchange between fluids circulating at low speed

  with very long routes.

  Despite a very low circulation speed inside the channels of the plate exchanger, a turbulent flow of the exchange fluids is obtained, which makes it possible to avoid the deposition of crystals in the fluid circulation channels in the case in which, inside the plate exchanger, a starting solution containing crystals and constituting the first exchange fluid and the crystallization of a substance are dissolved

  in the form of calibrated crystals, in a purified solution constituting the se-

  cond exchange fluid in the heat exchanger.

  The device according to the invention is therefore perfectly suited to the

  lisation of a crystallization process of a chemical substance.

  It is possible in particular to carry out, using a plate heat exchanger such as that of the invention, the crystallization of products such as paraxylene or dimethyl naphthalene or alternatively of aromatic carboxylic acids and anhydrides such as in particular terephthalic acid or isophthalic acid, optionally in purified form, trimellitic acid,

  trimellitic anhydride, naphthalene dicarboxylic acid, diphenic acid

  nyldicarboxylic, pyromellitic acid or pyromellitic anhydride and

oxybisbenzoic acid.

  The invention is not limited to the embodiment which has been described.

  Thus it is possible to use exchange elements constituted by stacks of plates having shapes different from those which have

  have been described, for example parallelepipedic rectangular section.

  These exchange elements can be arranged adjacent to each other in rows having a first direction and arranged themselves

  adjacent in a second direction perpendicular to the first

  mother. These exchange elements can be placed adjacent to each other,

  according to a general provision different from that which has been described.

  Collectors and connecting elements between the end parts

  exchange elements can also have different forms

  of those that have been described.

  Finally, the plate heat exchanger according to the invention can be used in other applications than the crystallization of

the chemical industry.

Claims (11)

  1.- Plate heat exchanger having a circulation path   with mutual heat exchange, a first and at least a second fluid   of, very long, in particular for an installation for crystallizing a substance contained in a liquid, for example aqueous solution, constituting one of the first and second fluids, characterized in that it comprises a plurality of heat exchange elements (6) between the first   and the second fluids each consisting of a stack of corrugated plates   stripes (7) between which spaces for circulation of the first   mier and second fluids, called channels (11), in a general direction   rale corresponding to a longitudinal direction of the plates, the elements (6) being arranged adjacent to each other in a first transverse direction parallel to the plates (7) and in a second transverse direction perpendicular to the plates (7), so as to be juxtaposed along their entire length and comprising, at their longitudinal ends, distribution and guide means (14, 14 '; 17ba', 17ca ', 17ad', 17bd ', 17cd', 17dd '; 18a, 18b , 18c, 18d, 18'a, 18'b, 18'c, 18'd, 18 "a, 18" b, 18 "c, 18" d; 19, 19 '; 20, 20') from the first and of the second fluids,   such that the first and second fluids flow against the current in each   one of the elements (6) and, along the entire length of each of the elements (6), successively.   2.- heat exchanger according to claim 1, characterized in that the heat exchange elements (6) has a general shape   rectangular or square base and are arranged ma-   adjacent, in rows (a, b, c, d, a ', b', c ', d') having a pre-   direction and arranged themselves adjacent to each other in a   second direction perpendicular to the first.   3.- Heat exchanger according to claim 2, characterized in that the means of distribution and guidance of the first and second exchange fluids comprise collectors (18a, 18b, 18c, 18d, 18'a, 18 ' b, 18'c, 18'd, 18 "a, 18" b, 18 "c, 18" d; 17ba ', 17ca', 17ad ', 17bd', 17cd ', 17dd'); 14, 14 '; 19, 19 '; 17ba ', 17ca', 17ad ', 17bd', 17cd ', 17dd') fixed along longitudinal end portions of the heat exchange elements (6), to ensure the passage of the first and second exchange fluids between the   end portions of adjacent heat exchange elements (6).   4.- Heat exchanger according to claim 3, characterized in that at least part of the collectors (14, 14 '; 18a, 18b, 18c, 18d, 18'a, 18'b, 18'c, 18 'd, 18 "a, 18" b, 18 "c, 18" d) has separating walls   ration (10) in two compartments for the distribution of the first and the second   cond exchange fluids, respectively.   5.- Heat exchanger according to any one of the claims.   tions 3 and 4, characterized in that the means of distribution and guidance   dage of the first and second fluids further comprise conduits (20, ') connecting between two distribution manifolds and receiving one   the first and second exchange fluids.   6.- Method of crystallization of a contained chemical substance   in a liquid, for example aqueous solution, in the form of pu-   rified and calibrated, in a plate heat exchanger according to one   conch of claims 1 to 5, characterized in that:   - which is circulated in the channels (11) of the plate heat exchanger (3) reserved for a first heat exchange fluid, a first   aqueous solution containing in suspension uncalibrated crystals containing   closing off impurities, - heating the first aqueous solution by heat exchange in the plate heat exchanger (3) with a second exchange fluid at a dissolving temperature of the non-calibrated crystals to obtain a first homogeneous solution containing impurities,   - that we introduce the first homogeneous solution containing impurities   retained in a chemical reactor (2), - that the impurities are separated from the first homogeneous solution in   the chemical reactor (2) to obtain a second homogeneous solution puri   , - that the second purified homogeneous solution is circulated in the channels (11) of the plate exchanger (3) reserved for the second exchange fluid, and - that we recover, at the outlet (lb) of the second exchange fluid   the plate heat exchanger (3) of water and purified and cali-   brés. 7. A crystallization process according to claim 6, characterized in that the first aqueous solution and the second purified homogeneous solution are circulated in the channels (11) of the heat exchanger at   plates (3) at a speed between 0.5 and 1 m / s.   8.- A crystallization process according to any one of the claims.   tions 6 and 7, characterized by the fact that the first and the second are circulated   as aqueous solutions in the channels (11) of the heat exchanger,   following an exchange route with a length between 20 and 300 m.   9.- Crystallization process according to any one of the claims.   tions 6, 7 and 8, characterized in that the first aqueous solution and the second aqueous solution are kept inside the channels (11) of the plate heat exchanger and in the chemical reactor (2) , to one   high pressure of the order of 200 bars.   10.- A crystallization process according to any one of the claims.   cations 6 to 9, characterized in that the first solution is heated   aqueous at a dissolution temperature of crystals containing im-   purities, of the order of 300 C, before introducing it into the chemical reactor (2).   11.- Method according to any one of claims 6 to 10, ca-   characterized by the fact that the chemical is one of the following substances   Vantes: paraxylene or dimethyl naphthalene or carbohydrates   aromatic boxyls and anhydrides such as in particular tere-   phthalic or isophthalic acid, optionally in purified form, trimellitic acid, trimellitic anhydride, naphthalene dicarboxylic acid,   diphenyldicarboxylic acid, pyromellitic acid or pyromel anhydride   litic and oxybisbenzoic acid.