GB2059402A - Modular Chlorine Dioxide Generation Systems - Google Patents

Abstract

A modular chlorine dioxide generating unit (10) is disclosed wherein recycled reaction mixture passes along a flow path (18) internally of the generator vessel (12) out of direct fluid flow communication with the reaction medium (13) from the lower end (16) of the vessel (12) to a discharge point in the vapour phase (15) above the liquid level (13) in the generator vessel (12). <IMAGE>

Description

SPECIFICATION Modular Chlorine Dioxide Generation Systems The present invention relates to modular chlorine dioxide generating systems or units and to the production of chlorine dioxide using such systems.

Chlorine dioxide is utilized in a variety of bleaching operations, particularly in the bleaching of cellulosic fibrous material, such as, wood pulp. In our prior U.S. Patents Nos. 3,895,100 and 3,975,506 (El 07) the disclosures of which are incorporated herein by reference, there is disclosed a chlorine dioxide generating process and equipment therefor wherein chlorine dioxide is formed by reduction of an alkali metal chlorate, usually sodium chlorate, with chloride ions in an aqueous acid reaction medium, in accordance with the equation::

The reaction medium in the generator is maintained at its boiling point, generally about 250 to about 900C, under a subatmospheric pressure, generally about 20 to about 400 mm Hg, to cause the deposition of a by-product salt from the reaction medium in the generation vessel once saturation has been achieved after start-up and to remove the chlorine dioxide and chlorine in gaseous admixture with steam.

The by-product salt which is precipitated depends on the acid used in the reaction medium, the by-product being a sodium sulphate when sulphuric acid is used and sodium chloride and/or hydrogen chloride provide the chloride ion reducing agent, and the by-product being sodium chloride when hydrochloric acid is used and also provides the reducing agent.

When sulphuric acid is used, the sodium sulphate may be in an acidic or neutral form depending on the total acid normality of the generator, with a neutral salt generally being obtained at total acid normalities of about 2 to about 5 normal and acid salts being obtained at higher values usually up to about 12 normal.

When the total acid normality of a sulphuric acid based system is such as to produce neutral sodium sulphate, it is preferred to use a reaction temperature above about 300C in order to obtain the anhydrous salt.

When hydrochloric acid is used, the actual hydrogen ion concentration in the reaction medium is maintained in the range of about 0.05 to about 0.3 normal. The term "actual hydrogen ion concentration", as used herein, is the value determined by a pH meter calibrated on an 0.1 normal solution of hydrochloric acid on the assumption that such a solution is 100% dissociated at this concentration.

As described in our earlier patents, the solid by-product is removed as a slurry in reaction medium from the generator vessel, sodium chlorate solution is mixed therewith, the mixture is recycled through a reboiler wherein the mixture is heated to the reaction temperature and forwarded to the reaction vessel after the addition of acid to the heated mixture. A portion of the slurry is withdrawn prior to the reboiler for crystal separation.

The heated mixture from the reboiler is passed through a venturi-like pipe which exerts sufficient back pressure on the mixture in the reboiler to prevent boiling therein, acid in concentrated form is introduced to the collar of the venturi and the resultant reaction mixture is allowed to expand at low acceleration to permit boiling and gaseous product formation to occur evenly and gradually and thereby avoid bumping and vibration, so that a fluid mixture of solid phase, liquid phase and gaseous phase enters the generator above the liquid level therein through a curved pipe joining the downstream end of the venturi-like pipe and the generation vessel inlet.

While this prior art procedure represents an excellent commercially-viable chlorine dioxide generating system having many advantages over prior art arrangements, as described in our earlier patents, it does suffer from the drawback that it is not capable of modular manufacture, transportation and installation but rather requires considerable on-site assembly of the reboiler, or other suitable heat exchanger, and recycle pipes. In addition, considerable lateral space beyond the dimensions of the generator vessel is required to accommodate the reboiler and recycle tubes.

The present invention is directed to improvements in our prior art system wherein a compact modular chlorine dioxide generation system or unit is provided having the heat exchanger and recycle pipes pre-assembled with the remainder of the generation system component parts, thereby avoiding the necessity and expense of on-site assembly and the space requirements of the prior art system.

The provision of the modular unit according to the invention is made possible by providing baffle means in the generation vessel which defines a recycle feed duct extending through the height of the reaction medium in the generation vessel and terminating above the liquid level. This duct communicates at its lower end with the venturi-like pipe and hence with the recycling heated reaction mixture and discharges that mixture into the vapor space of the generation vessel. Thus, the duct is out of direct fluid flow communication with the body of reaction medium in the generation vessel.

By providing the internal duct in this way, the heat exchanger may be located directly below the generation vessel and preferably within the diameter thereof and the outside curved recycle pipe of our prior art structure is eliminated. In this more compact form, the unit may be assembled as a complete module for shipping to the installation site. Further, since the curved outside recycle pipe is eliminated, the lateral on-site space requirements of the modular unit are decreased, as compared with the prior art.

The invention is described further, by way of illustration, with reference to the accompanying drawings, in which: Figure 1 is an elevational view of a chlorine dioxide generation unit constructed in accordance with one embodiment of the invention; Figure 2 is a sectional view taken on line 2-2 of Figure 1; Figure 3 is an elevational view of a modified form of the unit of Figure 1; Figure 4 is a sectional view taken on line 4--4 of Figure 3; and Figure 5 is an elevational view of a chlorine dioxide generation unit constructed in accordance with a second embodiment of the invention.

Referring first to Figures 1 and 2, a modular chlorine dioxide generating unit 10 comprises a generally cylindrical upright generator vessel 1 2 containing a chlorine dioxide generating reaction medium 13 which may be sulphuric acid-based or hydrochloric acid-based, as discussed in detail above. The reaction medium 1 3 is maintained at its boiling point to effect water evaporation and the vapor space 1 5 above the reaction medium 1 3 is maintained under subatmospheric pressure. Once the reaction medium 13 becomes saturated after start-up, a by-product salt is precipitated continuously from the reaction medium.

The generator vessel 12 may be constructed of any suitable corrosion resistant material, preferably titanium owing to its high corrosion resistance and strong structural qualities.

The generator vessel 12 is provided with an internal baffle 14 which extends upwardly through the reaction medium 1 3 from a lower outwardly dished closure wall 1 6 to above the height of the intended liquid level of the reaction medium 1 3 in the generator vessel 12 and defines an elongate passageway or duct 18 between the baffle 14 and the internal wall of the generator vessel 12 which is out of direct fluid flow communication with the reaction medium 1 3 in the generator vessel 12.

The provision of an outwardly dished closure wall 16 at the lower end of the generator vessel 12 contrasts with the conical structure utilized in the generation vessel described in our prior U.S. patents mentioned above. The dished structure is preferred in the illustrated embodiment of Figures 1 and 2 to accommodate in simple manner the other structural modifications required to provide the modified generation unit of this invention. A conical lower portion may be used, however, as is illustrated in the embodiment of Figures 3 and 4.

The lower outwardly dished closure wall 1 6 of the generator vessel 1 2 communicates with a vertical slurry removal pipe 20 for the removal of a slurry of by-product solid from the generator vessel 12. An inlet pipe 22 is provided to the slurry removal pipe 20 for the introduction of sodium chlorate solution to the slurry. The sodium chlorate solution also contains sodium chloride when sulphuric acid is the acidic reactant whereas the sodium chloride is omitted when hydrochloric acid is the acidic reactant. A separate sodium chloride feed stream may be used, if desired. If the acidic reactant is provided by a mixture of sulphuric and hydrochloric acids wherein the hydrochloric acid is sufficient to provide all the chloride ions for the process, then sodium chloride may be omitted.

A recycle pump 24 of any suitable construction is provided at the lower end of the slurry removal pipe 20 for pumping part of the slurry back to the generator vessel 12. A slurry discharge pipe 26 also is provided at the lower end of the slurry removal pipe 20 for transfer of part of the slurry to suitable separation equipment for the separation of the solid crystals from entrained reaction medium, the separated entrained reaction medium being recycled to the generator vessel 12 in any convenient manner.

A vertical reboiler 28, of any convenient construction, or other suitable heat exchanger, is provided extending substantially parallel to the slurry removal pipe 20 and in communication at its lower end with the discharge side of the recycle pump 24 to receive pumped slurry for heating to the reaction temperature therein. The relative positions of the pipe 20 and the heat exchanger 28 may be reversed but this arrangement is less preferred.

At its upper end, the reboiler 28 communicates with a first frusto-conical member 30 decreasing cross-sectional area in the direction of flow of the liquor and terminating in a cylindrical collar 32 having a plurality of acid inlets 34 located around the periphery thereof for feed of acid, preferably in concentrated form, into the recycling medium. Any desired number of such inlets 34 may be provided, although preferably four are provided in diametrically-opposed pairs right-angularly arranged with respect to each other.

On the downstream side of the collar 32, there is provided a second frusto-conical member 36 of increasing cross-sectional area in the direction of fluid flow and which terminates at the bottom closure 1 6 of the generator 12 in communication with the passageway 18.

As explained in detail in our U.S. Patents Nos. 3,895,100 and 3,975,506, the arrangement of frusto-conical members 30 and 36 on either side of the short cylindrical collar 32 in a venturi-like pipe structure results in acceleration of liquid leaving the reboiler 28 in the first frusto-conical member 30 to a high velocity while the back pressure induced by the decreasing cross-sectional dimension and acceleration exceeds the difference in saturation vapor pressure between that of the liquid present and that of the generator. In this way, boiling of recycled heated liquor in the tubes of the reboiler 28 is avoided.

Once the acid has been introduced in the collar 32 causing rapid mixing with the liquid, the mixture is expanded at low acceleration in the second frusto-conical member 36, so that boiling of the liquid and gaseous product release is brought about evenly and gradually. Since the liquid commences to boil and chlorine dioxide and chlorine start to form on the downstream side of the collar 32, the material which enters the passageway 1 8 is mainly gaseous with entrained amounts of liquid and solid suspended therein.

The resulting fluid mixture enters the gas space 1 5 of the generator vessel 12 at a fairly low velocity, for example, about 20 to about 50 ft/sec (about 7 to about 1 7 m/sec). A baffle 37 is provided at the upper end of the passageway 1 8 to direct the fluid generally horizontally and radially inwardly of the internal wall of the generator vessel 12.

In the embodiment illustrated in Figure 1, a gas discharge outlet 38 for removal of gaseous chlorine dioxide, chlorine and water vapor from the generator vessel 12 communicates with a pipe 40 which extends radially through the side of the generator vessel 1 2 adjacent the upper end thereof and is upwardly curved at its inner end to provide its inlet opening towards but spaced from a top closure 42 of the generator vessel 12 and located axially of the generator vessel 12. This arrangement assists in the separation of entrained liquid from the gaseous products in the vapor space 1 5 by inducing the gaseous material to turn through 1 800 to enter the pipe 40 before being able to exit from the generator vessel 1 2 through outlet 38.This movement induces entrained droplets to fall out of the gaseous phase towards the reaction medium 13, so that the discharged gaseous phase has a lower entrained liquid concentration.

This latter arrangement contrasts with the conventional system which has its gaseous outlet generally axially located in the top closure of the generator vessel so that the gaseous products pass out of the generator vessel 12 in a straight line path. Such a conventional arrangement may be used, if desired, as seen in Figures 3 and 4.

The illustrated arrangement for decreasing liquid entrainment in the removed gaseous phase contrasts with conventional demisting devices, generally consisting of a metal mesh structure, which have been suggested for chlorine dioxide generation systems of the evaporation type. Such demisting devices are prone to clogging by deposited solid material and must be cleaned from time to time, and hence are generally unsatisfactory.

In addition to the advantage of less liquor entrainment in the product gas stream using the illustrated structure of discharge outlet 38 and pipe 40, there is an additional advantage that less overall height is required since the outlet pipe 38 may communicate directly with the inlet of adjacent condensation equipment rather than requiring pipes which extend first vertically upwardly from a top closure outlet and then downwardly to the condensation equipment inlet.

The arrangement of the reboiler 28 and the reaction mixture recycle line in the illustrated embodiments in relation to the generator vessel 12 is quite different from the structure of our prior U.S.

patents described above. Thus, in the generation unit 10 of this invention, part of the recycle line from the acid inlet to the discharge above the liquid level in the generator vessel 12 is provided by the passageway 1 8 within the outer wall of the generator vessel 12 itself, in contrast to our prior arrangement where the recycle line included an inwardly-curved pipe located wholly outside the generator vessel 12.

The later provision of the vertically-directed passageway 1 8 from the lower closure wall 1 6 to above the level of the reaction medium 1 3 permits a much more compact overall structure to be provided which can be pre-assembled for shipping and then shipped in modular form, so that on-site assembly is avoided and hence this expense of the prior art structure is eliminated.

Additionally, since the external recycle pipe of the prior art structure is eliminated, the lateral space requirements of the overall unit 10 are decreased and this expense also is eliminated.

In the illustrated embodiment, the slurry removal pipe 20 is illustrated offset from, although parallel to, the axis of the generator vessel 12. This arrangement results from the diameter dimensions of the particular generation vessel 12 illustrated and the necessity to position the reboiler 28 below the generator vessel 12. For larger vessels 12 than that illustrated where more space is available below the vessel, the slurry removal pipe 20 usually is located coaxially with the vessel.

In the interests of maintaining a compact form, it is preferred to dimension the reboiler 28 such that it lies wholly within the lateral dimension of the generator vessel 12. The actual lateral dimension of the reboiler 28 depends on the capacity of the generator vessel 12, the diameter of the slurry removal pipe 20 and the capacity of the pump 24.

While the vertical passageway 1 8 is illustrated as being provided to one side of the generator vessel 12, and discharging diametrically thereof, this structure represents only one possible but convenient arrangement. Another possibility is to construct the passageway or duct 1 8 in such a way that it terminates at its upper end in a circular or other conveniently shaped orifice located centrally of the generator vessel 12 above the liquid level of the reaction medium 13. A baffle may be provided vertically upwardly spaced from such a central orifice to direct the fluid discharging therefrom towards the generator vessel walls.

As described above, in the embodiment of Figures 1 and 2, the fluid entering the generator 12 does so generally radially of the generator vessel 12 and of the reaction medium 13. The embodiment shown in Figures 3 and 4 illustrates a modification of this arrangement whereby the internal baffle 14 is curved round within the vessel at its upper end to provide a vertically elongate rectangular exit orifice 44 opening along the internal wall of the vessel 1 2 and arranged to project the incoming fluid generally tangentially to the internal wall of the generator 12. The tangential entry of recycled fluid assists in the separation of entrained liquid from the gaseous phase enabling the vapor separation space 1 5 of the generator vessel 12 to be decreased in size.

In the generator vessel 12 of Figures 3 and 4, a more conventional conical bottom closure 1 6 and axial gaseous outlet 38 are used, although the structures utilized in the embodiment of Figures 1 and 2 may also be utilized herein.

It will be seen from the above description of the embodiments of Figures 1 to 4, that the pump 24, the reboiler 28 and the acid entry collar 32 are provided outside the generator vessel 12, although forming part of the overall modular unit 1 0. This arrangement of component parts is quite beneficial in permitting ready maintenance of each of these parts without disassembly of the whole unit or requiring access to the interior of the vessel 12. Accessibility for maintenance can be a drawback in more integrated structures, such as are described further below.

The structures shown in Figures 1 to 4, therefore, combine the component parts into a compact modular chlorine dioxide producing unit without the necessity of an integrated one vessel assembly and in which the component parts requiring service are readily accessible.

Although less preferred than the embodiments illustrated in Figures 1 to 4 in view of possible servicing difficulties more integrated structures may be used, while still retaining the benefits of the internal baffle and recycle arrangement. In connection therewith, reference is made to Figure 5 wherein the frusto-conical portions 30 and 36 and the acid inlet collar 32 may be provided as parts of the internal baffling of the generator vessel 12, adjacent the lower end closure 1 6.

Additionally, the heat changer 28 is provided with a downflow and upflow abutting portions, communicating at the upper end directly with the generator vessel 1 2 and at the lower end with a pump-housing 24 to which the sodium chlorate solution is fed by line 22 and from which sodium sulphate slurry is removed by line 26. In this arrangement, the pipe 20 is eliminated. The pump housing 24 may be in the form illustrated, or in the more conventional form similar to that used in Figure 1.

The present invention, therefore, provides improved procedures and improved apparatus for chlorine dioxide production. Modifications are possible within the scope of the invention.

Claims (9)

Claims

1. A continuous method for the formation of chlorine dioxide, which comprises, in continuous manner, reacting an alkali metal chlorate with chloride ions in a reaction medium containing at least one strong mineral acid at the boiling point of the reaction medium under a subatmospheric pressure in an enclosed reaction zone while simultaneously depositing an alkali metal salt of the anion of the strong mineral acid to form a gaseous mixture of chlorine dioxide, chlorine and steam which is removed from the reaction zone, removing a slurry of the deposited alkali metal salt and reaction medium from the reaction zone and recovering at least part of the alkali metal salt from the slurry, and forming a recycle stream passing to the reaction medium consisting of a multiphase mass of steam, chlorine dioxide, chlorine, unrecovered alkali metal salt crystals and aqueous reactant-containing phase by heating alkali metal chlorate solution to the temperature of the reaction medium in admixture with the remainder of the slurry, and also in admixture with a source of the chloride ions when not provided by the strong mineral acid, while preventing the solution from boiling, and permitting the heated admixture to expand in controlled manner after addition of the strong mineral acid thereto to form the multiphase mass, characterized in that the recycle of the multiphase mass to the reaction medium includes passing the multiphase mass along a flow path upwardly within the enclosed reaction zone out of fluid flow communication with the reaction medium to a discharge location above the liquid level in the reaction zone.

2. The method claimed in claim 1, characterized in that the flow path of the multiphase mass extends generally parallel to the axis of a generally upright or cylindrical reaction zone and the multiphase mass is deflected generally radially of the reaction zone at the discharge location.

3. The method claimed in claim 2, characterized in that the flow path is located adjacent to and is defined in part by the internal lateral extremity of the reaction zone.

4. The method claimed in claim 1, characterized in that the flow path of the multiphase mass extends generally parallel to the axis of a generally upright cylindrical reaction zone for a lower portion of the length thereof and is curved in the upper region thereof to terminate in a discharge location causing tangential discharge of the multiphase mass into the vapor phase above the reaction medium in the reaction zone.

5. An apparatus for effecting the method of claim 1 including a generally cylindrical upright enclosed generator vessel for holding the reaction medium and having a vapor space therein, a gaseous product removal outlet in the upper region of the vessel for removal of the gaseous mixture of chlorine dioxide, chlorine and steam, a slurry removal pipe extending from the lower closure of the generator vessel generally parallel to the axis or coaxial with the generator vessel for removal of the slurry of reaction medium and alkali metal salt and terminating at its lower end in a pump for the driven recycle of the alkali metal chlorate solution, a feed pipe for alkali metal chlorate solution in communication with the slurry removal pipe, and a slurry recovery pipe also in communication with the slurry removal pipe for the recovery of the at least part of the alkali metal salt, a recycle pipe connecting the pump to the vapour space in the generator vessel for feeding the multiphase mass to the generator vessel, said recycle pipe having at some point a first frusto-conical portion of decreasing cross sectional area in the direction toward the reaction vessel terminating at its downstream end in a collar and a second frusto-conical portion of increasing cross-sectional area away from the collar, said collar being provided with inlet means for the introduction of strong mineral acid to the recycle liquid, heating means being provided on the slurry removal pipe or recycle pipe upstream of the first frustro-conical portion to heat the recycle liquid to temperature of the reaction medium characterised in that the recycle pipe includes a flow path (18) located wholly within the generator vessel (12) and extending in non-fluid flow relationship with respect to the reaction medium (13) into the vapor space (15).

6. The apparatus claimed in claim 5 characterized in that the flow path (18) is defined by a baffle (14) extending from the lower closure (16) of the generator vessel (12) and the adjacent internal wall of the generator vessel (12).

7. The apparatus claimed in claim 6 characterized in that a deflector plate (37) extends inwardly from the inner wall of the vessel (12) and is spaced upwardly from the upper end of the flow path (18).

8. The apparatus claimed in claim 5 characterized in that the flow path (18) is defined by a shaped baffle (14) extending first upwardly from a lower end closure (16) of the generator vessel (12) and then curved to follow the internal wall of the generator vessel (12) to terminate in an elongate opening (44) extending parallel to the axis of the vessel (12) for ejection of recycled fluid tangentially to the internal wall of the vessel (12).

9. The apparatus claimed in any one of claims 5 to 8, characterized in that the gaseous product outlet (38) includes a conduit (40) having a radial portion extending through the side wall of the generator (12) in an upper region thereof and a curved portion terminating in an upwardly-opening axially-located orifice. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~