GB2031613A - Production of chlorine dioxide - Google Patents

Abstract

An automatic continuous monitoring system for a chlorine dioxide generating process (10) which produces a gaseous mixture of chlorine dioxide and chlorine (12) is described. The automatic system eliminates the need for periodic manual analysis of the off-gas stream and enables changes in the process efficiency to be rapidly compensated for. The efficiency is an indication of the rate at which the desired reaction is proceeding in the face of competing reactions. The system preferably utilizes a gas-liquid chromatograph analyser (36) to analyse off-gases from the generator (10) and to provide signals (43, 44) corresponding to the measured amounts of chlorine dioxide and chlorine. These signals then are converted (46) to a signal (47) corresponding to the ratio of chlorine dioxide and chlorine in the off-gas stream. The ratio is then converted to the efficiency (48). The efficiency value so obtained (49) may be compared with a previously determined value and changes compensated for by catalyst and/or reactant feed variations. The gas analysis may be used in combination with actual production rate and required production rate determinations, possibly along with reaction medium analysis, in a machine controlled chlorine dioxide- producing process in which programmed integrated circuits are used to effect calculations and adjustments. <IMAGE>

Description

SPECIFICATION Control system for chlorine dioxide plants Field ofinvention The present invention is directed to the production of chlorine dioxide, and more particularly to the control of chlorine dioxide production by the measurement of key parameters.

Background to the invention In U.S. Patent No. 3,864,456 assigned to the assignee of this application, there is described a process for the production of chlorine dioxide which involves the reduction of sodium chlorate in an aqueous acid reaction medium which is maintained at its boiling point at the absolute pressure thereon while a sub-atmospheric pressure is applied to the reaction zone in which the reaction medium is located. The boiling temperature is greater than about 30"C and below the temperature above which substantial decomposition of chlorine dioxide occurs, preferably about 600 to about 80"C.

The reducing agent is chloride ions provided by added sodium chloride, hydrogen chloride or a mixture of the two while the acid is provided either wholly by sulphuric acid in the case where sodium chloride alone is used as the source of chloride ions or partly by sulphuric acid and partly by hydrochloric acid in the case where hydrogen chloride alone or in admixture with sodium chloride is used as the source of chloride ions.

The process operates on a continuous basis with reactants being continuously fed to the reaction medium and a gaseous mixture of chlorine dioxide, chlorine and water vapor being continuously removed from the reaction zone. The reaction medium is maintained at a total acid normality in the range of about 2 to about 4.8 normal and an hydrous neutral sodium sulphate is continuously crystallized from the reaction medium, once saturation is reached after start up. The crystallized sodium sulphate is removed from the reaction zone on a continuous or intermittent basis.

The just-described chlorine dioxide-producing process is commercially advantageous in that chlorine dioxide generation, water evaporation and by-product crystallization occur in the same vessel, an hydrous neutral sodium sulphate is formed directly, and a low total acid normality is employed. These and other qualities have led to wide commercial acceptance and implementation of the process.

It is known from U.S. Patent No. 3,563,702 to increase the efficiency of production of chlorine dioxide by the above-described process from about 90 to 92% to about 97 to 98% by the use of certain catalysts, and silver salts commonly are used in commercial practice.

By the term "efficiency" is meant the degree of conversion of sodium chlorate fed to the reaction medium to product chloride dioxide. The chlorine dioxide-producing reaction is represented by the following equation (1): NaClO3 + NaCI + H2S04Cl02 + 1/2 wiz + H2O + Na2SO4 -(1) There is a competing reaction which produces no chlorine dioxide and this reaction is represented by the following equation (2): NaClO3 + SNaCI + 3H2S0430l2 + 3H2O + 3Na2SO4 -(2) The efficiency of the process, therefore, is the extent to which the reaction of equation (1) can be made to predominate over the reaction of equation (2).

Since any decrease in efficiency of the process means that lesser quantities of sodium chlorate are converted to the desired chlorine dioxide product, and since sodium chlorate and silver salts are expensive raw materials, it is desirable to maintain the efficiency at as high a level as possible at all times. A number of factors can affect the efficiency of the process, mainly catalyst concentration and, to a lesser degree, mole ratio of chlorate ion to chloride ion in the reaction medium and temperature of the reaction medium.

In continuous plant operations, manual determinations of efficiency are effected to ensure operation at the desired efficiency level, any decrease in efficiency usually being compensated for by the addition of further quantities of catalyst, usually a silver salt, to the generator. Two types of determinations may be made, one based on the quantity of chlorate consumed and the quantity of chlorine dioxide produced. This determination provides an efficiency expressed as a percentage, signifying the percentage of one mole of chlorate which is reacted by equation (1) to form chlorine dioxide. This efficiency determination is only rarely made when a mass balance of the system is required, the chlorate feed and chlorine dioxide product values being monitored over the time interval and the determination being made from these monitored values.

The other manual determination which is made in the Gram Atom Percent Chlorine Dioxide (GA% C102) value of the product stream. GA% C102 is determined from the following equation (3): GAO/o C102 = CI in C102 x 100 -(3) in 0102 + Olin 012 by determining the chlorine atoms present in the product gas stream as chlorine dioxide and chlorine.

The GA% C102 value is an accurate representation of the chemical efficiency and 100% efficiency is reached at a GA% C102 value of 50%. This value is a valid determination of efficiency for the above-described process since chlorine is produced along with the chlorine dioxide and is present in the product gas stream, in contrast to some chlorine dioxide-producing processes wherein the chlorine is reduced in situ to form chloride ions and the GA% C102 value wou Id not represent a true indication of efficiency.

The determination of efficiency as the GA% C102 value is simpler to effect than the determination based on measurements of chlorate consumed and chloride dioxide formed, requiring the withdrawal of a sample of product gas and wet chemical analysis of the sample to determine chlorine dioxide and chloride contents.

The GA% C102 determination, however, is made at widely-spaced time intervals typically varying from once a shift to once a week. However, the product gas stream is at a high temperature and under subatmospheric pressure and operator skill is required to collect a representative sample for analysis.

Owing to the necessity for skilled operation and the problem of representative sample collection, the GA% C102 efficiency value determined may be incorrect. Further, variations in efficiency between the periodic determinations are not compensated for. As a result, the overall efficiency of the chlorine dioxide producing process on a long term basis may well be, and usually is, less than the optimum, leading to less overall chlorine dioxide production and increased chemical and catalyst usage than the optimum.

In large volume chlorine dioxide generators, the process is less sensitive to changes in conditions, such as, chloride to chlorate mole ratio and catalyst concentration in the reaction medium and temperature, than in smailer volume generators having the same chlorine dioxide production capacity. In view of the fabricating costs of chlorine dioxide generators, which are generally constructed of titanium, the trend is to smaller volume generators, which leads, as noted above, to greater sensitivity of the process to parameter variations.

For greater overall efficiency and increased chlorine dioxide production and hence decreased raw material costs in the form of sodium chlorate and catalyst, it is necessary to provide a continuous accurate and rapid determination of efficiency so that all variations in efficiency can be compensated for.

Summary ofinvention The present invention is directed to a continuous monitoring system for a chlorine dioxide generating process which produces chlorine dioxide and chlorine by reduction of sodium chlorate with added chloride ions in the substantial absence of reducing agents for the chlorine. The monitoring system determines efficiency continuously by analysis of the product gas stream so that close control of the operating parameters of the process may be effected. In this way, the human error possibilities of a manual system based on chemical analysis of product gas stream sample are avoided, the difficulties resulting from the periodic manual determinations are eliminated, and an overall improvement in efficiency, increased chlorine dioxide production, and decreased chemical and catalyst usage results are realized.

In the present invention, the following operations are effected on a continuous cyclic basis. A sample of product gas stream containing chlorine dioxide and chlorine is machine analyzed, preferably chromatographically, to provide two separate signals, one indicative of the quantity of chlorine dioxide present in the sample and the other indicative of the quantity of chlorine present in the sample. The signals are machine converted to a signal indicative of the mole ratio of chlorine dioxide and chlorine present in the sample, and the chemical efficiency is computed from the mole ratio signal by use of the equation: 6R Efficiency = 2 + 5R where R is the mole ratio of chlorine dioxide to chlorine in the sample.

The signal indicative of the efficiency determination is recorded. The recorded efficiency may be represented visually so that an operator can readily detect a decrease in efficiency for which he can compensate, usually by the addition of further quantities of catalyst to the reaction medium, or possibly by variation of other parameters. Alternatively, the recorded efficiency can be compared with previous determinations electronically and any variation can be compensated for by signal adjustment of catalyst flow valve setting or other feed chemical valve setting.

In addition, the present invention provides for a machine-controlled process for the production of chlorine dioxide which involves machine operated analyses and comparisons to maintain not only efficiency at optimum levels, but also production rate and chlorine dioxide solution inventory at desired levels.

General description ofinvention As noted previously, the chemical efficiency of a chlorine dioxide generating process is given by the ratio: moles (E) = mole of chlorine dioxide formed x 100% moles ( = moles of sodium chlorate consumed If the ratio of chlorine dioxide to chlorine in the product gas stream from a chlorine dioxide generator is represented by Rand if the quantity of sodium chlorate consumed by the reaction of equation (1 ) is represented by y, then, based on equations (1) and (2) above, for each mole of sodium chlorate consumed: Y Y = 2y y/2 + 3 (1-y) 6-5y from which it follows that:: 6R E -(4) Y - 2+5R - 100 Therefore, if the mole ratio of chlorine dioxide to chlorine in the product gas mixture is determined, then the chemical efficiency may be calculated from equation (4).

While the invention has particular applicability to the process of U.S. Patent No. 3,864,456, and will be described below with reference thereto, the invention is applicable to any chlorine dioxide generating process wherein chlorate is reduced with added chloride ion as the sole reducing agent and reducing agents for chlorine are substantially absent. Further, while the invention has particular applicability to the control of chlorine-dioxide-producing processes in which a gaseous mixture of chlorine dioxide, chlorine and evaporated water vapor is formed and the generator is maintained under a subatmospheric pressure, the invention may be used with processes which utilize atmospheric pressure and added diluent gas.

One particular chlorine dioxide-producing process to which the present invention may be applied is that described in U.S. Patent Nos. 3,929,974 and 4,075,308, assigned to the assignee of this application, wherein hydrogen chloride provides both the chloride ion reducing agent for the sodium chlorate and the acidity. In the latter process, operated at the boiling point of the reaction medium under a subatmospheric pressure, the by-product precipitated in the reaction vessel is sodium chloride. The acidity, expressed as actual hydrogen ion concentration, in this process is about 0.05 to about 0.3 normal.

Briefdescription of drawings Figure lisa schematic flow sheet of one embodiment of the invention; Figure 2 is a simplified logic diagram illustrating computer control of a chlorine dioxide-generating plant; and Figure 3 is a schematic flow diagram of the control system used in the computer operations of Figure 2.

Description ofpreferred embodiment Referring first to Figure 1, there is disclosed the presently-preferred embodiment of the invention. A chlorine dioxide generator 10 produces a gaseous mixture of chlorine dioxide, chlorine and steam in line 12 from an aqueous acid reaction medium operating in accordance with U.S. Patent No. 3,864,456 mentioned above, namely, under a subatmospheric pressure and at the boiling point of the reaction medium in the generator 10 under the absolute pressure thereon. An air bleed is provided corresponding to the subatmospheric pressure and gives rise to the presence of small amounts of air in the product gas stream 12.

Sodium chlorate, sodium chloride and sulphuric acid are fed to the generator vessel 10 by lines 14, 16 and 18 respectively, in any convenient manner, for example, as described in U.S. Patent No.3,895,100 assigned to the assignee of this application. A total acid normality of about 2 to about 4.8 normal, preferably about 2 to about 4.4 normal, is maintained in the reaction medium and anhydrous neutral sodium sulphate precipitates therefrom and is removed, on a continuous or intermittent basis, by line 20.

A silver salt is fed intermittently into the generator 10 by line 21 to provide catalyst to the generator 10 by line 21 to provide catalyst to the generator 10 as required to maintain the efficiency of the chlorine dioxide production at the desired level. Any other convenient catalyst may be used, or may be omitted entirely if the decreased efficiency resulting therefrom can be tolerated or if the chlorine dioxide-producing process is an inherently-efficient one.

The product gas stream in line 12 is cooled in an indirect cooler-condenser 22 to cause condensation of the bulk of the steam and the condensed water and remaining gaseous phase are forwarded to a chlorine dioxide absorber tower 24 wherein the chlorine dioxide, along with some of the chlorine, is dissolved in water fed by line 26 to form a product chlorine dioxide solution stream 28, which may be used in bleach plant operations for the bleaching of wood pulp, and a chlorine gas stream 30, which then may be further processed in known manner.

Samples of the vapor phase are taken at closely-spaced time intervals from line 32 after the cooler-condenser 22 by line 34 to a gas analyzer 36 in the form of a gas-liquid chromatograph. Each sample, after analysis, is returned to the main gaseous phase line 32 by line 38 in which a small water ejector 40 or suitable vacuum inducing means is located for drawing the sample through the chromatograph 36 by exerting a greater substmospheric pressure than exists in line 32.

This arrangement enables samples to be readily withdrawn from the high temperature subatmospheric pressure vapor phase for analysis and avoid the prior art necessity for skilled operator withdrawal of a representative sample.

The chromatograph 36 analyzes the incoming sample gas stream and provides a pneumatic output 41 to a detector unit 42 which detects pressure peaks in the pneumatic output 41 corresponding to the chlorine dioxide and chlorine of the sample, measures the height of each such peak above a baseline which is equivalent to the concentration of chlorine dioxide and chlorine respectively in the sample and transmits two separate pneumatic or other signals, depending on the form of the detector 42, and corresponding respectively to the analyzed quantities of chlorine dioxide and chlorine in the gas sample, and therefore corresponding to the quantities of these gases present in the condensed stream 32.

Any convenient analyzer 31 and detector 42 capable of achieving the above-described functions and providing the required output signals may be used. One suitable instrument which combines these operations is that known as the Model 91 PCT Analyzer sold by the Foxboro Company, Foxboro, Mass., U.S.A.

The pneumatic signals 43 and 44 respectively are forwarded from the peak detector 42 to a mole ratio calculator 46 wherein the signals are converted to a signal representative of the ratio of the molar amounts of chlorine dioxide and chlorine in the sample stream 34.

The conversion of the absolute values of chlorine dioxide and chlorine as measured by the peak detector 42 to a molar ratio thereof is important in that the conversion eliminates any zero drift and peak height variations which may be caused by variations in analyzer characteristics, such as, chromatograph absorbent characteristics, and temperature and pressure variations in the chromatograph unit.

The mole ratio signal in line 47 then is converted to the efficiency represented by the mole ratio signal in an efficiency calculator 48. The efficiency calculator 48 may take any convenient form to effect the efficiency calculation based on the equation (4): Y=2+5R x 100% where y is the efficiency and R is the mole ratio.

The efficiency value in line 49 then is recorded by a recorder 50 of any convenient form, such as, a pen recorder. The efficiency value so obtained represents the chemical efficiency of conversion of chlorate to chlorine dioxide in the generator 10 at the moment the gas sample was taken.

As the individual samples are taken, corresponding efficiency values are recorded by the recorder 50. An operator can recognize a trend towards decreased efficiency by observation of the penned recording.

Catalyst then is fed by line 21 to the generator to restore the efficiency to its desired level.

Alternatively, the recorder 50 may be provided with an alarm output 51 which is activated when the recorded efficiency drops to a predetermined value to alert an operator to the necessity for catalyst addition.

Where the chlorine dioxide process is one in which a catalyst is not used, the operator may vary other operating parameters, such as, reactant flows, to restore the desired efficiency.

In addition to, or alternative to, a visual readout of the determined efficiency, the recorder 50 may activate automatic feed of catalyst and/or other generator feeds to compensate for an undesirable fall in the efficiency, whereby the efficiency control is completely automatic and requires no operator activity.

At times it may be desired to operate intentionally at less than optimum efficiency when greater qualities of chlorine are required. The efficiency determinations may be used to maintain such an operation and indeed to maintain the individual productions of chlorine dioxide and chlorine at any desired levels.

The individual signals in line 43 and 44 may be independently recorded by recorder 50, as signified by lines 52 and 54, respectively, so that calibration of the recorded efficiency values may be effected by independent calculation from the recorded chlorine dioxide and chlorine values.

The system discussed above with reference to Figure 1, if desired, may be used to make efficiency determinations at widely-spaced time intervals, e.g., daily or twice daily, to determine long term variations in efficiency, in place of similarly-effected widely-spaced time interval manual determinations of GA% C102 values. However, the major benefit of the system lies in its ability to effect continuous monitoring of the efficiency of the chlorine dioxide generator 10 by taking samples as often as the chromatograph is capable of handling the same, for example, every 3 to 5 minutes.

By continuously monitoring the efficiency of chlorine dioxide generation in this way, significantly closer control of variations in efficiency of the process can be effected, as compared to the prior art discontinuous manual procedure, and hence an overall more efficient operation and an increased chlorine dioxide production are realized with consequential savings in raw material costs. These results are particularly significant in the smaller-sized generators currently used, in view of their greater sensitivity to parameter variations, as mentioned above.

Turning now to consideration of Figures 2 and 3, there is illustrated therein an automatic computercontrolled chlorine dioxide producing plant, in the form of a simplified logic flow diagram (Figure 2) and a hardware diagram (Figure 3).

In this computer control system, a number of measurements are continuously and automatically made, the measurements are used to determine parameters of the system, such as, efficiency of chlorine dioxide product and chlorine dioxide production rate, and when the plant is to be shutdown completely or shutdown to a standby condition, and the determined parameters are used to make adjustments as required.

The following description of the computer-controlled plant of Figures 2 and 3 is made with reference to a catalyzed chlorine dioxide-producing process, wheren the chlorine dioxide is produced using the procedure of U.S. Patent No. 3,864,456 and described above in connection with Figure 1. Certain modifications to the operations may be necessary if other chlorine dioxide-producing procedures are used.

Referring first to Figure 2, chlorine dioxide production efficiency and production rate are monitored and adjusted by a series of automatic operations. Generator off-gas is automatically machine analyzed at 110 to determine the concentrations of chlorine dioxide, chlorine and air therein, the efficiency is calculated therefrom at 112, the calculated efficiency is compared with flow data at 114, and the catalyst usage is read out at 116, so that additional catalyst can be added, if required to restore the efficiency at the desired level.

The calculated efficiency is compared with the flow rate data to ensure that a detected decrease in efficiency is not due to an improper flow rate of one of the reactants. If the detected decrease in efficiency were the result of such an improper flow rate, then the addition of catalyst would have little or no effect. Only if the flow rates are determined to be correct is the catalyst usage read out at 116 to indicate the necessity to compensate for a detected decrease in efficiency by catalyst addition.

The gas analysis and efficiency calculation may be effected using the efficiency monitoring system described above with respect to Figure 1 or by any other convenient automatic continuous system.

The strength of the chlorine dioxide solution which is formed in the chlorine dioxide generating process is analyzed at 118 and the flow rate of chilled water to the chlorine dioxide absorption tower is measured at 120. These determinations are used to calculate actual production rate at 122.

The concentration of chlorine dioxide, chlorine and air detected in automatic gas analysis at 110 also are used to calculate production rate at 122 by comparison of the ratios of chlorine dioxide to air and chlorine to air. The later calculation also is used to double check the calculation based on chlorine dioxide solution strength analysis and chilled water flow rate.

The level of chlorine dioxide solution in the chlorine dioxide product solution storage tank is continuously detected at 124 to give a measure of the required production rate and also to indicate plant shutdown to a standby basis should the detected inventory exceed a predetermined maximum level. The actual production rate determind at 122 is compared with the required production rate at 126 and a read out of these values is provided at 128. This comparison is effected to determine whether or not adjustment in chlorine dioxide production rate is required.

Provision is made at 130 for manual input to the production rate comparison at 126, so as to permit adjustment of the production rate in accordance with external factors.

Flow rates of reactants and other fluid materials and plant pressures and temperatures are continuously monitored at 132. The flow rate data is used in the flow rate and efficiency comparison at 1 14to determine whether or not catalyst flow adjustment is required in response to a decrease in efficiency.

The various plant operational parameters, representing actual plant controls, are compared at 134 with the controls required to achieve the required production rate in accordance with an indication of required production rate adjustment by the production rate comparison at 126.

The individual concentrations of species in the generator liquor may vary as a result of variations in concentrations of feeds and in losses of chemicals from the generator, for example, by way of spillage and with solid by-product. Any such variations result in either too high or too low concentrations of species in the liquor, so that a precise overall material balance cannot be determined directly. A comparison of theoretical generator liquor concentration and actual liquor concentration therefore is required.

The actual concentrations of species in the generator liquor may be automatically or periodically manually analyzed at 136. The actual liquor analysis is compared at 137 with the theoretical generator liquor concentration calculated at 138 from the plant operational data and the comparison of efficiency and production rate referred to below in order to determine differences.

The presence of differences indicates variations in individual species concentrations arising from one or more of the sources mentioned above. The differences are forwarded to the plant control comparison at 134 and are compensated for by suitable variation of the flow rates. When the differences exceed predetermined limits and indicate an undesirable plant operating condition, an alarm read out is provided at 139.

The generator liquor analysis may be omitted, if desired, although it is preferred to include the same, in view of the additional information that is provided.

The comparison of actual control and required control parameters at 134 provides one of three possibilities. The first possibility is the adjustment at 140 of the parameter controls to the required production rate. The second possibility is generated in response to a requirement for plant shutdown to standby at 142 as a result of a maximum inventory of chlorine dioxide solution product.

In the plant standby condition, reactant flows are set to zero and chilled water flow rate is decreased. The vacuum and reaction temperature are maintained, to permit immediate resumption of chlorine dioxide production once the reactant flow rates are re-established.

The third possibility is the complete plant interlock shutdown at 144 pursuant to an error signal generation at 146 from the detected plant operational data. An error signal may be generated by undesirably low or high generator temperatures, undesirably high cooler-condensortemperature, undesirably low heating steam pressure and undesirably low instrument air pressure. Upon plant shutdown, a purge system is actuated to clear the flow lines of gaseous and liquid materials.

In addition to the above-described utilization of the efficiency calculation at 112 and the production rate calculation at 122, periodic comparison of the efficiency and production rate may be effected at 148 to give a read out at 150, the read out being the production rate expressed as a percentage of efficiency and serving an informational function as to the overall performance of the chlorine dioxide producing procedure.

The chlorine dioxide solution strength analysis determined at 118, the generator liquor analysis determined at 136 and/or 138 and the flow rate data determined at 132 may be intermittently or continuously processed to calculate an approximate overall mass balance for the chlorine dioxide-producing process at 152, the calculated mass balance giving a read out at 154.

The various read outs at 128, 150 and 154 may be stored in any convenient manner for continuous, intermittent or alternate display by a visual display device, such as, a cathode ray tube (CRT) display unit and/or for continuous, intermittent or alternate print-out by any convenient printing device.

Figure 3 illustrates the hardware required to effect the operations set forth in Figure 2. A plurality of analog plant inputs is received by an analog plant inputs unit 210. These inputs respectively are acid flow in line 212, sodium chlorate solution flow in line 214, generator, liquid level in line 216, generator liquor density in line 218, heater steam flow in line 220, sodium chloride solution flow in line 222, chlorine dioxide absorber water flow in line 224, process air flow in line 226, generator pressure in line 228, chlorine dioxide solution storage tank level in line 230, off-gas stream gas analysis in line 232, chlorine dioxide solution strength in line 234 and generator liquor analysis in line 235.

These analog inputs are forwarded to a central processing unit 236 controlled by a real time clock 238. The central processing unit 236 consists of a plurality of integrated circuits which are program-controlled to effect the calculations described with respect to Figure 2. The analog inputs are processed in the central processing unit 236 to provide signals to a set point control module 240 which produces individual controls of flow valve settings, so that the desired production efficiency and production rate are attained or the plant is shut down to a standby condition in response to a high storage level signal in line 230.

The individual outputs of the set point control module 240 are acid flow in line 242, sodium chlorate solution flow in line 244, sodium chloride solution flow in line 246, heater steam flow in line 248, process air flow in line 252, generator pressure in line 254, and generator make up water flow in line 256.

An input terminal 258 is provided to permit operator input of process requirements and manual analysis to the central processing unit 236 to override and/or supplement the analog signals and/or to modify the program controlling the central processing unit 236. An output terminal 260 is provided to produce a printout in line 262 and a visual display, such as, a CRT display, in line 264, so that visual and printed records of current and past performance are maintained.

A digital plant inputs unit 266 is provided to receive digital signals corresponding to undesirable conditions of operation of the chlorine dioxide process which require complete interlock shutdown and to reset requirements. The digital signals corresponding to shutdown requirements are low instrument air pressure in line 268, low steam pressure in line 270, low generator gas temperature in line 272, high generator gas temperature in line 274 and high gas temperature in the cooler-condensor 276. Reset signals are generated by pushbutton resets of steam in line 278 and chemical feed in line 280.

The digital signals are fed to the central processing unit 236 and are processed therein to provide signals to a digital output unit 282 which produces a plurality of individual signals. The individual digital outputs are on-off signals to an acid flow solenoid valve in line 284, a sodium chlorate solution flow solenoid valve in line 286, a sodium chloride solution flow solenoid valve in line 288, steam flow solenoid valve 290, a purge media flow solenoid valve in line 292, motors in line 294 and external alarms in line 296.

By closely controlling the efficiency of chlorine dioxide production and the rate of production in response both to solution concentration and product demand in the above-described manner, uneven plant operation is minimized and optimum chemical usage is achieved. Since a chlorine dioxide solution of consistent concentration is provided, better control of bleach plant operations is possible with improved pulp quality and economic bleach solution usage.

Manpower requirements plant operation are considerabiy decreased since only occasional chemical analysis and visual checks of the read outs are required.

In summary of this disclosure, the present invention is directed to improvements in chlorine dioxide production processes by continuous monitoring of parameters of the system and making adjustments accordingly. Modifications are possible within the scope of the invention.

Claims (16)

1. A continuous process for the production of chlorine dioxide by reducing chlorate ions with chloride ions in the substantial absence of any reducing agent for chlorine and in an aqueous acid reaction medium to form a product gas stream containing chlorine dioxide and chlorine and controlling the efficiency of chlorine dioxide production by varying at least one efficiency-modifying operating parameter of the process in response to determinations of chlorine dioxide production efficiency, characterized in that the determinations of chlorine dioxide efficiency are effected continuously by repetitive operations of machine generating separate signals corresponding to the concentrations of chlorine dioxide and chlorine respectively in the product gas stream, machine converting the generated separate signals into a signal corresponding to the mole ratio of chlorine dioxide to chlorine in the product gas stream, machine converting the mole ratio signal into a signal corresponding to the efficiency of the process by machine computation from the equation: Efficiency = 6R 2 + SR 100% where R is the mole ratio, and machine recording the efficiency signal.

2. The process of claim 1 characterized in that the repetitive operations are effected on successive samples withdrawn from the product gas stream.

3. The process of claim 2 characterized in that the product gas stream exerts a subatmospheric pressure, the successive samples are withdrawn from the product gas stream by the application to the product gas stream of a subatmospheric pressure greater than the subatmospheric pressure of the product gas stream, and following analysis thereof, are returned to the produce gas stream downstream of the withdrawal.

4. The process of claim 2 or 3, characterized in that the samples are analyzed chromatographically and the separate concentration signals are generated from the analysis.

5. The process of any of claims 1 to 4, characterized in that the reduction of chlorate ions with chloride ions is effected at the boiling point of the reaction medium under a subatmospheric pressure and is effected in the presence of at least one chlorine dioxide-producing efficiency-improving catalyst, and chlorine dioxide production efficiency variation is effected by addition of controlled quantities of the catalyst to the reaction medium in response to decreased recorded efficiency.

6. A continuous process for the production of chlorine dioxide by reducing chlorate ions with chloride ions in the substantial absence of any reducing agent for chlorine in an aqueous acid reaction medium having variable operating parameters to form a product gas stream containing chlorine dioxide and chlorine from which an aqueous solution of chlorine dioxide is formed by contact with water, characterized in that (a) continuously machine analyzing the product gas stream to determine the relative concentrations of chlorine dioxide and chlorine thereof and continuously machine calculating from the relative concentrations the efficiency of production of chlorine dioxide by reduction of chlorate ions;; (b) continuously machine analyzing the concentration of chlorine dioxide in the aqueous solution thereof, continuously machine monitoring the flow rate of water to the product gas stream contact, and continuously machine calculating from the concentration analysis and flow rate determination the actual production of chlorine dioxide; and (c) continuously machine monitoring the operational data of the process, machine comparing such data with the operational data required to satisfy a change in production rate determined by machine comparison of the actual production rate and a continuously machine-determined desired production rate; and machine adjusting the operational data of the process to the required production rate.

7. The process of claim 6 further characterized by continuously machine comparing the calculated efficiency with machine monitored flow rates of reactants to the reaction medium and machine indicating any modification required in at least one efficiency-modifying parameter of the process in response to a decrease in efficiency.

8. The process of claim 7, characterized in that the reduction of chlorate ions with chloride ions is effected at the boiling point of the reaction medium under a subatmospheric pressure and is effected in the presence of at least one chlorine dioxide-producing efficiency-improving catalyst, and adding the catalyst to the reaction medium in response to the machine-indicated modification requirement.

9. The process of any one of claims 6 to 8, further characterized by machine determining the theoretical reaction medium ionic concentrations, machine comparing the latter values with actual reaction medium ionic concentrations and machine adjusting the operational data of the process to satisfy any determined difference in theoretical and actual ionic concentrations.

10. The process of any one of claims 6 to 9, characterized by machine determining a requirement for process shutdown in response to detected unacceptable values of at least one operating parameter of the process and machine activating shutdown of the process.

11. The process of any one of claims 6 to 10, characterized by manually inputting a required chloride dioxide production rate and machine adjusting the operational data of the process to the production rate required by the manual input irrespective of the comparison of the machine comparison of actual production rate and machine-determined production rate.

12. An efficiency determining and monitoring apparatus for use in the determination of efficiency of a chlorine dioxide producing process wherein chlorate ions are reduced with chloride ions in the substantial absence of any reducing agent for chlorine and in an acid aqueous reaction medium to form a product gas stream containing chlorine dioxide and chlorine, characterized by product gas analysis and signal generating device (36) for sampling the product gas stream and generating separate signals indicative of concentrations of chlorine dioxide and chlorine respectively in the sample, a converter device (46) for converting the separate concentration signals to a signal indicative of the mole ratio of chlorine dioxide to chlorine in the sample, an efficiency calculation device (48) for producing a signal corresponding to the efficiency of conversion of chlorate ions to chlorine dioxide in the process from the mole ratio signal by computation from the equation: 6R Efficiency = 2 + 5R x 100% wherein R is the mole ratio, and a recording device (50) for recording the efficiency value.

13. The apparatus of claim 12 characterized in that the product gas analysis and signal generating device (36) includes a gas-liquid chromatograph analyzer.

14. The apparatus of claim 12 or 13 characterized in that the product gas analysis and signal generating device (36) and the ratio calculator (46) produce pneumatic signals.

15. The apparatus of claim 12, 13 or 14, characterized in that the recording device (50) includes a comparator for comparing the recorded efficiency with previously-recorded values thereof and for generating a control signal in response to predetermined variations in recorded efficiency values.

16. Automatic apparatus for the control of the production of chlorine dioxide by reduction of chlorate ions with chloride ions in an acid aqueous reaction medium, characterized by: a central processing unit (236) comprising a plurality of integrated circuits programmed to process a plurality of individual analog inputs corresponding to determined parameters of the process received from an analog plant input unit (210), a plurality of digital inputs corresponding to other determined parameters of the process received from a digital plant input unit (212) and manually-actuated inputs received from an operator input terminal (258) and thereby to generate analog output signals to set a point control module (238) having a plurality of set-point controllers for controlling variable position flow values at settings required by the analog or manual inputs, digital output signals to a digital output unit (282) for control of on-off devices for shutdown or start up of the process in response to the digital signals, and data output signals to a recording device (262, 264).