8.31

ORP Controls D. M. GRAY

(1985)

R. R. JAIN

(1995)

R. H. MEEKER, JR.

(2005)

INTRODUCTION

The Nernst Equation

Oxidation-reduction potential (ORP) measurements are useful for the quantitative determination of ions, for monitoring chemical reactions, for determining the extent to which oxidizing or reducing reactions have taken place, and for determining the concentration of chemical species present. The ORP analyzer probes are described in Section 8.41 in Chapter 8 of the first volume of this handbook, Process Measurement and Analysis. While ORP measurements are somewhat similar to those of pH, the potential reading must be interpreted even more carefully for meaningful results. In this section a description is given of the oxidationreduction (redox) reactions and their control using oxidationreduction potential measurement. ORP and pH instrumentation is compared and an example of titration curves is provided to serve the understanding of the ORP response. The pH electrode is an ORP sensor that is specific to hydrogen and has been designed to selectively develop a hydrogen ion activity-related potential. Several major industrial applications for ORP control are described in this section, including cyanide oxidation, chrome reduction, sodium hypochlorite (bleach) production, and removal (scrubbing) of chlorine and chlorine dioxide from gaseous emissions. Both batch and continuous cyanide oxidation and chrome-reduction processes are discussed for waste treatment applications. The applications of bleaching of paper pulp and the treatment of swimming pool and spa water are also covered.

The measured electric potential developed by oxidationreduction reactions is described by the Nernst equation:

An oxidation-reduction reaction involves the transfer of electrons from one material (oxidation) to another (reduction). In industrial applications, oxidizing or reducing agents are used to promote the desired reaction. An example of such a reaction occurs between chromium and ferrous ions, as shown in Equation 8.31(1): 8.31(1)

In this process, the reducing agent (ferrous ion) donates electrons to the chromium, thus reducing chrome while the iron is oxidized. 2032 © 2006 by Béla Lipták

[α ]x [α ]y [α ]z K 2.303RT log X a Y b Z c nF [α A ] [α B ] [α C ] K

8.31(2)

where E = the developed potential (mV) E = the standard potential (mV) 0

−3

R = the gas law constant (e.g., 1.987 × 10 kcal/mole°K, in SI units) T = absolute temperature (e.g., 298°K, in SI units, for a reference temperature of 25°C) n = number of electrons transferred in the reaction −3

F = Faraday constant (e.g., 23.06 × 10 kcal/mV, in SI units) For a reaction written as a reduction equation: − aA + bB + cC + … + ne = xX + yY + zZ + … 8.31(3)

[αA], [αB], [αC] …, and [αX], [αY], [αZ], … are the activities of each respective ion (note, only ions, not neutral molecules, are counted in the equation); a, b, c, … and x, y, z, … are coefficients of the balanced chemical reaction. Activity α is related to the molar concentration (e.g., moles/liter) as follows:

α = γ [ A]

ORP AS A PROCESS VARIABLE

Cr +6 + 3Fe +2 → Cr +3 + 3Fe +3

E = E0 −

8.31(4)

where α = the ionic activity γ = the activity coefficient [A] = concentration of reactant A Generally, the more ions there are in solution, the more difficult it is for the exchange of electrons to occur that results in development of an electric potential. The activity coefficient reflects this. Most solutions of interest are dilute enough that the activity coefficient is very close to unity, though this is not always the case. Fortunately, in most process control applications it is ionic activity (the tendency of a component

8.31 ORP Controls

to participate in a reaction) that is of most interest and not the absolute concentration of the ions. With the constants for R, T, and F inserted and a reference temperature of 25°C assumed, Equation 8.31(2) can be written in a more commonly used form: E = E0 −

[α ]x [α ]y [α ]z ... 59.16 log X a Y b Z c n [α A ] [α B ] [α C ] ...

8.31(5)

0

The standard potential, E , is defined for any given halfcell reaction as the potential obtained when all ions or molecules in solution are at a concentration of 1 mole/liter and all 2 gases are at a partial pressure of 1 atmosphere. By convention, the standard potential is normally referenced to a temperature + of 25°C and to the hydrogen ion/hydrogen (H /H2) half-cell 0 reaction (E = 0 mV). As indicated by Equations 8.31(2) and 8.31(5), ORP is dependent upon concentrations of all of the reactants + involved. If any of the reactants are hydrogen (H ) or hydrox– ide (OH ) ions, then the ORP measurement is pH-dependent. This is evident by further refining the Nernst equation specifically for pH measurement: E = E 0 − 0.198Tk log

1 [α H + ]

8.31(6)

and

α H + = γ [H + ]

8.31(7)

The similarities between the ORP and pH measurements are evident in the fundamental principles upon which the measurements are based. It is helpful to realize the pH is just an ion-specific ORP sensor, where the measurement electrode has been designed to selectively develop a potential related to hydrogen ion activity. Similarly to hydrogen, other ion selective sensors can also be built. Measurement During the chemical reaction shown in Equation 8.31(1), an inert metal electrode is placed in contact with the solution and detects the solution’s ability to accept or donate electrons. The resulting oxidation-reduction potential (redox potential) is directly related to the progress of the reaction. A ferrous reduc+2 ing ion (Fe ) provides electrons and tends to make the elec+6 trode reading more negative. A chromium oxidizing ion (Cr ) accepts electrons and tends to make the electrode reading more positive. The resulting net electrode potential is related (logarithmically) to the ratio of concentrations of oxidizing and reducing ions in solution. ORP is an extremely sensitive measurement of the degree of treatment provided by the reaction. However, it is generally impractical to relate this measurement to a definite concen-

© 2006 by Béla Lipták

2033

tration of a reaction component. This is due to the dependence of the measurement upon the concentration ratios of all oxidants and reductants. Interpretation of the measurement is further complicated because of its nonlinearity with respect to concentration, plus the detector’s inaccuracy and the lack of temperature compensation. The exact potential to ensure complete treatment can be calculated theoretically, but in practice is subject to variations in reference electrode potential, pH, the presence of other waste stream contaminants, temperature, purity of reagents, 2 and so forth. The target potential is usually determined empirically by testing the treated process flow for trace levels of the material to be eliminated or minimized. The optimum control point (ORP reading) occurs when just enough reagent has been added to complete the reaction. Approximate suggested control points are given later in this section, but should be verified on-line by testing the actual samples. Instrumentation The detectors used for ORP measurement are very similar to that used for pH measurement. The reference electrode is identical to the pH reference electrode. The measuring electrode, however, consists of a noble metal instead of a glass bulb. The noble metal electrode is usually made from platinum, gold, or nickel, but carbon may also be used. The noble metal electrode is subject not only to coating, but also to poisoning, which can result in sluggish or inaccurate measurement. For this reason, it is advisable to use scrubber-type probe cleaners (Figure 8.31a) or retractable probes that in the retracted position can first be cleaned by a cleaning fluid and then can be recalibrated using a standard reference solution. ORP measurement electrodes must be maintained to provide a very clean metal surface. Routine cleaning of electrodes with a soft cloth, dilute acids, or cleaning agents is needed often to promote fast response. The noble metal comprising the measurement electrode is often platinum or gold. It is important to carefully select the electrode material for compatibility with the process. For example, gold may actually have a greater tendency to corrode than platinum in some solutions; however, gold is generally more resistant to strong oxidizing agents than platinum. Similarly, the composition and design of the reference electrode must be compatible. Incompatible reference electrode solutions can be quickly poisoned by the process fluid, rendering the measurement useless. At a constant solution temperature of 25°C, the slope of pH measurement will be 59.16 mV per pH unit, so full scale would be approximately +414 to −414 mV for 0−14 pH. Polarity of ORP measurements can be a source of confusion. Electrodes that measure pH produce a negative potential with upscale pH readings (because pH is defined as the negative log of the hydrogen ion concentration, and more hydrogen ion activity means a more acidic solution). Thus, the reference electrode connection of a pH instrument is the positive input.

2034

Control and Optimization of Unit Operations

TABLE 8.31b ORP Values (in Millivolts) of Quinhydrone-Saturated pH Buffer Solutions (Using Saturated Silver–Silver Chloride Reference Electrode) pH Buffer

FIG. 8.31a Scrubber-type probe cleaner. (Courtesy of Universal Analyzers Inc.)

Relative to the reference electrode, the more positive the potential of the ORP noble metal measurement electrode is, the greater the oxidizing environment of the solution in which it is placed, and conversely, the more negative, the more reducing the environment is. When ORP equipment is based on a pH instrument design, it may be necessary to connect the ORP electrode to the reference input and the reference electrode to the measuring terminal to achieve this response. When sensor connectors are of the BNC style, this may not be practical. Some combination pH/ORP transmitters have a jumper or configuration selection that will swap the polarity internally or permit the mA output range to be reversed. The applications described in this section assume the ORP sensor readings indicate increasing potential with increasing oxidizing environment. This is the most common convention, consistent with most chemistry texts and standard potential tables. Calibration ORP instruments are calibrated like voltmeters, measuring absolute millivolts (mV), although a standardized (zero) adjustment is often available on instruments designed also for pH measurement.

© 2006 by Béla Lipták

68°F (20°C)

77°F (25°C)

86°F (30°C)

4.010

267

263

259

6.86

100

94

88

7.00

92

86

80

9.00

−26

−32

−39

9.18

−36

−43

−49

To verify operation of electrodes, it is useful to have a known ORP solution composition using quinhydrone and pH buffer solutions. These must be made up fresh to prevent air oxidation and deterioration. A quinhydrone reference solution prepared in a pH 4 buffer solution should read about 264 mV at 25°C with a platinum measurement electrode and silver–silver chloride (Ag/AgCl) reference electrode. With quinhydrone in a pH 7 buffer solution, the same-type electrodes should read about 87 mV at 25°C. Quinhydrone-saturated pH buffers are used to establish known potentials as a check when a shift is detected (manually or automatically) in either the span or the potential. See Table 8.31b for ORP values in millivolts in quinhydronesaturated pH buffer solutions at various temperatures. A more stable ORP reference solution has been developed (also known as “Light's Solution”), consisting of 0.1 M ferrous ammonium sulfate, 0.1 M ammonium sulfate, and 1.0 M sulfuric acid. Its ORP is +476 mV when measured with a silver–silver chloride, saturated potassium chloride 3 reference electrode. The ORP measurement is displayed in millivolts. Temperature compensation is not used because the compensation would be different for each reaction, making it impractical to produce a generally useful temperature correction as can be done with pH.

ORP CONTROL The ORP control process (like pH control) requires that the control system designer understand the chemistry of the process that is to be controlled. In addition to vessel size, vessel geometry, agitation requirements (needed to guarantee uniform composition), and reagent delivery systems, solid removal problems must also be considered. In cases where one or both of the half-reactions (redox reaction) involve hydrogen ions, ORP measurement becomes pH-dependent. The potential changes measured by the ORP electrode will vary with the redox ratio, but the redox ratio will vary with pH. Therefore, it becomes necessary to experimentally determine the control point, and both pH and ORP measurements are required to control the process.

8.31 ORP Controls

As with pH, reliable ORP control requires vigorous mixing to ensure uniform composition throughout the reaction tank. For continuous control the tank should provide adequate retention time (filled tank volume divided by process flow 4 rate), typically 10 min or more. ORP measurement in relation to concentration of reactants (and, therefore, reactant flowrate) produces a nonlinear response similar in shape to the familiar pH titration curve. This nonlinearity in the ORP titration curves can make PID control difficult. The degree of difficulty is a function of how tightly the ORP is to be controlled, where the operating point is on the ORP curve, and over what range of conditions should the ORP be controlled. If necessary, techniques used to improve pH control, such as characterization of the error, can also be applied in the control of ORP (see Section 8.32). Complete treatment requires a slight excess of reagent and a control point that is slightly beyond the steep portion of the titration curve. Control in this plateau area, where process gain is relatively low, can be obtained by simple on/off control. Reagent feeders are typically metering pumps or solenoid valves. A needle valve in series with a solenoid valve can be used to set the reagent flow more accurately and to improve on/off control. Chrome Waste Treatment Chromates are used as corrosion inhibitors in cooling towers and in various metal finishing operations, including bright dip, conversion coating, and chrome plating. The resulting wastewater from rinse tanks, dumps, or cooling tower blow+6 down contains the toxic and soluble chromium ion (Cr ), which must be removed before discharge to comply with EPA regulations. The most frequently used technique for chrome removal is a two-stage chemical treatment process. In the first stage, acid is added to lower the pH, and reducing agent is added +6 +3 to convert the chrome from soluble Cr (toxic) to Cr (nontoxic). In the second stage the wastewater is neutralized, forming insoluble chromium hydroxide, which can then be removed. First Stage In the first stage, sulfuric acid is used to lower the pH to approximately 2.5 to speed up the reduction reaction and ensure complete treatment. The most commonly used reducing agents are sulfur dioxide, metabisulfite, and ferrous sulfate, but other reducers may also be used. The reducing agents react and form precipitates as shown in 5 Table 8.31c. Equation 8.31(8) describes the reduction reaction with chrome expressed as chromic acid, CrO3, which has a +6 charge on the chromium. The reducing agent is expressed as sulfurous acid (H2SO3), generated by sulfites at low pH. The result is chromium sulfate, Cr2(SO4)3, which has a +3 charge on the chromium. 2CrO3 + 3H 2SO3 → Cr2 (SO 4 )3 + 3H 2O

© 2006 by Béla Lipták

8.31(8)

2035

TABLE 8.31c Chrome Reduction and Precipitation Reactions Reducing Agent

Reaction

Ferrous sulfate (FeSO4)

2H2CrO4 + 6FeSO4 + 6H2SO4 → Cr2(SO4)3 + 3Fe2(SO4)3 + 8H2O Cr2(SO4)3 + 3Ca(OH)2 → 2Cr(OH)3 + 3CaSO4

Sodium metabisulfite (Na2S2O5)

Na2S2O5 + H2O → 2NaHSO3 2H2CrO4 + 3NaHSO3 + 3H2SO4 → Cr2(SO4)3 + 3NaHSO4 + 5H2O Cr2(SO4)3 + 3Ca(OH)2 → 2Cr(OH)3 + 3CaSO4

Sulfur dioxide (SO2)

SO2 + H2O → H2SO3 2H2CrO4 + 3H2SO3 → Cr2(SO4)3 + 5H2O Cr2(SO4)3 + 3Ca(OH)2 → 2Cr(OH)3 + 3CaSO4

Equation 8.31(8) describes the reaction if sulfur dioxide is the reducing agent. As shown in Figure 8.31d, the first-stage reaction is monitored and controlled by independent control loops: acid addition by pH control and reducing agent addition by ORP control. Acid is added under pH control whenever the pH rises above 2.5. Reducing agent is added under ORP control whenever the ORP rises above approximately +250 mV. (Refer to Section 8.39 on water treatment for other control configurations.) The ORP titration curve, Figure 8.31e, shows the entire +6 millivolt range that is covered when Cr chrome is treated in batches. With continuous treatment, however, operation is maintained in the completely reduced portion of the curve near the nominal +250 mV control point. The exact set point for a particular installation should be at a potential where all +6 the Cr has been reduced but without excess sulfite consumption, which is accompanied by the odor of sulfur dioxide. To complete the chrome reduction reaction takes 10– 15 min. The reaction time increases if pH is controlled at higher levels. Variations in pH also affect the measured ORP readings. Therefore, pH must be held constant to achieve consistent ORP control. Second Stage The wastewater is neutralized to precipitate +3 Cr as insoluble chromium hydroxide, Cr(OH)3, and also to the limits for pH, before the treated wastewater can be discharged. Sodium hydroxide or lime (Ca(OH)2) is used to raise the pH to 7.5–8.5, as shown by the reaction in Equation 8.31(9): Cr2 (SO 4 )3 + 6NaOH → 3Na 2SO 4 + 2Cr (OH)3

8.31(9)

In the second stage, it is more difficult to provide good pH control than in the first, because the control point is closer to the sensitive region of the titration curve near neutrality.

2036

Control and Optimization of Unit Operations

Acid

Base

Reducing agent

~250 mV ORPIC set point

S S

2.5 pH Set point

Proportional controller set point = 8 pH

PHIC

M

M

Influent wastewater First stage Cr+6

PHIC

To settling tank

Second stage Precipitate Cr+3

Cr+3

FIG. 8.31d Continuous chrome treatment.

Although the second-stage reaction is fast, retention time of at least 10 min is usually needed for continuous treatment processes, in order to achieve stable operation. In this stage, the pH controller can be a proportional controller. A subsequent settling tank or filter removes the suspended chromium hydroxide. Flocculating agents have been found helpful to assist in this separation. Batch Chrome Treatment Figure 8.31f shows the arrangement for a batch chrome treatment process in which all steps are accomplished in a single tank using a pH and an ORP controller. The steps of the treatment are sequenced, so the pH set point may be changed as needed. In the first stage, acid is added to lower the pH to 2.5, then reducing agent is added to lower ORP to approximately +250 mV.

ORP millivolts

600

500

400 Control point

300 Chrome reduced

After a few minutes have elapsed (ensuring complete +6 reaction) and after a grab sample test for Cr has been made, basic reagent is added in the second stage to raise pH to 8. A settling period then follows, or the batch is pumped into a separate tank or pond for settling. Cyanide Waste Treatment The metal-plating and metal-treating industries produce the largest amounts of cyanide waste. However, other industries also use cyanide compounds as intermediates. Cyanide solutions are used in plating baths for brass, copper, silver, gold, and zinc. The toxic rinse waters and dumps from these operations require destruction of the cyanide before discharge. The most frequently used technique for cyanide destruction is a one- or two-state chemical treatment process. The first stage raises the pH and oxidizes cyanide to the less-toxic cyanate form. When required, the second stage neutralizes and further oxidizes cyanate to harmless bicarbonate and nitrogen. The neutralization also allows the metals to be precipitated and separated from the effluent. First Stage Sodium hydroxide (NaOH) is generally used to raise the pH to approximately 11 to promote the oxidation reaction and ensure complete treatment. The oxidizing agent is generally chlorine (Cl2) or sodium hypochlorite (NaOCl). Alternately, oxidation reduction of cyanide wastes to less toxic by-products may be also be achieved by using ozone or hydrogen peroxide as oxidizing agents. The two-step chemical oxidation reaction between ozone and cyanide can be written as

200 Volume reducing agent added

FIG. 8.31e Chrome reduction titration curve.

© 2006 by Béla Lipták

CN − + O3 → CNO − + O 2 2CNO − + H 2O + 3O3 → 2HCO3 + N 2 + 3O 2

8.31(10) 8.31(11)

8.31 ORP Controls

2037

Acid Base Reducing agent S pH set points PHIC 1st stage = 2.5 2nd stage = 8.0

S First stage ORPIC ~250 mV set point

S

Influent wastewater

S

M

To settling tank

FIG. 8.31f Batch chrome treatment.

A total ozone dosage of approximately 3–6 O3/parts per million (ppm) CN is required for near-total cyanide destruction in industrial waste streams. A one-stage process using hydrogen peroxide and formaldehyde effectively destroys free cyanide and precipitates zinc and cadmium metals in electroplating rinse waters. The chemistry of the destruction of free cyanide cannot be expressed in a simple sequence of reactions, because destruction involves more than one sequence. The monitoring of cyanide rinse water treatment by ORP measurement (using a gold wire electrode) is a useful diagnostic tool for indicating whether the proper quantities of treatment chemicals have been added. The overall reaction for the first stage using sodium hypochlorite (NaOCl) is given here, with cyanide expressed in – ionic form (CN ), and the result expressed as sodium cyanate − (NaCNO) and chloride ion (Cl ): NaCOCl + CN − → NaCNO + Cl −

8.31(12)

For the case when the oxidizing agent is chlorine, refer to Section 8.39. As shown in Figure 8.31g, the first-stage reduction is monitored and controlled by independent control loops: base addition by pH control and oxidizing agent addition by ORP control. The pH controller adds base whenever the pH falls below 11. The ORP controller adds oxidizing agent whenever the ORP falls below approximately +450 mV. A slightly different design is described in Section 8.39.

© 2006 by Béla Lipták

The ORP titration curve, Figure 8.31h, shows the entire millivolt rage that is covered when cyanide is treated in batches. With continuous treatment, however, operation is maintained in the oxidized, positive region of the curve near the +450 mV set point. The exact set point is determined empirically by measuring the potential when all the cyanide has been oxidized but no excess reagent is present. This point can be verified with a sensitive colorimetric test. In this reaction, pH has a strong inverse effect on the ORP. Thus, pH must be closely controlled to achieve consistent ORP control, especially if hypochlorite is used as the oxidizing agent. Hypochlorite addition raises pH, which if left unchecked will lower the ORP, calling for additional hypochlorite and causing a runaway situation. To protect against this, the set point of the pH controller should be above the pH level at which hypochlorite has an influence. It is also necessary to move the ORP electrodes away from the hypochlorite addition point to prevent such interactions. Gold ORP electrodes have been found to give more reli6 able measurement than platinum for this application. Platinum may catalyze some additional reactions at its surface and is more subject to coating than is gold. The solubility of gold in cyanide solutions does not present a problem, because it is in contact primarily with cyanate. Any slight loss of gold actually serves to keep the electrode clean. Second Stage In this stage, the wastewater is neutralized to promote additional oxidation as well as to meet discharge

2038

Control and Optimization of Unit Operations

Base

Acid Oxidizing agent

Oxidizing agent

S S

ORPIC

11 pH set point

PHIC S

PHIC

Proportional control 8.5 pH set point

ORPIC

~600 mV set point M

M Influent wastewater First stage Cyanide (CN−)

~450 mV set point

To settling tank

Second stage

Cyanate (CNO−)

Cyanate (CNO−)

HCO3 + N2

FIG. 8.31g Continuous cyanide treatment.

pH limits. If the lowering of the pH is not required, that loop can be eliminated. Sulfuric acid is typically used to lower the pH to approximately 8.5, where the second oxidation occurs more rapidly. Acid addition must have a fail-safe design, because below neutrality (pH = 7) highly toxic hydrogen cyanide can be generated if the first-stage oxidation has not been completed. Hypochlorite is added either in proportion to that added in the first stage or by separate ORP control to complete the oxidation to sodium bicarbonate (NaHCO3), as shown by Equation 8.31(13):

−200

Batch Cyanide Treatment Figure 8.31i shows the arrangement for batch cyanide treatment with all steps accomplished in a single tank, which is provided with one pH and one ORP controller. (Other control configurations are described in Section 8.39.) In this control system, the steps are sequenced, changing the pH and ORP set points to obtain the required treatment, with the added assurance that treatment is complete before going on to the next step. Caustic is added to raise the pH to 11. Hypochlorite is added to raise the ORP to approximately +450 mV, simultaneously adding more caustic, as required, to maintain a pH of 11. An interlock must be provided to prevent acid addition before the completion of oxidation of all cyanide to cyanate. Then acid can be added to neutralize the batch and further hypochlorite oxidation completes the cyanate-to-bicarbonate conversion. A settling period can be used to remove solids, or the batch can be pumped to another tank or pond for settling.

−400

Sodium Hypochlorite Production

2 NaCNO + 3NaOCl + H 2O → 2NaHCO3 + N 2 + 3NaCl 8.31(13)

800 600

ORP millivolts

ORP control in the second stage is very similar to that in the first, except that the control point is near +600 mV. In the second stage, pH control is more difficult than in the first, because the control point is closer to the sensitive region of neutrality. The pH controller can be proportional only. A subsequent settling tank or filter can remove suspended metal hydroxides, although further treatment may be required.

400

Cyanate destroyed (pH = 8) Cyanide destroyed (pH = 11)

200

Second stage control point

First stage control point

0

Volume oxidizing agent added

FIG. 8.31h Cyanide oxidation titration curve.

© 2006 by Béla Lipták

Sodium hypochlorite (NaOCl) is produced by reacting chlorine gas (Cl2) and dilute sodium hydroxide (NaOH). Sodium hypochlorite is used both as an industrial and as a domestic

8.31 ORP Controls

2039

Acid Base Oxidizing agent S

S

First stage ~450 mV set point ORPIC Second stage ~600 mV set point

S

Influent wastewater

pH set points PHIC 1st stage = 11.0 2nd stage = 8.5

M

S

To settling tank

FIG. 8.31i Batch cyanide treatment.

bleaching agent. For industrial use, it is normally produced on-site and has an available chlorine strength of 12–15%. For domestic use, as household chlorine bleach, the strength is typically 3–6% available chlorine.

AY 1 FF

ORP is used as a measure of the available chlorine in the final product. The control system of the NaOCl production process is shown in Figure 8.31j. Caustic soda solution (sodium hydroxide) at 5% concentration is admitted on level

AIC 1 AT 1 ORP AE 1

Vent

Chlorine gas from vaporizer Reactor

Hypochlorite supply tank LIC 2

5% Caustic solution

LT 2

FIG. 8.31j Control system for the continuous production of sodium hypochlorite.

© 2006 by Béla Lipták

Sodium hypochlorite

2040

Control and Optimization of Unit Operations

control. This supply of caustic soda is normally diluted down from 50 to 5% concentration in two steps (not shown). The ORP controller throttles the ratio of the chlorine gas that is added to the flow of 5% caustic solution. The set point for the ORP controller will be somewhere between 500 and 750 mV, depending upon the initial caustic strength and the desired ratio of available chlorine to excess caustic. On-site sodium hypochlorite production is common in pulp and paper mills, where it is used in the pulp bleaching process. However, the industry trend has been to move away from chlorine bleaching (elemental chlorine or sodium hypochlorite), in favor of chlorine dioxide, hydrogen peroxide, oxygen, and other bleaching agents, which are considered to be preferred from a pollution point of view. Paper Pulp Bleaching The pulp and paper industry uses strong oxidizing agents (commonly chlorine, chlorine dioxide, hydrogen peroxide, oxygen, and ozone) for bleaching and delignification of pulp. ORP has been applied both to control the pulp bleaching and the delignification process and as emission controls for chlorine (Cl2) and chlorine dioxide (ClO2) gases. Bleaching and Delignification Control Bleaching and delignification are accomplished in several stages, with the residues of the bleaching operation extracted from the pulp in washers located between the stages. A traditional bleach plant sequence is chlorination (“C”), followed by caustic extraction (“E”), followed by hypochlorite bleaching (“H”), sometimes followed by caustic extraction (“E”), and finally, by treatment with chlorine dioxide (“D”). To eliminate chlorine, the CEHED or CEHD sequences have in many cases been replaced with DEDED, DED, or other variants that eliminate the C and H stages. ORP measurement can be used in the pulp discharge vat of the washer following the D stage to ensure the chlorine dioxide has been sufficiently rinsed from the pulp. Applying it to control actual chemical addition is more difficult. Because a variety of organic compounds are oxidized, it is not possible to write exact equations for the pulp bleaching process. The critical control parameters for the bleaching and delignification process are brightness, kappa number (an indication of lignin remaining, or pulp purity), chemical residual, pH, temperature, pulp consistency, pulp flowrate, chemical flowrate, and retention time (determined by tower size and flowrate). ORP can be used as an indication of the chemical residual. It is more apt to be useful for the measurement of lower concentrations of residuals, such as in the final stages or in the washers. Because ORP is so nonlinear over the potentially wide range of chemical concentrations in pulp bleaching, and because it can saturate at high residual levels, the preferred method of measurement has become the polarographic technique. Sensors based on polarography are now highly evolved for this application (see Sections 8.4, 8.42, and 8.43 in Chapter 8 in the first volume of this handbook).

© 2006 by Béla Lipták

Polarographic sensors can be located in the line a relatively short distance downstream from the point of chemical addition to rapidly measure how the reaction is proceeding and to be used in closed-loop control. While, in principle, ORP measurement could also be used, it has been found to be inadequate, because the ORP and reference electrodes require excessive maintenance in the harsh environment. When ORP is used to control the dosage of bleaching (chemical flow), it is best used in combination with on-line brightness, pH, temperature, pulp consistency and flow, and chemical flow. Brightness, residual (ORP or polarographic), and chemical flow can be used to calculate a “compensated brightness” used to actually control chemical dosage (pH or consistency can also be part of the compensation). Controlling dosage on residual alone, as measured by ORP or a polarographic instrument, will tend to overbleach for certain 7 process changes. It is also often necessary to independently control pH, depending upon the type of bleaching stage. In a ClO2 bleaching stage, for example, pH must be kept below 3.5 for maximum effectiveness. Chlorine/Chlorine Dioxide Scrubber Control A natural consequence of treating the pulp with strong oxidizing chemicals is the emission of noxious gases. Wet scrubbers are commonly used to “scrub” Cl2 and ClO2 from process gases prior to venting them to the environment. The most effective scrubbing media for this application are the white liquor or the weak wash, both of which contain large amounts of sodium 8 hydroxide (NaOH) and sodium sulfide (Na2S). The Cl2 and ClO2 gases rapidly oxidize the sulfides in − the white liquor or weak wash to form chlorite (ClO2 ) and − chloride (Cl ). A number of concurrent reactions are possible, the favored ones being: 8ClO2 + 5Na2S + 8NaOH → 5Na2SO4 + 8NaCl + 4H2O 8.31(14) 4Cl2 + Na2S + 8NaOH → Na2SO4 + 8NaCl + 4H2O 8.31(15) The scrubber consists of a vertical packed tower. The scrubbing liquid is continuously circulated from the bottom to spray nozzles at the top (Figure 8.31k). Fresh scrubbing make-up solution is added under ORP control to maintain the required excess of reducing agent in the circulated liquid. A continuous overflow provides for changeover of liquid in the system. The purpose of the double-headed positive displacementtype metering pump is to supply the required amount of scrubbing make-up solution to the scrubber, at the right concentration. The required dilution of the weak wash or white liquor is performed by manually adjusting the stroke of each head, thereby setting a constant ratio between these fluids and dilution water (usually condensate). If there was no excess reductant remaining in the recirculated scrubber fluid, the concentrations of Cl2 and ClO2

8.31 ORP Controls

Scrubbed vent gas

2041

AC AT ORP

Scrubber recirculation

SP

AE ORP 1 AT 1

AE 2 pH AT 2

AIC 1

AIC 2

AY 1

Weak wash or white liquor (NaOH, Na2S)

SP = 9

Cl2, ClO2 - laden gases

Make-up water

Cooling tower

Continuous purge overflow

>

Oxidizing biocide

SC Scrubbing make-up solution

Dilution water (condensate) Double-headed metering pump

FIG. 8.31k The control system for a chlorine/chlorine dioxide scrubber.

emissions in the vent gas will rise sharply. The object of the ORP control loop is to keep just enough excess reductant in the scrubbing fluid. The ORP set point would be in the reducing range, somewhere between –100 and –500 mV. For this process it is necessary to run emission tests and vary the ORP by varying fresh make-up to determine the operating point that results in design removal efficiency with only minimal excess chemical. In scrubbers that use white liquor or weak wash that contains sodium sulfide (Na2S), pH must be maintained above 7, because otherwise sulfur compounds will tend to form and deposit in the scrubber, increasing chemical consumption, decreasing removal efficiency, and causing plugging. Further, for liquor or weak wash containing sodium hydrosulfide (NaHS), pH must be maintained above 8.5 to avoid the for8 mation of hydrogen sulfide (H2S) gas. Consequently, the control system shown in Figure 8.31k includes a pH-based override controller. The output signal of this controller, through a high-signal selector (AY-1), overrides the ORP controller to ensure that enough white liquor or weak wash is added to keep the alkalinity at a pH of 9.0. Other ORP Control Systems There are numerous other industrial processes where it is important and useful to know the extent to which an oxidationreduction reaction has proceeded or the extent to which an

© 2006 by Béla Lipták

Process heat transfer equipment

Metering pump SC

FIG. 8.31l ORP-based control system for the control of biological growth in cooling towers.

oxidizing or reducing environment is being maintained. Treatment of process cooling tower water and the indigo dyeing process (textiles) are two examples. Microbial Control in Cooling Towers Biocides are frequently used in cooling towers to control the growth of algae, bacteria, fungi, barnacles, and even clams and mussels in the water. Typical oxidizing microbiocides for this application are chlorine (Cl2), bromine (Br2), sodium hypochlorite (NaOCl), chlorine dioxide (ClO2), and ozone (O3). ORP-based continuous control of the addition of biocide helps to maintain effective treatment without wasting chemicals (Figure 8.31l). The desired range of ORP control is typically between 550 and 650 mV. 9

Indigo Dye Process The process of dyeing cotton and cellulose fibers with indigo dye requires several additives to make the dye soluble in water. Sodium dithionite (Na2S2O4) is added to the bath to reduce the dye (leuco form), and sodium hydroxide (NaOH) is added to form phenolates, which are water-soluble. After the dye has been applied, it is reoxidized, fixing it into the fabric and making it once more insoluble (this can be done just by exposing it to air to dry). Control of the pH can be used to vary the behavior of the dye application (e.g., the degree of penetration) and, therefore, the final effect. ORP is maintained in the range of −760 to −860 mV, to keep the indigo dye in its reduced (leuco) form for solubility. ORP can also be adjusted in and around this range to affect the final shade of the dyed fabric. Swimming Pool and Spa Treatment ORP can provide a measure of sanitizer activity and water quality in swimming pools and spa water. In the United States many public pools and spas use ORP controllers for the automatic control of chlorine addition, but the controller readouts are usually labeled in

2042

Control and Optimization of Unit Operations

parts per million of free chlorine instead of in units of ORP (mV). When chlorine is introduced into the pool or spa water, it forms the active form of free chlorine, hypochlorous acid (HOCl), which is an excellent bactericide. The process of chlorine addition to water is described by the following equilibrium equation: +

−

Cl2(g) + H2O ↔ HOCl(aq) + H (aq) + Cl (aq)

8.31(16)

The position of the equilibrium in the reaction shown in Equation 8.31(16) strongly depends upon the pH and the other reactions going on in the process of oxidizing and disinfecting. The HOCl is a fast-acting sanitizer (weak acid). Its concentration decreases very rapidly with increasing pH in the range of interest for pools and spas (pH 7–8). For good bacteriological quality, it is essential to maintain a proper HOCl level in the water at all times. The recommended minimum ORP level should be controlled at between 650 and 750 mV. The ORP standards (650 or 750 mV) should apply to all sanitizers, including all forms of chlorine (with or without stabilizer) and bromine, as well as to systems using ozone or other sterilization methods. The recommended chlorine control parameters are given in Table 8.31m. Chemical controllers used in the pool and spa industry normally utilize an ORP sensor to monitor the sanitizer concentration, as well as a pH sensor to monitor the pH. The controller automatically turns the appropriate chemical feeders on and off as required to maintain the proper sanitizer and pH levels. This results in good water quality and elimination of chloramines and other undesirable by-products, as well as in savings in both chemical consumption and labor. Cyanuric acid, used to stabilize chlorine against degradation by sunlight in outdoor pools, will tend to reduce the ORP (and, therefore, the chlorine effectiveness) at a given pH and free chlorine residual level. Above 50 ppm cyanuric acid, ORP

controllers lose effectiveness, because additional chlorine called for by the ORP controller has a diminishing ability to increase free chlorine residual and, therefore, ORP. At about 70 ppm cyanuric acid, additional chlorine is reported to have 10 no effect, and ORP control is rendered useless. ORP control can also be used for changing other oxidizers used in pool and spa water sanitation, such as bromine and ozone. Automated pool and spa water treatment systems represent a major advance in pool operation and maintenance. ORP standards for pool and spa sanitation have been recognized by the World Health Organization (WHO), the Centers for Disease Control and Prevention (CDC), the National Spa and Pool Association (NSPA), and a number of state and 11 local health departments.

CONCLUSIONS ORP control is useful when the process to be controlled involves reduction and oxidation (redox) reactions. ORP measurement is best used to indicate the extent to which expected reactions have proceeded and to detect the relative strength of the oxidizing and reducing chemicals in the solution being measured. Successful ORP measurement and control requires an understanding of the ORP measurement principles and of the chemistry that governs the process. ORP control can be highly nonlinear, and therefore the same techniques and strategies that are used in controlling highly nonlinear pH processes (Section 8.32) can also be used for ORP control.

References 1. 2. 3.

TABLE 8.31m Basic Control Parameters for the Treatment of Pool and Spa Waters

4.

Parameters

6.

Recommended Control Levels

pH

7.4–7.8

Free chlorine residual

Minimum free chlorine residual 0.4 ppm

Total chlorine residual

Maximum total chlorine residual should not exceed free chlorine by more than 0.5 ppm

ORP control standard

650 or 750 mV

Total alkalinity*

80–150 ppm

Calcium hardness

Above 140 ppm

5.

7.

8.

9.

10. *Note: Total alkalinity times calcium hardness must equal 25,000–30,000 (this rule works when pH is 7.4–7.6 and temperature is 78–85°F).

© 2006 by Béla Lipták

11.

Masterton, W. L., and Slowinski, E. J., Chemical Principles, 4th edition, Philadelphia, PA: W.B. Saunders, 1977. Latimer, W. M., Oxidation Potentials, New York: Prentice Hall, 1952, Chapter 1. Light, T. S., “Standard Solution for Redox Potential Measurements,” Analytical Chemistry, 44:6, pp. 1038–1039, May 1972. pH Controllability, North Wales, PA: Leeds & Northrup, Application Bulletin C2.0001, October 1975. Lipták, B. G., Water Pollution Environmental Engineers' Handbook, Vol. 1, Radnor, PA: Chilton, 1974. Shinskey, F. G., pH and pIon Control in Process and Waste Streams, New York: John Wiley & Sons, 1973, p. 120. “TAPPI Bleach Plant Operations Short Course,” course notes, Orange Beach, AL: Technical Association of the Pulp and Paper Industry (TAPPI), 1994. Jain, A. K., and Dallons, V. J., “Technical Bulletin No. 616, Bleach Plant Chlorine and Chlorine Dioxide Emissions and Their Control,” New York: National Council for Air and Stream Improvements, (NCASI), 1991. Rosemount Analytical, “pH and ORP Monitoring for the Indigo Dye Process,” Application Data Sheet ADS2820-03/Rev.A, Emerson Process Management, 2002. Williams, K., “Cyanurics: Benefactor or Bomb,” Pumproom Press, Professional Pool Operators of America, 1997. Steininger, J. M., “ORP Control in Pools and Spas,” Santa Barbara Control Systems, 1998.

8.31 ORP Controls

Bibliography Benefield, L. D., Judkins, J. F., and Weand, B. L., Prosess Chemistry for Water and Wastewater Treatment, Englewood Cliffs, NJ: Prentice Hall, 1982, Chapter 13. Cali, G. V., and Galetti, B. J., “Plating Waste Control Instrumentation,” Pollution Engineering, March 1976, pp. 48–50. Chamberlin, N. S., and Snyder, H. B., “Technology of Treating Plating Wastes,” Tenth Industrial Waste Conference, Purdue University, May 1955. Devlin, P. M., Wang, H. J., Winchell, C. J., Day, S. G., Zura, R. D., and Edlich, R. F., “Automated Hydrotherapy Pool Water Treatment System,” Hydrotherapy Pool Management, Vol. 10, No. 1, January/ February 1989. Eckenfelder, W. W., Jr., Industrial Water Pollution Control, 2nd edition, New York: McGraw-Hill, 1989. Greer, W. N., “Measurement and Automatic Control of Etching Strengths of Ferric Chloride,” Plating, October 1961. Henkel Corporation, Energy Group, “Ozone Treatment of Cyanide Wastes,” Data Sheet 941C, June 1989. Kemmer, F. N., The NALCO Water Handbook, 2nd edition, New York: McGraw-Hill, 1988. Lanouette, K. H., “Heavy Metals Removal,” Chemical Engineering, October 1977, pp. 73–80. Lavigne, J. R., Instrumentation Applications for the Pulp and Paper Industry, California, Miller-Freeman, 1979. Lawes, B. C., “A Peroxygen System for Destroying Cyanide in Zinc and Cadmium Electroplating Rinse Waters,” Plating, September 1973.

© 2006 by Béla Lipták

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Leeds & Northrup, Reference Binder 5, Environmental Sciences, “Oxidation-Reduction (REDOX) Potentials,” June 1969. Masterton, W. L., and Slowinski, E. J., Chemical Principles, 4th Edition, Philadelphia, PA: W.B. Saunders, 1977, Chapters 22, 23. Mattock, G., “Automatic Control in Effluent Treatment,” Transactions of the Society of Instrument Technology, December 1964, pp. 1973–1989. McMillan, G. K, “Methods of Controlling pH,” Chemical Processing, July 1997. Record, R. G. H., “The Use of Redox Potentials in Chemical Process Control,” Instrument Engineer, October 1965, pp. 65–75, continued April 1966, pp. 95–102. Rosemount Analytical, “Fundamentals of ORP Measurement,” Application Data Sheet ADS43-014, Emerson Process Management, 2001. Ross, M., “Treating Water at ORP Speed,” Water Technology, May 1996. Shinskey, F. G., pH and pIon Control in Process and Waste Streams, New York: John Wiley & Sons, 1973, Chapters 1, 5. Shinskey, F. G., Problem Solving Software, Ver. 3.2, Foxboro, MA: The Foxboro Company (Invensys Systems), 1998. Shinskey, F. G., Process Control Systems, 4th edition, New York: McGrawHill, 1996, p. 93. Shinskey, F. G., “Sharpening pH Control,” Control, July 1998. Steininger, J. M., “Clean Up with ORP,” The Business Magazine for the Spa & Pool Professional, June 1990. Steininger, J. M., “PPM or ORP: Which Should be Used?” Swimming Pool & SPA Merchandiser, November 1985. Weast, R. C., Handbook of Chemistry and Physics, 50th edition, Cleveland, OH: The Chemical Rubber Company, 1969. Yorgey, W. B., “Updating a Wastewater Treatment Plant with Automatic Controls,” Plant Engineering, February 1973, pp. 1978–1982.