Quality Control in the Process Industries T h e concentration or composition of chemicals can often be measured and controlled with conventional industrial instruments by selection of the proper physical or physical chemical property on which to base the measurements. The use of boiling point rise to determine concentration of solutions and of motor load to determine viscosity represents two such measurements which have been reduced to commercial practice. As industry demands more and better quality control, the number of instruments available for specific controls will increase. However, these should not be used when the simpler instrumentation, described in this paper, fails to provide the required information.
John 6. Dobson
The Foxboro Co., Foxboro,MBSS.
I
NSTRUMENTS are applied to chemical processes to improve yield, increase throughput, or maintain quality. Of these, quality control is often the most compelling. Many instruments which do not actually measure quality may be said to be applied for quality control. For instance, a time-temperature cycle controller on a tire vulcanizer is applied to improve final tire quality. Such a measurement however, is only empirical; it does not take into account changes in rubber compounding. By proper choice of measured variable, conventional industrial instruments can often be applied to continuous measurements of composition or concentration to obtain laboratory accuracy. They offer advantages of lower cost and easier maintenance over specifically analytical instruments such as spectrographs. And because of their ability to measure quality in the flow stream, without the removal of samples, they minimize sampling errors and permit application of the measurement to continuous control. The application of instruments for such quality measurements can most readily be divided by the physical-chemical pqoperties on which the measurement depends (Table I).
In practice the temperature difference is measured between condensing chambers A and B (Figure 1). Condensing chamber A is continuously fed with steam which condenses in the central core and reboils. The temperature of the resistance bulb in chamber A therefore is equal to the temperature of saturated steam a t the total pressure actually existing in the relief line from the digester. The temperature of condensing chamber B is equal to the steam temperature in the line reduced to saturated condition by condensation on the walls of the chamber.
2 CONDENSING CHAMBER
Vapor Pressure Relations
*
*
B
I n mixtures of two or more boiling liquids, the complex relationship involving total pressure, partial pressure, boiling point temperature, superheat or quality of associated vapors, and composition is well known. If all but two of these variables are fixed, the measurement of one of the remaining two may be used as an indication or measure of the other. For instance, under fixed total pressure in a fractionating column, top temperature of the product can be used as a measure of overhead composition. However, because of the difficulty of maintaining precise control over some of these variables, particularly since changes of a few tenths of a degree of temperature or a few tenths per cent in pressure may make a substantial change in the measured variable, measurements utilizing relations among several of these variables can be more accurately and economically applied. For instance, if it is known that a vapor mixture contains one component which is in a saturated condition, it is possible by measuring the differential temperature between the boiling liquid of the pure component and the temperature of unknown mixed saturated vapor to determine the purity of the saturated vapor, under varying pressure conditions.
Pnlp Digester Ontgassing Such a measurement has been used successfully to determine and control the amount of noncondensable gases in alkaline pulp digesters.
k 4 " 4
TO DIGESTER C H A M B ~ R
Figure 1. Condensing Chambers Used for Control of Outgassing a Pulp Digester Left-hand chamber by condensing steam from relief line, measures temperature equal to saturated temperature of steam at partial steam pressure existing in condenser; right-hand chamber then measures evaporating temperature of steam condensate existing i n chamber thus measuring temperature of saturated steam at a pressure equal to total pressure i n condenser
When the temperatures in chambers A and B are equal, no noncondensable gases exist in the relief line. As the partial pressure of noncondensable gases increases, the temperature of B will decrease compared to the temperature of A . Therefore, by controlling the temperature difference, it is possible to vent off noncondensabjes without unnecessarily wasting steam. The usual range of the instrument used is 60' F. temperature difference. In making the installation, care must be taken that a
2695
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2696 Table I. Fundamental Property Utilized Partial pressure and boiling points
H e a t of solution Boiling point rise of solutions
Specific Gravity ’
PH
Vol. 43, No. 12
Physical-Chemical Properties Currently Being Used for Control of Industrial Processes
Actual Measurement Made Temp. difference between boiling known liquid and unknown mixed saturated vapor Temp. differencebetween boiling known liquid and known vapor of unknown superheat Vapor pressure difference between known liquid and unknown saturated vapor a t identical temp. Vapor pressure difference between saturated boiling liquid and same vanor of unknown
Instrument Used Temp. difference with resistance bulbs
Limitations on Measurement Unknown vapor must be saturated
Used to Measure or Control Composition of mixed vapors or mixed vapor and gases
Typical Application Digester outgassing
Temp. difference with resistance bulbs
Vapor must be puresame as reference liquid
%perheat of vapor
Steam desuperheaters
d V p cell
Vapor must be saturated
Vapor composition
Fractionation column
Vapor must be same composition as reference liquid
Superheat of vapor
Control of interstage refrigeration cooler
Ambient tem erature below e q u i d r i u m temperature Effect of heat of solution
Absolute humidity
Humidity control of natural gas
Pressure-temperature relation control d V p cell Pressure-temperature relation control Dewcel
Temp. difference recorder with resistance bulbs after reaction Difference in temperature Temp. difference recorder Effect of heat of reaction of solution before and with resistance bulbs after addition Difference in temperature Temp. difference recorder Evaporator pressure must between boiling diswith resistance bulbs not vary rapidly tilled water and boiling solution a t same pressure Differential p r e s s u r e Bell meter across air bubble svstem .......... Position of chain weighed Densitrol plummet Electrical potential of Self-balancing potentiom.......... glass calomel electrode eter system
OR potential
Electrical potential of Ptcalomel, Pt-Au, or similar electrode system
Self-balancing potentiometer
Solution electrical conductivity
Specific electric resistance of solution
Self-balancing resistance bridge
Dielectric constant
Change of capacitance of Self - balancing capacitance bridge fixed spacing electrical capacitor
Viscosity
Motor load on agitator or rotating drum Torque on rotating drum Loss across orifice plate
Motor load potentiometer Brookfield viscosimeter
..........
sample is always reaching the reference chamber since under conditions of no flow a zero temperature difference could exist, and the control system could “lock-up.” A similar measurement, comparing the temperature of a reference chamber with the temperature of steam flowing in a line, is used to measure and control the superheat of steam provided negligible noncondensables are present.
Alcohol Idss Control Similarly, by measuring the difference in temperature between a boiling pure liquid and a boiling liquid of unknown composition, it is possible to determine when additives affecting the boiling point of the liquid are present. This measurement has been used successfully to record and control the amount of alcohol reaching the bottom of a fractionating column in the first extraction, or beer column of a brandy or alcohol distillation train (Figure 2). In this instance, the temperature difference is measured between the temperature of water condensate boiling in the reference chamber and condensing distillate from the second or third
..........
Concentration solution
of
final
Pieparation of hypochlorite bleach
Concentration solution
of
final
hIaintaining fixed concentration of HzSOa
Concentration solution
of
final
Evaporators
Concentration Concentration
..........
..........
..........
..........
..........
..........
..........
..........
.......... ..........
..........
Control caustic bath concentration
Detection minute quantities of acid or alkali; obtain optimum reaction conditions for improved yield; control reaction time. determine end poidt of neutralization Detection of minute traces of oxidants and reductants (Cru’, Clz, 802,. HzS) in solutions or air (by scrubbing)’ determination of r e a d tion end points (CNClr) (SOz-CrIT‘). obtaining o p t i m u i conditions for bacteriological action Dilution or concentration to specific concentration of electrolytes; detecting contamination with electrolytes (boiler water) ; detecting efficiency of wash Moisture of sheet’ thickness of sheet; honcentration of certain organics
.......... ..........
tray from the bottom of the column. If any alcohol reaches this tray, it immediately depresses the temperature of the condensing distillate below the reference temperature. The instrument normally supplied has a range of -1 to + l o C. which permits accurate reading of as little as 0.02% of ethyl alcohol (Figure 3). The instrument may actuate an alarm when alcohol appears on the tray or, preferably, pneumatically set the control index on the flow controller to reduce feed to the column or to increase the flow of sparging steam; thus loss of alcohol in the slops is prevented. Changes in alcohol concentration of less than 0.005% can readily be read on the chart. However, when the instrument is used as a controller, the output air signal will be changed by deviations of as little as 0.0002% alcohol. These two examples show water (steam) as the pure component or reference, but other liquids may be as readily employed as a reference liquid. Naturally the practicality of such a measurement is contingent on the availability and cost of the required
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 1951
A pressure connection is made a t the same level on the tray and is connected to the opposite side of the same diaphragm. By means of a force-balance measuring and transmission system, the difference in pressure between the sealed system and the actual column pressure is transmitted as a pneumatic signal. This difference is a function of the difference of vapor pressure between the known and unknown materials a t the same temperature. The pneumatic signal is then used to operate an appropriate controller to modify the reflux ratio or make other suitable adjustment. Successful installations include columns separating isobutane and n-butane (8) and the rectifying column of an ethyl alcohol distillation train. The instrumentation on a typical isobutane-n-butane column is shown in Figure 5. Temperature control of the critiFigure 2. Instrumentation for Control of Loss of Alcohol in Beer Column cal tray is by means of a temperature controller resetting the flow controller By comparing condensing temperature of actual vapor in tray with condensing temperature of distilled water at same pressure, amount of alcohol being lost on tray may be measured and on steam to the reboiler. In order controlled to maintain desired composition on the critical tray, the d V p cell's pneumatic quantity of reference fluid. This method is of greatest value in signal is transmitted to a controller which modifies the pressure measuring small deviations in composition of a vapor or liquid on the column. from a pure material. A curve of vapor pressure versus temperature for this mixture is given in Figure 6. Since the d V p cell is available in ranges from 0 to 1.1 and 0 to 29 pounds per square inch (0 to 30 inches Composition Control of water and 0 to 800 inches of water) and sensitivities of better than 1% of range, the sensitivity in terms of composition can be When it is desired to meaaure and control the composition of a readily appreciated. mixture of substantial proportions of two or more components The desirable range far such an instrument is dependent on rather than small amounts of impurities in a single component, both rate of change of vapor pressure with composition and on the problem becomes more complex. However, if it is known existing column pressure, and must be computed for each tray that the vapor above a boiling liquid is saturated (and this condicomposition. tion may be ensured by having it in equilibrium with boiling liquid, as in a fractionating column), it is possible by comparing the vapor pressure of an unknown mixed vapor and the vapor pressure of a known mixed vapor a t the same temperature, to BOILING POINT DEPRESSION determine any deviations of the unknown from the composition ETHYL ALCOHOL-WATER MIXTURES of the known sample. This has proved to be valuable in the ATMOSPHERIC COLUMN control of composition of material on trays of a fractionating column, either in precision distillation of multiple-component mixtures or separation of components having very small differences in boiling points, particularly where composition of feed stocks vary. By measurement of the composition on a tray, a few trays above or below the feed tray, it is possible to prevent major upsets in the column by correcting reflux ratio or heat to the column, before changes in inventory of the entire column have occurred and off-quality overhead produced. In this way, differential vapor pressure control can maintain a column under more stable conditions than top temperature control (even under precise pressure control), since top temperature control cannot sense changes in composition in feed stock until they actually reach the top of the column. The device for measuring such deviation from predetermined composition is known as the d V p cell. It consists of a sealed system (bulb A ) which is filled with a mixture of exactly the composition desired as liquid on the tray in which the instrument is % ALCOHOL BY WEIGH1 installed. This sealed system bulb is immersed in the boiling Figure 3. Calibration Curve of Temperature Differliquid on the tray, and the resulting pressure developed in this ence us. Concentration of Alcohol Appearing on system is impressed on one side of a diaphragm (Figure 4). Extraction Column Tray Shown in Figure 2
+I=
c
2697
2698
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOl. 43, No. 12
that close control of moisture in the dried grain from tunnel dryers can be obtained by accurate control of dew point of the outlet air; the dew point instrument in turn resets the steam f l o ~to the heating coil*.
TO RECEIVER RECORDER CONTROLLER
Superheat MeasuromeIit Just as the measurement of partial pressure of vapors under saturated conditions is a measure of composition, so when the vapors are of known or fixed composition, their superheat can be similarly measured by a temperature difference instrument or a d V p cell, as previously mentioned. A related instrument is the pressure-temperature relation controller. This device consists of a LlPUlO IN BULB SAME sealed thermal system, which is filled AS LlOUID ON TRAY with the same vapor as is used in the process, and a second pressure measuring system. The bulb of the filled system is immersed in the vapor whose superheat is to be deAT SAME ELEVATION termined. The vapor pressure measFigure 4. Differential I‘apor Pressure Transmitter urement can readily be calibrated in ternis of temperature as in the faDifferences in vapor pressure are measured between desired sample of known composition and material actually existing on Ira) of fractionating column miliar vapor pressure syst’em. The controller operates to maint>aina fixed vapor pressure difference. Since this vapor pressure (temperature) difference is the difference betveen the Water Vapor Determination pressure of the saturated (sealed system) vapor and superheated vapot, the controller maintains constant superheat When moisture is the component of interest, other methods Such a pressure-temperature relation controller has been used of measurement, and control are available. The well-known 20 PSI AIR SUPPL
FLEXURE TUBE
relative humidity recorder, depending for its functioning on a length of an organic membrane which is in equilibrium n-ith thP moisture in the air, is a typical example. However; because of slow response, the membrane or hair element is not al\vays applicable to industrial control work. More suitable for such control is a measurement which depends on the temperature of equilibrium of a deliquescent salt and moisture in t,he air. Instruments operating on this principle are commercially available (3). Physicallj- the cell consists of an i n k t nick Ti-ound around a hollow core and saturated with lithium chloride. In operation, the cell is heated by an alternating elect,ric current (using the lithium chloride solution as the conducting path) until the lithiuni chloride in the n-icl; is substantially dry. The equilibrium temperature to obtain this condition then becomes a measurement of deiv point. This cell actually measures dew point or humiditj- in grains per pound of air and therefore is not affect’edby cooling or heating of the sample. It can be used in almost any atmosphere that does not cont,ain gases (such as ammonia) t’hat form conductive solutions. The actual cell temperature corresponding to a given devi point is shown in Figure 7 . Naturally, when the device is immersed in a gas or vapor whose temperature is higher than this “characteristic” temperature, it mill assume the temperature of the gas. It cannot, therefore, be used a t ambient temperatures higher than its characteristic temperature, and t’hesample must be cooled before measurement. Typical applications include the control of the addition of moisture to natural gas to maintain pipeline packing; control of dehumidifier for instrument air and for blanket gases; and con.tro1 of humidity in dryers. Experiments now in progress indicate
HEAD
UCT
REBOILER OIL
Figure 5.
Control System for Fractionating Column Separating Isobutane and n-Butane
Differential vapor pressure transmitter is used t o modify total column pressure to compensate for changes i n feed stock
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
December 1951 I
I
I
I
I
I
I
2699
A measurement is made of the temperature difference between the incoming lime solution and the outgoing hypochlorite solution, With known heat of reaction and virtually fixed specific heats in the solution, the temperature rise is practically an exact measure of the concentrations of the hypochlorite bleach liquor leaving the chlorine mixing tank (Figure 11). Similar arrangements, taking into account heat of dilution, can be used for dilution control on many compounds, for instance caustic or sulfuric acid (Figure 16). Boiling Point Rise
TEMPERATURE
%
‘F
s
Figure 6. Vapor Pressure Curve of Isobutane-n-Butane iMixture Showing Sensitivity Available in dVp Cell
to control the addition of liquid ammonia to an interstage cooler in a multistage ammonia compressor system (Figure 8). The advantages of interstage cooling, which results in increased volumetric efficiency and reduced final temperature, are well known. Without interstage cooling, the compression curve (Figure 9) would follow curve 1, 2, 3. Interstage cooling permits folloiving curve 1, 2, 212,3a. Point 2a should approach but not reach the saturation curve as “wet” gas will cause excessive compressor valve maintenance. The pressure-temperature relation controller permits the operator to control the exact superheat condition of point 2a. Once set, the controller maintains the superheat under widely varying pressure conditions. Although functionally this instrument is identical to the d V p cell, each has a distinct field of application. The d V p cell combines extremely high sensitivity with relathely narTOW range. The pressure-temperature relation controller combines a lower sensitivity with a substantially wider range.
The variation of the boiling point rise with concentration in polar solutions is used as a measure and means of control of the concentration of such solutions as they leave the last solution effect of a continuous evaporator. In practice, a measurement of temperature difference is made between the temperature of a condensing chamber in the outlet vapor line (equal to the boiling point of water a t the pressure in the evaporator, assuming negligible noncondensables present) and the temperature of a boiling polar solution in the body of the last liquid effect of the evaporator (Figure 12) A typical range of instrument on caustic (NaOH) evaporator is 100’ F. corresponding to 62% caustic. Following Duhring’s law, the boiling point rise is virtually independent of absolute pressure, and therefore the temperature difference required is not affected by the operating pressure of the evaporator. This is true under steady conditions, but rapid fluctuations in evaporator pressure will cause seriously erratic control. The reason for this is apparent when it is realized that it is often necessary to place the resistance thermometer measuring the boiling liquid temperature a t the discharge of the discharge pump in order to ensure a place of temperature measure-
Heat of Reaction
*
T
A common method for the preparation of calcium hypochlorite bleach liquor of fixed composition is to ratio, on a metered or weight basis, the addition of the lime solution and the chlorine to the mixing tank. However, when the chlorine gas is wet, the problem of metering such a highly corrosive Auid becomes difficult, and further when the chlorine is mixed with varying concentrations of inert gases the continuous preparation of a fixed concentration of hypochlorite bleach liquor by metering becomes impractical. The heat of reaction and the analogous property of heat of solution have made possible the solution of this and other similar problems. To prepare a hypochlorite solution, a lime solution of fixed composition is fed into the chlorine mixing tank (Figure 10)
100
Figure 7.
80
60
40
20
0
RELATIVE HUMIDITY Calibration Curve of Dewcel
For a given dew point temperature, the Dewcel will reach an equilibrium temperature which is used as a measure of dew point; right-hand chart shows range of ambient temperatures and relative humidities which are within range of the Dewcel
INDUSTRIAL AND ENGINEERING CHEMISTRY
2700
AllYlONIA
t
RECEIVER
xx
I
"3
t
'
"IIPOR
CYLINDER
CILlNDER
KhOCXOUT DRUM
Figure 8. Control System for Reducing Superheat Ammonia Vapor between Low and High Pressure Side of Ammonia Condenser Pressure-temperature relation controller bleeds i n liquid ammonia to reduce superheat from existing value to superheat temperature of a few degrees
Vol. 43, No. 12
bubble tubes and the pressure across a known reference chamber is a measure of the specific gravity of the unknown solution. The variation of specific gravity of sodium hydroxide solutions for concentrations of 0 to 28% is from 1.0 to 1.3. Recorder controllers can readily be built pith spans as narrov as 0.1 specific gravity, and since the instruments have sensitivity sub stant,ially better than 0.001 specific gravity (0.1% sodium hydroxide), more than adequate sensitivity is available for this application. Specific gravity may be used not only to measure the concentration of pure solutions but also of slurries. Figure 14 shows a method of preparing lime slurry on a continuous basis. Slurry is withdrawn from the tank intermittently depending on process requirements. Concentration of lime in the slurry is continuously measured by an air bubble system. As the level drops, the water flow increases and the speed of the dry feeder feeding lime is also increased proportionally. Because of the lack of an exact relation between speed and delivery on most volumetric feeders, the relation between water feed and feeder speed must be changed. This is automatically varied by the specific gravity controller continuously resetting the ratio between JTater input and feeder speed.
plf Sieasuremerrt 60
50
m
z
40
b" +'
;
30
0
II"
3 20
>
0
1 57 6.F
'l35'F
1166sF
TEYPERATJRE-'F
Figure 9. Volume-Temperature Chart Shows Improved Volumetric Efficiency and Decreased Final Temperature Due to Interstage Desuperheating of Vapor Curve 1, 2, 3 shows compression without interstage cooling; curve 1,2, Za, 3a shows similar curve with interstage cooling
Three electrochemical properties of solutions can now be measured by industrial instruments-namely, pH, oxidation reduction potential, and solution conductivity. The measurement of pH by determining the voltage of a glass-calomel electrode system with an instrument having an impedance in excess of 1014 ohms is well known. Recent advances in glass technology have permitted the measurement of higher pH's and the pH's of solutions of high salt concentration, and instrument improvements have made possible the measurement of p H under stray voltage conditions. pH is sometimes used as a means of detecting or measuring minute concentrations of acids or alkalies in otherwise neutral solutions. I n the range of a few parts per million, p€f is sometimes more sensitive than solution conductivity for such determinations although most materials reach a point when no further change in p H occurs because of increased concentrations when the concentration of the electrolyte reaches a few hundred parts per million. p H has the advantage of solution conductivity in measurement of minute concentrations in its ability to distinguish between acids and alkali contaminants However, the more complex electrode maintenance and instru-
ment a t rrhich the bulb will always be immersed. Naturally, under rapidly varying pressure conditions, the liquid temperature a t the pump discharge does not always remain in phase with the temperature of the liquid in the evaporator body, or the temperature of the condensing chamber. Because of this limitation, boiling point rise should only be applied to evaporators that are equipped with sensitive and rapid absolute pressure control.
Specific G r a v i t y
FDXBORD
I HYPOCHLORITE BLEbCH LlOUOfi.
1
Specific gravity as a measure of concentration of solutions has been accepted through the years in many process industries. The conventional hydrometer, of course, is not adaptable for control methods, although a modification of the hydrometer principle has recently been offered as an instrument suitable for control ( 7 ) . The common method of specific gravity measurement with control instruments is by means of the air bubble differential pressure principle. A common application is the control of the concentration of caustic in a continuous mercerizing bath (Figure 13). The difference between pressure across fixed
II
, I
\lEMPERbTURE BULB NO. 2
Figure 10. Control of Concentration of Calcium Hypochlorite Manufactured from Chlorine of Widely Varying Impurities Temperature rise due to heat of reaction is controlled
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
ment installation may result in solution conductivity being the favorite measurement. The p H of solutions is often maintained with a narrow limit in order to force reaction to the end point or to improve yield. For instance, in the treatment of cyanide waste by chlorination, control of the p H of the solution is necessary to prevent the formation of cyanogen chloride (g).
AVAlLA0LE CLo- GM/LITEA
2101
Because of this complexity, the interpretation of potential in terms of actual concentrations or activities of solution has been difficult. However, despite these limitations, oxidationreduction potential is finding increasing use in the industrial field. Oxidation-reduction potential measurement has been used for the determination and control of minute concentrations of oxidants and reductants in solution. It has been applied for the detection of cyanide, chlorine, sulfur dioxide, and hydrogen sulfide in water solutions. It has also been used to detect small quantities of these same materials in air by use of a countercurrent water scrubber and measurement of 0-R potential on the resulting solution. The measurement has the obvious advantage over solution conductivity in that it measures only the presence of oxidants and reductants and is insensitive to other contaminants such as carbon dioxide. It has also been used for determining the end point of reactions between oxidants and reductants in waste treatment work, including the end point of the reduction of hexavalent chrome to trivalent chrome by means of ferrous sulfate (6). The Texas Division of the Dow Chemical Co. is successfully using 0-R potential for the determination of chlorine residual in both their fresh- and salt-water installations. The calibration curves for three of their installations are shown in Figure 15.
Figure-11. Calibration Curve of Temperature,Difference Instrument Used to Maintainfcalcium Hypochlorite Concentration in Figure 10
" BULB
