Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D21/00—Control of chemical or physico-chemical variables, e.g. pH value
  • G05D21/02—Control of chemical or physico-chemical variables, e.g. pH value characterised by the use of electric means

Definitions

• the present invention consists of a number of steps, some of which are novel and separately appli- cable in other situations, which speed up the necessary mass transfer operations to the degree, required for successful total control of the reaction directly per ⁇ formed in the process line itself.
• This line may be a pipe, or a ditch as is common in wastewater pro- cessing.
• the process stream is mixed before and after reagent addition, by the use of passive mixers;
• a stream of neutral carrier liquid, or bypassed process liquid fed by a pump is used to entrain the output of the reagent control valves and to force this mixture through a multitude of orifices distributed over the cross-section of the process line.
• This approach has the advantage that powdered solid reagent, dispensed in a similar binary mode and then suspended in the carrier liquid, may be used instead of liquid reagent.
• Sensors, reagent orifices and passive mixer make up a control section in the process line.
• Two or more of these control sections preferably uti ⁇ lizing stepwise diluted reagent, more diluted in successive sections, may be cascaded for finer control.
• the output of the downstream sensors (even €ually as a final grid of sensors only, following the last control section), antilogged, digitized and averaged as described above, is not only used for control of its own section, but also far afeedback-correction of the reagent-command of the previous, more up ⁇ stream, section in order to establish correction for buffer of the process liquid.
• reagent dispensing systems in first and/or later sections may be doubled to provide for addition of either the alka ⁇ line or acid reagent as called for by the sensors.
• the electronic data processor should prevent os ⁇ cillating alternation of these reagent systems to preclude waste of reagents.
• the angle figure indicates- schematically in elevation the arrangement of two control sections, preceded by a flowmeter and a static backmixer, and followed by a final sensor section, which constitutes one embodiment of this invention.
• 2 designates a static mixer of the backmixing type, symbolized by a bed of "Raschig Rings". These rings are sections of thin-walled tubing, about as high as they are wide.
• Such a bed not only repeatedly splits and joins separate stream threads of the proces liquid, but within the cavities of the rings some process liquid is retained and released at a slower rate than the average process flow rate, producing the effect of backmixing.
• the end result is that a step-change of composition in the process stream is stretched into a ramp-change of composition, radially homogeneous from pipe-wall to pipeline center, rather than with a parabolic front as in plug flow.
• a grid across the process line carrying a number of composition sensors is designated by 3. These sensors have to operate reliably under total immersion under some ambient over-pressure.
• the sensors should be positioned such that shear flow across their sensitive faceplate maximizes fast response.
• the grid supporting the sensors may be followed by or be made part of, the distribution system which dispenses reagent evenly over the cross-section
• the static mixer 5 should be followed by the final sensor assembly 6 unless the control section is duplicated as discussed below.
• the sensors in grid 6 may be positioned as shown in grid 3, or in any different way which enhances shear flow of process liquid across their sensitive faceplates, as is shown in 6 by the use of flow deflectors.
• the final sensor -- supporting grid also carries a reference electrode 7, again preferably of the type described in U.S. Patent No. 4,133,732, issued January 9, 1979 consisting of an electrode embedded in gelled endpoint-composition process liquid. By locating this reference electrode at the end of the composition-adjustment sections, there is minimum possibility of contamination of the reference gel in the long run.
• This reference elec ⁇ trode designates the base potential from which the signa potential of each of the many sensor electrodes is measured.
• the electrical composition-related signals put out by the sensors are processed electronically in a way to be discussed further down, to one signal going to the reagent dispensing control valves 11, 12, 13, ... 18, 19.
• This command signal is digital, and has the form of a binary number or "word". Such a number consists of the digits 1 and 0, wherein a 1 causes the solenoid of a specific on-off solenoid valve to be energized and the valve opened. The appearance of a 0 for that particular valve, de-energizes the solenoid and causes the valve to close.
• every digit in the word may command its own valve.
• valves are placed parallel to each other between a reagent reservoir under constant overpressure and the process line, and if each adjacent valve counting from the valve controlled by the least significant bit, is provided with an orifice which allows twice as much re ⁇ agent to flow through as its smaller neighbor, every more significant bit in the control word commands a re ⁇ agent flow twice as large as the flow commanded by its adjacent less significant bit.
• the total flow of the control valve assembly now becomes directly related to the total numerical value of the control command word.
• a word of 10 binary digits or bits commands a total flow of from 0 to 1024 arbitrary units of flow although the diameters of the orifices in the 10 control valves only increase from 1 to 32 arbitrary length units.
• the subdivision in 10, or any other desired number of valves makes possible a very fast and very reproducible reagent flow control responding to commands for minute increases or decreases as fast as for large changes.
• Differential pressure controller 20 causes a constant pressure differential in the reagent liquid from reservoir 21 across the control valves to process line pressure at 22, e.g. by pressurizing the reagent reservoir with air.
• a flow of carrier liquid 23 in ⁇ sures rapid transportation of even small amounts of re- agent to the dispensing orifices in the process line at 4.
• This carrier liquid may be water or process liquid from the process stream, moved by a separate pump.
• the drawing shows addition of controlled amount of liquid reagent.
• slurry or powdered solid reagents can be added to the carrier liquid by feeders with sequentially doubling capacities, controlled by a binary control word.
• the second, downstream, section may be fed from a similar control valve assembly 24, which now advan ⁇ tageously dispenses diluted reagent 25 from dilutor 26 in order to make possible a wider range of controlled re ⁇ agent addition to the process stream.
• the reservoir of diluted reagent should be pressurized to a constant pressure differential over control valves 24, by differential pressure controller 27, just as was done with the previous control section.
• the second control section may advantageously narrow the final control range of the adjusted process liquid composition by using liquid reagent.
• the second, or any identical additional control section may control the addition of a different ⁇ reagent to correct a different deficiency an the process liquid, as e.g. acidic versus alkaline reagent in correcting the pH of the process stream which may be above as well as (at some other time) below the desired pH.
• the electrical signal derived from the sensors in that section will decide whether the section's reagent addition will be activated, or whether the pH calls for addition of different reagent from another section.
• the system of on-off control valves can shut off tight; something which should not be counted on with present-day control valves. Since leakage means extra chemicals to be neutralized, it represents waste.
• the electrical signal of the sensor electrodes has to be processed.
• Sensor electrodes by their nature have an • output which is the logarithm of actual ionic concen ⁇ tration in the liquid to be measured. Should different electrode outputs, representing locally different ionic concentrations, be averaged directly by connecting in parallel, an average of the logarithms of ionic con ⁇ centration differences would be obtained which mathe ⁇ matically may be called a geometric average. However, the actual difference in ion concentrations should be averaged by algebraic addition of these concentrations (and then division by the number of sensors) , an operation of which the logarithm cannot be taken. Hence not geometrical, but arithmetical averaging is required. This refinement becomes meaningful if unbuffered pro ⁇ cess liquids have to be neutralized.
• the averaged digital antilog output of the sensors lends itself directly to ;subtraction of.the number of ion equivalents of reagent introduced into the process stream to produce the desired endpoint concen- tration; this output can also be multiplied by a variable factor, like process flow rate from flowmeter 1 or buffer capacity of the process liquid, computed from the downstream sensor response to a certain addition of reagent. All these computational manipulations of the averaged sensor signal, and its process into a valve command word of the proper format, may be done inex ⁇ pensively by microprocessor 29 for the first, and 30, for the second control section in the drawing. Modern electronic technology may allow combination of all operations done by separately presented circuit units
• VLSI very large scale integrated circuit

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Control Of Non-Electrical Variables (AREA)
• Accessories For Mixers (AREA)
• Automatic Analysis And Handling Materials Therefor (AREA)

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Citations (6)

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• 1978
  • 1978-04-28 US US05/900,908 patent/US4181951A/en not_active Expired - Lifetime
• 1979
  • 1979-04-16 DE DE7979900433T patent/DE2965265D1/de not_active Expired
  • 1979-04-16 WO PCT/US1979/000236 patent/WO1979000998A1/en not_active Ceased
  • 1979-04-16 JP JP54500716A patent/JPS633323B2/ja not_active Expired
  • 1979-04-27 CA CA000326485A patent/CA1116719A/en not_active Expired
  • 1979-12-04 EP EP79900433A patent/EP0018971B1/en not_active Expired

Patent Citations (6)

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