DE10001911A1 - Reduction of deposits in water treatment plant by imposing an electrical impulse to alter the bicarbonate/carbonate equilibrium - Google Patents

Abstract

A process and apparatus are claimed for electrical impulse reduction of carbonate or carbonate-metal oxide mixed crystal deposits on heat-transfer surfaces and pipes in water containing dissolved alkaline earth bicarbonates in an amount to stabilise CO2, the process being effected such as to prevent corrosion and to lessen side-reactions. The process involves (A) introducing a current impulse of 10<4> Hz - 10<-3> Hz between 2 electrodes in the water; (B) using an impulse charge which, integrated over the period, constitutes 1 to a maximum of 20% of the charge of the biocarbonate anions dissolved in the water; and (C) using an impulse voltage below the decomposition voltage of water.

Description

State of the art

The previously published development of the prior art is in the documents DE 195 02 588 C1, DE 196 46 893 A1 and DE 19 84 689 A1 described in detail. In addition to the ion exchange, purely physical or physical-chemical chemical (electrochemical) way of reducing an uncontrolled solid education on heat exchanger surfaces of any kind can be achieved.

Basically, the following levels of efficacy can be identified after tests and publications (tests according to the differential method, treatment / blank value, methods according to the DVGW W 512 working guidelines and after determining the change in particle number):

• A) Magnet / capacitor fields have no visible effects single treatment with a low tendency to Cold water and subsequent separation at 80 ° C. In hot water Cycles under increased lime separation tendencies are efficiency (I fill et al., Trans I Chem E, Vol. 74, Part B, 1996).
• B) High-frequency alternating electric fields, which according to DE 195 02 588 directly in the water are introduced have an efficiency of around 10%.
• C) Up to 40% effectiveness can be achieved by combining higher frequencies Alternating fields with an affinity differentiation according to DE 198 07 336 A1.

The predominantly kinetic control of the homogeneous or in homogeneous agglomerate formation as well as their dissolution and the equilibrium shift due to the outgassing of carbon dioxide with increasing tempera responsible for this special case of a physical-chemical influence Lich.  

In this context, the document 195 02 588 C1 shows the abolition of a metastable state and the slight shift in equilibrium due to the kinetic energy; this is via the alternating field in the ion movement brought in. The result is the homogeneous formation and growth of crystallites in the liquids.

Evidence of such reactivities with differentiation in long-term kinetics control and the much faster equilibrium shifts is specified in DE 196 46 893 Al. There are also the basic approaches for the temperature and pressure-dependent equilibria or pH values summarized.

In DE 198 07 336 A1 the parameters energy, charge and alternating field are opti correlated with an affinity shift. Among the given there Conditions form on the inner surface of the structured crystallite conductive solid, while the hydrophilic carbon dioxide in the liquid remains solved or is withheld. Under the influence of the current and the Alternating field, crystallites detach in about nanometer to micrometer sizes and form nuclei for the homogeneous separation in the liquid, preferably in their warming.

Surprisingly, the alternating field and impedance were found to continue Investigations in the mhz range that are controllable for effectiveness well over 90 Percent can be reached without a noteworthy one known from the literature Shift into the pH range of the lime solution is necessary. Height Efficacy can therefore be achieved even at deposition rates that are far below the known rate in water electrolysis or cathode sludge electrochemical shear corrosion protection systems.

However, there are three problems with a technical application:

• A) Calcium carbonate deposits on or
• B) hydrogen formation at the cathode and
• C) Subsequent reduction of the nitrate present in ppm quantities in Drinking water to nitrite.

When analyzing the cathode reactions more precisely, it could be seen that the formation of calcium carbonate crystals on the ideal crystallite scale from nm to µm size, depending on the frequencies, took place preferentially in conductive pores or on fiber structures. In addition, the crystallites were considered washable. A reduction in the currents with constant voltage pulses due to z. B. Electrode contamination with non-conductive carbonate was not found at more than 10 ⁶ As in contrast to the smooth sheet metal electrode and direct current specifications. This assignment is exemplified in the literature reference K. Zeppenfeld, Chem. Ing. Tech. 71 (1999) p. 118.

Hydrogen formation was understandably increased at low frequencies on (AC electrolysis part), but it could by the so-called sub voltage separation (UspA) below the water electrolysis voltage almost be completely avoided. In this way, optimal control of the Surface layer formation and stone mining possible via current and frequency.

The intrinsic safety of the system when the water stops is both in the direction Detonating gas formation as well as towards nitrite enrichment guaranteed.

The present invention, described in claims 1 to 7, lies there forth the task, the balance between alkaline earth hydrogen carbo nat, alkaline earth carbonate and carbon dioxide so towards the poorly soluble Carbonate and dissolved carbon dioxide move that under in the liquid Voltage change and flow homogeneous crystallites arise and a ge controlled influencing of corrosion protection or stone formation, but also of Stone mining becomes possible. Furthermore, the intrinsic safety of the system in Guaranteed detonating gas formation and nitrate reduction.

This object is according to the invention, presented in the main claim and the follow-up claims, solved in that defined by modified electrodes electrical charges (As) introduced in a frequency range from mHz to kHz and the voltages are kept below the decomposition voltage of the water the.  

A separation that otherwise only takes place at higher temperatures occurs of the hydrogen carbonate in poorly soluble carbonate and liquid-dissolved Koh on the dioxide without the disadvantages of a disordered layer formation Heat exchanger surfaces of any kind.

The advantages achieved by the invention are, in particular, that during the operation of a device operating according to this method the formation of limestone deposits on pipeline walls, heating elements and heat exchanger surfaces can be controlled by 80 even at high water temperatures is reduced to 99%. When heating appropriate water solutions therefore limestone layers in pipes and on other heat exchanger surfaces limited and thus reduces energy losses, as well as the exchange, Cleaning and service intervals can be extended significantly.  

Description of special process variants and execution forms of the contraption 1st example General definition of the pulse currents and times

Typical throughputs of the water treatment plants are 2 to 100 l / min. In the case of hot water, the maximum quantities are reduced to average values of up to 30 l / min due to the time-dependent energy input. The amounts of the carbonates are between 4 and 8 mmol / l depending on the degree of hardness. Modern waterworks reduce this amount by adding lime milk to an average of 2-4 mmol / l. This results in charge equivalents for the implementation of the hydrogen carbonate anions of around 400 to 1600 As / l. Current strengths of 200 mA-1000 mA allow a specific current of 0.2 to 1 mA / cm ² with an electrode area of approximately 1000 cm ² . The conditions of a UspA ( Fig. 02) can thus easily be met.

With a treatment volume of the device of 5 l, a charge is obtained entry of 12 As / l (1 A, flow rate 5 l / min), d. H. 1-2% of the bicarbonate discharge quantity. Increase the usual reaction delay times of 600 seconds this proportion to 10 to 20%, d. H. it forms over 1 mmol calcium carbonate that is insoluble in water to a degree of over 90%.

In this way, not only are equilibria shifted, but also Crystalline nuclei are generated in sufficient numbers to be a preferred homogeneous Continued to grow in the liquid with thermal carbon dioxide outgassing guarantee.

2nd example Systems for reducing limestone deposits in Domestic water systems with degrees of hardness above 15 ° dH

For higher requirements, the treatment volume can be reduced to 10 l with two Electrode systems are expanded. That is, two stainless steel cartridges with graphite felt 20 cm by 50 cm winding electrodes and carbon rod anodes connected in series. The hydrogen carbonate treatment (10 l, 4 mmol / l, 8000 As) can switch from 1 mHz to 50 Hz at 0.5 A each after 30 minutes of downtime can be switched. The projection shows a possible device protected water consumption from 1200 l per day. This corresponds to the water needed in smaller apartment buildings.  

3rd example Heat exchanger and pipe protection in technical systems

Technical systems are characterized by a much larger variation of Operating conditions. Liquid mixtures, corrosion conditions, salt concentration tions, heat exchanger specifications justify and require a significantly higher level measurement and control effort (MRI). Here is primarily a pH control and, if required the measurement of dissolved gases, oxygen, hydrogen and carbon dioxide advantageous.

The frequency shift mentioned and tested in the claims with Variation of the currents in the UspA range offers the possibility of both in the cycle as well as deposits or protective layer formation in the inlet or bypass rivers.

Along with an addition of alkaline earth metal carbonates can at low frequencies active carbon dioxide generation for the transport of these layer formers is discontinued become. An increase in the frequencies then again represents the partial tendency homogeneous crystallite formation in the liquid and at the interfaces.

On the other hand, there remains dissolved hydrogen carbonate anions and earth alkalication the differentiation in layer solution at low frequencies and receive partial stratification at higher frequencies.

Experience has shown that the internal parameters of the Layer dissolution and layer formation very strongly with the application temperatures and the oxidation or reduction potentials. The separate setting of Fre sequences and the voltage-dependent currents allowed together with the Default parameters temperature, corrosion potential, pH value, buffering etc. the Formation of three-dimensional maps for optimal control or Reduction of system load.  

Formal relationships and description of the illustrations

Surprisingly, the impedance results mentioned above and below showed Investigations to determine the change in particle number and the efficiency according to W 512, the following combination which is not readily predictable by a person skilled in the art slopes:

1. A reduction of the bicarbonate anion by the reactions is sufficient

O ₂ + 2 H ₂ O + 4e ^- → 4 OH ^- and (21)

OH ^- + HCO ₃ ^- + Ca ²⁺ → H ₂ O + CaCO ₃ (22)

by about 10% to achieve a 90% stone formation reduction efficiency, if possible, the back reaction

CaCO ₃ + H ₂ O + CO ₂ → 2 HCO ₃ ^- + Ca ²⁺ (23)

to slow down significantly.

In the process and the corresponding devices, this is due to the use structured electrodes even when transitioning to lower frequencies pass.

The integral quantity of charge (As) that is necessary is used as a measure is to achieve comparable efficiencies. These efficiency measurements show a reduction in the back reaction rates from 90% to 10% the rate of formation when using structure-optimized electrodes.

2. In the case of precipitation without further separation, ie at low frequencies without the use of structured electrodes, an efficiency according to W 512 of 90% can also be obtained; the necessary reduction of the HCO ₃ ^- ions must then reach at least 75% of the amount in the liquid. This increases the concentration of free CO ₂ considerably. The pH value runs into a more dangerous area without buffering. Kinetic studies show the increase in the rate of the reverse reaction from 10% to 90%.

3. When increasing the frequencies to over 10 to 10,000 Hz and maintaining the structured electrodes (affinity separation crystallites / dissolved carbon dioxide) there is also an efficiency of the reduction. However, this is in one wide current range limited to a maximum of 40%. In a row switching (cascade) is probably a further slight increase in the reduction effect recognizable, but the effort considerably.

Kinetic and cryoscopic determinations show a shift in the time constant from 5.55 * 10 ^-4 s ^-1 at 1 mHz to 1.66 * 10 ^-4 s ^-1 at 50 Hz with a comparable reduction in the number of particles corresponding to the hardness of lime. Such values indicate the change in the rate-determining reactions from (21) to (24):

HCO ₃ ^- + HCO ₃ ^- → CO ₃ ^2- + CO ₂ + H ₂ O (24)

5. In the case of low frequencies, it is basically according to the status described technology (Zeppenfeld et al.) an enrichment of the crystallites is possible if also under the requirements of considerable side reactions and low charge efficiency. At lower and higher frequencies the back reaction is without structured affinity separation in over 90% of the educational response.

As compared to the speed of particle formation at higher frequencies at low frequencies but also drops to about 1/3, is at least among the Test conditions according to W 512 (withdrawal cycles 30-60 minutes), even with Cascade structure or cascade action, the limit of 40% reduction in Hardly increase solid formation on heat exchanger surfaces, as shown above bar.

An increase in energies does not work in relation to the reaction (24) increased charge rates during the deposition according to (22), they tend to lead to accelerating reduction resolution and to reduce efficiency according to W 512, as can be seen in some experiments.

6. The following boundary conditions thus result for the device and the method according to the present claims:

• - Voltage below the electrolysis limit (UspA) for water to avoid Oxyhydrogen and nitrite formation,
• - Shift of the frequencies in the mHz range with high hardness or temporary dismantling of disordered layers,
• - normal driving at higher frequencies and low energy consumption.

Fig. 01 shows the three areas of efficiency described:

• - Range ( 01 ) 0-10% effect, high-frequency alternating field with a back reaction close to 100%,
• - Range ( 02 ) efficiency by a maximum of 40%, with cascades close to 50% using higher-frequency alternating fields and structured electrodes (curve 04 ),
• - Area ( 03 ) up to 100% reduction in deposits.

This area ( 03 ) can be reached by electrolysis without structured electrodes (curve 05 ) or by undervoltage deposition with large-area structured electrodes (curve 06 ).

The diagram in Fig. 02 shows the current-voltage curves of a large-area winding module. At higher frequencies, the curve corresponds to a normal ohmic specification ( 10 ). When the frequency is reduced, the reaction polarization of the water decomposition becomes noticeable. However, the total resistance is made up of the sum of the reaction, concentration, diffusion and transition resistances. The extrapolated terminal voltage for hydrogen formation ( 08 ) is considerably higher than the thermodynamic size of 1.23 volts, it can be derived approximately from the curves ( 09 a). The curve ( 09 b) describes the oxygen conversion according to (21).

Fig. 03 shows the reduction of a relative particle density at higher frequencies by about 30 to 40% and an increase in this reduction by a factor of 2-3 when the frequencies are shifted (triangular voltage, constant maximum current, treatment time 4 h). In the direction of the arrow ( 11 ), in the direction of lower frequencies, the pH value drops and the CO ₂ content increases. The layer formation increases in the direction of the arrow ( 10 ) and the risk of corrosion drops; however, a reduction in uncontrolled stone formation persists over a wide frequency range.

Claims (7)

1. Method and device for intrinsically and corrosion-proof and in addition to low-reaction reduction of carbonate or carbonate-metal oxide mixed crystal formation on heat exchanger surfaces and pipelines in water which contain dissolved earth alkali hydrogen carbonates in the carbon dioxide-stabilized equilibrium, with the aid of electrical impulses, characterized in that

• a) these current pulses in the range from 10 ⁴ Hz (kHz) to 10 ^-3 Hz (mHz) are introduced into the water between two electrodes,
• b) the charge (As) of the current impulses, as an integral current value over time, accounts for 1 to a maximum of 20% of the charge quantities of the hydrogen carbonate anions dissolved in the process water volume,
• c) the voltages for generating the current pulses are kept below the decomposition voltage of the water.

2. The method and device according to claim 1, characterized in that the Have pulses in the frequency range mentioned rectangular, triangle or sine shape. 3. The method and device according to claims 1 and 2, characterized records that all electrode parts that are in contact with the water are made of oxidation stable or at least carbon materials. 4. The device according to claim 3, characterized in that the carbon materials from activated carbon, carbon, graphite structures, preferably from carbon, or Graphite felt or papers exist.   5. The method and device according to claim 1, characterized in that the Hydrogen carbonate / carbonate balance shift using pH sensor detec is tiert and the frequencies when the pH value increases or when the pH value can be lowered. 6. The method and device according to claim 1, characterized in that the Voltage setting is controlled or controlled via a dissolved gas sensor. 7. The method and device according to the preceding claims, characterized ge indicates that the formation of a uniform top layer by frequency shift to higher values (kHz) or the degradation of a disordered stone education achieved by shifting the frequency in the lower range (mHz) becomes.