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Electrodialysis equipment

Electrodialysis - the process of separating electrolytes from water using the permeation selectivity of an ion exchange membrane under the action of a direct current electric field.


Electrodialysis equipment
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    Electrodialysis operation principle technical description

    Electrodialysis - the process of separating electrolytes from water using the permeation selectivity of an ion exchange membrane under the action of a direct current electric field.

    Basic principles and characteristics

    The electrodialyzer is alternately arranged with many cations and anions, which are separated into small water chambers. When raw water enters these chambers, under the action of a direct current electric field, the ions in the solution migrate in a directional manner. The cation membrane only allows the passage of cations and traps the anions; the omentum only allows the passage of anions and traps the cations. As a result, a part of these chambers becomes a fresh water chamber with few ions, and the effluent water is called fresh water. The small chamber adjacent to the fresh water chamber becomes a concentrated water chamber that gathers a large number of ions, and the effluent water is called concentrated water. Thus, the ions are separated and concentrated, and the water is purified.

    Compared with ion exchange, electrodialysis has the following similarities and differences:

    (1) Although the working medium for separating ions is ion exchange resin, the former is a sheet-like film, and the latter is a spherical particle;

    (2) In terms of mechanism of action, ion exchange belongs to ion transfer substitution, and ion exchange resin undergoes ion exchange reaction in the process. Electrodialysis belongs to ion interception substitution, and ion exchange membranes play the role of ion selective permeation and interception in the process. So more precisely, ion exchange membranes should be called ion selective permeation membranes;

    (3) The working medium of electrodialysis does not need to be regenerated, but consumes electric energy; and the working medium of ion exchange must be regenerated, but does not consume electric energy. The characteristics of electrodialysis treatment of wastewater are that it does not need to consume chemicals, the equipment is simple, and the operation is convenient.

    Electrolatent bridge membrane (separate from exchange membrane)

    Electrodialysis membrane - an organic polymer with the same chemical structure as ion exchange resin as the skeleton. A resin membrane with a spatial network structure formed by bridging with a certain amount of cross-linking agent through transverse bonds.

    (1) Classification of ion exchange membranes? According to the different active groups, it is divided into cation exchange membranes, anion exchange membranes and special ion exchange membranes (also classified according to membrane structure).

    1) Cation exchange membrane: refers to an ion exchange membrane that can dissociate cations, or a membrane with acidic active groups in the membrane structure. It can selectively penetrate cations without allowing the anion to penetrate. These acidic groups can be divided into: strong acidity, such as sulfonic acid type (-SO3H); medium and strong acidity, such as phosphoric acid type (-OPO3H2), phosphoric acid type (-PO3H2); weak acidity, such as carboxylic acid type (-COOH), phenol type ().

    2) Anion exchange membrane: refers to an ion exchange membrane that can dissociate anions, or a membrane that combines basic active groups in the membrane structure. It can selectively penetrate anions without allowing cations to penetrate. These basic groups can be divided into: strongly basic, such as quaternary ammonium type [-N (CH3) 3OH]; weakly basic, such as primary amine type (-NH2), secondary amine type (-NHR), tertiary amine type (-NR2).

    3) Special ion exchange membrane (composite membrane): This membrane is composed of an anode membrane and an anode membrane. A layer of mesh (such as nylon cloth, etc.) can be separated between the two layers, or they can be directly pasted together. When working, the cathode is opposite to the anode, and the anode is anti cathode. Since ions outside the membrane cannot enter the membrane, the water molecules between the membranes are ionized, and H + ions penetrate the anode and tend to the cathode; O~ H-ions penetrate the cathode and tend to the anode, thereby completing the task of transmitting current. In addition, in wastewater treatment, H + or OH- ions generated by the composite membrane can also be used to combine with other ions in the wastewater to prepare certain products.

    According to the different membrane structure (or according to the manufacturing process), ion exchange membranes are divided into three types: heterogeneous membranes, homogeneous membranes and semi-homogeneous membranes.

    (2) Properties of ion exchange membranes

    1) The selective permeability of the membrane is determined by the properties of the membrane and depends on the porosity. Ion selective permeability in the membrane is expressed by selective permeability.

    2) Conductivity of the film

    Reflects the migration speed of ions in the membrane, and there are three influencing parameters

    ① Resistivity (Ω · cm) ② Conductivity (Ω-1 · cm-1)

    ③ Section resistance (Ω · cm2)

    3) Exchange capacity of the membrane

    The number of active groups contained in the film per unit weight is expressed in millimoles of exchangeable ions per gram of dry film (generally 1.5 to 3 mmol/g).

    4) The swelling rate and moisture content of the membrane are expressed in weight percent of the water contained in each gram of the membrane.

    5) Chemical stability of the membrane

    It is required to resist acid, alkali, oxidation reduction and biodegradation.

    6) Mechanical strength of the membrane

    Requires a certain tensile strength - the pressure (kg/cm2) that the membrane can withstand when it is pulled in parallel directions

    Burst strength - the pressure (kg/cm2) that the membrane can withstand when subjected to vertical pressure, generally > 5 kg/cm2

    (3) The performance requirements of the ion exchange membrane? 1) The selection of high permeability is required to be above 95%; 2) The conductivity is good, and its conductivity should be greater than that of the solution; 3) The exchange capacity is large; 4) The swelling rate and moisture content are appropriate: 5) Strong chemical stability; 6) High mechanical strength.?


    A device that uses the principle of electrodialysis to desalinate or treat wastewater is called an electrodialyzer.

    (1) The structure of the electrodialyzer is composed of three parts: membrane stack, polar region and pressing device.

    1) Membrane stack: Its structural unit includes a male membrane, a baffle, and a negative membrane. A structural unit is also called a membrane pair. An electrodialyzer is composed of many membrane pairs, and these membrane pairs are collectively referred to as membrane stacks. Baffles are often made of 1~ 2mm rigid polyvinyl chloride board, and there are water distribution holes, water distribution tanks, water channels, water collection tanks and water collection holes on the board. The function of baffle is to form a water chamber between the two membranes to form a water flow channel, and play the role of water distribution and water collection.

    2) Polar region: The main function of the polar region is to supply direct current to the electrodialyzer, introduce raw water into the water distribution hole of the membrane stack, discharge fresh water and concentrated water from the electrodialyzer, and pass in and discharge the polar water. The polar region is composed of a pallet, an electrode, a pole frame and an elastic backing plate. The function of the electrode pallet is to reinforce the pole plate and install the water inlet and outlet pipe. It is commonly made of thick rigid polyvinyl chloride plate. The function of the electrode is to connect the internal and external circuits to create a uniform DC electric field in the electrodialyzer. The anode is commonly used in materials such as graphite, lead, and iron wire coated nails; the cathode can be made of stainless steel and other materials. The pole frame is used to maintain a certain distance between the pole plate and the membrane stack to form a pole chamber and a channel for pole water. The pole frame is usually made of a coarse mesh multi-channel plastic plate with a thickness of 5~ 7mm. The backing plate plays the role of preventing water leakage and adjusting the uneven thickness, and is often made of rubber or soft PVC plate.

    3) Compression device: Its function is to form a water-tight electrodialyzer with polar regions and membrane piles. It can be tightened by pressure plates and bolts, or it can be pressed by hydraulic pressure.

    (2) Assembly of the electrodialyzer?? The basic assembly form of the electrodialyzer is shown in Figure 17-4. In practice, terms such as "stage", "segment" and "series" are usually used to distinguish various assembly forms. The number of electrode pairs in the electrodialyzer is called "stage", those with a pair of electrodes are called first stage, those with two pairs of electrodes are called second stage, and so on. In the electrodialyzer, the part of the membrane stack with the same direction of water inlet and outlet is called "section". Every time the direction of water flow changes, the number of "sections" increases by l.?

    Process technical problems and indicators of electrodialysis?

    (1) Polarization phenomenon and limit current density? As shown in Figure 17-5, during electrodialysis, on the fresh water side of the anion exchange membrane or cation exchange membrane, since the number of ions migrated in the membrane is greater than that in the solution The number of migrations makes the ion concentration C1 at the interface between the membrane and the solution smaller than the ion concentration Cl in the solution phase. Similarly, on the concentrated water side of the anion or cation membrane, the amount of ions migrated from the membrane is greater than the number of ions migrated in the solution, so that the concentration C2 at the interface of the phase is greater than the ion concentration C2 in the solution phase. In this way, a concentration difference is created on both sides of the membrane. Obviously, the greater the current intensity, the faster the ion migration, and the greater the concentration difference. If the current is increased to a considerable extent, there will be a situation where the C2 value tends to zero. At this time, the ionization of water molecules (H2O → H + ten OH-) occurs on the freshwater side, and the transfer current is supplemented by the migration of H + ions and OH- ions, a phenomenon called polarization.

    C '1 < C1, C' 1 Technologies 0 produces polarization.

    C '2 > C2, OH-enriched on the anode side to produce M (OH) m precipitation to produce knots. Ion migration is hindered.

    As a result of the polarization phenomenon, on the concentrated water-side of the anode membrane, due to the enrichment of OH-ions, the pH value of the water increases, and hydroxide precipitation occurs, resulting in scaling near the membrane surface; in addition, on the concentrated water side of the anode membrane, Since the ion concentration at the membrane surface is much larger than that of C2, it is also easy to cause scaling near the membrane surface. The result of scaling will inevitably lead to a decrease in the current efficiency, a decrease in the effective area of the membrane, and a shortening of life, which affects the normal progress of the electrodialysis process.

    An effective way to prevent polarization is to control the electrodialyzer to operate below the limit current density. In addition, regular electrode switching operations are performed to dissolve the accumulated precipitation on the membrane.

    The current passing through the membrane area per unit time is called the current density. The current density when the polarization phenomenon occurs in the membrane interface layer is called the limit current density (), and its theoretical value is:


    in the formula

    C - the ion concentration in the solution outside the interface layer;

    D - diffusion coefficient;

    F - Faraday constant;

    E - the migration number of counterions in the exchange membrane;?

    T - the migration number of counterions in solution;

    delta - the thickness of the interface layer.

    (2) Current efficiency When removing a certain amount of salts from the wastewater solution, the ratio of the amount of electricity theoretically required to the amount of electricity actually consumed is called current efficiency. It is an indicator to measure the current utilization rate of an electrodialyzer.

    (3) Voltage consumption and working voltage The higher the voltage required by the electrodialyzer, the greater the power consumption. The working voltage plant of the electrodialyzer can be decomposed into several parts in the following formula:


    Where Ed - the potential required for the electrode reaction, V;

    Em - the voltage required to overcome the membrane potential, V;

    I - working current, A;

    Rj - contact resistance, Ω;

    Rm - film resistance, Ω;

    Rs -- the electrical resistance of water, Ω.

    (4) Power consumption and power efficiency Power consumption is calculated as follows:

    Where V - operating voltage, V;

    I - working current, A;

    Qd - Freshwater production, m3/h.

    Electric energy efficiency is an indicator of the power utilization rate of the dialyzer, which is the ratio of the theoretical power consumption to the actual power consumption. The energy efficiency of an electrodialyzer is generally below 10%. In order to improve the power efficiency, the current efficiency and voltage efficiency must be improved, and the key to improving the voltage efficiency is to reduce the total resistance of the electrodialyzer.

    Process calculation of electrodialysis

    (1) Limit current density formula The limit current density formula is established under the critical condition of polarization. The practical limit current density (mA/cm2) is calculated as follows (Wilson formula):


    Where - the linear velocity of the water flow in the freshwater baffle, cm/s;

    Cm - logarithmic mean salt content of water in freshwater baffles, mol/L;

    - velocity coefficient;

    K - coefficient of hydraulic characteristics;?

    N - Valence.

    The voltage-current method is usually used to determine the limit current density and the sum of the coefficient ruler. This method is to make a voltage-current polarization curve on Cartesian coordinate paper through experiments with the measured membrane pair voltage and the corresponding current density (see Figure 17-6). The three parts of the curve are composed of: the two sections of OA and DE are approximately straight lines; the ABCD section is a curve, which is called the "polarization transition zone". OA and DE intersect P, and the vertical line through P intersects the curve at C. This point C is called the "standard polarization point", and the current density corresponding to point C is the limit current density. At different influent concentrations or flow rates, several groups, C and values of the electrodialyzer are measured, and then the following logarithmic formula of Wilson's formula is substituted, and the coefficients K and n can be obtained graphically (see Figure 17-7) or by solving equations.


    (2) Desalination formula? The desalination formula under the critical state of polarization is as follows:


    Where C. - the initial salt content of water in freshwater baffles, mol/L;

    C - the amount of salt in the freshwater baffle at the distance X from the starting point, mol/L;

    X - the length of the desalination process from the starting point, cm;

    - current efficiency;

    F - Faraday constant, equal to 96500n c/m, where n is the valence:

    D - Thickness of freshwater baffles, cm.

    The desalination formula (see Figure 17-8 for the curve) shows that when the flow rate and the length of the baffle process in the fresh water baffle are equal and operate under the limit critical state, the desalination ratio of each stage of the electrodialysis process is a constant, that is, C/C. = constant. According to this formula, there is the following relationship between the total desalination rate of multiple stages in series and the desalination rate of a single stage:


    Where G - total desalination rate;

    G - the desalination rate for a period;

    M - the number of segments in series.

    (3) Current efficiency formula The current efficiency of electrodialysis salt removal is the ratio of the amount of electricity actually used for salt removal to the amount of electricity connected to the electrodialyzer, that is


    Where CR--the salt content of the water at the entrance of the freshwater baffle, mol/L;

    Cch - the salt content of the water at the outlet of the freshwater baffle, mol/L;

    L - the length of the desalination process for freshwater baffles, cm;

    -- current density, mA/cm2;

    ? F, v, d are the same as before?

    (4) The logarithmic average of the concentration? In the practical application of the limit current density formula, since the decrease of the freshwater concentration along the process in the limit critical state changes exponentially (see Figure 17-8), the logarithmic average should be taken as follows:

    ? (17-9)

    Application of electrodialysis in wastewater treatment

    Electrodialysis was first used in seawater desalination to produce drinking water and industrial water, seawater concentration to produce table salt, and combination with other unit technologies to produce high-purity water, and later it was widely used in wastewater treatment.

    In wastewater treatment, there are two types of electrodialysis operations according to the process characteristics: one is an ordinary electrodialysis process that is alternately arranged by anode and anode membranes, which is mainly used to simply separate pollutant ions from wastewater, or to separate pollutant ions in wastewater from non-electrolyte contaminants, and then treat them by other methods; the other is a special electrodialysis nest process composed of composite membranes and anode membranes, which uses the polarization reaction in the composite membrane and the electrode reaction in the polar chamber to generate H + ions and OH- ions to produce acids and bases from wastewater.

    At present, electrodialysis is widely used in wastewater treatment practice: 1) Treating alkali papermaking waste liquid, recovering alkali from concentrated liquid, and recovering wood cable from light liquid; 2) From metal ions-containing

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