阅读说明：本技术 预估与原烟气直接接触换热后的脱硫废水的pH的方法 (Method for estimating pH value of desulfurization wastewater after direct contact heat exchange with raw flue gas ) 是由 吴文景 李茂清 纪连举 刘双华 刘振东 邓欣 李璟涛 李继宏 张怀宇 于 2020-03-31 设计创作，主要内容包括：本发明公开了一种预估与原烟气直接接触换热后的脱硫废水的pH的方法,所述预估与原烟气直接接触换热后的脱硫废水的pH的方法通过计算所述二氧化硫溶于所述脱硫废水中对氢离子浓度的影响,估算出所述二氧化硫和所述脱硫废水达到两相平衡时所述脱硫废水的pH。本发明的预估与原烟气直接接触换热后的脱硫废水的pH的方法可以在换热系统设计完成之前确定换热后脱硫废水的pH,为换热系统的设计选型提供参考依据,以有效解决换热系统的相关结构容易被酸腐蚀甚至造成泄漏的问题。(The invention discloses a method for estimating the pH value of desulfurization wastewater subjected to direct contact heat exchange with raw flue gas, which estimates the pH value of the desulfurization wastewater when the sulfur dioxide and the desulfurization wastewater reach two-phase balance by calculating the influence of the dissolution of the sulfur dioxide in the desulfurization wastewater on the concentration of hydrogen ions. The method for estimating the pH value of the desulfurization wastewater after the desulfurization wastewater directly contacts with the raw flue gas for heat exchange can determine the pH value of the desulfurization wastewater after heat exchange before the heat exchange system is designed, and provides a reference basis for the design and the model selection of the heat exchange system, so that the problem that the related structure of the heat exchange system is easily corroded by acid and even causes leakage is effectively solved.)

1. A method for estimating the pH value of desulfurization wastewater subjected to direct contact heat exchange with raw flue gas is characterized in that the raw flue gas contains sulfur dioxide, the desulfurization wastewater is approximately neutral, when the desulfurization wastewater is subjected to direct contact heat exchange with the raw flue gas, the sulfur dioxide is dissolved in the desulfurization wastewater and influences the concentration of hydrogen ions in the desulfurization wastewater, and the pH value of the desulfurization wastewater when the sulfur dioxide and the desulfurization wastewater reach two-phase balance is estimated by calculating the influence of the dissolution of the sulfur dioxide in the desulfurization wastewater on the concentration of the hydrogen ions.

2. The method for estimating the pH value of the desulfurization wastewater after the desulfurization wastewater is directly contacted with the raw flue gas for heat exchange according to claim 1, which comprises the following steps:

obtaining the total pressure p of the raw flue gas before heat exchange₁The volume fraction of sulfur dioxide in the raw flue gas before heat exchangeThe temperature T of the desulfurized wastewater after heat exchange;

consulting a Henry constant table to obtain a Henry constant k of sulfur dioxide dissolved in water at the temperature T of the heat-exchanged desulfurized wastewater_t；

Consulting an ionization equilibrium constant table to obtain a first-order ionization equilibrium constant K of sulfurous acid in water at the temperature T of the desulfurized wastewater after heat exchange_a1；

Estimating the pH value of the desulfurized wastewater after heat exchange according to the following formula:

3. the method for estimating the pH value of the desulfurization wastewater after direct contact heat exchange with the raw flue gas as recited in claim 2, wherein the step of obtaining the temperature T of the desulfurization wastewater after heat exchange comprises:

obtaining boundary conditions of heat exchange between the raw flue gas and the desulfurization wastewater;

and determining the temperature T of the desulfurized wastewater after heat exchange according to the boundary condition.

4. The method for estimating the pH of the desulfurization wastewater after direct contact heat exchange with raw flue gas as recited in claim 2, wherein the full pressure p of the raw flue gas before heat exchange is obtained₁Comprises the following steps:

measuring the total pressure p of the raw flue gas before heat exchange by a first measuring element₁。

5. According to the claims2 the method for estimating the pH of the desulfurization wastewater after the desulfurization wastewater is directly contacted with the raw flue gas for heat exchange, which is characterized in that the volume fraction of sulfur dioxide in the raw flue gas before heat exchange is obtainedComprises the following steps:

measuring the volume fraction of sulfur dioxide in the raw flue gas before heat exchange by a second measuring element

6. The method for estimating the pH of desulfurization waste water after direct contact heat exchange with raw flue gas as recited in claim 2, wherein the partial pressure of sulfur dioxide in the raw flue gas is kept constant before and after heat exchange, and the partial pressure p of sulfur dioxide in the raw flue gas is constant₂Comprises the following steps:

7. the method of estimating the pH of the desulfurization waste water after direct contact heat exchange with raw flue gas as recited in claim 2, wherein the temperature of the raw flue gas after heat exchange is substantially the same as the temperature T of the desulfurization waste water after heat exchange.

8. The method for estimating the pH value of the desulfurization wastewater after the desulfurization wastewater is directly contacted with the raw flue gas for heat exchange according to claim 1, which comprises the following steps:

measuring the total pressure p of the raw flue gas before heat exchange by a first measuring element₁；

Measuring the volume fraction of sulfur dioxide in the raw flue gas before heat exchange by a second measuring element

Acquiring boundary conditions of heat exchange between the original flue gas and the desulfurization wastewater, and determining the temperature T of the desulfurization wastewater after heat exchange according to the boundary conditions;

consulting a Henry constant table to obtain a Henry constant k of sulfur dioxide dissolved in water at the temperature T of the heat-exchanged desulfurized wastewater_t；

Consulting an ionization equilibrium constant table to obtain a first-order ionization equilibrium constant K of sulfurous acid in water at the temperature T of the desulfurized wastewater after heat exchange_a1；

Estimating the pH value of the desulfurized wastewater after heat exchange according to the following formula:

9. the method for estimating the pH of desulfurization waste water after direct contact heat exchange with raw flue gas as recited in claim 8, wherein the partial pressure of sulfur dioxide in the raw flue gas is kept constant before and after heat exchange, and the partial pressure p of sulfur dioxide in the raw flue gas is constant₂Comprises the following steps:

10. the method of estimating the pH of the desulfurization waste water after direct contact heat exchange with raw flue gas as recited in claim 8, wherein the temperature of the raw flue gas after heat exchange is substantially the same as the temperature T of the desulfurization waste water after heat exchange.

Technical Field

The invention relates to the technical field of flue gas recycling of power plants, in particular to a method for predicting the pH value of desulfurization wastewater after direct contact heat exchange with raw flue gas.

Background

The desulfurization wastewater treatment process of the thermal power plant is generally divided into a pretreatment stage, a concentration and decrement stage and a tail water solidification stage, the temperature of the desulfurization wastewater after the pretreatment stage is generally 35-40 ℃, and the desulfurization wastewater needs to be heated to more than 50 ℃ before concentration and decrement treatment is carried out by adopting a thermal method or a membrane distillation method. The temperature of the raw flue gas of the thermal power plant is usually 110-140 ℃, and the raw flue gas has a certain heat recovery value and can be used as a heat source for heating the desulfurization wastewater.

The heat exchange modes of the cold fluid and the hot fluid mainly comprise direct contact type heat exchange, heat accumulation type heat exchange and dividing wall type heat exchange. Because the direct contact type heat exchange has the advantages of good heat transfer effect, simple structure and the like, the problem of easy scaling of the dividing wall type heat exchange can be avoided. For the desulfurization waste water which is easy to scale in the heat accumulating type heat exchange and dividing wall type heat exchange processes, the heat exchange process of the desulfurization waste water and the original flue gas is more suitable for adopting a direct contact type heat exchange mode.

Disclosure of Invention

The present application is based on the discovery and recognition by the inventors of the following facts and problems:

the mass transfer process exists when the gas-liquid two-phase direct contact heat exchange occurs, and after the heat exchange occurs between the raw flue gas and the desulfurization wastewater, the acidic gases such as sulfur dioxide and the like contained in the raw flue gas can be dissolved into the desulfurization wastewater, so that the pH value of the desulfurization wastewater is reduced, namely the acidity is increased. The desulfurization waste water that acidity increases corrodes former flue gas and desulfurization waste water direct contact heat exchange system's relevant structure easily, for example heat exchanger inner shell, water conservancy pipeline inner wall, water pump impeller etc. and desulfurization waste water direct contact's equipment, cause the relevant structure to leak even, consequently need know the pH behind the desulfurization waste water heat transfer to avoid behind the desulfurization waste water heat transfer to the acid corrosion of the relevant structure in the heat exchange system and the leakage problem that causes, guarantee heat exchange system normal operating.

Drawings

FIG. 1 is a flow chart of a method for estimating the pH of desulfurized wastewater after direct contact heat exchange with raw flue gas according to an embodiment of the present invention.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. In the description of the present invention, it is to be understood that the terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," "circumferential," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the invention and to simplify the description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and are therefore not to be considered limiting of the invention.

As shown in fig. 1, according to the method for estimating the pH of the desulfurization wastewater after the desulfurization wastewater directly contacts and exchanges heat with the raw flue gas according to the embodiment of the present invention, the raw flue gas contains sulfur dioxide, the desulfurization wastewater is substantially neutral, when the desulfurization wastewater directly contacts and exchanges heat with the raw flue gas, the sulfur dioxide is dissolved in the desulfurization wastewater and affects the hydrogen ion concentration in the desulfurization wastewater, and the pH of the desulfurization wastewater when the sulfur dioxide and the desulfurization wastewater reach two-phase equilibrium is estimated by calculating the effect of the sulfur dioxide dissolved in the desulfurization wastewater on the hydrogen ion concentration.

The inventor is through research and analysis, when desulfurization waste water and former flue gas carried out the direct contact heat transfer, each component in former flue gas will be partly dissolved in desulfurization waste water, and former flue gas component generally includes carbon dioxide, oxygen, nitrogen gas, sulfur dioxide and water, and wherein can influence the pH of desulfurization waste water be carbon dioxide and sulfur dioxide. Because the solubility of the carbon dioxide at normal temperature is far less than that of the sulfur dioxide, the temperature of the desulfurization wastewater is higher when the desulfurization wastewater exchanges heat with the original flue gas, and the solubility of the carbon dioxide is further remarkably reduced, the components dissolved in the desulfurization wastewater in the original flue gas are mainly the sulfur dioxide. The pH of the desulfurization waste water is usually about 7, so that the hydrogen ion concentration (H) of the sulfur dioxide dissolved in water in the solution is calculated⁺) The influence of (3) can be estimated, namely the pH value of the desulfurization wastewater when the gas phase and the liquid phase are balanced.

According to the method for estimating the pH value of the desulfurization wastewater after the desulfurization wastewater directly contacts and exchanges heat with the raw flue gas, the influence of sulfur dioxide dissolved in the desulfurization wastewater on the concentration of hydrogen ions is calculated, the pH value of the desulfurization wastewater when the sulfur dioxide and the desulfurization wastewater reach two-phase balance is estimated, the pH value of the desulfurization wastewater after heat exchange can be determined before the heat exchange system is designed, a reference basis is provided for the design and selection of the heat exchange system, namely the selection of relevant heat exchange equipment, components and materials, and the problem that the relevant structure of the heat exchange system is easily corroded by acid and even causes leakage can be effectively solved.

In some embodiments, the method for estimating the pH of the desulfurization wastewater after direct contact heat exchange with the raw flue gas comprises the following steps:

obtaining the total pressure p of the original flue gas before heat exchange₁Volume fraction of sulfur dioxide in raw flue gas before heat exchangeThe temperature T of the desulfurized wastewater after heat exchange;

look up the Henry constant table to obtain the Henry constant k of sulfur dioxide dissolved in water at the temperature T of the desulfurized waste water after heat exchange_t；

Consulting the ionization equilibrium constant table to obtain a first-order ionization equilibrium constant K of sulfurous acid in water at the temperature T of the desulfurized wastewater after heat exchange_a1；

Estimating the pH value of the desulfurized wastewater after heat exchange according to the following formula:

the inventor analyzes that the desulfurization waste water and the raw flue gas are in direct contact for heat exchange, and the essence of the method is that the desulfurization waste water and the raw flue gas are directly subjected to gas-liquid two-phase mixing to realize efficient heat transfer, and a mass transfer process to a certain degree can also occur during the heat transfer. The raw flue gas contains gaseous sulfur dioxide generated after coal combustion, the gaseous sulfur dioxide has the property of being dissolved in water, and when the gaseous sulfur dioxide exchanges heat with the desulfurization wastewater, a part of the gaseous sulfur dioxide is transferred and distributed into the desulfurization wastewater (liquid phase) from the raw flue gas (gas phase). According to Henry's law, the partial pressure p of sulfur dioxide in the gas phase at a certain temperature in a gas-liquid two-phase equilibrium system₂Proportional to the molar concentration c of sulphur dioxide in the liquid phase:

in the formula, k_tIs the Henry constant, k, of the dissolution of sulfur dioxide in water at temperature T_tCan be obtained by looking up a Henry constant table, k_tUsually, the units are kPa, kg/mol, kPa, L/mol, etc., and k is a unit of the above three units_tCan be mutually converted; p is a radical of₂The partial pressure of sulfur dioxide can be p₁And volume fraction of sulfur dioxideAnd calculating to obtain:

sulfur dioxide dissolved in water will quickly form sulfurous acid (H) of the same molar quantity₂SO₃) Sulfurous acid will undergo the following ionization process in water:

the ionization process (1) is first-order ionization of sulfurous acid, and the ionization equilibrium constant K of the first-order ionization is_a1Is composed ofWherein [ H ]⁺]Is hydrogen ion concentration, [ HSO ]₃ ^-]Is bisulfite ion concentration, [ H ]₂SO₃]Is the concentration of sulfurous acid.

The ionization process (2) is the two-stage ionization of sulfurous acid, and the ionization equilibrium constant K of the sulfurous acid_a2Is composed ofWherein [ SO₃ ^2-]Is the concentration of sulfite ions。

Constant of ionization equilibrium K_a1And the ionization equilibrium constant K_a2Can be obtained by looking up a table of ionization balance constants.

For the two-stage ionization process of sulfurous acid in water, usually K_a1Ratio K_a2About 5 orders of magnitude larger, in calculating the hydrogen ion concentration [ H⁺]In time, the secondary ionization process is negligible compared to the primary ionization process. For first order ionization, equimolar amounts of hydrogen ions and bisulfite ions [ HSO ] can be ionized per mole of sulfurous acid₃ ^-]Since the desulfurized wastewater itself contains very few bisulfite ions, the concentration of the bisulfite ions and sulfite ions in the aqueous solution can be approximately considered to be equal in the state of equilibrium of ionization, i.e., [ H ]⁺]≈[HSO₃ ^-]And therefore the ionization constantTransforming the formula to obtain

According to the definition of pH, the pH of the desulfurization waste water after heat exchange can be estimated: pH-log [ H ]⁺]。

Since sulfur dioxide dissolved in water will quickly form sulfurous acid (H) of the same molar quantity₂SO₃) Thus sulfurous acid (H)₂SO₃) The concentration is equal to the concentration c of sulfur dioxide, thenSubstituting it into the above equation for estimating pH can yield:therefore, the pH value of the desulfurization waste water after heat exchange is as follows:

it can be understood that the method for estimating the pH value of the desulfurization wastewater after the desulfurization wastewater is directly contacted with the raw flue gas for heat exchange only needs to collectRelevant parameters of the raw flue gas to be heat-exchanged and of the desulphurised waste water, e.g. total pressure p of the raw flue gas before heat exchange₁Volume fraction of sulfur dioxide in raw flue gas before heat exchangeThe pH value of the desulfurized wastewater after heat exchange can be estimated without collecting relevant parameters of the original flue gas and the desulfurized wastewater after heat exchange. According to the invention, through a basic physical chemistry theory and reasonable approximate simplification, a formula for rapidly estimating the pH value is deduced, the pH value of the desulfurized wastewater after heat exchange can be obtained before the heat exchange equipment is prepared, so that a basis is provided for early design and model selection such as selection of related heat exchange equipment, components and materials in a direct contact type heat exchange system of the desulfurized wastewater and the raw flue gas, the problems of acid corrosion and even leakage of the heat exchange system can be effectively avoided, and the reliability and safety of system operation are greatly improved.

In some embodiments, the step of obtaining the temperature T of the heat-exchanged desulfurization wastewater comprises:

acquiring boundary conditions of heat exchange between the original flue gas and the desulfurization wastewater;

and determining the temperature T of the desulfurized wastewater after heat exchange according to the boundary conditions.

In other words, the temperature T of the desulfurized wastewater after heat exchange is determined according to the boundary condition of the heat exchange between the original flue gas and the desulfurized wastewater. In the method provided by the embodiment of the invention, only the heat exchange boundary condition of the original flue gas to be subjected to heat exchange and the desulfurization wastewater needs to be collected, the temperature T of the desulfurization wastewater after heat exchange is determined according to the boundary condition, and the temperature T does not need to be obtained after the heat exchange of the original flue gas and the desulfurization wastewater.

In some embodiments, the total pressure p of the raw flue gas before heat exchange is obtained₁Comprises the following steps:

measuring the total pressure p of the original flue gas before heat exchange by a first measuring element₁。

In other words, the total pressure p of the raw flue gas before heat exchange is measured by means of a first measuring element, for example a pressure sensor₁Thereby obtaining the total pressure p of the original flue gas before heat exchange₁. It can be understood that the invention obtains the total pressure p of the original flue gas before heat exchange₁Is not limited thereto, and for example, the phases may be consultedClosing the manual to obtain the total pressure p of the original flue gas before heat exchange₁。

In some embodiments, the volume fraction of sulfur dioxide in the raw flue gas before heat exchange is obtainedComprises the following steps:

measuring the volume fraction of sulfur dioxide in the original flue gas before heat exchange by a second measuring element

In other words, the volume fraction of sulphur dioxide in the raw flue gas before heat exchange is measured with a second measuring element, such as a flue gas analyzerThus obtaining the volume fraction of sulfur dioxide in the original flue gas before heat exchangeIt can be understood that the volume fraction of sulfur dioxide in the raw flue gas before heat exchange is obtained in the inventionFor example, the volume fraction of sulfur dioxide in the raw flue gas before heat exchange can be obtained by referring to a relevant manual

In some embodiments, the partial pressure of sulfur dioxide in the raw flue gas is kept constant before and after heat exchange, the partial pressure p of sulfur dioxide in the raw flue gas₂Comprises the following steps:

when the original flue gas and the desulfurization wastewater are subjected to direct contact type heat exchange, the flow of the original flue gas is far larger than that of the desulfurization wastewater, so that the relative content of sulfur dioxide in the original flue gas is not changed greatly, and the partial pressure of sulfur dioxide can be approximately considered to be constant before and after heat exchange.

In some embodiments, the temperature of the raw flue gas after heat exchange is substantially the same as the temperature T of the desulfurized wastewater after heat exchange. According to the method for estimating the pH value of the desulfurization wastewater after direct contact heat exchange with the raw flue gas, the Henry constant k of sulfur dioxide dissolved in water is determined according to the temperature of the desulfurization wastewater after heat exchange_tAnd first order ionization equilibrium constant K of sulfurous acid in water_a1. In order to realize quick estimation, the method approximately considers that the temperature of the raw flue gas after heat exchange is consistent with the temperature T of the desulfurization wastewater after heat exchange.

A method for estimating the pH of desulfurization waste water after heat exchange in direct contact with raw flue gas according to an embodiment of the present invention will be described with reference to fig. 1.

As shown in fig. 1, a method for estimating a pH of desulfurization waste water after heat exchange in direct contact with raw flue gas according to an embodiment of the present invention includes the steps of:

measuring the total pressure p of the original flue gas before heat exchange by a first measuring element₁(kPa)；

Measuring the volume fraction of sulfur dioxide in the original flue gas before heat exchange by a second measuring element

Acquiring boundary conditions of heat exchange between original flue gas and desulfurization wastewater, and determining the temperature T (K) of the desulfurization wastewater after heat exchange according to the boundary conditions;

look up the Henry constant table to obtain the Henry constant k of sulfur dioxide dissolved in water at the temperature T of the desulfurized waste water after heat exchange_t(kPa·L/mol)；

Consulting the ionization equilibrium constant table to obtain a first-order ionization equilibrium constant K of sulfurous acid in water at the temperature T of the desulfurized wastewater after heat exchange_a1(dimensionless);

estimating the pH value of the desulfurized wastewater after heat exchange according to the following formula:

further, the partial pressure of sulfur dioxide in the original flue gas is kept constant before and after heat exchange, and the partial pressure p of sulfur dioxide in the original flue gas₂Comprises the following steps:

furthermore, the temperature of the original flue gas after heat exchange is approximately the same as the temperature T of the desulfurization wastewater after heat exchange.

In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above are not necessarily intended to refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, various embodiments or examples and features of different embodiments or examples described in this specification can be combined and combined by one skilled in the art without contradiction.

Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.

In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; may be mechanically coupled, may be electrically coupled or may be in communication with each other; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.

In the present invention, unless otherwise expressly stated or limited, the first feature "on" or "under" the second feature may be directly contacting the first and second features or indirectly contacting the first and second features through an intermediate. Also, a first feature "on," "over," and "above" a second feature may be directly or diagonally above the second feature, or may simply indicate that the first feature is at a higher level than the second feature. A first feature being "under," "below," and "beneath" a second feature may be directly under or obliquely under the first feature, or may simply mean that the first feature is at a lesser elevation than the second feature.

Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made to the above embodiments by those of ordinary skill in the art within the scope of the present invention.