WO2010133726A1

WO2010133726A1 - Rankine cycle with absorption step using hygroscopic compounds

Info

Publication number: WO2010133726A1
Application number: PCT/ES2010/000208
Authority: WO
Inventors: Francisco Javier Rubio Serrano
Original assignee: Francisco Javier Rubio Serrano
Priority date: 2009-05-18
Filing date: 2010-05-13
Publication date: 2010-11-25

Abstract

The hygroscopic cycle is a power cycle that operates with steam and hygroscopic compounds. These compounds absorb steam at low pressure and temperature that has been expanded in the turbine and desorbed therefrom in the generator under the required operating conditions. In the absorber, the enthalpy of the steam is utilized and increased in the generator with the provision of heat. The turbine is able to work at high vacuums and cooling temperatures above the exit temperature of the steam in the turbine. The invention comprises the following main items of equipment: absorber, dissolution pump, condensation pump, heat recovery means, thermal degasser, steam generator, steam separator, expansion valve, air cooler or cooling tower, superheater, steam turbine and, optionally, hydraulic turbine, inverse-osmosis system and absorption/adsorption machine. It may include many of the improvements made to the Rankine cycle, being characterized by the incorporation of the physical and chemical principles of absorption machines for achieving better performance from the Rankine cycle and improved cooling conditions.

Description

RANKINE CYCLE WITH ABSORPTION STAGE THROUGH HYGROSCOPIC COMPOUNDS

The Rankine cycle is a power cycle that operates with steam. It is currently used in the generation of electricity in thermoelectric or thermal steam plants. Water is used as a working fluid through different equipment and producing mechanical work thanks to its expansion (steam state) inside a steam turbine; A generator connected to the turbine outlet delivers the electrical power produced.

The performance of the Rankine cycle is determined as the ratio between the subtraction of the work0 produced by the turbine and the one consumed by the pump, split from the heat transferred to the steam from the steam generator (boiler plus superheater).

There are numerous improvements that have been introduced to the Rankine cycle in order to increase the cycle performance, and thereby improve the efficiency of the power plant5 treated. In addition to increasing the performance of the teams participating in the cycle (pump, turbine, boiler ...), it has become necessary to make changes to the layout of the plant. Among the main improvements made to increase the performance of the Rankine cycle are the following: 0 • Steam overheating at the beginning of expansion (Hirn cycle).

• Modifications of the operating conditions at the beginning and end of expansion (increase in the pressure of expansion start, decrease in pressure of expansion term, supercritical conditions).

• Reheating (working with higher pressures avoiding the formation of moisture at the end of the expansion, for this purpose the steam is completely extracted in an intermediate pressure stage and reheated in the boiler to an average temperature subsequently taking it to a new expansion ).

• Regeneration (feed water is preheated before it reaches the boiler using open or closed heaters). 0 • Binary Cycle (two circuits in parallel: one working at high temperature and one at low temperature).

The decrease in the expansion term pressure is limited by the temperature of the cooling agent (mainly water or air) in the condenser. 5

The hygroscopic compounds are all those substances that attract water in the form of steam or liquid from their environment, hence its main application as desiccants. Many of them react chemically with water such as hydrides or alkali metals. Others catch it as hydration water in its crystalline structure as is the case with sodium sulfate. Water It can also be physically adsorbed. In these last two cases, the retention is reversible and the water can be desorbed. In the first case, having reacted, it cannot be recovered simply.

Deliquescent materials are substances (mostly salts) that have a strong chemical affinity for moisture and absorb relatively high amounts of water if they are exposed to the atmosphere, forming a liquid solution. Examples of deliquescent substances are: calcium chloride, ferric chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium hydroxide and sodium hydroxide. The presence of these compounds in dilution with water modifies their properties in relation to their pure state. These modifications are known as properties of a solution, classified as constitutive (viscosity, density, electrical conductivity, etc.) and colligative or collective (decrease in solvent vapor pressure, increase in boiling point, decrease in freezing point and pressure osmotic) of special interest in this patent.

One of the main applications of hygroscopic compounds is the absorption cycles used for refrigeration. These machines began to be marketed at the beginning of the 50s. The absorption cycles are physically based on the ability of some substances, such as water and some salts such as lithium bromide, to absorb, in liquid phase, other vapors substances such as ammonia and water, respectively. By similarity, in this cycle the water would be the cooling fluid and the hygroscopic compound the absorbent.

The hygroscopic cycle is a power plant cycle that works with water and with this type of compounds or materials, which must have the following characteristics:

• They must be highly hygroscopic compounds, deliquescent materials.

• They must be less volatile than water (vapor pressure less than water) and easily separable, the retention is reversible and the steam can be easily desorbed in the generator, obtaining a clean, unmixed steam. In the absorber, the vapor pressure of the absorbent / water solution depends on the nature of the absorbent, its temperature and its concentration. At a lower temperature of the absorbent and at a higher concentration, the lowest vapor pressure will be in the solution.

• Good water solubility at low or moderate temperatures.

• They must work correctly and be chemically stable at the pressures and temperatures of work (the high of the generator and the low of the absorber) to which it will be subjected.

• Non-toxic or flammable fluids are advised.

Some examples of the best known hygroscopic compounds are: - Sodium Chloride (Halite) (ClNa) Calcium Chloride (CaCl ₂ )

- Sodium Hydroxide (NaOH) - Sulfuric acid (H ₂ SO ₄ )

- Copper sulfate (CuSO ₄ )

Phosphorus pentoxide (P ₂ O ₅ or more correctly P ₄ O ₁₀ ) Silica gel

- Hydrated salts such as Na ₂ SO ₄ -IOH ₂ O - LiBr (the most commonly used today, especially in absorption machines for cold generation)

- LiCl

The hygroscopic cycle incorporates the physical and chemical principles of absorption machines to provide the Rankine cycle with better performance and better cooling conditions. It comprises the following main equipment: a) Steam turbine. b) Absorber. It can be a venturi, jet type, scrubber, spray tower gas scrubber. c) Condensate pump. d) Dissolution pump. e) Heat recovery. It can be a shell and tube or plate type heat exchanger. f) Thermal degasser. g) Steam generator. It can be evaporator, boiler or shell and tube exchanger. h) Steam separator. Deposit with "demister" (nebulizer) or rectification column. i) Superheater. j) Expansion valve. k) Air cooler, cooling tower or heat exchanger.

I) Hydro turbine (optional). m) Reverse osmosis, nanofiltration, ultrafiltration or microfiltration system (optional). n) Absorption or adsorption machine (optional).

This cycle may include many of the improvements made to the Rankine cycle (pressure increase start of expansion, decreasing the pressure term expansion, vapor superheat, reheat, regeneration, supercπ ^conditions' tics) described above and incorporating any equipment that allows the steam to be in contact with the chosen hygroscopic compound (both absorption and adsorption). Following the process diagram in Figure 1, it will start from saturated steam at low pressure and temperature (from 0.01 to 0.2 bar (a) generally) leaving the steam turbine. This vapor is directed to the absorber where it is absorbed and diluted in contact with the selected hygroscopic compounds (mainly salts), for example lithium bromide (LiBr), lithium chloride (LiCl), sodium sulfate (Na ₂ SO ₄ ), etc. , whose concentration will depend on the pressure of the vapor to be absorbed and the available cooling conditions. The thermal energy released during the absorption process comes from the heat of condensation, sensible heat and heat of dissolution or dilution. Due to this heat of absorption, the solution contained in the absorber is usually at a higher temperature than that of the feed water vapor. This temperature difference depends on the hygroscopic compound or mixture chosen and its concentration. To reduce the corrosion produced by the dissolution of the hygroscopic compounds it is recommended to use inhibitors.

This diluted solution from the absorber is aspirated by the condensate pump and one part is driven, after passing through the enthalpy recuperator, to the thermal degasser or deaerator tank of the steam generator and the other part acts as a reflux of the absorber. The thermal degasifier is a tank used to extract oxygen and other dissolved gases from the solution and heat and store the feed water to the steam generator. This equipment can be connected to a low pressure steam line from a bleeding of the steam turbine in order to remove the oxygen and dissolved gases and favor its elimination. The heat recuperator is an exchanger that preheats the diluted solution from the absorber, with the hot solution returning from the steam separator. Its objective is to increase the temperature of the solution that feeds the degasser and cool the concentrated solution that returns to the absorber.

The diluted solution from the degasser is driven at high pressure to the steam generator by means of the dissolution pump. In the generator the necessary energy to boil at high pressure and temperature is supplied in order to concentrate the absorbent solution and obtain a clean water vapor that will be fed to the steam turbine, previously passing through the superheater, from which a Superheated steam needed to increase the process performance and achieve a drier steam at the turbine outlet. Steam purity is achieved thanks to the steam separator with built-in "demister" (de-fogger) or a rectification tower. The energy provided to the steam generator and superheater can come from a thermal fluid, nuclear energy, solar energy, a combustion process, whose energy sources can be the heat of the exhaust gases of an internal combustion engine, heat of the exhaust gases of a gas turbine, heat obtained from the burning of a fuel or biomass, waste heat from industrial processes ... The concentrated solution that leaves the heat recuperator is directed to the air cooler, heat exchanger or cooling tower in order to dissipate the absorption heat (condensation heat plus dissolution heat) to prevent the temperature from rising and thus be absorbed a maximum amount of water vapor. After cooling, it enters the absorber at low pressure after passing through the expansion valve where it loses almost all the pressure that the fluid has.

The outlet steam of the superheater (live steam) is at the necessary conditions to be expanded in the steam turbine, which is connected to a generator that will produce the electric power thanks to the work produced. The steam at low pressure and temperature would return to the cycle described above.

The process represented in Figure 2 is included in this patent. It is the same process explained above but optionally including a series of equipment that improves the performance of the cycle and thereby the overall performance. The teams are as follows:

1. Hydro turbine. Its mission is to recover most of the energy that causes the pressure jump of the concentrated solution from the steam separator to the absorber. The expansion valve in this case would lose very little pressure.

2. System, membrane or filters of reverse osmosis, nanofiltration, ultrafiltration or microfiltration. Depending on the type of hygroscopic compounds selected, one or the other type of system will be installed. The objective is to separate most of the hygroscopic compounds from water, which are recirculated to the absorber. Thanks to this, the steam pressure in the steam generator can be increased which is affected by the presence of the selected hygroscopic compounds and their concentration. The cost of the materials to be used in the steam generator is also reduced and the useful life of the equipment is increased. The energy required in this type of filters is mostly in the form of pressure, whose pump will be part of the selected filtration system.

3. Absorption / adsorption machine. After thermal use in the steam generator, most of the time we have energy available at low temperature which can be used to cool the diluted solution that must return to the absorber. When temperature levels from 80 ⁰ C are required in the case of absorption machines and 50 ⁰ C for adsorption machines, these equipments allow energy optimization of the process and provide low cooling temperatures necessary to lower the condensation pressure of the turbine In gas engines, diesel or others, the energy that needs to dissipate in the high temperature cooling circuit can be used for this purpose.

As an important annotation, it is recommended to choose compounds or combinations between them where the heat of dissolution is endothermic (endothermic mixing), to compensate for the heat of exothermic condensation released in the absorption or adsorption phase, thereby increasing the cycle performance and saving energy in the chosen cooling system

(heat exchanger, air cooler or cooling tower) with the possibility of eliminating it. The heat of absorption that keeps the diluted solution at a temperature higher than that of the steam leaving the turbine will then be used to produce steam useful to the process. For this reason, part of the enthalpy of the steam coming out of the turbine in the absorber is used and the generator is increased with the heat input.

It is also recommended to work with hygroscopic compounds with good solubility at low or moderate absorber temperatures (<100 ° C) and low solubility at high temperatures

(> 1 OO ⁰ C) existing in the steam generator (decreasing solubility with temperature or inverse solubility). At this point condensation in the absorber at temperatures higher than the feed vapor is favored and vapor pressures close to those of pure water in the steam generator are achieved. In this case, the solution from the steam separator would be supersaturated, diluting completely by decreasing its temperature by passing through the enthalpy recuperator. An example of a hygroscopic compound with such behavior is sodium sulfate (Na ₂ SO ₄ ).

In this cycle the condensation of the steam takes place on top of a film of liquid of the absorbent where some molecules condense immediately and dissolve in the solution film leaving space for new molecules thus reducing the condensation pressure. For this, it is necessary to dissipate the heat of absorption mentioned above so that the process is not interrupted. The cooling conditions of this system are less demanding than the current Rankine cycles working under vacuum.

The cycle performance (η) is determined as the ratio between the subtraction of the work produced by the turbines (steam turbine plus hydraulic turbine (if any), W1 and W4 respectively) and that consumed by the condensate pump (W2), dissolution pump (W3) and reverse osmosis, nanofiltration, ultrafiltration or microfiltration system (W5), if any, split from heat transferred to steam from the steam generator (Q2), superheater (OJ) and absorption machine (Q3) , if the latter exists. It is represented in the following formula: WI + W4- W2 - W3 - W5

7 = QI + Q2 + Q3

The performance depends on the selected operating conditions and the hygroscopic compounds chosen and their concentration. Typical yields are between 1 and 5% above the current Rankine cycles.

The hygroscopic cycle achieves:

> High yields when taking advantage of part of the enthalpy of steam at the exit of the turbine, reducing thermal losses, taking advantage of the thermal energy of the concentrated solution from the steam generator to provide the diluted solution with the energy needed to reach the conditions of degassing, and above all for being able to work the turbine with greater pressure difference (high vacuum at the turbine outlet) without having limited the cooling temperature. The heat of absorption can be used to produce steam at a temperature higher than that of the feed steam of the absorber. This increase in performance can be seen in Figure 3 (T-S diagram of the Rankine cycle).

> Improve the cooling conditions of the cycle. The heat energy dissipated in the air cooler, cooling tower or heat exchanger is decreased.

Depending on the hygroscopic compounds chosen and their concentration, the necessary refrigeration temperature of the cycle is normally between 1 and 80 ° C above the steam temperature at the turbine outlet, with the possibility of eliminating cooling (endothermic mixing). This effect allows working in more severe vacuum conditions (around 0.01 bar) without having the cooling temperature limited.

Industrial applications

In all those plants that use or can use a Rankine cycle or similar, for the generation of mechanical work, or production of electrical energy. The main power plants where it can be installed are thermoelectric plants (oil, coal and natural gas, other fuels), nuclear, biomass and solar thermal.

Also in those cogeneration or trigeneration plants where the heat of condensation that must be released in the air cooler, cooling tower or heat exchanger is recovered as thermal energy for another process. Explanation of the drawings

Figure 1 represents the process diagram of the hygroscopic cycle including the main equipment as detailed in the description.

Figure 2 represents the process diagram of the hygroscopic cycle including as optional equipment a hydraulic turbine, a reverse osmosis filtration system, ultrafiltration or microfiltration nanofiltration and an absorption or adsorption machine; equipment explained above.

Figure 3 represents a typical temperature - entropy diagram of Rankine cycles.

There are four different processes in the development of the cycle, which change the state of the fluid. The processes are as follows (assuming an ideal cycle with internally reversible processes):

• Process 1 - 2 ^' : Isoentropic expansion of the working fluid in the turbine from the generator pressure to the absorber pressure.

• Process 2 ^' - 3 ^' : Steam heat transmission with the hygroscopic compound at constant pressure in the absorber to the state of saturated liquid. • Process 3 ^' - 4 ^' : Isoentropic compression in the pump. It increases the pressure of the fluid through a compressor or pump that is given a certain job.

• Process 4 ^' - 1 ^' : Heat transmission to the working fluid at constant pressure in the generator and superheater. The processes are not internally reversible, there are different irreversibilities and losses.

This figure details the performance equation for two cases whose main difference lies in the temperature conditions at the turbine outlet and the slight overheating provided to maintain the same degree of humidity at the outlet. Performance 2, whose points are 1 ^' , 2 ^' , 3 ^' and 4 ^' is greater than performance 1 of the first case, whose points are 1, 2, 3 and 4. This increase in performance can be observed in the scratched area , being typical of hygroscopic cycles versus Rankine or similar cycles, since it is possible to work with higher vacuum conditions (lower temperature) for the same cooling conditions. In addition, the heat of absorption can be used to produce steam at a temperature higher than that of the steam supplied to the absorber.

Figure 4 shows the consumption or generation of energy in the form of work or heat of existing equipment, showing at each point of the process a description of each current.

Claims

1. Hygroscopic cycle based on the properties of hygroscopic compounds to absorb or adsorb water in the form of steam. It incorporates the physical and chemical principles of absorption machines to provide the Rankine cycle with better performance and better cooling conditions. It is characterized in that these compounds absorb water vapor at low pressure and temperature that has been expanded in the turbine and are desorbed from the water in the generator at the necessary operating conditions. The enthalpy of the steam is used in the absorber and is increased in the generator with the heat input. The hygroscopic compounds must be chemically stable, with good solubility at low and moderate temperatures with water, easily separable (reversible retention) and less volatile than water, providing high affinity for it. The turbine can work with high voids (around 0.01 bar) and depending on the hygroscopic compounds chosen and their concentration, the cooling conditions of the process are normally between 1 and 80 ⁰ C above the steam outlet temperature in the turbine, with the possibility of eliminating cooling (endothermic mixing of the compounds).

This cycle can include many of the improvements made to the Rankine cycle (increase in the expansion start pressure, decrease in the expansion end pressure, steam overheating, overheating, regeneration, supercritical conditions), and applied in plants where there is or may exist a Rankine cycle or similar, to perform a mechanical work or generation of electrical energy (thermoelectric, nuclear, biomass, thermosolar plants).

It comprises the following main equipment:

a) Steam turbine b) Absorber c) Condensate pump d) Dissolution pump e) Heat recovery f) Thermal degasser g) Steam generator h) Steam separator i) Superheater j) Expansion valve k) Air cooler, tower cooling or heat exchanger

2. An installation according to claim 1 applied to a cogeneration or trigeneration plant. In this case the heat of condensation that must be released in the air cooler, cooling tower or heat exchanger is recovered as thermal energy for another process.

3. An installation according to claim 1 incorporating a hydraulic turbine in order to recover most of the energy caused by the pressure drop of the concentrated solution from the steam separator to the absorber. This increases the overall performance of the cycle.

4. An installation according to claim 1 incorporating a filtration system or membranes by reverse osmosis, nanofiltration, ultrafiltration or microfiltration (depending on the size of the selected hygroscopic compounds) to separate most of the hygroscopic compounds from the diluted solution and recirculate them absorber in order to further dilute the diluted solution before reaching the steam generator, increasing steam pressures in said equipment. This increases the overall cycle performance and life of the equipment.

5. An installation according to claim 1 incorporating an absorption or adsorption machine to cool the diluted solution back to the absorber taking advantage of the energy still available in the process. This increases the overall performance of the cycle.

6. Emphasis is placed on an important characteristic that selected hygroscopic compounds must have. Hygroscopic compounds with good solubility at low or moderate temperatures of the absorber and low solubility at the high temperatures existing in the steam generator (decreasing solubility with temperature or inverse solubility) are recommended. At this point condensation in the absorber at temperatures higher than the feed vapor is favored and vapor pressures close to those of pure water in the steam generator are achieved. In this case, the solution from the steam separator would be supersaturated, diluting completely by decreasing its temperature by passing through the enthalpy recuperator. An example of a hygroscopic compound with such behavior is sodium sulfate (Na ₂ SO ₄ ).