RU2500950C1 - Preparation method of natural gas for transportation - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: method for obtaining gas hydrates is implemented at the temperature of +0.2°C and pressure of 1 MPa. In order to cool down gas with water there used is ice-water pulp, preferably with particle size of not more than 10 mcm, which are uniformly distributed along the reactor volume; ice content is about 50% of its volume.

EFFECT: use of this invention allows reducing power, capital and current costs for production of gas hydrate, and reducing material consumption of equipment required for implementation of the method.

3 cl, 3 dwg

Description

The invention relates to the gas industry and can be used in the receipt, storage and, preferably, pipe-free transport of natural gas.

Gasification of facilities remote from main pipelines with low-pressure bends with significantly different costs for them in the spring-summer and autumn-winter periods is unprofitable, and in some part unprofitable. Therefore, it is relevant to expand the network of pipelineless supplies of natural gas, satisfactory in terms of profitability and ease of implementation for both the supplier and the consumer.

A known method of processing natural gas and supplying it to the consumer in the form of liquefied natural gas (LPG) upon receipt of the latter at gas reduction stations (GDS) using turbo expanders (Vasiliev Yu.N. “Motor fuels of the future”, “Gas industry” 1995, No. one).

The disadvantage of this method is the difficulty of manufacturing turbo-expanders at high costs, working in the field of cryogenic temperatures, the need to use special cryogenic structural materials for the manufacture of the expander, and therefore the high capital costs, the need for deep gas purification from components that are otherwise high-boiling compared to methane and freeze disable the turbo-expander, the fundamental impossibility of continuous operation of the one-expander system, while while redundancy leads to higher costs, the complexity of controlling the expander's operating modes under varying pressures, costs and temperatures of natural gas passing through the gas distribution system.

There is also a known method of processing natural gas and delivering it as fuel to the consumer in the form of LNG upon receipt of the latter at the gas distribution system using throttle-vortex technology (see No. 2202078, F25J 1/00, 2001). This method is simple in hardware design, efficiently uses the heat and cold obtained when the pressure of natural gas is throttled during throttling at the gas distribution system, it has been industrially tested and evaluated as promising.

The disadvantage of the described method is that it allows you to get in the form of liquefied gas no more than 4-6% of the natural gas passing through the gas distribution station, expensive cryogenic equipment is required for storage, transportation and use of LNG as fuel gas (product temperature in the transport tank or storage capacity is approximately equal to 125 ° C at a pressure of about 0.6 MPa, which leads to an increase in the cost of fuel). The gas leaving the LNG production unit has a lowered temperature, which is not always acceptable for the operation of a low pressure pipeline due to its possible freezing with the laying soil, groundwater migration to the freezing front, deformation of the pipeline, and as a result of its damage.

There is also known a method of preparing natural gas for transportation, including the production of gas hydrates by mixing gas with water in a reactor, continuous cooling and maintaining the required temperature of the resulting mixture while maintaining a pressure not lower than the equilibrium pressure necessary for hydrate formation (see RU No. 2200727, C07C 5 / 02, 1997).

The disadvantages of the method include the fact that the process of obtaining hydrates, if necessary, is highly energy-consuming, because repeated com- bination and subsequent cooling of the gas is required, and the use of the same energy to create conditions for hydrate formation and preservation of hydrates. In addition, for the implementation of the method, it is necessary to carry out preliminary gas purification from heavy hydrocarbons.

The problem to which the invention is directed is expressed in reducing energy costs for hydrates.

The technical result expected from the use of this invention is to reduce energy, capital and current costs for obtaining gas hydrate. In addition, the material consumption of the set of equipment necessary for implementing the method is reduced

The specified technical result is achieved in that the method of preparing natural gas for transportation, including the production of gas hydrates by mixing gas with water in the reactor, continuous cooling and maintaining the required temperatures of the resulting mixture while maintaining the pressure not lower than the equilibrium required for hydrate formation, is characterized in that, the process of producing gas hydrates is carried out at a temperature of + 0.2 ° C and a pressure of 1 MPa, while ice-cold PU is used to cool the gas-water mixture Sny, preferably with a particle size of not more than 10 microns are evenly distributed along the reactor volume, wherein the ice content is about 50% of its volume. In addition, an ice maker is used to produce ice, which ensures that the refrigeration coefficient reaches at least 12 at a boiling point of -3 ° C and condensation + 6 ° C. In addition, mineralized, preferably seawater, is used to produce ice.

A comparison of the features of the claimed solution with the features of analogues and prototype indicates its compliance with the criterion of "novelty."

The features of the characterizing part of the claims solve the following functional tasks:

Signs ... "the process of producing gas hydrates is carried out at a temperature of 0.2 ° C and a pressure of 1 MPa" provide the possibility of implementing the method in fairly simple conditions from the standpoint of the requirements for the materials used for the manufacture of equipment and, in addition, the energy consumed.

Signs ... "ice-water pulp is used to cool the gas-water mixture" provide high heat removal efficiency - the heat energy generated during the formation of hydrate particles is effectively absorbed by the melting particles of water ice (the heat of hydration of natural gas is 410 kJ / kg and the heat of fusion of water ice is 335 kJ / kg). At the same time, 1 kg of ice-water pulp (at a 30% concentration of water ice particles in it) is 5 times more effective in the cold storage capacity of any single-phase refrigerant carriers, including water. Moreover, water ice particles serve as centers of nucleation of a new phase of gas hydrate (see Olga Zatsepina. HYDRATE FORMATION IN ENVIROMENT. University of British Colambia. 1997), providing a heterogeneous mechanism for the growth of hydrate particles, because natural gas bubbles are adsorbed on them (Ramm V.M. Gas adsorption. M .: Chemistry, 1976 - 549 p.), which are a component of the hydrate.

The signs indicating that it is preferable to use particles with a particle size of "not more than 10 microns, which are evenly distributed over the reactor volume" provide the realization of the thesis known in the theory of heat transfer that the best type of heat transfer surface is its absence. Heat is removed from the resulting hydrate particles of comparable size and in close proximity and in contact with them ice particles constituting an ice-containing slurry, while the intensity of the interphase heat transfer thus provided (heat transfer coefficient α, W / m ² * K) between the surface of the growing hydrate particles and melting particles of water ice with a size of 3 ... 5 μm reaches 3000 ... 5000 W / m ² * K (P. Pronk, I. Celigueta Azurmendi, JW Meewisse and CA Infante Ferreira. FLUT-DIZED BED FOR ICE SLURRY PRODUCTION, PHASE 2, SECOND PROGRESS REPORT DELFT UNIVERSITY OF TECHNOLOGY. Faculty of Design, Construction and Production M echanical Engineering and Marine Technology, July 2002 to December 2002), which is comparable in effect to immersion of hydrate particles in boiling Freon-22 (Perelshtein II, Parushin EB. Thermodynamic and thermophysical properties of working substances of refrigeration machines and heat pumps . - M., Food industry, 1998, 232 S.).

Signs indicating that in an ice-containing pulp “ice content is about 50% of its volume” provide the ability to pump the pulp with a pump at a relatively low expenditure of energy for pumping.

The signs of the second claim provide the effectiveness of the ice generation process, as a process that determines the effectiveness of the claimed method.

The features of the third claim reduce the energy costs of the ice generation process.

The invention is illustrated by drawings, where in Fig.1 schematically shows an installation that ensures the implementation of the claimed method; figure 2 shows a diagram of the formation of hydrate; figure 3 is an illustration of some modes of unsteady heat transfer in thermally thin bodies.

The drawings show reactor 1, its first 2 and second 3 inlets, a natural gas source 4, an ice-containing pulp storage device 5, first 6 and second 7 reactor exits, a gas hydrate storage unit 8, pumps 9, 10 and 11, respectively, for pumping an ice-containing pulp , for pumping a mixture of recirculated water with ice, for pumping brine, a turbocharger 12, an ice generator 13, an ice separator from a brine 14, an ice and recirculating water mixer 15, a source of feed water 16, gas pipelines 17 and 18, slurry pipelines 19-21, respectively, for pumping gas hydrate pulp, for pumping ice-containing pulp and for pumping ice-containing brine pulp, pipelines 22-25, respectively, for pumping recycled water, for pumping brine, pumping feed water and ice. Shut-off and safety valves, instrumentation and other auxiliary devices necessary for the operation of the installation, ensuring the implementation of the claimed method are not shown in the drawings.

As reactor 1, a thermally insulated tank is used, which can withstand pressure of more than 10 bar, equipped with appropriate shutoff valves and instrumentation.

The source of natural gas 4 (for example, the main gas pipeline) is connected by gas pipelines 17 and 18, respectively, with the first inlet 2 of the reactor 1 and a gas turbine that ensures the operation of the turbocompressor 12.

The first outlet 6 of reactor 1 is communicated by slurry conduit 19 with a gas hydrate storage unit 8, and its second outlet 7 is communicated by conduit 22 with a mixer of ice and recirculated water 15.

The storage of ice-containing pulp 5 by slurry pipelines 20 for pumping the ice-containing pulp is communicated with the second input 3 of the reactor 1 (via pump 9) and, in addition, through the pump 10, is connected to the mixer of ice and recirculated water 15. As the storage of ice-containing pulp 5, a heat-insulated tank is used, the output of which is communicated with the second input 3 of the reactor 1.

As the gas hydrate storage unit 8, a thermally insulated tank (or several tanks) is used, made with the possibility of maintaining the thermodynamic equilibrium of the gas hydrate pulp stored in them and equipped with means for shipping the material to the consumer.

As an ice generator 13, a vacuum ice machine, preferably of the IDE Tech brand, is used, driven by a turbocharger 12. Structurally, it is a hollow tank filled with any aqueous solution, aggregated by a turbocompressor, which creates a vacuum in the tank equal to the pressure of the triple point of the solution used (in this case, sea water).

In this vacuum ice maker, the refrigeration coefficient is 12 at a boiling point of -3 ° C and condensation + 6 ° C, while the ammonia refrigeration unit at a condensation temperature of + 6 ° C has a refrigeration coefficient of not more than 5, because must have a boiling point of -10 ° C (due to the fact that it is impossible to provide direct contact between boiling ammonia and crystallized sea water in the evaporator). An additional advantage of a vacuum ice maker over traditional ones is the use of a turbocompressor, which uses natural gas as an energy carrier, which can significantly reduce energy consumption in the production of gas hydrate pulps.

The input of the ice generator 13 is communicated by a pipe 24 with a source of feed water 16, which is used as a seawater intake of known design.

As a separator of ice from brine 14 using a known device for a similar purpose, the performance of which corresponds to the performance of the installation.

The products of separation of the ice-containing brine pulp into fresh ice and brine are used as follows - by gravity, ice is discharged by pipeline 25 into the ice and recirculation water mixer 15, and the brine has a higher salt concentration than either in the original seawater or into the sea, or (as shown in the drawing), through the pipe 23 returns to the ice generator 13.

The hydrate formation takes place on the hydrate formation lines (Fig. 2), which are separated from the equilibrium line of hydrate-gas-water by zones of a meta-stable state (a-b, g-d, g-h).

In a gas hydrate pulp generator (FIG. 1), a natural gas hydrate (GPG) is formed in water from a solution of natural gas (GH: methane - 90%, ethane - 5%, propane - 3%) in water. Points a, d, g (Fig. 2) correspond to the equilibrium state of the "hydrate-gas-water" system, and this state cannot go into the hydrate formation process (a-b, g-d, g-h) until no specific “driving force” of hydrate formation will be applied to the system (Gibbs potential G, chemical potential Δµ, supercooling Δt, supersaturation σ = Δµ / RT). All special cases of the manifestation of the driving force behind the nucleation and growth of a new phase are united by the Gibbs potential, at negative values of which the passage of all phase transitions is possible). It is known that, ceteris paribus, the hydrate formation process begins earlier and faster if various mechanical inclusions, gas bubbles, or molecular complexes-associates are always present in the water, which are always the centers of formation of a new phase, in this case hydrated (heterogeneous nucleation). The beginning of the hydrate formation process coincides with the achievement of the figurative point of the spinodal gas-water system (Fig. 2). The distance from the equilibrium line to the region of the stable state of the hydrate illustrates the increase in the "driving force" of hydrate formation. In this case, the “driving force” of hydrate formation is represented by supercooling of the gas-water system (temperature gradient of supercooling Δt _OVER = t _a -t _b ; t _g -t _d ; t _w -t _z ) with respect to the equilibrium state (points a , g, g in figure 2). It is obvious that when the temperature of the gas-water system is reduced to the same value (for example, to 0.2 ° C), the gradient, expressed in supercooling, will be different at different pressures. This makes it possible to reduce the pressure and, accordingly, energy consumption in the gas hydrate generator, having a high hydration gradient potential obtained due to interphase heat transfer, which sharply reduces the temperature gradient between the growing hydrate particles and the coolant and, accordingly, increases the supercooling gradient Δt _OVERCOOL .

In addition to creating a gradient that ensures hydrate formation in the gas-water system, it is necessary to ensure the removal of heat of hydration, which for methane hydrate is 410 kJ / kg.

Usually, in the process of hydrate formation, simultaneously with the formation of hydrate particles, their dissociation proceeds due to local temperature fluctuations, which always accompany exothermic phase transitions. They arise due to the impossibility of efficient heat removal from each nascent and growing particle of a new phase, due to their remoteness from the heat exchange surface. Statistical and molecular physics introduce, as a parameter of the intensity of growth or destruction of a phase, an indicator of the excess of the intensity of one process over another, or their equality, when the emerging and disappearing particles of a new phase are equal per unit time (dynamic equilibrium). Obviously, at an infinitely high intensity of heat removal from each nascent and growing hydrate particle, the temperature fluctuations, and accordingly the number of dissociations of individual hydrate particles per unit time, will tend to zero, while the energy efficiency of the hydrate formation process will tend to its theoretical maximum.

The claimed method implements the thesis known in the theory of heat transfer that the best type of heat transfer surface is its absence. In a gas hydrate generator that implements the claimed method, there are no heat exchange surfaces, because the heat generated by the generated particles of gas hydrate is removed from them by comparable size and located in close proximity to them (including contact) particles of ice-containing pulp. In this case, the intensity of interphase heat transfer thus ensured (heat transfer coefficient α, W / m2 * K) between the surface of growing hydrate particles and melting water ice particles of 3 ... 5 μm in size reaches 3000 ... 5000 W / m ² * K, which is comparable with the effect immersion of hydrate particles in boiling Freon-22.

и Фурье $$\left(Fo=\frac{a\tau }{R^2}\right).$$

Установлено, что в термически тонких телах, при расстоянии от их термического центра до поверхности (R) порядка 5-10 мкм, скорость изменения температуры внутри объекта не зависит от теплопроводности, а определяется его размерами. На фиг.3 дана иллюстрация некоторым режимам нестационарного теплообмена в термически тонких телах.The reason for such a significant influence of the sizes of crystals of ice-containing pulp on the rate of their melting and, ultimately, on the intensity of heat removal from growing hydrate particles lies in the essence of the Bio numbers $$\left(Bi=\frac{\alpha R}{\lambda }\right)$$

and fourier $$\left(Fo=\frac{a\tau }{R^2}\right).$$

It is established that in thermally thin bodies, at a distance from their thermal center to the surface (R) of the order of 5-10 μm, the rate of change of temperature inside the object does not depend on thermal conductivity, but is determined by its size. Figure 3 is an illustration of some modes of unsteady heat transfer in thermally thin bodies.

When the dimensionless time is Fo = 20 (for the number Bi = 0.1), the actual duration of the process of melting a water-ice crystal with a size of 100 microns is 0.2 seconds, and with a size of 5 microns - 4 * 10 ^-4 seconds.

Thus, during the nucleation and growth of hydrate particles surrounded by particles of water ice, the value of local temperature fluctuations will be reduced to its theoretical minimum and practically equal to 0.

In this case, water ice particles simultaneously serve as centers of nucleation of a new phase of gas hydrate, providing a heterogeneous mechanism for the growth of hydrate particles, since natural gas bubbles are adsorbed on them, which are a component of the hydrate. Upon nucleation, hydrate particles begin to release thermal energy, which is immediately absorbed by the melting particles of water ice present directly at the hydrate nucleation site. The uniform distribution of particles of water ice and hydrate is achieved by a constant supply of ice-water pulp to the reactor and recirculation of recirculated water (Fig. 1).

The prototype uses the principle of heat removal due to direct contact of the formed hydrate particles with a single-phase coolant (circulating water), which is cooled to perform the function of a coolant. Its disadvantage is the low specific cold storage capacity of all single-phase refrigerant carriers, including water (the heat capacity of water is 4.19 kJ / kg * K, which, when the temperature difference in the heat exchanger is 5 ° C, makes it possible to remove 21 kg of heat from the cooling object with one kilogram of coolant - Q = cmΔt = 4.19 * 1 * 5 = 21 kJ, while melting an ice-containing pulp at a 30% concentration of water ice particles in it allows one kilogram of pulp to remove 110 kJ of heat from the cooling object - Q = 0.3 * r * m = 0.3 * 335 * 1 = 110 kJ, and at a 50% concentration of about 170 kJ).

The heat of hydrate formation of natural gas is 410 kJ / kg, and the heat of fusion of water ice is 335 kJ / kg.

The low temperature gradient between the resulting gas hydrate and melting water ice is the main factor in the energy efficiency of the claimed method. When using contact-type heat exchangers of the most modern designs, the temperature difference between the media is 9 ° C (when used in ammonia), 12 ° C for freons, while the application of the effect of interphase heat transfer through the use of pulps as a coolant, reduces the temperature difference (distance b-c; d-e; s-i, Fig. 2) up to 0.2 ° C. In this case, the points a , d, g (Fig.2) will shift to the isotherm 0.2 ° C, and the distance a-b; gd; _gh (temperature gradient Δt _OVERHEAD , as the "driving force" of hydrate formation) will increase to its maximum possible value. It is obvious that a decrease in the temperature gradient between the hydrate particles formed and the coolant increases the hydrate formation gradient (supercooling of the gas-water system Δt _OVERCOOL relative to the equilibrium temperatures t ₁ , t ₄ , t ₇ , Fig. 2). An increase in the “driving force” of hydrate formation reduces the delay time for the nucleation of hydrate particles and, accordingly, increases the productivity of the process of generating gas hydrate pulp.

An additional factor that increases the efficiency of hydrate formation is the infinitely large heat transfer area between an infinitely large number of thermally thin bodies (hydrate particles and water ice), which is the reason for maintaining high heat flux between growing hydrate particles and melting particles of water ice, with a temperature gradient between almost equal to zero.

When ice is generated, seawater begins to solidify at a temperature of -2 ° C and a pressure of 420 Pa (the boiling - hardening temperature decreases to -3 ° C, when 30% of the solid phase is frozen out of water and, to -5 ° C, when 50% of solid is frozen) phase), while ice is chemically pure water in a solid state of aggregation. The water ice obtained in the cavity of the vacuum ice maker forms an ice-containing brine pulp with the liquid phase of the solution, which is transferred to the ice separator from the brine. After separation of the ice-containing brine pulp into fresh ice and brine, the ice is gravity fed into the ice and recirculation water mixer 15, and the brine is either discharged into the sea or returned to the ice generator 13.

Ice-containing pulp (including fine ice and fresh water) is accumulated in accumulator 5, from where it is pumped into the gas hydrate pulp generator. In a gas hydrate pulp generator, water ice particles melt during heat removal from the resulting hydrate particles and are removed as a recirculation water by a pump into an ice and recirculation water mixer 15.

The finished gas hydrate pulp is accumulated in the gas hydrate storage unit 8, from where it is shipped to the consumer.

Claims (3)

1. A method of preparing natural gas for transportation, including obtaining gas hydrates by mixing gas with water in a reactor, continuously cooling and maintaining the required temperatures of the resulting mixture while maintaining the pressure not lower than the equilibrium required for hydrate formation, characterized in that the process for producing gas hydrates is carried out at a temperature of + 0.2 ° C and a pressure of 1 MPa, while ice-water pulp is used to cool the gas-water mixture, preferably with a particle size of not more than 10 microns, which are evenly distributed throughout the reactor volume, while the ice content is about 50% of its volume. 2. The method according to claim 1, characterized in that an ice generator is used to produce ice, which ensures that the refrigeration coefficient reaches at least 12, at a boiling point of -3 ° C and condensation + 6 ° C. 3. The method according to claim 1, characterized in that mineralized, preferably sea water, is used to produce ice.

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