Process for capturing a heavy metal contained in a moist gas, integrating a heat pump to heat the gas introduced into a capture mass

Abstract

Capturing at least one heavy metal, from mercury and arsenic, contained in a moist gas comprising water vapor, by the following steps: a) heating the moist gas by heat exchange with a compressed heat transfer fluid obtained in step e) in order to obtain a condensed heat transfer fluid and a gas reheated to a temperature Tc; b) bringing the reheated gas into contact with a heavy metal capture mass in order to obtain a gas depleted in heavy metal; c) decompressing the cooled heat transfer fluid; d) cooling the gas depleted in heavy metal by heat exchange with the heat transfer fluid produced in step c) in order to obtain a cooled gas at a temperature Tf, the heat transfer fluid being vaporized; e) compressing the vaporized heat transfer fluid in a manner such as to obtain a compressed heat transfer fluid, the compressed heat transfer fluid being recycled.

Claims

1. A process for capturing at least one heavy metal, selected from mercury and arsenic, contained in a moist gas comprising water vapour, in which the following steps are carried out: extracting a natural gas from an underground deposit; then introducing the natural gas into a sludge elimination device; then introducing the natural gas into a gas-liquid separation device; then sending the natural gas to the following step a); a) heating (1001) the moist gas by heat exchange with a compressed heat transfer fluid obtained in step e) in order to obtain a condensed heat transfer fluid and a gas reheated to a temperature Tc with a reduced water vapour content; b) bringing the reheated gas with a reduced water vapour content obtained in step a) into contact with a heavy metal capture mass (2000) in order to obtain a gas which is depleted in heavy metal, whereby the water vapour content of the moist gas was sufficiently reduced in step a) so that capillary condensation on the heavy metal capture mass is reduced or avoided; c) decompressing (4000) the cooled heat transfer fluid obtained in step a); d) cooling (1003) the gas depleted in heavy metal by heat exchange with the heat transfer fluid produced in step c) in order to obtain a cooled gas at a temperature Tf, the heat transfer fluid being vaporized during step d); and e) compressing (3000) the vaporized heat transfer fluid obtained in step d) in a manner such as to obtain a compressed heat transfer fluid, the compressed heat transfer fluid being recycled to step a) to complete a heat pump circuit; then, after step e), the following steps are carried out: eliminating at least a portion of the H.sub.2S and CO.sub.2 contained in the gas depleted in heavy metal; and then eliminating at least a portion of the water vapour contained in the gas depleted in heavy metal.

2. The process according to claim 1 in which, before carrying out step a), a heat exchange is carried out between the moist gas and the gas depleted in heavy metal obtained from step b).

3. The process according to claim 1, in which the temperature Tc is determined in a manner such that the reheated gas with a reduced water vapour content obtained in step a) has a relative humidity of less than 90%.

4. The process according to claim 1, in which the difference between Tc and Tf is less than 50 C.

5. The process according to claim 1, in which the capture mass comprises an active phase selected from at least one metallic sulphide based on a metal selected from the group constituted by copper (Cu), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni).

6. The process according to claim 1, in which the capture mass comprises an active phase composed of elemental sulphur.

7. The process according to claim 1 in which, when the moist gas contains H.sub.2S, the capture mass comprises an active phase composed of at least one metallic oxide of a metal selected from copper (Cu), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni).

8. The process according to claim 1, in which the capture mass comprises an active phase composed of at least one metallic oxide of a metal selected from copper (Cu) and lead (Pb).

9. The process according to claim 5, in which the active phase is distributed over a porous support, the porous support being selected from the group constituted by aluminas, phosphorus-containing aluminas, silica-aluminas, silicas, clays, activated carbons, zeolites, titanium oxides, zirconium oxides and mixtures thereof.

10. The process according claim 1, in which the heat transfer fluid is selected from hydrocarbons containing 1 to 7 carbon atoms and optionally one or more heteroatomic ligands.

11. The process according to claim 1, in which at least one of the following operations is carried out: before carrying out step e), carrying out a supplemental step for heating the vaporised heat transfer fluid obtained in step d); before carrying out step b), carrying out a supplemental step for further heating the reheated gas obtained in step a); before carrying out step c), carrying out a supplemental step for further cooling the cooled heat transfer fluid obtained in step a).

12. The process according to claim 11, in which an exchange of heat of the vaporized heat transfer fluid obtained in step d) with the cooled heat transfer fluid obtained in step a) is carried out.

13. The process according to claim 1, in which the moist gas is selected from one of the following gases: a natural gas, a shale gas, a coal gas, a synthesis gas, combustion fumes, a gaseous hydrocarbon effluent, chlor-alkali plant exhaust gases, and rare earth production plant exhaust gases.

14. The process according to claim 1, in which the moist gas is introduced into step a) at a pressure in the range of 2 to 10 MPa, at a temperature in the range of 20 C. to 80 C., and at a relative humidity in the range of 30% to 100%.

15. The process according to claim 1, in which the moist gas introduced into step a) contains mercury in a proportion of between 10 nanograms and 1 gram of mercury per Nm.sup.3 of the moist gas.

16. The process according to claim 1, in which, in step a) the heat exchange heats the moist gas by 1 C. to 20 C.

17. The process according to claim 1, in which the reheated gas with a reduced water vapour content obtained in step a) has a relative humidity of less than 80%.

18. The process according to claim 1, in which the reheated gas with a reduced water vapour content obtained in step a) has a relative humidity of less than 75%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the invention will become more apparent from the following description made with reference to the accompanying drawings, in which:

(2) FIG. 1 represents the process for the reduction of the partial pressure of water vapour of a moist gas in accordance with the invention;

(3) FIG. 2 represents a process for the production of a natural gas integrating the process of the invention;

(4) FIG. 3 represents a variation of the process of FIG. 1.

(5) The present invention consists of controlling the relative humidity of a moist gas by increasing the temperature upstream of the capture mass in a manner such as to avoid capillary condensation. In order to minimize the energy to be supplied to the moist gas to be treated, a charge-effluent thermal integration is carried out around the capture mass in a manner such that the hot treated moist gas obtained from the capture mass reheats the cold moist gas entering the capture mass. In order to provide the complement of the heat and to be able to precisely control the relative humidity of the moist gas, heat from the cooled treated gas is transferred downstream of the feed-effluent exchanger to the reheated moist gas downstream of the feed-effluent exchanger and upstream of the capture mass using a heat pump system. The thermal integration of the invention may be carried out either using a heat pump system alone or using a feed-effluent exchanger around the capture mass supplemented by a heat pump system.

(6) Referring to FIG. 1, the moist gas arrives via the conduit 100. The gas arriving via the conduit 100 may be at a pressure in the range 2 to 10 MPa, preferably at a pressure in the range 5 to 9 MPa, and at a temperature in the range 20 C. to 80 C., preferably at a temperature in the range 25 C. to 70 C. The moist gas comprises water vapour, for example at a relative humidity in the range 30% to 100%. The moist gas also comprises at least one heavy metal, for example mercury and/or arsenic, in proportions which may vary between 10 nanograms and 1 gram of mercury per Nm.sup.3 of gas.

(7) Depending on the selected configuration and the site conditions, it may be selected to treat all or a portion of the moist gas in order to reduce its partial pressure of water. If only a portion is treated, the treated portion will be the stream moving in the conduit 101 and the untreated portion will be the stream evacuated via the conduit 110. The distribution between the streams 101 and 110 will be determined such that the treated stream moving in the conduit 104 and the untreated stream moving in the conduit 110, when they are combined, provide a gas which is depleted in heavy metals, in particular in mercury, to below an acceptable limit. Preferably, the whole feed is treated and the stream 110 is zero.

(8) The stream of water-saturated gas arriving via the conduit 101 is reheated using a heat exchanger 1001 by heat exchange with the heat transfer fluid of the heat pump system arriving via the conduit 301. This exchange of heat is preferably carried out in counter-current mode. The heat transfer fluid is condensed in the heat exchanger 1001. Optionally and in a complementary manner, the reheated gas obtained from the heat exchanger 1001 via the conduit 102 may be reheated using a heat exchanger 1002 in order to obtain a relative humidity which is suitable and to avoid capillary condensation of water on the capture mass 2000. The heat exchanger 1002 may, for example, be a heat exchanger with an external heat source such as a hot oil circuit, or fumes obtained from a furnace, or a burner. The gas arriving via the conduit 101 may be heated to a value in the range 1 C. to 20 C., preferably in the range 2 C. to 10 C. In this case, the relative humidity of the gas obtained from the heat exchanger 1002 via the conduit 103 is satisfactory. Preferably, there is no need for complementary heat to be provided via 1002 because the gas moving in the conduit 102 satisfies the relative humidity specifications. In this case, the conduit 102 is connected directly to the conduit 103. Preferably, in accordance with the invention, the gas is heated in 1001, optionally with supplemental heating in 1002, in a manner such that the gas has a relative humidity of less than 90% of saturation, preferably less than 80% and highly preferably less than 75% of saturation. This threshold for the moisture content depends on the pore characteristics of the heavy metal capture mass.

(9) The moist and reheated gas moving in the conduit 103 is brought into contact with the heavy metal guard bed or capture mass 2000. This produces a heavy metal-depleted gas evacuated from the capture mass 2000 via the conduit 104.

(10) All or a portion of the untreated fraction of the gas moving in the conduit 110 may be mixed via the conduit 111 with the treated gas arriving via the conduit 104 in order to produce the mixture moving in the conduit 106. The stream moving in the conduit 106 is at a temperature above the initial temperature of the gas arriving via the conduit 100 and thus has the capacity to give up yet another part of its heat. In accordance with the invention, a portion of the heat from the stream 106 is supplied to the heat pump system in a heat exchanger 1003: the stream 106 is cooled by heat exchange in 1003 with the heat transfer fluid arriving via the conduit 303 in order to generate a vaporized fluid evacuated via the conduit 304. The gas cooled in the heat exchanger 1003 is evacuated via the conduit 107, which may then be mixed with a fraction of untreated gas arriving via the conduit 112 in order to form a gas which is depleted in heavy metal evacuated via the conduit 108. The gas arriving via the conduit 104 may be cooled to a value in the range 1 C. to 20 C., preferably in the range 2 C. to 10 C.

(11) The guard bed 2000 is composed of capture masses which have the capacity to adsorb a heavy metal. It is possible to use any type of capture mass which is known to the skilled person. The capture mass employed may be selected from those known to the skilled person. The capture mass comprises a compound, usually known as the active phase, which reacts with the mercury in order to capture the heavy metal on the capture mass. The capture mass is preferably in the form of a bed composed of granules.

(12) In particular to capture mercury, the active phase of the capture mass may comprise metals which react with mercury in their sulphide form. The metallic sulphide or sulphides contained in the capture mass of the invention are based on a metal selected from the group constituted by copper (Cu), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni). Preferably, the metal or metals of the metallic sulphide or sulphides are selected from the group constituted by copper (Cu), manganese (Mn), iron (Fe) and nickel (Ni). Highly preferably, if a single metallic sulphide is present, copper sulphide is selected.

(13) The active phase used may also be elemental sulphur, for example as described in the patent document FR 2 529 802.

(14) The mercury guard bed is disposed upstream of the deacidification units (for example an amine unit), and so the gas to be treated contains H.sub.2S. Thus, it is also possible to use the metallic oxide precursor, the metal being selected from the group constituted by copper (Cu), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni), preferably copper oxide, which can be used to jointly eliminate H.sub.2S and mercury by being sulphided by H.sub.2S in a first step into a metallic sulphide, then by being reacted with mercury.

(15) In order to capture arsenic, and in particular in its gaseous form as AsH.sub.3, the active phase of the capture mass may comprise metals which react with arsenic in their oxide form. The metallic oxide or oxides contained in the capture mass of the invention are based on a metal selected from the group constituted by copper (Cu) and lead (Pb). Preferably, if a single metallic oxide is present, copper (II) oxide, also termed cupric oxide (CuO), is used. Highly preferably, if a single metallic oxide is present, lead (II) oxide (PbO) is used.

(16) Preferably, the capture mass may be constituted by an active phase as described above, distributed on a porous support.

(17) The porous support may preferably be selected from aluminas, phosphorus-containing aluminas, silica-aluminas, silicas, clays, activated carbon, zeolites, titanium oxides, zirconium oxides and mixtures thereof.

(18) A capture mass containing a support and copper sulphide is described in the document U.S. Pat. No. 4,094,777, for example.

(19) The capture mass may be obtained using any preparation method which is known to the skilled person, such as impregnation or co-granulation, for example.

(20) Processing the gas treated by the capture mass 5000 is preferably carried out by injecting the effluent to be treated into a reactor containing the capture mass in the form of a fixed bed.

(21) In the equipment 2000, the effluent to be treated may be brought into contact with the capture mass at a temperature in the range 50 C. to 115 C., preferably in the range 0 C. to 110 C., and more preferably in the range 20 C. to 100 C. Furthermore, it may be carried out at an absolute pressure in the range 0.01 MPa (0.1 bar) to 20 MPa (200 bar), preferably in the range 0.1 MPa (1 bar) to 15 MPa (150 bar), and more preferably in the range 0.1 MPa (1 bar) to 12 MPa (120 bar).

(22) In addition, this step for bringing the effluent to be treated into contact with the capture mass may be carried out at a HSV in the range 0.1 h.sup.1 to 50000 h.sup.1. The term HSV means the hourly space velocity of the gaseous effluent in the capture mass, i.e. the volume of gaseous effluent per volume of reactor per hour. For a gaseous effluent to be treated, the HSV may preferably be in the range 50 h.sup.1 to 500 h.sup.1.

(23) Contact with the capture mass may advantageously be carried out in order to capture the heavy metals, in particular mercury, contained in the effluent to be treated, and to obtain an effluent with a reduced heavy metals content, in particular of mercury, compared with the initial content of the effluent, or even to totally eliminate the heavy metals from the effluent.

(24) Advantageously, the reduction in the total content by weight of heavy metal between the gaseous effluent before treatment and the effluent obtained after treatment with the capture mass may represent at least 90%, preferably at least 95% and more preferably at least 99%.

(25) The heat pump circuit uses a heat transfer fluid. The heat transfer fluid supplying the heat pump circuit may be selected from the group formed by refrigerating fluids defined by the organization known as ASHRAE in the document Standards 34-2010.

(26) The refrigerating fluid is preferably selected from the group of light hydrocarbons containing 1 to 7 carbon atoms and comprising or not comprising one or more heteroatomic ligands. This group comprises, for example, all alcohols in which the carbon backbone contains 1 to 7 carbon atoms irrespective of whether this backbone is linear or branched, such as methanol, ethanol, propanol, butanol, pentanol, hexanol or heptanol, or the corresponding diols or triols, as well as their isomers. The heat transfer fluid may also contain molecules comprising at least one fluorine or chlorine atom such as, for example, hexaflorocene C.sub.3F.sub.6, or chlorofluorocarbon gases such as dichlorodifluoromethane CCl.sub.2F.sub.2, or indeed hydrochlorofluorocarbon gases such as chlorodifluoromethane, CHClF.sub.2. Still more preferably, the refrigerating fluid comprises at least 80% by weight of at least one of the following elements: propane, butane, pentane, hexane, heptane or any mixture of at least two of the mentioned five elements.

(27) The heat pump circuit is described below. The evaporated and optionally superheated heat transfer fluid may be introduced into the heat exchanger 1005 via the conduit 304 in order to undergo optional supplemental superheating, for example by heat exchange with the condensed heat transfer fluid 302 moving in the conduit 302 (in this case the heat exchangers 1005 and 1004 form a single piece of equipment). The evaporated and optionally superheated heat transfer fluid moving in the conduit 300 is compressed using a compressor 3000, for example a compressor which may be a centrifugal, reciprocating, screw or gear compressor. The choice of the technology depends on the flow rate of the heat transfer fluid and its nature (self-lubricating or not, molar mass, corrosive or non-corrosive nature, etc). Compression raises the temperature and pressure of the heat transfer fluid. The pressurized fluid obtained from the compressor via the conduit 301 is in the vapour form. The fluid 301 is condensed in the heat exchanger 1001 by heat exchange with natural gas arriving via the conduit 101. Condensation in the exchanger 1001 has the effect of moving the heat transfer fluid into the liquid phase and fixing the temperature at the condensation temperature under the selected pressure conditions. This results in the condensed heat transfer fluid being evacuated from the exchanger 1001 via the conduit 302, as a liquid, the temperature of which may optionally be reduced by heat exchange in the heat exchanger 1004, for example by heat exchange with the evaporated heat transfer fluid moving in the conduit 304 (in this case the heat exchangers 1005 and 1004 form one and the same piece of equipment). The condensed and optionally cooled fluid moving in the conduit 302b is decompressed in equipment 4000 producing a pressure drop, for example a valve or a reduction orifice, in order to produce the decompressed fluid moving in the conduit 303. The decompressed fluid has undergone partial evaporation, which causes a reduction in its temperature. The partially evaporated fluid is evaporated and optionally superheated by heat exchange with the natural gas depleted in mercury arriving via the conduit 106 in the heat exchanger 1003 which closes the heat pump circuit.

(28) The temperature of the moist gas may vary as a function of the production site. However, the invention is implemented in a manner such that the temperature Tc of the natural gas 103 introduced into the guard bed is determined in a manner such that the natural gas 103 has a relative humidity of less than 90%, preferably less than 80%, highly preferably less than 75%. The temperature Tf corresponds to the temperature of the gas moving in the conduit 107.

(29) The pressure of the heat transfer fluid on the side of the exchanger 1003, i.e. the evaporator, is selected such that the temperature of evaporation of the heat transfer fluid is in the range between the desired temperature Tf for the condensation point of the saturated gas 103 and 10 C. below Tf, preferably 2 C. to 5 C. below Tf.

(30) The pressure of the heat transfer fluid entering the heat exchanger 1001, i.e. the condenser, is selected in a manner such that the temperature of condensation of the fluid is in the range between the desired temperature Tc for reheating the gas 103 and 10 C. above Tc, preferably between 2 C. and 5 C. above Tc.

(31) In optimal manner, the difference between Tc and Tf is less than 50 C., and preferably TcTf<30 C.; still more preferably, TcTf<20 C., in order to obtain the best efficiency of the heat pump circuit.

(32) FIG. 2 diagrammatically shows an example of the natural gas production process from extraction to the deacidification operation, in which the process of the invention is carried out. The references in FIG. 2 which are identical to the references in FIG. 1 designate the same elements.

(33) The natural gas extracted from the underground deposit 1 is sent via the conduit 2 to a device 3 for trapping sludge, usually termed a slug catcher. The sludge is evacuated from the slug catcher via the conduit 4. The natural gas obtained from the device 3 is introduced via the conduit 5 into a primary gas-liquid separator 6 which can be used to eliminate a liquid stream comprising water and hydrocarbons via the conduit 7.

(34) The moist gas is then introduced into a second gas-liquid separator 20. The function of this equipment is to separate the moist gas from droplets of liquid resulting from saturation of the condensables at the temperature of the system. This equipment 20 is, for example, a coalescer. The document EP 2 473 250 A2 describes an example of an embodiment of a coalescer. The coalescer 20 may be composed of a chamber provided with internal elements such as baffles and/or mats of fibres promoting condensation of droplets of liquid contained in the gas. Eliminating the droplets of liquid is preferable, as this prevents them from being present in a system where there is a saturated gas which also then entrains condensed liquids which will then become fixed on the mercury guard bed.

(35) The liquid water is evacuated from the equipment 20 via the conduit 21. The gas, which has been freed from droplets in the equipment 20, is introduced into the heat exchanger 1001 via the conduit 101 for heating by heat exchange with the heat transfer fluid arriving via the conduit 301. The heated gas is evacuated from the exchanger 1001 via the conduit 102 for introduction into the mercury guard bed 2000. The gas, depleted in mercury, is evacuated from the equipment 2000 and is introduced into the heat exchanger 1003 via the conduit 104 for cooling by heat exchange with the heat transfer fluid arriving via the conduit 303. The reheated gas obtained from 1003 is introduced into a deacidification unit 8 via the conduit 107.

(36) The heat transfer fluid obtained from the exchanger 1001 is introduced into the decompression device 4000 via the conduit 302. The decompressed fluid moving in the conduit 303 is reheated in the heat exchanger 1003 by heat exchange with the natural gas. The reheated heat transfer fluid is evacuated from the exchanger 1003 via the conduit 304 for introduction into the compressor 3000. The compressed fluid is introduced via the conduit 301 into the exchanger 1001 for cooling by heat exchange with the natural gas.

(37) In the unit 8, the gas is brought into contact with a liquid amine solution which absorbs H.sub.2S and optionally CO.sub.2 contained in the gas. The gas, which is depleted in acidic compounds, is introduced into the dehydration unit 10 via the conduit 9. In the unit 10, the natural gas may be brought into contact with a liquid glycol solution which absorbs the water contained in the gas. Alternatively, in the unit 10, the natural gas may be brought into contact with solid water-adsorbing masses. The gas evacuated from the unit 10 via the conduit 11 may be sold.

(38) FIG. 3 represents a variation of the process described in FIG. 1, in which the thermal integration is optimized by using the heat exchanger 1000. The references in FIG. 3 which are identical to the references in FIG. 1 designate the same elements.

(39) Referring to FIG. 3, the moist gas arriving via the conduit 101 is reheated in a heat exchanger 1000 by the heavy metal-depleted gas arriving via the conduit 104. The reheated moist gas is evacuated from the exchanger 1000 for introduction into the exchanger 1001 via the conduit 102. The heavy metal-depleted gas is evacuated from the exchanger 1000 via the conduit 105. Next, the gas may be mixed with a fraction of untreated gas arriving via the conduit 111, the mixture then being introduced into the heat exchanger 1003 via the conduit 106.

(40) The heat exchanger 1000 can be used to recover the heat from the heavy metal-depleted gas and to optimize the thermodynamic cycle of the heat pump by reducing the quantity of heat transferred to the heat exchanger 1003 which condenses the heat transfer fluid.

(41) The process of the invention is particularly suitable for the treatment of natural gas. However, the process of the invention may be applied to other types of gaseous effluents, such as combustion fumes, synthesis gas or indeed a gaseous hydrocarbon effluent, for example a shale gas, a coal gas, a gaseous oil cut or a refinery fuel gas, chlor-alkali plant exhaust gases, or rare earth production exhaust gases. The gaseous effluent comprises water in varying proportions which in particular depend on its origin and earlier treatments which it has undergone. Typically, the relative humidity of the fumes is in the range 30% to 100%.

(42) The combustion fumes are in particular produced by the combustion of hydrocarbons, biogas, coal in a boiler or a combustion gas turbine, for example for the purposes of the production of electricity. These fumes are at a temperature in the range 20 C. to 60 C., a pressure in the range 1 to 5 bars (1 bar=0.1 MPa) and may comprise in the range 50% to 80% of nitrogen, in the range 5% to 40% of carbon dioxide, in the range 1% to 20% of oxygen, and several impurities such as SO.sub.x and NO.sub.x, if they have not been eliminated upstream by a deacidification process. Synthesis gas contains carbon monoxide CO, hydrogen H.sub.2 (generally in a H.sub.2/CO ratio of 2), water vapour (generally at saturation at the temperature at which scrubbing is carried out) and carbon dioxide CO.sub.2 (of the order of 10%). The pressure is generally in the range 20 to 30 bar, but may be up to 70 bar. It also further contains sulphur-containing impurities (H.sub.2S, COS, etc), nitrogen-containing impurities (NH.sub.3, HCN) and halogenated impurities. Depending on the positioning of the demercuration step in the synthesis gas purification chain, its relative humidity is in the range 5% to 100%.

(43) The following examples demonstrate the advantages of the invention.

EXAMPLE 1 (CONTEXT)

(44) In this example, two commercial mesoporous solids A and B were considered and used for demercuration and characterized by the pore volume distributions shown in Table 1.

(45) TABLE-US-00001 TABLE 1 Proportion of pore types of solids A and B under consideration % pore volume Solid A Solid B Micropores (d < 2 nm) 0 0 Mesopores (2 < d < 50 nm) 49 60 Macropores (d > 50 nm) 51 40
in which d corresponds to the pore diameter of the solid, measured using a mercury porosimetry method, for example the method described in the document by Rouquerol F.; Rouqerol J.; Singh K. Adsorption by powers & porous solids: Principle, methodology and applications, Academic Press, 1999.

(46) Assuming that the pores are cylindrical, Kelvin's law (equation 2) can be applied to determine, as a function of the moisture content, the proportion of the total pore volume filled with liquid water. The assumed conditions were a fluid which completely wets the surface (=0), a surface tension equal to that of an air/water system (=67.9 mN.Math.m.sup.1 at T=50 C.) and the temperature T=50 C. The saturated vapour pressure P.sub.0 of the water was determined using Antoine's law (P.sub.0=105.203891733.926/(T39.485)). The results of this calculation are shown in Table 2.

(47) TABLE-US-00002 TABLE 2 Percentage of pore volume filled as a function of moisture content % of total pore volume filled in the humidity range Moisture content Solid A Solid B 0 < P/P.sub.0 < 25 0 0 25 < P/P.sub.0 < 50 0.02 0.01 50 < P/P.sub.0 < 70 1.55 1.27 70 < P/P.sub.0 < 80 6.52 8.01 80 < P/P.sub.0 < 90 30.72 42.40 90 < P/P.sub.0 < 99.9 57.62 45.55

(48) It can clearly be seen that the majority of the pores were not affected beyond a moisture content of approximately 80%. Beyond that, a substantial increase in the total pore volume filled with liquid water was observed, up to approximately 100% for moisture contents close to saturation.

(49) Assuming a homologous distribution of the active phase over the surface of the support, it was possible to calculate the quantity of active phase rendered inactive, because it is inaccessible due to filling of the pores, as a function of the moisture content.

(50) TABLE-US-00003 % of active phase which is inaccessible in the moisture content range Moisture content Solid A Solid B 0 < P/P.sub.0 < 25 0 0 25 < P/P.sub.0 < 50 0 1.10 50 < P/P.sub.0 < 70 7.06 5.08 70 < P/P.sub.0 < 80 21.29 23.03 80 < P/P.sub.0 < 90 46.17 58.75 90 < P/P.sub.0 < 99.9 25.48 12.04

(51) In agreement with these data, a dynamic mercury capture test carried out on 1 g of capture mass A showed that the purification efficiency was 100% after one hour for a moisture content of 25%; 100% after one hour for a moisture content of 50%; 90% after one hour for a moisture content of 75%; 25% after one hour for a moisture content of 90% and 0% after one hour for a moisture content of 100%.

(52) In agreement with these data, a dynamic mercury capture test carried out on 1 g of capture mass B showed that the purification efficiency was 100% after one hour for a moisture content of 25%; 100% after one hour for a moisture content of 50%; 90% after one hour for a moisture content of 75%; 15% after one hour for a moisture content of 90% and 0% after one hour for a moisture content of 100%.

(53) The function of the capture masses was thus only altered to a moderate extent up to a moisture content of 80%. A fortiori, in order to provide a safety margin, it is recommended to maintain the relative moisture content of the gas to be treated in the demercuration step at a relative humidity of less than 75%.

EXAMPLE 2 (NOT IN ACCORDANCE WITH THE INVENTION)

(54) This example is given by way of comparison. In this example, the water content of a moist gas saturated with water was to be reduced to below 75% in accordance with that disclosed in Example 1.

(55) It was intended to evaluate the quantity of energy necessary to reduce the relative moisture content of a natural gas saturated with water to 75% relative humidity by reheating the whole of the stream to be treated. This was based on the following initial conditions: the moist gas was a natural gas saturated with water produced at 70 bar and 30 C. and with a dry flow rate of 140000 Nm.sup.3/h. Its dry composition is given in Table 3.

(56) TABLE-US-00004 TABLE 3 Composition of dry gas Compound % mol CO.sub.2 10.7 O.sub.2 0.8 N.sub.2 0.9 C.sub.1 77.2 C.sub.2 6.6 C.sub.3 2.2 iC.sub.4 0.2 nC.sub.4 0.5 iC.sub.5 0.2 nC.sub.5 0.3 C.sub.6 0.2 C.sub.7 0.1 C.sub.8 0.1

(57) When this gas was saturated with water, its water vapour content was 880 ppm mol.

(58) The results presented below result from a simulation using Aspen Hysys v7.2 software. The SRK (Soave-Redlich-Kwong) model was the thermodynamic model which was selected.

(59) The temperature at which a relative humidity of 75% was obtained was 35.5 C. The thermal power necessary to heat the flow of gas from 30 C. to 35.5 C. was 500 kWth (kilowatt thermal).

(60) Next, the reheated gas was treated on a mercury guard bed.

EXAMPLE 3 (IN ACCORDANCE WITH THE INVENTION)

(61) This example in accordance with the invention was aimed at reducing the water content of a moist gas below 75% in agreement with the disclosure in Examples 1 and 2 in order to prevent any major phenomena of capillary condensation in the mercury capture masses as described in Example 1. The unit for partially drying gas located upstream of the demercuration unit is described in FIG. 3, in which the heat exchangers 1002, 1004 and 1005 were not used.

(62) The results presented below result from a simulation using Aspen Hysys v7.2 software. The SRK (Soave-Redlich-Kwong) model was the thermodynamic model which was selected.

(63) The following initial conditions were employed: the moist gas was a natural gas saturated with water produced at 70 bar and 30 C. with a dry flow rate of 140000 Nm.sup.3/h. Its dry composition is given in Table 3 of Example 2.

(64) When the gas was saturated with water, its water vapour content was 880 ppm mol.

(65) In this example, the whole of the feed 100 was treated. Thus, the stream of gas in the conduit 110 was zero. The moist gas 101 entered the feed-effluent exchanger 1000 and exchanged 276 kWth with the stream 104 which entered the exchanger at 35.5 C. This produced a stream 102 of reheated gas to be treated at 33 C. and a stream of cooled treated gas 105 at 32.4 C. The reheated gas to be treated 102 entered the condenser of the heat pump system 1001 and exchanged 232 kWth with the heat transfer fluid 301. This produced a stream of moist gas to be treated 103 at a temperature of 35.5 C. The relative humidity of 103 was 74.5% of saturation which under these pressure and temperature conditions would correspond to 1181 ppm mol. The gas was then treated on the capture mass 2000. This produced a treated gas 104 which would give up heat in the heat exchanger 1000 to the entering gas in order to produce a partially cooled treated gas 105 at 32.4 C. In the event that the whole of the feed is treated, the stream 111 is zero in this example and the stream moving in the conduit 106 has the same composition and is under the same conditions as that moving in the conduit 105. The stream 106 gives up 220 kWth to the heat transfer fluid 303 in the evaporator 1003 of the heat pump system. The temperature of the treated gas 107, and thus of the gas produced 108 because in this example the stream 112 was zero, was 30.2 C.

(66) The heat pump circuit started with a vaporized heat transfer fluid 300 composed of butane at 27.7 C. and 2.64 bar which moved at 39.2 kmol/h. Its pressure was raised using a compressor 3000. This corresponded to a consumption of 11.3 kWe (electrical) with a compression efficiency of 75%. This produced a fluid 301 at 3.65 bar and 38.5 C. This fluid was condensed at 1001 to produce a stream of liquid butane 302 at 38.5 C. The condensed fluid was decompressed in the valve 4000 to produce a stream of partially vaporized butane at 27.7 C. and 2.64 bar. This fluid was then vaporized in the heat exchanger 1003 to recover the fluid moving in the conduit 3000 and close the circuit of the heat pump.

(67) In summary, the stream of moist natural gas 100 received a total of 508 kWth for an energy consumption of 11.3 kWe. If it is assumed that this electricity is produced by combustion of gas in a turbine with a net electrical efficiency of 40%, this would correspond to an equivalent consumption of 28 kWth of primary energy such as that used in Example 2 to produce the thermal power.

EXAMPLE 4 (IN ACCORDANCE WITH THE INVENTION)

(68) This example in accordance with the invention was aimed at reducing the water content of a moist gas below 75% in agreement with the disclosure in Examples 1 and 2 in order to prevent any major phenomena of capillary condensation in the mercury capture masses as described in Example 1. The unit for partially drying gas located upstream of the demercuration unit is described in FIG. 1, in which the heat exchangers 1002, 1004 and 1005 were not used.

(69) The results presented below result from a simulation using Aspen Hysys v7.2 software. The SRK (Soave-Redlich-Kwong) model was the thermodynamic model which was selected.

(70) The following initial conditions were employed: the moist gas was a natural gas saturated with water produced at 70 bar and 30 C. with a dry flow rate of 140000 Nm.sup.3/h. Its dry composition is given in Table 3 of Example 2.

(71) When the gas was saturated with water, its water vapour content was 880 ppm mol.

(72) In this example, the whole of the feed 100 was treated. Thus, the stream of gas in the conduit 110 was zero. The initial feed which moved in the conduit 100 then 101 was a saturated moist gas (880 ppm mole of water) which was reheated in the condenser 1001 of a heat pump system by transferring 501 kWth with a heat transfer fluid (this value differs from Example 1 because of a smaller pressure drop due to missing out one exchanger). This produced a stream 102 of reheated moist gas at 35.5 C. at 74.8% saturation (which is 1177 ppm mol under these conditions). The treated gas 106 restored 477 kWth to the heat transfer fluid in the evaporator 1003. The result was a gas 108 produced at 30.2 C.

(73) In the heat pump circuit, the vaporized heat transfer fluid 300 was composed of butane at 27.7 C. and 2.64 bar which moved at 85 kmol/h. Its pressure was raised using a compressor 3000. This corresponded to a consumption of 24.5 kWe (electrical) with a compression efficiency of 75%. This produced a fluid 301 at 3.65 bar and 38.5 C. This fluid was condensed in the exchanger 1001 to produce a stream of liquid butane 302 at 38.5 C. The condensed fluid was decompressed by the valve 4000 to produce a stream of partially vaporized butane at 27.7 C. and 2.64 bar. This fluid was then vaporized in the heat exchanger 1003 to recover the fluid moving in the conduit 300 and close the circuit of the heat pump.

(74) In summary, the stream of moist natural gas 100 received a total of 501 kWth for an energy consumption of 24.5 kWe. If it is assumed that this electricity is produced by combustion of gas in a turbine with a net electrical efficiency of 40%, this would correspond to an equivalent consumption of 61 kWth of primary energy such as that used in Example 2 to produce the thermal power.

CONCLUSIONS FOR EXAMPLES 2, 3 AND 4

(75) In Example 2 (not in accordance with the invention), the thermal power consumed to obtain 75% relative humidity was 500 kWth.

(76) In Example 4 (in accordance with the invention), the equivalent thermal power to obtain 74.8% relative humidity was 61 kWth.

(77) In Example 3 (in accordance with the invention), the equivalent thermal power to obtain 74.5% relative humidity was 28 kWth.

(78) Using a heat pump system in accordance with the invention means that the energy consumed can be reduced by a factor of at least 8 compared with the solution described in Example 2, and by a factor of close to 20 if it is supplemented by a feed-effluent exchange.