EP0770667B1 - Drying process for gases making use of glycol including the separation of gaseous effluents - Google Patents

Description

The invention relates to a method for dehydrating gas using a liquid desiccant (glycol) including an effluent purification step gaseous emitted during the regeneration of said liquid desiccant. The invention relates more particularly to a process making it possible to reduce pollution due to gaseous emissions from natural gas drying units, pollution mainly due to the following aromatic compounds: benzene, toluene, ethyl benzene, xylene (BTEX).

Dehydration of a gas, for example a natural gas or a gas of refinery, is a classic operation. It allows to control the point of dew "water" of the gas, to avoid the formation of hydrates or ice during transportation or use of this gas, reduce the risk of corrosion, or for all other reasons.

For this purpose, it is common to put the gas in contact with a desiccant hydrophilic liquid; among these, the chemical family of glycols is of a widespread use. Most often, in almost 95% of cases, the triethylene glycol (TEG) because of its strong affinity for water, its stability chemical and its low cost. However, for some applications, the monoethylene glycol (MEG), diethylene glycol (DEG), or tetraethylene glycol (T4EG) may be preferred.

In a conventional gas dehydration unit with a liquid desiccant, for example a glycol, as shown diagrammatically in FIG. 1 annexed, the wet gas enters, by line 1, at the bottom of a column of absorption A1, operating under pressure, where it contacts by circulation to counter-current the liquid desiccant introduced at the head by line 3. During this contact, the water contained in the gas is absorbed by the desiccant. The gas dehydrated comes out at high pressure from the head of the absorption column A1 by the line 2. Leaving the bottom of column A1, the desiccant loaded with water is sent by line 4 to the head of the regeneration unit R1 where it is used as coolant. After the heat exchange, the desiccant charged with water is sent to a flash separation tank S1, in which the pressure is lower than in the absorption column A1. In some case, it is possible to send the desiccant loaded with water first in the flash separation flask before using as coolant at the head of the regeneration unit R1. Much of the gas absorbed at high pressure by the desiccant is separated from the liquid phase in this balloon S1. This gas can either be released to the atmosphere through line 5, or used as fuel gas during the desiccant regeneration stage. It is then sent to the reboiler burner R2 of the regeneration device R1.

The liquid desiccant containing water but being separated from the gas absorbed at strong pressure comes from the flash separation balloon through line 7. After its passage in at least one heat exchanger El, it is sent by the line 7 in the thermal regeneration device R1, in which part of the water absorbed by the desiccant will be vaporized and eliminated at the top by the line 8, while the regenerated desiccant which comes out at the bottom through line 3 crosses the exchanger El and is sent by a pump P1, in a cooler E4, then at the top of the absorption column A1.

However, it is well known that water cannot be completely separated from desiccant thermally at atmospheric pressure, when the latter degrades at a temperature below its normal boiling point. For example, the TEG boils at around 285 ° C, but we generally limit ourselves to 204 ° C during regeneration to limit its degradation. At this temperature, the purity of the regenerated TEG is close to 98.7% by mass.

If you want a higher purity for the liquid desiccant (glycol) so to obtain further dehydration of the gas, a conventional means consists in following the thermal reconcentration step with a step of stripping with dry gas or with a low water content, for example part of the gas stream dehydrated by the desiccant, as described in particular in US-A-3,105,748.

• 1. une étape de rebouillage du dessiccant liquide chargé en eau,
• 2. une étape de distillation dudit dessiccant comprenant au moins un étage de distillation,
• 3. une étape de strippage du dessiccant liquide partiellement régénéré lors des étapes 1 et 2, par l'agent de strippage vaporisé,
• 4. une étape de condensation de la vapeur sortant de l'étape de distillation 2, condensation générant deux phases liquides, l'une majoritaire en eau, l'autre majoritaire en agent de strippage,
• 5. le chauffage de la phase liquide riche en agent de strippage issue de l'étape 4, chauffage régénérant une phase vapeur plus riche en eau que ladite phase liquide et une phase liquide appauvrie en eau,
• 6. le renvoi de la phase liquide constituée essentiellement d'agent de strippage issue de l'étape 5 vers l'étape 3.

Another technique consists in following the reconcentration step with a stripping step, using a liquid stripping agent at ambient temperature and pressure and forming a heteroazeotropic with water. This configuration, described in particular in patent FR-B-2698017, comprises:

• 1. a reboiling step of the liquid desiccant loaded with water,
• 2. a step of distilling said desiccant comprising at least one distillation stage,
• 3. a step of stripping the liquid desiccant partially regenerated during steps 1 and 2, by the stripping agent vaporized,
• 4. a stage of condensation of the steam leaving the distillation stage 2, condensation generating two liquid phases, one mainly in water, the other mainly in stripping agent,
• 5. heating the liquid phase rich in stripping agent from step 4, heating regenerating a vapor phase richer in water than said liquid phase and a liquid phase depleted in water,
• 6. returning the liquid phase consisting essentially of stripping agent from step 5 to step 3.

When, in dehydration processes, natural gas or treated refinery contains aromatic compounds (BTEX: benzene, toluene, ethyl benzene and xylene), during the absorption phase, the desiccant - generally TEG- which is also a solvent for the compounds aromatic, is charged in the so-called BTEX.

Because of the boiling temperatures of BTEX at atmospheric pressure, which are between 80 and 144 ° C, few of these compounds are separated from desiccant in the flash separation flask described above, which operates at low pressure and high temperature. Most compounds aromatics are separated from the desiccant when heated in the column of regeneration.

The vapors emitted by a TEG reboiling unit can have a very high total aromatic content (greater than 30%). A specific composition (Treatment of the Whitney Canyon natural gas field, Wyoming, United States) is given for information below (% by weight): - Water 45.2% - Nitrogen 7.7% - Benzene 4.6% - Toluene 15.6% - Ethyl benzene 0.9% - Xylene 12.7% - Other hydrocarbons 13.3%

The composition of the discharges varies according to the nature of the gas to be treated, the temperature and flow rate of TEG circulating in the installation. These releases must be reduced in order to respond to the new constraints linked to toxic products in the atmosphere. For example, in the United States, the "Clean Air Act Amendment", published in 1990, drastically reduces rates acceptable releases of BTEX to the atmosphere in the United States. Any unit discharging more than 100 tonnes / year of BTEX or 25 tonnes / year of any combination of these 4 compounds is subject to control and regulation.

In order to meet the new constraints on toxic product emissions in the atmosphere, the manufacturers concerned have modified the dehydration of existing gases and used conventional techniques following:

Incineration of vapors, which can be carried out by a flame incinerator powered by the fuel oil produced by the unit, has the disadvantage of requiring a very important investment.

Condensation of vapors to produce water and BTEX and separation by gravity in a three-phase separation flask are described in detail in US-A-3,867,736 and shown schematically in Figure 2. According to this technique, the gaseous discharges leaving the head of the thermal regeneration R1 are sent by line 8 in a condenser C1, usually an air cooler. The different fluids from the condenser C1 are sent to a three-phase separation flask B1, where are separated by gravity a liquid phase mainly containing water, discharged via line 11, and a liquid phase mainly containing hydrocarbons, withdrawn laterally by line 10. The gaseous phase leaving of this three-phase balloon B1 via line 9 is made up of water vapor and contains a residual rate of hydrocarbons frequently exceeding environmental constraints, as will be shown in example 2 presented later.

We know an industrial process that uses two condensers such as C1 and two three-phase balloons such as B1, this process making it possible to treat the vapors emitted by the flash separation flask S1 and by the column of regeneration R1.

US-A-5,209,762 describes an improvement to the previous process eliminating the aromatics dissolved in the liquid water extracted from the three-phase balloon.

Another technique involves installing a steam circuit on the primary condenser, followed by a screw compressor, vapors not condensables being reintroduced into the processing unit.

Yet another technique implements the drying and treatment of a gas, using a solvent composed of glycol, N-methyl-caprolactam and water, the glycol concentration (preferably TEG) being between 80 and 97%. This method is described in US-A-4,479,811.

• un gaz qui contient toujours les BTEX,
• une solution contenant de l'eau et du TEG, qui peut être régénérée sans risque d'émission de BTEX.

Finally, the use of gas permeation for this application is described in US-A-5 399 188. A mixture of water and TEG circulates inside a bundle of hollow fibers placed in a chamber. The humid gas containing the BTEX is sent into this chamber. Only the water mixed with the glycol passes through the membrane. At the exit of the room we recover:

• a gas which always contains BTEX,
• a solution containing water and TEG, which can be regenerated without risk of BTEX emission.

The invention relates to a new process using condensation vapors from the desiccant regeneration device.

The process of the invention has the particular advantage of producing purified gaseous effluents, which can be discharged directly to the atmosphere or to a conventional torch system (without incinerator) or well to reuse in the installation.

(a) une étape d'absorption de l'eau et les BTEX par contact entre ledit gaz humide et le dessiccant liquide régénéré dans l'étape (c), produisant un effluent gazeux sec et un flux de dessiccant liquide chargé en eau et en BTEX,

(b) une étape de séparation dudit dessiccant liquide chargé en une vapeur contenant principalement du méthane, de la vapeur d'eau et une partie des BTEX et une phase liquide contenant principalement le dessiccant liquide chargé en eau et en BTEX,

(c) une étape de régénération dudit dessiccant liquide comprenant une zone de rebouillage et une zone de distillation, dans laquelle le dessiccant liquide chargé est envoyé dans ladite zone de distillation, d'où sort une vapeur contenant de l'eau et des BTEX et ledit dessiccant liquide régénéré, qui est renvoyé comme dessiccant à l'entrée de ladite zone d'absorption, dans l'étape (a),

(d) une étape de condensation de la vapeur issue de ladite zone de distillation, suivie de la séparation de trois phases : un effluent gazeux contenant des BTEX, une phase liquide hydrocarbonée contenant des BTEX et une phase liquide aqueuse ; et

(e) le traitement d'au moins ledit effluent gazeux contenant des BTEX dans une zone de lavage par absorption des BTEX par une fraction de dessiccant liquide régénéré prélevée en un point du procédé et renvoi dudit dessiccant ayant absorbé les BTEX vers un point de la zone de régénération de l'étape (b), l'effluent gazeux sortant de ladite zone de lavage étant ainsi débarrassé des BTEX.

In general, the invention provides a method of dehydration by means of a hydrophilic liquid desiccant of a wet gas chosen from natural gas and refinery gases, essentially comprising methane and other light alkanes, BTEX, water and optionally carbon dioxide, nitrogen and / or hydrogen sulfide, with regeneration of said liquid desiccant, said process comprising:

(a) a step of absorbing water and BTEX by contact between said wet gas and the liquid desiccant regenerated in step (c), producing a dry gaseous effluent and a flow of liquid desiccant charged with water and BTEX,

(b) a step of separating said liquid desiccant loaded into a vapor containing mainly methane, water vapor and part of the BTEX and a liquid phase containing mainly the liquid desiccant loaded with water and BTEX,

(c) a step of regenerating said liquid desiccant comprising a reboiling zone and a distillation zone, in which the charged liquid desiccant is sent to said distillation zone, from which a vapor containing water and BTEX emits, and said regenerated liquid desiccant, which is returned as desiccant at the entrance to said absorption zone, in step (a),

(d) a step of condensing the vapor from said distillation zone, followed by the separation of three phases: a gaseous effluent containing BTEX, a liquid hydrocarbon phase containing BTEX and an aqueous liquid phase; and

(e) the treatment of at least said gaseous effluent containing BTEX in a washing zone by absorption of BTEX with a fraction of regenerated liquid desiccant sampled at a point in the process and return of said desiccant having absorbed BTEX to a point in the regeneration zone of step (b), the gaseous effluent leaving said washing zone thus being rid of BTEX.

The process of the invention is described below in more detail in relation to the figure 4:

In step (a), the flow of wet gas 1 is brought into contact with the flow of liquid desiccant 3, against the current in an absorption column A1, this which produces a dry gaseous effluent 2 leaving at the top and a flow of desiccant liquid 4 loaded with water and BTEX, leaving at the bottom of said column absorption A1.

In this stage, the wet gas enters the production pressure (generally from 20 to 150 bar) and at a temperature below 50 ° C. If the temperature of gas production is greater than this value, said gas will be cooled, by example by an air cooler, before entering column A1. The liquid desiccant introduced at the head of column A1 is conventionally at a temperature approximately 5 ° C higher than that of the gas to be treated.

In step (b), the flow of charged liquid desiccant 4 is sent to a flash separation tank S1, in which a vapor effluent is separated 5 leaving the head, mainly containing methane, water vapor and BTEX and, leaving the bottom, a liquid phase 7 containing mainly the desiccant liquid loaded with water and BTEX.

In this step, the flow of liquid desiccant loaded with water and BTEX leaves by line 4 at the temperature of the gas to be treated; it is usually sent as cooling fluid at the top of the distillation column D1 of the regeneration device R1, where the temperature of said desiccant increases in general about 10 ° C. The desiccant, then sent to the flask flash separation S1, is relaxed at a pressure of 2 to 5 bar, its temperature, depending on operating conditions, which can vary from 50 to 85 ° C.

In step (c), the flow of liquid desiccant 7 is sent through a heat exchanger El, to the distillation column D1 of the regeneration R1, which further comprises a reboiler R2; of said device regeneration R1, it leaves a steam effluent 8 at the head containing water and BTEX and in the bottom a liquid effluent 3, constituting the liquid desiccant regenerated, which is sent, through the heat exchanger El and the pump P1, at the top of the absorption column A1 of step (a).

In this step, the flow of liquid desiccant is heated in the exchanger El, dimensioned so as to cause a temperature variation of at least minus around 100 ° C on stream 7 (warmed) and stream 3 (cooled). The effluent steam 8 from the distillation column D1 generally leaves at a temperature about 80 to 90 ° C and at atmospheric pressure. The liquid desiccant regenerated leaves the reboiler R2 at the bottom at a temperature of around 200 ° C. and undergoes a temperature drop of at least about 100 ° C in the exchanger El as already indicated above. The temperature of the regenerated desiccant is adapted to the conditions of column A1: it is cooled, generally in a exchanger E4, up to a temperature approximately 5 ° C higher than that of gas to be treated. Its pressure is also adapted, by the pump P1, to that prevailing in the absorption column A1.

In step (d), said gaseous effluent 8 leaving at the head of the column of distillation D1 of the regeneration device R1 is condensed in a condenser C1 and sent to a three-phase separation flask B1, hence exits, at the top, a gaseous effluent 9 containing BTEX, laterally, a hydrocarbon phase 10 containing BTEX and at the bottom, an aqueous liquid phase 11.

The overhead effluent from the distillation column D1 is cooled through the condenser C1, generally an air cooler, up to about 50 ° C, or less depending on operating conditions. The three-phase separation flask B1 is at this temperature and at atmospheric pressure; the same is true of the gaseous effluent 9.

Finally, in step (e), the gaseous effluent 9 is sent in updraft in a washing column L1, in which it is brought into contact against the current with a liquid flow 12, taken from the liquid desiccant circuit regenerated. From said washing column L1, a desiccant flow comes out at the bottom liquid 13 having absorbed BTEX, which is returned to the device for regeneration R1, and at the top a gaseous effluent free of BTEX.

In this step, the regenerated liquid desiccant flow used for washing generally represents 3 to 10% of the flow injected into the supply of the absorption column A1. For washing to be effective, the temperature of the desiccant used is advantageously at least 5 ° C higher than that of the gaseous effluent to be treated. This temperature will be adapted to the conditions operating, generally by means of an E3 heat exchanger. The injected desiccant comes out at the bottom of the washing column L1 at temperature of the gaseous effluent to be treated.

Different configurations can be envisaged to implement the method of the invention.

Thus, the regenerated desiccant used for washing the gaseous effluents of the three-phase separator B1 can be taken from the supply of the absorber A1, according to the arrangement shown in FIGS. 4 to 6. This configuration avoids the installation of an exchanger and a pump on site.

In this case, the desiccant loaded with BTEX, leaving at the bottom of the wash L1 by line 13 can be sent to feed 7 of the column distillation D1 upstream of the heat exchanger E1, as shown figure 4.

The desiccant loaded with BTEX leaving the washing column L1 by the line 13 can also be sent to the feed 7 of the distillation column D1 downstream of said heat exchanger E1, as shown in FIG. 5. It can also be injected directly at the top of the distillation column D1 of the regeneration device R1, or at an intermediate level as indicated in dotted lines in Figure 5.

In these different cases, the additional energy consumption induced at the reboiler by the addition of this cold fluid is low, given that a small fraction of the desiccant flow is dedicated to this function of washing.

It is also possible to carry out a heat exchange between the desiccant leaving the column L1 and the head of the regeneration column causing partial reflux as shown in Figure 6. This arrangement allows warm the desiccant while ensuring all or part of the condensation required at the top of regeneration column D1.

In the process of the invention, the stream of regenerated liquid desiccant 12 supplying the washing column L1 at the head can still be taken from the reboiler R2 by a pump P2 and pass through a heat exchanger E2, and if necessary in an exchanger E3, in which it is cooled, and the liquid desiccant 13 having absorbed BTEX and leaving the bottom of the column L1 is returned through the E2 heat exchanger, where it is reheated to reboiler R2. Such a configuration is shown in figure 3.

1) une étape de rebouillage du dessiccant liquide chargé en eau,

2) une étape de distillation dudit dessiccant comprenant au moins un étage de distillation,

3) une étape de strippage du dessiccant liquide partiellement régénéré, lors des étapes 1 et 2, par l'agent de strippage vaporisé,

4) une étape de condensation de la vapeur sortant de l'étape de distillation 2, condensation générant deux phases liquides, l'une majoritaire en eau, l'autre majoritaire en agent de strippage,

5) le chauffage de la phase liquide riche en agent de strippage issue de l'étape 4, chauffage générant une phase vapeur plus riche en eau que ladite phase liquide et une phase liquide appauvrie en eau, et

6) le renvoi de la phase liquide constituée essentiellement d'agent de strippage issu de l'étape 5 vers l'étape 3.

In order to appreciably improve the dehydration of a natural gas or of a refinery gas, the regeneration of the liquid desiccant, in the process of the invention, can include a stripping operation for example by means of a stripping agent liquid at room temperature and pressure and forming a heteroazeotrope with water. Generally the stripping agent is a mixture of hydrocarbons mainly containing benzene. The regeneration process of the liquid desiccant can then be subdivided into the following 6 steps.

1) a step of reboiling the liquid desiccant loaded with water,

2) a step of distilling said desiccant comprising at least one distillation stage,

3) a step of stripping the partially regenerated liquid desiccant, during steps 1 and 2, with the vaporized stripping agent,

4) a step of condensing the steam leaving the distillation step 2, condensation generating two liquid phases, one predominantly in water, the other predominantly in stripping agent,

5) the heating of the liquid phase rich in stripping agent resulting from step 4, heating generating a vapor phase richer in water than said liquid phase and a liquid phase depleted in water, and

6) the return of the liquid phase consisting essentially of stripping agent from step 5 to step 3.

A particular embodiment of the method is described in more detail below in connection with FIG. 7. In this embodiment, the stripping agent liquid from step 4 is partially vaporized during a first heating stage, generating a vapor phase enriched with water, which is returned upstream of step 4 and a liquid phase depleted in water, which is vaporized before being sent to step 1.

This arrangement allows stripping the liquid desiccant by a vapor phase containing practically no more water and thus being able to obtain a very thorough regeneration of the liquid desiccant.

The charge to be treated arrives via line 4 at the head of the distillation device D1. After passing through the flash separation tank S1, it is sent by the line 7 to the exchanger E1, where it is heated by the liquid desiccant regenerated arriving by line 3. Leaving the exchanger El by line 7, the charge enters the distillation device D1, which overcomes successively from top to bottom a reboiling zone R2, a zone of stripping S2 and a tank B2.

The temperature in the reboiling zone R2 is generally understood between 150 ° C and 250 ° C.

The absolute pressure in the assembly consisting of the distillation device D1, reboiler R2, stripping zone S2 and balloon B2 is generally between 0.5 and 2 bar.

Most of the water and lighter products in the reboiler R2 as the desiccant absorbed by the latter are vaporized. The desiccant liquid depleted in water falls by gravity from reboiler R2 into the stripping S2, where it is brought into counter-current contact with the dehydrated stripping arriving in balloon B2 via line 15.

The regenerated liquid desiccant leaves the flask B2 through line 3, crosses exchanger E1, where it is cooled by the load arriving via line 7, and is reinjected at the head of the absorption column A1, by the pump P1.

Water, stripping agent and other products sprayed into the reboiler R2 leave the distillation device D1 via line 8, are mixed, if necessary if necessary, with the steam coming from the tank B3 via line 16, and cooled in the condenser C1. The partially condensed mixture enters the balloon B1.

From there, the lighter compounds are removed from the process in the form gas via line 9; the water is evacuated from the process by line 11 with the other hydrophilic products; stripping agent and other products hydrophobic are sent, saturated with water, by line 10 and through the pump P2, to the exchanger E5, where they are partially vaporized and sent by line 17 to balloon B3.

In general, the vapor phase generated in the E5 exchanger, plus rich in water that the liquid arriving via line 10, can be drained from the process. However, it is more advantageous to return it by line 16 in upstream of the condenser C1 with the steam leaving the distillation device D1 by line 8.

The liquid phase leaving balloon B3 via line 18, leaner in water than liquid arriving via line 10, is divided so as to keep constant the flow of stripping agent in the loop: a fixed part is sent to the evaporator E6 via line 20; possible excess, due to absorption by the desiccant of part of the gas stream treated during the step of dehydration, is evacuated from the process by line 19.

The vapor phase leaving the evaporator E6 via line 15 is sent to balloon B2.

We know that during the exploitation of a natural gas field, the composition of said gas may vary and have a variable richness in compounds aromatic, as described in "Glycol Experience in the Brae Field", J.H. Miller and K.A. O'Donnell, presented in London, at the Congress "Developments in Production Separation Systems "in March 1993. Also, the establishment of a stripping step, as described above, must be accompanied by a monitoring of the level of stripping agent. In case of gas production rich in aromatic compounds, the volume of stripping agent will increase during step 3, and occasionally the separator three-phase B1 must be purged and the excess of aromatic compounds sent to flash splitter B3. If the gas does not contain compounds aromatic, it will take charge of these compounds during stage 3. During from step 4, the majority liquid phase in water will condense, while the second majority liquid phase in stripping agent will have a low volume or will be nonexistent. As a result, the volume of stripping agent contained in the process may decrease and require top-ups. A mode of operation used in the North Sea to compensate for variations in the aromatics contained in the gas produced consists of alternating periods of normal use of the process with periods during which fuel gas is used as stripping agent. These last periods allow the constitution of a reserve of stripping agent.

When the stripping step is associated with the process of the invention, such a way of operating is no longer necessary. In fact, almost all of the compounds aromatic BTEX is recovered and concentrated in the flask three-phase B1 and BTEX can be advantageously used to overcome variations in volume of stripping agent.

The aromatics arriving in the charge accumulate in balloon B1 and the purge carried out through line 19 can be operated so as to maintain the constant amount of stripping agent in the B1 flask, per example by controlling the purge flow by level regulation.

The purge can be carried out either at the outlet of the B1 tank under control of level in balloon B1, i.e. at the exit of balloon B3 under level control in ball B3. The latter arrangement has the advantage of producing a dehydrated liquid fraction. This liquid fraction can either be remixed with the gas while being vaporized, or valued separately.

In the process of the invention, it may be advantageous to use at least one part 6 of the gaseous effluent 5 of the flash separation flask S1 as a gas fuel for reboiler R2.

Furthermore, the gaseous effluent 5 from the flash separation flask S1 can be injected into the three-phase balloon B1, where it can be injected partly condensed. The vapor joins that already separated in the balloon B1 and which leaves therefrom line 9 to be treated in the washing column L1 according to the invention. This possibility is shown in dotted lines in Figure 4.

It is still possible, alternatively, to install on the gaseous effluent 5 of the flash separation tank S1 a washing column L2 supplied at the head by regenerated liquid desiccant, with the same sampling possibilities and than those described above for the washing column L1.

The gaseous effluent leaving column L1 via line 14 is freed from the fraction of BTEX but is also dehydrated. It can therefore be recompressed by a compressor K1 and mixed with the gas treated as indicated on the diagram in Figure 4. Optionally, and depending on the composition of the gas to treat, relative flows of effluents 2, 5 and 14, effluent 5 or effluent gas from a washing column L2 treating effluent 5 can be associated with effluent 14. We can thus improve the production yield treated gas, which is an additional advantage of the process. Said effluent 14 can also be used as fuel for heating of reboiler R2 of regeneration system R1.

The following examples illustrate the invention.

EXAMPLES

In these examples, we consider a field of natural gas, producing 220 MSCFD (Millions of Standard Cubic Feet per Day) or 5.896 million (n) m ³ / day of gas whose dry composition is given in column 1 of Table 1. The molar mass of the dry gas is 21.5 g / mole, of which 0.37% by weight of BTEX. This gas is saturated with water at production temperature and pressure (51 ° C, 61 bar) and contains 390 kg of water per million m ³ .

Example 1 (comparative)

The gas is sent to a conventional working dehydration unit with TEG, as shown in Figure 1.

• le débit de TEG circulant dans le procédé est de 32 000 m³/j
• le TEG régénéré injecté en tète d'absorbeur A1 contient 1,2 % poids d'eau résiduelle,
• l'absorbeur A1 fonctionne à 51°C et 61 bar,
• le ballon de séparation flash S1 fonctionne à 85°C et 5 bar. La teneur en BTEX de l'effluent gazeux (7,49 kg/h) permet son utilisation comme fuel gaz. Toutefois les conditions locales ou une législation sévère peuvent entraíner son traitement.
• la température dans le rebouilleur de la colonne de régénération 4 est de 204°C,
• la régénération est faite à pression atmosphérique.

In this example:

• the flow of TEG circulating in the process is 32,000 m ³ / d
• the regenerated TEG injected at the top of the absorber A1 contains 1.2% by weight of residual water,
• the absorber A1 operates at 51 ° C and 61 bar,
• the flash separation tank S1 operates at 85 ° C and 5 bar. The BTEX content of the gaseous effluent (7.49 kg / h) allows its use as a fuel gas. However local conditions or severe legislation may cause its treatment.
• the temperature in the reboiler of the regeneration column 4 is 204 ° C.,
• regeneration is carried out at atmospheric pressure.

The composition of effluent 8 from regenerator R1 is described in column 2 of table 1. Such a unit rejects 56.9 kg / h of BTEX.

Example 2 (comparative)

The gas is dehydrated with a conventional unit having a condenser, lowering the temperature of the vapors from the regeneration column R1 at 55 ° C, and a three-phase gravity separation flask (Figure 2). All the operating conditions are identical to those of the example described above.

The composition of the gaseous effluent 9 of the three-phase flask is described in column 3 of table 1. Such a unit rejects 29.8 kg / h of BTEX.

Example 3 (according to the invention)

The gas is dehydrated with a unit having a condenser, lowering the temperature of the vapors from the regeneration column 4 at 55 ° C. and a three-phase gravity separation balloon. The vapors coming out of this balloon are taken up in a washing column L1 described in FIG. 4.

• la colonne de lavage comprend au moins trois étages théoriques,
• le débit de TEG régénéré 12 issu de la colonne de régénération et injecté en tête de colonne de lavage est de 500 kg/h.

In this example:

• the washing column comprises at least three theoretical stages,
• the flow rate of regenerated TEG 12 coming from the regeneration column and injected at the top of the washing column is 500 kg / h.

The composition of the effluent 14 from this column is described in column 4 of Table 1. Such a unit rejects only 3.9 kg / h of BTEX.

Claims (18)

1. A process for dehydrating a wet gas selected from natural gas and refinery gases comprising methane and other light alkanes, BTEX (benzene, toluene, ethylbenzene, xylene), water and possibly carbon dioxide, nitrogen and/or hydrogen sulphide using a hydrophilic liquid desiccant, with regeneration of said liquid desiccant, said process comprising:

   (a) a step for absorbing water and BTEX by contacting said wet gas with the liquid desiccant which has been regenerated in step (c), producing a dry effluent gas and a stream of liquid desiccant charged with water and BTEX;

   (b) a step for separating said charged liquid desiccant into a vapour containing mainly methane, water vapour and a portion of the BTEX, and a liquid phase containing mainly the liquid desiccant charged with water and BTEX;

   (c) a step for regenerating said liquid desiccant, comprising a reboiling zone and a distillation zone, in which the liquid desiccant charged with water and BTEX is sent to said distillation zone, from which a vapour containing water and BTEX and said regenerated liquid desiccant are extracted, which latter is sent as the desiccant to the inlet to said absorption zone of step (a);

   (d) a step for condensing the vapour from said distillation zone, followed by separation into three phases: a gaseous effluent containing BTEX, a liquid hydrocarbon phase containing BTEX and an aqueous liquid phase; and

   (e) treating at least said gaseous effluent containing BTEX in a washing zone by absorbing the BTEX with a fraction of the regenerated liquid desiccant taken from a point in the process and returning said desiccant, having absorbed the BTEX, to a point in the regeneration zone of step (b), the gaseous effluent leaving said washing zone having been freed of the BTEX.
2. A process according to claim 1, characterized in that:

   in step (a), the wet gas stream 1 is brought into contact with a counter-current of liquid desiccant 3 in absorption column A1, producing a dry gaseous effluent 2 leaving overhead and a stream of liquid desiccant 4 charged with water and BTEX which leaves the bottom of said absorption column A1;

   in step (b), the charged liquid desiccant 4 is sent, after passing inside the head of distillation column D1, to a flash separation drum S1, in which a vapour effluent 5 is separated which leaves overhead, containing mainly methane, water vapour and BTEX, and a liquid phase 7, containing mainly the liquid desiccant charged with water and BTEX, leaves from the bottom;

   in step (c), the desiccant stream 7 which is charged with water and BTEX is passed through a heat exchanger E1 to distillation column D1 of regeneration apparatus R1, which also includes a reboiler R2; from said regeneration apparatus, a vapour effluent 8 leaves overhead which contains water and BTEX and a liquid effluent 3 which constitutes the regenerated liquid desiccant leaves from the bottom, passes through heat exchanger E1 and is sent to the head of absorption column A1 of step (a);

   in step (d), said gaseous effluent 8 leaving overhead from distillation column D1 of regeneration apparatus R1 is condensed in a condenser C1 and sent to a three-phase separation drum B1, from which a gaseous effluent 9 containing BTEX leaves from its upper portion, a hydrocarbon phase 10 containing BTEX leaves as a side stream and an aqueous liquid phase 11 leaves the bottom;

   and in step (e), the gaseous effluent 9 is sent as an upflow to washing column L1, in which it is brought into contact with a counter-current of a liquid stream 12 which has been removed from the regenerated liquid desiccant circuit; a stream of liquid desiccant 13 which has absorbed the BTEX leaves the bottom of said washing column L1 and is returned to regeneration apparatus R1, and a gaseous effluent which is free of BTEX leaves overhead.
3. A process according to claim 2, characterized in that the stream of regenerated liquid desiccant 12 supplying the head of washing column L1 is removed from the regenerated liquid desiccant supply 3 to the absorption column A1.
4. A process according to claim 3, characterized in that the liquid desiccant 13, having absorbed the BTEX and leaving the bottom of washing column L1, is returned to the supply 7 to distillation column D1 of regeneration apparatus R1, upstream of heat exchanger E1.
5. A process according to claim 3, characterized in that the liquid desiccant 13, having absorbed the BTEX and leaving the bottom of washing column L1, is returned to the supply 7 to distillation column Dl of regeneration apparatus R1, downstream of heat exchanger E1.
6. A process according to claim 3, characterized in that the liquid desiccant 13, having absorbed the BTEX and leaving the bottom of washing column L1, is returned directly to the head of distillation column D1 of regeneration apparatus R1.
7. A process according to claim 2, characterized in that the stream of regenerated liquid desiccant 12 supplying the head of washing column L1 is removed from the reboiler R2 via a pump P1 and through a heat exchanger E2, in which it is cooled, and liquid desiccant 13, having absorbed the BTEX and leaving the bottom of washing column L1 is returned through heat exchanger E2, in which it is reheated, to reboiler R2.
8. A process according to any one of claims 2 to 7, characterized in that it further comprises a stripping step for the liquid desiccant to be regenerated.
9. A process according to claim 8, characterized in that stripping is carried out using a fraction of dry gas recovered as an effluent from absorption column A1.
10. A process according to claim 8, characterized in that a liquid stripping agent is used at ambient pressure and temperature and forms a heteroazeotrope with the water, the liquid desiccant regeneration process thus comprising:

    1) a reboiling step for the liquid desiccant charged with water;

    2) a distillation step for said desiccant comprising at least one distillation stage;

    3) a stripping step for the liquid desiccant which has been partially regenerated during steps 1 and 2, using the vaporised stripping agent;

    4) a step for condensing the vapour leaving distillation step 2, condensation generating two liquid phases, one of which is mainly water, the other of which is mainly stripping agent;

    5) heating the liquid phase which is rich in stripping agent from step 4, heating generating a vapour phase which is richer in water than said liquid phase and a liquid phase which is depleted in water; and

    6) returning the liquid phase which is constituted essentially by stripping agent from step 5 to step 3.
11. A process according to claim 10, characterized in that the stripping agent comprises aromatic hydrocarbons.
12. A process according to claim 10 or claim 11, characterized in that the hydrocarbon phase 10 containing BTEX leaving the three-phase drum B1 as a side stream is used to make up the stripping agent.
13. A process according to any one of claims 2 to 13, characterized in that at least a portion 6 of the gaseous effluent from flash separation drum S1 is used as a fuel to heat reboiler R2.
14. A process according to any one of claims 2 to 13, characterized in that the gaseous effluent 5 from flash separation drum S1 is injected into the three-phase drum B1.
15. A process according to any one of claims 2 to 14, characterized in that a washing column L2 supplied with regenerated liquid desiccant is installed for the gaseous effluent from flash separation drum S1.
16. A process according to any one of claims 2 to 15, characterized in that the gaseous effluent 14 from washing column L1 is recompressed and injected into dry gaseous effluent 2.
17. A process according to any one of claims 1 to 16, characterized in that said liquid desiccant is a glycol.
18. A process according to claim 17, characterized in that said glycol is triethyleneglycol.

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