METHOD FOR PURIFYING METHANE-COMPRISING GAS

Abstract

The invention is directed to the purification of gas streams comprising methane and hydrophobic pollutants. The invention provides a method for purifying a gas stream comprising methane and one or more hydrophobic pollutants comprising the steps of contacting the gas stream with a lean liquid stream comprising micelles of a surfactant or a separate surfactant phase. The resulting rich liquid stream comprises at least part of the hydrophobic pollutants. By subsequently changing the temperature of the rich liquid stream, a system of a pollutants-poor phase and a pollutants-rich phase is obtained. These phases are optionally separated and from the aqueous stream the lean liquid stream comprising micelles and/or surfactant can be regenerated.

Claims

1. A method for purifying a gas stream comprising methane and one or more hydrophobic pollutants having a logP-value of at least 0.5, said method comprising the steps of a) contacting said gas stream with a lean liquid stream comprising an aqueous liquid and a surfactant to obtain a purified gas stream and a rich liquid stream comprising at least part of said hydrophobic pollutants, wherein the temperature of the lean liquid is above the critical micelle temperature; b) changing the temperature of the rich liquid stream to obtain a pollutants-poor phase and a pollutants-rich phase; c) optionally separating said pollutants-poor and pollutants-rich phases to obtain a separated aqueous stream comprising said liquid and a separated pollutants stream comprising said pollutants; and d) regenerating said lean liquid comprising surfactants.

2. The method according to claim 1 wherein the hydrophobic pollutants comprise one or more compounds selected from the group consisting of siloxanes, organo sulfur components, hydrocarbons comprising more than five carbon atoms and combinations thereof.

3. The method according to claim 1 wherein the surfactant is a non-ionic surfactant.

4. The method according to claim 1 wherein changing the temperature in step b) is increasing the temperature of the rich aqueous liquid to above the cloud point temperature of the liquid.

5. The method according to claim 1, wherein changing the temperature in step b) is decreasing the temperature to below the critical micelle temperature of the liquid.

6. The method according to claim 1 wherein said lean aqueous liquid further comprises an additive that influences the cloud point temperature and/or critical micelle temperature.

7. The method according to claim 1 wherein step b) further comprises the addition of an additive that influences the cloud point temperature and/or the critical micelle temperature of the rich liquid.

8. The method according to claim 1 wherein step a) takes place at a temperature between 0 to 50 C. and/or at a pressure of about atmospheric pressure.

9. The method according to claim 1 wherein the step of regenerating said lean liquid comprises changing the temperature of the separated aqueous stream in which the surfactant is present.

10. The method according to claim 1 wherein the surfactant is a block-copolymer.

11. The method according to claim 1 wherein the surfactant is a poloxamer.

Description

EXAMPLE 1

[0051] 200 g aqueous solution of 1 wt % Pluronic L31 was prepared having a cloud point temperature of about 40 C. The solution was heated to 35 C. and a gas mixture, which contained about 2 g/m.sup.3 pinene (pollutant) in a 100 ml/min N.sub.2 flow, was continuously added to the solution. The gas was dispersed into the liquid as small bubbles using a sparger. The contact time between the gas and the liquid was estimated to be less than 1 second. At given time intervals, the solution was visually inspected to assess the presence of a foam layer, and the pinene concentration in the outlet gas was measured using gas-chromatography. Almost complete pinene removal was observed at the start of the experiment, and negligible foaming was observed during the entire experiment. As time progressed, the outlet pinene concentration increased, reaching a more or less stable concentration after 49 min.

EXAMPLE 2

[0052] Example 1 was repeated, but as aqueous solution a 1 wt % Pluronic L81 having a cloud point of 19 C. was used. The solution was at room temperature while the gas mixture containing about 2 g/m.sup.3 of pinene in a N2 flow of 100 ml/min was continuously added to the solution. Results were comparable to Example 1 and summarized in Table 1.

[0053] The results from Examples 1 and 2 illustrate the absorbance ability of the surfactant solutions to remove terpenes contaminants from a gas stream.

TABLE-US-00001 TABLE 1 Example 1 2 Surfactant type Pluronic L31 Pluronic L81 Surfactant concentration (wt %) 1 1 Solution cloud point temperature ( C.) 40 19 Solution initial temperature ( C.) 35 20 Solution mass (g) 200 200 Gas flow (ml/min) 100 100 Contaminant type Pinene Pinene Contaminant feed concentration (g/m.sup.3) 1.7 1.8 Minimum contaminant outlet concentration <0.05 <0.05 (g/m.sup.3) Stabilization time for contaminant outlet 49 34 concentration (min)

EXAMPLE 3

[0054] Three liter of aqueous solution of 5 wt % Pluronic L81 was prepared having a cloud point temperature of about 17 C. The solution was heated to 25 C. obtaining a clouded system comprising a dispersion of an aqueous solution and a surfactant-rich phase. A gas mixture, which contained about 0.8 g/m3 limonene (pollutant) in a 500 ml/min N.sub.2 flow, was continuously added to the solution. The gas was dispersed into the liquid as small bubbles using a sparger. The contact time between the gas and the liquid was estimated to be less than 10 seconds.

[0055] At given time intervals, the solution was visually inspected to assess the presence of a foam layer, and the limonene concentration in the outlet gas was measured using gas-chromatography (see FIG. 6). Almost complete limonene removal was observed at the start of the experiment, and negligible foaming was observed during the entire experiment. As time progressed, the outlet limonene concentration slowly increased, reaching a level of about 20% of the feed concentration after 6 hours 45 min, upon which the absorption experiment was stopped. It is noted that the conditions in this test are such that the solution absorbs the pollutant above the cloud point temperature.

[0056] Subsequently, the solution was cooled to 5 C., obtaining a clear solution, and a 500 ml/min N2 flow was continuously added to the solution, acting as a purge flow. As detected by gas-chromatography (see FIG. 7), this caused the release of limonene from the solution, at even higher concentrations than the feed concentration during absorption, indicating a fast release of the contaminant limonene. After 6 hours of purging, the limonene concentration in the outlet gas flow was almost zero again, indicating that the absorption solution was regenerated and suitable for absorption again.

EXAMPLE 4

[0057] In order to determine the cloud point temperatures of various surfactant solutions, and the effect that additives can have on this temperature, cloud point measurements have been performed. For about 1 mL of solution of known composition, the temperature was slowly (0.25 C./min) increased and decreased within a predefined temperature range, while continuously stirring and monitoring the solution's light transmission properties. The cloud point temperature could be detected by a sudden change in light transmission. Table 2 gives an overview of the performed cloud point measurement results.

TABLE-US-00002 TABLE 2 Surfactant Additive Cloud point Type Concentration Type Concentration Temperature L31 5 wt % 32.7 C. L31 20 wt % 25.0 C. L81 1 wt % 20.6 C. L81 5 wt % 19.4 C. L81 10 wt % 17.1 C. L81 5 wt % SDS 29 mg/kg 22.1 C. L81 5 wt % SDS 91 mg/kg 32.9 C. L81 10 wt % SDS 29 mg/kg 26.9 C. L81 10 wt % SDS 91 mg/kg 31.3 C. L81 10 wt % SDS 288 mg/kg 36.3 C.

[0058] The results indicate that the cloud point temperature of the solutions can i.a. be increased by: 1) decreasing the size of the Pluronic molecules, 2) decreasing the concentration of the surfactant, 3) increasing the concentration of the additive SDS (sodium dodecyl sulfate).