Process for removing pollutants from a flue gas

Abstract

A process for removing impurities from a flue gas, comprising treating the flue gas with a liquid absorbent comprising (i) a precursor of chlorine dioxide and (ii) an organic ionic liquid, and releasing a purified flue gas into the atmosphere. The process is useful for removing Hg, SO.sub.2 and NOx.

Claims

1. A process for removing pollutants from a flue gas, comprising treating the flue gas with a liquid absorbent comprising (i) a precursor of chlorine dioxide, which is an aqueous solution of chlorite (ClO.sub.2.sup.) and (ii) an organic ionic liquid, wherein the aqueous chlorite and the ionic liquid are essentially immiscible with one another, and releasing a purified flue gas into the atmosphere, wherein the treatment comprises passing said flue gas through a treatment zone, contacting said flue gas with a circulated liquid absorbent comprising the aqueous chlorite solution and the organic ionic liquid, wherein the circulation of the liquid absorbent comprises discharging the absorbent from the treatment zone after it has been in contact with the flue gas, separating the discharged absorbent into aqueous and organic streams, driving said streams through a first and second lines, respectively, combining the separate aqueous organic streams and introducing the combined streams back into the treatment zone.

2. A process according to claim 1, wherein the absorbent is free from an acid other than an indigenously formed acid.

3. A process according to claim 1, wherein the pollutant is selected from the group consisting of Hg.sup.0, SO.sub.2, NO.sub.x and mixtures thereof.

4. A process according to claim 3, which further comprises recovering sulfate and/or nitrate from the aqueous stream flowing through the first circulation line or directing said aqueous stream to a site of use.

5. A process according to claim 1, which further comprises injecting fresh chlorite into the first circulation line.

Description

(1) In the drawings:

(2) FIG. 1 provides an illustration of a wet scrubber suitable for carrying out the process according to the invention.

(3) FIG. 2 provides photographs of a mixture of water and aliquat 336 before (A) and after the addition of the chlorite (B).

(4) FIG. 3 is a GC chromatogram.

(5) FIG. 4 is a schematic illustration of an experimental set-up used for the measurement of mercury absorption.

(6) FIG. 5 is a graph showing the results of the experiment for mercury absorption.

(7) FIG. 6 is a schematic illustration of an experimental set-up used for the measurement of SO.sub.2 or NOx absorption.

(8) FIG. 7 is a graph showing the results of the experiment for SO.sub.2 absorption.

(9) FIG. 8 is a graph showing the results of the experiment for NOx absorption.

(10) FIG. 9 is a graph showing the results of the experiment for NOx absorption.

EXAMPLES

(11) Materials

(12) Sulfur dioxide 5% (w/w) in nitrogen was purchased from Maxima gas supplier, Israel.

(13) Nitrogen oxide 5% (w/w) in nitrogen was purchased from Maxima gas supplier, Israel.

(14) Mercury, Methyl trioctyl ammonium Chloride/bromide, diethyl ether, butyl chloride, butyl bromide, 1-methylimidazole were purchased from Sigma Aldrich, Israel.

(15) Measurements

(16) Mercury concentration was determined using HG-MONITOR 3000 by Seefelder Messtechnik, Germany.

(17) SO.sub.2 concentration was determined using a 3SF CiTiceL analyzer from City Technology Ltd, gas analyzer manufactured by Emproco ltd Israel.

(18) NO.sub.x concentration was determined using T2NFF and T3NDH CiTiceL analyzer from City Technology Ltd, gas analyzer manufactured by Emproco ltd Israel.

(19) UV-Vis spectra were obtained using Cary 100 Bio spectrophotometer by Varian.

(20) Gas chromatography (GC) analysis was performed using Trace GC ULTRA manufactured by Thermo with TCD detector, RT Q plot 30 m 0.53 mm ID column and N.sub.2 gas as the carrier.

(21) XRD measurements were performed on D8 Advance of Bruker AXS.

Preparations 1-2

Preparation of 1-butyl-3-methylimidazolium Halide (Chloride and Bromide)

(22) 1-bromobutane (110 mmol, 15.07 grams) and 1-methylimidazole (100 mmol, 8.21 grams) were added to a 250 mL flask. The reaction mixture was stirred for 48 hours at 80 C. The resulting ionic liquid was then cooled, washed with ether (325 mL) to remove unreacted starting materials, and the product was dried under vacuum at 80 C. for 4 hours to afford 1-butyl-3-methylimidazolium bromide [BMIMBr] in a yield of 93% with 96% purity. The procedure was repeated using 1-chlorobutane to give the corresponding chloride.

Example 1

Preparation of a Liquid Absorbent and Identification of Chlorine Dioxide Formed In-Situ in the Absorbent

(23) Water and aliquat 336 are added to a flask. A mixture consisting of a lower aqueous phase and an upper organic phase is formed. Sodium chlorite is then added to the flask. The organic phase undergoes a visible color change, exhibiting a strong yellow color indicative of the formation of chlorine dioxide and its complexation in the organic phase. Photographs of the mixture before and after the addition of the chlorite are shown in FIG. 2, designated by the capital letters A and B, respectively. For example, 5 gr of water are mixed with 1 gr of Aliquat 336 (mixture A), followed by the addition of 0.1 gr of sodium chlorite (Mixture B). The color of the upper phase turns from pale yellow to strong yellow. IR analysis confirmed the formation of ClO.sub.2 in Mixture B.

(24) GC is used to qualitatively identify the chlorine dioxide generated in the mixture. The gas liberated upon heating the mixture was subjected to GC analysis and the chromatogram produced in shown in FIG. 3. The retention time of chlorine dioxide is 3.3 min. As indicated by the chromatogram, the reference ClO.sub.2 and the gas that was liberated from the mixture during the heating process are the same.

Example 2

Mercury Absorption

(25) FIG. 4 is a schematic illustration of the experimental setup used for the measurement of mercury absorption. An air stream flowing at a constant rate of about 1.5 liter/minute is introduced into a flask 101 which contains elemental mercury. The air stream 102 exiting flask 101, which has been contaminated with mercury vapors, is directed into a gas trap 103 loaded with 12 grams of the liquid absorbent described in Example 1. The outgoing air stream 104 which leaves the gas trap is analyzed by an analyzer 105 for the presence of mercury. The experimental setup includes a bypass 106 through which the mercury-containing air stream 102 is directly conveyed to the analyzer, without being subjected to the treatment with the liquid absorbent in 103.

(26) The results of the experiment are presented graphically in FIG. 5, in which the concentration of mercury is plotted as a function of time over an interval of about 450 seconds. At the beginning of the experiment the bypass 106 is open. Then, at time t.sub.150 seconds the bypass is closed such that the mercury-containing air stream 102 is forced to pass through the liquid absorbent 103. An instantaneous, sharp decrease in the concentration of the mercury in the outgoing air stream 104 is noted. Upon reopening bypass 106 at t.sub.250 seconds, the concentration of elemental mercury measured by the analyzer starts increasing again.

Example 3

SO.SUB.2 .Absorption

(27) The experimental setup used in this example is shown schematically in FIG. 6. A mixture of air and sulfur dioxide is made to flow through mass flow controller (by AALBORG) 202 into a gas trap 203 loaded with 12 g of the liquid absorbent of Example 1. The SO.sub.2 source 201 was a commercial 5% SO.sub.2 gas cylinder (in N.sub.2). The flow rates for the air and the 5% SO.sub.2 streams were 1 L/minute and 15 ml/minute, respectively. The concentration of SO.sub.2 in the outgoing air stream 204 leaving the gas trap was analyzed by analyzer 205. A bypass 206 is also provided, allowing the gaseous mixture to flow directly from the mass flow controller 202 to the analyzer 205, without being subjected to the treatment with the liquid absorbent in 203.

(28) The results are shown in FIG. 7 in the form of a graph in which the concentration of the contaminant sulfur dioxide in the treated, outgoing stream 204 is presented versus time. At the beginning of the experiment the bypass 206 is open. Then, at time t.sub.1100 seconds the bypass is closed such that the mixed air/SO.sub.2 stream is forced to pass through the liquid absorbent in 203. The concentration of sulfur dioxide in the outgoing stream 204 drops almost instantaneously to 0 ppm, indicating SO.sub.2 removal efficiency of 100% during the period of time in which the liquid absorbent of the invention was operating (at t.sub.2=1100 seconds the bypass 206 is reopened).

(29) The presence of the oxidation product, i.e. the sulfate, in the aqueous phase of the absorbent was confirmed through the addition of calcium chloride. The addition resulted in the formation of a precipitate in the aqueous phase of the absorbent. X-ray powder diffraction analysis confirmed that the precipitate formed is calcium sulfate.

Example 4

NO.SUB.x .Absorption

(30) The experimental setup shown in FIG. 6 was used also for absorbing nitric oxide from an air stream. To this end, instead of the SO.sub.2 cylinder, NO cylinder (5% w/w in nitrogen) was used. The flow arrangement and conditions are as set forth in Example 3.

(31) The concentration against time curve for the experiment is shown in FIG. 8. A gradual decrease at the concentration of NO.sub.x in the outgoing air stream took place at the time interval between t.sub.210 to t.sub.220 seconds, following the closure of the bypass. At the time interval between 20 to about 60 seconds, during which the bypass was closed and the air stream was forced to flow through the absorbent, NO.sub.x removal efficiency of 100% was achieved. The increase at the NO.sub.x concentration measured in the outgoing air stream at t.sub.360 seconds and afterward is due to the reopening of the bypass.

(32) The concentration of the oxidation productthe nitrate ionin the aqueous phase of the absorbent was measured as a function of time and the results are graphically presented in FIG. 9. The concentration versus time curve exhibits linear relationship, indicating the gradual accumulation of the nitrate in the aqueous phase.