Hydrogen Sulfide Removal in Liquid and Gas Streams

Abstract

The invention relates to water-soluble and/or dispersible ferric hydroxide in a strong base that is suitable for hydrogen sulfide removal in contaminated liquid and gas streams. The water-soluble and/or dispersible ferric hydroxide in a strong base may be introduced into a liquid stream to remove hydrogen sulfide. The water-soluble and/or dispersible ferric hydroxide may be further incorporated into a solid substrate for gas phase hydrogen sulfide removal.

Claims

1. Compositions and products comprising water-soluble/dispersible ferric hydroxide in a strong base may be prepared: A first aqueous solution of iron salts at a concentration ranging preferably from 10 to 80% by weight, more preferably from 20 to 70%, most preferably from 30 to 60%, wherein the water-soluble iron salts may include iron chloride, iron sulfate, and iron nitrate; a second aqueous solution of small organic molecules containing one or more hydroxyl and/or carboxylic functional groups at a concentration ranging preferably from 10 to 80% by weight, more preferably from 20 to 80%, most preferably from 40 to 80%, wherein the small organic molecules include hydroxyl acetic acid, tartaric acid, ascorbic acid, and gluconic acid; a third aqueous solution of a strong base at a concentration ranging preferably from 10 to 60% by weight, more preferably from 20 m to 50%, most preferably from 30 to 50%, wherein the strong base includes sodium hydroxide, potassium hydroxide, and lithium hydroxide; the first solution is mixed with the second solution in a molar ratio preferably from 1:10 to 10:1, more preferably from 1:5 to 5:1, most preferably from 1:2 to 2:1; the third solution is allowed to react with the above mixture to afford water-soluble and/or dispersible ferric hydroxide in a strong base.

2. The compositions and products in claim 1 may be injected into a liquid stream contaminated with hydrogen sulfide to raise the pH of the liquid stream and to reactively remove hydrogen sulfide without concerns for excessive solid production in the liquid stream.

3. The compositions and products in claim 1 may be incorporated into solid substrates, preferably biobased substrates, wherein the biobased substrates include wood chips, wood shavings, shredded crop hulls, husks, stalks, coconut shells, and other plant-derived organic matters, to form solid scavengers. Such scavengers may be packed in a fixed bed system for the removal of hydrogen sulfide in contaminated gas streams.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows the reaction kinetics between nanosized ferric hydroxide and a sulfide solution

[0012] FIG. 2 compares hydrogen sulfide breakthrough in chemical scrubbing of hydrogen sulfide from biogas

[0013] FIG. 3 compares hydrogen sulfide breakthrough for activated carbon and nanosized ferric hydroxide impregnated into a biobased substrate

DETAILED DESCRIPTION OF THE INVENTION

[0014] One or more embodiments relate generally to the treatment of hydrogen sulfide in contaminated liquid and gas streams, such as in wastewater collection systems and treatment facilities. For liquid phase application, the invention discloses compositions comprising water-soluble and/or dispersible ferric hydroxide in a strong base and methods for the preparation of said compositions. For gas-phase application, the invention discloses compositions comprising water-soluble and/or dispersible ferric hydroxide in a strong incorporated into solid substrates to form solid scavengers and methods for the preparation of said solid scavengers.

[0015] One or more embodiments relate generally to reaction products between iron salts, organic compounds containing both hydroxyl and carboxylic functional groups, and strong bases. The products may be prepared through a procedure described below, but modifications may be made without departing from the spirit of this invention. A first aqueous solution of iron salts is prepared at a concentration preferably ranging from 10 to 80% by weight, more preferably from 20 to 70%, most preferably from 30 to 60%, wherein the water-soluble iron salts may include iron chloride, iron sulfate, and iron nitrate; a second aqueous solution of water-soluble small organic molecules containing one or more hydroxyl and/or carboxylic functional groups at a concentration preferably ranging from 10 to 80% by weight, more preferably from 20 to 80%, most preferably from 40 to 80%, wherein the small organic molecules include hydroxyl acetic acid, tartaric acid, ascorbic acid, and gluconic acid; a third aqueous solution of a strong base at a concentration preferably ranging from 10 to 60% by weight, more preferably from 20 to 50%, most preferably from 30 to 50%, wherein the strong base includes sodium hydroxide, potassium hydroxide, lithium hydroxide; the first solution is mixed with the second solution at a molar ratio preferably from 1:10 to 10:1, more preferably from 1:5 to 5:1, most preferably from 1:2 to 2:1; the third solution is allowed to react with the above mixture to afford water-soluble and/or dispersible ferric hydroxide in a strong base.

[0016] Without wishing to be bound by any particular theory, the water solubility and/or dispersibility of ferric hydroxide is postulated to be attributable to the formation of nanosized ferric hydroxide particles, which are assisted and stabilized by the organic compounds containing hydroxyl and carboxylic function groups. The particle size distribution of a sample prepared in accordance with the above-described procedure is in the range of 11-77 nm. The particle size distribution analysis is conducted on a Malvern Zetasizer Nano ZS dynamic light scattering instrument, following ISO 22412:2008 Particle Size Analysis—Dynamic Light Scattering (DLS) and ASTM E2490-09 (2015) Standard Guide for Measurement of Particle Size Distribution of Nanomaterials in Suspension by Photon Correlation Spectroscopy (PCS).

[0017] In accordance with one or more embodiments, the nanosized ferric hydroxide in a strong base may be injected into liquid streams in wastewater collection systems to control hydrogen sulfide. Hydrogen sulfide control products used in liquid phase treatment generally fall into one of three main categories: inhibition of sulfide formation, reactive removal of sulfide after its formation, and elevation of pH to keep hydrogen sulfide from gas off. The disclosed compositions of nanosized ferric hydroxide in a strong base combines the performance attributes of two categories of products: reactive removal of hydrogen sulfide and pH elevation. The nanosized ferric hydroxide is responsible for fast and selective reaction with sulfide while the strong base is responsible for raising the pH of the liquid stream. Advantageously, a single product may be dosed into the wastewater collection system with a single chemical dosing system rather than two separate chemical dosing systems at two different sites as disclosed in prior arts to achieve the same level of treatment. Cost savings in both capital and operating expense may be realized as a result.

[0018] The reaction kinetics between nanosized ferric hydroxide and sulfide is shown in FIG. 1. In the experiment, a standard sulfide solution of 86 ppm is prepared and an equal molar ratio of nanosized ferric hydroxide is added to the solution. The reaction is followed with a Thermo Scientific Orion Silver/Sulfide combination ion-selective electrode, using a Thermo Scientific Orion 920A+ Advanced ISE/pH/MV/ORP meter with a BNC connection. The reaction is near completion after 60 seconds, with >90% reductions in sulfide concentration.

[0019] To demonstrate the superior performance of disclosed compositions to existing products, a laboratory-scale experiment is described. Wastewater samples are taken from a wastewater treatment plant and dissolved sulfide concentration is measured. Aliquots of 250 ml wastewater samples are transferred into BOD bottles and are dosed with magnesium hydroxide slurry, sodium hydroxide solution, nanosized ferric hydroxide, and ferric chloride solution respectively, at an equal molar ratio to dissolved sulfide. Gas-phase hydrogen sulfide is monitored qualitatively using lead acetate impregnated filter paper which is placed above the opening of the BOD bottles. The result showed that nano ferric hydroxide is most effective in controlling sulfide as indicated by almost no color change on the lead acetate paper. In comparison, wastewater samples treated with the other conventional products all showed variable shades of black colors on the lead acetate paper.

[0020] In accordance with one or more embodiments, the disclosed compositions overcome the drawbacks associated with conventional iron salt which tends to act as a coagulant if not first reacted with sulfide. Such products may be advantageously dosed into the wastewater collection systems to control hydrogen sulfide without concerns for excessive solid production. To demonstrate this, two of three 50 ml wastewater samples were added 10 microliters of 45% ferric chloride solution and 10 microliters of nanosized ferric hydroxide respectively, and the third sample was used as control. Total suspended solids (TSS) were measured after a contact time of one hour. The nanosized ferric hydroxide treated sample has a TSS of 204 ppm, which is the same as the control sample, while the ferric chloride treated sample has a TSS of 226 ppm, which represents an 11% increase in solid production.

[0021] In accordance with one or more embodiments, the nanosized ferric hydroxide in a strong base may be used to chemically scrub hydrogen sulfide from biogas produced in anaerobic digesters. In a laboratory-scale trial, a bubbler filled with 100 mml scrubbing solution is connected to digester gas at a wastewater treatment plant at a flow rate of 100 ml/minute. The concentration of hydrogen sulfide is measured continuously from the outlet of the bubbler with a hydrogen sulfide data logger until a breakthrough concentration of 15 ppm is reached. As shown in FIG. 2, the ferric chloride solution showed almost immediate breakthrough while the breakthrough for sodium hydroxide solution didn't occur until after 30 minutes. In comparison, the breakthrough for nano ferric hydroxide in a strong base didn't occur until after 130 minutes.

[0022] In accordance with one or more embodiments, the nanosized ferric hydroxide may be incorporated into a solid substrate to form a solid scavenger, wherein the solid scavenger may be used to remove hydrogen sulfide from contaminated gas streams. Surprisingly, solid scavengers prepared by incorporating nanosized ferric hydroxide into lignocellulose or cellulose biobased substrates, wherein the biobased substrates include wood, crop hulls, husks, stalks, coconut shells, and other plant-derived organic matters, were found to possess high removal capacity for hydrogen sulfide at fast reaction rate equivalent to activated carbon when evaluated under the same test conditions. FIG. 3 is the comparison between nano ferric hydroxide impregnated wood shavings and a high capacity GAC for gas-phase hydrogen sulfide removal. The evaluation follows ASTM D6646-03, Standard Method for Determination of the Accelerated Hydrogen Sulfide Breakthrough Capacity of Granular and Pelletized Activated Carbon. For the gas-phase treatment of hydrogen sulfide in the wastewater collection systems and treatment facilities, activated carbon has been and remains to be the solid scavenger of choice in packed/fixed-bed treatment systems. The favorable performance attribute provided by activated carbon is attributed to its highly porous structure and extremely high surface area. The biobased substrate, on the other hand, has a very low surface area and non-porous structure. The fact that a solid scavenger made with a biobased substrate has equivalent hydrogen sulfide removal capacity to activated carbon is unexpected. Activated carbon is produced in an energy-intensive process and thus is expensive. The disclosed compositions may bring about substantial economic and environmental benefits to gas stream treatment.

EXAMPLE 1

[0023] The first solution of 40% ferric chloride is prepared by dissolved 40 g anhydrous ferric chloride in 100 g water. The second solution of 50% gluconic acid is prepared by dissolving 50 g gluconic acid in 100 g water. The first solution is mixed with the second solution in a molar ratio of between 1:0.5 to 1:2. The third solution of 40% sodium hydroxide is prepared by dissolving 40 g sodium hydroxide in 100 g water. Sodium hydroxide solution is added to the mixture of ferric chloride and gluconic acid until a water-soluble dispersion is obtained.

EXAMPLE 2

[0024] 70 ml of products prepared in Example 1 is mixed with 70 ml 10N sodium hydroxide solution. The mixture is added to 400 ml shredded wood and dried in an oven at 100° C. for 2 hours. Its hydrogen sulfide breakthrough capacity is measured following ASTM D 6646-3.

[0025] While particular embodiments of the compositions, products, and methods for the preparation of said compositions and products have been shown and described, it will be appreciated by those skilled in the art that changes and modifications may be made without departing from the inventions in its broader aspects and as outlined in the following claims.