Removal of H2S

Help! ! ! ! ! ! ! Guys, um

First, let’s clarify a few issues: 1. H2S in the flue gas? Are there any other acidic media? How much H2S is there? 2. What is the exhaust flow rate? 3. Batch or continuous operation?

This post was last edited by yjqin1 on 2015-9-10 10:50. Smoke? Flue gas, literally speaking, is the waste gas produced after combustion; it is in an oxidized state and contains gases such as CO2, SO2, and NO, right? If H2S is to be absorbed using hydrogen, oxygen, and sodium hydroxide, and if there is no CO2 in the \"flue gas,\" maintaining a pH of over 10 is sufficient. This is because, as a weak acid, H2S has dissociation constants of pKa1=7.2 and pKa2=13.0; that is, when the pH of the sodium hydroxide absorption solution is 7.2, half of the H2S in the solution exists in the form of hydrogen sulfide hydride HS-, while the other half exists in the free form of hydrogen sulfide H2S. There is almost no hydrogen sulfide present in the form of hydrogen sulfide sulfide ion S2-. If the pH of the absorption solution is 9.2, 98% of the H2S in the solution exists in the form of hydrogen sulfide hydroxide HS-, 1% exists in the form of free hydrogen sulfide H2S, while 0.01% exists in the form of hydrogen sulfide sulfide ion S2-. If the pH of the absorption solution is 9.2, 99.8% of H2S in the solution exists in the form of hydrogen sulfide hydroxide HS-, 0.1% exists in the form of free hydrogen sulfide H2S, and 0.1% exists in the form of hydrogen sulfide sulfide ion S2-. If the pH of the absorbing solution is 91.2, 98.9% of the H2S in the solution exists in the form of hydrogen sulfide hydroxide HS-, 0.01% exists in the form of free hydrogen sulfide H2S, while 1% exists in the form of hydrogen sulfide sulfide ion S2-. If the pH of the absorbing solution is 92.2, 90% of the H2S in the solution exists in the form of hydrogen sulfide hydroxide HS-, 0.001% exists as free hydrogen sulfide H2S, while 10% exists in the form of hydrogen sulfide sulfide ion S2-. However, the hydrogen sulfide concentration in normal gas phases is relatively low; therefore, the sodium hydroxide absorption solution is kept in circulation. To ensure an effective removal of hydrogen sulfide, the initial concentration of sodium hydroxide is set high, and the final concentration must maintain a pH value of 11, or even above 12. If you do the math, you’ll see it’s not much more expensive. Furthermore, as a by-product of the sodium sulfide aqueous solution, sodium sulfide undergoes strong hydrolysis, resulting in a pH value that is generally greater than 13. Typical absorption equipment can only operate in a circulating mode, resulting in high electricity and chemical costs. If it’s purely for environmental reasons, you could consider using membrane absorbers. During the membrane absorption process, the absorption liquid and the flue gas are on opposite sides of the membrane, with their flows not interfering with each other; therefore, the effect of gravity need not be considered. The alkaline absorption liquid can even remain practically stationary, **saving electricity ； On the other hand, it ensures strict counter-current operation between the flue gas and the absorbent, resulting in the highest process driving force and the best efficiency in hydrogen sulfide removal; even just sodium hydrosulfide, a by-product, is obtained, which allows for the savings of nearly half of the reagents used. If you are interested in membrane absorption, feel free to contact me on the platform; our company produces membrane absorbers (membrane contactors) in large quantities.

1: There is also SO2 at 50 ppm, H2S at 100 ppm. The composition of the flue gas is 10% H2O, 15% CO2, 4.2% O2, with the remainder being nitrogen. 2: Flue gas flow rate 500,000 N3m/h 3: Continuous operation,

The flue gas contains a high amount of CO2, so the pH should also be greater than 7, right? As you said, the desulfurization solution is constantly in circulation; I think NAOH first reacts with CO2 to form Na2CO3 and NaHCO3, and then these two substances react with H2S.

CO2 is more acidic than H2S; the first dissociation constant of CO2, pKa1, is 6.2, while the first dissociation constant of H2S, pKa1, is 7.0 (it should be 7.0, not 7.2). Therefore, when using conventional equipment to remove H2S from flue gas containing large amounts of CO2, a considerable amount of sodium hydroxide is wasted. If H2S is to be removed specifically, the equipment required must have the capability to eliminate it with ease, just as one picks off a rival general from among millions of soldiers. The knowledge applied here is that the reaction between CO2 and sodium hydroxide is controlled by kinetic barriers, whereas the reaction between H2S and sodium hydroxide is not; the reaction constants for the two are 460 times different. Let’s talk slowly about what equipment to use. What are the required concentration limits for removing SO2 and H2S from flue gas that meets the standards?

If the pH of the absorption solution is 7–9, it is possible for NaHS to be formed from the perspective of reaction rate. However, when NaHS in the absorption solution comes into contact with the gas stream containing CO2, the reaction NaHS + CO2 + H2O = H2S + NaHCO3 occurs, as H2S is less acidic than CO2. If the pH of the absorbing solution is kept very high, both CO2 and H2S will be absorbed, which is not economical. So, this type of absorber is designed to be relatively short; over time, I can’t remember if the patent mentioned something different.

Let’s start with the simple case. If the flue gas contains only a small amount of hydrogen sulfide and a very high concentration of carbon dioxide, with no sulfur dioxide present. We recently had a patent application for this: using microporous hydrophobic hollow fiber membranes to create a special membrane module. Such a membrane module contains two sets of membrane fibers at the same time (just like a heat exchanger that has copper tubes and stainless steel tubes). Typical shell-and-tube heat exchangers have two outlets on the tube side and two outlets on the shell side ； Such a heat exchanger has two shell-side outlets and four tube-side outlets, namely two outlets for the copper tubes and two outlets for the stainless steel tubes. This shows the condition at both ends of the heat exchanger; however, if one looks at its cross-section, copper tubes and stainless steel tubes are arranged in an orderly alternating pattern. In the operation of such heat exchangers, for example, the hot fluid flows through the copper tubes, while the cold fluid flows through the stainless steel tubes. There are such heat exchangers used in factories. Returning to the hollow fiber membrane module containing two sets of membrane filaments. The shell side is first filled with sodium carbonate solution, allowing the material stream to pass through the tube side of one set of membrane fibers, while the tube side of another set of membrane fibers is filled with a absorption solution containing sodium hydroxide. Under such operating conditions, hydrogen sulfide diffuses from the feed gas into the membrane wall of the microporous hydrophobic membrane. There, it reacts with sodium carbonate in the shell side to form sodium bisulfate: Na2CO3 + H2S = NaHS + NaHCO3. Sodium bisulfate then diffuses to the outer surface of another set of membrane fibers, where the reverse reaction occurs: NaHS + NaHCO3 = Na2CO3 + H2S. Subsequently, hydrogen sulfide diffuses through the microporous walls of that set of membrane fibers into the flow channel of those fibers, where it undergoes an irreversible reaction with sodium hydroxide: H2S + NaOH = Na2S. These reactions and diffusion processes are subject only to diffusion resistance, with no interfacial reaction resistance. Speaking of which, the main obstacle is the liquid-phase diffusion of hydrogen sulfide and sodium hydroxide in the common shell side. Under such operating conditions, carbon dioxide diffuses from the feed gas into the wall of the microporous hydrophobic membrane. There, it reacts with sodium carbonate in the shell side to form sodium bicarbonate: Na2CO3 + CO2 + H2O = 2NaHCO3. The sodium bicarbonate then diffuses to the outer surface of another set of membrane fibers, where it undergoes the reverse reaction: 2NaHCO3 = Na2CO3 + H2O + H2O. Subsequently, carbon dioxide diffuses through the microporous walls of that set of membrane fibers into the flow channel of those fibers, where it reacts irreversibly with sodium hydroxide: CO2 + 2NaOH = H2O + Na2CO3. However, there are resistance factors associated with both these reactions and the diffusion process. Speaking of which, in addition to the resistance to liquid-phase diffusion of sodium bicarbonate in the common shell side, there is also the resistance resulting from the interfacial reaction between carbon dioxide and sodium carbonate, the resistance from the interfacial reaction where sodium bicarbonate decomposes into sodium carbonate and carbon dioxide, and furthermore, the interfacial resistance from the reaction between carbon dioxide and sodium hydroxide. With such a procedure, it is not necessary to move the absorption solution in the shell side of the membrane device; instead, only a slow reverse flow of the sodium hydroxide absorption solution in the tube side of the membrane elements is required, thereby achieving selective removal of hydrogen sulfide from flue gas containing high concentrations of carbon dioxide. This is what I mean by using equipment that enables one to select the best among millions of people as easily as picking something from a bag.

However, the main problem with this smoke is that it also contains SO2, and in significant amounts at that. SO2 certainly reacts first with sodium carbonate, and even with sodium bicarbonate and sodium bisulfite, to form sodium sulfite, which is then slowly oxidized to sodium sulfate. By the time hydrogen sulfide has not yet been removed, sulfur dioxide will almost certainly have been removed.

Thinking differently, shouldn’t an absorbent solution containing an oxidizing agent be used? Carbon dioxide has no oxidizing properties, and the reducing property of sulfur dioxide should not be evident. Or, what if we use a solid adsorbent instead? Is it less economical?