A device for waste gas scrubbing in a urea plant may be configured such that a waste gas passes along a transportation direction in the duct. The duct may include a first region for removing urea dust particles from the waste gas and a second region for removing chemical compounds from the waste gas, which can be integrated by an acid-base reaction into an aqueous liquid phase. A cross-sectional area of the duct extending perpendicular to the transportation direction in the second region may be greater than a cross-sectional area extending perpendicular to the transportation direction in the first region. Further, the device may be configured such that the duct extends horizontally at least in sections and/or the transportation direction of the waste gas through the duct extends horizontally in an installed state.

A processing unit for a liquid washing medium contaminated with sulphur oxides and/or nitrogen oxides, has an evaporation stage for concentrating the active components of the washing medium by an evaporator and/or by a heat exchanger, and has a collecting tank connected to the evaporator and/or to the heat exchanger. The collecting tank is configured as a crystallizer for removing sulfur oxides from the washing medium by crystallization of a sulphate, in particular of potassium sulphate. A separating device for carbon dioxide has a corresponding processing unit, and a method for processing a washing medium contaminated with sulphur oxides and/or nitrogen oxides uses a corresponding processing unit.

An absorbent for selective removal of hydrogen sulfide over carbon dioxide from a fluid stream comprises an aqueous solution of a) a tertiary amine, b) a sterically hindered secondary amine of the general formula (I)

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in which R.sub.1 and R.sub.2 are each independently selected from C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl; R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are each independently selected from hydrogen, C.sub.1-4-alkyl and C.sub.1-4-hydroxyalkyl, with the proviso that at least one R.sub.4 and/or R.sub.5 radical on the carbon atom bonded directly to the nitrogen atom is C.sub.1-4-alkyl or C.sub.1-4-hydroxyalkyl when R.sub.3 is hydrogen; x and y are integers from 2 to 4 and z is an integer from 1 to 4; where the molar ratio of b) to a) is in the range from 0.05 to 1.0, and c) an acid in an amount, calculated as neutralization equivalent relative to the protonatable nitrogen atoms in a) and b), of 0.05 to 15.0%. One preferred amine of the formula I is 2-(2-tert-butylaminoethoxy)ethanol. The absorbent allows a defined H.sub.2S selectivity to be set at pressures of the kind typical in natural gas processing.

Disclosed are methods and systems for removing submicron particles from a gas stream, in particular from urea prilling off-gas, wherein a Venturi ejector is used. A method comprises contacting a gas stream containing submicron particles in a Venturi ejector with an injected high velocity scrubbing liquid to provide a pumping action, wherein the scrubbing liquid has an initial velocity of at least 25 m/s and wherein the ratio of scrubbing liquid and gas flow is between 0.0005 and 0.0015 (m.sup.3/h)/(m.sup.3/h).

A process for purifying exhaust air from a granulation plant for producing a urea-containing granulate may involve causing a gas stream containing a urea-containing dust and ammonia to contact a sulfuric acid solution or a nitric acid solution in a scrubbing process. The scrubbing process may comprise scrubbing the gas stream with a first weakly acidic scrubbing solution in a first scrubbing stage and then scrubbing the gas stream exiting the first scrubbing stage with a second scrubbing solution having a lower pH than the first weakly acidic scrubbing solution in a second scrubbing stage. The acidic scrubbing solution generated in the second scrubbing stage may be recycled into the first scrubbing stage via a conduit and used therein as the first weakly acidic scrubbing solution. A pre-scrubbing stage where dust is scrubbed out of the gas stream may additionally be arranged upstream of the first scrubbing stage.

A complex malodor removing equipment includes: a neutralizing module which dissolves a portion of malodor-causing substances, in malodorous gas introduced from malodor-producing equipment, in liquid water and removes same, which includes an acidity neutralizing module that introduces an alkaline substance from outside and removes an acidic malodor-causing substance from the malodor-causing substances, and an alkaline neutralizing module that introduces an acidic substance from outside and removes an alkaline malodor-causing substance from the malodor-causing substances, and which connects the acidity neutralizing module and the alkaline neutralizing module; and a balancing module which dissolves the remainder of the malodor-causing substances, in the malodorous gas introduced from the neutralizing module, in water and removes same, which includes an oxidation balancing module that introduces an oxidizing agent from outside and balances the malodor-causing substances, and a reduction balancing module that introduces a reducing agent from outside and balances the malodor-causing substances.

Disclosed is a method for the removal of ammonia from the off-gas of a finishing section of a urea production plant. Also disclosed is a corresponding urea plant, and a method of accordingly modifying a pre-existing urea plant. In a scrubbing section, the off-gas is brought into contact with an acidic scrubbing liquid so as to provide a scrubbed off-gas and a utilized scrubbing liquid comprising ammonium salt. The method specifies an evaporation section, which is part of the urea plant that produces urea melt, that is divided into first and second stages. The first stage is part of the urea melt production plant. The second stage decoupled, as regards recirculation of liquids other than the urea product stream, from the urea melt production plant. This is accomplished by sending utilized scrubbing liquid that contains ammonium salts to the second stage evaporation section, and by sending condensed vapours from said second stage evaporation section to said scrubbing section.

A method for the removal of ammonia from the off-gas of a finishing section of a urea production plant, a corresponding urea plant, and a method of accordingly modifying a pre-existing urea plant. In a scrubbing section, off-gas is brought into contact with acidic scrubbing liquid to provide a scrubbed off-gas and a utilized scrubbing liquid comprising ammonium salt. The method specifies an evaporation section, which is part of the urea plant that produces urea melt, that is divided into first and second stages. The first stage is part of the urea melt production plant. The second stage is decoupled, as regards recirculation of liquids other than the urea product stream, from the urea melt production plant. This is accomplished by sending utilized scrubbing liquid containing ammonium salts to the second stage evaporation section, and condensed vapours from said second stage evaporation section to said scrubbing section.

A process for substantially removing hydrogen sulfide from an ammonium sulfate solution. The process involves maintaining the pH of the solution to a range that optimally removes entrained hydrogen sulfide. The hydrogen sulfide is then reacted with sulfites, bisulfites, or sulfur dioxide to form ammonium thiosulfate.

Disclosed are methods and systems for removing submicron particles from a gas stream, in particular from urea prilling off-gas, wherein a Venturi ejector is used. A method comprises contacting a gas stream containing submicron particles in a Venturi ejector with an injected high velocity scrubbing liquid to provide a pumping action, wherein the scrubbing liquid has an initial velocity of at least 2 m/s and wherein the ratio of scrubbing liquid and gas flow is between 0.0005 and 0.0015 (m.sup.3/h)/(m.sup.3/h). The disclosure also pertains to a prilling tower having a gas stream treatment system comprising a Venturi ejector at the top of the prilling tower, and to a method of modifying an existing prilling tower.