NO761914L - - Google Patents

Description

Procedure for purification of

nitrous-containing exhaust gases.

In a number of high-tech processes such as the production of nitric acid and fertilizers or the nitration of organic compounds, nitrous gases with the formula N0x (nitrogen oxide NO or nitrogen dioxide N02) appear as by-products, which can be found in the oxygen-containing exhaust gases and of which only insufficient amounts can be removed by usual methods. For the purification of these exhaust gases, a number of methods are available which are based on the principles of absorption, adsorption and catalytic conversion (Chemiker-Zeitung 89, 632, (1965)). In the absorption process, e.g. the exhaust gases with acidic urea solution (German patent no. 568.8l4 a US patent no. 3,565,575) with caustic soda, soda, water or nitric acid. According to German patent no. 1,085,640, a method is known for removing nitrogen oxide, which is contained in smaller amounts of up to 50 ppm in coke oven gases, generator gases and flue gases. In the method, in a first step, NO is oxidized under pressure to N02, HN02 and HNO^ and in a second step, N02 is washed with a reducing agent, e.g. an aqueous alkaline solution of thiosulphate.

In the adsorption method, the NOx contained in the exhaust gas is adsorbed onto a carrier. This has the disadvantage that the carrier must be regularly renewed or regenerated.

The catalytic conversion depends on turnover of

NO- with ammonia or methane in the presence of e.g. vanadium-containing catalysts to N£ + H20 (+ C02). With this method, a high cleaning effect is achieved, however, the high operating temperature of 200 to 300°C and the necessary regular renewal of the catalyst cause considerable running costs.

In summary, the usual methods are either too expensive to use or they give unsatisfactory results, especially with high NO A concentrations. Absorption experiments with NO -containing mixtures of high concentration (4,000 volume ppm) using the method according to US patent no. 3•565-575 always gave final concentrations of N0x of over equal to 400 volume ppm. The method according to German patent no. 1,085-640 can be used

for the removal of very small concentrations of N0Xi, such as e.g. can be found in city gas etc. In this case, the amount of added chlorine - approximately equimolar with NOX - is small and it therefore contributes only slightly to the pollution of waste gas and waste water.

However, if larger quantities of NOx are to be removed, as they e.g. appears during the production of nitric acid or fertiliser, then very considerable amounts of chlorine must already be added. These would enter the atmosphere with the exhaust gas. As this is not favorable for ecological reasons, chlorine had to be removed in a further step with considerable amounts of lye from the exhaust gas, whereby the method becomes uneconomical. In addition, an excessive amount of chlorine-containing products produces an additional waste water load that requires special precautions.

The task therefore further consists in lowering the N0x~ content in nitrous-containing exhaust gases in a cheaper way than is possible with the known methods.

The object of the invention is therefore a method which is characterized by passing the NOx-containing exhaust gas through a basic thiosulphate solution, the pH value of the basic thiosulphate solution being at least 8 and the molar ratio S020-^2-/NOX being at least 0.4 to 1 Preferably, one proceeds in such a way that one brings the exhaust gas in a counter-flow process using columns in intense contact with the thiosulphate solution. Bell-bottom columns are particularly suitable for this, as a very favorable liquid/gas volume ratio can thus be achieved, however other bottom columns are also taken into consideration.

Aqueous solutions of alkali and alkaline earth hydroxides or -carbonates, e.g. KOH, NaOH, Ca(OH) 2 , Na 2 CO 2 ; ammonia is also suitable. As thiosulphates, sodium, potassium and ammonium thiosulphate come first.

To prevent the precipitation of elemental sulphur, the pH value of the aqueous solution should preferably be at least 8 and the molar ratio.S20j _ o/NOX preferably at least 0.4 to 1. The pH optimum lies at 8 to 12, the ratio thiosulphate/NOx preferably between 0 ,4 and 1.5 to 1. An increase in the pH value resp. of the thiosulphate concentration above these limits does not entail any additional benefits. The molar ratio base/NOx is preferably 1.2 to 6 to 1, the lower value corresponding to the lower thiosulphate concentration.

The overall concentration of the solution (thiosulphate + lye) naturally depends on the NOx concentration of the exhaust gas to be cleaned. At high exhaust gas concentrations of approx. 4000 volume ppm it should amount to approx. 2 to 6 wt$, at lower exhaust gas concentrations correspondingly less.

During the reaction, N0V .X is converted into nitrite and nitrate, thiosulphate into sulphate. The method according to the invention makes it possible in a simple and cheap way, in particular, to purify exhaust gases with a relatively high NO X content, as the NO part of NO X can amount to up to 70% in order of magnitude. Thereby, final purities of less than 100 volume ppm NOx are achieved, which is a considerable improvement compared to the usual absorption methods. The exhaust air escaping from the process is free of SO2. Example 1.

A bell-bottom column (diameter 5 cm) with 30 bottoms was supplied with air from below with a concentration of 4000 volume ppm of pure N02 at a rate of 1000 litres/hour. The aqueous washing liquor filled at the top of the column contained 3*2 wt$ of Na2S20-^

and 2.0 wt% NaOH. It was fed in at a rate of 0.5.7 litres/hour. The waste liquor exiting at the top of the column also contained 57 volumes ppm N0X, of which 9.5 volumes p'pm N02- The washing liquor contains as the reaction product 3.7 wt% Na2SO^ (62% of the total sulfur was present as sulfate); 2.3 wt$ NaN02 and approx. 0.2 wt$ NaNO-j.

Example 2.

A bell-bottom column (diameter 5 cm) with 30 bottoms was supplied with air at a concentration of 4000 volume ppm of NOX.

(of which 25$ NO) with a speed of 1000 litres/hour. The aqueous washing liquor contained 3.2 wt Na 2 O 3 and 3.0 wt NaOH and was fed at a rate of 0.5 liters/hour. The exhaust air contained 56.5 volumes ppm N0X, of which 9.5 volumes ppm N02> The exhaust contained 4.0 wt% Na2S0^ (64% of the total sulfur was present as sulfate); 1.5% by weight NaN02 and approx. 0.9 wt$ NaNO^.

Example 3»

A bell-bottom column (diameter 5 cm) with 30 bottoms was supplied with air with a concentration of 4000 volume ppm of N0X (of which 50% NO) at a rate of 500 liters/hour. The wash liquor contained 1.9% by weight Na 2 S 2 O-j and 2.0% by weight NaOH and was fed at a rate of 1 liter/hour. The exhaust air also contained 82.5 volumes ppm N0x, of which equal to or less than 10 volumes ppm N02. Example 4.

A bell-bottom column (diameter 5 cm) with 30 bottoms was supplied with air with a concentration of 4000 volume ppm of pure N0 2 at a rate of 500 liters/hour. The washing lye contained

1.9 wt% Na 2 S 2 O and 1.2 wt % NaOH and was fed at a rate of 1 liter/hour. The exhaust air contained 43.5 volume ppm N0X, of which equal to or less than 10 volume ppm N02-

Example 5.

A bell-bottom column (diameter 5 cm) with 30 bottoms was supplied with air with a concentration of 1000 volume ppm of pure N0 2 at a rate of 1000 liters/hour. The filled washing liquor contained 0.8% by weight Na 2 S 2 O and 1.55% by weight Na 2 ©.0^ and was fed at a rate of 0.5 liters/hour. The exhaust air contained 28 volume ppm NOx, of which 18 volume ppm N<0>2.

Claims (4)

1. Process for purifying nitrous-containing exhaust gases, characterized by passing the exhaust gases through a basic thiosulphate solution, the pH value of the basic thiosulphate solution being at least 8 and the molar ratio S20-^2-/N0x being at least 0.4 to 1. 2. Method according to claim 1, characterized in that aqueous solutions of alkali or alkaline earth hydroxides or carbonates or of ammonia are used as the basic medium. 3- Method according to claim 2, characterized in that aqueous solutions of KOH, NaOH, Ca(OH) 2 or Na 2 CO 2 are used as the basic medium. 4. Process according to claims 1-3, characterized in that sodium, potassium or ammonium thiosulphate is used as thiosulphate.