DE2141944A1 - METHOD OF ABSORPTION OF NITROSE-CONTAINING EXHAUST GASES - Google Patents

Description

Gelsenkirchen-Buer, August 18, 1971

Veba-Chemle Aktiengesellschaft Gelsenkirchen-Buer

Process for the absorption of nitrous gases

The invention relates to a method for the economic use of the nitrous content of exhaust gases, for example during production of nitric acid through the absorption of nitrogen oxides, during the digestion of rock phosphate with nitric acid or in pickling plants, in which nitric acid is used "

Numerous methods are already known for the absorption of residual nitrogen oxides in exhaust gases · They either work with alkaline Absorbents, e.g. with milk of lime or caustic soda, or with reagents specific for nitrogen oxides, e.g. B. with sulfamic acids In many cases an additional oxidation is used to aid the absorption of the nitrous gases that are usually present are between the valence state +2 and +4 and which are oxidized to nitric acid with pentavalent nitrogen 'should be carried out. For example, ozone, hydrogen peroxide, peracids or chlorine are used for this.

All of these processes can either be carried out very laboriously, or else they deliver products with only a small amount Usability, e.g. B. calcium nitrate, in many cases processing is only carried out in order to comply with environmental hygiene regulations To do enough.

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One has therefore already gone over to the nitrogen oxides catalytic combustion and similar work-up methods, see below destroy,

With the help of the invention it is possible to use both the To meet hygienic requirements, as well as to use the nitrogen oxides largely economically.

The method according to the invention for the absorption of 110 / NO ₂ from nitrous-containing exhaust gases consists in using either phosphate-containing, granular raw materials in the presence of water in the form of steam or spray mist or aqueous suspensions of very fine-grained phosphate rock as absorbents Absorption masses are worked up by conventional phosphate conversion processes known per se, for example to give fertilizers, animal feed or phosphoric acid.

The invention evaluates the observation that rock phosphate and other raw materials containing PpOc are suitable for absorbing nitrogen oxide © yv if the gases are moistened so much that they pass through a fine-grained, phosphate-containing layer! capillary condensation promotes the absorption of nitrogen oxides. Good absorption build-up is also achieved if r _; -: n fine-grained phosphate suspensions suspended in water are used for the absorption of dry nitrogen oxides,

These absorption processes occur JIal2iumn.itrat and citric acid and usually v / ater-soluble Kalziu> iiphosp]) ate _e Γ Le occurring phosphates with their v / asserlöslichen Shares must continuously bsi suitable action / "chargeiivoir.e from the absorption chamber removed and replaced with fresh material.

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The phosphate-containing rock is so to speak by the Treatment with nitrogen oxides pre-digested. The absorption masses obtained can then be subjected to a further digestion process subject to, whereby a saving of digestion acid can be taken into account »

In contrast to the previously usual absorbents, in the case of rock containing phosphate, the absorption mass is fully usable « In the case of the absorption devices developed, there is only a relatively low outlay in terms of equipment and minimal maintenance necessary so that the process can also be used in companies that are forced to use their absorption masses treated with nitrous oxide Selling phosphate digestion plants is economical.

The process according to the invention was made with numerous rock phosphate " Tried varieties, of which only the initial analysis of two varieties is listed in Table 1:

Tobelle 1

Ingredient of Moroccan phosphate, Taibaphosphate

Gesanit-P ₂ 0 ₅ 34.5 % 37.7

Neutral citrate soluble , _{n ηΛ c / n Ar}

P ₂ O ₁ - (Fresenius-Heubauer) ^υ » ^{υι / 0 U} » ^DD

Al-imon-citrate-soluble y η 01 «> / η

P ₂ O ₅ (after Püterraaim) ' ^{/ 0}

Water soluble P ₂ O, - <0 _f 01 % 0.01

Total "CaO 49.0 % 52.6

V / asserlBsliclies CaO <0 _? 02% 0 _s l4 All: ali-fcät

Carbonates as ¹ A C0 _? 3.85 ^ o - 1 _? 3

pH value (5 igc solution) 5.8 4.8

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However, other phosphate-containing substances can also be used in the same way Use rocks such as phosphorite, thomas flour or superphosphate,

While one for the process with the use of dry phosphate and humidified, nitrous-containing exhaust gas can expediently use a relatively coarse-grained starting product, it recommends to use a phosphate in the suspension process in which at most 1% by weight of the particles is coarser than 1 mm. Of the The Moroccophosphate listed above were samples with the grain size distributions given in Table 2 in the two types of process used. The fine-grained sample was obtained by appropriate grinding and sieving of the coarser sample.

Table_2

Grain size distribution for the dry and the wet process

description of 5 > 5 sample coarse-grained fine-grained f U, b Jo 48.0% Shares 4th to 4 mm 0.5 # 51.4 ° / s 3 to 3 mm 0.1 % ι J 2 Ms 2 mm 0.1% 1 until 1 mm 1.4 % to O mm OO θ /
C | O / 0 < ° , 2 mm • 47.0 ^ , 2 mm 48.1 #

For the absorption devices, two embodiments each are described below for the dry process and for the wet process. However, corresponding others can be used Use appropriate facilities as well.

Two of the apparatuses used are shown in Figures la, lb and Ic and 2 are shown.

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Figure 1a shows the material flow in a schematic way in one form of the apparatus. of the absorption of the moistened nitrogen oxide-containing exhaust gases to dry, coarse-grained Phosphate took place The phosphate granulate flows continuously from the bunker 1 between two sieve plates. Distance d or through two touching parallel rows of filter candles ^, (see Fig 1c) with the diameters D through. The · phosphate beds in this plate? -or in the double-row tube arrangement Filter for the transverse stream of the nitrous The filter space 3 must therefore for de.n entire transverse. . cut of the exhaust gas flow constantly filled with absorbent.

The phosphate sluices 2 under the bunker 1 always remain intact opened, as a result, as much phosphate always flows in as. is withdrawn from the drainage port 4 of the filter chamber.

It was found, that the absorption effect is only insignificantly dependent on the thickness of the absorbent phosphate layer. Since it is advisable to keep the pressure loss in the exhaust gas flow as low as possible, the effective thickness of the filter layer should be chosen to be as small as possible. of 1000 mm and the total length ± 2 of the filter 3 was 1550 mm, a filter diameter of d = 150 mm has proven itself. For the double rows of candles, a candle diameter x * of 100 mm is appropriate.

A fan 16 (not shown) draws the nitrous-containing exhaust gases through the filter 3 o via a steam injection 17 or 13, the nitrous exhaust gases are grounded by low-pressure steam, 100 % saturated with water. For the separation of liquid particles is

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between the steam injection 13 and the. Filter 3 a mist eliminator 14 is provided, which can be drained via the valve 18 can.

If you the raw phosphate throughput through the filter apparatus 3 to If you have chosen low, it happens that the calcium nitrate formed in the phosphate-containing rock destroys the pores of the filter plates stuffs, so that the pressure loss in the flow of nitrous gases then increases. In such a case, the filter plates must be cleaned become ... This is achieved by turning valves 2, 12 and 15 closes and first drains all of the rock phosphate by the valve 6 of the vent (for example 100 mm 0) opens. The drain valve 4 is then also closed and through the compressed air nozzle 7 in front of and behind the filter arrangement Compressed air eirageblaseii. Then the filter raura 3 can be filled again with frischera absorbent from bunker 1.

Figure 1b shows a section with a projection perpendicular through the image plane of Figure 1a, and Figure Ic shows the ■ arrangement of filter candles in double rows in a plan view. A corresponding representation of the filter cross-section when using . Plate filters were dispensed with.

With correct adjustment of the phosphate throughput through the filter— Room 3 with the exhaust gas flow rate from 12 to -15, the absorption can be tion £ inasse continuously withdrawing through the drain valve 4, you is most appropriate to the digestion tank for the production of Compound fertilizer !! or fed by phosphoric acid.

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Figure 2 shows one of the possible ways to absorb the NO / NOp content of "dry" nitrous gases to aqueous Suspensions shown. The finely ground phosphate rock coming from a bunker is poured over the pouring opening 31 introduced into the premixer 32 and in this vessel with, the The stirrer 33 is intimately mixed with the water flowing in via the filler neck 30 in a ratio of 1: 2. The phosphate-water suspension obtained is continuously fed into the template 35 via the drain valve 34. The suspension is also through in the template a stirrer 36 kept constantly in motion.

The phosphate suspension from the absorption vessel 35 is via the bottom outlet 37 with the aid of the pump 38 with the riser 39 to Washing tower 51 promoted. In the washing tower, the suspension gradually trickles downwards over grids arranged in layers the stream of nitrous gases flows in the opposite direction. It finally returns to the absorption vessel 35 via the inlet, to be pumped again. However, part of the suspension is constantly overflowing from the connecting piece extending from the riser pipe 39 the throttle 54 led to the (not shown) post-digestion of the phosphate. In the same way you give over 34 fresh ones Suspension from the storage vessel 32 after.

The nitrous gases arrive from their place of origin 61 either under their own flow pressure or accelerated by a suitable blower via the inlet valve 62 and the nozzle 63 into the absorption vessel 35 »where it hits the surface the suspension can be inflated. They then flow through the trays 52, which are arranged on several floors, in order to escape via the ventilation pipe 64 into the droplet separator 65 and the residual gas chorus block 66. In the droplet separator 65 excess water vapor is deposited and ir> via the downpipe 67 returned to the absorption vessel.

1. Use of an absorption apparatus as shown in Figure 1

The examples a to e carried out are shown in Table 3 in their essential data are recorded.

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Table 3.

Example abode

Filter area m ² 1.0

Phosphate layer thickness m 0.15

Amount of phosphate / filling kg 240

Gas throughput έΡ / ti 360

Gas velocity at

Filter m / s. 0.1

Residence time of the gas s 1.5

Type of used. Phosphate H.arokko-Rohphosphat Taibaphosphat

coarse coarse

Nitrous content (NO / NO calculated as g N0 / Nm3)

in the feed gas g N0 / Nm ⁵ 8.2 6.2 5.6 3.5. 1.3 in the residual gas g NO / Nm ⁵ 1.2 1.2 1.0. 0.7 0.23 • Efficiency% 85.3 80.7 82.3 80.1 82.3 Amount of Hitrose absorbed kg NO / h 2.52 1.8 1.66 1.01 0.385

Provided that 1 mole of CaO binds 2 moles of KO implemented in example la 4.6 kg of rock phosphate.

In order to prevent cementing of the phosphate, in general 50 kg of rock phosphate renewed per hour.

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2. Experimental examples with an absorption apparatus according to Figure 2
Table 4

Example . a '^^ "."., .__._._ .., P

Type of phosphate Moroccan phosphate

Washing tower height m 5

Washing tower diameter m 1.0 internals

Art

Mesh size mm

Distance from-. each other's number

Gas throughput Nnr / h Gas velocity in the tower m / s

Dwell time s ratio

Rock phosphate: water 1: 2

total suspension amount in the circuit nr 5 'pumped suspension vc? / h 10 withdrawn or increased ^^^. ". ^ .- ^'" ~ - ^ - "· ~~ * s new suspension kg / h 250 200 nitrous content (N0 / N0 ₂
calculated as g NO / Nnr)

5.5 1.1 80

17.6

32

V 4 A - • Grids 5 O, 5 10 4000 It 4th 3, 5

in the feed gas g NO / Nm- * kg / h _ 7th in the residual gas g NO / Nm ³ that 1 mole of CaO 2 moles of NO 3098 1.5 Efficiency % tie) 79 Amount of absorb. Nitrous kg, 'NO / h 22nd Amount of sales Rock phosphate (under the ' Prerequisite 40 10 - 08/1304

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The gas supplied to the apparatus should always be cooled to temperatures below 30 ° C., because with increasing temperatures Temperatures, the absorption yield drops - just like with the. simple absorption of nitrogen oxides in water - strong, ab. Deeper Temperatures, on the other hand, favor absorption.

Surprisingly, as examples 1a to e show, the absorption yield decreases with decreasing nitrous content of the substance used Practically no exhaust gas. So it is very possible to connect several devices one behind the other and so to extremely low To get nitrous content in the residual gas »

It is also possible to use the moistened method Nitrous gas stream to add gaseous oxidizing agents, in particular ozone, to the gas stream. In the case of the pumping over of Rock phosphate suspensions through washing towers can reduce the absorption yield by adding hydrogen peroxide or by others Oxidizing agents also increase.

It is also advisable to add suitable wetting agents to the suspensions, add silicone oils, alkyl sulfonates, for example.

The absorption yield can also be increased by adding alkaline substances or the reagents mentioned in the introduction (on page 1) to the suspensions. The compounds obtained by the absorption of the nitrous-containing gases in the fine-grained phosphates are generally easily soluble. The main product is the readily soluble Ca (NO ^) ₂ 4 HpO. Secondary and primary calcium phosphates are also formed. For example, in test series 1a (see Table 3), the values of neutral citrate-soluble, ammonium citrate-soluble and water-soluble Pp ^ 5 ^from "less than 0.01 % (see Table 1) to 3.30, 3 , 03 and 2.56 % .

It is also possible to carry out the two variants of the process in a fluidized bed. At this point you can do both use the dry, phosphate-containing rocks as.

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even those that have been moistened with water to which a wetting agent has been added have been. In the former case, the stirring up must be accelerated by appropriately accelerated, beforehand by suitable hatreds humidified nitrous-containing exhaust gas flows take place. In the second case, you can use the "dry" one containing nitrous Use exhaust gas flow after appropriate acceleration.

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Claims (1)

Patent claims : Ιλ. Process for the absorption of NO / NO ^ from nitrous-containing exhaust gases by bringing them into contact with solid absorbents, characterized in that either phosphorus-containing, granular raw materials in the presence of water in the form of steam or spray mist or aqueous suspensions of very fine-grained phosphate rock are used as the absorbent used, 2 _{Λ ι} method according to claim 1, characterized in that the absorbent is used dry and the nitrous-containing waste gas stream is completely saturated with water vapor either by steam injection or by a trickle tower. The method of claim 1 and _t 2, characterized in that through pouring the granular phosphate rock con- 'continuously between filter surfaces while passing sends containing nitrogen gas stream transverse thereto. . Process according to Claims 1 and 2, characterized in that very finely ground, phosphate-containing Rocks through a correspondingly accelerated, nitrogenous exhaust gas flow saturated with water vapor in a can be fluidized continuously operating fluidized bed arrangement. Method according to claim 1, characterized in that that one suspended phosphate-containing rocks in the finest grinding in water and with the nitrogen oxide-containing Brings gas stream into intimate contact. - 13 - 309808/1304 13 .- 2U194A 6 ^ The method according to claim 1 and 5, characterized in " that an aqueous suspension of finely ground phosphate rock through a Wash tower with multi-level hordes in the cycle and in this tower in intimate contact with The nitrogen oxide-containing exhaust gas stream is first blown onto the surface of the suspension and then through Absorption layers from its nitrous content and, thanks to droplet separators, from excess water content practically is released. · 7 * Process according to claim 1 and 5, characterized in that previously finely ground, phosphate-containing rocks wetted with water are whirled up in a continuously operating fluidized bed system by a dry, nitrous-containing exhaust gas stream. 8_, _m Process according to Claims 1 and 5 and / or 6 and / or 7, characterized in that wetting agents known per se are added to the water used to moisten the very finely ground phosphate rock. 9. The method according to claims 1 and 5 and / or 6 and / or 7 and / or 8, characterized in that known reagents promoting the absorption of nitrogen oxides are added to the suspension, 10 ^ ₁ Process according to claims 1 to 9 »characterized in that the absorption of nitrogen oxides is accelerated by adding oxidizing agents such as ozone, hydrogen peroxide or peracids. 11 .. Use of the according to claims 1 to 10 after absorption of the nitrogen oxides obtained, partially digested absorption masses for processing according to known ones usual phosphate digestion process for fertilizers, Feed or phosphoric acid. dr.wu-ra 309 808/130 4