FI66657B - FOERFARANDE FOER MINSKNING AV KORROSIONEN I PAULING-KITTLAR - Google Patents

Description

1 66657

Method for reducing corrosion in Pauling boilers

The present invention relates to a method for reducing corrosion in Pauling boilers by heating the boiler as evenly as possible, in particular in area 5 above and below its support flange.

Many industrial processes contain more or less sulfuric acids which have been contaminated with other substances in different concentrations. These acids most often need to be treated for reuse, i. concentrate and 10 possibly purified.

In the known Pauling method (cf. Ullman. Enzyklopädie der technischen Chemie, 3rd edition, 15th volume 1964, p.443) sulfuric acids are concentrated in externally heated casting boilers at normal pressure and possibly purified by the addition of nitric acid.

This method is technically relatively simple to implement, but has significant drawbacks. In normal use, a corrosion ring is formed on the inner wall of the boiler at the level of the liquid level.

20 The boiler wall therefore gradually becomes thinner, so that it must be inspected regularly. In addition, the boiler must normally be replaced after approx. 2..3 years of use.

Surprisingly, we have now found that the increased corrosion can be substantially reduced by heating the boiler as evenly as possible at the height of its support flange and thus at the height of the liquid level.

The present invention therefore relates to a method for reducing corrosion of Pauling boilers, and this method is characterized in that the boiler is heated as evenly as possible in the region of its support flange.

According to the invention, the boiler is thus heated in such a way that the temperature of the boiler wall above the support flange and the wall temperature below the support flange are as uniform (equal) as possible. These temperatures are schematically denoted T. and T in Figure 1.

2

According to the invention, the heating or flue gases are led by suitable structures, such as baffles in the form of a split casting ring, which have a slightly larger inner diameter than the boiler and are clearly smaller than the outer edge of the flange or specially shaped stones, vert. Figures 2a and 2b, 10 in the masonry for supporting the casting boiler, so that they also directly hit the boiler wall between the support flange and the cover flange over the entire range. This special conduction of the flue gases avoids the temperature differences which have occurred so far and achieves a small temperature rise above the boiler support flange. The same purpose as the baffles or specially shaped stones is also achieved, for example, by a casting ring which is placed in the masonry or also by a shaped stone with a "cam". In principle, it is also possible to provide the support flange running around the boiler 20 with corresponding notches, however, this means casting problems.

The temperature dependence of the attack of concentrated sulfuric acid on gray cast iron in the temperature range of about 15 to 300 ° C is known (DECHEMA-Werkstoff-Tabelle, 25 Chemische Beständigkeit, Schwifelsäure, December 1969).

According to this table, the corrosion maximum passes at a temperature of about 215 ° C. Above a temperature of about 220 ° C, the reduction in corrosion has been described to be so small that no significant improvement in corrosion behavior could be expected by extrapolating the literature values to the boiling point of concentrated sulfuric acid.

fr 3 66657

This was all the more surprising because the method according to the invention was able to achieve a technically significant advance in this respect and the average service life of the casting space could be more than doubled.

Fig. 1 schematically shows a casting boiler with a large support flange, Figs. 2a and 2b show shaped stones, and Fig. 3 shows a corrosion behavior according to exemplary values.

In the figures, the reference numerals denote the following: 10 Figure 1: 1 = support flange 2 = boiler 3 = liquid level 4 = base, eg casting ring 15 5 = masonry

Figure 2: a, b = shaped stones 6 = flue gas inlet 7 = flue gas outlet 20 There is a possibility for the heating gas to penetrate through the base 4, which must be formed accordingly and arrive in the area T 1 on the shaped stones placed above it. On the other side of the stone may be arranged in the same arrangement even when the flue gas 25 through passage 3 through the masonry is made possible.

Figure 3: A = groove depth, mm B = service life, months 30 9 ... 14: values of examples 1 ... 3

The present method is explained in more detail below by means of examples. The gray cast iron Pauling boiler mentioned in the examples had the following dimensions: 4 66657 ....... . . 3200 mm s 1 s ah time a. S i j a height 2400 mm outer diameter of the flange 3560 mm wall thickness at the top 70 ... 80 mm wall thickness at the bottom 65 mm 3 5 nominal volume 12 m

Example 1 A cooking pot filled with 96% sulfuric acid boiling under low vacuum was agitated according to the 2Q Pauling method with 72% sulfuric acid used from the nitration process.

The depth of the groove formed at the height of the acid level was measured with periodic precision supports.

The following measured values were obtained: 15 Operating time 205 348 420 481 541 729 days

Groove depth 20 35 42 45 48 60 mm In the renovation building of this boiler, the masonry was equipped with the shaped stones according to Figure 2 and a new boiler with the same dimensions and operated under the same conditions was built in. The groove depth measurement gave the following values here: Operating time 105 185 255 371 483 563 677 734 830 days

Groove depth 8 8 15 20 20 22 26 27.5 nm 25 These results are shown graphically by curves (8) and (9) in Figure 3.

Example 2

Under comparable Pauling conditions, two boilers were operated in succession in the same plant, with the first roof having standard flue gas control and the second having flue gas control according to the invention. Depending on the operating time, the following groove depths were obtained: 5 66657

Usual Operating Time 128,221,299,431 days

Flue gas control Groove depth 20 45 50 75 suction

Improved Operating Time 78,210,309,425 days

Flue gas control Groove depth 3 5 5 7.5 suction 5 Boiler in use Curves (10) and (11) in figure 3.

Example 3

In the Pauling concentration unit, three casting boilers were used in succession in the usual way. Only a third had the control of heating gases according to the invention. Measurement of groove depths gave the following values: 15 Normal Operating Time 113,163,286,374 days

Flue gas control Groove depth 19 22 23 31 nm

Usual Usage Time 265 days

Flue gas control Groove depth 60 irm 2q Operating time 288 461 days

Improved Groove Depth 6 7 nm

Flue gas control

The last boiler is still in use.