CS257156B1 - ) Method of elimination of interference of current counterelectrodes in electrochemical protection of tube heat exchangers - Google Patents

Abstract

Around each current counter electrode during electrochemical corrosion protection of tubular heat exchangers, a free space of cylindrical shape (if the counter electrode is rod or tubular) or spherical shape if the current counter electrode has a point character is created. The minimum dimensions of this space are calculated from data obtained in advance in the laboratory by substituting into the relation (1).

Description

The invention solves the problem of eliminating unwanted interference during electrochemical corrosion protection of tubular heat exchangers.

Electrochemically protected heat exchangers are structurally complex and their internal spaces are completely filled with various built-ins, which is why the placement of current counter electrodes is difficult. For these reasons, there are cases where a certain part or even most of the counter electrode surface is in close proximity to the protected surface and so-called overprotection may occur, i.e. the shift of the protective potential of the protected device to an undesirable area, where, on the contrary, the corrosion rate is increased instead of decreased. Overprotection usually develops uneven forms of corrosion (intergranular, point, hydrogen embrittlement, corrosion cracking, etc.), which lead to very rapid destruction of the protected device.

All the above-mentioned shortcomings are eliminated by the solution according to the invention, which consists in creating a sufficiently free space around the current counter electrodes, the required size of this space being able to be determined in advance by the following calculation.

List of symbols used

E _m ..... the maximum permissible potential of the heat exchanger construction material in a given environment (V);

E _q ..... protective potential (V);

I ..... protective current (A);

J ..... length current density at the cathode (A.111^), „2 jp ..... passive current density (A.m );

l _m in ··· minimum possible distance of the electrochemically protected metal surface from the surface of the current counter electrode (m);

n ..... number of exchanger tubes;

R^ ..... cathode radius (m);

^R ref ''' ^{not less} than 5 distance of the reference electrode from the axis of the elongated counter electrode or the center of the point electrode (m)

R _fc ..... radius of the exchanger tubes (m);

R _v ..... inner radius of the exchanger (m);

H .....specific electrical conductivity of the given environment (Sm ³ ).

For an exchanger with one counter electrode in its longitudinal axis, the following relation applies: (1) n^.^.Rp \ - ^R k ^min ^{+ R} k>

2.Ί .H(E„ - E ).(1 - n

R ² - R ^{3 R} in ^R to ref. + R, min k + j _r

n.2.# ,R _t

R-R,

V k ^min ^{+ R} k - ^R ref> ⁺

With sufficient accuracy for practice, the following relation ⁽²⁾ can also be used: ^in ^{= R} ref ^exp jj

n.2.# .R,

R.. - R,

2.ÍY.H(1 - n -^5—^—5·) /E - E ) + j _T <*k - ^R ref>

n.2.W.R.

rfef _r 2 _ 2''“o“m'' ^J p _ ^R k ³ nvkvkkvk

For an exchanger with current counter electrodes perpendicular to its longitudinal axis, the ^{following relation applies:} ^in = ^R ref ^{exp 2} ·«ΙΓ.Η(Εο - E _m )

If the counter electrode has a point character, the relation (4) i = ... __ ^rer _ „ _R min 4.Ί .HR _ref (E _m - Ε _θ ) ₊ I V holds.

In anodic protection, the quantities JC and J have a positive sign, in cathodic protection they have a negative sign.

The advantage of creating a free space around the current counter electrode during electrochemical protection of tubular heat exchangers is the elimination of unwanted interference with the internal components of the exchanger, which has been proven by laboratory and operational tests. Furthermore, it is not necessary to use various shielding covers, housings and sleeves on the current counter electrodes, which complicated their design, made their production more expensive and was insoluble in some aggressive environments.

Fig. 1 shows a plan view of a vertical tubular exchanger showing the necessary free space around the rod current counter electrode positioned parallel to the longitudinal axis of the exchanger.

Fig. 2 shows a plan view of a vertical exchanger showing the minimum free space around the current counter electrodes placed perpendicular to the longitudinal axis of the exchanger. Fig. 3 and 4 show a cross-sectional and longitudinal view of the minimum free space around the point counter electrode in the heating register.

The calculation of the minimum dimensions of the free space around the current counter electrodes according to the invention in specific exchangers is best explained by the examples given below.

Example 1 - Fig. 1 (schematic cross-section of the exchanger)

Anodically protected cooler 98.5% H-SO. from 95 to 65 °C has an exchange area of 220 m^

2 4 (total protected area is 250 m ) and is made of stainless steel ČSN 17 348. The diameter of the shell .1 is 800 mm, 489 tubes 2 with a diameter of 25 x 2.5 mm have a length of 6,090 mm. The cathode 2 ^is located in the longitudinal axis of the cooler and has a diameter of 22 mm, an effective length of 6,090 mm. In the laboratory, it was found from potentiostatic and potentiokinetic curves that the optimal protective potential of steel ČSN 17 348 is in this case +0.6 V against MSE at a passive current density of 0.3 A/m , the highest permissible potential is +0.9 V (MSE), The lowest electrical conductivity is sulfuric acid at 65 °C, namely 25 Sm \

Substituting into relation (2) at E = 0.9 V, E = 0.6 V, j - 0.3 Am J = 250.0.3/6.09 = η ΤΠ O p = 12.3 Am , n = 489, R _r = 0.011 m, R _ref = 0.4 m, R _fc = 0.025 m, R _y = 0.4 m, H = 25 Sm we get 1 = 0.0124 m. The minimum free space _4 around cathode 2 is therefore the shape of a cylinder with a diameter of 2(R, + 1 . ) = 0.0468 m.

to the min

Example 2 - Fig. 2 (schematic cross-section of the exchanger)

20% formic acid is heated with glycerin from 40 to 55 °C in a small exchanger with a shell diameter 1 equal to 200 mm and a length of 4,000 mm made of stainless steel CSN 17 249. The acid flows in the intertube space, which is anodically protected, in tubes 2 with a diameter of 12x1.5 mm there is glycerin. The specific electrical conductivity of the acid is 1.2 S.m at 40 °C and 1.4 S.m at °C. Across the exchanger (perpendicular to the longitudinal axis) _§. cathodes 3 made of Hastelloy B alloy with a diameter of 6 nm, regularly spaced, are inserted into the intertube space. There are 94 tubes, the protected area is 17 m 2 , the required current protection density reaches 0.22 A.m 2 . The optimal protective potential was found to be 0.3 V against the silver chloride reference electrode, the reference electrode is located in the exchanger shell. The highest permissible potential of steel CSN 17 249 was found to be 0.95 V (ACLE).

We substitute E^ = 0.95 V, E _q = 0.3 V, J = 17.0.22/8.0.2 - 2.34 Am ¹ , R^ = 0.003 m,

R , = 0.1 m, H = 1.2 Sm“ ¹ into relation (3) and we obtain 1 . = 0.0063 m. The minimum tube spacing ref min around the cathode must be 2.(R^ + l _m ^ _n ⁺ ~ 0.0306 m.

Example 3 - Fig. 3 and 4

The heating register in the evaporator for a solution with a predominance of NaOH is made of chromium-nickel tubes ČSN 17 241 with a diameter of 18x2 mm and is heated with steam to 180 °C. The specific electrical conductivity of the mixture is 120 Sm ^-1 . The protective potential is - 0.4 V (molybdenum reference electrodes). The reference electrode is located in the lower part of the evaporator shell at a distance of 1,010 mm from the anode J3. The lowest permissible potential of steel ČSN 17 241 in terms of hydrogen embrittlement is - 0.55 V (Mo). The U-tubes 2 are shaped so that a free space of spherical shape 4 is created in the center of the register, in the center of which a platinum anode 2 ^with a lead 5 is placed, with which the register is cathodically protected by a current of 21 A. To calculate the minimum size of the free space 2, we use the equation (4) and substitute Em = -0.55 V, Εθ = -0.4 V, I = -21 A, Rref = 1.01 m, H = 120 Sm ^-i . We obtain 1 f - 0.086 m. The internal tubes of the register must be shaped so that they circumscribe a sphere with a diameter of 172 mm.

As we can see from the last example, the solution according to the invention can also be used for heat exchange devices (registers) that are part of other larger apparatuses - evaporators, reactors, columns, etc. In addition to eliminating the undesirable increase in the corrosion rate, the solution according to the invention has other effects. It prevents or greatly reduces undesirable electrochemical reactions on the protected surface, which could either lead to decomposition or change the properties of the medium inside the exchanger or consume most of the protective current and thus reduce or completely eliminate the effect of electrochemical protection.

Claims (2)

OBJECT OF THE INVENTION A method for eliminating undesirable interference of current counterelectrodes in electrochemical corrosion protection of tube heat exchangers, characterized in that a free space is formed around each current counterelectrode, the minimum dimensions of which can be determined according to J + j _y n.2. (jf. R _fc ^R v - ^R k ⁽¹ min ^{+ R} k>) ref 2. <XH, E _o - E _ra ) (1 - -ATS '° ^R in ^R k (1) where E ^ ..... the limit permissible potential of the exchanger construction material in the given environment (V); Εθ ..... protective potential (V); J ..... cathode length current density (Am ¹ ) t 2 j ..... passive current density (Am); Min ² ''^{', m} and ^{n: m} i- alni possible distance electrochemically protected metal surface from the surface of the counter electrode current (m); n ..... number of coil tubes; R 4 ... cathode radius (m); ^R REF '·· * · or ^{less non-reference} electrode distance from the axis of the elongate counterelectrodes or center point electrodes (m); R _t ..... exchanger tube radius (m); R ..... exchanger inner radius (m); ^V -I H ..... specific electrical conductivity of the medium! (S.m). and the free space has a cylindrical shape around the elongated shape counter electrode, and a spherical shape around the point counter electrode. 2. A method according to claim 1, characterized in that the heat exchanger tubes are arranged in such a way that they fill at least two separate spaces spaced apart by at least a value of [mu] m and having the above-mentioned meaning. to be equidistant from the two separate spaces.