RU2204101C1 - Method of chemical cleaning of internal surface of heat- exchange equipment - Google Patents

Abstract

FIELD: removal of deposits difficult for solution from internal surface of pipe space of heat- exchange apparatuses producing carbamide. SUBSTANCE: cleaning is carried out with use of mixture of phosphoric and nitric acids with concentration of H₃PO₄ equal to 10-30 per cent by mass, of HNO₃ equal to 15- 25 per cent by mass and corrosion inhibitor in the capacity of flushing solution. Potassium bichromate in the amount of 4.0-6.0 g per liter of flushing solution is utilized as corrosion inhibitor. Deposits are removed at boiling temperature 95-110 C of flushing solution without circulation by way of multiple filling of pipe clearance with subsequent flushing with condensate of water vapor. EFFECT: improved method of cleaning of internal surface of pipe clearance in heat exchange equipment that can be utilized in apparatuses made of any corrosion-resistant materials, titanium included, used in production of carbamide. 1 tbl

Description

 The invention relates to chemical methods for cleaning insoluble deposits from the inner surface of the tube space of heat exchangers for the production of urea.

Known methods for removing iron oxide deposits using phosphoric acid as a washing solution of various concentrations (5-40%) when heated to 100-110 ^o C (Chemical cleaning of heat-exchange equipment. Ed. T. Kh. Margulova. M., Energy, 1987 g .; RF Patent 1772578, MKI F 28 G 9/00, Bull. 40, 1992). The disadvantage of these methods is that the precipitates dissolve very slowly, it takes a lot of time to flush and energy to circulate the reagents. In addition, they are not applicable in the case of complete or partial absence of patency in the tubes.

Closest to the proposed is a method of cleaning the inner surface of the heat exchange equipment for the production of urea (Pat. RF 2148227, IPC F 28 G 9/00, 08 V 3/08 from 05/05/1999), which proposes to use phosphoric to remove iron oxide deposits acid concentration of 60-65% when heated to 110-120 ^o C without circulation by filling an open tube with subsequent washing with demineralized water. The disadvantages of this method are the high consumption of expensive phosphoric acid and the inability to use it for devices made of titanium.

As you know, titanium in concentrated phosphoric acid at a temperature above 100 ^o C dissolves, therefore, the prototype method can only be used for devices made of chromium-nickel-molybdenum steel type 08X17H13M2T.

 The aim of this invention is to improve the method of cleaning the inner surface of the tube space of heat-exchange equipment, which allows it to be used for apparatuses made of any materials that are corrosion-resistant in urea production environments, including titanium. This is especially true because titanium is increasingly used as a structural material, replacing more expensive special steels.

The problem is solved due to the fact that a mixture of phosphoric and nitric acids with a concentration of H ₃ PO ₄ 10-30 wt.%, HNO ₃ 15- is used to clean the inner surface of the tube space of heat exchange equipment, mainly for the production of urea from iron oxide deposits. 25 wt.% With the addition of a corrosion inhibitor - potassium dichromate in an amount of 4-6 g per 1 liter of washing solution at a temperature of 95-110 ^o C.

It has been established, for example, that during the interaction of titanium samples of grade VT 1-0 at a boiling point of a mixture of acids in the presence of an inhibitor of K ₂ Cr ₂ O ₇ - 5 g / l, the corrosion rate is:
Nitric acid (wt.%) 15; 17; 20; fifteen; fifteen.

 Phosphoric acid (wt.%) 10; fifteen; 20; thirty; 40.

 The corrosion rate of titanium (mm / year) 0.1; 0.1; 0.2; 0.2; 0.4.

Example:
The urea of the second stage of urea distillation is a vertical heat exchange apparatus consisting of 2840 tubes with an inner diameter of 7 mm and a tube part height of 8 m. The heat exchanger was manufactured in 1991. After 10 years of operation, the tubes of the apparatus were completely clogged with iron oxide sediment. The chemical composition of the precipitate: Fe ₂ O ₃ - 95%; Ti ₂ O ₃ - 1.6%; Cr ₂ O ₃ - 1.6%; Mo ₂ O ₃ - 0.1%; Ni - traces; soluble in CCl ₄ - 1.4%.

To dissolve deposits in the tank, a solution is prepared: H ₃ PO ₄ - 17 wt.%, HNO ₃ - 16 wt.%, Corrosion inhibitor K ₂ Cr ₂ O ₇ - 5 g / l. With this solution, the tube space of the apparatus is filled to a level of 10-20 cm above the tube plate, and steam is supplied into the annulus to heat the wash solution to the boiling point. After 1 h, the solution is poured into a container. The operation is repeated 8-10 times, after which the heat exchanger is washed with steam condensate. Then, the tube space is again filled with washing solution and boiled for 1-2 hours, alternating with washing with condensate. The total number of filling 20-25 times. The total contact time with the washing solution during boiling is 35-40 hours.

 Throughout the entire cleaning process, analytical concentrations of acid concentrations and temperature of the wash solution are maintained. The content of iron, ammonium and titanium in the washing solution after contact with the tube is also controlled. These indicators during washing are maintained in the following limits (g / l) (see table).

 After a control wash with steam condensate, the heat exchanger is fully restored.

Claims (1)

The method of chemical cleaning of the inner surface of heat exchange equipment, mainly for the production of urea, characterized in that a mixture of phosphoric and nitric acids with a concentration of H ₃ PO _{4 of} 10-30 wt. %, HNO ₃ 15-25 wt. % and a corrosion inhibitor, which is used potassium dichromate in an amount of 4-6 g per 1 liter of washing solution.

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