GB2260992A

GB2260992A - Cleaning the metal surface of a component by circulating aqueous soda and then washing with water

Info

Publication number: GB2260992A
Application number: GB9222313A
Authority: GB
Inventors: Isabelle Garcin; Claude Ginisty; Jean Leybros
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1991-10-31
Filing date: 1992-10-23
Publication date: 1993-05-05
Anticipated expiration: 2012-10-23
Also published as: FR2683165A1; JP3231102B2; FR2683165B1; JPH05263280A; GB9222313D0; GB2260992B

Abstract

A process for cleaning the metal surface of an installation component which has been used for the treatment of organic effluents, particularly organic effluent evaporators is characterised by circulation (by pipes 9, 13 and 11) on the inner surface of component (eg tubes (3) thereof) of an aqueous soda solution containing 4 to 11 mole/1 of NaOH, at ambient temperature and then rinsing with water is carried out. Preferably, the soda solution contains a wetting agent, e.g. an equimolar quantity of dibutyl phosphate and monobutyl phosphate and at least 15 hours elapse before rinsing with water is performed in order that the soda which has impregnated the deposits to be eliminated has time to convert them into water-soluble products. in one embodiment circulation of the soda solution takes place for approximately 5 min, followed by a 24 hour wait, and then water rinsing takes place for approximately 30 secs. <IMAGE>

Description

Process for cleaning the metal surface of an installation component used for the treatment of organic effluents.

The present invention relates to a process for cleaning the metal surface of an installation component which has been used for the treatment of organic effluents, particularly organic effluent evaporators.

The evaporation of organic effluents leads to the formation of deposits based on mineral and organic substances, resulting from the decomposition of organic solvents on the surface of components such as evaporators and every so often these deposits must be eliminated in order to ensure a satisfactory operation of the evaporators. These deposits e.g. have the compositions given in uglg of deposit in the attached table, which refers to two evaporator types which are at present in operation.

In order to eliminate these deposits, use can be made of decontamination processes employing solutions able to attack and dissolve these deposits. Thus, FR-A-2,641,404, describes a process for the decontamination of installations such as distillation apparatus, with a view to eliminating deposits of metallic compounds containing dibutyl phosphate ions and consisting of rinsing the apparatus by means of a solution containing a diamide, an organic diluent such as butyric acid, and an inorganic acid such as nitric acid, followed by the rinsing of the equipment with an organic diluent.

In this document, it is also stated that it is possible to decontaminate apparatus contaminated by metallic compounds containing dibutyl phosphate ions by means of concentrated hot soda.

However, in the case of the deposits present in organic effluent evaporators, such as evaporators I and II of the attached table.

the use of hot concentrated soda does not make it possible to obtain a satisfactory cleaning, because 74% by weight of deposit remains after washing with lON NaOH at 900C for 2 hours.

In addition, following research carried out on various organic or mineral solvents permitting a satisfactory cleaning of the surface of organic effluent treatment installations, a cleaning process has been developed, which also uses soda, but still makes it possible to obtain a deposit elimination rate well above that obtained with lON soda at 90 C.

According to the invention, the process for cleaning the metallic surface of an installation component used for organic effluent treatment comprises the following successive stages: a) circulating on said surface at ambient temperature, an aqueous soda solution containing 4 to 11 mole/l of NaOH and b) rinsing the thus treated surface with water.

In this process, stage a) of circulating the aqueous soda solution over the surface to be cleaned makes it possible to impregnate with soda the said deposits and attack the same. The water rinsing stage b) has the effect of solubilizing the attacked deposits and of permitting their elimination in the rinsing water, whereas said elimination was impossible with the soda solution, due to the limited solubility of the deposits in said solution.

Preferably, according to the invention, the soda solution used in stage a) also incorporates at least one wetting agent. This makes it possible to impregnate all the deposits on the surface of the component and thus facilitate their attacking and elimination.

According to the invention, the wetting agent used more particularly depends on the nature of the deposits, i.e. that of the organic effluents treated in the installation.

In treatment installations for organic effluents also containing tributyl phosphate, such as evaporators, it is in particular possible to use as the wetting agent dibutyl phosphate (DBP), monobutyl phosphate (MBP) and mixtures thereof. Preference is given to the use of an equimolar mixture of dibutyl phosphate and monobutyl phosphate.

Advantageously, the wetting agent quantity represents 0.01 to 1% by volume of the soda solution.

For performing the process according to the invention, stage a) of circulating the soda solution is carried out and then there is generally a wait before carrying out the water rinsing stage b) to enable the soda solution impregnating the deposit to be eliminated to have time to attack the entire deposit.

Thus, the water rinsing stage preferably takes place at least 15h following the end of the soda solution circulating stage a).

Other features and advantages of the invention can be gathered from the following description given in a non-limitative, illustrative manner with reference to the single drawing, which shows in diagrammatic manner an installation for cleaning an evaporator having vertical tubes.

The drawing shows an evaporator 1 to be decontaminated, which comprises vertical tubes 3 internally having deposits formed during the evaporation of organic effluents. These tubes are arranged between two tube plates, which respectively define with the enclosure of the evaporator an effluent supply inlet 5 and an outlet collector 7.

For cleaning the inner surface of the tubes 3, into the inlet 5 of the evaporator is introduced the soda solution or the rinsing water by means of an auxiliary pipe 9. Following the passage into the evaporator tubes, the solution or rinsing water which passes out into the collector 7 can be recycled by a pipe 11 equipped with a pump 13 into an auxiliary supply pipe 9, or can be discharged by a pipe 15 equipped with a valve 17.

Thus, in stage a), circulation takes place in the tubes 3 of the soda solution and the solution is recycled to the inlet of the evaporator 5 for the desired time for impregnating with soda solution the deposits formed in the tubes.

There is then a wait of at least 15 hours, e.g. 18 to 36 hours, so as to enable the soda solution impregnating the deposits to have time to attack and transform them into water-soluble products and then water rinsing takes place by circulating the water in the tubes 3 and recycling or not the rinsing water to the evaporator inlet 5, in order to completely eliminate the deposits. A rinsing time of approximately 30s is generally sufficient.

For these different operations, the soda solution and rinsing water flow rates are chosen as a function of the viscosity of the solutions and the characteristics of the evaporator, so as to ensure the formation of a regular liquid film within all the evaporator tubes.

For example, it is possible to use flow rates of 2.5 to 5.5 m3/h, which corresponds to a linear charge of 890 to 1960 1/hem in the case of existing evaporators.

Thus, a flow rate below 2.5 m3/h is inadequate to ensure an even mediocre wetting. The wetting quality becomes acceptable as from 2.5 and more particularly 3m3/h.

According to the invention, working takes place at ambient temperature, namely a temperature of 15 to 40 C. The pressure can be atmospheric pressure.

The following examples relating to the cleaning of evaporators with tubes with a diameter of 36.6 to 110 mm and a length of 2 to 5m and having up to approximately 2g/m2 of deposits, illustrate the results obtained with the process according to the invention.

Example 1.

In this example, working takes place at 200C and use is made of a 10 molell soda solution in stage a). This solution is circulated at a flow rate of 1000 1/hem on the inner surface of the evaporator tubes for 5 min. There is then a 24h wait, followed by the water rinsing, once again using a flow rate of 1000 1/hem for 15s.

At the end of the operation, there is a satisfactory elimination of the deposits present in the evaporator tubes.

Example 2.

The same operating procedure as in Example 1 is adopted, but to the soda solution is added 0.05% by volume of an equimolar mixture of dibutyl phosphate (DBP) and monobutyl phosphate (NDP).

Under these conditions, there is a better deposit elimination than in Example 1 due to an improved wetting of the deposits by the soda solution.

Example 3.

In this example, working takes place at 200C using a 5 mole/l soda solution. This solution is circulated in the evaporator tubes at a rate of 3 m3/h, i.e. 1070 1/hem for 5 min. There is then a 24h wait, followed by water rinsing for 30s, once again at a rate of 3 m3/h. Under these conditions, there is a satisfactory elimination of the deposits formed on the evaporator surfaces.

Example 4.

This example adopts the same operating procedure as in Example 3, but the soda solution also contains 0.05% by volume of an equimolar mixture of DBP and MBP and a solution flow rate of 2.5 m /h (890 1/hem) is used. Under these conditions, very good results are obtained with a lower circulation rate, because the wetting is better than in Example 3.

Comparative Example 1.

This example makes use of a 10 mole/l soda solution, which is circulated in the evaporator tubes for 15h at a flow rate of 100 l/h at 200C.

After said 15h, the deposit which was a chestnut colour at the start has become black. This colour change is due to the impregnation of the deposit by the soda solution, but the colouring of the soda solution has not significantly evolved during said 15h due to the limited solubility of the deposit in said solution.

However, if following the said operation a rinsing with water takes place, there is a perfect cleaning of the evaporator surface and the rinsing water, unlike the washing soda solution, is black. Thus, the rinsing stage is essential in the process according to the invention.

Comparative Example 2.

This example makes use of a 10 mole/l soda solution at 90 0C and it is circulated for 2h in the evaporator tubes. After said two hours of treatment, it is found that approximately 74% by weight of the deposits are still present in the evaporator. Thus, the hot soda also has a limited effectiveness and does not make it possible to dissolve the deposits.

TABLE Elements Evaporator I Evaporator II (ug/g of deposit) (pg/g of deposit) Al 75 300 Ca 80 400 Cl 25 50 Cr 500 100 Cu 25 20 Fe 40000 500 K 50 N.D.

Mg 50 80 Mn 15 15 Ni 100 100 Pb 5 80 S 1600 4500 Si 800 500 Ti 9 100 Zn 50 15 U 500 C 54.104 50.4.104 N 2.4.104 3.6.104 0 22.2.104 28.6.104 P 6.7.104

Claims

CLAIMS 1. Process for cleaning the metallic surface of an installation component which has been used for the treatment of organic effluents, characterized in that it comprises the following successive stages: a) circulating on said surface at ambient temperature, an aqueous soda solution comprising 4 to 11 mole/l of NaOH and b) rinsing the thus treated surface with water.
2. Process according to claim 1, characterized in that the soda solution used in stage a) also contains at least one wetting agent.
3. Process according to claim 2, characterized in that the wetting agent is chosen from among dibutyl phosphate, mono butyl phosphate and mixtures thereof.
4. Process according to claim 3, characterized in that the wetting agent is constituted by an equimolar mixture of dibutyl phosphate and monobutyl phosphate.
5. Process according to any one of the claims 2 to 4, charact erized in that the wetting agent quantity is 0.01 to 1% by volume of the soda solution.
6. Process according to claim 2, characterized in that the aqueous soda solution comprises 5 mole/l of NaOH and 0.05% by volume of an equimolar mixture of dibutyl phosphate and monobutyl phosphate.
7. Process according to any one of the claims 1 to 6, charact erized in that the water rinsing takes place at least 15 hours after the end of the stage a) of circulating the soda solution.
8. Process according to claim 7, characterized in that the circulation of the soda solution takes place for approx imately 5 min, followed by a 24 hour wait and then water rinsing takes place for approximately 30s.
9. Process according to any one of the claims 1 to 8, charact erized in that the component is an organic effluent evapora tor.
10. Process for cleaning the metallic surface of an installation component substantially as hereinbefore described with reference to the accompanying drawings.