GB2049181A - Detection of acidic solvent decomposition products - Google Patents

Abstract

A method for detecting an unacceptable degree of acid-forming decomposition of a halogenated hydrocarbon solvent, for example in a metal-degreasing or a solvent-drying bath, which comprises contacting vapour emitted by the solvent, in the presence of water, with a fuse made of a material which is soluble in the acid decomposition product(s) of the solvent, and detecting rupture of the fuse. In a specific embodiment a zinc fuse is used for detecting an unacceptable degree of decomposition in chlorinated hydrocarbon solvents.

Description

SPECIFICATION Method for detecting solvent decomposition This invention relates to a method for detecting decomposition of halogenated hydrocarbon solvents and particularly to a method for detecting decomposition of chlorohydrocarbon solvents which results in the formation of acid, usually hydrochloric acid, for example decomposition induced by heat, metal ions and/or by hydrolysis of the solvent in the presence of water. The invention is especially concerned with a method for determining the point at which decomposition of chlorohydrocarbon solvents reaches an unacceptable level.

Halogenated hydrocarbon solvents containing chlorine, notably chlorohydrocarbon solvents, are used extensively in industry, for example for cleaning and/or drying metal and plastics articles, the most notable instances of such use being chlorohydrocarbon solvents and especially 1,1,1 -trichloroethane in cleaning and degreasing metals for fabrication into articles, and trichloroethylene and perchloroethylene in cleaning and drying metal articles.

The chlorohydrocarbon solvents are often used at elevated temperatures up to and including their boiling points and in many instances cleaning or drying is effected at least partially in the vapour of the solvent.

The chlorohydrocarbon solvents are often used under conditions whereby they come into contact with finely-divided metals and/or water which may be present on articles being cleaned or dried or which may, especially in the case of drying compositions, be added as part of a cleaning/drying formulation. Under these conditions, and especially at locally elevated temperatures, i.e. hot-spots, the solvent may be decomposed to yield acid, especially hydrochloric acid, amongst the decomposition products. A low degree of solvent decomposition (acid formation) may be acceptable before the ability of the solvent to fulfil its intended purpose is seriously impaired, and the acidacceptance value of the solvent may be increased by incorporating acid accepting stabilizers, but eventually it becomes necessary to replace the solvent to achieve satisfactory results.By acid acceptance value is meant the capacity of the solvent to absorb acid expressed as equivalent concentration by weight of caustic soda.

There is usually no visible evidence of chlorohydrocarbon solvent decomposition, which therefore is usually estimated on the basis of the acid acceptance value of the liquid solvent. Frequently such an estimate of acidity, for example in metal degreasing baths is made by the process operator removing a sample of the liquid solvent and carrying out a back titration on the sample to determine its acid acceptance value. This method provides an accurate estimate of decomposition of the solvent but is time-consuming and troublesome to its operator so that checks on the solvent frequently are overlooked or are not carried out with sufficient regularity.The present invention resides in a method of detecting chlorohydrocarbon solvent decomposition, and particularly for detecting the stage at which decomposition reaches an unacceptable level, which does not involve the process operator in removing and analysing samples of the solvent and which provides an accurate and reliable estimate of an unacceptable degree of decomposition of the solvent.

According to the present invention, there is provided a method for detecting an unacceptable degree of acid-forming decomposition of a halogenated hydrocarbon solvent which comprises contacting vapour emitted from the solvent with a fuse comprising a material soluble in the acid decomposition product(s), and detecting rupture of the fuse.

The fuse will normally be made of a metal, for example iron, copper, brass or zinc and usually will be in the form of a wire or an evaporated film. Since the products of decomposition of the solvent will normally include hydrochloric acid, we prefer to use a metal fuse which is readily soluble in hydrochloric acid, especially a zinc fuse. In the case of a wire fuse the diameter of the wire is not critical though in general the smaller the diameter of the wire the greater will be the ease of rupture of the fuse and the more sensitive will the fuse be to acid. It is a simple matter of trial to determine the optimum combination of wire diameter and material for the wire to provide a suitable fuse for any selected unacceptable degree of solvent decomposition.As a guide we have obtained satisfactory detection of unacceptable decomposition of 1,1,1trichloroethane in metal degreasing baths us inga zinc wire fuse of diameter 50 microns, and in general we regard zinc wire of diameter in the range from 20 microns to 500 microns as suitable materials for the fuse.

Apparatus for using chlorohydrocarbon solvent in cleaning and drying processes usually comprises a cleaning tank having heating means for the solvent and cooling coils located above the liquid solvent level in the tank to condense solvent vapour and retain the solvent in the tank. In this way a solvent vapour zone is established above the liquid solvent. When applying the method of the invention to detection of an unacceptable degree of decomposition of the solvent in such a bath, the fuse normally will be located above the cooling coils so that it is outside the solvent vapour zone created by the cooling coils. Location of the fuse above the liquid solvent but in the solvent vapour zone may result in the fuse being ruptured too quickly at only low degrees of decomposition of the solvent, often in advance of the stage at which decomposition of the solvent becomes unacceptable.Preferably, the fuse is located immediately above and in particular less than 5 cm above, e.g. 2 or 3 cm above, the solvent vapour zone (as defined by the uppermost of the cooling coils creating the vapour zone) in the so-called freeboard zone.

Preferably the fuse is located in a position such that it does not hinder the movement of articles to be cleaned into and out of the cleaning bath. A convenient -location for the fuse in common solvent degreasing baths is at the side of the tank in which case the fuse may be incorporated in a probe, for example a narrow tube, extending through the side wall of the tank. In most proprietory solvent degreasing baths, the cooling coils aye located beneath a ledge so that they are ,protected from damage during movement of articles in and out of the tank. In such an arangement, a probe containing the fuse can usually be located under the ledge in the gap between the ledge and the uppermost of the cooling coils.An advantage of mounting the fuse in.a probe extending through a side wall of the tank is that the detection system for detecting rupture of the fuse is not exposed to corrosive vapours emitted from the tank and can be contained in a box adjacent to or directly attached to the cleaning tank.

Rupture of the fuse results from contact of the fuse wire with the products) of decomposition of the solvent present in the vapour emitted from the solvent. The build-up of free water in the solvent may reduce the amount of the decomposition products in the vapour emitted and in this way affect the ability of the fuse to accurately detect an unacceptable degree of decomposition in the solvent. For this reason it is preferred to incorporate a water-separation device in the system to avoid the build- up of water in the solvent.

Rupture of the fuse is readily detected in a variety of ways and we prefer to employ an electronic detection system which upon rupture of the fuse produces a signal in the form of an electric current. The signal can be used to actuate one or more alarms and/or to put in motion one or more actions to shut down the cleaning operation. For example, the signal can be used to actuate visual and/or audible alarms, which may be continuous or intermittent, and/or to switch off the supply of energy to heaters in the cleaning bath and in the case of automatic operation cease the supply of contaminated articles to the bath.

The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows a schematic representation of one side of a conventional vapour degreas- ing bath provided with a probe containing a fuse for detecting an unacceptable degree of decomposition of the solvent used in the bath, and Figures 2 and 3 show alternative forms of probes containing a fuse.

Referring to Fig. 1 the degreasing bath comprises a lower tank 1 containing liquid solvent 2 and an upper tank section 3 having venting ducts 4. Heaters (not shown) are located in the lower part of the tank for heating the liquid solvent to its boiling point.

The solvent may be, for example 1,1,1-trichlo- roethane. Cooling coils 5 are provided along at least one side of the lower tank for condensing solvent vapour and substantially preventing escape of vapour to the atmosphere.

The cooling coils establish a solvent vapour zone 6 in the tank.

A probe 7 carrying a fuse is mounted through the side of the tank 1 such that the probe is above the level of the uppermost cooling coils 5 in the tank, and so is above the solvent vapour zone 6 and in the freeboard zone.

The probe 7 is connected in an electric circuit for electronic detection of rupture of the fuse contained in the probe; for simplicity the circuit is not shown in the drawing.

Alternative forms of probe 6 are shown in Figs. 2 and 3. The probe in Fig. 2 comprises a rod 8 made of polytetrafluoroethylene in which are embedded wires 9 and 10. These wires respectively connect terminal pins 11 and 1 2 on one end of the probe with sockets 1 3 and 14 in the other end of the probe, the socket providing a means of connecting the probe in an electric circuit by means of a pinand-socket joint 1 5 and cable 1 6. The probe carries a fuse in the form of a thin wire 1 7 mounted between terminal pins 11 and 1 2.

The fuse at the end of the rod 8 is protected from accidental damage by a perforated cover 18.

The probe 7 shown in Fig. 3 is similar to that shown in Fig. 2 but comprises an evaporated metal film fuse instead of a thin wire fuse. This fuse comprises a thin film 1 9 of metal making electrical connection between terminals 20 and 21.

In using the degreasing bath, the solvent 2 is boiled and the solvent vapour zone 6 is thereby created. Articles such as contaminated metal articles to be cleaned and degreased are lowered into the solvent vapour zone 6 where solvent condenses on them and drains back into the body of liquid solvent. The articles are held in the solvent vapour until cleaned and then are removed. Though not shown in the drawing, automatic continuous feed of articles to and from the bath may be provided. As is evident from Fig. 1 the cooling coils 5 and probe 7 are located below the venting ducts 4 of the upper tank section 3 and in this way are protected against accidental damage by articles being lowered into or withdrawn from the solvent vapour zone 6 in lower tank 1.

Operation of the bath as described results in a slight escape of solvent vapour from the vapour zone 6 to the atmosphere and a portion of escaping vapour will contact the probe 6. Water vapour escaping from the bath and/ or adventitious water vapour likewise will contact the probe 6. In normal conditions where the solvent is not appreciably decomposed, little if any acid vapour will escape from the bath and the fuse in probe 7 will remain intact.

However, as decomposition of the solvent increases and the amount of acid in the vapour zone increases in consequence, more and more acid vapour will escape from the bath and contact the probe 6. When decomposition of the solvent reaches an unacceptable level the amount of acid contacting the fuse will, in the presence of moisture, be sufficient to dissolve the metal of the fuse wire 1 7 (Fig. 2) or fuse film 19 (Fig. 3) and rupture the fuse. In two specific embodiments of probe 7 (Fig. 2), the fuse wire 1 7 is a zinc wire of diameter 50 microns and 1 25 microns respectively.

The electric current signal generated upon rupture of the fuse can be employed to actuate one or more alarms and/or to switch off the supply of energy to the heaters. The contaminated solvent in the bath is replaced with fresh solvent and the fuse wire is renewed, and the bath is ready for use again.

Claims (15)

1. A method for detecting an unacceptable degree of acid-forming decomposition of a halogenated hydrocarbon solvent which comprises contacting vapour emitted from the solvent with a fuse comprising a material soluble in the acid decomposition product(s), and detecting rupture of the fuse.

2. A method as claimed in claim 1 wherein the fuse is made of a metal.

3. A method as claimed in claim 2 wherein the products of decomposition of the solvent include hydrochloric acid and the fuse is made of zinc.

4. A method as claimed in claim 2 or 3 wherein the metal fuse is in the form of a wire of diameter in the range of from 50 microns to 500 microns.

5. A method as claimed in any one of claims 1 to 4 for detecting an unacceptable degree of decomposition of a halogenated hydrocarbon solvent contained in a tank having cooling coils located above the liquid solvent level to establish a solvent vapour zone above the liquid solvent, wherein the fuse is located above the cooling coils and outside the solvent vapour zone

6. A method as claimed in claim 5 wherein the fuse is located above the cooling coils at a distance of less than 5 cm above the uppermost cooling coil.

7. A method as claimed in claim 5 or 6 wherein the fuse is located at the side of the tank.

8. A method as claimed in claim 7 wherein the fuse is located beneath a ledge so that it is protected from damage during movement of articles in and out of the tank.

9. A method as claimed in claim 7 or 8 wherein the fuse is contained in a probe extending through the side wall of the tank.

10. A method as claimed in any one of the preceding claims wherein rupture of the fuse is detected electronically.

11. A method as claimed in any one of the preceding claims wherein rupture of the fuse is caused to actuate one or more alarms.

1 2. A method as claimed in any one of the preceding claims wherein the solvent is in use in a cleaning operation and wherein rupture of the fuse puts in motion one or more actions to shut down the cleaning operation.

1 3. Apparatus for using a halogenated hydrocarbon solvent and for detecting an unacceptable degree of acid-forming decomposition of the solvent during its use, comprising a tank for containing liquid solvent, one or more cooling coils within the tank at a level above that at which liquid solvent is to be present in the tank and a fuse located above the cooling coil or the uppermost cooling coil, the fuse comprising a material which is soluble in the product(s) of decomposition of the solvent.

14. Apparatus as claimed in claim 1 3 wherein the fuse is contained in a probe which extends through a side wall of the tank

1 5. A method for detecting an unacceptable degree of decomposition of a halogenated hydrocarbon solvent substantially as described with particular reference to the drawings