RISK ASSESSMENT: DISTILLATION OF SOLVENTS

Name and status of Assessor: Dr. K. MacNeil, DSA.

This Risk Assessment should be read in conjunction with that for "Transport, Storage and Use of Sovents and other Flammable Liquids".

Additional Hazard

With distillation, a specific chemical drying agent is used for each solvent which may react violently with other solvents. Particularly dangerous in this respect are sodium and lithium aluminium hydride.

Remaining Risks

Risks should normally be low if the rules below derived from experience are followed carefully, the stills are constructed to the standard design and sources of ignition are controlled. However, distillation does involve risks and an important first question must be do you really need a solvent still? Can you use commercial dried solvent or share a still?

Training

New sudents must take the training course in the use of stills given at the beginning of the Session.

Recommendations for Solvent Still Operation and Construction

1. Location of Stills
   1. Solvent stills are potentially dangerous and should be located in a fume cupboard dedicated to this purpose whereever possible. Stills for diethyl ether or for toxic solvents like benzene, must be located in a fumecupboard.
   2. If stills have to be located in a fumecupboard which is also used for other activities, a barrier should be erected to prevent dangerous interaction between these activities.
   3. If stills have to be located in the open laboratory they should be away from areas where people sit, from any source of ignition and should be protected from damage if a flood brings down ceiling tiles.

2. Water Supply
   1. All water connections, from the tap to the still and from the still to the drain, are to be secured using nylon ties.
   2. Water tubing to be made of a material such as translucent polyvinyl chloride, which is resistant to perishing.
   3. The outlet water from a still or from from the last still in a group must pass through a flow switch (see electrical requirement).
   4. Stopcock to be inserted into water supply to limit maximum flow rate where possible.
   5. Water flow switches must be cleaned regularly as algae may stop correct operation.

3. Nitrogen Supply and Vent
   1. On no account must it be possible to seal off a still from its vent and so create a pressurised system.
   2. Each still must have its own individual nitrogen supply valve and bubbler.
   3. Flexible connections are to be made using translucent polyvinyl chloride tubing, and to be secured using a nylon tie to the supply valve, the still 'Tee' pieces and the bubbler.
   4. Flexible connections in the case of a tetrahydrofuran still should be made from polytetrafluorethylene tubing, which is resistant to this solvent. For advice on connecting this tubing to the still head and bubbler, see Adrian Stevenson (E317) T el 4261.
   5. All exit tubing to be attached so as to minimise the danger of kinking and vented to a fumehood or the exterior of the building.
   6. Flexible tubing to be used to attach vent from the bubbler to the copper manifold, which is in turn connected via 10 mm copper pipe to a fume hood or the exterior of the building.

4. Electricity Supply
   1. The electricity supply must be controlled by a water flow switch which will turn off the electricity if the water supply drops below a minimum safe flow rate. These devices are available from the Electronic Workshop, E.102, and are rated at 10 A, 240 V.
   2. The electricity supply from this water flow switch is then connected to a range of individual switched sockets, and finally via fused plugs and simmerstats, to the heating mantles.
   3. The solvent catch trays should be earthed.

5. Miscellaneous
   1. All still flasks must be labelled in plain English, no chemical shorthand, stating the solvent and the drying agent. Labels available from Adrian Stevenson E.317.
   2. Each still to have its own individual catch tray made of aluminium. These may be ordered from the Mechanical Workshop. A list of recommended sizes is available.
   3. All flammable solvent stills must only be filled or re-filled when they are at room temperature. There should be no electrical device turned on in the fume cupboard when the still is being filled and all heating devices such as mantles or hotpla tes must have been turned off for at least 5 minutes to avoid chance ignition of vapour.
   4. Stills to be turned off when left unattended.
   5. The joint between the still flask and the reciever must be fitted with a heavy duty p.t.f.e joint sleeve.

6. Recommended Solvent Drying Agents

   Solvent	Drying Agent
   Tetrahydrofuran	Sodium wire/benzophenone
   Ethanol	Magnesium
   Acetonitrile	Calcium hydride
   Acetone	Calcium chloride
   Dichloromethane	Calcium hydride
   Ethyl acetate	Calcium hydride
   (2-Methoxyethyl)ether	Sodium
   40/60 Petrol ether	Calcium hydride or Sodium wire/benzophenone/triglyme
   Toluene	Sodium
   Diethyl ether	Sodium wire/benzophenone
   Methanol	Magnesium
   Hexane	Calcium hydride or Sodium wire/benzophenone/triglyme
   Pentane	Calcium hydride or Sodium wire
   Heptane	Calcium hydride or Sodium wire
   Benzene	Calcium hydride or Sodium wire
   Xylene	Sodium

   This list is not exhaustive, but gives combinations of solvents and drying agents in use in the Inorganic Section. The routine use of potassium or its alloys is not recommended because of the risk of fire; if potassium must be used, a Special Assessment has to be made under COSHH with particular concern for its disposal.

7. Safe Disposal of Drying Agents in Stills

   The reactivity of the above drying agents means that disposal has to be carried out with great care. In the procedures outlined below for in situ decommissioning of stills, it is assumed that the stills are at room temperature and electrically isolated and that they remain connected to a flow of cooling water and nitrogen. Put a warning notice on the still during decommissioning so it is not accidentally turned on.

   (a) Disposal of sodium (10 g or less)

   The flask must be at least 1/3 full of solvent before the addition of reagents is commenced. About 30 cm3 of ethanol is syringed into the solvent receiver with the tap closed. The ethanol is then added slowly to react with the drying age nt.There may be a delay in hydrogen evolution whilst surface contamination of the drying agent is dissolved. It is important to swirl the flask during the addition to ensure adequate mixing. When the evolution of hydrogen has ceased, a small amount (ca .15 cm³ ) of a 1:1 ethanol/water mixture is cautiously added. If no further hydrogen evolution occurs, water is cautiously added to fill the flask and it is left until no more bubbles of gas can be seen. The flask can now be detached, the u pper organic layer separated and put into the waste solvent container and the lower, aqueous layer washed down the sink with lots of water (take special care with tetrahydrofuran as this is fairly soluble in water).

   (b) Disposal of calcium hydride or magnesium

   The procedure is similar to that used for sodium except that a mixture of 90% ethanol and 10% water is used in place of pure ethanol. When hydrogen evolution ceases, water may be added cautiously and the still dismantled when no bubbles c an be seen.

   If an aqueous and an organic layer have formed, these must be separated, the solvent put into the appropriate waste solvent bottle and the aqueous layer washed down the sink with lots of water. If the water and solvent are miscible, often the whole contents of the flask can be washed down the sink with lots of water to render the mixture non-flammable - if you have doubts about what is allowed, check with the University Code of Practice on Disposal of Chemical Waste or with the DSA.

   • Glass bottles which contain sodium and solvent should be handled with extreme care. An incident has occurred in the School of Chemistry when such a bottle, which was being carried, struck and smashed on the edge of a bench. The person involved s lipped in the spilled solvent and fell into the pool on the floor absorbing the solvent into their clothing. It was only the speed of a co-worker in dealing with the exposed sodium wire that prevented an horrific burning accident.

Emergency Procedure

1. Personal injury or fire:- follow the procedures outlined in the School of Chemistry Safety Handbook under "Coping with an Emergency".
2. Spillage, no fire. Beware of drying agent! Users should be fully conversant with the procedures required to make safe any drying agent that is in danger of being exposed to air o r water.
   1. Serious - toxic or large quantities. Evacuate and ventilate the affected area, closing doors and eliminating sources of ignition if it is safe to do so. Telephone the PORTER (7458)and the DSA (8311) or SECURITY out of hours (7848) and OPERATE THE NEAREST FIRE ALARM POINT. Go to the Porters' Lodge to advise the emergency team of the nature of the incident. DO NOT ATTEMPT TO CLEAN UP A MAJOR SPILLAGE BY YOURSELF.
   2. Minor. Ventilate the affected area and eliminate any sources of ignition. Inform a member of Staff and your co-workers. Decide on and use suitable protective equipment such as gloves, lab. coats, respirators. The liquid may be absorbed onto absorption granules available at the Fire Points (have them replenished after use) and, as appropriate, transferred to a fumehood to evaporate or to a suitable sealed container for waste disposal. In a well ventilated area such as a labo ratory, the best procedure may be simply to turn off sources of ignition, ventilate, evacuate and seal and secure the room.