The treatment of recovered oil is a chain: collection and temporary storage, washing, dehydration and vacuum degassing, filtration, and only then testing. If the front end is not done cleanly, the best equipment at the back can only separate the contaminants; it cannot remove them. The first gate on the whole chain is the washing, and it decides whether this batch of oil can subsequently be brought up to the specified standard.
The purpose of the washing is to separate out the water, the sediment and the residues of some additives that have got into the oil. The common requirement of water washing and solvent washing is that the washing medium itself must be clean and must not carry new contaminants into the oil. Washing once with water that carries salts or iron ions amounts to introducing new inorganic contaminants into the oil; the same applies to a solvent that contains water or non-volatile residues, since the residue goes into the next process stage together with the oil.
What each stage has to solve
| Stage | Problem to be solved | What happens if it is done badly | How it is judged |
|---|---|---|---|
| Collection and temporary storage | Prevent water and debris from entering at the source | Sediment and free water are carried through into the later stages | Whether the containers are dedicated and whether water has been mixed in |
| Washing | Separate the water, the sediment and the additive residues | Emulsified water and dissolved water are carried into the next stage | Sample after washing and check the water content and the cleanliness |
| Dehydration and vacuum degassing | Remove the dissolved water and the dissolved gas | The water content stays outside ≤3 ppm (GB/T 7600) | GB/T 7600 laboratory testing |
| Fine filtration | Intercept the fine particles | The cleanliness does not reach NAS 1638 class 5 | NAS 1638 |
Why the quality at the front end sets the ceiling on the treatment capacity
Vacuum degassing removes the water dissolved in the oil and the dissolved gas, on the premise that the water is present in a dissolved state. If the washing is not thorough, a large quantity of emulsified water stays in the oil, and this water does not vaporise easily under negative pressure, so the water content stops outside the standard. The same applies to the filtration stage: if the sediment and the coarse particles reach the fine filtration stage first, the media are saturated very quickly, the differential pressure rises, and the cleanliness cannot reach NAS 1638 class 5.
The treatment capacity of the machine therefore cannot make up for the washing quality at the front end. A 6000 L/h vacuum oil purifier may run perfectly smoothly on clean oil, but when it is given a batch of recovered oil that has not been washed properly, the result is still not up to standard.
There is a relation here that is easily overlooked: the washing quality determines the initial condition of the oil that enters the machine, while the level to which the machine can bring the oil depends on how much contaminant it can remove. The dirtier the oil, the higher the purification cost carried by each litre for the same throughput, and the faster the media are consumed. Contaminants cannot be saved at the source, so everything downstream has to be made up with consumables and labour.
The judgements that can be made first on site
- Sample and test the washing medium separately first. If the condition of the water or the solvent itself is not known, there is no basis on which to interpret the later results.
- Send the oil for testing after washing and before it goes into the machine. Run the water content and the cleanliness through first, deal with whatever is outside the range, and do not expect the machine to make it up.
- Separate the sediment physically first. Settling or coarse filtration placed before the washing noticeably reduces the load on the media later.
- Write the replacement interval for the washing medium into the work documents. Both water and solvent have a service life, and replacing them when the time is up is less troublesome than judging afterwards whether they are contaminated.
- Record the origin and the treatment route of every batch of recovered oil. The same machine gives very different results on recovered oil from different origins, and the information on the origin is itself a basis for judgement.
The machine can separate more cleanly what has already been separated, but however much of what cannot be separated there is, the machine has no way to deal with it.
