The TL series works on old oil that has turned black and smells of burning, using adsorptive media such as activated carbon and silica gel to catch the oxidation products and the polar impurities in the oil on the surface of the medium, which is then thrown away together with the medium. To understand this equipment, the key is to understand adsorption capacity: the amount that a material surface can adsorb is limited, and it does not keep growing with time.
How the adsorption takes place
The oil flows through the adsorption cylinder filled with the medium, the oxidation products and the polar impurities are held back on the surface of the medium, and the oil that passes through becomes transparent again. The process has two characteristics. First, it takes place on the surface of the medium, so the capacity is set by the surface area and the pore structure of the medium. Second, adsorption is selective: the medium captures specific types of molecule. That is why the oil passing through a spent medium becomes noticeably clearer without being clean, since the medium cannot capture water or gas at all and the vacuum stage still has to look after them. Decolouring equipment is normally connected after the vacuum degassing, as the last stage of the whole line.
The saturation curve: fast at the beginning, slow at the end
The change in adsorption capacity with time is not linear. At the start of contact there are many fresh adsorption sites on the surface of the medium, the contaminants are captured quickly and the decolouring effect improves fast. As the available sites are filled one by one the adsorption rate falls gradually and the curve enters its flat section. Once the medium is saturated, running it longer no longer improves the decolouring capability at all; it only continues to pass oil through the medium. This curve explains a great many things seen on site: the drum of oil that starts running comes out noticeably lighter, and after a while the same oil goes in and the colour rebounds.
- Early stage: the decolouring is pronounced and the differential pressure is basically stable.
- Middle stage: the decolouring effect starts to deteriorate and the differential pressure rises slowly.
- After saturation: the colour of the oil rebounds, the differential pressure rises markedly and the flow rate starts to fall.
Three external signals before and after saturation
The most direct one is the rebound in colour: the colour difference across the same batch of oil becomes much smaller, and may even approach the colour of the incoming oil. The second is the rise in differential pressure, with the inlet and outlet pressure gauges showing more resistance, because the surface of the medium is covered with contaminants and deposits, the pores narrow, and the oil needs a higher pressure to pass through. The third is the fall in the flow rate: at the same opening the outlet flow is smaller than it was when the medium was new. The three do not necessarily appear at the same moment; the differential pressure often appears before the change in colour, so using the differential pressure as the early signal is more reliable.
| Stage | Decolouring effect | Differential pressure | Action on site |
|---|---|---|---|
| Newly fitted medium | Pronounced | Low | Record the initial differential pressure as the baseline |
| Approaching saturation | Starting to deteriorate | Rising steadily | Have medium of the same specification ready |
| Saturated | The colour of the oil rebounds | Rises markedly | Withdraw the whole assembly and replace the medium |
| Medium can be regenerated | Depends on the degree of regeneration | Falls back | Decide on regeneration or direct replacement according to the medium |
Replace or regenerate
Whether regeneration is possible depends on the type of the medium and on the nature of the contaminants. A medium that has adsorbed desorbable contaminants can be restored by driving the contaminants off with heat or steam, and the medium becomes active again. But if what has been adsorbed is oxidation products that have already changed chemically, or if metal contaminants have been mixed in, very little of the capacity comes back after regeneration. There are two criteria that can be used on site. First, record the cumulative volume processed and the curve of the differential pressure against time, and see whether the medium is saturating abnormally quickly. Second, withdraw the medium and look at its colour and its condition; if the surface is coked or has formed lumps, replacing it outright is better value than regenerating it repeatedly. The adsorption cylinder of the TL-50 uses a quick-release structure, so withdrawing the whole assembly and replacing the medium does not require the pipework to be disturbed, and the operation of changing the medium should be able to be done within a few minutes.
Saturation of the medium can be read off, and the differential pressure gives the warning before the colour does; by the time the colour of the oil rebounds and you start to act, the decolouring opportunity for that batch has already been lost.