Glycol dehydration: handling and operational problems pH - salt - oxidation
5. GLYCOL pH CONTROL
The pH of a glycol solution is the measure of its acidity or alkalinity, and is measured on a scale of 0 - 14. A pH of less than 7 is an acid solution , 7 is neutral and, greater than 7 is an alkaline solution.
The corrosion rate of equipment increases rapidly with a decrease in the glycol pH. The formation of organic acids, resulting from the oxidation of glycol, thermal decomposition products or acid gases picked up from the gas stream, are the most troublesome corrosive compounds. Therefore, the glycol pH should be checked periodically and kept on the basic side by neutralising the acidic compounds with borax, Ethanol-amines or other suitable alkaline chemicals to maintain the pH at 7.5 to 8.0. A glycol solution that is too alkaline - i.e. pH greater than 9.00, tends to foam and emulsify .
6. SALT CONTAMINATION
Salt deposits accelerate equipment corrosion, reduce heat transfer in the glycol reboiler and change the specific gravity readings when a hydrometer is used to determine glycol concentration. These troublesome compounds cannot be removed by normal regeneration processes. Salts should be prevented by the use of effective filters or an efficient scrubber.
7. GLYCOL OXIDATION
Oxygen can enter the glycol system via the vapour space of an un-blanketed storage tank or through the glycol make-up pump packing glands ... etc. The glycol will oxidise readily in the presence of oxygen (air) and form corrosive organic acids
Precautions should be taken to prevent glycol oxidation. It is highly recommended, that process vessels that can draw in air as the liquid level is lowered, should contain a gas blanket to keep oxygen (air) out of the system. Oxidation inhibitors, such as Hydrazine can be used to prevent the formation of corrosive, organic acids.
8. SLUDGE FORMATION
Accumulation of solid particles and tarry hydrocarbons very often forms in the glycol solution. This sludge is suspended in the circulating glycol and, over a period of time, the accumulation becomes large enough to settle out.
This action results in the formation of a black, sticky and abrasive gum which can cause erosion of the equipment. It usually occurs when the glycol pH is low and becomes very hard and brittle when deposited on the absorber trays, still column parts and other areas in the circulating system. Good, effective filtration will prevent the build-up of sludge in the glycol system.
