| Near ten thousand coolers are used for oilfield extract gas in Daqing oilfield. The deposit comes into being in the coolers during use. This leads reduction of cooler heat efficiency and increase of energy cost . And this causes bottleneck in production. How to remove the deposit is an urgent problem.The main components of the deposit in the cooler are sulfur, ferric oxide and sulfide. Most coolers have complex structure, so cleaning the deposit with acid is a good way. The primary principle for dissolving the deposit: the acid may dissolve some of the deposit and loosen some others.1. Dissolution2.loosening while some components in the sulfuric deposit dissolve in the acids, other components not dissolve will loosen and fall off. On the other hand, the H2S gas create from the washing will accelerate the loosening of the sulfuric deposit.The advantage of inorganic acid washing is removing deposit quickly and low cost. The disadvantage is that the acids may erode the equipment to some extent .The acids used in washing are sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid,hydrofluoric acid and so on.Sulfuric acid as acid washing component is low cost and no corrosion to stainless steel and aluminum alloy equipment. Since sulfuric acid solution is steady to heat, when heating, washing rate increases. When sulfuric acid solution(5%-15%) as acid washing component, temperature may reach about 60℃.Hydrochloric acid as acid washing component is removing deposit fast and low cost. But hydrochloric acid has volatility and it is generally used at lower than 40℃and can't used to stainless steel.Nitric acid as acid washing component is used to the equipment of stainless steel, carbon steel and brass. But at low concentration, nitric acid has strong corrosion to most metals, especially to carbon steel.The main organic acids used in acid washing are aminosulfonic acid , alkyl radical acetic acid, citric acid, EDTA and so on. Compared with inorganic acid washing, the organic acid washing is high cost, used at higher temperature, long washing time. But organic acids have low corrosion and stronger removing deposit ability.Aminosulfonic acid is appropriate for calmagite carbonate deposit but not for dissolve rust. To dissolve rust better, Aminosulfonic acid and sodium chloride are mixed to create hydrochloric acid slowly. They are hydrolyzed to create ammonium sulfate and ammonium bisulfate at high temperature. So washing temperature need to be controlled lower than 60℃to reduce hydrolyzing.Citric acid is a kind of organic acid used most in acid washing. It can dissolve ferric oxide and cupric oxide deposit. It is used to wash the rust and cupric oxide deposit of different material property equipment. Generally, the washing solution temperature is between 90℃and 105℃, not lower than 0℃.EDTA has low solubility at room temperature, so when EDTA used as acid washing component, the washing temperature is higher, generally more than 100℃.Since the coolers to be washed are made from stainless steel, carbon steel, so sulfuric acid is chosen as acid washing component. Testing its dissolving deposit ability at different concentrations and temperatures, considering the factors of practicability and economics at sites the best acid washing components and acid washing conditions are chosen.To low the acid's corrosion to the surface of coolers, mental corrosion-inhibitors are added during acid washing. The mental corrosion-inhibitors in common use are DA-6, Lan-826 and LK-45.The factors that affect corrosion-inhibitor are the concentration of corrosion-inhibitor, time, temperature and velocity of flow. Corrosion-inhibitor's performance is appraised by sample strip .Lan-826 is the best. It's delay corrosion efficiency are not affected by acid washing temperature, Fe3+, acid washing time and liquid velocity of flow. Delay corrosion efficiency reach the top when it's concentration is at 0.30%. The research and application of corrosion-inhibitor develop fast in China. It is characteristic to extract corrosion-inhibitor from plants, chemical and medical byproduct in China.To finish acid washing at sites, a scientific washing system is needed. A compulsion circulation washing system is introduced according to cooler's structure. The acid solution may not only wash the sulfuric deposit but also take away the sulfuric deposit by the compulsion circulation washing system and avoid jam in pipelines.To acquire good result, it is needed to reinforce Supervision and analysis to acid wash solution. The acidity of acid wash solution and concentration of dissolved metal ion are needed to supervise and analyze. The acidity of acid wash solution affects washing rate directly, and also reflects washing extent indirectly. The concentrations of dissolved metal ion(Fe3+,Cu2+) are needed to measure. The concentration of Fe3+ is an important target to control. The concentration of Fe3+does not get higher is a criterion in the washing when the main components are rust in the deposit.Supervision and analysis are made by sample strip. The material property of the sample strip selected is the same with the equipment's material property to be washed.The site washing scheme: temperature 50±1℃, sulfuric acid solution ( 5% ) wash for 40 min, then add another half sulfuric acid solution ( 10% ). During washing, measure the concentration of sulfuric acid solution every 30 min, keep the concentration between 7.5% and 10%.By the site washing, the whole process is easy to operate and control. The acid washing component and corrosion-inhibitors selected have the advantages of removing the deposit efficiently (more than 95%), light corrosion to equipment and low cost.The washing scheme is not only applicable to removing sulfuric deposit in the coolers for oilfield extract gas but also for the deposit in the pipeline to translate oilfield extract gas. But with environment protection and safety are paid more attention and the development of science and technology, organic acid washing such as aminosulfonic acid washing, alkyl radical acetic acid washing, citric acid washing, EDTA washing and physical washing like microwave washing will apply more.
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