Study On EDTA Cleaning Dispersing Agent And Its Process

Boiler is one of the main equipment of thermal power plant.Boiler scaling can not only have an influence on boiler thermal efficiency and electric energy production,but also cause local overheating of boiler tubes,resulting in the emergence of red tubes or even explosion accident.Therefore,it is essential for power plant to carry out boiler cleaning operations as indispensable overhaul procedures every year.However,the truth is that boiler which has been cleaned is more prone to the emergence of red tubes in the actual operation,leading to re-cleaning or even blocking parts of them.Based on the field investigation and analysis of parts of red tubes,the main reason why boiler arises of red tube failure was that when conducting boiler cleaning operations,the boiler scale particles were not discharged in time,but deposited on the bottom or the corner of the boiler tubes.With the improvement of boiler capacity,pressure,safety operation and requirements of energy saving and envirornmental protection,a simple acid bath for descaling can not meet the requirements of power plant boiler.Thus,a higher standard was put forward to solve the problem of boiler descaling.In order to avoid the deposition of scale particles washed off from boiler tubes,the dispersants used for scale particle by EDTA boiler chemical cleaning and the corrosion inhibition of dispersants to cleaning agent were studied in this work by means of gravity sedimentation,electrochemical polarization and AC impedance method.The process of EDTA chemical cleaning applied in power plant boiler was also investigated.The results were concluded as follows:(1)Three kinds of dispersants(sodium dodecyl sulfonate,sodium hexametaphosphate and PBTCA)had good dispersibility to boiler scale particles within 10 minutes.However,scale parnicles began to deposit due to the action of gravity after 20 minutes,leading to the reducion of the dispersibility.When the addition of the dispersant was 0.1-0.15 wt.%,its dispersibility was sodium hexametaphosphate>PBTCA.(2)Three kinds of dispersants had good corrosion inhibition to chemical cleaning of boiler.When the concentration of DTPA was 5 wt.%,the corrosion rates of sodium dodecyl sulfonate,PBTCA and sodium hexametaphosphate were 30%,57%and 65%,respectively,as their contents respectively exceeding 0.1 wt.%at room temperature.(3)When sodium hexametaphosphate was used as the dispersant,the maximum value of the initial speed of the film formation was that the adsorption film was formed within about 1 second.However,However,the current began to oscillate after formation of the adsorption film,indicating that some ions escaped from the surface of it and some new ions absorbed on the surface of it,leading to the result that ions in the surface film were in dynamic equilibrium.When the dispersant was sodium dodecyl sulfonate,the adsorption film was formed within 2 seconds.When PBTCA was used as the dispersant,some organic groups firstly absorbed,followed by adsorption of the relatively large inorganic groups,which made the adsorption film noncompact.As time goes by,the adsorption film became thicker and more compact.The growth of the adsorption film with respect to the dispersant,was firstly adsorbed onto on the surface of boiler steel to form a film,and then the further growth was the formation of a denser film layer by ions penetrating through the adsorption film.(4)During the actual cleaning operation,a composite dispersant system,based on sodium dodecyl sulfonate with addition of 0.1 wt.%and sodium hexametaphosphate with addition of 0.3 wt.%,was appropriate to the cleaning agent with high inhibitive efficiency.In addition,a composite dispersant system,based on sodium dodecyl sulfonate with addition of 0.3 wt.%and sodium hexametaphosphate with addition of 0.3 wt.%,was suitable for the condition of being strict with the dispersibility of scale particles.(5)Cyclic cleaning method must be adopted to the cleaning agent of EDTA chemical cleaning.The appropriate flow velocity,cleaning temperature and pH of the cleaning agent were 3.85-5.1(m/s),85-90 ? and 7-8,respectively.