Study On The Relationships Of Circulation Cooling Water Quality And Water Treatment Effect In The Refinery Device

The development of circulation cooling water treatment technology has experienced two leaps in the recent 70 years, the marks of which are adding acid to controll pH technique and nature pH running technique respectively. The technique of pH controlled with acid has well resolved the problem of heat exchanger scaling and ensured the equipment of cooling water system operating uninterruptedly for more than one year, and the nature pH running technique resolved two problems of heat exchanger corrosion and scaling and ensured the equipment of cooling water system operating uninterruptedly for more than 3 years. These two techniques have been adopted and applied widely, which play a great role of ensuring the equipment running effectively and continuously. However, along with competition growing in products selling, supply and demand contradiction of water resource emerging and environmental protection rules becoming strict, it's strongly required to improve water treatment effect further for prolonging equipment operating period, to increase circulation cooling water concentration factor further for saving fresh water and to reduce pollution output according to environmental demand. The work of improving water treatment effect, saving fresh water, reducing pollution output to the greatest extent, exploring the direction of the third circulation cooling water technology transformation, studying the composition, effect and relationship with water quality of the water treatment agents in use completely, and investigating the internal rules between them, is valuable for scientific research and application.Many methods are used in the experiments, for examples: NMR (nuclear magnetic resonance) and GC-MS (gas chromatography and mass spectrum) were applied to analyze the composition and structures of water treatment agent samples. Rotary coupon corrosion test, electrochemical and static scaling inhibition test methods were applied to study corrosion, electrochemical course and scaling inhibition and dispersion performance for water quality and water treatment agent. Microbe colony counting was applied to evaluate the effect of biocide and slime stripping and cleaning agent. Auger electron spectroscopy was applied to analyze the composition of anti-corrosion film on the metal surface. Electron microscope was applied to study the formation of biologic slime and microorganism. The results of the experiments are following:1. The widely used 34 typical domestic and overseas water treatment agents can be categorized into seven classes. The corrosion and scaling inhibition effects of the seven classes of water treatment agent have been evaluated comprehensively in typical water qualities. The result shows that various types of corrosion and scaling inhibitor can bring different effect, and the primary factors include agent concentration, water turbidity, iron ion concentration, calcium hardness, basicity and biocide type. These factors influence the effect of corrosion and scaling inhibition to different extent on different types of agent. The corrosion inhibition effect increases with the agent concentration increasing, the water calcium hardness and basicity increasing, and the turbidity and iron ion concentration decreasing. Different biocides influence the anti-corrosion effect on seven classes of corrosion and scaling inhibitors in different ways, among which some types perform synergetic effect but some types perform contradictive effect. The scaling inhibition and dispersion effect increases with the corrosion and scaling inhibitor concentration increasing, the water turbidity, iron ion concentration, calcium hardness and basicity decreasing. Different types of biocide influence the effect of scaling inhibition on these agents in different ways, among which some types perform synergetic effects but some types perform contradictive effects. Under the same condition, the biocide exerts more influence on calcium phosphate scaling inhibition than on calcium carbonate scaling inhibition. Especially, the anti-corrosion capability and the scaling inhibition and dispersion capability of composite corrosion and scaling inhibitor are relative to water quality. When the anti-corrosion capability and the scaling inhibition capability reach some equilibrium, the composite agent exhibits good anti-corrosion efficiency and good scaling inhibition and dispersion efficiency at the same time. Otherwise, the composite agent can not present good anti-corrosion efficiency or good scaling inhibition and dispersion efficiency at the same time.2. There are two opposite courses in pre-filming process, which are metal dissolution by corrosion and pre-filming agent deposition. In the first 5 minutes of pre-filming, the rate of metal dissolution by corrosion and the rate of pre-filming agent deposition are the fastest. At 120 minutes, the rate of metal dissolution by corrosion and the rate of pre-filming agent deposition reach equilibrium, so the corrosion rate becomes stable. Per-filming rate and film performance are relative to water quality. For the water quality with the same hardness, the corrosion rate decreases with the basicity increasing in pre-filming process. For the water quality with the same basicity, the lowest corrosion rate can be reached at the calcium hardness of 50mg/L. When the rate of metal corrosion dissolution and the rate of pre-filming agent depositing on metal surface reach equilibrium, the best efficiency of anti-corrosion film forming can be gained. Otherwise, the anti-corrosion film will not form well.3. Biologic slime in circulation cooling water contains a great amount of heterotrophic bacteria, epiphyte, iron bacteria and sulfate reducing bacteria, which is much higher than the amount in water at the same time. The biologic slime is composed of protein and polypeptide mostly, and exhibits obvious biological characteristics. The slime stripping agent that is developed according to the characteristics of biologic slime can reach the average slime stripping rate of 90.8% and the biocide rate of greater than 99.9% for heterotrophic bacteria, which exerts excellent dissolution effect on biologic slime and microbe, raising the COD of the test solution from 228.6mg/L to 1556.7mg/L. The primary reason is that the slime stripping agent can perform bacteriolysis to kill bacteria and make them dissolve in water, which causes the protein and polypeptide in microbe cytoplasm to be broken, denaturalized and dissolved.4. Adjusting the pH of water can causes an obvious change to water quality. The corrosiveness of water to carbon steel decreases with the pH increasing, and the least corrosion can be reached at the pH of 10.9. When the passive film forms in pH≥10, corrosion course transforms from cathode control to anode control, and the influence of chlorine ion on carbon steel corrosion rate tends to decrease with the pH of water increasing. Calcium concentration in water decreases with pH increasing. Calcium concentration holds at 20mg/L in the pH≥12, and the scale formation changes with pH changing. The crystal form of amorphous calcium carbonate decreases and calcite increases gradually. The high pH value performs obvious restraining effect on both heterotrophic bacteria and sulfate reducing bacteria. With pH increasing, reaction time extended, the bacteria restraining effect increases. For instance, the inhibition rate of sulfate reducing bacteria reaches a high percentage of 99.2% at pH of 9, and more than 90% at pH of 11. When the pH is higher than 10.5, the corrosiveness, scaling situation and microbe propagation rate in water decrease to a great degree, and the corrosion rate, scaling rate and bacteria amount meet the demand of circulated cooling water system running for refinery equipment without additional corrosion inhibitor, scaling inhibitor and biocide.5. The concepts of corrosion factor (F_water) and scaling factor (J_water) are put forward on the basis of previous work, which can be used to evaluate the actual water quality more precisely. The greater F_water is, the more corrosiveness capability water possesses. On the contrary, the smaller F_water is, the less corrosiveness capability water possesses. The greater J_water is, the more scaling capability water possesses. On the contrary, the smaller F_water is, the less scaling capability water possesses.When J_water=1, water quality reaches equilibrium.When J_water <1, water quality becomes unscalable, and previous scale will dissolve.When J_water >1, water tends to scale.