Concentration ratio of circulating water

A Brief Analysis of the Factors Affecting the Increase in the Concentration Ratio of Circulating Water in Refineries -------------------------------------------------------------------------------- Publication Date: September 15, 2006 Lu Hui (Research Institute of Dushanzi Petrochemical Company, Xindu*shanzi 833600) In light of the continuous rise in the concentration ratio of the circulating water system at the Dushanzi Refinery, despite the ongoing efforts to reach the high standard of 5–6, this paper analyzes the factors affecting such an increase from aspects such as system design, chemical treatment agents, and material leaks. It also offers some suggestions for improving the concentration ratio in refineries. Circulating water ； Concentration ratio ； Factor analysis ； Improvement Measure 1: Current status of the concentration ratio of circulating water in the refinery. Table 1: Statistics on the concentration ratio of the circulating water system. 1999, 2000, 2001, 2002, 2003, 2004: North Line – 2.01, 2.61, 3.81, 3.87, 3.86, 4.69; South Line – 2.33, 3.12, 4.17, 4.57, 4.57, 5.12; Second Circulation Line – 2.16, 2.32, 3.78, 3.62, 3.59, 4.28; Cleaning Line – 2.09, 1.78, 2.32, 2.44, 2.45, 2.50. Figure 1: Trend chart of the concentration ratio of the circulating water system. The Dushanszi Refinery has four circulating water systems: the South Line, North Line, Second Circulation Line, and Cleaning Line, which supply 245 water coolers used in the 18 units within the refinery. In recent years, the concentration factors of these four systems have continued to increase, but it has yet to be possible to reach the high standard of 5–6, as shown in Table 1 and Figure 1. 2 Factors Affecting the Increase in Concentration Ratio and Recommendations The meaning of the concentration ratio N is the ratio of the concentration CR of a certain substance in the circulating water to its concentration CM in the make-up water (usually expressed as N+), that is, N = CR/CM. Further derivation yields: N = CR/CM = M/(B + D + F) = 1 + E/(B + D + F) (1) Where M is the supplementary water volume, in m3/h ； E is the amount of water evaporated, in m3/h ； B is the wastewater discharge volume, in m3/h ； D is the wind-induced loss, in m3/h ； F is the leakage loss, in m3/h. Further derivation of the evaporation amount E yields: E = a(R – D) = e(t1 – t2)(R – B) = eΔt(R – B) (2) Where a is the evaporation loss rate, in % ； R is the circulating water volume in m3/h ； Δt is the temperature difference between the inlet and outlet,℃ ； e is the loss coefficient. Because the loss coefficient e in (2) varies with the ambient temperature, it can be considered a constant at a certain temperature ； Under normal circumstances, the circulation water volume R is much larger than the waste discharge volume B, such that R – B » R; therefore, equation (2) can be simplified to: E = eΔtR (3). By substituting equation (3) into equation (1), we obtain: N = 1 + eΔtR / (B + D + F) (4). Equation (1) shows that the concentration factor is proportional to the amount of water evaporated, while equation (4) indicates that the concentration factor is inversely proportional to the waste discharge volume. 2.1 The low amount of water evaporated results in a too low Δt during actual operation, making it difficult to increase the concentration factor. In the design of the cooling water system, the heat load is estimated too high, which leads to a designed Δt that is greater than the Δt during actual operation. This is mainly reflected in the fact that, during the actual operation of the circulating water system, the heat load is insufficient, resulting in a low evaporation rate E; typically, the temperature difference is only around 4–6°C, which makes it difficult to increase the concentration factor. In cases such as the second circulation system and cleaning processes in refineries, the thermal load was overestimated during design, which led to an oversized design for the circulation water volume. In actual operation, the temperature difference is only around 4–6°C; coupled with system leaks and wind-induced losses, it becomes difficult to increase the concentration factor. Based on the specific conditions of our plant, we recommend the following improvement measures: (1) Re-evaluate the heat load of all the water coolers in the plant; for those with lower heat loads, reduce the flow rate of circulating water as appropriate. When the heat load remains unchanged, reducing R will increase Δt, thereby raising the concentration ratio of the circulating water. However, when reducing the circulating water flow rate, the flow velocity of the circulating water should not be too low; it is preferable that it be no less than 0.75 m/s. ⑵ If the flow rate of the circulating water is too low, Δt can be increased by connecting heat exchangers in series; that is, water coolers with lower heat loads are connected in pairs in series, which meets the requirement for flow rate while also increasing the temperature difference. 2.2 A high amount of wastewater discharge is an important factor limiting the increase in the concentration ratio. As mentioned earlier, N increases as B decreases. When designing a circulating water system, the value of B is generally determined as follows: E is calculated based on R and Δt, and then B, which represents the amount of wastewater that must be discharged from the cooling tower, is determined based on the target value for N. Due to wind action, the salt concentration in the water is the same as B. Therefore, the water loss due to wind, denoted as D, is generally included in B; D is a value determined empirically, usually ranging from (0.07% to 0.1%) of R. However, in actual production operations, even when all the cooling tower drain valves are closed (i.e., B=0), the concentration ratio still fails to reach the designed value. For example, in the north line circulating water system in October 2001, even when the wastewater discharge volume B was 0, the concentration ratio N ranged between 2 and 3 (with a design value of between 4 and 6). Subsequent investigations revealed that there was circulating water being discharged from workshops, and the total amount of such discharge **exceeded the wastewater volume B corresponding to an N value of 3. Similar situations exist in other circulating water systems at refineries as well. This is an important factor contributing to the low concentration ratio. The reasons for the high amount of wastewater discharged include: (1) The cooling water for the pumps is recycled water that is not returned to the system; especially in installations with many small pumps, it is discharged directly due to difficulties in recovering the water. Over time, this results in a considerable amount of wastewater being discharged outside the system. ⑵ Leaks within the circulating water system itself, such as leaks in the underground pipelines for circulating water, as well as leaks from pipelines and equipment. ⑶ Reasons such as outdated technology, poor quality, inadequate performance, and damage to cooling tower air separators lead to an increase in the D value. In light of the above circumstances, we recommend the following improvement measures: (1) For the circulating water that is difficult to recover, collection tanks have been added to gather the cooling circulating water first, and then pressure pumps are installed to transport this water back under pressure ； ⑵ Regarding leaks in the circulating water itself, strict management systems are implemented in the circulating water workshop to promptly address any leaks or drips in the equipment and pipelines. 2.3 A high ratio of system volume to circulation water volume (V/R) increases the difficulty of raising the concentration factor. Theoretical calculations show that, when the B value remains constant, the lower the V/R value, the shorter the time it takes for the system to reach a certain concentration factor. Therefore, a lower V/R value means that the system is less affected by external drainage, making it easier to increase the concentration factor. As can be seen from Table 2, the average V/R value for the four circulating water systems in the refinery is 0.61, with a maximum of 0.92 and a minimum of 0.50. This value is significantly different from the range specified in industrial circulating water design standards, which require V/R to be between 1/5 and 1/3, thereby increasing the difficulty of improving the concentration ratio in the refinery. Table 2: V/R Statistics for the Four Circulating Water Systems in the Refinery – South Line, North Line, Second Circulation System, and Cleaning System. System volume V: 1800, 1300, 2500, 2500; Circulating water volume R: 3500, 2500, 5000, 2700; V/R ratio: 0.51, 0.52, 0.50, 0.92; Average V/R ratio: 0.61. Based on the specific circumstances, we recommend taking corrective measures: For the existing circulating water systems, the designed ratio of system volume to circulating water volume (V/R) has already been determined; modifying them is difficult and costly. After thorough discussion, we decided to adopt low-liquid-level operation to reduce the system volume. Specifically, while ensuring that the water pump does not run dry, the liquid level in the cold water tank is reduced as much as possible to decrease the system volume, thereby lowering the V/R ratio and enabling a faster increase in the concentration factor. 2.4 Chemical water treatment agents are an essential means of increasing the concentration ratio. As the concentration ratio in a circulating water system increases, the amount of scaling and corrosive ions in the water rises significantly, and the residence time of water within the system also increases. Therefore, higher demands are placed on the corrosion-inhibiting, scale-preventing, and stability properties of these chemical treatment agents. To ensure good treatment results when operating at high concentration ratios, it is necessary to select appropriate chemical treatment agents based on the actual conditions of the system, such as water quality, the material of the water coolers, heat transfer efficiency, and the residence time of the chemicals. Otherwise, although the concentration ratio increases, it ultimately leads to a shortened lifespan of the water cooler and a decrease in heat transfer efficiency, thereby affecting the normal operation of the processing equipment – resulting in greater losses than benefits. 2.5 Frequent system leaks severely hinder the increase in concentration ratios. For various reasons, the heat exchange medium leaks into the circulating water, causing contamination of this water. A large amount of inorganic, organic, and microbial sludge is generated, which reduces the heat transfer efficiency of the water cooler; in severe cases, it even leads to blockages. As a result, the processing units are forced to reduce production or even stop operating, and such incidents occur frequently in oil refineries. To meet the operational needs of refineries and to avoid shutting down for maintenance, the most common approach to improving water quality and reducing sludge is to lower the concentration ratio of the system, that is, by increasing the amount of water added and the amount of wastewater discharged. After strengthening the sterilization and stripping process, large-volume wastewater discharge for replacement will inevitably lead to a decrease in the concentration ratio. Therefore, to keep the circulating water system operating at a high concentration ratio, it is necessary to avoid leakage incidents. Common causes of leaks in water coolers and corresponding suggestions are: (1) Poor quality in the manufacturing and maintenance of the water cooler, resulting in leaks as soon as it is put into use. In response to this situation, it is recommended that the plant’s water supply and drainage department establish strict maintenance procedures, ensure proper quality control during the manufacturing and maintenance of water coolers, and conduct inspections in strict accordance with the standards set by the head office; these are also the most important measures to prevent leaks in water coolers. ⑵ The strong corrosivity of the heat medium causes corrosion and perforation on the process side, leading to leaks. It is recommended to conduct a comprehensive inspection of all the water coolers in the plant. For those water coolers subject to strong corrosion from heat transfer fluids, corresponding anti-corrosion measures should be taken, such as changing the material of the water coolers or applying anti-corrosion coatings. ⑶ Corrosion and leakage occur due to untimely updates of the water cooler. Although chemical treatment is applied to the circulating water to reduce water corrosion on metals, this does not mean that there is no corrosion at all, and the water cooler can be used indefinitely. There is always a service life for water coolers made of certain materials; if they are not replaced in time, leaks become inevitable. It is said that there is such a regulation in the country: a water cooler must have more than 10% of its tubes blocked before it can be scrapped and replaced. It is hard to imagine how it is possible to ensure no leakage incidents occur during the production cycles of two or three years between maintenance sessions, when the water coolers are filled with blocked tubes. Some of the water coolers in the refinery have been in use for over a decade; some of them are leaking or have blocked tubes, yet they continue to be used, resulting in a persistent problem of leakage incidents. For long-term operation, a fixed-period replacement system must be adopted; that is, the cooler is replaced after reaching a certain service life, regardless of whether it is leaking or not. Considering all factors, this approach may prove to be economically viable. 3 Conclusion Raising the concentration ratio cannot be achieved through the efforts of the circulating water system alone; it involves various aspects such as design, equipment, chemicals, operation, and management. It is a systematic project that requires the joint efforts of all relevant parties. Only by improving the standards in all these areas can the overall level of concentration ratio truly be enhanced.

I think the title should be changed to ’‘What chemicals should be used at high concentration ratios in circulating water?’’ It’s better that way, because the concentration factor is controlled manually, not by the chemical itself!