Proposal of Cost Reduction for Electrochlorination Plant

Transcription

1 Proposal of Cost Reduction for Electrochlorination Plant (Direct Injection System on Electrochlorination Plant) Naokazu Kumagai*, Tadashi Kimura*, Shinichi Adachi*, Chisei Murayama*. *Electrolytic System Engineering Devision, Daiki Ataka Engineering Co., Ltd, Shintofofuta 11, Kashiwashi, Chiba Japan Introduction Biofouling can cause serious operational and maintenance issue with the seawater cooling system for thermal power plants and various other plants along coastal industrial zone. Electrochlorination is the best method to prevent/control biofouling. Combination systems of continuous and shock chlorination are widely used for biofouling control in the world. However, shock chlorination is not adopted in Japan due to its legal limitation. Therefore, the method of preventing biofouling with ultra low concentration chlorination has been developed. Montani et al 1) revealed that 99% of attached organisms reduced at 0.06 ppm with continuous basis. Furthermore, recent studies 2), 3) revealed that shock chlorination is not effective to control biofouling since mussels can stay alive for 252 hrs under 3 mg/l of residual chlorine by means of closing their valves. If it is, we can delete the equipments, instruments, pipes and buildings/civil and their maintenance works for shock chlorination, and it results big cost reduction for Electrochlorination (EC) plant. The present paper aims to verify EC plant without shock chlorination. Chlorination procedure Since gas chlorination found several disadvantages (hazards due to leakage of chlorine gas, frequent maintenance, space requirement for chlorine gas storage, plant life and license from authorities) in comparison to EC plant, Daiki Ataka Engineering (DAE) created EC plant 40 years ago. Chlorination procedure in Japan has been only continuous chlorination because shock chlorination is not allowed due to protecting aquaculture. Therefore, the method of preventing biofouling with ultra low concentration chlorination has been developed. Montani et al 1) studied on the effect of residual chlorine below 0.1 ppm to seek for the lower limitation. Fig.1 shows their results, 99% of organisms can reduce at 0.06 ppm with continuous basis. On the other hand, the combination systems of continuous and shock chlorination have been generally used in the world except Japan. Shock chlorination is a legacy of gas chlorination because gas chlorination was carried out intermittently. Furthermore it has been believed that shock chlorination would be effective to eliminate macrofouling-attachment of larger organisms when they could take place by any possibility under control with continuous chlorination. Therefore, the combinations of continuous and shock chlorination are 1.0 Number of species 0.1 Number of organisms % Wet weight ppm ppm Fig.1 Relationship between residual chlorine and attached organism 1) - 1 -

2 concentration has been developed. DAE has a top share of Electrochlorination plants in Japan and cumulative know-how that are marine growth prevention procedure with only continuous chlorination. Can marine biofouling be protected with only continuous dosing? You can see considering the mechanism of biofouling. Mechanism of biofouling It is well known that biofouling formation proceeds as shown in Fig.3. Fig.2 3) Cumulative mortality (%) of three species subjected to continuous and intermittent chlorination at different concentrations (TRC). Eighty mussels were used at each chlorine dose for each species. carried out nowadays in most of EC plants. However, according to the recent studies, intermittent chlorination is not effective to control larger organisms. Rajagopal et al reported that mussels exposed continuously to 1mg/l chlorine took 588 hr to reach 100% mortality 2), whereas intermittent chlorination between 1-3 mg/l is not effective to control macrofouling (0-5% mortality) 3) as shown in Fig.2. Shock chlorination is generally performed for 15 min every 8 hours at 2-5 ppm in general. The intermittent chlorination was carried out between 1-3 mg/l applied at 4 h on and 4 h off cycle in the above study. This condition is more sever to larger organisms than the shock chlorination. Thus, the studies reveal that shock chlorination is not effective to eliminate macrofouling, but only continuous chlorination is effective to prevent biofouling including micro and macro fouling. Continuous chlorination As mentioned the above, shock chlorination has not been carried out in Japan due to very strict discharge criteria. Accordingly, continuous chlorination with low chlorine Fig.3 Macrofouling sequence (1) All fouling events in natural seawater begin with a spontaneous deposition of a "primer" coat of natural, high-polymer film. (Biofilm) This film is a prerequisite to initiation of the fouling succession. (2) Secondary, bacteria and diatoms appear on the biofilm and multiply rapidly together with derbies and other organic particulate materials. (Micro-fouling) (3) The animals and plants that make up the next stage of succession in fouling communities and grow up to 100mm thickness. (Macrofouling) Because continuous low level residual chlorine is very effective to microbe which can form biofilm and microfouling, it is also effective to - 2-

3 Kobe and Tokyo bay in Japan. As can be seen in Fig.3, the concentration of TRC at out fall exceeds the 0.2 mg/l limitation at mg/l of chlorine injection rate since it generally takes minutes after intake to reach to the out fall. Taking account of that the shock chlorination is carried out at 2-5 mg/l, it definitely exceeds the discharge criteria during shock chlorination. (a) Biocide bacteria with electricity control subsequent macrofouling. Evaluation on affection of preventing biofilm to macrofouling formation We disinfected initiate biofilm with electrical biocide in order to prove if biofilm formation could be blocked, Macrofouling was avoidable. Fig 4 shows comparison between rubber lined steel plates with electrical disinfection and without disinfection. The electrical disinfection kills microbes and bacteria which form primary biofilm. This result means that if primary biofilm is eliminated, subsequent macrofouling can be controlled. Since continuous chlorination can stop the biofilm formation, continuous chlorination with low concentration is effective to control biofouling. (b) No treatment Fig. 4 Submerged rubber lined steel in seawater for 50 days chemical usage or further minimization. In addition to the social requirement, the authorities such as EPA and World Bank also specified 0.2 mg/l of discharge limitation of total residual chlorine (TRC). The local limitation will be stricter taking into account of influence to aquaculture. However, the TRC mostly is over 0.2 mg/l of the limitation during shock chlorination. Fig.5 shows TRC decay curves with lapse of time measured in Electrochlorination Plant without shock chlorination facility Electrochlorination plant using combination of continuous and shock chlorination consist of ; (1)Seawater booster pumps (if required) (2) Seawater strainers (3) Electrolyzer (4) Transformer/Rectifiers dedicated to Electrolyzers (5) Local control panel (6) MCC, SWGR (7) Hypochlorite storage and hydrogen dilute tanks with level gauges and transmitters (8)Hydrogen dilute fans (duty/standby) (9) Air duct for hydrogen dilute fans Environmental requirement Along with increasing environmental concerns over-usage of chlorine as a biocide and it is desired to reduce Fig. 5 Residual chlorine decay curves in Kobe and Tokyo Bay - 3-

4 Fig. 6 Deletable equipment in case of adopting DIS with flow meters and transmitters (10)Continuous dosing pumps (duty/standby) (11) Shock dosing pumps (duty/standby) (12)Continuous dosing pipe lines with control flow valves (13) Shock dosing pipe lines with control flow valves. Since we revealed that shock chlorination is not effective to control biofouling and resulted in the exceeding discharge criteria, shock chlorination can be deleted in EC Plant. And thus, the above item numbers (7), (8), (9), (10), (12) and (13) can be deleted. Furthermore, the building/shelter/civil works for these items also can be deleted. Direct Injection System (DIS), which is the system for continuous chlorination only, in 1982 and has more than 100 experiences of DIS in the world. Direct Injection System DIS is very simple system as shown in Fig.7. The purpose of dehydrogen drum is to remove hydrogen gas from chlorinated seawater prior to injection to seawater intake with keeping pressure. Hydrogen removed is exhausted from the top of the drum directly to the atmosphere. Fig.8 shows the mechanism of dehydrogen drum. Chlorinated seawater including hydrogen gas comes into dehydrogen drum tangentially, makes spiral flow in the drum. Hydrogen bubbles gather to the center of the drum by means of the spiral flow, and make a space in top of the drum. And then, hydrogen gas was discharge to atmosphere from the float Fig. 6 shows P&I of EC plant for combination of continuous and shock chlorination. When you select only continuous chlorination, the equipments indicated with hatching is not necessary, the EC system become very simple and ease the maintenance. DAIKI ATAKA Engineering developed Fig. 7 Schematic drawing of DIS - 4-

5 valve installed at top of the drum. Since this drum can remove hydrogen gas from the chlorinated seawater with maintaining the pressure, the chlorinated seawater having the pressure can flow to the injection point without dosing pumps. In DIS, flow rate of chlorinated seawater is a constant, injection rate can be varied by means of adjusting DC out put current generated from DC power supply (Transformer/Rectifier). 1. System becomes very simple Maintenance activity becomes ease. Fig. 8 Degassing valves Therefore, response time for change injection ratio can be changed quickly and widely (10%-100%). It can be seen in some other EC plants to vary injection rate by means of flow rate, but such control method make much stress to the pipe system, easy to clog with scale generated in electrolyzers. Advantages of Direct Injection System 2. Cost reduction Storage tanks, hydrogen dilution blowers, continuous dosing pumps, shock dosing pumps and associated instruments are not necessary. Since there are no signals from these equipments and instruments, I/O numbers for control can be reduced. 3. Civil and building cost reduction The civil and building size become smaller since the above equipments are not necessary. Conclusion 1) Shock chlorination has been believed that it is effective to eliminate macrofouling attachment organisms if it could settle in the cooling system under continuous chlorination control. 2) However, recent studies revealed that intermittent chlorination such as shock chlorination is not effective to control macrofouling. 3) We would like to propose DIS EC plant to our customer, because the system can reduce the equipments cost and construction cost. 4) Furthermore, DIS has several advantages comparing with storage tank system for the combination procedure of continuous and shock chlorination as shown in Fig.9. References 1) Mitsuto Montani, Yumiko Fujita, Atsushi Kawabe. Ultra-low concentration chlorination for macrofouling control. Proceedings of International Conference on "Corrosion in refinery petrochemical and power generation" P , ) Sanjeevi Rajagopal, Gerard Van der Velde, Henk A. Jenner. Effect of low level chlorination on zebra mussel, Dreissena polymorpha. Water Research, 36 P , ) Sanjeevi Rajagopal, Gerard Van der Velde, Marinus Van der Gaag, Henk A. Jenner. Effect of low level chlorination on zebra mussel, Dreissena polymorpha. Water Research, 37 P , Response to change injection ratio becomes very quickly No retention time due to storage tank Fig. 9 Advantages of DIS - 5-