Article pubs.acs.org/IECR
Technology Integration for Sustainable Manufacturing: An Applied Study on Integrated Profitable Pollution Prevention in Surface Finishing Systems Jie Xiao† and Yinlun Huang‡ Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, Michigan 48202, United States ABSTRACT: Technology improvement and innovation are of utmost importance in achieving greenness while ensuring economic incentives in manufacturing. Aiming at Profitable Pollution Prevention (P3), design and operation technologies can be developed to enhance manufacturing sustainability through energy and material efficiency increment, source waste reduction, production efficiency improvement, safety assurance, etc. This paper explores opportunities for an effective integration of P3oriented technologies in a systematic way. It will be shown that the integrated P3 (IP3) technology can be identified through a technology implementation approach based on process fundamentals and engineering practicability needed for process design modification and operational strategy development. A successful application of an IP3 technology in an electroplating process, a main type of surface finishing system, demonstrates the methodological efficacy.
1. INTRODUCTION Pollution prevention (P2) can be defined as any activity that is aimed at reducing, to the extent feasible, the release of undesirable substances to the environment.1 It has been accepted as a key policy for manufacturing activities in a wide variety of process industries. Numerous P2 technologies are available for reducing waste, promoting nontoxic or less-toxic substances and advocating environmentally benign processing, etc.2 However, a majority of those technologies are essentially postprocess based, focusing on on-site (pre)treatment of the waste generated from production lines, which makes the effluent waste streams of the plants reduced to meet environmental regulations. Note that any postprocess based P2 technology is fundamentally passive from the viewpoint of source reduction, because it only reacts on the generated waste. A proactive approach must be in-process oriented, i.e., it should aim at minimizing waste in the first placethe production lines. In manufacturing systems, waste can be usually divided into two categories: the unavoidable waste and the avoidable waste. The former is referred to the amount that must be generated due to manufacturing requirements, whereas the latter is the amount generated mostly due to improper process design and operation. Minimization of the avoidable waste can reduce not only the total production cost but also the waste treatment cost, thereby helping improve manufacturing profitability. In the past decade, a wide variety of process synthesis, analysis, design, and optimization methods have been developed in order to achieve proactive P2 or money-saving P2.1,3,4 Through collaborating with the surface finishing industry, Huang and associates introduced the concept and theory of Profitable Pollution Prevention (P3) by resorting to the fundamentals of process systems science, environmental engineering, and engineering economics.5,6 A number of P3 technologies have been subsequently developed for manufacturing sustainability improvement. An application of the Switchable Water Allocation Network (SWAN) design technology in an electroplating system, for example, has led to very significant fresh and © 2012 American Chemical Society
wastewater reduction; the ratio of the annual profit generated by this technology to the total annualized cost for using the technology reached 17:1.7,8 Process sustainability should be further improved if the P3 technologies can be integrated and applied in a systematic way. This work explores opportunities for developing an integrated P3 (or simply IP3) methodology. In the following text, a review of the P3 fundamentals and existing P3 technologies are presented at the outset. Then, a systematic approach for integrating P3 technologies is delineated. Finally, an application of an IP3 technology in a bronze cyanide plating system is described, which can demonstrate how it helps achieve economically sound and environmentally benign manufacturing.
2. P3 FUNDAMENTALS AND TECHNOLOGY DEVELOPMENT The target of P3 is to maximize economic benefits while minimizing adverse environmental impact. The focal point in achieving this is to change the P2 practice from reducing “end-ofplant” waste to minimizing “end-of-process” waste. To minimize the avoidable waste in a process is much riskier than handling the postprocess waste, since any process waste reduction activity could potentially cause problems in product quality assurance and process efficiency. Note that source reduction faces a number of technical challenges: (i) how to determine if a waste is avoidable or not, (ii) what the waste generation mechanism and waste propagation paths are, (iii) what a technically feasible way to break the paths is, (iv) how to assess the effectiveness of waste reduction and economic benefits including the assurance of product quality, operational efficiency, and operating cost, and (v) what the best technology implementation strategies are. Such strategies can be identified based on a full understanding of Received: Revised: Accepted: Published: 11434
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Figure 1. Typical electroplating process.
Table 1. Key Process Parameters and Performance Targets in Each P3 Technology key parameters process
targets
chemical concentration chemical feeding policy rinse water flow rate rinse water flow pattern cleaning time rinse time plating time cycle time product quality assurance/ improvement chemical consumption reduction water consumption reduction waste generation reduction operating cost reduction production rate increase
Tech 1 (optimal cleaning and rinse)
Tech 2 (water network design)
√ √ √ √ √
√
Tech 4 (near-zero discharge)
√
√ √ √
√
√
√
√ √ √
√
√
√
√ √
√ √ √
√
√ √ √ √
Tech 3 (sludge reduction)
√ √
Tech 5 (dynamic hoist scheduling)
√ √ √ √ √
√ √
manufacturing fundamentals, especially process dynamics and product formation, from which waste generation mechanism can be identified, product quality assurance can be defined, and process efficiency can be quantified. In this work, the electroplating process, a main type of surface finishing system, is studied to elaborate P3 concepts and strategies. 2.1. Waste Generation Paths in Electroplating. A typical electroplating process is given in Figure 1. Depending on the size and geometry of the parts (or workpieces) to be plated, they may be loaded in a barrel or mounted on a rack before passing through three types of process units (i.e., cleaning tanks, rinsing tanks, and plating tanks). Multistage, multipurpose cleaning is used to strip off the dirt (such as soil, oils, and oxides) from the surface of parts through chemical and electro-based processing. The stripped dirt mixed with a certain amount of used chemicals is deposited as sludge at the bottom of cleaning units. The cleaned parts covered by a thin layer of dirt-chemical mixture (called drag-out) on the surface are sent to the succeeding rinse units for one or more steps of rinse. Note that the drag-out from the
cleaning steps is a major source of hazardous or even toxic pollutants in the wastewater discharged from the rinse units. Subsequently, the parts enter an electroplating unit to receive a thin layer of metallic coating through electro-deposition. The plated parts must be rinsed to remove the plating solution left on their surface (another kind of drag-out) before being discharged from the production line. The postplating rinse steps will generate wastewater that contains various types of pollutants different from those in the preplating wastewater. The parts cleaning, rinsing, and plating steps are all operated in the batch mode, but with significantly different processing times. It may take, for example, 3 min for a barrel of parts to be processed in a soak cleaning tank, one minute in two consecutive rinsing tanks, and 30 min in a plating tank. In practice, key operational parameters are usually set quite conservatively and adjusted based on experience only. It makes process operations inefficient, and waste generation, in the forms of wastewater, sludge, spent solutions, and emissions, always excessive.9,10 For example, extensive cleaning and rinsing may not improve the surface 11435
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Figure 2. P3 technology application for manufacturing sustainability in electroplating.
whereas rinse qualities in different rinse steps can be all guaranteed. Technology 3Process Sludge Reduction Technology. In cleaning operations, most of the dirt on the surface of parts will be removed by the chemicals in the cleaning tanks or rinsed off in the succeeding rinse tanks. The sludge reduction technology classifies the sludge into two categories: the avoidable one and the unavoidable one. It contains a method for precisely determining and then reducing the amount of the avoidable sludge caused by an excessive use of chemicals, insufficient parts surface pretreatment, and unnecessarily long cleaning time.16 Technology 4Near-Zero Chemical and Metal Discharge Technology. Electroplating operations consume huge amounts of chemical solvents and plating solutions daily. It is known that chemical loss from chemical cleaning and electroplating steps can be as high as 60 and 30% of the consumption, respectively, in normal production. The loss of chemicals and metal ions contained solution can cause a dramatic increment of production cost (due to unnecessary consumption of solvents, plating solutions, and freshwater), as well as waste treatment cost (because of extra waste loads to the system). The developed technology can be used to design an effective direct recovery system based on a reverse drag-out concept that can minimize drag-out related chemical/metal loss safely.17,18 It can also be used to identify critical process parameters (e.g., evaporation rate, drag-out rate, rinse cycle time, etc.) and generate detailed environmental and economic analysis, which is essential to the optimality of system design. Technology 5Environmentally Conscious Dynamic Hoist Scheduling Technology. It has been shown that in production, hoist scheduling may also help reduce source waste.19 A key element in such type of scheduling is to calculate the amount of waste to be generated in different process units together with the production rate and the product quality. The developed technology is capable of generating a dynamically adjustable production schedule, based on the evaluation of job order change, waste generation in different process units, chemical and energy consumption, etc. Industrial applications have shown that the technology can contribute significantly to the minimization of the quantity and toxicity of wastewater while maintaining the production rate.19,20
pretreatment quality noticeably, but increase chemical and water consumption substantially and decrease production rate. These have motivated the development of P3 technologies for minimizing avoidable wastes and maximizing profit simultaneously. 2.2. P3 Technologies for Electroplating. A number of electroplating specific P3 technologies have been developed by integrated process design and operational optimization. Five of them are briefly described below. System performance targets and associated key process parameters are listed in Table 1. Technology 1Cleaning and Rinse Operation Optimization Technology. In any plating line, each step of cleaning (e.g., presoaking, soaking, electro-cleaning, and acid cleaning) is always followed by one or two steps of rinse. Chemical concentration setting, chemical feeding policy, rinsewater flow rate, as well as cleaning and rinse time for each barrel or rack of parts are critical to chemical conservation and wastewater reduction. The technology is developed using a two-layered hierarchical dynamic optimization technique.11,12 In the lower layer, the optimal settings for chemical concentration and rinsewater flow rate are identified for unit-based consumption minimization. In the upper layer, the processing time distributions for all the cleaning and rinse operations are adjusted so as to explore the global opportunities of minimizing the overall operating cost and waste generation. Technology 2Optimal Water Use and Reuse Network Design Technology. In an electroplating line, freshwater is sent to different rinse units for rinsing off the dirt and solution residues on the surface of parts. The laboratory measurement of pollutant levels in used water from some rinse units shows that it can be either partially or entirely reused in some other rinse steps. In plants, however, rinsewater reuse is mostly experience based and thus is always set in a very conservative way to ensure rinse quality; this is especially true as the initial dirtiness of parts before entering the production line may vary greatly, and thus the pollutant levels in used rinsewater could fluctuate significantly. By this technology, an optimal water allocation network can be designed for a plating line of any capacity, and the optimal operation strategy for the network can also be developed based on rinsewater flow dynamics.13−15 The operation strategy can provide control policies for switching water flow patterns during operation, which permits water reuse to the maximum extent, 11436
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3. P3 TECHNOLOGY INTEGRATION Each P3 technology11−21 was developed for a specific application in electroplating for manufacturing sustainability. Figure 2 depicts the application areas of the five P3 technologies described in the preceding section. As shown, each focuses on the profit and environmental quality issues associated with one or two of the five flows in the system, i.e., the parts processing flow, the water flow, the chemical flow, the energy flow, and the waste flow. If more flows can be focused at the same time, then more environmental and economic benefits could be possible. Note that, however, the benefits cannot be achieved by a simple combination of individual P3 technologies. Different P3 technologies can affect the applicability and effectiveness of each other if they are not carefully integrated; note that as shown in Table 1, a change of key process parameter settings will affect the performance of more than one technology. This renders a need to develop an approach for technology integration. In this section, a hierarchical decision-making strategy as well as integrated system characterization, analysis, and optimization methods are introduced for the development of an IP3 technology. 3.1. Integrated Modeling for System Characterization. A fundamental basis for applying P3 technologies is firstprinciples-based modeling of an electroplating system, as such models can provide quantitatively all necessary information on product formation, process operation, material and energy flows, and waste generation. The plant-wide model system can be served as a virtual electroplating plant, to which the P3 technologies can be applied and the environmental and economic benefits can be evaluated. The model system consists of the integrated models for all key unit operations, which are described below. Detailed model development information can be found in our previous work.5,11,18 Cleaning and Rinse System Modeling. In cleaning tanks, the dirt sticking to the part surface is removed by chemical solvents. The dirt removal dynamics can be characterized as follows.12 Ap
dWpc(t ) dt
= −rpc(t )
Ap
dWri(t ) = −rri(t ) dt
where Wri is the amount of dirt on the parts in the rinse tank. The contamination of the rinsewater is modeled as11 Vr
dxr(t ) = rri(t ) + Fw(t )(zr(t ) − xr(t )) dt
(4)
where Vr is the capacity of the rinse tank; xr(t) is the pollutant concentration in the rinsewater; Fw is the flow rate of the rinsewater; zr is the pollutant concentration in the influent rinsewater. Plating and Rinse System Modeling. The dynamics of chemical concentrations in an electroplating tank, with the consideration of solution recovery from the succeeding countercurrent rinse tanks, are governed by17,18 V
E
dCjE(t )
N Rk )(U (t − t isE) = D ∑ [y k C jR k(t ini,os
d (t )
k=1
− U (t − t ieE))] − DCjE(t )[U (t − tosE) E )] + f j (R a(t ), R c(t ))A p − U (t − toe
i[U (t − t isE) − U (t − tosE)] + FrC jR1(t ) E
(5)
CEj (t) CRj k (t)
where V is the volume of the plating tank; is the concentration of chemical j in the plating tank; is the concentration of chemical j in the kth rinse tank; f j is the reaction rate of chemical j, which is a function of the anodic current efficiency Ra(t) and the cathodic current efficiency Rc(t); i is the plating current density; Fr is the recovery flow rate; D is the flow rate of drag-in or drag-out; yk is a binary variable integer (0 or 1) determining the existence of rinse tanks after or before the plating tank; U(t − ta) is a unit step function at time instant ta; k tRini,os is the starting time of the initial drag-out from the kth rinse tank; tEis and tEie are, respectively, the starting and the ending time of drag-in into the electroplating tank; tEos and tEoe are, respectively, the starting and the ending time of drag-out from the electroplating tank; N is the total number of rinse tanks after the electroplating operation. The concentrations of chemicals in the countercurrent rinse tanks are described by18
(1)
where Wpc(t) is the amount of dirt on parts at time t and Ap is the total surface area of parts. The dirt removal rate rpc(t) in the cleaning tank is determined by the chemical concentration in the tank, Ca(t), the remaining amount of dirt on parts, and the looseness of dirt. In addition to the consumption of chemicals for dirt removal, chemical addition together with drag-in and drag-out can lead to the change of chemical concentrations in the cleaning tank. Their correlation can be described below.
VR
dC jR k(t ) d (t )
= Fr(C jR k +1(t ) − C jR k(t )) + DC jR k −1(tosRk −1) [U (t − t isR k) − U (t − t ieR k)] Rk )] − DC jR k(t )[U (t − tosRk) − U (t − toe N
−
Vc
(3)
rpc(t ) dCa(t ) =− + Wc(t ) + kdA p(C0(t ) − Ca(t )) dt η
R ) ∑ [yk DC jR (t )(U (t − t ini,os k
k
k=1 Rk ))] − U (t − t ini,oe
(2)
(6)
where VR is the volume of the rinse tank; tRisk and tRiek are, respectively, the starting and the ending time of drag-in into the R R kth rinse tank; tosk and toek are, respectively, the starting and the k k ending time of drag-out from the kth rinse tank; tRini,os and tRini,oe are, respectively, the starting and the ending time of the initial drag-out from the kth rinse tank. 3.2. IP3 Technology Development. The aim of developing an IP3 technology is to achieve economically optimal and environmentally benign manufacturing through optimizing both
where Vc is the capacity of the cleaning tank; Wc(t) is the flow rate of chemical addition at time t; η is the chemical capacity for dirt removal; kd is the drag-out coefficient determined by the temperature and surface tension of the solution, parts drainage time, and the shape of the parts; C0 is the chemical concentration in the preceding cleaning tank. After cleaning, the flowing water in the succeeding rinse tank washes off dirt from the parts. This process can be modeled as11 11437
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will be given to the technology with a higher rank (see the ranking process in the Level-1 decision making). Level 3Operational Strategy Development. If the process flowsheet is modified, and/or if the process operational settings are not optimal, then a new set of process operational strategies should be developed. In this step, the integrated process modeling system should be used to provide the key information of process dynamics and determine the profit and waste generation data under the given process and production specifications. The results are then used to compare with the real production data. The comparison will show where and to what extent the waste can be reduced and the profit can be improved. In this level, analysis is based on rigorous mathematical models. The “win−win” and “trade-off” aspects between key process parameters can be thoroughly investigated by comprehensive parametric analysis. Economic and Environmental Performance Analysis. As shown in Figure 3, the outputs of the three-level decision making modules should all be sent to the Economic and Environmental Performance Analysis module. A detailed report on the manufacturing sustainability data due to the application of the IP3 technology will be generated for the plant management. IP3 Technology Development Procedure. To effectively implement the IP3 technology, we introduced the following procedure. Step 1. Collect the process and product information of the electroplating system under investigation. Step 2. Characterize the entire system through dynamic modeling (eqs 1−6) and validate the integrated modeling system using the data collected from the production line. Step 3. Select appropriate P3 technologies to form an integrated P3 technology set. Step 4. Identify process design modification plans based on the process and production specification information as well as specific features of each P3 technology in the selected technology set. Step 5. Determine the optimal operational settings for each process modification plan by analyzing the process and product dynamic data generated from the integrated modeling system. Step 6. Compare the economic and environmental performance for all system modification plans and implement the most appropriate one based on preferences of the decision maker.
process design and process operation. The development can be realized through a three-level decision making process (see Figure 3).
Figure 3. Decision making hierarchy for IP3 technology development.
Level 1Technology Identification. On the basis of the defined P3 objectives on economic and environmental benefits and process design and production specification information, a suitable set of P3 technologies should be determined. This can be achieved by the analysis of the source waste generation data about the electroplating system under study. Guidelines for technology selection are related to the feasibility of technology use. For instance, for a manually operated production line, it is impractical to implement very complicated processing procedures and chemical/water feeding policies. Also, in the cases where the plating quality is sensitive to the purity change of electroplating baths, it is forbidden to recycle used rinsewater to the electroplating tank. The identified technology candidates can be ranked based on the preset P3 objectives. A general criterion is that a better technology can simultaneously bring more types of benefits, such as (i) a simultaneous reduction of chemicals, water, and energy consumption, (ii) a reduction of waste transferring among process units, (iii) a reduction of waste generation in individual process unit, (iv) an improvement of cleaning, rinse and plating quality, (v) an increment of productivity, and (vi) a reduction of capital and operating costs. Note that the priority of economic or environmental benefits should also be taken into account in this ranking process. Based on the functionality and capacity of the known P3 technologies, appropriated ones can be finally selected to form an integrated P3 technology set. Level 2Process Design Modification. The IP3 technology may contain some P3 technologies that require process modification. Figure 3 lists additional information needed for developing a modified process flowsheet. Process design guidelines for individual P3 technologies, such as near-zero discharge and water use and reuse network, have been discussed in detail in our previous work.13,14,17 When the selected technologies are to be implemented simultaneously, the design will be modified to satisfy all process requirements of the suitable P3 technologies. If some requirements cannot be satisfied (e.g., because of the restrictions on space, time and budget), priority
4. CASE STUDY The introduced IP3 technology development methodology has been successfully used to improve the economic and environmental performance of a manually operated bronze cyanide plating line, which is presented in this section. In this application, the plant expects to reduce at least 60% of the chemical solvent loss in the cleaning systems and the plating solution loss in plating, and to reduce the cost for rinsewater consumption and wastewater treatment. 4.1. Data Acquisition and System Characterization. To gain a better understanding of the original system, we collected detailed process and product data and constructed a virtual electroplating line using the first-principles-based process and product models that are described in the preceding section. Process Design. The original design of the plating system is given in Figure 4a. In production, each rack of parts first undergoes soak cleaning and alkaline cleaning and then flow11438
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Figure 4. Process design for the bronze plating system: (a) the original process, (b) the modification plan A, and (c) the modification plan B.
Table 2. Operational Settings for the Bronze Plating Line cleaning and plating tank
capacity (gal)
temp. (F)
processing time (min)
chemicals used
quantity
soak clean T1 electro clean T2 acid clean T4 predip T6
1100 440 330 1100
160−200 160−200 room temp. room temp.
5−20 5−20 0.25 − 0.75 NA
plating T7
1350
135−150
50−75
soak cleaner alkaline deruster hydrochloric acid sodium cyanide caustic soda copper (metal) free sodium cyanide caustic soda sodium carbonate
8 −12 (% vol) 20 − 50 (% vol) 30 − 60 (% vol) 0.5 − 1.5 oz/gal 0.2 − 1 oz/gal 1.2 − 4.0 oz/gal 2.0 − 3.5 oz/gal 0.9 − 2.0 oz/gal 3.0 − 50.0 oz/gal
rinse tank
capacity (gal)
processing time (min)
rinse water flow rate (gal/min)
rinse T3, T5, T8, T9 and T10
220
0.5−1.5
5
water rinsing. After these, a step of acid cleaning is followed. Through a second flow-water rinse step, the parts are predipped by cyanide. Then, a composite coating that contains copper and tin is plated onto the surface of parts through the electroplating step. Finally, the plated parts are required to go through a threestage countercurrent rinse.
The key chemicals contained in the soak cleaner are: sodium hydroxide (NaOH) for saponifying oils, sodium metasilicate (Na2SiO3) as an emulsifying and deflocculating agent, sodium carbonate (Na2CO3) for providing good buffering and water softening, sodium tripolyphosphate (Na5P3O10) for softening water and tying up metal ions, and sodium linear alkylate 11439
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plating system, to which appropriate P3 technologies can be applied for performance evaluation. 4.2. Technology Selection. To gain a deep understanding of the original electroplating system is essential for the identification of the key waste reduction opportunities and hence promoting selection of the most effective P3 technologies to be implemented. The optimal cleaning and rinse technology and the dynamic hoist scheduling technology cannot be implemented in this manual line, since a strict control of the chemical addition schedule and parts processing time in each operation is not feasible. After eliminating these two infeasible technologies, we further evaluate the remaining ones. It is noticed that the original bronze cyanide plating system contains flow rinse tanks only, which causes a significant chemical and metal loss (up to 60% of the consumption) through drag-out to the wastewater treatment facility (WWTF) (see Figure 4a). The wastewater contains the most toxic chemical used in the metal finishing industry, i.e., cyanide, whose discharge is severely limited by EPA. This has led to a substantial expenditure for wastewater treatment. Thus, the drag-out of chemicals and metals to the flow rinse tanks must be minimized for waste reduction and cost saving. A near-zero discharge technology can fulfill this objective satisfactorily.17,18 The fundamental approach is the reversed drag-out, i.e., to collect the lost solvent or solution directly in relevant static rinse systems and to pump it back periodically to the original cleaning or plating tanks. Note that a key advantage of the cyanide-based electroplating process is its wide tolerance for impurities and variations in bath composition, which allows the application of a reversed drag-out technique. Moreover, the conservative operational strategy given in Table 2 suggests opportunities to reduce chemical and water consumption through operational improvement. Rinse water saving technology15 is another feasible approach to realize P3. Note that the reduction of drag-out and rinsewater consumption is a principal component of the minimum sludge generation technology.16 Consequently, the IP3 technology to be explored is an integrated near-zero discharge, water saving, and sludge minimization technology, which should lead to better environmental and economic performance than any individual P3 technology does. It is understandable that, however, several major technical difficulties must be overcome to ensure the effectiveness of the IP3 technology. These include: (i) how to restructure rinse systems in the most economical way, (ii) when and how much the lost solvent or solution contained rinsewater should be pumped back, (iii) how to change the rinsewater flow rate, and (iv) how to ensure cleaning, rinsing, and plating qualities without sacrificing production efficiency. All these issues have to be fully addressed through optimal design and operation of the electroplating process, which are the focuses of the remaining decision making tasks. 4.3. Process Design Modification. To implement the nearzero discharge technology, cleaning and electroplating tanks must be followed by integrated static and flow rinse systems. Pumps have to be added to send the chemicals collected in the static rinse tank back to the cleaning and electroplating tanks. Moreover, the water saving technology requires the addition of valves to restrict water flow rates. Through analyzing process and product information collected from the original plating line, two process modification plans are identified to meet these special requirements for implementing the selected P3 technology set. In Plan A (Figure 4b), a new rinse tank T11 is to be installed next to tank T3 in order to recover the chemical solvents from
sulfonate as a surfactant. The key reaction involved for soil and dirt removal from the parts surface is (RO)3 C3H5 + 3NaOH → 3RONa + C3H5(OH)3
The acid cleaning operation uses hydrochloric acid (HCl), where the key reactions for metal scale and rust removal are FeO + 2HCl → FeCl 2 + H 2O
Fe3O4 + Fe + 8HCl → 4FeCl 2 + 4H 2O Fe + 2HCl → FeCl 2 + H 2
The electroplating solution contains copper cyanide (CuCN), sodium cyanide (NaCN), and sodium hydroxide (NaOH). The reactions occurred at the anode are Cu + 3CN− → [Cu(CN)3 ]2 − + e− 4OH− → O2 + 2H 2O + 4e−
and the reactions at the cathode are [Cu(CN)3 ]2 − + e− → Cu + 3CN− [SnO3]2 − + 3H 2O + 4e− → Sn + 6OH−
Product Quality Control. Similar to most manufacturing systems, in this manual plating line, product quality is managed through complying with process specifications. Table 2 lists the nominal operating condition settings for the cleaning, rinsing, and plating steps, which include the process solution temperature, chemical and metal concentrations, parts processing time, and rinsewater flow rate. In addition to specifying operating strategy, postprocess quality inspection is also conducted, e.g., the measurement of coating composition (i.e., a key quality indicator) for randomly selected plated parts. The percentage of tin in the coating should be within 7−12% in order to achieve satisfactory wear resistance performance. System Characterization. The generic models for the cleaning and rinse subsystem (eqs 1−4) and the plating and rinse subsystem (eqs 5 and 6) can be readily applied to modeling the complete bronze plating line. Figure 5 gives concentration
Figure 5. Chemical and metal concentration dynamics in the plating tank: comparison between model predictions (continuous lines) and experimental data (values collected every 10 min).
predictions of four key chemical and metal ions (Cu+, Sn4+, CN−, and OH−) involved in the electroplating stepthe most critical and complicated process in this system It shows that model predictions can well fit the real data collected from the production line. The validated model will then serve as a virtual 11440
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cleaning systems. Rinse tank T3 needs to be changed for static rinse, while the new tank will be for flow rinse. A pump should be used to pump the solution in tank T3 back periodically to electrocleaning tank T2. To minimize plating solution loss from the plating tank, the three-stage countercurrent flow rinse system needs to be modified. Rinse tank T10 maintains its flow rinse, whereas tanks T8 and T9 are changed for static rinse. The water flow in the three rinse tanks after the electroplating tank should also be changed accordingly. A pump will be needed to pump the water from tank T8 back periodically to the electroplating tank. Because of the opportunities for reducing rinsewater consumption and consequently reducing the wastewater treatment cost, several valves will be needed to control the rinsewater flow (see the differences between panels a and b in Figure 4). Plan B is more conservative when compared with Plan A. In Plan B (Figure 4c), rinse tanks T9 and T10 maintain their flow rinse status, while rinse tank T8 is changed for static rinse. Moreover, rinsewater control is imposed only for the new rinse tank. It is clear that Plan A can lead to more waste reduction, water saving, and chemical and metal recovery. But Plan B can ensure better rinse quality, because it has one more flow rinse tank (i.e., tank T9) and the rinsewater flow is not restricted. Both plans will be investigated further. 4.4. Operational Strategy Derivation. In this section, operational strategy and settings are determined based on rigorous model-based analysis by resorting to the process modeling system. The system performance in terms of waste reduction and cost saving is evaluated by quantifying the improvement percentage as compared with the original process. The performance for both process modification plans will be evaluated and compared. Analysis on Plan A. The chemical recovery and rinsewater saving achieved through Plan A application are analyzed for each of the two subsystems: i.e., the cleaning and rinse subsystem (CRS) and the plating and rinse subsystem (PRS). Chemical Recovery for the CRS. The amount of water pumped from tank T3 back to tank T2 can be determined based on the steady-state system analysis. Because the solution in the static rinse tank T3 is used to replenish the evaporation loss in tank T2, this flow rate should be equal to the evaporation rate in tank T2 (i.e., about 2.7 gal/h). Given a drag-out amount of about 0.106 gal/rack, chemical recovery from the electro-cleaning tank can be estimated as well.17 Simulation results show that the chemical concentration in the drag-out from tank T3 is about 10% of the concentration of the drag-out from tank T2, which indicates 90% recovery of the chemical loss from the electrocleaning tank. Rinse Water Saving for the CRS. The rinsewater saving strategy is derived on the basis of analyzing contamination level dynamics in the rinse tank and on the parts. An attractive strategy should minimize the rinsewater consumption without compromising the rinse quality. In the cleaning and rinse subsystem, rinsewater in the new rinse tank will wash out the dirt and chemicals on the parts and in the drag-out from tank T3. The rinse quality can be evaluated by quantifying the contaminant concentration in the solution attached to the parts as the rack leaves the rinse tank, which should be lower than 0.085 g/L for this electroplating system. Dynamic simulation shows that when the parts leave tank T11, the rinse quality is quite acceptable since the contaminant concentration is only 0.0825 g/L (Figure 6a). At the end of one 20-min rinse cycle, the contaminant concentration becomes far
Figure 6. Contamination analysis of the new rinse tank T11 with 5 gal/ min rinsewater flow: (a) the original flow rinse and (b) the new rinse system with water cutoff.
below the initial contamination level. It suggests that the rinse quality for the next rack of parts will be better, which is not necessary. The water can be cut off once the contamination is reduced to the initial level (see Figure 6b). In this case, the maximum rinsewater cutoff period is 65% of one rinse cycle, which gives about 65% reduction of rinsewater consumption. Chemical Recovery for the PRS. Similar to the analysis for the cleaning and rinse subsystem, the evaporation rate of the plating tank is estimated (i.e., about 9.8 gal/h). It is specified as the flow rate of the solution sent from tank T8 back to the plating tank. A steady-state system analysis suggests that the chemical concentration in the drag-out from tank T9 is about 0.1% of the chemical concentration in the drag-out from the plating tank, which indicates more than 99% plating solution recovery. Rinse Water Saving for the PRS. In order to ensure product quality, the amount of contaminant on the parts leaving the production line should be smaller than 0.018 g/cm2. As observed in Figure 7, with three countercurrent flow rinse tanks, the equilibrium status is achieved within 600 min (i.e., the processing time for 30 racks) and the contaminant level becomes much lower than the maximum permissible level (Figure 7b), which is unnecessary. It is understandable that the change of tanks T8 and T9 for static rinse will result in more contaminant residual on the parts when leaving tank T9, as compared with the original system that has three flow rinse tanks. To achieve satisfactory rinse quality, two approaches can be applied: (1) to increase the water flow rate for rinse tank T10, and (2) to leave the rack in rinse tank T10 for a longer duration. Because the production is not busy and rinsewater consumption should be minimized, the second approach is selected. The rinsewater flow rate for tank T10 is kept at 5 gal/min, while the rinse time in tank T10 is increased from 1 to 2 min and a total of 4 min in the tank idle mode have the water cut off. With these settings, the simulation results for the new system (Plan A in Figure 4b) are shown in Figure 8. The rinse quality requirement is satisfied for each rack of parts. Because there is 11441
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a 4 min water-cutoff period in each rack processing cycle (i.e., 20 min), 20% of rinsewater can be saved. Analysis on Plan B. The major difference between Plan A (Figure 4b) and Plan B (Figure 4c) is in the plating and rinse subsystem. Plan B has one less static rinse tank, and the plating solution recovery is around 96% (i.e., 3% lower than Plan A). Instead of cutting off rinsewater in each rinse cycle, a water flow restriction strategy is used to reduce rinsewater consumption for the rinse step after the electro-cleaning. Forty percent reduction can be achieved by reducing the water flow rate from 5 gal/min to 3 gal/min with rinse quality simultaneously guaranteed. Comparison between Plans A and B. In summary, the implementation of Plan A will lead to higher plating solution recovery and less rinsewater consumption. However, the operation becomes more complicated, since the water flow needs to be switched on and off in each cycle of the rinse operation. On the other hand, Plan B can ensure better rinse quality and thus better product quality. In operation, the water flow restriction strategy is much simpler than the water cutoff strategy. Decision makers in the plant prefer a conservative plan (i.e., Plan B), which is finally implemented. 4.5. Performance Assessment. The post implementation analysis shows very promising results for the chemical and metal recovery and cost reduction. Environmental Benefits. The IP3 technology has led to significant chemical, metal, water, and waste reduction (see the data listed in Tables 3 and 4). The information of hazardous substance classification and toxicity is listed in the second and third columns of Tables 3 and 4. The data are from the U.S. EPA Hazardous Substance List22 and the Integrated Risk Information System (IRIS).23 The toxicity data are evaluated by the inhalation Reference Concentration (RfC) or the oral Reference Dose (RfD)an estimate of a daily exposure to the human population that is likely to be without an appreciable risk of deleterious effects during a lifetime. It is of utmost importance to show specifically how individual hazardous/toxic chemicals are reduced in use and in emission. Figure 9 provides convincing information. Clearly, the air quality in the workshop area has been significantly improved after the IP3 technology implementation. Economic Benefits. In the electroplating line, the achieved annual profit is ∼$56 700/year, whereas the total annualized cost for the technology implementation is ∼$3370/year. This means that the ratio of the annual profit to the total annualized cost reaches 16.8 to 1. Tables 3 and 4 summarize the cost reduction due to the implementation of the IP3 technology (calculation basis: 3 600 000 parts/year). Product Quality Assurance. It must be pointed out that the parts cleaning, rinsing and plating qualities are fully satisfied after
Figure 7. Contaminant dynamic simulation for the original system with three flow rinse tanks: (a) contaminant on parts (30 racks), (b) contaminant on parts (the 30th rack), (c) contaminant concentration in rinse tank T10 (30 racks), and (d) contaminant concentration in rinse tank T10 (the 30th rack).
Figure 8. Contaminant dynamic simulation for the new system with one flow rinse tank and two static rinse tanks: (a) contaminant on parts (30 racks), (b) contaminant on parts (the 30th rack), (c) contaminant concentration in rinse tank T10 (30 racks), and (d) contaminant concentration in rinse tank T10 (the 30th rack).
Table 3. Chemical Source Reduction from IP3 Technology Implementation (Cleaning and Rinse Subsystem) consumption (gal/yr) chemicals
substance classification
deruster (NaOH, Na2SiO3, Na2CO3, Na5P3O10) hazardous soak cleaner (NaOH, Na5P3O10, Na2SiO3, Na2CO3) hazardous acid (HCl) hazardous source reduction benefits before tech. implementation rinse water wastewater treatment
toxicity (RfC) (mg/m3)
before
not available 817 not available 154 2 × 10−2 2635 after tech. implementation
2 160 000 gal/yr $32 400/yr
1 296 000 gal/yr $9720/yr 11442
after
chemical reduction (%)
savings ($/year)
493 40 0 100 2430 8 cost reduction
2520 1560 360 savings ($/year)
40 70
3 720 22 680
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Table 4. Chemical and Metal Source Reduction from IP3 Technology Implementation (Plating and Rinse Subsystem) consumption (lb/yr) chemicals
substance classification
toxicity (RfD) (mg/kg/day) −2
NaCN hazardous (P106) 4 × 10 NaOH hazardous not available CuCN hazardous (P029) 5 × 10−3 Na2SnO3 unknown not available source reduction benefits before tech. impl. ($/year) wastewater treatment
before
after
1458 583 3719 1649 after tech. impl.
796 428 2981 1278
32 400
chemical reduction (%)
savings ($/year)
45 26 20 22 cost reduction (%)
960 480 10 440 2340 savings ($/year)
36
11 640
20 760
have demonstrated practicability and effectiveness in simultaneous waste reduction and profit making. In this paper, a systematic approach is introduced for developing integrated P3 (IP3) technologies. The methodology integrates process design with operation optimization, focuses on the profit and environmental quality issues associated with multiple flows in an entire electroplating system, and thus promotes the best performance in waste reduction and cost saving. The attractiveness of the IP3 technology has been demonstrated by its successful implementation in a bronze cyanide plating system. The hierarchical decision making strategy and the integrated system characterization, analysis and optimization methods are generic rather than chemical/ process/product specific, and should be applicable to a wide range of surface finishing industries for realizing sustainable manufacturing, i.e., drastically reducing the consumption and the loss of hazardous/toxic chemicals and minimizing waste in the most economical way.
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Figure 9. Environmental benefits: (a) the consumption of hazardous substances before and after the implementation of the IP3 technology and (b) emission-reduction-based working environmental air quality improvement data.
AUTHOR INFORMATION
Corresponding Author
‡Phone: 313-577-3771. Fax: 313-577-3810. E-mail: yhuang@ wayne.edu.
the technology implementation. Figure 10 gives an example, where the coating composition data collected during a onemonth period are well within the acceptable limits.
Present Address †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is in part supported by NSF (0730383 and 1140000) and Michigan Department of Environmental Quality. Technical assistance from KC Jones Plating Company, Michigan is gratefully appreciated.
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REFERENCES
(1) El-Halwagi, M. M. Pollution Prevention through Process Integration: Systematic Design Tools; Academic Press Limited: London, 1997. (2) Higgins, T. E. Pollution Prevention Handbook; Lewis Publishers: Boca Raton, FL, 1995. (3) Foo, D. C. Y.; El-Halwagi, M. M.; Tan, R. R. Recent Advances in Sustainable Process Design and Optimization; World Scientific Publishing: Singapore, 2012. (4) Cheremisinoff, N. P. Handbook of Pollution Prevention Practices; Marcel Dekker: New York, 2001. (5) Lou, H. R.; Huang, Y. L. Profitable pollution prevention: concept, fundamentals and development. J. Plat. Surf. Finish. 2000, 87, 59−66. (6) Lou, H. H.; Huang, Y. L. Profitable pollution prevention in electroplating: an in-process focused approach. Toward Zero Discharge: Innovative Methodology and Technologies for Process Pollution Prevention. Das, T. K.. Wiley-Interscience: NY, 2005, 476−491.
Figure 10. Postimplementation coating composition data.
5. CONCLUSIONS The profitable pollution prevention (P3) concept not only inherits the merits of traditional P2, but takes economic incentive into account as well. In the past decade, a number of P3 technologies have been developed for the electroplating industry. The implementation of these technologies usually involves a slight modification of process and the improvement of operation, which requires little to no capital investment. P3 technologies 11443
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(7) Zhou, Q.; Lou, H. H.; Huang, Y. L. Design of a switchable water allocation network based on process dynamics. Ind. Eng. Chem. Res. 2001, 40, 4866−4873. (8) Huang, Y. L. Demonstration of a Novel Source Waste Reduction Technology in Electroplating Plants; Technical Report to Michigan Department of Environmental Quality; Michigan Department of Environmental Quality: Lansing, MI, 2002. (9) Ford, C.; Delaney, S. Metal Finishing Industry Module; Toxics Use Reduction Institute: Lowell, MA, 1994. (10) Cushnie, G. C. Pollution Prevention and Control Technology for Plating Operations; National Center for Manufacturing Sciences: Ann Arbor, MI, 1994. (11) Gong, J. P.; Luo, K. Q.; Huang, Y. L. Dynamic modeling and simulation for environmentally benign cleaning and rinsing. J. Plat. Surf. Finish. 1997, 84, 63−70. (12) Zhou, Q.; Huang, Y. L. Hierarchical optimization of cleaning and rinsing operations in barrel plating. J. Plat. Surf. Finish. 2002, 89, 68−71. (13) Yang, Y. H.; Lou, H. R.; Huang, Y. L. Optimal design of a water reuse system in an electroplating plant. J. Plat. Surf. Finish. 1999, 86, 80− 84. (14) Yang, Y. H.; Lou, H. H.; Huang, Y. L. Synthesis of an optimal wastewater reuse network. Waste Manage. 2000, 20, 311−319. (15) Zhou, Q.; Lou, H. H.; Huang, Y. L. Design of a switchable water allocation network based on process dynamics. Ind. Eng. Chem. Res. 2001, 40, 4866−4873. (16) Luo, K. Q.; Gong, J. P.; Huang, Y. L. Modeling for sludge estimation and reduction. J. Plat. Surf. Finish. 1998, 85, 59−64. (17) Xu, Q.; Huang, Y. L. Design of an optimal reversed drag-out network for maximum chemical recovery in electroplating systems. J. Plat. Surf. Finish. 2005, 92, 44−48. (18) Xu, Q.; Telukdarie, A.; Huang, Y. L. Integrated electroplating system modeling and simulation for near zero discharge of chemicals and metals. Ind. Eng. Chem. Res. 2005, 44, 2156−2164. (19) Xu, Q.; Huang, Y. L. Graph-assisted optimal cyclic hoist scheduling for environmentally benign electroplating. Ind. Eng. Chem. Res. 2004, 43, 8307−8316. (20) Kuntay, I.; Uygun, K.; Xu, Q.; Huang, Y. L. Environmentally conscious hoist scheduling in material handling processes. Chem. Eng. Commun. 2006, 193, 273−293. (21) Huang, Y. L. Demonstration of a Chemical-Metal Zero-Discharge Technology for Profitable Pollution Prevention in Electroplating Processes; Technical Report to Michigan Department of Environmental Quality ; Michigan Department of Environmental Quality: Lansing, MI, 2008 (22) http://www.epa.gov/oust/fedlaws/hazsubs.htm (23) http://cfpub.epa.gov/ncea/iris/index.cfm?fuseaction=iris. showSubstanceList
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