An Innovative Approach to Water System Modeling for Phosphoric Acid Plants FINAL REPORT. Ronald P. Tebbetts and Kenneth J Cygan

Transcription

1 An Innovative Approach to Water System Modeling for Phosphoric Acid Plants FINAL REPORT Ronald P. Tebbetts and Kenneth J Cygan Nalco, an Ecolab Company Yarmouth, Maine April 2015

2 ABSTRACT Water finds a use in nearly every process in phosphoric acid production, from steam and power generation, vacuum creation for evaporators, as both a heat source and as wash water for gypsum filtration, as well as providing cooling for flash coolers and heat removal in the sulfuric acid plant. It can be safely said that phosphoric acid plants cannot operate without water and the proper management of that water is critical to the bottom line. As industry searches for ways to conserve, reuse and recycle their available water resource, or as they plan for plant expansions or modifications, it has become apparent that the good intensions of many these efforts often ignore the negative unintended consequences from a water treatment perspective. Often these efforts use a simple hydraulic model to predict an outcome while ignoring the effect that any change may have on the chemistry of the system. This paper describes an innovative method where a water system is modeled in its entirety, using a software based program that combines the familiarly of a hydraulic model with the accuracy and confirmation of an ionic or salt balance. An example of an actual simulation and a completed project is provided.

3 INTRODUCTION Heavy industries by their very nature often require large quantities of water and the phosphate industry is no exception. The wet process manufacture of phosphoric acid, as practiced in Florida and many other parts of the world, requires copious quantities of what is called process water. This water finds a use in nearly every process, from steam and power generation, vacuum creation for evaporators, as both a heat source and as wash water for gypsum filtration, as well as providing cooling for flash coolers and heat removal in the sulfuric acid plant. It can be safely said that phosphoric acid plants cannot operate without water and the proper management of that water is becoming more and more critical to the bottom line. Historically, many industries considered water a cheap and readily available commodity and little was done to minimize its use. Industry purposely located next to easily accessible water bodies in order to use them as a source of water and energy as well as a convenient depository for their waste. In many instances, these water sources also provided the means to transport their finished goods to market. Recent climatological events in many parts of the U.S. have brought to light a new paradigm that water is not as universally plentiful nor as inexpensive as first thought. Drought conditions being experienced from Texas to California are obvious precursors to the need to conserve water. But even in seemingly water rich states such as Minnesota, with its 10,000 lakes, water conservation efforts are being stressed because of the overuse of the ground water resource near the larger metropolitan areas (Freshwater Society, 2013). To make matters worse, many industries are finding that the ever expanding urban areas are fast encroaching upon their property boundaries with competition for a share of the available water resource becoming more and more common. At the current rate of growth, Florida is expected to have a population of nearly 21 million people by 2020 (World Population Review, 2015), each with a need for a reliable, clean and safe water supply. Competing needs between domestic, industrial and in many areas agricultural users have demonstrated that an unlimited supply of inexpensive water can no longer be taken for granted in many parts of the world. Many industries have responded quickly and appropriately to the real risk that a dwindling water supply represents to their long term financial survival. The first step many took involved the recognition that sustainability of all resources, including water should be a prominent component of their operating objectives. A review of numerous corporate websites now show whole sections dedicated to sustainability, of which water is a prominent component. It is apparent that all industry now recognizes the value of protecting the water resources of their communities if for no other reason than to be a good neighbor.

4 In some jurisdictions however, voluntary compliance with local water use regulations is no longer sufficient, rather, mandatory water use reduction is now the norm. Suddenly, many plants are finding that they no longer should, but now must reduce their water intake. An extreme example of this is happening in California where the governor recently instituted a mandatory water reduction plan for the entire state with a goal to reduce water use by 25% across the board (New York Times, 2015). Consequently, water conservation efforts have proliferated in recent years with the goal of reducing a plants water footprint. Most conservation projects take a single minded approach by focusing all of their efforts on ways to use less water in a single process, or on ways to reuse a single waste stream, either from a process internal to the plant or from a locally available source. For instance, California has actively encouraged water reuse for many years and its industries are actively encouraged to use Title 22 water, which is recycled municipal wastewater. El Paso, Texas is also in the forefront of communities actively encouraging the use of reclaimed wastewater and has been doing so since 1963 (EPWU, 2014). Locating and capturing the easy water or low hanging fruit as it s commonly referred to, has been the mainstay of most efforts. However, by taking this single minded approach, the implications of the effect that the conservation effort may have on the downstream components that may have been will be receiving this water are often overlooked or ignored. For instance, sending reverse osmosis (RO) reject water, which is considered to be clean, to the cooling system to replace some of the make-up may seem reasonable and logical if you only consider the volume of water you are replacing. Unfortunately, focusing simply on quantity of water being recycled and ignoring the effect that the quality or more specifically, the ionic concentration of the waste stream may have on the receiving process has often led to what can be called the unintended consequences of good intensions. WHAT IS THE ISSUE? The one characteristic common to most major industries is their ability to concentrate ions into their process and waste streams. These ions are a combination of those found in the influent water being used by the plant, ions contributed by the unique characteristics of the process itself and purchased ions from the various treatment chemistries, such as acids or other chemistries, used in the processes. In most cases, the amount of purchased ions can far outweigh those coming in with the influent. Converting this water to steam or evaporating it in a cooling tower or pond results in the ions that remain behind becoming much more concentrated. Ions also concentrate in the waste streams from many processes that may be used to pretreat the water going to the steam generators, like RO and demineralizers. All of these ions, comprised mostly of calcium, magnesium, sodium, silica, sulfate and chlorides can be problematic to various systems from a scaling or corrosion perspective, or may become toxic in discharge waters if sufficient concentrations are achieved. As these concentrated salt streams move through the system on their way to a discharge facility, they may be diluted by other water streams that are not as concentrated, producing a final ionic concentration that is

5 acceptable for direct discharge to a final receiving body. In a way, a plant s overall water/waste streams eventually reach something of an equilibrium for both hydraulic loading and ionic concentrations, allowing all processes the ability to function sufficiently well enough to allow the plant to meet its discharge limits. Upsetting this equilibrium does have its consequences. IMPEDIMENTS TO WATER REUSE Significant water reuse efforts can be hampered by several key characteristics of the facility, namely the arrangement of the various units in the plant, the condition of the infrastructure and the internal knowledge of the plant s water infrastructure. Unfortunately, few of the older existing facilities were built with water conservation in mind. Consequently the means to allow the return of water streams that could be easily reused are generally not readily available. Most conventional water conservation efforts tend to focus on discrete process within a plant, such as a cooling system. Hydraulic modeling is typically used to predict an outcome but because water has never been considered a high value resource, system monitoring devices like flow meters and pressure gauges are either not available, unreliable or inaccurate. A few of the more advanced planners may add a third party ionic modeling program but this is done simply to predict a change in the scaling potential of the receiving system caused by the addition of the reused water. Since the focus is on a single system or unit, what is often overlooked is the disruptive effect that recycling these waste streams may have on the entire plant water system s equilibrium. For instance, if the waste plant is generally accustomed to receiving a certain volume of wastewater containing a certain concentration of food, a smaller but more concentrated volume of waste could be either toxic to the biology or contain an insufficient level of food to support the existing population. Additionally, many jurisdictions are now tightening total dissolved solids (TDS) limits on discharge permits. Overzealous cycling of systems and excessive reuse of the more dilute waste streams can concentrate dissolved solids in the final effluent stream well above these tighter limits resulting in a permit violation. You still have to retain the ability to discharge the final waste from the plant and losing sight of the fact that until capital is spent to remove some of these salts completely from the system, the solution to pollution is dilution will still prevail. AN INNOVATIVE APPROACH It became obvious that to avoid the unintended consequences common to many conservation efforts, a more holistic or whole plant approach was needed. Rather than focusing on a single process, the entire system must be modeled to ensure that changing conditions in one component would not negatively affect it or other equipment elsewhere in the system. Since most of the unintended consequences always seemed to be chemistry related, it became clear that hydraulic modeling described only one piece of the puzzle and that ionic modeling must be fully integrated into the model to show the full

6 picture. It also became obvious that a simple spreadsheet based program was incapable of handling the advanced logic required to properly loop back flows to other points within the model while simultaneously adjusting the characteristics of the water downstream of the receiving process. In response to these issues, a new innovative approach was taken, whereby a non-spreadsheet based, proprietary, software program, which incorporated both hydraulic and ionic modeling with recycling capabilities, was created. The program is called the Advanced Modeling System (AMS Water ). Development of the AMS Water software started in 2001 and continues today as enhancements and improvements are constantly being made to ensure that the most recent technological advances in water treatment equipment and theory are represented. The model has become the core feature of a comprehensive water audit process that has been used nearly 100 times in a variety of industries such as Refining, Power, Petrochemical and CPI plants. THE WATER AUDIT PROCESS The first step of the audit process is to develop an accurate process flow diagram (PFD) of the entire water system, from influent to effluent. This step relies heavily upon any original engineering drawings of the plants water systems. Numerous hours are spent with plant personnel to properly define the various water using systems in the facility. Often, ad hoc sketches done by the various process engineers responsible for specific systems within the plant are also used to update these diagrams. An on-site walk-through of the entire system is done to confirm the information obtained from any original site design diagrams, or to correct any deviations that may exist between actual conditions and those shown on the drawings. At some point during the walk-through process, key water sample points are identified and water samples obtained. These samples are sent to an off-site lab for analysis using Inductively Coupled Plasma Analysis (ICP) for cations and Ion Chromatography (IC) for the anions. In addition, all pertinent operating data, including purchased chemicals and dosages and any flow data that may be available is collected. This entire data collection process including sample preparation and analysis, can take up to three weeks or more to complete, depending upon the complexity and size of the system and usually involves two to three modeling engineers and one or two of the plants engineers. Once the initial PFD is created using a commercially available drawing program, the information is returned to the plant engineering staff for red-lining and approval. This is often done in an on-site group session to get as many of the plant personnel involved as possible. Senior operations staff is also consulted as they tend to be most familiar with the plant history and infrastructure and often offer valuable insight and knowledge about their systems. An example of a typical PFD developed during this phase of the project is shown is Figure 1.

7 Figure 1: Process Flow Diagram (PFD) Example. As the PFD is built, the information is transferred to the modeling software where each component in the water system, along with its design and operating characteristics is added as a small icon. Clicking on each icon will open the data form for that icon, an example of which is shown in Figure 2. The input forms contain all of the design and operating data for that piece of equipment. A screenshot of a model under construction is shown in Figure 3. Color coded connectors (elbows and tees) are used to link the equipment together. Blue connectors denote product water from non-heated sources, red connectors denote product water from steam generators and brown connectors denote waste streams. Intermediate points on the blue lines between connectors are called connections and when clicked show the characteristics of the water at that point.

8 Figure 2: Input form for Cooling Tower Data. Figure 3: Screenshot of a model under construction. These water characteristics will either be based on an actual water sample analysis or on the model s prediction of what the water quality should be based upon the operating characteristics of the process that the water just passed through. Upon completion of the model which now includes all equipment parameters, operating data, available flow data, water sample data and purchased chemistry dosages, a first run is conducted to see how well the model agrees with the actual sample data. Any deviation between actual sample data and predicted sample data greater than ± 5% would be investigated to determine what is causing this difference. The difference could simply be the result of an overlooked process chemistry that may have been left out of the

9 model. On one occasion, the difference was caused by the addition of spent caustic to the inlet of an API. The API was acting much like a cold lime softener, removing hardness from the effluent. Once this was recognized and appropriate adjustments were made to the model, the model balanced. Once all of the adjustments are made and the AMS-Water model sufficiently mirrors the actual water chemistry and system performance, variations in system configuration can now be evaluated. This fully balanced model is designated as the Base Configuration or Base Case, and all other evaluations and revisions will be compared to it. Additional features of the software include the ability to estimate capital expenses (± 50%) and the estimation of savings achieved in water and purchased chemistry costs for each evaluation. SYSTEM EVALUATIONS Any number of system configuration changes can be made to the base case by simply dropping and dragging icons that represent new or additional treatment methodologies, such as RO, cold lime softening and evaporators, into the model. Water can also be re-routed from one process to another with simple mouse clicks. Unlimited What If scenarios can be made to test out different configurations to achieve a cost effective solution. However, it still takes the knowledge and experience of a water treatment professional to select the most appropriate technology to use or to know when and where water can be safely and effectively be re-purposed. The software simply does all of the tedious calculations and numerous iterations required to appropriately model the new conditions. All audits are designed to meet the pre- defined needs and objectives of the plant management. The overall objective could be as simple as the need for an accurate and current Base Configuration for future use or as complex as determining what it would take to make the plant a zero liquid discharge (ZLD) facility. In any event, once the model is prepared, it is available and ready to be used for a variety of water related purposes such as 1) Water Conservation Efforts 2) Plant Expansions Planning 3) Water System Debottlenecking 4) Current Equipment Evaluation or 5) to satisfy Regulatory Requirements, to name just a few reasons. One such example is provided in the case study that follows. CASE STUDY A western U.S. coal mine typically disposed of its mine water in a nearby retention pond. Evaporation was typically greater than the flow of water from the mine, so the pond was able to adequately handle the volume of water generated. Conditions changed where the volume of water requiring disposal exceeded the evaporation capacity of the retention pond. Consequently, the mine had to find another disposal source for its excess water. A nearby power plant that used the coal from the mine wanted to know if

10 they could take this water and if so, what were the ramifications of doing so to their processes. The power plant had two significant users of water; the cooling tower and the flue gas desulfurizer (FGD). The mine water was unusual in that it was alkaline in nature (M Alkalinity ~ 775 ppm as CaCO 3 ), yet contained very low levels of hardness. The main constituent was sodium bicarbonate. Since the power plant had to purchase low grade sodium carbonate to use in the FGD, it was hoped that the mine water could help to reduce the volume and cost of sodium carbonate being purchased. Unfortunately, two problems became evident when the mine water was used in the FGD unit. The first problem involved the coal fines that were present in the mine water. These fines plugged up the spray nozzles and demister pads of the FGD unit, effectively rendering the system ineffective. The second problem involved the volume of mine water requiring disposal. The power plant used approximately 1,700 gpm of water in the FGD unit but the volume of mine water exceeded 3,000 gpm. Consequently the FGD could not effectively handle the entire volume of excess mine water. It was then decided to send the mine water to two of the four cooling towers to be used as cooling tower make-up. Unfortunately, the high alkalinity of the mine water required a much larger volume of acid be used to maintain the ph in the cooling system. This increased acid usage rate resulted in a significant cost increase to the power plant. The power plant requested assistance from Nalco to formulate a water balance analysis to determine the best way to utilize this mine water in a safe and cost effective manner. Three goals for the evaluation were established by the plant management: 1. Evaluate the mine water chemistry to establish its best use in the facility. 2. Minimize mine water processing and required materials. 3. Minimize chemical additive usage (cost, real estate). A base case model was constructed and a total of seventeen different evaluations were conducted to provide the site with a variety of options to achieve their goals. It was readily apparent that any major treatment of the mine water was going to require capital and equipment. Most of the evaluations involved some combination of a sodium zeolite softener and a reverse osmosis (RO) unit to treat the mine water. This process would result in a very acceptable, low alkalinity, low hardness make-up source for the cooling towers and a significantly lower volume, highly alkaline waste stream for the FGD. This would significantly reduce the amount of acid required at the cooling tower while also turning a waste into a raw material, replacing some of the purchased soda ash used in the FGD. A summary of these evaluations are shown in appendix A.

11 CONCLUSIONS The need to reduce or reuse industrial water has become critical in many parts of the world as environmental conditions literally dry up water sources or competing needs from a growing population stress the available supply. As industry searches for ways to conserve, reuse and recycle their available water resource, it has become apparent that the good intensions of many these efforts are often overshadowed by the negative unintended consequences from a water treatment perspective. Focusing only on discrete systems within a facility and using a simple hydraulic model will not provide the information needed to properly predict a low risk outcome. Improved models may include a salt balance to provide a more accurate representation of the true risk involved, but few take a holistic approach by looking at a plant s water system in its entirety. Furthermore, what all of these spreadsheet based models lack is the ability to simulate the recycling of flows for anything more than a single process. The case study demonstrated that the model is capable of providing the user with a variety of water reuse options. An evaluation of each option based upon not only the water savings but also upon total cost of operation and estimated capital costs associated with each is also provided. The end user is then able to make a more informed decision based upon a much more realistic assessment of the potential risks of any water conservation effort. It needs to be emphasized that once the easy water is conserved, additional water savings comes at a price. Reducing water usage by cycling up individual systems has a finite limit and will eventually become counter-productive. At some point, a portion of the salts has to be taken out of the system and this will require equipment, capital, engineering services and space.

12 Appendix A SUMMARY OF CASE STUDY OPTIONS

13 Appendix A Case Study: Summary of Model Options Base Case Rev_001 Used to establish flows throughout the facility as the basis for all other design work and modeling. It utilizes a known FGD reagent (soda ash) for modeling chemistry confirmation of the soda FGDs. Case Rev_002 Utilizes a sodium zeolite softener and RO to treat the mine water to minimize plant cooling tower acid and FGD soda liquor use. It utilizes a known FGD reagent (soda ash) for modeling chemistry confirmation of the soda FGDs. Case Rev_003 Is Rev_001 but introduces the Soda Liquor Solution as reagent into the water flow to the Flue Gas Desulfurizers as is currently utilized at the plant. Case Rev_004 Combines Rev_002 and Rev_003 and optimizes the mine water usage by processing it with the sodium zeolite softener and RO, thus reducing acid requirements for the cooling towers and soda liquor consumption for the FGD units. This is a recommended Revision for the facility. Case Rev_005 Utilizes Rev_004 and has units 1 and 2 off line. This optimizes the mine water usage by processing it with the sodium zeolite softener and RO, thus reducing acid requirements for the cooling towers and soda liquor consumption for the FGD units. Case Rev_006 Utilizes Rev_004 and has units 2 and 3 off line. This optimizes the mine water usage by processing it with the sodium zeolite softener and RO, thus reducing acid requirements for the cooling towers and soda liquor consumption for the FGD units. Case Rev_007 Utilizes Rev_004 water balance but now replaces the Green River influent with Mine Water RO effluent. The boiler cycle make up cold lime softener and filters are eliminated. Case Rev_008 Utilizes Rev_004 water balance but reduces the volume of mine water processed with the sodium zeolite and reverse osmosis system to 1,000 gpm. Case Rev_009 Utilizes Rev_004 water balance but reduces the volume of mine water processed with the sodium zeolite and reverse osmosis system to 2,000 gpm. Case Rev_010 Is Rev_003 but introduces 800 gpm of clarified mine water to the water flow to the Flue Gas Desulfurizers. It introduces 2,200 gpm of mine water to cooling tower 3&4. This is a recommended Revision for the facility Case Rev_011 Is Rev_003 but introduces 800 gpm of clarified mine water to the water flow to the Flue Gas Desulfurizers. It introduces 3,000 gpm of mine water to cooling tower 3&4.

14 Appendix A Case Rev_012 Is a 50% capacity factor revision with units 3 and 4 operating and 1 and 2 offline. It is used to create a base line cost for this configuration. Case Rev_013 Is a 50% capacity factor revision with units 3 and 4 operating and 1 and 2 offline. It introduces 400 gpm of clarified mine water to unit 3 and 4 FGDs. It directs 3,000 gpm of clarified mine water is to towers 3&4. Case Rev_014 Is a 50% capacity factor revision with units 1 and 4 operating and 2 and 3 offline. It introduces 400 gpm of clarified mine water to unit 1 and 4 FGDs. It directs 3,000 gpm of clarified mine water is to tower 4. Case Rev_015 Is a 50% capacity factor revision with units 1 and 4 operating and 2 and 3 offline. It is used to create a base line cost for this configuration. Case Rev_017 Utilizes the Rev_003 water balance but sends 100 gpm of the 3,000 gpm of mine water through a spare FGD to produce 50 gpm of acidic water for the cooling towers to reduce purchased acid.

15 References El Paso Water Utilities. (2014, May 26). Reclaimed Water Water Shouldn t Only Be Used Once. Retrieved from EPWU Website: Gleick, P.H. (1994). Water and Energy. Annual Review of Energy and the Environment, 1994, 19: Freshwater Society (2013, April). Minnesota s Groundwater: Is our use sustainable? Retrieved from Freshwater Society Website: freshwater.org/wpcontent/uploads/2013/04/updated-mns-groundwater-paper-lo-res.pdf World Population Review. (2014, Oct 19). Florida Population Retrieved from World Population Review Website: worldpopulationreview.com/states/floridapopulation/ New York Times. (2015 April 1). California Imposes First Mandatory Water Restrictions to Deal With Drought. Retrieved from New York Times Website: