Table of Contents
Understanding Material Balance Discrepancies in Industrial Operations
Material balance discrepancies represent one of the most persistent challenges facing industrial plants across multiple sectors, from chemical processing and pharmaceuticals to oil and gas production. These discrepancies can significantly impact operational efficiency, product quality, safety standards, and ultimately, profitability. Understanding the fundamental principles behind material balance and developing systematic approaches to identify and resolve discrepancies is essential for maintaining optimal plant performance and ensuring regulatory compliance.
Material balance compares the quantity of material that entered an organization/department/area during a certain period to the quantity reported as leaving that same area. This fundamental accounting principle serves as the backbone of process engineering and operational management in industrial facilities. When properly implemented, material balance calculations provide critical insights into process efficiency, help detect losses or leaks, and ensure that operations remain within acceptable parameters.
In physics, a mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique. This powerful analytical tool enables engineers and plant operators to maintain tight control over their processes and quickly identify when something goes wrong.
The Fundamental Principles of Material Balance
At its core, material balance is rooted in the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system. This fundamental principle applies to virtually all industrial processes, excluding nuclear reactions. Mass balances are used widely in engineering and environmental analyses. For example, mass balance theory is used to design chemical reactors, to analyse alternative processes to produce chemicals, as well as to model pollution dispersion and other processes of physical systems.
The General Material Balance Equation
The general material balance equation provides a comprehensive framework for tracking materials through any process. The equation accounts for all material entering the system, leaving the system, accumulating within the system, and any generation or consumption that occurs due to chemical reactions. For systems at steady state, where properties do not change over time, the accumulation term becomes zero, simplifying the calculations significantly.
Understanding when to apply different forms of the material balance equation is crucial for accurate analysis. In processes without chemical reactions, such as blending, drying, or physical separation, the generation and consumption terms are eliminated. This simplification makes the calculations more straightforward but requires careful attention to ensure all input and output streams are properly accounted for.
System Boundaries and Control Volumes
Defining appropriate system boundaries is critical for conducting accurate material balance calculations. The system or region needs to be defined by an imaginary closed box which is called the system boundary, which should always be drawn to avoid making any mistakes. A system can be one single process unit, a collection of process units or an entire process. The choice of system boundaries can significantly impact the complexity of the analysis and the insights gained from the calculations.
When troubleshooting material balance discrepancies, engineers often need to draw multiple system boundaries at different scales. Starting with an overall plant balance can help identify which sections of the process are experiencing problems, while more detailed balances around individual units can pinpoint specific sources of discrepancies. This hierarchical approach to material balance analysis is particularly effective in complex, multi-unit operations.
Common Causes of Material Balance Discrepancies
Material balance discrepancies can arise from numerous sources, ranging from simple measurement errors to complex process phenomena. Understanding these potential causes is the first step toward developing effective troubleshooting strategies and implementing preventive measures.
Measurement and Instrumentation Errors
The flow and composition measurements have a variety of errors associated with them. First, sensor readings have noise (variations in the measurement due to more or less random variations in the readings that do not correspond to changes in the process). The sensor readings can also be inaccurate for a wide variety of other reasons. These measurement inaccuracies represent one of the most common sources of material balance discrepancies in industrial plants.
A sensor may require recalibration because it degrades, or it may be used for a measurement for which it was not designed. Instrumentation drift over time is inevitable, and without regular calibration and maintenance, even high-quality sensors can provide increasingly inaccurate readings. Flow meters, in particular, are susceptible to fouling, wear, and changes in process conditions that can affect their accuracy.
Temperature and pressure variations can also impact measurement accuracy. Many flow measurement devices are calibrated for specific operating conditions, and deviations from these conditions can introduce systematic errors. Density changes due to temperature fluctuations, for example, can cause volumetric flow measurements to misrepresent actual mass flows, leading to apparent material balance discrepancies.
Process-Related Factors
Industrial processes are almost always in a state of flux, and rarely reach precise steady-state behavior. This inherent variability in industrial operations can make material balance calculations challenging, particularly when attempting to close balances over short time periods. Accumulation within the process equipment, changes in inventory levels, and transient operating conditions all contribute to apparent discrepancies.
A component of interest may be generated or consumed inside the process by reactions that the process engineer has not considered. Unexpected side reactions, degradation of materials, or previously unidentified reaction pathways can lead to material appearing or disappearing from the balance. These phenomena are particularly common in chemical processes operating at elevated temperatures or pressures, where reaction kinetics may differ from laboratory-scale predictions.
Some materials vaporize or burn during the process. There is no magic solution; we need to estimate the percentage lost during each production operation. Evaporation losses, particularly of volatile components, can be significant but difficult to measure directly. Similarly, materials that undergo thermal decomposition or oxidation may contribute to material balance discrepancies if these losses are not properly accounted for in the calculations.
Physical Losses and Leaks
Equipment leaks represent a significant source of material loss in many industrial facilities. These leaks can occur at flanges, valve stems, pump seals, and other connection points throughout the process. While individual leaks may be small, their cumulative effect can result in substantial material balance discrepancies. Detecting and quantifying these losses requires systematic inspection programs and, in some cases, specialized leak detection equipment.
Performing a mass balance on these process units (or the overall plant) is critical for identifying a number of issues, including leaks, faulty sensors, meter calibration issues, process inefficiencies, and more. Regular material balance monitoring can serve as an early warning system for developing leaks, allowing maintenance teams to address problems before they become severe.
Inventory and Accounting Issues
Inventory counts can be a nightmare. A mid-range inventory count will arbitrarily change the inventory. In this case, you need to take the overages or shortages that arose from the inventory count and add or subtract them to/from the output. Inaccurate inventory tracking, particularly of in-process materials and intermediate products, can create significant apparent discrepancies in material balances.
Changes in material holdup within process equipment, such as reactors, distillation columns, and storage tanks, must be carefully accounted for when performing material balances. Failure to properly track these inventory changes can lead to substantial errors, particularly in batch processes or during process startups and shutdowns. Establishing clear procedures for inventory measurement and reconciliation is essential for maintaining accurate material balances.
Scrap, Waste, and Unaccounted Transfers
Usually, we find them in one or more of the following options: Scrap: Material that is rejected or recycled. If we can estimate the percentage of the material in the scrap, we can add it to the calculations as part of the process inventory. Rejected materials, off-specification products, and waste streams must be properly quantified and included in material balance calculations. These streams are sometimes overlooked or inadequately measured, leading to apparent losses.
Transfers outside the black box: Make sure you didn’t transfer material to a warehouse that was not counted or lent to another department or any other occurrence. Material transfers between different areas of the plant, temporary storage locations, or even to other facilities must be carefully tracked. These transfers can easily be missed in material balance calculations if communication between departments is inadequate or if proper documentation procedures are not followed.
Undocumented Process Adjustments
For example, taking our diet candies with 25% sugar, when we test them at the end of the production process in the laboratory, we don’t know exactly how much sugar is in them. Let’s suppose that the R&D team defined that the theoretical glucose content should be X% +/- 1%. The production team knows that to ensure the QA glucose test always passes, they should add an extra 2%-3% sugar to the formula. They don’t report it to anyone, and they do it to avoid stopping for QA reasons. This example illustrates how undocumented process modifications can create persistent material balance discrepancies that are difficult to identify and resolve.
Operators may make adjustments to process conditions or material additions to compensate for quality variations or to meet production targets. While these adjustments may be well-intentioned, if they are not properly documented and communicated, they can create significant confusion when attempting to reconcile material balances. Establishing clear protocols for documenting all process changes is essential for maintaining accurate material accounting.
Acceptable Closure Limits and Industry Standards
Understanding what constitutes an acceptable material balance closure is important for setting realistic expectations and determining when discrepancies require investigation. Material balance closure to within 5% for material balances for most industrial processes is considered reasonable. This tolerance reflects the inherent uncertainties in measurement systems and the natural variability of industrial processes.
However, acceptable closure limits can vary significantly depending on the specific industry, process, and materials involved. Processes handling high-value materials or hazardous substances may require much tighter closure limits, potentially in the range of 1-2%. Conversely, processes dealing with bulk materials or those with inherently high measurement uncertainty may accept closure limits of 10% or more.
Establishing appropriate closure criteria requires consideration of several factors, including the accuracy of available measurement instruments, the economic value of the materials being processed, safety and environmental regulations, and the potential consequences of undetected losses. Regular review and adjustment of these criteria based on operational experience and improvements in measurement technology is recommended.
Systematic Approach to Troubleshooting Material Balance Discrepancies
Developing a systematic, methodical approach to troubleshooting material balance discrepancies is essential for efficient problem resolution. A well-structured troubleshooting process helps ensure that all potential causes are considered and that solutions address root causes rather than symptoms.
Step 1: Verify Data Quality and Measurement Accuracy
The first step in troubleshooting any material balance discrepancy should be to verify the quality and accuracy of the data being used in the calculations. This includes checking that all measurement instruments are functioning properly, properly calibrated, and being used within their design specifications. Flow meters should be inspected for fouling or damage, and their calibration records should be reviewed to ensure they are current.
Composition analyzers, including gas chromatographs, spectrophotometers, and other analytical instruments, should be verified using standard samples or reference materials. Systematic errors in composition measurements can have a significant impact on material balance calculations, particularly for multi-component systems. Cross-checking analytical results using alternative methods or independent laboratories can help identify measurement biases.
Temperature and pressure measurements should also be verified, as these parameters can affect density calculations and, consequently, mass flow determinations. Ensuring that all instruments are reading correctly and that data acquisition systems are functioning properly is a critical first step that can often resolve apparent discrepancies without further investigation.
Step 2: Review System Boundaries and Balance Equations
After verifying data quality, the next step is to carefully review the system boundaries used in the material balance calculations and ensure that all relevant streams have been included. It is surprisingly common for material balance discrepancies to result from overlooked streams, such as small purge flows, sampling lines, or intermittent waste streams.
The material balance equations themselves should be reviewed to ensure they are appropriate for the process being analyzed. This includes verifying that the correct form of the balance equation is being used (steady-state vs. unsteady-state, reactive vs. non-reactive) and that all relevant terms have been included. Special attention should be paid to accumulation terms, which are often neglected but can be significant during process transients or in batch operations.
Step 3: Conduct Physical Inspections
Physical inspection of process equipment and piping systems is an essential component of material balance troubleshooting. This includes visual inspection for obvious leaks, checking valve positions to ensure they match process flow diagrams, and verifying that all equipment is operating as intended. Thermal imaging cameras can be useful for detecting leaks of volatile materials or identifying areas of unexpected heat loss or gain.
Inspection should also include verification of material storage and handling systems. Tank levels should be checked against inventory records, and any discrepancies should be investigated. Sampling systems should be inspected to ensure they are representative and that sample volumes are properly accounted for in material balances.
Step 4: Analyze Process Data and Trends
Detailed analysis of process data over time can provide valuable insights into the nature and causes of material balance discrepancies. Trending key process variables, such as flow rates, compositions, temperatures, and pressures, can help identify patterns or correlations that point to specific problems. For example, a material balance discrepancy that increases with throughput might indicate a flow measurement problem, while one that varies with ambient temperature might suggest evaporation losses.
Statistical analysis techniques can be particularly useful for identifying subtle trends or relationships in process data. Control charts, regression analysis, and other statistical tools can help distinguish between normal process variability and systematic problems that require correction. In industrial process plants, using the fact that the mass entering and leaving any portion of a process plant must balance, data validation and reconciliation algorithms may be employed to correct measured flows, provided that enough redundancy of flow measurements exist to permit statistical reconciliation and exclusion of detectably erroneous measurements. Since all real world measured values contain inherent error, the reconciled measurements provide a better basis than the measured values do for financial reporting, optimization, and regulatory reporting.
Step 5: Consider Process Chemistry and Reactions
For processes involving chemical reactions, careful consideration of reaction stoichiometry, conversion rates, and potential side reactions is essential. Unexpected or unaccounted-for reactions can create significant material balance discrepancies. This may require laboratory analysis of process samples to identify unknown components or verify that reactions are proceeding as expected.
Changes in process conditions, such as temperature, pressure, or catalyst activity, can affect reaction rates and selectivity, potentially leading to material balance discrepancies. Comparing current process performance with historical data or design specifications can help identify whether process chemistry has changed over time.
Step 6: Evaluate Inventory Changes and Accumulation
Careful evaluation of inventory changes within the process system is critical, particularly for batch processes or during periods of changing operating conditions. This includes not only obvious inventory locations such as storage tanks but also material holdup in process equipment such as reactors, separators, and piping systems.
For continuous processes, changes in operating conditions can lead to transient accumulation or depletion of materials within the system. For example, increasing throughput may temporarily increase the holdup in distillation columns or other separation equipment. These transient effects must be accounted for when performing material balances over short time periods.
Step 7: Implement Corrective Actions and Monitor Results
Once the root cause of a material balance discrepancy has been identified, appropriate corrective actions should be implemented. These might include recalibrating instruments, repairing leaks, modifying operating procedures, or updating material balance calculations to account for previously overlooked streams or phenomena.
After implementing corrective actions, it is essential to monitor material balance closure over time to verify that the problem has been resolved and that no new issues have been introduced. Establishing ongoing monitoring procedures and setting up alerts for material balance discrepancies that exceed acceptable limits can help prevent problems from recurring and enable early detection of new issues.
Advanced Techniques for Material Balance Analysis
Data Reconciliation and Validation
Data reconciliation is a sophisticated mathematical technique that uses redundant measurements and material balance constraints to improve the accuracy of process data. By applying statistical methods and optimization algorithms, data reconciliation can identify and correct measurement errors, providing more reliable estimates of process flows and compositions than raw measurements alone.
Modern data reconciliation software can handle complex process networks with multiple units and recycle streams, automatically identifying gross errors in measurements and providing statistically optimal estimates of all process variables. These tools are particularly valuable in large, complex facilities where manual material balance calculations would be impractical.
Real-Time Material Balance Monitoring
To ensure mass balance calculations are accurate, reliable, and up-to-speed, process manufacturing operations use Seeq to calculate and monitor their plants’ mass balance. This mass balance can run continuously to track changes over time and identify discrepancies between inlet and outlet streams. Real-time monitoring systems enable immediate detection of material balance problems, allowing operators to respond quickly before small discrepancies become major issues.
Implementing real-time material balance monitoring requires integration with plant distributed control systems (DCS) or supervisory control and data acquisition (SCADA) systems to access process measurements continuously. Automated alerts can be configured to notify operators and engineers when material balance discrepancies exceed predefined thresholds, enabling rapid response and investigation.
Material Balance Reconciliation Software
Specialized software tools for material balance reconciliation have become increasingly sophisticated and accessible. These tools can handle complex process configurations, including recycle streams, multiple phases, and chemical reactions. They typically incorporate statistical methods to account for measurement uncertainty and can identify which measurements are most likely to be in error.
Many modern material balance software packages also include optimization capabilities, allowing engineers to determine optimal operating conditions while maintaining material balance constraints. Integration with process simulation software enables comparison of actual plant performance with design specifications or theoretical predictions, facilitating identification of process inefficiencies or equipment degradation.
Best Practices for Maintaining Accurate Material Balances
Establish Robust Measurement Systems
Investing in high-quality measurement instrumentation and maintaining it properly is fundamental to achieving accurate material balances. This includes selecting instruments appropriate for the specific application, ensuring they are installed correctly, and establishing regular calibration and maintenance schedules. Redundant measurements at critical points in the process can provide valuable cross-checks and enable data reconciliation techniques.
Measurement uncertainty should be quantified and documented for all key instruments. Understanding the accuracy and precision of measurements allows for realistic assessment of material balance closure and helps prioritize improvement efforts. Regular audits of measurement systems can identify instruments that are degrading or operating outside their design specifications before they cause significant problems.
Develop Clear Procedures and Documentation
Establishing clear, written procedures for performing material balance calculations and troubleshooting discrepancies ensures consistency and helps prevent errors. These procedures should specify system boundaries, calculation methods, acceptable closure criteria, and escalation procedures for discrepancies that exceed acceptable limits.
Documentation of all process changes, including modifications to equipment, operating procedures, or material specifications, is essential for maintaining accurate material balances over time. A formal management of change (MOC) process should include assessment of potential impacts on material balances and updating of balance calculations as needed.
Implement Regular Material Balance Reviews
Conducting regular material balance reviews, even when no obvious problems exist, can help identify developing issues before they become serious. These reviews should include examination of trends in material balance closure over time, comparison with historical performance, and assessment of whether any systematic biases are present.
Periodic comprehensive material balance audits, involving detailed verification of all measurements and calculations, can uncover problems that might not be apparent from routine monitoring. These audits might be conducted annually or when significant process changes occur, and should involve personnel with expertise in material balance analysis and process engineering.
Foster Cross-Functional Collaboration
Effective material balance management requires collaboration between multiple departments, including operations, engineering, maintenance, and laboratory personnel. Regular communication and information sharing help ensure that all relevant data is available for material balance calculations and that problems are identified and addressed quickly.
Training programs should ensure that all personnel understand the importance of accurate material balances and their role in maintaining them. Operators should be trained to recognize signs of potential material balance problems and to report them promptly. Engineers and technicians should be proficient in material balance calculation methods and troubleshooting techniques.
Leverage Technology and Automation
Modern process control and information management systems offer powerful capabilities for automated material balance monitoring and analysis. Implementing these systems can reduce the manual effort required for material balance calculations, improve accuracy, and enable real-time detection of problems.
Integration of material balance monitoring with other plant systems, such as laboratory information management systems (LIMS), maintenance management systems, and enterprise resource planning (ERP) systems, can provide a more comprehensive view of plant operations and facilitate identification of relationships between material balance performance and other operational parameters.
Industry-Specific Considerations
Chemical and Petrochemical Industries
In chemical and petrochemical facilities, material balance discrepancies can have significant economic and safety implications. Complex reaction networks, multiple recycle streams, and the presence of numerous chemical species make material balance calculations particularly challenging. Accurate tracking of catalyst usage and deactivation, proper accounting for side reactions and byproducts, and careful monitoring of separation unit performance are all critical for maintaining accurate material balances.
The presence of hazardous materials in many chemical processes makes early detection of leaks through material balance monitoring particularly important for safety and environmental protection. Regulatory requirements for emissions monitoring and reporting also necessitate accurate material accounting for many chemical facilities.
Pharmaceutical Manufacturing
Pharmaceutical manufacturing faces unique challenges related to material balance, including stringent regulatory requirements for material accountability, the need to track high-value active pharmaceutical ingredients (APIs) with extreme precision, and complex batch processes with multiple intermediate stages. Material balance discrepancies in pharmaceutical manufacturing can trigger regulatory investigations and may require batch rejection, making accurate material accounting essential.
Good Manufacturing Practice (GMP) regulations require detailed documentation of all material movements and reconciliation of material balances for each batch. Establishing robust procedures for material tracking, implementing appropriate measurement systems, and maintaining comprehensive records are all critical for compliance and product quality assurance.
Oil and Gas Production
In oil and gas production facilities, material balance calculations are used for reservoir management, production allocation, custody transfer, and regulatory reporting. The multiphase nature of production streams, variations in fluid properties, and the presence of water and gas along with oil create unique challenges for material balance calculations.
Accurate measurement of multiphase flows is particularly challenging and often requires specialized instrumentation. Regular testing of wells and production facilities to determine fluid properties and compositions is essential for maintaining accurate material balances. Material balance calculations are also used to monitor reservoir performance and optimize production strategies over the life of the field.
Food and Beverage Processing
Food and beverage processing facilities must maintain accurate material balances to ensure product quality, optimize ingredient usage, and comply with labeling regulations. The biological variability of raw materials, moisture content changes during processing, and the presence of multiple product streams and byproducts all contribute to material balance complexity.
Tracking of allergens and other regulated ingredients requires particularly careful material accounting to ensure compliance with food safety regulations. Material balance monitoring can also help identify opportunities for waste reduction and improved utilization of byproducts, contributing to sustainability goals.
Economic and Environmental Benefits of Accurate Material Balances
Cost Reduction and Profitability
Maintaining accurate material balances directly impacts plant profitability through multiple mechanisms. Identifying and eliminating material losses reduces raw material costs and can significantly improve margins, particularly for high-value products. Early detection of process inefficiencies through material balance monitoring enables timely corrective action, preventing prolonged periods of suboptimal operation.
Accurate material accounting also supports better inventory management, reducing working capital requirements and minimizing the risk of stockouts or excess inventory. For facilities involved in custody transfer or toll processing, precise material balances are essential for fair commercial transactions and dispute resolution.
Environmental Compliance and Sustainability
Material balance monitoring plays a crucial role in environmental compliance by enabling accurate tracking of emissions, effluents, and waste generation. Many environmental regulations require facilities to report material usage and waste generation, making accurate material balances essential for regulatory compliance. Early detection of leaks or unintended releases through material balance discrepancies can prevent environmental incidents and associated penalties.
From a sustainability perspective, accurate material balances support efforts to reduce waste, improve resource efficiency, and minimize environmental impact. Identifying opportunities for material recovery, recycling, or reuse requires detailed understanding of material flows throughout the facility, which material balance analysis provides. Many companies are increasingly focused on circular economy principles, and material balance analysis is fundamental to implementing these concepts effectively.
Safety and Risk Management
Material balance discrepancies can be early indicators of safety problems, such as equipment leaks, process upsets, or unexpected reactions. Prompt detection and investigation of these discrepancies can prevent incidents that might otherwise result in injuries, environmental releases, or property damage. For facilities handling flammable, toxic, or reactive materials, material balance monitoring is an important layer of protection in a comprehensive safety management system.
Regular material balance analysis also supports process safety management by providing data for process hazard analyses, helping to identify potential loss of containment scenarios, and verifying that safety systems are functioning as intended. Integration of material balance monitoring with other safety systems can enhance overall facility safety performance.
Future Trends in Material Balance Management
Artificial Intelligence and Machine Learning
Artificial intelligence and machine learning technologies are beginning to transform material balance management in industrial facilities. These technologies can analyze large volumes of process data to identify patterns and relationships that might not be apparent through traditional analysis methods. Machine learning algorithms can be trained to predict normal material balance behavior and automatically flag anomalies that require investigation.
Advanced analytics can also help identify the root causes of material balance discrepancies by correlating them with other process variables, maintenance activities, or external factors. As these technologies mature, they promise to make material balance monitoring more automated, accurate, and actionable, enabling faster response to problems and more proactive management of plant operations.
Digital Twin Technology
Digital twin technology, which creates virtual replicas of physical assets and processes, offers exciting possibilities for material balance management. By combining real-time process data with detailed process models, digital twins can provide more accurate predictions of expected material balance behavior and help identify discrepancies more quickly. They can also be used to test hypotheses about the causes of discrepancies without disrupting actual plant operations.
As digital twin technology becomes more sophisticated and accessible, it is likely to become an increasingly important tool for material balance analysis and process optimization. Integration of digital twins with other Industry 4.0 technologies, such as the Industrial Internet of Things (IIoT) and cloud computing, will enable more comprehensive and powerful material balance management capabilities.
Enhanced Measurement Technologies
Ongoing advances in measurement technology continue to improve the accuracy and reliability of material balance calculations. New sensor technologies, including advanced spectroscopic methods, wireless sensors, and non-invasive measurement techniques, are making it possible to measure process variables that were previously difficult or impossible to quantify accurately.
Improvements in multiphase flow measurement, online composition analysis, and real-time moisture measurement are particularly relevant for material balance applications. As these technologies become more mature and cost-effective, they will enable more accurate and comprehensive material balance monitoring across a wider range of industrial applications.
Conclusion
Material balance discrepancies represent a persistent challenge in industrial plant operations, but with systematic approaches to troubleshooting, appropriate measurement systems, and effective management practices, these discrepancies can be minimized and their impacts mitigated. Understanding the fundamental principles of material balance, recognizing common causes of discrepancies, and implementing robust monitoring and analysis procedures are essential for maintaining optimal plant performance.
The economic, environmental, and safety benefits of accurate material balances make investment in material balance management systems and practices highly worthwhile. As technology continues to advance, new tools and techniques will make material balance monitoring more automated, accurate, and actionable, enabling industrial facilities to operate more efficiently, safely, and sustainably.
Success in managing material balance discrepancies requires commitment from all levels of the organization, from operators who collect and report data to engineers who analyze it and management who provide resources and support. By fostering a culture that values accurate material accounting and continuous improvement, industrial facilities can achieve and maintain the high levels of performance necessary to compete in today’s demanding business environment.
For additional information on process engineering fundamentals, visit the American Institute of Chemical Engineers. To learn more about industrial measurement and instrumentation best practices, explore resources from the International Society of Automation. For guidance on process safety management and material accountability, consult the Center for Chemical Process Safety.