William J. Rogers

A review of estimation methods for flash points and flammability limits

INTRODUCTION Flash point measurements of various types are used as one measure of the flammability of liquid materials. The flash point is also related to the lower flammability limit, which is the minimum content of the combustible in a combustible–air mixture that will propagate flame [1]. Many manufacturing processes involve flammable chemicals; therefore, flash points and flammability limits are essential to maximize safety in process design and operational procedures. Flammability is an important factor in the development of safe practices for handling and storage of liquid mixtures. Regulatory authorities use flash point determinations made from small-scale test apparatus to classify flammable and combustible liquids. These determinations are based mainly on the use of the closed cup flash point temperature as the key property for classifying liquids according to their degree of flammability. Based on these classifications, regulators then specify or provide guidance on the appropriate methods for transporting, handling, packaging, storing, dispensing, and protecting these materials [2]. Also, federal regulations require that the mixture flash point be provided as part of materials safety data information. Industry works with mixtures under different conditions of temperature, pressure, and oxygen concentrations according to the process involved. The flash point can be used to determine the level of risk in different stages of a process because it is the temperature at which sufficient vapor is generated to bring the concentration of flammable vapor above the lower flammability limit. One question that must be answered to ensure a safe level of operation in a certain process is “What is the minimum oxygen concentration requirement for flame propagation?” Or, in terms more directly applicable to process operations, “What is the minimum amount of inerts required to prevent flame propagation?” The quantity of air that is required to decrease the combustible vapor concentration to a safe level in a particular process carried out at a specific temperature should be based on flammability measurements at that temperature. Knowledge of flammable limits at elevated temperatures and pressures is needed for safe and economical operation of some chemical processes. This information may be needed in order to start up a reactor without passing through a flammable range, to operate the reactor safely and economically, or to store or ship the product safely [3]. Flash points are available for most pure liquids, but the information for mixtures is very limited and is usually at ambient pressures. For mixtures of flammable liquids, or more importantly, liquid mixtures containing both flammable and nonflammable constituents, the precise level of risk is more difficult to predict. Mixtures of flammable and nonflammable constituents are especially significant because the vapor phase composition differs from the liquid composition, and it can change from nonflammable to flammable as the mixtures evaporates or its temperatures changes. Flammability limits obtained experimentally under conditions similar to those found in practice are most reliable for designing installations that are safe and assessing potential gas-explosion hazards. Predictive theoretical methods are needed to estimate the flash point of mixtures when experimental data are unavailable. Flash point determinations for mixtures generally are based on the Le Chatelier equation together with a vapor–liquid equilibrium model calculation of the vapor composition when liquids are involved. Most existing predictive methods are appli© 2004 American Institute of Chemical Engineers

Prediction of reactive hazards based on molecular structure.

There is considerable interest in prediction of reactive hazards based on chemical structure. Calorimetric measurements to determine reactivity can be resource consuming, so computational methods to predict reactivity hazards present an attractive option. This paper reviews some of the commonly employed theoretical hazard evaluation techniques, including the oxygen-balance method, ASTM CHETAH, and calculated adiabatic reaction temperature (CART). It also discusses the development of a study table to correlate and predict calorimetric properties of pure compounds. Quantitative structure-property relationships (QSPR) based on quantum mechanical calculations can be employed to correlate calorimetrically measured onset temperatures, T(o), and energies of reaction, -deltaH, with molecular properties. To test the feasibility of this approach, the QSPR technique is used to correlate differential scanning calorimeter (DSC) data, T(o) and -deltaH, with molecular properties for 19 nitro compounds.

Development of a Fuzzy Logic-Based Inherent Safety Index

Inherent safety has been recognized as a design approach useful to remove hazards or reduce hazards at the source instead of controlling them with add-on protective barriers. However, inherent safety is based on qualitative principles that cannot easily be evaluated and analysed, and this is one of the major difficulties for the systematic application and quantification of inherent safety in plant design. The present paper introduces the use of fuzzy logic for the measurement of inherent safety. The proposed methodology describes the development of an overall index for use in process simulation and process synthesis to generate inherently safer alternatives and to evaluate them in a systematic and rapid way. The application to process simulation is expected to be useful for the application of inherent safety to operating plants. The use of fuzzy logic is helpful modeling uncertainty and subjectivities implied in evaluation of certain variables and it is helpful for combining quantitative data with qualitative information. This paper focuses only on the development of a fuzzy logic-based inherent safety index, which constitutes the first step toward a systematic application of inherent safety.

Adiabatic calorimetric decomposition studies of 50 wt.% hydroxylamine/water.

Calorimetric data can provide a basis for determining potential hazards in reactions, storage, and transportation of process chemicals. This work provides calorimetric data for the thermal decomposition behavior in air of 50wt.% hydroxylamine/water (HA), both with and without added stabilizers, which was measured in closed cells with an automatic pressure tracking adiabatic calorimeter (APTAC). Among the data provided are onset temperatures, reaction order, activation energies, pressures of noncondensable products, thermal stability at 100 degrees C, and the effect of HA storage time. Discussed also are the catalytic effects of carbon steel, stainless steel, stainless steel with silica coating, inconel, titanium, and titanium with silica coating on the reaction self-heat rates and onset temperatures. In borosilicate glass cells, HA was relatively stable at temperatures up to 133 degrees C, where the HA decomposition self-heat rate reached 0.05 degrees C/min. The added stabilizers appeared to reduce HA decomposition rates in glass cells and at ambient temperatures. The tested metals and metal surfaces coated with silica acted as catalysts to lower the onset temperatures and increase the self-heat rates.

Layer of protection analysis for reactive chemical risk assessment

Reactive chemical hazards have been a significant concern for the chemical process industries (CPI). Without sufficient control and mitigation of chemical reaction hazards, reactive incidents have led to severe consequences, such as release of flammable and toxic materials, fires and explosions, and threats to human lives, properties, and the environment. Consequence of reactive hazards can be well understood through calorimetric testing and computational techniques. However, risks of incidents caused by reactive chemicals have not been well addressed due partly to sparse failure frequency data. In this paper, the semi-quantitative layer of protection analysis (LOPA) approach is used to estimate reactive chemical risk, and the probabilities or frequencies of failure scenarios are addressed. Using LOPA, reactive risks can be evaluated with respect to predefined criteria, and the effectiveness of risk reduction measures can be assessed. The hydroxylamine (HA) production system is employed as a case study to demonstrate the application of LOPA to reactive chemical risk assessment.

Thermal Decomposition Pathways of Hydroxylamine: Theoretical Investigation on the Initial Steps

Hydroxylamine (NH(2)OH) is an unstable compound at room temperature, and it has been involved in two tragic industrial incidents. Although experimental studies have been carried out to study the thermal stability of hydroxylamine, the detailed decomposition mechanism is still in debate. In this work, several density functional and ab initio methods were used in conjunction with several basis sets to investigate the initial thermal decomposition steps of hydroxylamine, including both unimolecular and bimolecular reaction pathways. The theoretical investigation shows that simple bond dissociations and unimolecular reactions are unlikely to occur. The energetically favorable initial step of decomposition pathways was determined as a bimolecular isomerization of hydroxylamine into ammonia oxide with an activation barrier of approximately 25 kcal/mol at the MPW1K level of theory. Because hydroxylamine is available only in aqueous solutions, solvent effects on the initial decomposition pathways were also studied using water cluster methods and the polarizable continuum model (PCM). In water, the activation barrier of the bimolecular isomerization reaction decreases to approximately 16 kcal/mol. The results indicate that the bimolecular isomerization pathway of hydroxylamine is more favorable in aqueous solutions. However, the bimolecular nature of this reaction means that more dilute aqueous solution will be more stable.

Predicting the Impact Sensitivities of Polynitro Compounds Using Quantum Chemical Descriptors

ABSTRACT There has been considerable interest in predicting the stabilities of energetic materials to improve safety during manufacture, handling, storage, and transportation. Although a variety of experimental techniques are available to test the properties of energetic materials, computational screening techniques can harness the convenience of modern computers to reduce the cost of destructive tests. In this paper quantitative structure–property relationships (QSPRs) based on quantum mechanical calculations were employed to correlate the measured impact sensitivities from shock or impact tests with molecular properties. Molecular descriptors were evaluated using both the Hartree-Fock method with a STO-3G basis set and the semiempirical method PM3. Equations that correlate impact sensitivities to the energy of lowest unoccupied molecular orbital (ϵLUMO), energy of highest occupied molecular orbital (ϵHOMO), midpoint potential (MPP), ionization potential (IP), dipole moment (DM), and total energy (E) of the molecules were developed.

A holistic approach to control process safety risks: Possible ways forward

Pursuing process safety in a world of continuously increasing requirements is not a simple matter. Keeping balance between producing quality and volume under budget constraints while maintaining an adequate safety level proves time and time again a difficult task given that evidently major accidents cannot be avoided. Lack of resilience from an organizational point of view to absorb unwanted and unforeseen disturbances has in recent years been put forward as a major cause, while organizational erosive drift is shown to be responsible for complacency and degradation of safety attitude. A systems approach to safety provides a new paradigm with the promise of new comprehensive tools. At the same time, one realizes that risk assessment will fall short of identifying and quantifying all possible scenarios. First, human error is in most assessments not included. It is even argued that determining human failure probability by decomposing it to basic elements of error is not possible. Second, the crux of the systemic approach is that safety is an emergent property, which means the same holds for the technological aspect: risk is not fully predictable from failure of components. By surveying and applying recent literature, besides analysing, this paper proposes a way forward by considering resilience of a socio-technical system both from an organizational and a technical side. The latter will for a large part be determined by the plant design. Sufficient redundancy and reserve shall be kept to preserve sufficient resilience, but the question that rises is how. Available methods are risk assessment and process simulation. It is helpful that the relation between risk and resilience analysis has been recently defined. Also, in a preliminary study the elements of resilience of a process have become listed. In the latter, receiving and interpreting weak signals to boost situational awareness plays an important role. To maintain alertness on the functioning of a safety management system, the process industry is monitoring safety performance indicators. The critical intensity level upon which management must be alarmed is less simple. Risk assessment may be improved, made dynamic, and be a tool of process control by taking account of short-term risk fluctuations based on sensor signals and the influence of human factors with its long-term changes via indicators. Bayesian network can provide the infrastructure. The paper will describe various complexities when applying a holistic control of safety to a process plant in general, and it will more specifically focus on safeguarding measures such as barriers and other controls with some examples.

Effect of air in the thermal decomposition of 50 mass% hydroxylamine/water.

This paper presents experimental measurements of 50 mass% hydroxylamine (HA)/water thermal decomposition in air and vacuum environments using an automatic pressure tracking adiabatic calorimeter (APTAC). Overall kinetics, onset temperatures, non-condensable pressures, times to maximum rate, heat and pressure rates versus temperature, and mixture vapor pressures for the experiments in vacuum were similar when compared to the corresponding data for HA decomposition in air. Determined was an overall activation energy of 119+/-8 kJ/mol (29+/-2 kcal/mol), which is low compared to 257 kJ/mol (61.3 kcal/mol) required to break the H(2)N-OH bond reported in the literature. The availability of oxygen from air did not affect detected runaway decomposition products, which were H(2), N(2), N(2)O, NO, and NH(3), for samples run in vacuum or with air above the sample. A delta H(rxn) of -117 kJ/mol (28 kcal/mol) was estimated for the HA decomposition reaction under runaway conditions.

Calculated flame temperature (CFT) modeling of fuel mixture lower flammability limits.

Heat loss can affect experimental flammability limits, and it becomes indispensable to quantify flammability limits when apparatus quenching effect becomes significant. In this research, the lower flammability limits of binary hydrocarbon mixtures are predicted using calculated flame temperature (CFT) modeling, which is based on the principle of energy conservation. Specifically, the hydrocarbon mixture lower flammability limit is quantitatively correlated to its final flame temperature at non-adiabatic conditions. The modeling predictions are compared with experimental observations to verify the validity of CFT modeling, and the minor deviations between them indicated that CFT modeling can represent experimental measurements very well. Moreover, the CFT modeling results and Le Chateliers Law predictions are also compared, and the agreement between them indicates that CFT modeling provides a theoretical justification for the Le Chateliers Law.