Saul M. Lemkowitz

Dust explosions in spherical vessels: The role of flame thickness in the validity of the ‘cube-root law’

Abstract A well known limitation of the ‘cube-root law’ is that it becomes invalid when the flame thickness is significant with respect to the vessel radius. In the literature, flame thicknesses in dust-air mixtures ranging from 15 to 80 cm have been reported, which exceed the radii of the 20 litre sphere and the 1 m3 vessel. Therefore, we have developed a model (the three-zone model) for the pressure evolution of confined dust explosions in spherical vessels which takes the flame thickness into account. The pressure-time curves that are generated with this model show a good resemblance with those measured in practice. It is shown by numerical simulations that the maximum rate of pressure rise can be normalized with respect to the vessel volume as well as to the flame thickness and that the ‘cube-root law’ becomes inaccurate for relative flame thicknesses exceeding 1%. Furthermore, the actual burning velocity and the flame thickness during real dust explosions can be obtained by fitting the model to the experimental pressure-time curve.

The relation of cool flames and auto-ignition phenomena to process safety at elevated pressure and temperature

The cool-flame phenomenon can occur in fuel-oxygen (air) mixtures within the flammable range and outside the flammable range, at fuel-rich compositions, at temperatures below the auto-ignition temperature (AIT). It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favoured by elevated pressure. The hazards that cool flames generate are described. These vary from spoiling a product specification through contamination and explosive decomposition of condensed peroxides to the appearance of unexpected normal (hot) flame (two-stage ignition).

An interpretation of dust explosion phenomena on the basis of time scales

Abstract This paper reports some tests of dust explosibility made in vessels of two different sizes. Although most powders tested in the 1 m 3 vessel and the 20 l sphere give similar results, in some cases the values of K St measured in the 20-l sphere are somewhat higher than the values found in the 1 m 3 vessel. Measurements of the explosion indices of activated carbon illustrate this phenomenon. This paper proposes an explanation on the basis of turbulence measurements and a comparison of the time scales which are relevant to dust explosions. Also some remarks are made on the validity of the cubic law in the 20-l sphere.

Gas chromatographic determination of water: A source of systematic error introduced by interactions of polar compounds on porous polymer gas chromatographic columns

Abstract The accuracy of the determination of water was found to be dependent on two major effects. The first is related to the adsorption properties of porous polymers. Under normal conditions of analysis a considerable amount of water is adsorbed on the column. In a non-polar sample matrix this adsorbed water does not interfere with water introduced by the sample. In a polar matrix, however, a certain amount of water is desorbed, which is seen as a virtual peak. This virtual peak is co-eluted with the first eluted polar compound. The second effect is related to the water content in the carrier gas, which should be controlled to ensure a constant analytical performance. The water concentration in the carrier gas is set by the desired sample blank value.

Improving on chemical process safety through distributed computer assisted knowledge analysis of preliminary design

Abstract Because accidents cause delays that have become increasingly expensive, the chemical industry should strive to improve safety by shifting from secondary protection to primary protection. The current design approach, that is basically reactive, is not fully adequate, as opportunities for technology are most efficiently realised in early design. A proactive framework, using the concept of ‘anomalies’, is proposed to identify desired as well as undesired relations in a system being designed. The proposed methodology also presents a strategy to deal with system anomalies. It efficiently prevents incidents that cause delays, helping to ensure profitability.

Assessment and Control of Fire and Explosion Hazards and Risks of Particulates

Fires and explosions of particulates have caused, and continue to cause, substantial financial loss and loss of life. This Chapter provides knowledge and insight into preventing or at least mitigating particulate fire and explosion hazards and risks. To this end we distinguish between the concepts hazard and risk, first discussing in some detail the nature of fire and explosion hazards of particulates, the indexes expressing these hazards, the many factors that determine such hazards, and a widely used and internationally accepted system for ranking fire and explosion hazards of particulates. All of these topics relate to a given particulate; i.e., a particulate of a given chemical composition and given physical form (e.g., particle shape and size distribution). We next discuss fire and explosion risks of particulates. Risk is a much more extensive and complex topic than hazard, as risk involves factors extensive to the particulate itself, such as: amounts of particulate; where (region, country) it is made, transported, and stored; the (process) conditions under which it is made, transported, and stored; relevant legislation; and management. Rather than discuss the complex topic of particulate risk as such, we concentrate on a number of fundamental approaches to reduce risk, approaches which are increasingly applied in modern (chemical) engineering design. One general method to reduce risk is to negate the basic factors causing risk. This is the basic logic of Inherently Safer Design (ISD), which, essentially, aims at avoiding or at least greatly reducing hazard by clever design/choice of materials, process, and conditions. Inherently Safer Design forms the starting point of designing layers of defense against mishap. The analysis of the degree of risk reduction by the layers is called Layers of Protection Analysis (LOPA). LOPA recognizes that in practice Inherently Safe Design rarely can eliminate all hazards. Thus, starting from the ideal of Inherently Safer Design, LOPA applies ‘layers of protection’ around manufacturing the particulate. These ‘layers of protection’ are applied at various scales, starting from the process itself (e.g., safety devices, such as a device to suppress explosion) to the neighborhood in which the plant manufacturing the particulate is located (e.g., a disaster plan involving local government). The goal of LOPA is to reduce risk to an acceptable level. We discuss Inherently Safer Design and LOPA in some detail, including examples. Relevant legislation and management aspects are also absolutely essential topics in particulate safety. These are mentioned, but because this Chapter focuses more on scientific and technical aspects, these two important topics are discussed in less detail. The Chapter ends with a note on research and also provides an extensive literature list.

Incorporation of sustainability demands into the conceptual process design phase of chemical plants

Abstract Incorporating sustainability issues into the chemical industry requires a society-focussed methodology for the design of processes and products. In this paper, we present our design methodology that is able to support both students and experienced process engineers. We constructed this methodology from existing chemical process design practices, insights from various engineering disciplines, and the main issues from the societal debate on sustainability. The methodology is illustrated by a conceptual design for the conversion of biomass into methanol, which is done by students trained in the methodology. We found some striking differences in comparing this case to an existing industrial case.

Process safety at elevated temperatures and pressures: Cool flames and auto-ignition phenomena

Publisher Summary The chapter explains the concept of process safety at elevated temperatures and pressures. The hazards due to cool flames generation are also described. Partial oxidation processes, carried out at elevated conditions are used in the chemical industry. Detailed knowledge about relevant explosion indices is essential for a safe and economic operation in the most efficient way. Such explosion indices must be known under realistic process conditions—high temperature, high pressure, and high turbulence conditions. The cool flame phenomenon can occur in fuel (-air) -oxygen mixtures, within the flammable range and outside the flammable range, at fuel-rich composition and at temperatures below the auto-ignition temperature. It is caused by chemical reactions occurring spontaneously at relatively low temperatures and is favored by elevated pressure. The hazards of cool flames vary from spoiling a product specification through contamination and the appearance of unexpected normal (hot) flame (two-stage ignition) to explosive decomposition of condensed peroxides. Therefore, cool flame temperature and limits should be considered as a safety parameter for processes operating at elevated temperatures and pressures.

A Deposition Model for Fibres in the Deep Parts of the Lung Based on Similarities of Bead Beds and Human Lungs

Fibrous particles are those with an elongated shape. They include glass fibres and asbestos. Asbestos has become very widely used because it is a low-cost material with very desirable chemical and physical properties, for instance, excellent heat and fire resistance. This unique combination makes the economical replacement of asbestos very difficult in many applications.