DOYLE Process Control Modules: A Software Laboratory for Control Design ELLIOT AND LIRA Introductory Chemical Engineering Thermodynamics FOGLER Elements of Chemical Reaction Engineering, 3rd edition. Library of Congress Cataloging-in-Publication Data Crowl, Daniel A. Chemical Process Safety : Fundamentals with Applications I Daniel A. Prentice Hall (International Series in Physical and Chemical Engineering Sciences).
9 Relief Sizing 383
10 Hazards Identification 429
11 Risk Assessment 471
Accident Investigations 515
13 Case Histories 535
Appendix A: Unit Conversion Constants 561
Appendix B: Flammability Data for Selected Hydrocarbons 565 Appendix C: Detailed Equations for Flammability Diagrams 571
Preface
We also continue to thank all members of the Undergraduate Education Committee of the Center for Chemical Process Safety and the Safety and Loss Prevention Committee of the American Institute of Chemical Engineers. Finally, we continue to thank our families who were patient, understanding and encouraging during the writing of the first and second editions.
Nomenclature
Gibbs free energy (energylmole) or mass flux (mass larea time) mass flux during relief (mass larea time).
Greek Letters
Introduction
The word "safety" used to mean the older strategy of accident prevention through the use of hard hats, safety shoes and a variety of rules and regulations. Safety or loss prevention: the prevention of accidents by using appropriate technologies to identify the hazards of a chemical plant and eliminate them before an accident occurs.
1-1 Safety Programs
The most effective way to implement a safety program is to make it the responsibility of everyone in the chemical process plant. A good security program eliminates existing threats when they are identified, while an exceptional security program prevents threats from existing.
1-2 Engineering Ethics
This includes time to study, time to do the work, time to record results (for history), time to share experiences and time to train or be trained. The commonly used management systems aimed at eliminating the existence of hazards include safety reviews, safety audits, hazard identification techniques, checklists and proper application of technical knowledge.
1-3 Accident and Loss Statistics
Both the OSHA incidence rate and the FAR depend on the number of hours exposed. A FAR can be converted to a fatality rate (or vice versa) if the number of hours of exposure is known.
1-4 Acceptable Risk
As organizations focus on the root causes of employee injuries, it is helpful to analyze how workplace fatalities occur (see Figure 1-4). Is it enough to design a process with a risk comparable to the risk of sitting at home?
1-5 Public Perceptions
1-6 The Nature of the Accident Process
Major Property Damage Losses in Hydrocarbon Chemical Industries: A Thirty Year Review (New York: J & H Marsh & McLennan Inc., 1998), p Source: Major Property Damage Losses in Hydrocarbon Chemical Industries: A Thirty Year Review (New York: J & H Marsh.
1-7 Inherent Safety
Risk A measure of human injury, environmental damage, or economic loss in terms of both the likelihood of an incident and the magnitude of the loss or injury. The types of inherent safety techniques used in the chemical industry are illustrated in Table 1-9 and described more fully below.
1-8 Four Significant Disasters
The bypass pipe section is believed to have ruptured due to insufficient support and overbending of the pipe section due to internal reactor pressures. When the bypass was breached, an estimated 30 tons of cyclohexane volatilized and formed a large vapor cloud.
Bhopal, India
An alternative reaction scheme is shown at the bottom of the figure and involves the less dangerous chloroformate intermediate. One such design produces and consumes MIC in a highly localized process area, with an MIC inventory of less than 20 pounds.
Seveso, Italy
About 2 kg of TCDD was released through the relief system in a white cloud over Seves. TCDD is so toxic and persistent that in 1963, a smaller but similar release of TCDD at Duphar, India finally led to the factory being dismantled brick by brick, encased in concrete and dumped in the ocean.
Suggested Reading
General Aspects of Chemical Process Safety
Bhopal
Seveso
Flixborough
General Case Histories
Problems
Based on the causes of the largest losses (Figure 1-7), what would you emphasize in a safety program. If the roof is flooded with 8 inches of water (equal to the maximum pressure), what is the total weight (in pounds) of the water.
Toxicology
2-1 How Toxicants Enter Biological Organisms
The skin plays an important role in both the dermal absorption and injection routes. The absorption properties of the skin vary as a function of location and degree of hydration.
Respiratory System
The presence of water increases skin hydration and leads to increased permeability and absorption.
2-2 How Toxicants Are Eliminated from Biological Organisms
2-3 Effects of Toxicants on Biological Organisms
2-4 Toxicological Studies
2-5 Dose versus Response
Specifically, the response interval within one standard deviation of the mean represents 68% of individuals, as shown in Figure 2-4a. For convenience, the response is plotted against the logarithm of the dose, as shown in Figure 2-7.
2-6 Models for Dose and Response Curves
There are many methods of representing the dose-response curve.' For a single exposure, the probit method (probit = probability unit) is particularly suitable, providing a straight line equivalent to the dose-response curve. The probit ratio of Equation 2-4 changes the sigmoid shape of the normal dose-response curve to a straight line when plotted using a linear probit scale, as shown in Figure 2-10.
2-7 Relative Toxicity
2-8 Threshold Limit Values
TLVs shall not be used for (1) a relative toxicity index, (2) air pollution control work, or (3) assessment of the toxic hazard of continuous, uninterrupted exposure. The ACGIH clearly states that the TLVs should not be used as a relative toxicity index (see Figure 2-8), should not be used for air pollution control work, and should not be used to assess the impact of continued exposure to toxic substances. .
Probit Analysis
All efforts should be made to reduce workers' exposure to toxins below the PEL and, if possible, lower it. Burson, ed., Industrial Toxicology: Safety and Health Applications in the Workplace (New York: Van Nostrand Reinhold, 1985).
Threshold Limit Values
Estimate the number of deaths expected from pulmonary hemorrhage and the number of eardrums ruptured as a result of this explosion. Use the NIOSH website to determine the number of deaths that occurred in 1992 from asbestos.
Industrial Hygiene
3-1 Government Regulations
Laws and Regulations
Creating a Law
Creating a Regulation
In 1970, the US Congress passed a health and safety law that continues to have a significant impact on industrial hygiene practice in the chemical industry: the Occupational Safety and Health Act of 1970 (OSHAct). Poison Prevention Packaging Act of 1970 Water Quality Improvement Act of 1970 Federal Railroad Safety Act of 1970 Resource Recovery Act of 1970 Occupational Safety and Health Act Noise Control Act of 1972.
OSHA: Process Safety Management
Under the audit section of the PSM standard, employers are required to certify at least every three years that they have evaluated their compliance with the standard. The trade secrets section of the PSM standard ensures that all contractors receive all information relevant to working safely in the plant.
EPA: Risk Management Plan
Under the management of change section of the PSM standard, employees are required to develop and implement documented procedures to manage changes in the process chemistry, process equipment and operating procedures. Of greatest concern are the release scenarios that have the potential to reach the public.
3-2 Industrial Hygiene: Identification
The MSDS lists the physical properties of the substance that may be necessary to determine the potential hazards of the substance. An industrial hygienist or safety professional must interpret the physical and toxicological properties to determine the hazards associated with the chemical.
Solution
3-3 Industrial Hygiene: Evaluation
If problems are evident, control measures should be implemented immediately; temporary control measures such as personal protective equipment can be used. After obtaining the exposure data, it is necessary to compare actual exposure levels with acceptable occupational health standards, such as TLVs, PELS or IDLH concentrations.
Evaluating Exposures to Volatile Toxicants by Monitoring
If the sum of the toxicant concentrations in the mixture exceeds this amount, workers are overexposed. In vapor mixtures, the TLVs of individual species in the mixture are significantly lower than the TLVs of the pure substance.
Evaluation of Worker Exposures to Dusts
Evaluating Worker Exposures to Noise
Estimating Worker Exposures to Toxic Vapors
V , be the volume of the container (volume), r, be the constant filling rate of the container (time-'),. If pv is the density of the volatile vapor, rfVcpv is also the mass rate of volatiles displaced from the container (mass time).
3-4 Industrial Hygiene: Control
Environmental control reduces exposure by reducing the concentration of toxic substances in the work environment. Personal protection prevents or reduces exposure by providing a barrier between the worker and the work environment.
Respirators
For this design, bypass air is supplied through a grill at the top of the hood. Calculate the concentration (inppm) of the equilibrium vapor with air above a solution of 50 mol % toluene and benzene.
Source Models
4-1 Introduction to Source Models
Selection of Source Model
CHAPTER 2 Selection of Effect Model
For liquids, a leak below the liquid level in the tank results in a stream of escaping liquid. A leak into the vapor space above the liquid can result in a vapor stream or a two-phase stream consisting of vapor and liquid, depending on the physical properties of the material.
4-2 Flow of Liquid through a Hole
The pressure of the fluid in the process unit is converted to kinetic energy as the fluid escapes through the leak. The energy resulting from the pressure of the fluid height above the leak is converted to kinetic energy as the fluid exits the hole.
4-3 Flow of Liquid through a Hole in a Tank
A 1-in puncture occurs in the tank 5 ft from the ground due to careless driving of a forklift truck. Estimate (a) the gallons of benzene spilled, (b) the time required for the benzene to flow, and (c) the maximum mass flow rate of benzene through the leak.
4-4 Flow of Liquids through Pipes
The Fanning friction factor f is a function of the Reynolds number Re and the roughness of the pipe E. Determine the excess head loss terms for the pipe (use Equation 4-30), for the fittings (use Equation 4-38), and for any entrance and exit effects (use Equation 4-39).
4-5 Flow of Vapor through Holes
For downstream pressures less than PC, the following statements are valid: (1) The fluid velocity at the nozzle is the speed of sound under prevailing conditions, and (2) the velocity and mass flow rate can no longer increase. further reducing the downstream pressure; they are independent of downstream conditions. M is the molecular weight of the escaping vapor or gas, To is the temperature of the source and.
4-6 Flow of Gases through Pipes
Adiabatic Flows
If the upstream pressure is increased or if the downstream pressure is decreased, the gas velocity at the end of the pipe remains constant at the sonic velocity. ZThe equation used to fit the expansion factor and the sonic pressure drop ratio is of the form.
The pressure at the end of the pipe is equal to the pressure of the environment. In the event of a failure of the nitrogen regulator, the vessel will be exposed to the full 200 psig pressure of the nitrogen source.
4-7 Flashing Liquids
A good approximation is to assume a choked pressure equal to the saturation vapor pressure of the flashing liquid. Note that the temperature Tin Equation 4-104 is the absolute temperature from the Clausius-Clapyron equation and is not related to the heat capacity.
4-8 Liquid Pool Evaporation or Boiling
For liquids boiling from a pool, the rate of boiling is limited by the heat transfer from the surroundings to the liquid in the pool. The initial stage of cooking is usually controlled by the heat transfer from the ground.
4-9 Realistic and Worst-Case Releases
4-1 0 Conservative Analysis
For diameters greater than 4 inches, assume a break area equal to 20% of the pipe's cross-sectional area. For the area of the hole, a possible decision is to consider the area of the largest pipe connected to the tank, because pipe disconnections are a common source of tank leaks.
Consequence Modeling
Unfortunately, this process can produce consequences that are many times greater than actual, leading to possible over-design of mitigation procedures or safety systems.
Flow of Liquid through Holes
Flow of Liquid through Pipes
Flow of Vapor through Holes
Flow of Vapor through Pipes
Flashing Liquids
Estimate the maximum mass flow rate (in kgl) of the gas if the initial pressure in the tank is 800 kPa gauge. What is the driving force that pushes the water out of the hole in the tank.
Toxic Release and Dispersion Models
Several options are available, based on the predictions of the toxic release model, for example, (1) developing an emergency response plan with the surrounding community, (2) developing engineering modifications to eliminate the source of the release, (3) contain the potential release and add appropriate vent scrubbers or other vapor removal equipment, (4) reduce the inventory of hazardous materials to reduce the amount released, and (5) add area monitors to detect incipient leaks and provide block valves and engineering controls to eliminate dangerous levels of spills and leaks.
5-1 Parameters Affecting Dispersion
As the release height increases, ground-level concentrations decrease because the plume must disperse a greater distance vertically. If the gas has a density less than air, the initially released gas will be positively buoyant and rise.
5-2 Neutrally Buoyant Dispersion Models
For the puff model, a typical example is the sudden release of a fixed amount of material due to the rupture of a storage container. It follows that the concentration C will also fluctuate as a result of the velocity field; so.
