Chemical Engineering Design

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2 The safe design and operation of facilities is of paramount importance to every company that is involved in the manufacture of fuels, chemicals and pharmaceuticals

3 Processes must meet acceptable safety and environmental performance standards because: It is required by law The costs (human, social, economic) of noncompliance can be catastrophic Negligent attitudes are reflected in insurance premiums, stock prices Moral and ethical obligations

4 Factory and Machinery Act (FMA) regulations of FMA 1967: 1. STEAM BOILER AND UPV ELECTRIC PASSENGER AND GOODS LIFT FENCING OF MACHINERY AND SAFETY SAFETY, HEALTH AND WELFARE CERTIFICATES OF COMPETENCY EXAMINATIONS NOTIFICATION, CERTIFICATE OF FITNESS AND INSPECTION LEAD ASBESTOS PROCESS BUILDING OPERATIONS AND WORKS OF ENGINEERING CONSTRUCTION (SAFETY) NOISE EXPOSURE MINERAL DUST 1989

5 The Occupational Safety and Health Act (OSHA) 1994 Employers must provide a place of employment free from recognized hazards to safety and health, such as exposure to toxic chemicals, excessive noise levels, mechanical dangers, heat or cold stress, or unsanitary conditions. Seven regulations of OSHA 1994: 1. Employers Safety and Health General Policy Statement (Exception) Regulation Control of Industry Major Hazards (CIMAH) Regulations Safety and Health Committee Regulations Classification, Packaging and Labeling of Hazardous Chemicals (CPL) Regulations Safety and Health Officer Regulations Use and Standards of Exposure of Chemicals Hazardous to Health (USECHH) Regulations Notification of Accident, Dangerous Occurrence, Occupational Poisoning and Occupational Disease (NADOPOD) Regulation 2004

6 The Occupational Safety and Health Act (OSHA) 1994 For all industries If >5 Employees-Safety & Health Policy 40 Employees (S30) -Safety & Health Policy + Safety & Health Committee For high risk industries (i.e. construction, ship building, gas etc.) >100 Employees (Order 1997) -Safety & Health Policy + Safety & Health Committee + a Certified Safety & Health Officer For low risk industries (other than the above mentioned industries) >500 Employees (Order 1997) -Safety & Health Policy + Safety & Health Committee + a Certified Safety & Health Officer

7 i. Physical Hazards e.g. height, noise, vibration, force, lighting etc. ii. Chemical Hazards e.g. gas, liquid, vapor, fumes, mist, dust etc. iii. Biological Hazards e.g. microbes, animals, arthropod, toxin plants iv. Electrical Hazards e.g. current, voltage, sparks v. Radiation Hazards e.g UV light, lasers vi. Psychological Hazards e.g. workplace space, organization culture, space.

8 To design a safe process or product we need to understand and mitigate the associated hazards Materials hazards Toxicity Flammability Incompatibility (corrosivity and reactivity) Process hazards Overpressure Explosions Loss of containment Noise

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10 Almost every chemical is toxic if you get enough of it Source of exposure inhalation Chemical plants tend to have large enough amounts to cause serious concern for workers and local residents Process design needs to consider Elimination or substitution of the most hazardous compounds Prevention of releases Containment Disposal (via effective collection or vent systems) Ventilation Emergency procedures

11 Acute Effects Symptoms develop rapidly (e.g. burns to skin after direct contact) Normally the result of short-term exposures Chronic Effects Symptoms develop over a long period of time (e.g. cancer) Often but not always the result of long-term exposure Chronic conditions usually persist or recur frequently LD 50 Lethal dose at which 50% of test animals are killed Usually expressed in mg/kg body mass Indicates acute effects only Threshold Limit Value (TLV) or Permissible Exposure Limit (PEL) Concentration that it is believed the average worker can safely be exposed to for 40 hr work week Recommended PEL values are published by OSHA Recommended TLV values are provided by the American Conference of Government Industrial Hygienists (ACGIH)

12 Examples: Compound PEL (ppm) LD50 (mg/kg) Source: OSHA Carbon monoxide Carbon disulfide Chlorine Chlorine dioxide Chloroform Cyclohexane 300 Dioxane Ethylbenzene Formic acid Furfural Hydrogen chloride Hydrogen cyanide Isopropyl alcohol Toluene Xylene Ethanol LD50 = 3450 (oral, mouse) 7060 (oral, rat) 1440 (intravenous, rat)

13 A fire requires three things: A sufficient amount of fuel A sufficient amount of oxidant A source of ignition (but not always - see autoignition) Possible ignition sources include Electrical equipment such as motors, actuators Usually specified as flame-proof or non-sparking when fuels are present Open flames from furnaces, incinerators & flare stacks Static electricity From any flow, hence pipes, vessels & flanges are always grounded Miscellaneous sources Matches, lighters & mobile phones are usually banned

14 Flash point The lowest temperature at which the material will ignite from an open flame Function of vapor pressure and flammability limits Autoignition temperature Temperature at which the substance ignites in air spontaneously Indicates maximum temperature the material can be heated to in air, e.g., in drying Flammability limits Highest and lowest concentrations in air at normal temperature and pressure (ntp) at which a flame will propagate through the mixture Vary widely for different materials Data can be found in Materials Safety Data Sheets (MSDS) or safety handbooks

15 Flame arrestors (flame traps) are specified on vent lines of equipment that contains flammable materials to prevent a flame from propagating back from the vent Various proprietary designs are available Basic principle: Remove the heat source (high temperature) Provide high metal surface area to act as a sink for heat and free radicals Enardo detonation flame arrestors Source: Enardo LLC

16 Mixtures of incompatible materials may undergo violent reaction (exothermic, temperature runaway) Acids and bases Acids and metals Fuels and oxidants Free radical initiators and epoxides, peroxides, unsaturates, Incompatibility with materials of construction can lead to loss of containment Corrosion of vessels, internals, instruments Softening of gaskets, seals, linings Materials incompatibility is one of the major sources of incidents

17 Material Safety Data Sheets (MSDSs) must be provided to employees and customers by law in the U.S.A. (OSHA Hazard Communication Standard 29 CFR Part ) MSDS contains the information needed to begin analyzing materials and process hazards Most MSDSs contain a disclaimer stating that the user should also make their own evaluation of compatibility and fitness for use

18 Always collect MSDS of all components used in the process at as early a stage as possible Sources: manufacturers, manufacturer s web sites, libraries, etc. Because of disclaimers, it is worth checking > 1 source Good starting points are or Use MSDS information to improve intrinsic safety of process Eliminate incompatible mixtures Substitute less hazardous chemicals when possible (e.g. toluene instead of benzene as solvent) Ensure that design meets regulatory requirements Vapor recovery Other emissions

19 Substitution use something less toxic and hazardous Containment Sound design of plant and equipment For example, use welded joints instead of flanges Prevention of releases By design of equipment and disposal systems Ventilation Use open plant structure or engineered ventilation system Disposal Effective vent stacks and scrubbers Collection and treatment of run-off water and liquid from relief systems Provision of emergency equipment

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21 Occurs when mass, moles or energy accumulate in a contained volume (or space with restricted outflow) Rate of accumulation determines the pressure rise Process controls may not be able to respond quickly enough If pressure is not relieved by pressure safety valve then outcomes could include Vessel rupture Explosion Other loss of containment

22 A fire requires a flammable mixture and an ignition source Fires in chemical plants can quickly lead to damage to control systems and equipment, causing overpressure, loss of containment and explosions Fire protection guidelines are given in several standards NFPA 30, API RP 2001 Legal requirements for fire protection are set by Uniform Building By-Laws 1984

23 Can you think of possible sources of ignition on a chemical plant? Sparking of electrical equipment Motors, actuators, lighting, electric heaters, Process flames Furnaces, flare stacks, incinerators These should always be sited well away from plant, usually upwind Static electricity See API RP 2003 and NFPA 77 Lightning Vehicles (engines, electrical systems and exhausts) Portable electrical devices Welding and cutting equipment Miscellaneous sources (matches, lighters, etc. are usually banned)

24 The use of electrical equipment in chemical plants is regulated by law (OSHA) and by industry design codes National Electrical Code NFPA 70 NFPA standards 496, 497, API RP 500, 505 NFPA 70 defines classified areas in which flammable materials may be present at high enough concentrations to be ignitable Specific precautions must be taken depending on the classification Equipment must be designed and installed in accordance with code

25 Codes should be consulted before selecting equipment for use in classified areas Codes also govern electrical maintenance work (NFPA 70B). Companies usually have strict Lock-out, tag-out procedures to prevent electric shock accidents

26 An explosion is the sudden, catastrophic release of energy causing a pressure wave (blast wave) Explosions can be caused by ignition of a flammable mixture Liquid Vapor Solid (e.g., finely dispersed dust) Explosions can also be caused by release of thermal energy Boiler rupture BLEVE (boiling liquid expanding vapor explosion)

27 Deflagration Combustion zone propagates at (subsonic) flame speed, usually < 30 m/s Pressure wave generated usually < 10 bar Principal heating mechanism is combustion Detonation Combustion zone propagates at supersonic velocity, m/s Pressure wave up to 20 bar Principal heating mechanism is shock compression Usually requires confinement or a high-intensity source Deflagration can turn into detonation when propagating along a pipe Expansion factor Measure of the increase in volume resulting from combustion E = (molar density of reagents)/(molar density of products) Maximum value of E is for adiabatic combustion Flame speed The rate of propogation of a flame front through a flammable mixture, with respect to a fixed observer

28 Vol% gas at Maximum Adiabatic Autoignition Flammability Limits (vol%) Expansion Fuel Formula max flame flame speed flame Temp temperature factor Upper Lower speed (m/s) (K) (ºC) Hydrogen H Methane CH Ethane C 2 H Propane C 3 H n-butane C 4 H Pentane C 5 H Hexane C 6 H Heptane C 7 H Acetylene C 2 H Ethylene C 2 H Propylene C 3 H Butylene C 4 H Benzene C 6 H Cyclohexane C 6 H Dugdale, D. An introduction to Fire Dynamics, Wiley, New York, 1985

29 Design to prevent explosions from happening Prevent formation of explosive mixtures whenever possible Operate outside flammability envelope Consider confined explosion as a pressure relief scenario and ensure that PRV is sized to allow adequate relief load to prevent detonation Use flame suppressors to prevent deflagration from propagating into detonation

30 The primary means of protecting the public from toxic chemicals is containment by the plant itself Loss of containment can occur due to: Pressure relief Operator error (e.g. leaving a sample point open) Poor maintenance procedures Failure to drain and purge properly Failure to put everything back together properly Leaks from degraded equipment Corrosion Damaged seals, gaskets These are mostly operational issues, but design may need to provide for secondary containment if the potential impact of a release is high

31 Chemical plants can be very noisy, especially compressors, turbines, motors and solids handling Chronic effects include permanent damage to hearing Sound is measured in decibels, defined by: RMSsound pressure Pa Sound level 20 log (Note: log scale) Ear protection should be required in areas where noise > 80 db Permanent damage can be caused by noise > 85 db db