Process Design Closed-Book Exam

hazard

an inherent chemical or physical characteristic that has the potential to cause damage to people, property, or the environment

They are all examples of hazards

Which of the following is NOT an example of a hazard: highly energetic chemical reaction, high-pressure steam, a line carrying liquid nitrogen through an occupied area, maintenance work requiring confined space entry, combustion dues, flammable gas, toxic components (like a railcar of chlorine)

consequence

measure of some adverse effects from an incident

They are all examples of hazards

Which of the following is NOT an example of a consequence: overpressure magnitude at specified distance away from vapor cloud explosion, heat radiation level at specified distance from a pool fire, concentration downwind from toxic vapor cloud release, quantity of toxic of flammable material released in an incident, dollar value of property damage from an incident, serious injury from an injury, fatalities from an incident

1. What can go wrong?

2. How bad can it be?

3. How often might it happen?

Hazard Identification and Risk Analysis (HIRA)

Risk = Frequency x Consequence

What does risk equal?

risk matrix

used to represent the frequency and consequence and flag processes with unacceptably high risk

Process Safety Management System

What does the Center for Chemical Process Safety (CCPS) have 20 pillars for?

temperature, pressure, flow, static electricity, level

What are the five physical hazards?

design pressure

minimum pressure to which the vessel must be designed in order to perform its intended function

maximum allowable working pressure (MAWP)

maximum allowable pressure at the top of a vessel at a designated temperature

True

T/F: The maximum allowable working pressure (MAWP) must be greater than or equal to the design pressure

That portion of the vessel will eventually reach a temperature that the steel cannot withstand and with high internal pressure will cause a violent tank rupture

In an incident where a vessel containing a liquid is caught in an external fire, what would happen to the unwetted upper portion of the vessel when the vapor above the liquid reaches a high temperature?

charge accumulation and rapid discharge can cause a spark energetic enough to ignite a flammable gas or dust

Why is static electricity a hazard?

1. Limit the velocity of low conductivity liquids in a pipe

2. Avoid splash filling

3. Bonding

4. Grounding

What precautions can be taken to reduce static charge accumulation?

bonding

low resistance electrical connections between equipment and bond equipment

grounding

provides a low resistance electrical pathway to drain charge off of equipment to safely dissipate in the ground

ERPG-1

the maximum airborne concentration below which nearly all individuals could be exposed for up to one hour without experiencing effects more than mild transient adverse health effects or without a clearly defined objectional order

ERPG-2

the maximum airborne concentration below which nearly all individuals could be exposed for up to one hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair an individual’s ability to take protective action

ERPG-3

the maximum airborne concentration below which nearly all individuals could be exposed for up to one hour without experiencing or developing life-threatening health effects

release modeling

can be used to identify areas containing people at risk of high exposure and to predict where a flammable concentration occurs

lower flammability limit (LFL)

fuel concentration below which a gas mixture is too lean to sustain burning

upper flammability limit (UFL)

fuel concentration above which a gas mixture is too rich to sustain burning

limiting oxygen concentration (LOC)

minimum oxygen required for combustion of a flammable cloud at any concentration

False; they do not - it is much easier to reduce oxygen to below the LOC than to zero

T/F: Oxygen levels need to be reduced to zero to prevent fires

minimum ignition energy (MIE)

lowest spark energy capable of igniting a flammable cloud of optimum concentration

False; it will partially damage a structure - 5 psi overpressure will cause many fatalities

T/F: 1 psi overpressure will not cause any damage to structures

Adding inerting gases (ex: Nitrogen and CO2)

What can create an oxygen-deprived atmosphere?

asphixiant

substance, usually a gas, that causes suffocation

15-19% O2

At what oxygen concentration does the first signs of hypoxia occur?

False; confined spaces

T/F: Asphyxiation is often associated with open spaces

flow-through purging

-purging method that introduces an inert gas in one end that flows through vapor system to purge unwanted fumes out the other end

-open system

-used for vessels that cannot hold any pressure or vacuum

-for any vessel designed to operate close to atmospheric pressure, including large storage tanks

pressure-swing purging

-vessel is filled and pressurized with inert gas to reduce the oxygen mole fraction in the vapor space and then vented to atmosphere

-closed system (cycle of filling and venting)

-repeated as many times as needed to drop oxygen mole fraction below the LOC, without exceeding the design pressure

vacuum-swing purging

-vacuum is pulled on a vessel and then the vessel is pressurized back to 1 atm with inerts to reduce the oxygen mole fraction in the vapor space

-repeated as many times as needed to drop the oxygen mole fraction below the LOC

-vessel must be rated for vacuum to perform

safeguard

any, device, system, or action that either interrupts the chain of events following an initiating event or that mitigates the consequence

initiating event

a mechanical failure, human action, or external cause that puts a normally operating process into an abnormal process

loss event

an irreversible physical event with potential for loss and harm impacts, such as fires, explosions, and/or toxic releases

impact

-loss and harm resulting from a loss event

-includes the number of injuries/fatalities, damage to environment, property loss

preventative safeguards

-safeguards that interrupt the chain of events following an initiating event and keep a loss event from occurring

-ex: operator response to process deviation, automatic shutdown valve, relief valve

mitigative safeguards

-safeguards that reduce the impact of a loss event

-ex: secondary containment, release detection and isolation, fire suppression systems

Hazard —> Initiating Event —> (Preventative Safeguards) —> Loss Event —> (Mitigative Safeguards) —> Impact

What is the order of a potential hazard situation from start to finish?

1. The basic process control system

2. Human response action

3. Automatic safety systems

4. Pressure protection systems

Four Common Types of Preventative Safeguards

True

T/F: Risk can be assessed and reduced at all points along the process life cycle

1. Primary containment system (mostly preventative safeguards)

2. Process controls (mostly preventative safeguards)

3. Siting and layout of the facility (mostly mitigative safeguards)

Three aspects of process design that are particularly related to safety and safeguards are:

inherently safer design (ISD)

a way of thinking about the design of chemical processes that focuses on the elimination or reduction of hazards rather than on their management and control

True

T/F: ISD should be utilized early in the process life cycle, but can also be utilized at later stages

1. Inherently Safer Design (ISD)

2. Passive Controls

3. Active Controls

4. Procedural Controls

What are the 4 Strategies for Safety in Process Design in order of reliability

1. Minimize - store smaller quantities of flammable chemicals

2. Moderate - operate reactor at lower pressure to reduce consequence of vessel rupture

3. Substitute - substitute a more toxic chemical for a less toxic chemical

4. Modify - eliminate unnecessary valves and piping connections that could introduce potential for leaking

4 Main Strategies for Inherently Safer Design (ISD)

active control

-control that requires power, detection, and mechanical movement to work

-ex: temperature sensor in a tank turns on a heater when a pre-set low temperature is reachedp

passive control

-control that relies purely on physical design and require no activation, power, or human input to function

-ex: a containment dike wall is installed around a storage tank containing flammable material

procedural control

-control that relies on written instructions, standard operating procedures (SOPs), safety policies, training, and worker actions to prevent or mitigate hazardous chemical and energy releases

-ex: standard operating procedure must be followed before starting an exothermic batch reaction

No because the process chemistry during runaway is not understood. If we changed conditions, perhaps the pressure could peak at 20 barg

Given the following example, is the passive design strategy guaranteed to be safe?

The reaction is exothermic and the maximum adiabatic pressure is 10 barg (This assumes 100% reaction with no heat removal. No specific reaction chemistry can be defined for the runaway reaction, but pressure is generated by boiling reagent and/or decomposition of reagent or products). The vessel is designed to contain 17 barg pressure.

True

T/F: Few engineers are involved in quantitative safety calculations daily, but most engineers are regularly involved in managing process safety

1. Management of Change

2. Emergency Management

3. Operational Readiness

4. Conduct of Operations

Operational Components of Managing Process Safety

management of change

-process that requires a written request, some form of risk evaluation, and some level of management approval

-many parties may all need to be involved in the change evaluation

-ex: if something is not an exact replacement, a risk evaluation must be performed before implementing the change

leading metric

-applied before an incident happens to track safety performance

-more proactive in driving safety improvements

-ex: # MOCs performed per month, % MOCs for which workers were misinformed or untrained, % of MOCs reviewed that did not have adequate hazard risk assessment

lagging metric

-counts the number of bad things that happened

-ex: # of incidents, # lost work hours, # unplanned shutdowns, etc

1. The workers might need more training because they are not recognized when MOC requests are needed. (unfavorable interpretation)

2. There could be a safety culture issue when people feel MOC requests are a bother and they don’t submit them. (unfavorable interpretation)

3. The facility might be showing more stability with less change being needed. (favorable interpretation)

You are monitoring a facility tracking system for change management and see a trend of gradually decreasing MOC requests. How can this be interpreted?

False

T/F: Organizational charts only provide benefit

RXN1: Benzene + Ethylene —> Ethylbenzene
RXN2: Ethylbenzene + Ethylene —> Diethylbenzene
RXN3: Diethylbenzene + Benzene —> 2 Ethylbenzene
RXN4: Toluene + 2 Ethylene —> Ethylbenzene + Propylene

Write the chemical reactions involved in the ethylbenzene process. You may represent each chemical component by its chemical name or formula

False; the loss of reagents like benzene and ethylbenzene will impact process economics because it will increase raw material costs

T/F: The loss of reagent when removing the light components from the ethylbenzene base case design has little impact on process economics

False; O2 sensors are used

T/F: Accidents involving nitrogen asphyxiation are rare because nitrogen sensors can accurately measure the concentration of nitrogen

False; not necessarily because Risk = Frequency x Consequences

T/F: A chemical process must involve chemicals with low hazards for the process to have an acceptable level of risk

True; A fuel-rich mixture that is initially nonflammable can become flammable when diluted with air

T/F: A gas mixture with a fuel concentration above the upper flammability limit can form a flammable mixture when released in air

False; A gas mixture with a fuel concentration below the lower flammability limit (LFL) is too lean in fuel to ignite. When released into air, the mixture becomes further diluted, decreasing the fuel concentration even more. Therefore, it cannot enter the flammable range through dilution with air alone

T/F: A gas mixture with a fuel concentration below the lower flammability limit can form a flammable mixture when released in air

False; must be below the LOC (usually around 10% O2)

T/F: To make a gas mixture nonflammable, the oxygen concentration in the gas mixture must be above the limiting oxygen concentration

False; Fp = tcalc/tmin; the vessel is built to the greater of the two thickness requirements. A calculated pressure thickness below the mechanical minimum doesn't mean the vessel will actually be built too thin. If the mechanical minimum is the greater of the two, Fp = 1.0 (baseline), but if the calculated wall thickness is above the minimum, then tcalc is the greater thickness and Fp must be corrected (greater than 1.0)

T/F: If the calculated wall thickness to hold the pressure inside a process vessel is less than the minimum wall thickness for mechanical integrity, the calculated Fp will be greater than 1.0

False; it correct time

T/F: The Chemical Engineering Cost Index is used to correct equipment cost estimates for different materials of construction

False; it’s to maintain an efficient heat exchanger design

T/F: A minimum thermal driving force of 10*C between the hot and cold streams of a heat exchanger is required for heat exchanger safety

False; if you’re using a reboiler to heat the process fluid, the saturation temperature of the steam must be at least 10*C higher than the process fluid temperature

T/F: The steam pressure used in a distillation column reboiler is selected to ensure that the saturation temperature of the steam is at least 10*C lower than the process fluid temperature

False

T/F: The factor for material of construction has little impact on equipment cost estimates

True; tau = V/v*; v* (gas) = n*RT/P; lower P —> higher v* —> lower tau

T/F: For a gas phase reaction, lowering the reactor pressure without making any changes to the reactor configuration in Aspen will reduce the residence time for the reactor

False; increase in feed flow rate would increase the column diameter and heating/cooling loads

T/F: The number of stages required for a separation will increase if the feed flow rate is increased

True; Higher reflux ratios generally reduce the number of theoretical stages required; feed composition, separation specifications, and relative volatility all affect the number of stages as well

T/F: The number of stages required for a separation will decrease if the reflux ratio is increased

False; it only prevents ignitions caused by static electricity

T/F: Electrical bonding of equipment prevents all possible ignitions

False; thermodynamics do not indicate kinetics

T/F: If a reaction is thermodynamically possible, the reaction rate will always be fast at high temperature

False; Procedural controls are generally less reliable than passive controls because they depend on human actions, while passive controls operate without human intervention.

T/F: Procedural controls to enhance process safety are more reliable than passive controls

1 bar = 750 Torr —> very low pressure drop per tray. A smaller pressure drop results in a lower bottom-column pressure. A lower pressure generally decreases the boiling temperature of the liquid. Consequently, the simulated bottom product temperature would be lower than expected. Lower pressure —> Lower boiling temperature

T/F: If a 0.007 Torr per tray pressure drop is mistakenly entered for a distillation column, the simulation results will give an unrealistically high temperature of the bottom product stream

False; exothermic reactions generate heat and lead to increased reaction rates; endothermic reactions absorb heat and do not self-accelerate like exothermic reactions

T/F: A runaway reaction is a potential hazard only for endothermic reactions

False

T/F: All process vessels are made to hold both internal pressure and vacuum

With low single pass conversion, unreacted reactants remain in the process stream. This incurs a higher recycle flow rates, requiring more powerful pumps/compressors. It also means that to produce the same amount of product with lower single-pass conversion, the process must handle greater quantities of reactants. Also, unreacted reactants must often be separated from the desired products before being recycled.

Explain why low single pass conversion across a reactor leads to higher process costs

Inherently Safer Design - substituting a more toxic reactant for a less toxic reactant

Passive Control - installing a containment dike around a storage tank

Active Control - pressure relief valve when pressure in a vessel gets too high

Procedural Control - standard operating procedure (SOP)

Identify the four strategies for safety in process design and provide an example of each.

Preventative Safeguard - safeguard taken into effect directly after the initiating event to keep a loss event from occurring; ex: pressure relief valve when pressure inside a vessel gets to high

Mitigative Safeguard - safeguard that reduces the impact of a loss event; ex: secondary containment

Explain the difference between a preventative safeguard and a mitigative safeguard and provide an example of each.

-how easy is it to remove the impurity upstream vs downsteam

-is the impurity toxic, corrosive, etc. and will it poison the catalyst, damage the vessel, cause side reactions

-how will it effect purity

Describe the factors that impact the decision of whether to remove a feed impurity upstream of a reactor.

This is because it prevents unwanted side reactions from occurring by discouraging the reaction ethylbenzene and ethylene to form diethylbenzene

In the ethylbenzene base case design, a high ratio of benzene to ethylene is maintained in the reactor feed streams. Explain why this might be selected for the design

-the distillate-to-feed ratio, which essentially sets the material balance, may not be set accordingly

-the feed tray location may not be set accordingly either, which can reduce separation efficiency

Explain reasons why a distillation column simulation may not reach the desired distillate product purity even with a large number of stages and high reflux ratio

As you increase the number of stages, the Cbm of the tower increases and the Cbm of the trays increase

As you increase the reflux ratio, the Cbm of the condenser and reboiler increases, the operating costs of the condenser and reboiler increase, where the heating/cooling loads increase, and column diameter increase (because more volumetric flow in diameter), so Cbm of tower and trays increase

For a distillation column, explain cost tradeoffs between the number of stages and reflux ratio

Increasing temperature favors the endothermic reaction, according to Le Chatelier’s Principle.

For an exothermic reaction, as temperature increases, K decreases because the reverse (endothermic) reaction is favored
For an endothermic reaction, as temperature increases, K increases because the forward (endothermic) reaction is favored

Describe how the reaction equilibrium constant changes with temperature for an exothermic and an endothermic reaction

Leading - proactive, measures activities intended to prevent incidents before they occur; ex: % of MOCs completed on time

Lagging - reactive, measures incidents or failures after they occur; ex: number of fatalities

Explain the difference between a leading indicator and a lagging indicator in monitoring safety performance and provide an example of each.

A longer residence time increases ethylene conversion but may promote unwanted diethylbenzene formation and require a larger reactor. A shorter residence time can improve ethylbenzene selectivity and reduce reactor size, but lower conversion increases reactant recycling and separation costs. The optimal residence time minimizes total process costs while maintaining sufficient conversion and selectivity.

What are the tradeoffs between a long and short residence time for the reaction of ethylene with benzene?

1. Measure

2. Decide

3. Act

What are the three logical actions in a control loop?