Economic

CHAPTER (6) Tradeoffs

Presented by Hein Min Htike
TRADEOFFS IN PROCESS DESIGN

Energy

Raw Process
Capital
Material topology

Operation
 TWO BROAD CLASSES OF TRADEOFFS

Local tradeoffs

Global tradeoffs
 OPTIMIZATION OF REACTOR CONVERSION
Single Reactions Multiple Reactions

For the above reaction, optimization can be carried out by Optimization of reactor conversion for the above
minimizing a cost function or maximizing economic potential. reactions is presented in Figure.

The annualized reactor cost increases because high conversion At high conversions, the raw materials costs due to
requires a large volume. byproduct formation are dominant.

The annualized separation and recycle cost decreases with The selectivity becomes very poor at high conversion
increasing reactor conversion because the amount of because the reaction to the undesired BYPRODUCT is
unreacted FEED to separate and recycle decreases. series in nature.
The cost of heat exchanger network and utilities decreases The byproduct formation cost forces the optimum to
with increasing conversion because the separation duty is lower values of conversion.
decreased and the heating and cooling duties in the recycle
decrease.
Heat integration and the cost of heat exchanger network The tradeoffs for the alternative flowsheets will be
and utilities have a major influence on the optimal different.
conversion.
Figure: Overall cost tradeoffs decomposed Figure: Cost tradeoffs for a process with
according to the onion model byproduct formation
 Chapter-9
Safety and Health Consideration
Three major hazards in process plants
1. Fire
2. Explosion and
3. Toxic release

Fire
-The important factors in assessing fire as a hazard.

Flash point
the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an
ignitable mixture with the air near the surface of the liquid.

Autoignition temperature
the minimum temperature at which a substance in air must be heated to initiate or cause self-sustaining combustion
independent of the heating source.

Flammability limits
The entire range of concentrations of a mixture of flammable vapor or gas in air (expressed as volume percent) over
which a flash will occur or a flame will travel if the mixture is ignited.
Explosion
-explosion is a sudden and violent release of energy.
-the energy released in an explosion in a process plant is either chemical or physical:
1.Chemical energy
Chemical energy derives from a chemical reaction.
The source of the chemical energy is exothermic chemical reactions, which
includes combustion of flammable material.
Explosions based on chemical energy can either uniform or propagating.
An explosion in a vessel will tend to be a uniform explosion, while explosion. an
explosion in a long pipe will tend to be a propagating explosion.
2.Physical energy
- Physical energy may be pressure energy in gases, thermal energy, strain energy in
metals, or electrical energy.
There are two basic kinds of explosions involving the release of chemical energy:
1.Deflagration

A deflagration is an explosion where the flame speed is lower than the speed of sound,
which is approximately equal to 335 m/sec (750 mph).

Explosives that deflagrate are known as low explosives.

The actual speed of the explosion can vary from 1–350 m/s (2–780 mph).

Peak pressures produced by low explosives are orders of magnitude lower than those
produced by high explosives, and the damage inflicted by low explosives can vary
greatly depending on the fuel and confinement.

For example, if black powder is ignited outside of containment, it just fizzles, but when
it is confined, it creates an explosion that can propel bullets.

In addition to the black powder example, examples of deflagrations involving low
explosives include the ignition of propane gas for a cooking grill and fuel powering of a
combustion engine in a car.
2.Detonation
A detonation is an explosion where the flame speed is greater than the speed of
sound.
Detonations are louder and often more destructive than deflagrations.
While deflagration occurs when a fuel and oxidizer (typically air) mix, a detonation
doesn’t always need an external oxidizer.
Explosives that detonate are referred to as high explosives and have a detonation
speed in the range of 2,000–8,200 m/sec (4,500–18,000 mph).
High explosives are typically designed to cause destruction—often for demolition,
mining, or warfare.
Examples of high explosives that detonate include dynamite, TNT, and C4, a
plastic-based explosive.
Toxic release
The third of the major hazards and the one with the greatest disaster is the release of toxic
chemicals.
The hazard posed by toxic releases depends not only on the chemical species but also on the
conditions of exposure.
The high disaster potential from toxic release crises in situations where large numbers of
people are briefly exposed to high concentrations of toxic materials, ie, acute exposure.
However, the long-term health risks associated with prolonged exposure at low
concentrations, ie, chronic exposure, also present serious hazards.
For a chemical to affect healthy alsubstance must come into contact with an exposed bgdy
surface.
The three ways. in which this happens are by inhalation, skin contact, and inigestion, the latter
being rare.
The acceptable limits for toxic exposure depend on whether the exposure is brief or
prolonged.
 Chapter-10
Waste Minimization
Two types of waste from chemical processes

 Process waste

 Utility waste

Process waste
-The process waste is waste byproducts, purge, etc.
There are three sources of process waste:
Reactors
-Waste is created in reactors through the formation of waste byproducts, etc.
Separation and recycle systems
-Waste is produced from separation and recycles systems through the inadequate recovery and recycling of valuable
materials from waste streams.
Process operations
-The third source of process waste is process operations. Operations such as start-up and shutdown of continuous
processes, product changeover, equipment cleaning for maintenance, tank filling, etc. all produce waste.
Utility Waste

The utility waste is composed of the products of fuel combustion, waste from the production of boiler
feed water for steam generation, etc.

However, the design of the utility system is closely tied together with the design of the heat exchanger
network.

The principal sources of utility waste are associated with hot utilities.

Furnaces, steam boilers, gas turbines, and diesel engines all produce waste as gaseous combustion
products.

These combustion products contain carbon dioxide, oxides of sulfur and nitrogen, and particulates
which contribute in various ways to the greenhouse effect, acid rain, and the formation of smog.

In addition to gaseous waste, steam generation creates aqueous waste from boiler feed water
treatment, etc.
 Life-cycle analysis

There are three components in a life-cycle analysis: Raw Materials

1. Identification of significant issues for the product

Disposal Manufacturing
or process being analyzed, which are based on
the life-cycle inventory

2. Evaluation, which considers completeness,

sensitivity and consistency checks Use Packaging

3. Reporting Distribution
 Chapter-11
Effluent Treatment
Types of wastewater treatment processes

Wastewater treatment processes are generally classified in

 Primary (or pretreatment)

 Secondary ( or biological)

 Tertiary ( or polishing)

1.Primary or pretreatment methods

Primary or pretreatment processes serve two purposes:

 Recover useful material where possible

 Prepare the aqueous waste for biological treatment by removing excessive load or components that
will inhibit the biological processes.
2. Biological treatment
In secondary or biological treatment, a concentrated mass of
microorganisms is used to break down organic matter into stabilized wastes.
There are two main types of biological reaction, aerobic and anaerobic:
a. Aerobic
Aerobic reaction take place only in the presence of free
oxygen and produce stable, relatively inert end products such as carbon dioxide and water. Aerobic reactions
are by far the most widely used, being capable of removing up to 95 percent of BOD.
b. Anaerobic
Anaerobic reactions function without the presence of free
oxygen and derive their energy from organic compounds in the waste. Anaerobic reactions proceed relatively
slowly and lead to end products which are unstable and contain considerable amounts of energy such as
methane and hydrogen sulfide.
The performance of anaerobic digestion processes varies
according to the type of unit, throughput, and feed concentration, but such processes are typically capable of
removing between 75 and 85 percent of COD.
3.Tertiary treatment

Tertiary or polishing treatment prepares the aqueous waste for

final discharge. The final quality of the effluent depends on the nature and flow of the receiving water.
Tertiary treatment processes vary, but they constitute the final stage of effluent treatment to ensure that
the effluent meets specifications for disposal. Processes used include the following:

a. Filtration

b. Ultrafiltration

c. Adsorption

d. Nitrogen and phosphorus removal

e. Disinfection
Types of incinerators are used in process industry