CHE 425 Engineering Economics and Design Principlesfaculty.kfupm.edu.sa/che/alamer/ChE_425/CHE_425_Ch2.pdf · Prof. Adnan Alamer Chemical Engineering Dept., KFUPM. 1 CHE 425 Engineering

Prof. Adnan AlamerChemical Engineering Dept., KFUPM.

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CHE 425 CHE 425

Engineering Economics and Engineering Economics and Design PrinciplesDesign Principles


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CHAPTER 2CHAPTER 2

The Structure and Synthesis The Structure and Synthesis of Process Flow Diagramsof Process Flow Diagrams


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PURPOSE OF CHAPTER 2PURPOSE OF CHAPTER 2

To show that the evolution of every process To show that the evolution of every process follows a similar path.follows a similar path.

To provide a framework to generate To provide a framework to generate alternativealternativePFDsPFDs for a given process.for a given process.

Factors determining choice of alternative routeFactors determining choice of alternative routeCost of raw materialsValue of by-productsComplexity of the synthesisEnvironmental impact of waste materials


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Hierarchal Approach to Conceptual DesignHierarchal Approach to Conceptual Design

Five step process to tackle a conceptual process designFive step process to tackle a conceptual process design

Batch vs. continuous Batch vs. continuous Identify the InputIdentify the Input--output structure output structure Identify and define recycle structure of processIdentify and define recycle structure of processIdentify and design general structure of separation Identify and design general structure of separation systemsystemIdentify and design heatIdentify and design heat--exchanger network or exchanger network or

process energy recovery systemprocess energy recovery system

Designing New Process: [1Designing New Process: [1--22--33--44--5]5]Analyzing Existing Process: [5Analyzing Existing Process: [5--44--33--22--1]1]


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Batch vs. ContinuousBatch vs. Continuous

A batch processA batch process is one in which a finite quantity is one in which a finite quantity (batch) of product is made during a period of a few (batch) of product is made during a period of a few hours or days.hours or days.

Continuous processContinuous process is one in which feed is sent is one in which feed is sent continuously to a series of equipment, with each continuously to a series of equipment, with each piece usually performing a single unit operation.piece usually performing a single unit operation.


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Batch vs. Continuous (cont)Batch vs. Continuous (cont)

Variables to Consider: Variables to Consider: Size Size

Batch < 500 Batch < 500 tonnetonne/yr ~ 1.5 /yr ~ 1.5 tonnetonne/day /day

(< 2 m3 of liquid or solid per day) (< 2 m3 of liquid or solid per day) Continuous > 5000 Continuous > 5000 tonnetonne/yr/yr

Flexibility Flexibility Batch can handle many different feeds and products Batch can handle many different feeds and products ––

more flexible more flexible Continuous is better for smaller product slate and fewer Continuous is better for smaller product slate and fewer

feedsfeeds


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Batch vs. Continuous (cont)Batch vs. Continuous (cont)

Continuous will allow you to benefit from the Continuous will allow you to benefit from the ““Economy of ScaleEconomy of Scale”” but you will pay the price for less but you will pay the price for less flexibilityflexibility

Other Issues Other Issues Accountability and quality control Accountability and quality control –– Government Government

should require batch accountability should require batch accountability Safety Safety –– batch is more accident prone batch is more accident prone Scheduling of equipment Scheduling of equipment –– may be most important may be most important

issue issue Seasonal demands Seasonal demands –– ex. Antifreeze, food ex. Antifreeze, food productsproducts


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Input Input –– Output StructureOutput Structure(Process Concept Diagram)(Process Concept Diagram)


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Input Input –– Output Structure of The PFD Output Structure of The PFD


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Input Input –– Output Structure of The PFD (cont)Output Structure of The PFD (cont)

Important factors to consider in analyzing the overall inputImportant factors to consider in analyzing the overall inputoutput structure of a PFDoutput structure of a PFD

Chemicals not consumed are either required to operate a piece of equipment or are inert material.

Any chemical leaving a process must have either entered in one of the feed streams or have been produced by a chemical reaction within the process.

Utility streams are treated differently from process streams. Utility streams rarely directly contact the process streams. They usually provide or remove thermal energy or work.


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Input Input –– Output (Utility Streams)Output (Utility Streams)


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Generic Structure of PFDGeneric Structure of PFD


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Generic Structure of PFD (cont)Generic Structure of PFD (cont)


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Other Considerations for Input Other Considerations for Input –– Output StructureOutput Structure

Feed Purity and Trace Components Feed Purity and Trace Components

Small Quantities and Small Quantities and ““InertsInerts”” –– Do Not Separate Do Not Separate

E.g. E.g. HH22 in feed to THDA process contains CHin feed to THDA process contains CH44

CHCH44 does not react does not react

so, do not removeso, do not remove


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Other Considerations for Input Other Considerations for Input –– Output StructureOutput Structure

If Separation of Impurities is Difficult If Separation of Impurities is Difficult –– Do Not Separate Do Not Separate

•• AzeotropeAzeotrope –– (water and ethanol) (water and ethanol)

•• Gases Gases –– (requires high P and low T)(requires high P and low T)

If impurities foul or poison catalyst then If impurities foul or poison catalyst then separate separate

•• Sulfur Sulfur –– Group VIII Metals such as iron, nickel, and Group VIII Metals such as iron, nickel, and platinum platinum

•• A guard bed of activated carbon (or Zinc Oxide) is placed A guard bed of activated carbon (or Zinc Oxide) is placed upstream of the reactor for protectionupstream of the reactor for protection


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Other Considerations for Input Other Considerations for Input –– Output Structure (cont)Output Structure (cont)

If impurity reacts to form difficultIf impurity reacts to form difficult--toto--separate material or separate material or hazardous product then hazardous product then separate separate

•• Phosgene Example Phosgene Example CO + ClCO + Cl22 COClCOCl22

CHCH44 + H+ H22O O CO + 3HCO + 3H22

Any HAny H22 HClHCl

Impurity in large quantities then Impurity in large quantities then purifypurify –– why? why?

•• A notable exception is airA notable exception is air


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ADD MATERIAL TO FEEDADD MATERIAL TO FEED

Stabilize Products Stabilize Products

Enable Separation/Minimize Side Reactions Enable Separation/Minimize Side Reactions

AntiAnti--Oxidants and Scavengers Oxidants and Scavengers

Solvents and CatalystsSolvents and Catalysts


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INERT FEEDINERT FEED

Control Exothermic Reactions Control Exothermic Reactions

Steam for oxidation reactions Steam for oxidation reactions

Control Equilibrium Control Equilibrium

Adding Adding inertsinerts shifts equilibrium to the right shifts equilibrium to the right

Example: styrene reaction Example: styrene reaction

CC66HH55CHCH22CHCH33 ←→←→ CC66HH55CHCHCHCH22 + H+ H22


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Information Obtained From InputInformation Obtained From Input--Output DiagramOutput Diagram

Basic economic analysis on profit margin.Basic economic analysis on profit margin.

What chemical components must enter with the feed What chemical components must enter with the feed and leave as products.and leave as products.

All the reactions, both desired and undesired, that All the reactions, both desired and undesired, that take place.take place.


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Profit Margin (PM)Profit Margin (PM)

If PM < 0, then donIf PM < 0, then don’’t bother to pursue this process but start t bother to pursue this process but start looking for an alternate route looking for an alternate route

Toluene HDA vs. Toluene Toluene HDA vs. Toluene DisproportionationDisproportionation

CC66HH55CHCH33 + H+ H22 →→ CC66HH66 + CH+ CH44

Toluene benzene Toluene benzene

2C2C66HH55CHCH33 →→ CC66HH66 + C+ C66HH44(CH(CH33))22

Toluene benzene Toluene benzene xylenexylene

PM = Value of products – Cost of raw materials

Toluene used more

efficiently


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Profit Margin (PM)Profit Margin (PM)

Some processes are more sensitive to product and Some processes are more sensitive to product and feed prices than others.feed prices than others.

Average cost data over a period of several years Average cost data over a period of several years should be used in evaluating PM.should be used in evaluating PM.

Cost of raw materials play a role in deciding which Cost of raw materials play a role in deciding which chemical path to choose to produce a given product.chemical path to choose to produce a given product.


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Example 2.1Example 2.1

Evaluate the profit margin for the HDA process.Evaluate the profit margin for the HDA process.

From Tables 6.3 and 6.4 (Year 2001)From Tables 6.3 and 6.4 (Year 2001)Benzene = $ 0.349/kgBenzene = $ 0.349/kgToluene = $ 0.322/kgToluene = $ 0.322/kg

Natural gas (methane and ethane, MW =18) = $ 6.00/GJ = $ 6.7/1000 std. ft' = $ 0.293/kg

Hydrogen = $ 0.721 /kg

Using 1 kmol of toluene feed as a basis


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Example 2.1 (cont.)Example 2.1 (cont.)

Cost of Raw Materials92 kg of Toluene = (92 kg)($ 0.322/kg) = $ 29.62 2 kg of Hydrogen = (2 kg)($ 0.721/kg) = $ 1.44

Value of Products78 kg of Benzene = (78 kg)($ 0.349/kg) = $ 27.22 16 kg of Methane = (16 kg)($ 0.293/kg) = $ 4.69

Profit MarginProfit Margin = (27.22 + 4.69) - (29.62 + 1.44) = $ 0.85

or $ 0.0092/kg toluene

ConclusionFurther investigation of this process is probably marginallywarranted


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Example 2.1 (cont.)Example 2.1 (cont.)Consider 1993 pricesConsider 1993 prices

Cost of Raw Materials92 kg of Toluene = (92 kg)($ 0.24kg) = $ 22.082 kg of Hydrogen = (2 kg)($ 0.312/kg) = $ 0.624

Value of Products78 kg of Benzene = (78 kg)($ 0.27 /kg) = $ 21.0616 kg of Methane = (16 kg)($ 0.126/kg) = $ 2.016

Profit MarginProfit Margin = (21.06 + 2.016) - (22.08 + 0.644) = $ 0.372 or

$ 0.0040/kg of toluene

ConclusionThis is significantly lower than the 2001 margin. Production ofbenzene via the HDA of toluene would probably not be economical


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Evaluate the profit margin for the toluene Evaluate the profit margin for the toluene disproportionationdisproportionationprocess.process.

From Tables 6.4 (Year 2001)From Tables 6.4 (Year 2001)Mixed Mixed XylenesXylenes = 0.323 $/kg= 0.323 $/kgUsing 2 Using 2 kmolskmols of toluene feed as a basisof toluene feed as a basis

Cost of Raw Materials184 kg of Toluene = (184 kg)($0.322/kg) = $ 59.25

Value of Products78 kg of Benzene = (78 kg)($0.349/kg)= $ 27.22 106 kg of xylene = (106 kg)($0.323/kg) = $ 34.24


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MarginMarginProfit Margin = 34.24 + 27.22 Profit Margin = 34.24 + 27.22 -- 59.25 = $ 2.21 59.25 = $ 2.21

or $ 0.012/kg toluene feedor $ 0.012/kg toluene feed

Currently this process is significantly better than thehydrodealkylation process.

Year 1993Year 1993Mixed Mixed XylenesXylenes = 0.323 $/kg= 0.323 $/kgMargin = (78)(0.27) + (106)(0.25) Margin = (78)(0.27) + (106)(0.25) -- (184)(0.24) = $ 3.4 (184)(0.24) = $ 3.4

or $ 0.0185/kg of tolueneor $ 0.0185/kg of toluene

We conclude that the margin for the disproportionation process issignificantly less sensitive to product and feed prices than is thehydrodealkylation process.


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Recycle Structure of the ProcessRecycle Structure of the Process

Raw materials are very valuable.

They make up 25 to 75% of total operating costs.

Separation and recycling of unused reactants is very important.

Exception is when raw materials are very cheap

Extent of recycling of unused reactants depends on ease of separation.


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Efficiency of Raw Material UsageEfficiency of Raw Material Usage

reactant consumed in reactionSingle-pass Conversionreactant fed to the reactor

=

reactant consumed in processOverall Conversionreactant fed to the process

=

moles of reactant to produce desired productYieldmoles of limiting reactant reacted

=


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Single-pass Conversion (SPC)

SPC tells us how much of the reactant that enters the SPC tells us how much of the reactant that enters the reactor is reacted.reactor is reacted.

The lower the SPC the greater the amount of recycle.The lower the SPC the greater the amount of recycle.

SPC affects equipment size and utility flows.SPC affects equipment size and utility flows.

Raw material costs are not changed significantly by SPC.Raw material costs are not changed significantly by SPC.

SPC of hydrogen in HDA process is kept low to reduce SPC of hydrogen in HDA process is kept low to reduce coking of the catalyst.coking of the catalyst.


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Overall Conversion (OC)

OC tells us tells us what fraction of the reactant in the feed to the process is converted to product.

High OC (e.g 99.3 %) is typical for chemical processes.

High OC shows that unreacted raw materials are not being lost from the process.

Low OC of hydrogen in the HDA process indicates poor raw material usage.


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Yield

Yield tells us what fraction of the limiting reactant ends up in our desired product.

Competing or side reactions may reduce the yield.

Yields for hydrodealkylation process are generally high (e.g 98-99 % for DETOL, Lummus).


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3 Basic Recycle Structures3 Basic Recycle Structures

Separate and purify unSeparate and purify un--reacted feed from products reacted feed from products and then recycle, e.g., Toluene and then recycle, e.g., Toluene

Recycle feed and products together and use a Recycle feed and products together and use a purge stream, e.g., hydrogen with purge as fuel purge stream, e.g., hydrogen with purge as fuel gas gas

Recycle feed and products together but do not use Recycle feed and products together but do not use a purge stream a purge stream -- must come to Equilibrium must come to Equilibrium

2C2C66HH66 CC1212HH1010 + H+ H22


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Separate and purifySeparate and purify

Ease of separation depends on:Ease of separation depends on:

What conditions (T and P) are necessary to What conditions (T and P) are necessary to operate the process?operate the process?

Are the differences in physical and chemical Are the differences in physical and chemical properties for the species to be separated large or properties for the species to be separated large or small?small?


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What process should be used in the separation of toluene andWhat process should be used in the separation of toluene andbenzene?benzene?

DistillationDistillation

Normal boiling point of benzene = 79.8 Normal boiling point of benzene = 79.8 °°C C Normal boiling point of toluene = 110 Normal boiling point of toluene = 110 °°CC

Separation should be easy using distillation, and neitherSeparation should be easy using distillation, and neitherexcessive temperatures nor pressures will be needed. This is aexcessive temperatures nor pressures will be needed. This is aviable operation for this separation of benzene and toluene inviable operation for this separation of benzene and toluene inthe HDA process.the HDA process.


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Recycle Structure in PFD


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Consider the following two process alternatives for the toluene HDA process when the side reaction of benzene to form diphenyl occurs.

Alternative A

As recycle increases, Equip andOp costs increases


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Alternative B

Extra tower with associated operating

costs


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Clearly for Alternative B, we require an additional separator, Clearly for Alternative B, we require an additional separator, shown here as a second distillation column Tshown here as a second distillation column T--102, along with 102, along with the associated equipment (not shown) and extra utilities to the associated equipment (not shown) and extra utilities to carry out the separation. For Alternative A, we avoid the cost carry out the separation. For Alternative A, we avoid the cost of additional equipment, but the recycle stream (Stream 11) of additional equipment, but the recycle stream (Stream 11) will be larger (because it now contains toluene and will be larger (because it now contains toluene and DiphenylDiphenyl), ), and the utilities and equipment through which this stream and the utilities and equipment through which this stream pass (Hpass (H--101, E101, E--101, R101, R--101, E101, E--102, V102, V--102, V102, V--103, T103, T--101, E101, E--106) will all be greater. Which is the bet106) will all be greater. Which is the bet--ter (economically ter (economically preferable) alternative?preferable) alternative?

The answer depends on the value of The answer depends on the value of KeqKeq


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Assume that reaction has reached equilibriumAssume that reaction has reached equilibriumUsing data from Table 1.5Using data from Table 1.5

x = 1.36 x = 1.36 kmol/hkmol/h. . Thus, the toluene recycle, Stream 11, will be increased from Thus, the toluene recycle, Stream 11, will be increased from

35.7 to 37.06 35.7 to 37.06 kmol/hkmol/h, an increase of 4%, while the increases , an increase of 4%, while the increases in Streams 4 and 6 will be approximately 0.1%. in Streams 4 and 6 will be approximately 0.1%.

Based on this result, Alternative A will probably be better (lesBased on this result, Alternative A will probably be better (less s expensive) than Alternative B.expensive) than Alternative B.


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Other Issues on RecycleOther Issues on Recycle

Number of recycle streams Number of recycle streams Consider valuable reactant species.Consider valuable reactant species.If SPC < 99 %, then consider recycling.If SPC < 99 %, then consider recycling.

How does excess reactant effect recycle structure? How does excess reactant effect recycle structure? Size of Recycle Loop Size of Recycle Loop

HH22 : Toluene = 5 : 1 : Toluene = 5 : 1

Reasons for multiple reactors:Reasons for multiple reactors:Approach to equilibriumApproach to equilibriumTemperature controlTemperature controlConcentration controlConcentration controlOptimization of conditions for multiple reactions.Optimization of conditions for multiple reactions.


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Other Issues on Recycle (cont.)Other Issues on Recycle (cont.)

Do we need to purify prior to recycling ? Do we need to purify prior to recycling ?

Is recycling of Is recycling of inertsinerts warranted ?warranted ?

Can recycling an unwanted inert material push Can recycling an unwanted inert material push equilibrium to the right? equilibrium to the right?

Gasification of coal Gasification of coal –– COCO22 recyclerecycle ..


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Other Issues on Recycle (cont.)Other Issues on Recycle (cont.)

Can recycling an unwanted inert control reaction ?Can recycling an unwanted inert control reaction ?

CO2 in CO2 in GasifierGasifier

Phase of Recycle Stream?Phase of Recycle Stream?


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Example 2.6Example 2.6Illustrative Example Showing the Input/Output and Recycle

Structure Decisions Leading to the Generation of Flowsheet Alternatives for a Process.

Consider the conversion of a mixed feed stream of methanol (88 mol%), ethanol (11 mol%), and water (1 mol%) via the following dehydration reactions:


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Example 2.6 (cont.)Example 2.6 (cont.)Components in reactor effluent listed in order of

decreasing volatility

Ethylene (C2H4)Dimethyl ether (DME) Diethyl ether (DEE) Methanol (MeOH)Ethanol (EtOH) Water (H20)


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Example 2.6 (cont.)Example 2.6 (cont.)Azeotropes

• DME - H2O (but not with significant alcohol present) • DME – EtOH• DEE - EtOH• DEE - H2O • EtOH - H2O

The mixed alcohol stream is available at a relatively low price from a local source ($0.25/kg). However, pure methanol ($0.22/kg) and/or ethanol ($0.60/kg) streams may be purchased if necessary. The selling price for DME, DEE, and ethylene are $0.95/kg, $1.27/kg, and $0.57/kg, respectively. Preliminary market surveys indicate that we can sell up to 15,000 tonne/yof DEE and up to 10,000 tonne/y of ethylene.

For a proposed process to produce 50,000 For a proposed process to produce 50,000 tonnes/ytonnes/y of DME,of DME,determine what are the viable process alternatives?determine what are the viable process alternatives?


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Example 2.6 (cont.)Example 2.6 (cont.)Important Points

Because making ethylene is far less profitable. Inaddition, the maximum amount of DEE that the marketcan support is not currently being produced. Therefore,supplementing the feed with ethanol should beconsidered.

Since the main feed stream contains both reactants andan impurity (water), separation or purification of the feedprior to processing should be considered.

In order to minimize the production of by-products(ethylene), the selectivity of the DEE reaction should beoptimized.


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Course Outcomes Course Outcomes

Upon successful completion of this course, the student will be able to:

Understand the process flow diagrams of a chemical process.

Understand and justify the process conditions

Be able to use heuristics in process design and analysis


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Course Outcomes (cont.) Course Outcomes (cont.)

Estimate capital investment.

Estimate manufacturing cost.

Understand engineering economics and perform profitability analysis.

Be familiar with using simulation for equipment design.

Documents

CHE 425 Engineering Economics and Design Principlesfaculty.kfupm.edu.sa/che/alamer/ChE_425/CHE_425_Ch2.pdf · Prof. Adnan Alamer Chemical Engineering Dept., KFUPM. 1 CHE 425 Engineering