CHE 372 Project Report

8/10/2019 CHE 372 Project Report

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Ammonia Synthesis

by

Mahek

Sarah Lipp

GJ Sequeira

Submitted to

Dr. Thomas Edison

Chemical Engineering 253M

McKetta Department of Chemical Engineering

Cockrell School of EngineeringThe University of Texas at Austin

Fall 2014


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Ammonia Synthesis

Abstract

[Add the text of your abstract here in one paragraph. Read the information provided onthe website to learn how to write this important section. Do not indent the first line, andsingle-space the text. Your abstract should include the following:

* the purpose or principal objectives of the experiment

* the methods employed* quantitative results

* conclusions

Do not include illustrations. Make sure the abstract is self-contained and that it includesno information or conclusion not stated in the report. Keep the length to 250-500 words.]


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Contents

[NOTE: Check the page numbers LAST to make sure they conform to the placement ofyour major headings, tables, and figures.]

Introduction #

Methods #

Safety #

Sample Calculations #

Results #

Conclusions and Recommendations #

Appendices [List appendices as subheadings below.][Appendix 1. (data)] #[Appendix 2. (questions)] #[Appendix 3. (supporting material)] #

References [use noodleBiB and APA format] #

List of FiguresFigure 1: [Title of Figure] #Figure 2: [Title of Figure] #

List of TablesTable 1: [Title of Table] #Table 2: [Title of Table] #


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[Your title]

Introduction

Ammonia synthesis is common in industrial applications. The basic reaction for

ammonia synthesis is described by the reversible reaction

This reaction favors formation of ammonia at high pressures and low temperatures. In

order to optimize the process, these two parameters must be manipulated. Most processes

utilize a syngas feed stream flowing into a catalytic converter bed. We used a three stageammonia synthesis process. The goal of simulating an ammonia synthesis process is to

find the optimum operating conditions for a three stage, adiabatic, plug flow reactor

(PFR). The optimum conditions will be determined by creating adiabatic and equilibrium

operating lines at three different pressures; the temperature and conversion of the

reactants will be determined based on an input feed temperature and constant feed flowrate. In addition, to model realistic applications more closely, cold-shot cooling and inter-

stage cooling between stages are simulated to maximize the conversion while minimizingthe amount of catalyst needed. No recycle stream is included in the simulations in for

ease of calculation. All cases will be compared and an optimal feed temperature, reactor

design, process design, and weight of catalyst will be determined.The synthesis gas from the converter is composed of hydrogen, nitrogen,

ammonia, methane, and argon. Methane and argon are inert gases in the feed in order to

absorb heat. The composition of the feed is

Figure 1: Composition of Synthesis Gas Feed

Synthesis of ammonia requires large activation energies in order to occur

spontaneously. In order to decrease the activation energy, a triply promoted (K2O-CaO-Al2O3) iron-oxide catalyst is used. The catalyst is granules about 6-10mm in diameter. Its

associated physical properties are

Figure 2: Physical Properties of Catalyst


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It is important to note that the catalyst is deactivated at operating temperatures above

985F, which puts a constraint on the reactor design. Furthermore, the catalyst can bepoisoned by hydrocarbons, such as lubricating oils and olefins. These species can crack

and plug pores in the catalyst. The model used does not incorporate the influence of

poisons.Although the overall reaction is presented, the rate of the reaction follows themechanism which consists of the adsorption of nitrogen and hydrogen, and the surface

reaction of intermediate NH, NH2, and H compounds in order to facilitate the desorption

of ammonia.

Figure 3: Reaction Mechanism

This mechanism influences the reaction rate, and consequently the amount of catalyst.

The reactor vessel design will be modeled similar to industrial applications, which

involves a reactor that is split into portions of catalyst and heat exchanger areas.

Figure 4: Reactor Vessel Design Basis

The rate and design equations associated with the vessel are used to calculate the weightof catalyst and heat requirements at each temperature, pressure, and design specification.

These calculations are too rigorous to be performed by hand, so POLYMATH is used for

ease of calculation. We expect the direct contact quench will provide a better coolingalternative than indirect (heat exchanger) cooling. A combination of the two may

improve conversion at each stage.


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Methods

The base case design algorithm involved using the rate equation based on theactivities of each species

For the sake of preliminary calculations, the effectiveness factor is unity. The rate

equation is used to determine the equilibrium lines by setting the left hand side equal to 0.The adiabatic operating lines for a single bed are determined by solving the design

material and energy balances simultaneously.

The specific heat capacities and enthalpy of reaction are assumed to be temperature

dependent. The heat capacities and enthalpy relations are provided in Appendix 1.

The inter-stage cooling method uses the same equations for each stage. However,between stages, the stream temperature is cooled using a shell and tube heat exchanger

embedded in the reactor vessel. The heat consumption is governed by the equation

The heat lost from the incoming stream is a simple heat transfer equation

where the heat capacity is still the sum of the individual heat capacity multiplied by theflow rate of each component. These heat capacities are still temperature dependent. The

inlet temperature to the next stage is input to a desired value.

The next design case involves cold shot cooling. The design equations remain thesame, but the temperature of the inlet to the subsequent stages changes based on the

outlet temperature of the preceding stage and the temperature of the cold shot stream.

The heat capacity dependency is still applicable; however, the inlet temperature to thenext stage is determined based on the fraction of initial feed taken from the initial feedstream. In addition, the composition of the feed changes; this behavior differs from the

constant feed composition between stages in inter-stage cooling.


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Relevant equations

[Give a brief description of the experimental apparatus. Please cite the lab manual andonly specify any modifications to the standard procedure. You may reference a fuller,

illustrated description in the appendix if you choose. Give distinctive features and critical

dimensions. What did you measure and how?]

Safety

[Describe the important safety issues that need to be considered when carrying our theexperiment you described above. There are some safety issues that are common to

several of the experiments and some that are unique to each. Both the general and

specific safety considerations should be described]

Sample Calculations

[Include a brief discussion of the theoretical basis of this experiment. This section shouldlist the equations that represent the theoretical result. The reader will refer to this section

to learn how the data were reduced and how the quantitative results were derived. This

section is expected to represent the result of a group effort in all of your reports and it canbe identical in the reports of each member of your group. ]

Results

Three operating pressures are simulated. The three adiabatic and equilibrium lines

and their corresponding equations are presented below

Three Pressure Ad and EquilLines

At higher pressure, the conversion is greater. However, the heat required to operate thevessel adiabatically is

Important points to consider:

The size of the reactor for a single vs three stage

What temperature do you want to go down to between stages

The heat transfer area needed for the exchangers

Inclusion of recycle and how it would affect the results

Why the conversion is so low- comment on thermodynamic limits and why 21%conversion is the maximum amount possible


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Each of the laboratories has some unique and important safety issues that need to be

considered and there are some safety issues that are common to several of theexperiments.

[Begin this section with an overview, summarizing the key results. Next, present yourdata in figures or tables. (See FAQs for instructions in preparing figures and tables.)Specify what data you are presenting, how you analyzed them, and what you concluded

from your analysis. Compare your results to the theory, and discuss the implications.

Remember to maintain consistency with the Methods section and the Sample calculationsand do not introduce new theory here.

Be sure to discuss this section with your TA to insure that you know the specific issues

for each experiment.]

Conclusions/Recommendations

[Conclude with a summary of the most important conclusions you developed in the

Results section. The conclusion should not introduce new information. You are restating

important information succinctly both for emphasis and convenience to your reader.]

Appendix 1.

Figure A1: Fugacity Coefficients

Figure A2: Effectiveness Factor

Figure A3: Enthalpy of Reaction

Figure A4: Heat Capacity of Ammonia


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[Put your data in Appendix 1. Some experiments may require only one appendix for raw

data. Others may need several. Multiple appendices should be labeled in sequential

numbers (Appendix 1., Appendix 2., etc.), and each new appendix begins on a new page.If you have only one appendix, just title it Appendix, not Appendix 1. Single-space

the text of the appendices.]


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Appendix 2.

[If the laboratory write-up includes questions for discussion, please provide yourresponse to those questions in Appendix 2.]


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Appendix 3.

[Appendix 3. any other supporting documentation for your report]


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References

[All sources cited in the text and appendices should be included in a list of references.Use NoodleBib to generate your list of references according to APA style. You may

single-space the text of the references themselves, but add a space between each

reference. ]

Appendices

[List the equations that represent the theoretical result. (See the Writing Websites FAQs

for how to handle numbers and equations.) Provide a complete sample calculation that

allows the reader to follow your analysis of the data. Again, in the Results section, youwill compare your results to the theory, so prepare your reader for that discussion here.]

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CHE 372 Project Report