CHEMCAD

 

This is a CHEMCAD assignment.

 

A proposed synthetic fuel refinery will separate 4200 kg/h of a synthetic crude oil into a gasoline precursor, a kerosene fuel, a heavy oil stream, and a heavy residual. You can assume that for this model, the synthetic crude oil contains 12.5 wt% n-heptane, 37.5 wt% n-dodecane, 18.4 wt% n-octadecane, and 31.6 wt% n-pentacosane.The stream enters the separation system at 1 atm and 65°C.

 

Your job is to develop a separation sequence that will recover 99% of each of these hydrocarbons into a stream that is at least 99 wht% pure.  Your goal is to minimize the total annualized cost of the system; a simplified cost model is given on the next page.

 

Please note the following limitations on your design:

 

  1. You will simulate your process on CHEMCAD. A list of additional CHEMCAD simulation blocks you may use is given on the next page.  Do not introduce new chemical species,use liquid-liquid extractors, or use other unit operations without prior approval.

 

  1. No stream in your process can be cooled below 25°C.

 

  1. No distillation column in your process may contain more than 40 equilibrium stages.

 

  1. You will need to cool the condensers and provide heat to the reboilers.

 

Note that this is an open-ended problem: there are many flow sheets that will meet the design criteria. Some suggestions for tackling this project are attached.

 

 

 

Submit your design and analysis in a brief, typed, formal report[1] that describes the following elements:

 

  1. An analysis of the thermodynamics of the problem.
  2. A design that meets the project criteria given above.
  3. A flow sheet of your process.
  4. A discussion of how your CHEMCAD simulation. Note that you must complete a rigorous simulation of every distillation column in your process, using either the TOWR, TOWR+, or SCDS module.
  5. Plots showing how the liquid mole fractions of every species vary within each tower.
  6. A unit operations list, giving the key dimensions and operating parameters[2]of each unit operation.
  7. A stream table listing for each stream the temperature (in ˚C), pressure (in atm), vapor fraction, and molar flows (in kmol/h) of each species.  The table should be in the following format:

 

Stream (1) (2) (3)
Pressure (atm)      
Temperature (˚C)      
Vapor Fraction      
Liquid Fraction      
Total Flow (kmol/h)      
Component Flow Rates(kmol/h)      
n-heptane      
n-heptadecane      
n-octadecane      
n-pentacosane      

 

 

 

 

 

Allowed CHEMCAD simulation blocks:

 

Feeds  

Products

Process Streams
Mixers  

Dividers (Pipe Ts)

 

 

Controllers

Flash #1 (VLE equilibrium stage, LL equilibrium stage, or VLLE equilibrium stage)  

TOWR

TOWR+

SCDS

 

Distillation Blocks

Others with Instructor Permission

 


 

  1. Report Outline.

 

Your report must contain the following sections:

 

  1. Title page.This is one page with a descriptive title, date, course name, list of authors, and an executive summary. The executive summary should be 150-250 words long (according to MS Word ‘Word Count’).

 

  1. Your introduction should

 

  1. Describe the design problem clearly and completely and in enough detail that a student in a separations class at another university would clearly understand the assigned problem.
  2. Demonstrate a thorough understanding of the assigned task and the larger context of the problem.

 

  1. Background and Design Strategy. This section should
    1. State how phase equilibrium was modeled and explain why your thermodynamic modeling approach is appropriate for this particular system.
    2. Provide appropriate figures and diagrams
    3. Summarize your design strategy in enough detail that a student in a separations class at another university would be able to explain, defend, replicate and extend your work.
    4. A discussion of any alternative designs you considered and how you chose between them.

 

  1. Process Design. This section should contain
    1. A process narrative that walks the reader through your process
    2. A flow sheet of your process
    3. A stream table that uses the format given earlier, and lists the temperature, pressure, phase(s), flow rate, and contaminant levels for every stream in your process.
  2. A unit operations table listing all process equipment needed for the design and giving the key specifications for each piece of equipment.
  3. A discussion of any optimization or sensitivity studies you performed, described in enough detail that a student in a separations class at another university would be able to explain, defend, replicate and extend your work.

 

  1. This section should assess the quality of the final design

 

  1. Summary and Conclusions.This section should summarize the design approach and the final process design.

 

  1. Future Work. This section should provide recommendations for addressing any weaknesses in the analysis, developing any unexplored but promising design alternatives, and identifying the next step(s) in developing this process.

 

 

 

  1. Simplified cost model

 

Here, is the total installed capital cost of the equipment and  is the sum of the annual costs for steam, cooling water, and electricity.

 

Capital costs:

Unit Total Installed Capital Cost (2012 dollars) Variables
Heat Exchanger $430

 

Q is the heat duty of the exchanger in kW
Column Shell  and Internals  

$1800D

D = column diameter in feet.

H= column height in feet.  H=4N + 10, where N is the number of equilibrium stages within the tower.

 

The capital cost of valves and liquid pumps can be ignored at this stage of the design.Your design should not include a gas compressor.

 

Operating Costs

Heating Utility:

Cooling Utility:

 

You can assume that your plant operates for 8500 h/yr.

You may use the following formula to estimate the column diameter:

Here,

  • D has units of feet
  • V is the internal molar vapor flow rate, in
  • is the molecular weight of the internal vapor
  • is the density of the internal vapor, in units of .

The column diameter should be evaluated at the bottom of the tower and just above the feed plate.  Choose the larger value.

 

  • How to approach this project.

 

This is an open-ended design problem.  Here are some strategies you should consider.

 

  1. Think about the separation problem you are trying to solve. What sort of phase behavior do you expect? Ideal VLE? Azeotropes? Two liquid phases? How can you use CHEMCAD to check your intuition?

 

  1. Use your result from step one to develop a separation strategy. Develop a rough flowsheet that implements your strategy. Estimate the temperature and pressure of each stream. Solve the mass balances around this flowsheet so that you have estimates of flow rates and compositions.  Do not begin CHEMCAD simulation of your process until you have worked out this strategy on paper.

 

  1. Simulate your process on CHEMCAD.

 

  1. Make sure that your report includes all the items requested in the problem statement and meets the report guidelines on the next page.Ask yourself what information you needed to find and assumptions you need to make in order complete the project, and make sure it is all in your report.

 

 

  1. Additional Report Guidelines.

 

  1. The written content of your report should logically organized, clearly and concisely presented, in standard American English, and free of grammatical, spelling, or usage errors.

 

  1. All figures, tables and equations should be professional and formatted correctly. Figures and tables must have accurate and descriptive captions.

 

  1. All sources must be cited correctly within the text and listed in the References section.

 

  1. Text may not be copied or closely paraphrased from another source. The source of a copied figure must be cited in the caption. Table titles should be above the title. Figure captions should be below the title.

 

  1. Please be mindful of some common mistakes and report annoyances.

 

  1. Misuse of terms from optimization.An optimum is the minimum or maximum of a specific objective function. An optimum design might minimize cost or maximize profit.  An optimum design is not simply a practical or reasonable combination of design parameters.  In fact, optimizing some objectives leads to completely impractical designs.

 

  1. Poorly organized reports.Technical reports should be organized by technical subjects, not your personal experience with the project you are reporting upon. If you took a tortuous path to solve a particular problem, don’t drag your reader down that same path.

 

  1. Be concise.

[1] A brief set of report guidelines are attached.

[2] You should provide enough information that a student who does not have access to CHEMCAD could size and cost your equipment.  Providing temperature, pressure, heat duty, reflux ratios, etc.is appropriate.   “Flash Mode 1” is not.

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