Rev: Sept 5 08
3rd Year Integrated Design Problem 2008
You should expect that as in all design problems, the document below is incomplete in
terms of the information that will be required to solve the problem. Making engineering
decisions and assumptions is an integral part of this assignment. With the exception of
the 1st deliverable, all work will be done in groups of four.
The Cost of Synthesis Gas Conversion and Purification to Hydrogen
Synthesis Gas is a mixture of CO2, CO, H2, and H2O. It has a multitude of uses which
include energy generation via gas turbines, as a precursor to a variety of chemicals
including ammonia for fertilizer, ubiquitous methanol, or liquid fuels (see Fischer
Tropsch) for transportation. It is usually generated in Canada from “gasification” of
natural gas, but elsewhere in the world gasification of coal is a common raw material.
However, for a number of years the use of biomass as raw material has been of great
interest since it holds the promise of reducing greenhouse gases and the dependency of
foreign energy sources. In this project, you will only be involved with a part of the
overall process.
Figure 1 presents the scope of the project. It represents a preliminary process for shifting
the ratio of CO2, CO, H2 and H2O and purifying it to obtain hydrogen. This is typically
important in processes that utilize a large amount of H2 (i.e. ammonia , refinery
hydrotreating, fuel cells). The reaction that occurs is:
CO + H2O  H2 + CO2 (exothermic)
The equilibrium of the reaction changes with temperature. Lower temperatures favour
the production of H2, but higher temperatures favour higher reaction rates and therefore
smaller less costly reactors (when you take into account the cost of the vessel and the
mass of catalyst).
Our situation is that H2 is desired (as would be the case if a fuel cell were to be powered
from a biomass source). A preliminary design is being provided to you (fig. 1), and as
you complete the tasks listed below, you will be making recommendations for
improvements to the design.
A stream of syngas at a rate of 10,000 kg/hr (for the communication course you may
suggest other capacities, but the plant will be designed for 10k) is being fed to our
process unit. Exiting our process is a pure stream of H2 and a second stream comprised
of the other components (with some unintended H2). Your ultimate goal is to present
the cost of manufacturing the hydrogen for a plant that for economics analysis will
operate for 10 years. Since we do not have the syngas costs, you may calculate the
‘upgrading’ price of our syngas.
Good design process follows a route of background research and understanding the
constraints of the problem followed by idea generation and evaluation. It’s important to
understand the intent of the design (business and technical), and how it fits into the larger
picture, so that you may make better informed design decisions.
Process Details and Deliverables
1) [for CHEE 360, Technical Communications II] Write a concise (less than 3 page)
introduction to this design problem that summarizes the major business issues and
the technical constraints or optimizations you feel are necessary. This individual
report will require research and planning. Due in Week 4, the material can serve
as the background section of your final (group) project report and will also be the
basis of a CHEE 360 oral group presentation (Week 5-7; exact dates to be
announced) in which all members of your group will be asked to speak to an
audience knowledgeable in technical and business aspects of the process. You
must convince them that you understand the big picture and have a firm grasp of
the technical and business challenges, and that you have a plan to complete the
project on time and with sufficient quality in your design. Only students
registered for CHEE 360 need to complete this section. (For further details, see
http://appsci.queensu.ca/courses/engcomm/CHEE360/assignments/index.php and
see the slides from the project introduction)
2) The process includes two packed-bed reactors operating in series. As part of
CHEE 321, you will explore the tradeoffs between high temperature (faster
reaction rates) and low temperature (higher equilibrium conversion) operation.
You will also learn how to estimate the pressure drop caused by the catalyst
packing, and examine reactor energy balances. This understanding will be useful
as you consider the tradeoffs between capital and operating costs of the overall
process. A few hints are in order:
a. The Ergun equation can be used to determine pressure drops of fluids
passing through packed beds. It is helpful to know that as you increase the
diameter of the bed, the velocity of the gas decreases, thus the pressure
drop decreases. Pressure drop is an important consideration because of the
recycle (and the need to keep the H2 at high pressure).
b. A part of your report should document the physical dimensions of the
reactors. These dimensions are something you need to determine (versus
the preliminary values in the model provided to you). The reaction is an
equilibrium one, and thus a mixture of reactants and products leave the
two reactors. Upon separation, the CO and H2O is recycled back to the
reactor feeds. A compressor will be required to offset the pressure drop
through the beds and its size and energy requirements are important in the
overall economics of this process.
c. The reactor design should consider costs (operating and capital). You can
assume its tan-tan (straight side) Length to Diameter ratio is 1.5:1 (but
other values are acceptable).
3) The Flash System: The method for separating CO2 from the syngas is commonly
done by absorbing or reacting the CO2 with a liquid and allowing the CO, H2,
and H2O to pass through. CO2 is then separated from the absorbent. One
industrial process for separating CO2 is marketed by Dow Chemical as Selexol
(other processes utilizing amine solutions such as MDEA also work). Selexol is a
physical absorbent and therefore decreasing the temperature and increasing the
pressure causes CO2 to dissolve into the liquid. Decreasing the pressure allows
the solvent to degas (in the case of amine solutions a reaction occurs and heat is
required to reverse the reaction to remove the CO2). Our process utilizes the
physical absorbency principle, but there remains some unanswered questions at
this time that require answering:
a. Determination of Binary Coefficients: The model currently uses the
UNIQUAC property package which assumes the fluids are not ideal. The
property package (thermodynamic method) utilizes binary coefficients to
calculate the gas solubility. The binary coefficients for the component
mixtures we are interested in were not in the database. Instead, they have
been generated theoretically via the UNIFAC method. You should
validate the predicted solubility vs the manufacturer’s data (see attached
file). If the solubilities are not correctly calculated you should modify the
binary coefficients accordingly.
Fig – finished tutorial model
b. The number of flash drums and their operating pressure is yet to be
confirmed, as is the recirculation rate of Selexol. You should analyze the
current design and develop recommendations for improvements.
4) You will be required to provide the design and costs for the heat exchangers.
Two methods will be utilized.
a. Provide a detailed design for the heat exchanger that cools the
recirculating absorbent (“Selexol Cooler”). The detailed design should be
for a shell and tube heat exchanger and include the shell diameter, number
of tubes, baffle spacing, etc.
b. Provide a quick sizing for the other heat exchangers (given heat transfer
film coefficients, determine the surface area requirements).
Assume the values for flow and pressure etc. that are in your optimized
simulation.
5) You should provide a short analysis of the process hazards that are present in this
process.
6) You will need to explain your design through a process flow diagram that shows
the equipment and major control loops and a short description of how design
works.
Attachments: Included with this assignment is a UNISIM simulation file.
The Final Report:
The final project report will be marked for CHEE 321, CHEE 311 and CHEE 318; a
portion of your grade will be course specific, and a portion will be assigned to your
overall design and discussion. The report will eventually be marked, after a round of
feedback and individual editing, for CHEE 360 in the winter term. (CHEE 360 in the
winter term also includes an individual presentation based upon your project.) The report
will be submitted electronically (format to be confirmed later) to the applied science
communication course website, as well as by hardcopy (5 copies).
The report should be as concise as possible (< 15 pages in 12 point Times New Roman
font, not including appendices). It should have the following content: (This list is
preliminary, and may be modified later in the semester.)
1. An executive summary (½ to 1 page)
2. Project background – problem definition and brief summary of background
research
3. A process flow diagram (complete with the major control loops) and brief
description explaining how the process works.
4. Identification of process hazards.
5. The major assumptions, a summary of conclusions and findings of your work
(including the financial analysis), leaving most of the calculations for
appendices.
6. (Optional) Recommendations for future work.
7. Appendices of related engineering calculations, clearly stating all
assumptions. Calculations should be complete and written so they can be
followed by someone other than yourself. Indicate how you provided quality
assurance in your calculations.
Fig 1: Preliminary Process Flow Diagram