Re-sit exam spm4352
August 17th, 2009, 9.00 – 12.00
This exam consists of 5 questions; the points per question are indicated.
Read the questions carefully.
It is not allowed to consult books, or notes during the exam.
Make assumptions and clarify them if you need to.
Good luck!
1. Amine gas treating [25 pt]
Gases containing H2S or both H2S and CO2 are commonly referred to as sour gases or acid gases in the hydrocarbon processing industries.
A typical amine gas treating process (as shown in the flow diagram below) includes an absorber unit and a regenerator unit as well as accessory equipment. In the absorber, the down flowing amine solution absorbs H2S and CO2 from the up flowing sour gas to produce a sweetened gas stream (i.e., an H2S- free gas) as a product and an amine solution rich in the absorbed acid gases. The resultant "rich" amine is then routed into the regenerator (a stripper with a reboiler) to produce regenerated or "lean" amine that is recycled for reuse in the absorber. The stripped overhead gas from the regenerator is concentrated H2S and CO2. In oil refineries, that stripped gas is mostly H2S, much of which often comes from a sulfur-removing process called hydrodesulphurization. This H2S-rich gas stream is then usually routed into a Claus process to convert it into elemental sulfur. In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide in 2005 was byproduct sulfur from refineries and other hydrocarbon processing plants.
[from: Wikipedia.com]
a) Identify at least five design variables for the design of this process, indicate what the designer’s options could have been and identify the chosen option in the PFD.
b) Execute a brief HAZOP-analysis on the Absorber. Use at least three guidewords and draw meaningful conclusions.
c) Sketch a McCabe Thiele diagram for the Regenerator column. Clearly define what is on the axis (not just say x and y), and what the various lines you drew represent. Clearly list your assumptions.
d) What other qualitative and quantitative data would you need to complete the diagram? Specify for each piece of data why you need it and how it would influence the McCabe-Thiele diagram.
e) In some plants, more than one amine absorber unit may share a common regenerator unit. Explain, by using your knowledge of separation processes and the associated economic trade-offs why this can be a more economic plant design in some cases, than “one regenerator unit to one amine absorber unit”.
f) If you were to redesign an existing amine gas treating unit to accommodate flows with higher concentrations of H2S, what design options would you consider?
2. Paper and pulp industry [25 pt]
In a paper production process, wood and recycled paper is converted into a slurry of water and fibers. The fibers present in ‘fresh’ wood typically contain much lignin (a kind of cellulose), which needs to be removed (bio)chemically, as the lignin will make the paper turn yellow when aging (newspapers turn yellow after a while because the lignin has not been removed). This is done by using an enzyme called ligninase which breaks down the (polymer-type) lignin molecules.
In biochemical reactions, such as this reaction, between a substrate (wood cells) and an enzyme (ligninase) the reaction rate follows a specific pattern as a function of the substrate’s concentration, as shown in the graph below (ignore the Km, Vmax and 1/2Vmax labels). This typical type of kinetics is called “Michaelis-Menten” kinetics.
[source: Wikipedia.com]
The most often used reactor type for enzymatic reactions is a CSTR. The design equation for the CSTR for a first-order reaction is given below:
in which component A represents the substrate ‘wood fibers’ and k follows the reaction rate curve as depicted in the graph above (it is labeled in the graph as V, but this would be similar to k (in s-1).
Assume a first order reaction for this enzymatic reaction.
a) In designing a paper plant, following Douglas’ hierarchy, the third step would be to determine the reactor and recycle structure. For the ligninase reaction section, following the MM-kinetics, often multiple CSTR reactors would be placed in series. Explain why this is a good design, using the MM- kinetics graph and the CSTR design equation (note, it is not necessary to carry out detailed calculations).
b) The first reactor needs to obtain a conversion rate of 50%. Assume that the substrate concentration in the feedstock is 4000 and calculate the required residence time in the reactor.
c) What would be the designer’s main design variable(s) in order to try to satisfy this required residence time?
Consider the following pinch analysis done at the same paper and pulp plant [from: Pinch analysis: For the efficient use of energy, Water & hydrogen; Pulp and paper industry Energy recovery and effluent cooling at a TMP plant, Natural Resources Canada, 2003]. (TMP = thermomechanical pulp).
The analysts considered a small section of the plant only, comprising 2 hot and 3 cold streams:
They created the following composite curve diagram.
d) Explain briefly how above diagram is to be used in a pinch analysis
e) Deduce from the data given in the table and the diagram, what would happen to the minimal heating and cooling loads when the specific heat of the reboiler’s condensate would be much higher than anticipated? What does this mean in practice in the paper plant?
- questions f and g continued on next page –
The analysts also carried out an economic analysis, resulting in the following graph:
f) Explain the overall shape of the curve (ignore the little bumps and dents).
g) Assume you are the plant manager faced with this curve. You have to decide about which ΔTmin the designers must use to design the heat exchanger network, to purchase the equipment, etc. Other interesting projects offered to you by other design teams in other parts of the plant have typical payback times of 10-11 months. What would your decision be for the ΔTmin of this project? Explain your answer.
3. Going Solar – CO2-free Energy for Society? (PART I) [20 pt]
Recently, concentrated solar power or CSP has been advocated as a CO2 free route for electricity generation. Desertec proposes to create a network of CSP’s in the Maghreb (the desert states in Northern Africa that border the Mediterranean) and transport the electricity to the EU via high- voltage, high capacity cables. Desertec may complete between 2020 and 2030, for example, a set of 20 installations with a total output of 50.000 MWe or 15% of EU power needs. Below drawing is a schematic process diagram of a CSP power plant.
[from http://www.volker-quaschning.de/articles/fundamentals2/index_e.html]
The scheme shows the solar collector where solar heat is concentrated and transferred to the circulating medium, which can be special heating oil, liquid lithium etc. The hot medium is circulated through a cascade of heat-exchangers where the concentrated heat is transferred to water/steam. Steam is expanded in a two-stage turbine with intermediate reheat. A ‘simple’ CSP thus consists of a solar heat concentrator, heat transport and transfer and a Rankine cycle where heat is converted into power.
A CSP completed in Spain in 2007 has a peak electric power output of 10 MWe. While its concentrator captures 75% of incoming solar radiation, the overall solar to electric power efficiency during daytime operation is only 20%. The investment costs for the facility amounted to 100 Million Euro’s, 30% of which was spent on the solar concentrators, 70% on the Rankine cycle apparatus.
State-of-the art power plants have single train capacities of up to some 1250 MWe. The economy of scale that can be achieved when increasing the capacity of solar concentrators is limited, however.
a) Heat-to-power conversion is always done via a Rankine- or modified Carnot-cycle. Draw a principle process-scheme schematic of a Rankine cycle
b) Indicate and explain the main energy flows through the Rankine cycle of the above depicted CSP.
c) Calculate an estimate of the temperature (oC) of the medium leaving the solar concentrators in the Spanish CSP
d) The plant cost index in 2008 was 400. Assuming this index increases to an average of 600 between 2020 and 2030, calculate a first +/- 50% “order-of-magnitude” investment cost for any of the CSP installations Desertec is planning. Make suitable but credible assumptions.
e) What other costs will need to be covered by the revenue obtained from electric power sold? How is an estimate of these costs obtained in the very early phases of design?
f) The “Functional Unit Method” could be used to arrive at a somewhat more accurate investment cost estimate. Which functional units would you discern in the depicted system? Explain.
g) Why it is difficult if not impossible to use the Lang Factor method in the very early phases of design?
4. Going Solar – CO2-free Energy for Society? (PART II) [15 pt]
[From http://www.volker-quaschning.de/articles/fundamentals2/index_e.html]
Desertec has argued its network of CSP’s will be an attractive solution for resource constricted, power hungry Europe. They have targeted a number of inland and coastal locations in the Maghreb. They have not decided yet whether to go for peak-load (daytime + evening) power delivery only, or base-load (24 hour constant power delivery) yet.
Above diagram indicates possible modes of operation of a CSP system that could be labeled a ‘complex’
CSP, which could also be labeled a ‘hybrid’ CSP.
[If you have skipped question 3, you may find the information provided there useful to answer this question]
a) Underpin what you consider to be the main function of a network of CSP’s in the Maghreb.
b) Argue what design objective(s) and constraints are relevant for the development of Desertec’s principle design for their CSP’s to be built in the Maghreb.
c) What options do you see for inclusion of system elements in a ‘complex’ CSP design where ‘burning’
fossil fuels, e.g. natural gas occurs?
d) Briefly discuss options for realizing the cooling subsystem in the Maghreb.
e) Including the elements indicated under c) and d) and using the information given in question 3 and this question, develop a generic system diagram (“superstructure”) that envelop ‘basic’ and ‘complex’ CSP system designs for Desertec’s CSP to be built in the Maghreb. Set up your diagram to allow a discussion on
“system design alternatives” and infrastructure and investment planning decisions for the Desertec consortium and relate to the objectives and constraints given in b). Select a system boundary and use a suitable system aggregation level that allows you to be as complete as possible, without losing oversight.
[continued on next page]
5. Gas Infrastructure [15 pt]
A couple of weeks ago, the 50th anniversary of the Dutch Slochteren field discovery was celebrated.
This discovery of what proved to be one of only a few mega-gas fields in the world sparked the development of the Dutch gas infrastructure. Within a decade it led to an almost ubiquitous presence of natural gas infrastructure in the Netherlands. The initial grid and gas-firing equipment was designed for Groningen gas quality (G-gas). Presently, however, 50% of the original reserve in the Slochteren field has been depleted. To maintain G-gas-delivery capacity (Nm3 / hr], large storage and recompression facilities have been built connected to the Slochteren field. In Callantsoog (North- Holland) and Ommen (almost at the Eastern border of the Netherlands) nitrogen production plants have been built to allow feed-in of Hi-cal natural gas from the North Sea and transported from Norway and Russia.
a) Explain briefly why storage and recompression is needed to maintain G-gas delivery capacity of the Slochteren field
b) Discuss system design alternatives for maintaining G-gas delivery capacity, Include storage/(re-compression)/nitrogen mixing and available empty gas fields in your discussion.
G-gas is also exported all the way to Switzerland (800 km) and from thereon to the North of Italy and South-East of France (another 200-400 km). A recently graduated SEPAM M.Sc suggests to replace this export by Hi-cal gas and to build a nitrogen plant and mixing facility in Switzerland.
She claims this not only will reduce transport (energy) cost but also that this will open the way to remove a capacity bottleneck and export more gas to Switzerland to meet increasing Swiss demand.
c) Do you (dis)agree with the SEPAM M.Sc? Underpin your answer.
Nitrogen is produced from air by cryogenic distillation. For mixing into natural gas of H-cal quality the used nitrogen must be almost pure. Notably water ([H2O]air = 1-5 vol.%) and carbon dioxide ([CO2]air= 380 ppm) must be removed to a level < 5 ppm, because even small amounts will build up in pipeline pockets and may cause equipment damage (corrosion, compressor blade impingement etc.).
d) Would water and CO2 removal be pretreatment or post treatment (i.e. before or after the cryogenic distillation)? Indicate why!
e) What separation systems could one think of to remove carbon dioxide and water from nitrogen or air? Provide concise arguments.
[end of exam questions]
