Tentamen spm4352
August 23, 2010, 9.00 – 12.00 h
This exam consists of 4 questions, each are worth 25 points.
Read the questions carefully. Answer concisely.
It is not allowed to consult books, or notes during the exam.
Make assumptions and clarify them if you need to.
Good luck!
1. Black liquor gasification1 [25 pt]
CO2 neutral biofuel is expected to replace fossil fuel when increasingly strict targets for greenhouse gas emission reductions are to be reached. Since biofuel is a limited resource, it is important to use it efficiently.
The kraft process (also known as kraft pulping or sulfate process) describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers. The process entails treatment of wood chips with a mixture of sodium hydroxide and sodium sulfide, known as white liquor, that break the bonds that link lignin to the cellulose.
The most common way to produce the steam necessary for kraft pulp mill operation is a Tomlinson recovery boiler fired with the wood residue (black liquor) that remains after the fibres for paper production have been removed. The recovery boiler is also used for the recovery of spent cooking chemicals for reuse in the pulping process. Process developments are expected to result in an energy surplus for future pulp mills. This surplus can be used in different ways. One option is to use the excess steam from the Tomlinson boiler for power generation in a condensing turbine. Another option is to extract lignin from the black liquor and export it as a fuel. It is also possible to gasify the black liquor to obtain an energy rich syngas with a high content of CO and H2. This gas can be fired in a gas turbine combined cycle CHP unit with a high power-to-heat ratio or it can be used for synthesis of commercial fuels such as hydrogen or methanol.
The gasification process has been studied for many years and a number of pilot plants have been successfully operated. A process flow diagram is attached at the end of this exam.
a) Study the PFD. Draw the input-output diagram.
b) Your colleagues state that the second option mentioned in the description above (lignin extraction) is a more feasible process than gasification. Use Douglas’ hierarchy to design a process for such lignin extraction. Make assumptions where needed.
c) Back to the gasification process. The gasifier is currently operated as a fluidized bed and the designers decided that the reactor can be modeled as an ideal PFR. Give three reasons why any simulation results using an ideal PFR model will differ from real life measurements to the reactor. In one of your argumentations, use the (structure of the) design equation (ln (CA / CA,0) = kτ).
d) There are several separation units in this PFD. Identify at least 4 of these units and for each unit indicate the following items, follow the example (cannot be found in PFD!) in the table below:
- what is separated from what
- possible type of separation process (indicate a feasible one, that matches with the icon in the PFD)
Unit label (if any) Separates Mechanism
Column T5 Benzene toluene Distillation by boiling point difference
e) The PSA unit on the flowsheet is a PressureSwingAdsorption unit. Explain in detail, accompanied by clear system drawings how a PSA unit works. Also explain what gas the “off-gas” is.
f) There is one clear example of heat integration in this flowsheet. Where is it and what does it do? Argue whether or not you think a full pinch analysis will yield other integration options.
1 From: E. Andersson, S. Harvey, System analysis of hydrogen production from gasified black liquor, Energy, Volume 31, Issue 15, December 2006, Pages 3426-3434
2. Natural gas cleaning [25 pt]
Natural gas, coming from a well, contains a number of contaminants, e.g. heavier hydrocarbons, sulfur, carbon dioxide, etc. Before the gas can be piped, it needs to be cleaned and brought up to specifications with regard to its heating value, purity and other requirements. A natural gas processing plant therefore consists of a number of different separation units. The first step is usually the removal of sulfur from the sour gas. Many different processes can be used to this extent, and historically
“amine treating” is the most common process; amine is used to ‘wash’ the gas, after which the amine is recovered (stripped) by using steam to extract the sulfur.
More recently, membrane technology has become technically and economically attractive as an alternative to amine treatment. Added advantage is that the membrane is also able capture carbon dioxide.
The designers of a new gas treatment unit still consider both options (amine and membrane) at this stage of the design process.
A laboratory has done the following experiment with gas coming from the well:
1. mix 1 mol of sour gas (natural gas with sulfur, G1) with 5 mol of amine (A1) 2. shake vigorously
3. let the mixture settle for 5 minutes
4. separate the amine phase (A2) from the gas phase (G2) and analyse sulfur contents.
Results:
Phase Total Sulfur (mmol) A1 32 A2 712 G1 858 G2 178
a) Define and calculate the equilibrium constant. If you need to make assumptions, state them clearly.
b) What would happen to the value of the equilibrium constant if the lab had used 10 moles of amine?
c) Draw/sketch the McCabeThiele diagram if the following extra data is given for the actual gas treatment plant. Make sure you label the axes, curves and points; make assumptions if needed. How many equilibrium stages are needed for this separation?
Sour gas flow 100 moles / s Target purity gas 100 mmol S/mol Amine start purity 10 mmol S/ mol
Amine flow 50 moles / s
d) The designers wonder if they can optimize the cost of the amine treatment plant by varying the amine flow. Explain what the impact of this variable is on the total cost of the unit.
e) How would you determine graphically the minimal amount of amine that can still achieve the cleaning of this gas? Estimate this amount using your McCabeThiele diagram.
Membrane technology is a competitive technology and the designers consider this as an alternative for the amine treatment unit
f) Discuss and compare the various cost elements of the amine unit and membrane technology. Estimate what the main cost drivers will be for both technologies.
There are two membrane units on the market that will clean sour gas at steady state operation. They are not equally reliable as reflected in the following data:
Membranes’R’Us Clean’up
Capacity: 35 moles / s 40 moles / s
Capital Cost: € 73,000 € 50,000
Availability: 90% 70%
g) The designers want to minimize their investment cost. Calculate what company you would want to do business with. If you need to make assumptions, state them clearly.
3. Going Bio – CO2 free Energy for TU Delft [25 pt]
First read the text and drawing on page 5!
a) Create a schematic drawing of a bio-cogeneration facility to answer below questions b-d
b) Use your schematic to describe and explain the basic steps of the Rankine- or modified Carnot-cycle in this cogeneration facility.
c) Explain the differences in conversion efficiency between this cogeneration facility and large-scale (state-of-the-art), coal and gas-fired stand-alone power plants.
d) Use power system design principles to explain why biomass is almost exclusively fired in co- generation facilities.
e) What is/are the main function(s) of the biomass cogeneration system depicted?
The TU Delft strategic plan 2010 proposes to transform the TU Delft Campus and its energy infrastructure to create a climate-neutral campus. Meanwhile, deep-geothermal energy is developed on Campus (20 MW heat at 80oC) and underground heat storage is considered.
f) Argue what design objective(s) and constraints respectively Eneco, one of the local energy companies, would use when planning for a bio-cogeneration facility on and for the TU Delft campus.
g) Develop and draw a generic system diagram (“superstructure”) for Eneco on supply of “energy products” to student-housing, TU Delft offices and labs and small business on campus. Your diagram must be useful for
(i) discussion of sources, products and sinks (ii) decision on major system elements of.
(ii) discussion on “system design alternatives” and “feedstock” alternatives.
(iv) select a system boundary system aggregation level that allows you to be as complete as possible, without losing oversight.
h) Briefly explain your diagram per items g (i-iv), and argue what system option ought to be explored further by Eneco.
4. Natural Gas [25 pt]
In Vlissingen, the Port of Rotterdam and Groningen Seaports gas companies have are developing large- scale LNG terminals, which including storage and regasification.
And, with small fields depleted and the Groningen field depleting, the Netherlands is transforming its gas infrastructure to a gas hub, which must enable European players to trade and transport via the Dutch grid.
a) What are the two main gas qualities transported in the Netherlands?
b) What other objectives and constraints must be met by the Dutch gas infrastructure system?
c) Argue where you would locate large-scale air-separation plants producing nitrogen to be added to LNG and gas imported from Russia and Norway to cater for Dutch domestic demand and meet above design objectives and constraints.
d) What does the label “Stranded gas” mean?
e) Explain the options other than LNG for utilizing ‘Stranded gas’ in Trinidad and Tobago (in the middle of the Caribbean) and bring it to the energy-hungry markets in Europe and the US.
f) The stranded gas in Trinidad and Tobago contains environmentally harmful contaminants in significant quantities, i.e. 5 vol. % H2S and 10vol.% CO2 . Explain what system you would elect/advise and where you would situate it to enable supply of clean gas as LNG to the US or European gas markets.
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Figure & Text for Question 3
Going Bio – CO2-free
Using biomass has been advocated as a CO2 free route for electricity generation and for district heating. Utilizing domestically grown biomass would make the EU and the Dutch economy less dependent on foreign coal and gas. Biomass, however, has become the subject of a heated debate, because its use for energy purposes would compete for food; Essent had to abandon its plans for palm- oil fired installations as in South-East Asia rainforest makes way for palm-oil plantations. On the basis of extensive environmental Life-Cycle Analysis (LCA), the Cramer Committee (2007) concluded that biomass is only CO2 free for 30-70%. Meanwhile the Platform Energy Transition advocates the use of
‘Secondary biomass’ only, biomass from non-food related sources.
Below drawing is a process diagram of a biomass fired small –scale cogeneration facility as developed by Wärtsila.
The scheme shows a special reactor where biomass is fed into and combusted with air. The heat liberated during combustion is extracted from the flue gas in a number of heat-exchangers and used to produce medium-pressure steam. Thus it is converted to shaft-power via a conventional Rankine cycle. As indicated, low pressure steam from the turbine is used for district-heating. In this case, waste heat is rejected to the environment via an ‘air-cooler’ or small cooling tower.
At an operating temperature of the biomass reactor of 500 oC the plant generates 5 [MW] electric power to the grid (ambient air temperature (10 oC) and 15 [MW] to the district heating system (water temperature 120 oC. The turndown of the facility is 50%, while the heat to power ratio can be varied between 4:1 to 3:1.
With one such facility, the TU Delft campus could be supplied with all of its electricity demand and half of its winter heat demand by connecting the facility to the existing heat grid on campus.
A typical coal fired plant can be used to co-fire a maximum of 15% biomass. With a typical electric power rating of 1000 MW, a furnace operating temperature of some 1100 oC, its conversion efficiency would be some 45% (LHV). In the Netherlands, several natural gas fired power plants are connected to the grid, of which the newest and largest in the Eemshaven has a power output of max. 2500 MW at a conversion efficiency of 55%.
Fuel
Steam Condensate
Water/liquid Flue gas Ash Air
TO DISTRICT HEATING NETWORK
FROM
DISTRICT HEATING NETWORK
