Exam spm4352 / spm4351 June 24, 2008,

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1 Exam spm4352 / spm4351 June 24, 2008, This exam consists of 4 questions, each are worth 25 points. 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. Total Isomerisation Package [25 pt] Process Flow Diagram on the last page of this exam An isomerisation process takes naphtha (a mixture of light gasoline fractions), and improves the octane number by reacting it with hydrogen. The feed stream must be clean (ie. no excess metals, sulphur or nitrogen) and dry. This is because these contaminants will poison the catalyst. There are several types of isomerisation process, and the Total Isomerisation Package (TIP) is one of them. Others are variations of the same reaction techniques, but licensed by different companies. A Process Flow Diagram of the Total Isomerisation Package is shown on the last page. Study this PFD and answer the following questions. The translated legend is given below: P1 feed pump F9 furnace H17 condensor H2 feed pre heater C10 H2 recycle compressor V18 reflux drum F3 feed furnace F11 furnace P19 reflux pump R4 isomerisation reactor T12 integrated adsorption/ P20 pump desorption column H5 cooler H13 cooler H21 reboiler V6 vapor liquid separator V14 accumulator P22 product pump P7 pump P15 pump H23 product cooler H8 heat exchanger T16 stabilizer (distillation) Stookolie = heavy oil Stream numbers are in ovals, temperatures are in rectangles, and pressures are in circles. Use the drawing on the last page a) Identify at least five design variables for the design of this process, indicate what the options could have been and identify the chosen option in the PFD. b) Execute a brief HAZOP-analysis on the feed furnace F3. Use at least three guidewords and draw conclusions. The isomerisation reactor is depicted as a fixed-bed reactor, i.e. it contains two areas in which the feedstock flows through a solid material, the catalyst bed (R4). c) Assume that you have to make size calculations for this reactor. What reactor model would you choose and why? Discuss your choice, your assumptions and what data are necessary to perform the size calculation. d) There is one distillation column in the process, T16. Assume that the plant manager wishes to increase the purity of the bottom product (Totaal Isomeraat). Design two ways to achieve this. Indicate what the consequences are for the column s main pieces of equipment (numbers 17-22) in terms of material flows, energy flows, and in terms of money. e) The design includes three process furnaces, F3, F9 and F11. Briefly describe the analytical/design procedure you would use to determine whether these furnaces can be eliminated from the design. Exam spm4352 / spm4351, June 24th, /6

2 2. Fischer Tropsch [25 pt] From Wikipedia, the free encyclopedia The Fischer-Tropsch process (or Fischer-Tropsch Synthesis) is a catalyzed chemical reaction in which synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons of various forms. The most common catalysts are based on iron and cobalt, although nickel and ruthenium have also been used. The principal purpose of this process is to produce a synthetic petroleum substitute, typically from coal, natural gas or biomass, for use as synthetic lubrication oil or as synthetic fuel. This synthetic fuel runs trucks, cars,and some aircraft engines. The use of diesel is increasing in recent years. Combination of biomass gasification (BG), to produce syngas, and Fischer-Tropsch (FT) synthesis is considered by some a very promising route to produce renewable transportation fuels (biofuels). The Fischer-Tropsch process involves a variety of competing chemical reactions, which lead to a series of desirable products and undesirable byproducts. The most important reactions are those resulting in the formation of alkanes (alkanes are molecules with structure C n H (2n+2) ; the longer the chain, the higher the boiling point of the alkane). These can be described by chemical equations of the form: (2n+1)H 2 + nco C n H (2n+2) + nh 2 O where 'n' is a positive integer. The simplest of these (n=1), results in formation of methane, which is generally considered an unwanted byproduct. Process conditions and catalyst composition are usually chosen, so as to favor higher order reactions (n>1) and thus minimize methane formation. Most of the alkanes produced tend to be straight-chained, although some branched alkanes are also formed. In addition to alkane formation, competing reactions result in the formation of alkenes (C n H 2n ), as well as alcohols (with OH groups) and other oxygenated hydrocarbons. Usually, only relatively small quantities of these non-alkane products are formed, although catalysts favoring some of these products have been developed. Generally, the Fischer-Tropsch process is operated in the temperature range of C. Higher temperatures lead to faster reactions and higher conversion rates, but also tend to favor methane production. As a result the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors formation of long-chained alkanes both of which are desirable. Typical pressures are in the range of one to several tens of atmospheres. Chemically, even higher pressures would be favorable, but the benefits may not justify the additional costs of high-pressure equipment. A variety of synthesis gas compositions can be used. For cobalt-based catalysts the optimal H 2 :CO ratio is around Iron-based catalysts can tolerate significantly lower ratios. This can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H 2 :CO ratios (<1). A variety of catalysts can be used for the Fischer-Tropsch process, but the most common are Cobalt, Iron, and Ruthenium. Nickel can also be used, but tends to favor methane formation. Cobalt seems to be the most active catalyst, although iron also performs well and can be more suitable for low-hydrogencontent synthesis gases such as those derived from coal. a) Design the Fischer Tropsch process, using Douglas hierarchy and rules of thumb. Assume that you want to produce at least one rather pure C 3 H 8 flow. Clarify assumptions. b) Discuss three trade-offs that the designer may run into while designing this plant. c) What is a purge? Explain whether or not your process needs a purge. d) The feed for the Fischer Tropsch process has to be dry. Sketch a McCabe-Thiele diagram for a possible drying process. Explain what the various curves mean. Also draw an acutal piece of equipment that follows from your sketched McCabe-Thiele diagram. e) What would happen to the design of the process when the designer would apply a life cycle costing approach? Use at least the concepts of MTBF and maintenance in your answer. Exam spm4352 / spm4351, June 24th, /6

3 3. Going Nuclear - Clean Energy for Society [25 pt] First read the text and drawing on page 5! a) Briefly describe the basic steps of the steam-cycle (Rankine- or modified Carnot-cycle) b) Explain the differences in conversion efficiency of the Dutch (state-of-the-art) nuclear, coal and gasfired power plants. c) Use power system design principles to explain why nuclear fission never has been used for cogeneration systems. d) Draw a schematic of the complete system around nuclear fission (the infrastructure to use Uranium-235). e) What is/are the main function(s) of this system? Argue what design objective(s) and constraints respectively the Dutch government would use. f) Develop a generic system diagram ( superstructure ) for the Dutch government on supply of energy products to industry and households. Your diagram must be useful for discussion of sources, products and sinks, decision on major system elements of a new Dutch energy infrastructure. Set up your diagram to allow a discussion on system design alternatives and feedstock alternatives. Select a system boundary and use a suitable system aggregation level that allows you to be as complete as possible, without losing oversight g) By law, the Dutch government must plan and arrange for a 25 year horizon of energy supply. Using your diagram developed under f), discuss what you expect from feedstock alternatives versus system design alternatives and select a preferred fuel/system mix. Use societal design objectives and constraints to underpin your answer. 4. Natural Gas [25 pt] In Vlissingen, the Port of Rotterdam, Amsterdam and Groningen Seaports new large-scale gas-fired cogeneration facilities are planned. All these facilities will use local industry as a continuous, highlevel heat-sink, and export electricity to the grid. They will all depend on a sufficient supply of natural gas. How to ensure on-site availability of quality fuel is of paramount important for the investors, energy companies, as well as what product/market combinations must we develop. a) What is the main characteristic of natural gas that is used in gas burner design? b) Home-owners, small consumers of gas, take the supply of natural gas -anywhere anytime- for granted. Investors in large-scale natural gas fired power plants should not. Use a simple schematic of gas pipeline infrastructure to explain. Natural gas of G-gas quality can be obtained by mixing H-cal gas with Nitrogen (N 2 ) to achieve a 84 vol.% methane, 16 vol.% nitrogen gas composition. Nitrogen is produced from air in an air separation plants. Generally it is considered a waste by-product of oxygen production. It can be transported for decentralized use. In that case, it is stored in insulated nitrogen tanks. The density of liquid nitrogen is some 800 kg/m 3. The owner of a small-size newly discovered gas-field in the Netherlands containing H-Cal gas has brokered a lucrative contract to deliver G-Gas. While the field can produce Nm 3 /hr H-gas, which will decrease after 10 years, the owner has signed a 10-year contract to deliver a base-load of Nm 3 /hr, and each day a peak load for the duration of the equivalent of 2 hours at Nm 3 /hr. He will use an adjacent empty gas-field, where up to 1 Million Nm 3 of gas can be stored, to allow delivery at peak load. c) Calculate the amount of nitrogen that needs to be mixed in the H-gas in base-load operation to achieve G-gas quality. Would you elect for an air-separation plant on location? d) Identify the functions of the system for the delivery of G-gas that will ensure that the owners lives up to his contract. Use these to develop a first conceptual design. Explain your design choices. e) Briefly discuss what system one may use to extract the significant quantities of water from the H-gas. Exam spm4352 / spm4351, June 24th, /6

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5 3. Going Nuclear - Clean Energy for Society [25 pt] Going nuclear has been advocated recently as a CO2 free route for large-scale electricity generation that also would make the Dutch economy less dependent on foreign coal and gas. In the Energy Report 2008 the Dutch government has reconfirmed, however, that no new plans for nuclear power plants will be approved before Energy infrastructure planning and design, however, continues! Below drawing is a process diagram of a simple Nuclear Power Plant of the Light-Water Graphite Reactor, or LWGR type that was in use in Tsjernobyl. The scheme shows that the reactor consists of a matrix of nuclear fuel rods interspersed with control rods for moderation. The latter must control the speed of the nuclear fission reactions. The heat liberated during fission is extracted from the reactor and converted to shaft-power via a conventional steam cycle. In this case, waste heat is rejected to the environment via an air-condensing plant, also known as a cooling tower. The nuclear power plant in Borssele is of another type, a Pressurized Water Reactor (PWR). At an operating temperature of PWR reactor of 320 o C the plant generates 450 MW electric power to the grid (sea water temperature (10 o C). In Borssele, sea water is used as a sink for the power plant s waste heat. The power plant is run continuously at 100% capacity; 33 % of the heat released from nuclear fission is converted to electric power. Spent fuel rods are sent to a special processing facility in Cap- LaHague, France where nuclear waste and useful Uranium-235 are separated. The nuclear waste is highly radioactive for some 200 years and remains moderately radioactive for > years. A coal fired plant is located adjacent to the nuclear plant in Borssele. Its electric power rating is 900 MW. The furnace operating temperature is some 700 o C. The conversion efficiency of this plant is 40% (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 MW at a conversion efficiency of 55%. Exam spm4352 / spm4351, June 24th, /6

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