4) I saperi tradizionali della gondola veneziana
2.4 Un approfondimento della Convenzione sul concetto di comunità: L’Expert Meeting on Community Involvement Community Involvement
Objective:
After reading this unit, you should able to Know what reforming is.Know to produce hydrogen.
Know what steam methane reforming is.
Know the reforming process keywords.
Reading 1. Hydrogen Production
Fuel cell development has seen remarkable progress in the past decade because of an increasing need to improve energy efficiency as well as to address concerns about the environmental consequences of using fossil fuel for producing electricity and for propulsion of vehicles. The lack of an infrastructure for producing and distributing H2 has led to a research effort to develop on-board fuel processing technology for reforming hydrocarbon fuels to generate H2. The primary focus is on reforming gasoline, because a production and distribution infrastructure for gasoline already exists to supply internal combustion engines. Existing reforming technology for the production of H2 from hydrocarbon feedstocks used in large-scale manufacturing processes, such as ammonia synthesis, is cost prohibitive when scaled down to the size of the fuel processor required for transportation applications (50-80 KWe) nor is it designed to meet the varying power demands and frequent shutoffs and restarts that will be experienced during normal drive cycles. To meet the performance targets required of a fuel processor for transportation applications will require new reforming reactor technology developed to meet the volume, weight, cost, and operational characteristics for transportation applications and the development of new reforming catalysts that exhibit a higher activity and better thermal and mechanical stability than reforming catalysts currently used in the production of H2 for large-scale manufacturing processes.
The conversion of hydrocarbon fuels to H2 can be carried out by several reaction processes, including steam reforming (SR), partial oxidation (PO), and autothermal reforming (ATR). Steam reforming involves the reaction of steam with the fuel in the presence of a catalyst to produce H2 and CO. Since steam reforming is endothermic, some of the fuel must be burned and the heat transferred to the reformer
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via heat exchangers. Partial oxidation involves the reaction of oxygen with fuel to produce H2 and CO when the oxygen-to-fuel ratio is less than that required for total combustion, i.e., complete conversion to CO2 and H2O. Partial oxidation can be conducted with a catalyst (catalytic partial oxidation) or without a catalyst (non-catalytic partial oxidation). The reaction rates are much higher for partial oxidation than for steam reforming, but the H2 yield per carbon in the fuel is lower. Non-catalytic partial oxidation requires reaction temperatures above 1000ºC to achieve rapid reaction rates. Although the reaction is exothermic, some of the fuel must be combusted because the amount of heat generated by the reaction is not sufficient to preheat the feed to achieve optimal rates. Recently, there has been an interest in catalytic partial oxidation since it operates at lower temperatures than the non-catalytic route. The lower operating temperatures provide better control over the reaction, thus minimizing coke formation and allowing for a wider choice of materials of construction for the reactor. Autothermal reforming involves the reaction of oxygen, steam, and fuel to produce H2 and CO2, and can be viewed as a combination of partial oxidation and steam reforming.
TASK 1. COMPREHENSION QUESTIONS
Answer the questions below.1. Why can gasoline be considered as the primary sources for hydrogen production?
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2. How many processes can be used to convert of hydrocarbon fuels to H2?
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3. What is steam reforming?
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4.
What is Partial oxidation?...
5. What is Autothermal reforming?
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6. Why do we need to supply heat to the steam reforming reactor? How can we do that?
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7. Is non-catalytic partial oxidation reaction is exothermic or endothermic? If this process is exothermic, why do we need to supply heat to the reactor?
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140 Reading 2. Catalytic reforming
Catalytic reforming is a process whereby light petroleum distillates (naphthas) are contacted with a platinum-containing catalyst at elevated temperatures and hydrogen pressures ranging from 345 to 3,450 kPa (50–500 psig) for the purpose of raising the octane number of the hydrocarbon feed stream. The low octane, paraffin-rich naphtha feed is converted to a high-octane liquid product that is rich in aromatic compounds. Hydrogen and other light hydrocarbons are also produced as reaction by-products. In addition to the use of reformate as a blending component of motor fuels, it is also a primary source of aromatics used in the petrochemical industry.
The first catalytic reforming units were designed as semiregenerative (SR), or fixed-bed units, using Pt/alumina catalysts. Semiregenerative reforming units are periodically shut down for catalyst regeneration. This involves burning off coke and reconditioning the catalyst‘s active metals. To minimize catalyst deactivation, these units were operated at high pressures in the range of 2,760 to 3,450 kPa (400–
500 psig). High hydrogen pressure decreases coking and deactivation rates.
Catalytic reforming processes were improved by introducing bimetallic catalysts. These catalysts allowed lower pressure, higher severity operation: ∼1,380–2,070 kPa (200–300 psig), at 95–98 octane with typical cycle lengths of one year. Cyclic reforming was developed to allow operation at increased severity. Cyclic reforming still employs fixed-bed reforming, but each reactor in a series of reactors can be removed from the process flow, regenerated, and put back into service without shutting down the unit and losing production. With cyclic reforming, reactor pressures are approximately 200 psig, producing reformates with octanes near 100.
TASK 2: TRUE OR FALSE
Decide whether the following statements are true (T) or false (F).
1. Silver-containing catalysts are usually met in catalytic reforming process.
2. The purpose of the catalytic reforming process is to increase the octane number.
3. Hydrogen is one of the main products of this process.
4. First catalytic reforming units were designed with a series of fixed-bed reactors.
5. Semiregenerative reforming units can operate continuously.
6. Hydrogen pressure affects the amount of coke formation during steam reforming process.
141 Reading 3. Steam methane reforming, SMR
The use of hydrogen for petrochemicals, fertilizers and as energy carrier in connection with renewable energy production will increase substantially in the next 5-10 years as even more stringent environmental legislation is enforced. Low sulfur gasoline and diesel fuels will become mandatory and harmful emissions will be reduced drastically. Hydrogen will be required by refiners and specialty chemical manufacturers to meet the global need for cleaner products. In spite of efforts to produce hydrogen by processes involving solar energy, wind energy, nuclear energy and biofuels, fossil fuels remain the most feasible feedstock in the near term, and for commercial scale production of pure hydrogen, steam reforming remains the most economic and efficient technology for a wide range of hydrocarbon feedstocks.
Steam methane reforming, SMR
The steam methane reforming (SMR) process is the most widespread method to generate hydrogen-rich synthesis gas from light carbohydrates. The feed material can be natural gas liquid gas or naphtha. They are converted endothermically with steam into synthesis gas in catalytic tube reactors. Process heat as well as flue gases are used for generation of steam.
It consists of two steps
1. Reformation process
The first step of the SMR process involves a light hydrocarbon reacting with steam at 750-800 oC to produce a synthesis gas or syngas, which is a mixture primarily made up of hydrogen, H2 and carbon monoxide, CO. The desulfurized hydrocarbon feed is mixed with superheated process steam in accordance with the steam/carbon relationship necessary for the reforming process. This gas mixture is heated up and then distributed on the catalyst-filled reformer tubes. The gas mixture flows from top to bottom through tubes arranged in vertical rows. While following through the tubes heated from the outside, the hydrocarbon/steam mixture reacts, forming hydrogen and carbon monoxide according to CnHm + n H2O nCO + (2n+m)/2H2
2. Shift reaction
The second step, known as a Water Gas Shift (WGS) reaction, the CO produced in the first reaction is reacted with steam over a catalyst to form H2 and CO2. This process occurs in two stages, consisting of a High Temperature Shift (HTS) at 350 oC endothermic reaction
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CH4 + H2O CO + 3H2And a Low Temperature Shift (LTS) at 190-210 oC exothermic reaction CO + H2O CO2 + H2
To minimize the CH4 content in the synthesis gas while simultaneously maximizing the H2 yield and preventing the formation of elemental carbon and keeping it from getting deposited on the catalyst, the reformer is operated with a higher steam/carbon relationship than theoretically necessary.
As the process is endothermic, the required heat must be produced by external firing. The burners for the firing are arranged on the ceiling of the firing area between the tube rows and fire vertically downward. The residual gas from the pressure swing adsorption unit as well as heating gas from battery limits is used as flue gas. The flue gas is then cooled down in a convection zone, generating steam.
TASK 3. COMPREHENSION QUESTIONS
Answer the questions below.1. How many steps in steam methane reforming? What are they?
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2. What are the compositions of the syngas?
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3. Where does the reaction between steam and hydrocarbon occur?
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4.
What are the products of the reformation process?...
5. How many stages are there in the ―ship reaction‖?
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6. Why should the reformer be operated with a higher steam/carbon ratio?
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7. Whether we should add heat to the reformation process?
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TASK 4: How to find good keywordsFind the keywords in reading paragraphs 1, 2 and 3.
TASK 5: Summary
In about 5 sentences, summarize the main idea in paragraphs 1, 2 and 3.
TASK 10: Glossary
Search your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 autothermal 2 biofuel 3 efficiency 4 endothermic 5 fertilizer 6 fixed-bed 7 fossil fuel 8 fuel cell 9 fuel processor 10 mechanical stability 11 partial oxidation 12 petrochemical 13 reforming
14 reforming catalysts 15 semiregenerative 16 shutoffs
17 steam reforming 18 syngas