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3.3 Dimethyl Ether Synthesis 3.3.1 Syngas Clean-up The major components of the syngas at the outlet of an entrained flow slagging gasifier are CO, H2, and CO2. Some N2, CH4 and small amounts of H2S are present. Some amount of char (unconverted carbon) and ash are also contained in the exit flow of the gasifier. Table 26 provides the composition of the raw syngas. Table 26: The composition and components of the raw syngas. Components Mole percentage Mass percentage CO 45.7 0.62683 H2 37.0 0.03659 CH4 1.2 0.00916 CO2 13.5 0.29094 N2 1.9 0.02560 H2S 0.7 0.01084 Syngas quenching The quench ensures that the char and ash particles will be non-sticky to prevent fouling issues. After capture in a filter, these particles will be recycled to the gasifier to increase the carbon conversion efficiency. Among the four main alternatives for quenching, a water quench will be used in the gas cleaning system which uses sensible heat from the syngas to vaporize water. The hot syngas will be totally quenched with water to about 300 o C. Particle removal Wet solids removal systems use water scrubbers operating at a temperature lower than the dewpoint of the gas so that the smallest solid particles can act as nuclei for condensation and ensure efficient operation. Dry solids removal systems use candle filters that can remove all solids from the gas at temperatures between 300 and 500 o C. Above 500 o C, alkali compounds may pass the filters in significant amounts. Below 300 o C, the filters may be blinded of deposits of ammonium chloride (NH4Cl) [10] (C. Higman and M. van der Burgt, “Gasification”, Elsevier, 2003). Sulfur compounds The major part (>90 %) of the sulfur components in the feed are hydrogen sulfide (H2S). Up to 99.8 % of the coal sulfur can be removed in the acid gas removal process. Since the syngas will be used to produce DME, deep sulfur removal will be required to protect the catalyst downstream. Usually, Rectisol physical solvent (methanol as solvent) AGR (acid gas removal) process will be applied. But the relatively low temperature (40 o C) in the AGE process will result in great heat loss in the gas cleaning process. Herein, zinc oxide will be used as sorbent for H2S removal. In this case, remarkably higher desulfurization temperatures of ca. 400 o C will be obtained[101]. 52
Nitrogen compounds Nitrogen from the gasifier can be molecular nitrogen and fuel bound nitrogen, such as hydrogen cyanide (HCN) and ammonia (NH3), which have very high solubilities in water, and may therefore be removed in water scrubbing[102]. Since the main component in the raw is molecular nitrogen not fuel bound nitrogen, we do need to use water scrubbing to remove them. And molecular nitrogen in the raw syngas flow will not be removed. 3.3.2 DME synthesis 3.3.2.1 Characteristics of DME synthesis reactions In the introduction part, we have reviewed the existing commercial technologies of DME production. Herein, due to the syngas H2/CO ratio (~0.81), we are going to employ a single-step reaction to synthesize DME from syngas based on JFE and Air Products & Chemicals technologies[48, 58]. The reactions are shown below: The overall reaction: 3CO + 3H2 ↔ CH3OCH3 + CO2 H = −244.95 kJ/mol (1) In practice, the reactions shown in the formulae (2), (3), and (4) below occur simultaneously: CO + 2H2 ↔ CH3OH H = −90.29 kJ/mol (2) 2CH3OH ↔ CH3OCH3 + H2O H = −23.41 kJ/mol (3) H2O+CO ↔ CO2 + H2 H = −40.96 kJ/mol (4) The two molecules of methanol synthesized from CO and H2 in formula (2) are dehydrated in formula (3) to produce DME. The water produced in formula (3) is recycled as hydrogen in formula (4). In the overall reaction, the H2/CO ratio is 1:1. When H2/CO is not 1:1, the reaction formulas concerning DME synthesis are as follows: 2CO + 4H2 ↔ CH3OCH3 + H2O (5) 2CO + 4H2 ↔ 2CH3OCH3 H = −244.95 kJ/mol (6) Figure 12 shows equilibrium conversion (ideal) of synthesis gas (CO conversion plus H2 conversion) for the three DME synthesis reactions, formula (1), (5) and (6). In each reaction, the equilibrium conversion has its maximum peak where the H2/CO ration corresponds to the stoichiometric value. The maximum equilibrium conversion for formula (1) is much higher than those for formula (5) and (6)[48]. 53
Page 1 and 2: Novel Design of an Integrated Pulp
Page 3 and 4: Table of Contents List of Figures .
Page 5 and 6: List of Figures Figure 1: Price of
Page 7 and 8: 1 Introduction The global transport
Page 9 and 10: 2 Background 2.1 Pulp Mill Backgrou
Page 11 and 12: 2.1.2.2 Chemical Pulping The most c
Page 13 and 14: sodium sulfide and sodium carbonate
Page 15 and 16: process described in the section on
Page 17 and 18: 2.2.1 Low-Temperature Black Liquor
Page 19 and 20: Figure 2: Chemrec gasification proc
Page 21 and 22: Table 3: Average syngas composition
Page 23 and 24: DME than that could be obtained fro
Page 25 and 26: U.S. Patent[58] and European Patent
Page 27 and 28: Figure 4: Topsøe gas phase technol
Page 29 and 30: DME Workshop, Japan (Sept. 7, 2000)
Page 31 and 32: 2.4.1. Fischer-Tropsch Reactors The
Page 33 and 34: kpH 2 pCO Cobalt rate = 2 (2.4.1) (
Page 35 and 36: invention of Fischer-Tropsch in the
Page 37 and 38: Table 7: hydrocarbons and associate
Page 39 and 40: 3 Process Design 3.1 Pulp Mill 3.1.
Page 41 and 42: carbonate is fired in a lime kiln t
Page 43 and 44: lost and the pulp that provides tha
Page 45 and 46: Table 16: General operating paramet
Page 47 and 48: Table 19: Ash analysis of Pittsburg
Page 49 and 50: Table 22: Experimental syngas compo
Page 51: 3.2.5 Process Options and Flexibili
Page 55 and 56: 3.3.2.3 Reactors A single-step gas
Page 57 and 58: Table 27: FT-diesel fuel synthesis
Page 59 and 60: 3.5.2 Power Generation Process Desi
Page 61 and 62: Figure 21: Schematic of biorefinery
Page 63 and 64: ar) reacts with the fuel in the com
Page 65 and 66: Figure 25showed the detailed energy
Page 67 and 68: 4. Design Summary The DME design an
Page 69 and 70: Figure 27: Mass and energy flow of
Page 71 and 72: References 1. Åhman, M., G. Modig,
Page 73 and 74: 42. R.A. Peascoe, J.R.K., C.R. Hubb
Page 75 and 76: PerformanceSimulation. 2006, Prince
Page 77 and 78: Appendix Appendix A MP Steam Demand
Page 79 and 80: Appendix B: Composite Fuel Blend to
Page 81 and 82: DME Density = 0.67 kg/L (at 20 °C)
Page 83 and 84: Air Separation Unit Requirements Ba
Page 85 and 86: Purge gas: CO conversion is 60%, 40
Page 87 and 88: Appendix D: FTD Synthesis Two diffe
Page 89 and 90: Table D.3: The hydrocarbon product
Page 91 and 92: Figure D.3 Mass and energy balance
Page 93 and 94: Table D.3 Mass and energy balance o
Page 95 and 96: Appendix E: Heat and Power Generati
Page 97 and 98: Energy H2O absorbed MW 46.71 Using
Page 99 and 100: Combustion 2CO+O2→ 2CO2 + 566 kJ/
Page 101 and 102: Specific Heat (cp) b kJ/kgK 1.072 E
Page 103 and 104:
Specific Heat of Steam (cp) kJ/kgK
Page 105:
Appendix F: Concept Map 105
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