CHE2162 Material and Energy Balances Assignment 1 and 2 Combined Handout - Methanol Production Problem Statement Societal Need Methanol (CH3OH) is a key raw material for many chemical products and

has potential as a transport fuel. According to the Methanol Institute [1], the annual global production is currently around 65 million metric tonnes (Mta). In a report by IRENA [2], approximately 70% of the methanol produced worldwide is used in chemical syntheses. The following products (in order of quantity of methanol consumed) are: Formaldehyde, methyl tert-butyl ether (MTBE), acetic acid, dimethyl ether (DME), propene, methyl methacrylate and dimethyl terephthalate (DMT). However, there are many more that are produced from methanol to a lesser extent. More than 95% of methanol currently produced comes from natural gas. However, methanol may be produced from carbon sources, and therefore there is also strong potential for it to play an important role in reducing carbon footprint of our society. There is clearly a need to produce many of the products currently made using methanol with a smaller carbon footprint. Methanol can also play an important role in diesel fuel substitutes. Methanol is a significant feedstock in the manufacture of biodiesel. In addition, the methanol derivative dimethyl ether (DME) is regarded as an ideal substitute for diesel in most diesel engines, Volvo [3] and even if this was to occur at a small fraction of the global diesel consumption (720Mta), this would increase significantly the global methanol production. According to IRENA [2], methanol can be blended directly with gasoline up to 20 vol%, but it is more frequently converted to MTBE. Methanol Synthesis The conventional method of producing methanol on a large scale is by steam reforming natural gas as shown in Figure 1. CH4+H₂O → CO + 3H₂ (1) AH = 206 kJ.mol-¹ The methanol synthesis proceeds via two reactions simultaneously CO + 2H₂ CH3OH ΔΗ -98 kJ.mol-¹ CO2 + 3H₂ → CH₂OH + H₂O ^H = -58 kJ.mol-1 = (2) (3) The mismatch in the H₂/CO ratio between the reforming (reaction 1) and synthesis section (reaction 2) leads to different options in the plant design, which include steam-only reforming, CO2 or "dry" reforming, and auto-thermal reforming (which involves partial oxidation). In order not to waste H2, the molar ratio of H₂ to CO should be just above 2.0 going into the methanol synthesis loop. Using reaction (3), methanol can also be produced from H2 and CO2. If H2 is available from a carbon neutral process and CO2 can be captured as a waste stream and reused, the methanol produced would be carbon neutral, providing that any other utilities can also be provided with no net CO2 emissions. In this project you will be developing a Process Flow Diagram for methanol production from CO₂ captured from point sources (flue gas) or Direct Air Capture (DAC) and green hydrogen (H₂ produced from electrolysis of water). Alternatively, methanol can also be produced from carbon derived from biological sources, such as biomass. There are two pathways for converting biomass into methanol feedstock. First is the gasification route, in which solid biomass containing approximately 10 wt% moisture or lower is gasified in oxygen deficient conditions to produce synthesis gas (a mixture of mainly CO, CO2 and H₂), which can be fed to the methanol synthesis process. The second route is anerobic fermentation, in which biomass containing approximately 30 wt% moisture or more is digested in a bioreactor to produce biogas (a mixture of mainly CO2 and CH4), which can be upgraded via dry- or bi- reforming to make synthesis gas. Dry reforming of methane is show in reaction (4), whereas bi-reforming is a combination of dry reforming and steam reforming (reactions (1) and (4) combined). CH4 + CO₂ → 2CO + 2H₂ ΔΗ = 247.3 kJ mol-1 (4) Process water Natural gas Process fuel Steam production Process fuel Steamreforming Sulphur from desulphurisation Electricity Heat recovery Synthesis gas Cooling water Methanol synthesis Emission to air CO, CO2, NOX, VOC Purge gas Hydrogen Catalysts: CoMo, NiMo, ZnO, CuO, Al2O3 Spent catalysts Raw methanol Waste heat Light ends Methanol purification Figure 1: Simplified Process Flow Diagram for a natural gas based methanol plant (from [4]) Waste water Methanol, at plant Assignment 1 (Mass Balances) Part 1.A In the first assignment (Mass Balances) you will conduct "hand-calculation" of feedstock preparation and methanol synthesis route. Biomass (sugarcane bagasse) is gasified in a novel Reactive Flash Volatilisation (RFV) process to produce synthesis gas, along with biochar and water-soluble tar as by- products. In a pilot plant RFV process, pulverised biomass is feed to catalytic gasifier at a rate of 10.0 kg/h, along with 4.2 L(STP)/h of O₂ and 6.4 kg/h of steam as the gasifying agents. The biomass and oxygen are supplied at room temperature, whereas the steam is fed at 150°C and the gasifier operates at 800°C. The steam is supplied in excess; therefore, the unreacted water vapour exiting the gasifier with the synthesis gas is condensed at room temperature before the synthesis gas is sent for further processing. The condensed water can be assumed to contain all of the water-soluble tar produced in the gasifier. The biochar is periodically drawn from the gasifier and sent for analysis. Table 1 provides the ultimate and proximate analysis of sugarcane bagasse, whereas Table 2 shows the raw data for synthesis gas flow rate (wet basis) and composition (nitrogen and water free basis) measured at the outlet of the condenser. Table 3 provides the ultimate analysis of char and tar collected from the gasifier and condenser, respectively. Table 1 - Proximate and Ultimate Analysis of Sugarcane Bagasse a 1. Conduct an atom species balance for C, N, H and O for the duration of the given measurement and estimate the average carbon conversion efficiency (percentage of carbon conversion into gas phase). (18 Marks) 2. Report the errors in mass balance analysis and reflect upon the reasons for those errors. What could be the reasons behind mass balances not closing properly? (8 Marks) Ultimate Analysis Carbon (C) Hydrogen (H) Nitrogen (N) Sulphur (S) Oxygen (0) Proximate Analysis Moisture Ash Volatile Matter Fixed Carbon a Calculated by difference. Calculated by difference. 46.40 6.19 0.44 0.05 44.42 Due: 8 September 23:55 26 Marks (% db) 79.16 18.13 (% as received, db except moisture) 10.0 Table 2 - Time on stream gas measured flow rate and composition Time on Stream (min) 30 60 90 120 150 180 210 240 270 300 330 360 390 Sample name Biochar Tar Synthesis gas flow Part 1.B rate (Nm³/h)* 15.89 15.94 15.66 16.61 16.46 14.61 16.70 15.88 14.82 13.75 16.51 16.22 15.66 420 16.75 450 15.66 480 14.48 510 16.13 540 15.21 570 13.75 600 12.25 Normal conditions = 25°C, 1 atm Table 3 - Ultimate analysis of biochar and tar Yield (kg/h) CO₂ 0.262 1.552 Synthesis gas composition (mol fraction) 0.321 0.355 0.275 0.346 0.409 0.306 0.322 0.348 0.319 0.334 0.320 0.307 0.367 0.375 0.375 0.381 0.367 0.366 0.381 0.421 N 0.472 0.794 H₂ 0.474 0.471 0.503 0.488 0.438 0.497 0.480 0.469 0.480 0.478 0.474 0.470 0.459 0.490 0.498 0.527 0.466 0.519 0.499 0.414 CH4 0.024 0.023 0.034 0.025 0.022 0.034 0.034 0.031 0.041 0.033 0.038 0.042 0.031 0.030 0.034 0.039 0.032 0.044 0.036 0.032 Composition (wt%) C H 50.833 0.419 0.243 7.458 CO 0.186 0.155 0.188 0.149 0.132 0.164 0.165 0.153 0.162 0.158 0.171 0.183 0.144 0.130 0.149 0.158 0.149 0.181 0.147 0.131 S N.D. N.D. SO₂ N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 26 Marks The synthesis gas produced from the RFV process is pretreated to make the feed for methanol synthesis process. The pretreated synthesis gas feed enters the methanol synthesis plant at a flow rate of 100.0 kmol/h and contains 1.0% methane (inert) with the remainder being a stoichiometric mixture of CO and H₂. This stream is mixed with a recycle stream to produce a stream which is fed to the reactor. The product of the reactor is at equilibrium conversion based on reaction (2) and enters a condenser which condenses some of the methanol. The overhead gas product contains 5.0 mol% methane, 5.0% methanol, 30.0% CO and the remaining H₂. 5.0% of the overhead gas is purged to prevent build-up of the methane and the rest is recycled. The liquid product from the condenser contains pure methanol. A. Draw and clearly label the block flow diagram. Calculate the following - B. The molar flow rate of the purge stream. C. The molar flow rate of the liquid methanol stream. D. The equilibrium constant K based on the mole fractions. E. The composition of the recycle stream on a mass basis. F. The overall and single pass fractional conversion of CO in the reactor. (4 Marks) (3 Marks) (3 Marks) (10 Marks) (3 Marks) (3 Marks)