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The Production of Methanol
Methanol can be made from a wide range of feed stocks making it one of the most versatile energy sources available today. It has various applications in our daily life such as being a transportation fuel, Electricity generation as it’s a fuel for driving the turbines, plastics production, paints and dyes manufacturing and many other numerous applications. The main mean of producing methanol nowadays is via the synthesis of natural gas however there are other alternative production methods available that can produce a higher purity of
methanol, specially the green technologies developed to preserve our valuable environment.
An insight into methanol’s history: Methanol, also known as wood alcohol or methyl alcohol has the chemical formula of CH3OH, it was produced in the old days by the destructive distillation of wood. It is the simplest alcohol, a light volatile, colourless and flammable liquid. It was first isolated in 1661 by Robert Boyle.
1 Production of methanol using natural gas: Typically, methanol is produced using natural gas by the means of three steps. The first step is to convert the feed stocks of natural gas into syngas that consists of CO, CO2, H2O and hydrogen. This is performed by the catalytic reforming of natural gas with steam: 2 CH4 + 3 H2O -----> CO + CO2 + 7 H2 (Synthesis Gas)
Diagram 1: The simplified steam reforming of natural gas with catalyst aid.
Diagram 2: Detailed syngas formation loop.
Then in order to make methanol, the second step involved is the catalytic synthesis of methanol from the syngas: CO + CO2 + 7 H2 -----> 2 CH3OH + 2 H2 + H2O
Diagram 3: The simplified methanol synthesis loop.
This method creates an excess amount of hydrogen gas that when reacted with external carbondioxide gas if available, then it would produce more methanol:
CH4 + ½O2 -> CO + 2 H2 -> CH3OH CH4 + O2 -> CO2 + 2 H2 In the last equation the carbondioxide and hydrogen produced would react with more hydrogen from the previous equations and hence more methanol is produced and the overall efficiency is ameliorated however this comes at an additional capital cost.
The third step involved is the purification of methanol. The crude methanol produced from the second step has approximately 18% water as an impurity
and is stored in specially made tanks as a feed to the following step to remove impurities via two separate distillation columns.
Diagram 4: Purification of crude methanol.
The first column is also called as the topping column and its purpose is to remove materials with low boiling points from the top of the column. The second column is also called as the refining column where water is separated from the desired methanol. It is a very tall column that operates at a high temperature and methanol vapour is collected from the top of it and condensed to give the required pure methanol that can reach 99% purity levels.
2 Making methanol from wood waste: Any carbonaceous material such as coal, lignite, wood waste, agricultural residue and garbage can be benefited from and synthesised into methanol. In order to make methanol from wood, we need to partially oxidise the wood to produce a crude gas consisting of hydrogen, carbondioxide and carbonmonoxide. The initial gasification step to obtain syngas from wood produces a limiting supply of hydrogen gas and thus steam is reacted with the carbonmonoxide to bring the ratio of hydrogen :carbonmonoxide to 2:1 CO + H2O ————> CO2 + H2 The sequence of reactions occurring in wood gasification is as follows: Drying (100° - 200°C) Moist wood and heat ————> Dry wood and water vapor Pyrolysis (200°-500°C) Dry wood and heat ————> Char + CO + CO2 + H2 +CH4+ + tars and pyroligenous acids Gasification (500°C+) Char + O2 + H2O... ————> CO + H2 + CO2
Diagram 5: The process flow diagram of producing methanol from wood.
Gas Purification and Shift Conversion The raw gas is then purified to remove all but hydrogen and carbon monoxide. This mix is reacted to convert part of the CO to H2 So that the final mixture contains a 2:1 ratio of H2 to CO. In this conversion, additional CO2 is formed and must be removed before methanol synthesis.
Raw gas is passed to the scrubber where it is cooled down and tar and acid are removed. Later on, it is compressed and treated in two steps with a hot
potassium carbonate solution and then a monoethanolamine to reduce the carbondioxide content. This treated gas is then passed through a cryogenic system which removes the remaining CO2 ,water vapour, CH4, and other hydrocarbons along with nitrogen. The gas is compressed for the shift reaction where CO reacts with water vapour in the presence of an iron catalyst to form additional hydrogen to maintain the exit gas with a ratio of 2:1 , H2:CO. Rescrubbing with additional potassium carbonate absorption system is required in this step as extra CO2 is generated. The syngas is compressed at a very high pressure and passed into the methanol synthesis reactor where about 95% of the gas is converted into methanol with the aid of a zinc-chromium catalyst. Unreacted gases are recycled back to be converted into more methanol and the methanol is finally purified by distillation.
3 The production of methanol from Biomass: Biomass is a general term for material derived from growing plants or from animal wastes. It is a renewable source of energy. We can obtain Biomethanol through the gasification and treatment of biomass such as cow dung. Some recent surveys have showed that we can produce about 186 gallons of methanol from one ton of biomass feedstock. The cow dung or biomass is collected and allowed to be sundried or treated in massive air dryers to remove moisture and prevent unwanted microorganisms from inhibiting it. Then mass scale slurry is prepared from the biomass by adding water and transferred to agitators for anaerobic digestion to take place and agitated daily. Internal biogas starts to form bubbles during the fermentation process. This gas is passed to a scrubber with NaOH to remove the carbondioxide and also flowed over alkaline pyrogallol solution to remove any free oxygen in the formed gas. This biogas is later passed on a third reactor containing iron fillings in order to remove the hydrogen sulphide component. Another reactor containing CaCl2 is used to absorb water vapour and as a result dehydrated methanol is formed. The purified methane gas produced is set to react with weak sulphuric acid to form methane-sulphonic acid which is then introduced to a heat medium where it breaks into methanol and SO2. The SO2 is passed over water to form H2SO3 acid which is reintroduced into the system. Other by-products of methyl-methane sulfonate and dimethyl-ether also boost the yield of methanol via hydrolysis which is achieved by introducing water and sulphuric acid in excess.
Here are the reactions involved:
Discussion: This form of production of methanol is very environmentally friendly and efficient plus the spent cow dung can be reused as a fertilizer for plants. Also, feed stocks of animal waste are abundant. Nevertheless some disadvantages are that SO2 gas is produced which needs to be treated as it harms the environment. Also the set up cost and capital of the plant is relatively high. Methane gas is hard to store. The wood waste method disadvantages are : it produces a low capital and low energy yield. It is harming to the environment due to the emissions of greenhouse gases to the atmosphere. However, wood is a renewable resource of energy which is an advantage. The production of methanol from natural gas remains the most conventional way with a high conversion rate, however, it is bound to change as preserving the natural gas reserves is vital for our future generations and humans are ought to be flexible and not rely only on one source as a raw material when there are many alternatives available for development that can incur higher purity yields of methanol at a cheaper cost. Also a main disadvantage is that it is a non-renewable energy source.
Conclusion: I suggest the use of animal waste or biomass method as the best bet due to the fact that it produces less greenhouse gases and animal waste is a very cheap raw material. It is a successful method that yields high purity methanol.
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