Methanol, production purification

Feedstock Purification Manufacture of Synthesis Gases Hydrogen, Ammonia, Methanol, product bulletin. United Catalysts, Inc., Louisville, Ky. 

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

Development ofother fuels for feeding fuel cells (natural gas, methanol, bioalcohols, oil fractions etc.) relative to profitable and clean production, purification and infrastructure development. 

Methanol synthesis plants utilizing the low-pressure process currently operate at capacities of 2 x 105 to 2 x 106 metric tons per year [15]. Such installations are composed of a synthesis gas production unit, the actual methanol synthesis reactor, and a separation and purification section. The production and purification of synthesis gas accounts for 50%-80% of the total cost of methanol production, with the remaining cost associated with the actual synthesis and purification of methanol [2, 8], Although a variety of carbonaceous feedstocks can be transformed into synthesis gas, the steam reforming of natural gas (Equation [4]) is by far the most common option, especially for large plants [2, 15-16] ... 

Thus, although hydrogen is used in methanol production, it can be taken straight from the steam-hydrocarbon reformer and does not require further purification and treatment as in the case of pure hydrogen production or ammonia production. The economics of methanol production are significantly affected by the thermal integration of the reformer (or other gas generation unit) with the rest of the plant. 

In a methanol plant (Figure 4.3), the synthesis gas passes from the reformer furnace to a heat recovery section where it is cooled to room temperature. The synthesis gas is then compressed to 750 to 1,500 psia (5,170 to 10,345 kPa) and fed to the converter vessel through preheat exchangers. Methanol is formed as the gas passes over catalyst beds in the converter vessel at 400 to 600° F (205 to 315 C). The methanol product is then cooled and fed to separators and then to fractionators to complete the purification. 

Commodity chemicals share the same benefits of economies of scale. Typical process units are 1-2 orders of magnitude smaller than refinery units, although large methanol synthesis plants can produce up to several Mt/a (Section 4.7.1). Since a chemically pure material is being produced, often in a stoichiometric reaction, the catalyst system now becomes more specialized, the reactor may require special metallurgy, and product purification starts to be an issue. 

A mixture of 1.0 mmol of the 5-hydroxyalkene, 2.2 mmol of copper(I) chloride and 0.10 mmol of bis(ace-tonitrile)dichloropalladium in 3 mL of CH3OH under carbon monoxide (1.11 bar) is stirred at 23 °C until the reaction is complete (3.5 10 h, TLC monitoring). After removal of the methanol, the residue is triturated with pentane and the pentane solution is concentrated to give the crude organic products. Purification by short-path vacuum distillation provides the cyclization products. The stereochemistry is determined by an analysis of the H-NMR chemical shift of the hydrogen at C-5 this signal is 0.1 -0.2 ppm further downfield when the proton is cis to the ester side chain compared to the trans arrangement. 

The first commercial processes for the production of DMT made use of nitric acid oxidation of p-xylene to crude terephthalic acid, followed by esterification with methanol and purification by distillation [3]. Air oxidation of p-xylene displaced the use of nitric acid with the development of the Witten process [5]. In the Witten process, p-xylene is air-oxidized at 140-180 °C and 0.5-2 MPa over a homogeneous cobalt or cobalt/manganese catalyst system to give p-toluic acid, which is then esterified to methyl p-toluate, oxidized again over the cobalt/manganese catalyst, and finally esterified to DMT (see Scheme 1). The four process steps are accomplished in two reactors (see Figure 1). The Witten process uses no solvent. 

In the second step, a dilute H P-methanol solution is introduced in a fixed-bed epoxidation reactor. Make-up propene, recycled propene and HP from the product purification stage are fed into the reactor. The reaction is catalyzed by titanium silicalite, and takes place at 40-50 °C and 300 psi. HP per-pass conversion is initially 96% but drops down to 63% after 400 hours. PO selectivity is 95 mol.% propene per-pass conversion is 39.8%. This technology gives capital savings compared to conventional hydroperoxidation technologies however, it is likely that the operating costs of such a plant are higher than that of the latter. 

The Rh carbonyl complexes present in the product stream must be stable at low CO partial pressures and the temperatures in the distillation columns. Complexes of the less expensive Co are catalytically active for methanol carbonylation in the presence of an iodide cocatalyst, but the carbonyl complexes require CO partial pressures of 10 N/m for stability, which complicates the product-catalyst separation. In the 0x0 process high pressures are required for the Co catalysts to maintain their stability. Product purification... 

A number of appropriate summaries exist describing the suitability of wash processes and wash liquors in general [2.1] and for the purification of coal gas in particular [2.2]. The sections below wiU, however, describe only those processes which appear to be best suited to the purification of coal gas for methanol production. 

The two BASF processes, A/l zzid M (potassium monomethylamino-propionate) and Alkazid-DIK (potassium dimethylaminoacetate), which had been frequently used in the past to remove minute quantities of CO2 and H2S or to remove H2S selectively, have no more chance today of competing with modem coal gas purification processes for methanol production. 

Wherever the absorption or adsorption processes used to clean the gases have a high absorptivity for H2S but can eliminate COS only at high cost or not at all, the processes are designed in such a way that the sulfur contained in the gas reaches the gas purification unit in the form of H2S. Since, however, most modem gas purification units are capable of removing not only H2S but also COS and other organic sulfur components efficiently enough to meet the requirements for methanol production, COS hydrolysis is today used only in special cases. 

As many types of coal contain little sulfur, but have a considerable surplus of carbon for methanol production, the H2S content of the resulting sour gases is frequently around or even less than 5 vol. %. Although the sulfur concentration can be increased in the gas purification section, this is always a costly undertaking. Certain direct treatment processes may therefore be used for such sulfur gases although they lead to sulfuric acid rather than to the more easily manageable and normally more conveniently marketable elemental sulfur. 

Even though the process is generally regarded as being highly selective, the crude methanol product still requires purification. This is usually accomplished by distillation which removes dimethyl ether and the higher molecular weight alcohols. 

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