Synthesis loop

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

Retrofitting features of the more efficient reactor types have been the principal thmst of older methanol plant modernization (17). Conversion of quench converters to radial flow improves mixing and distribution, while reducing pressure drop. Installing an additional converter on the synthesis loop purge or before the final stage of the synthesis gas compressor has been proposed as a debotdenecking measure. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Ammonia Synthesis and Recovery. The purified synthesis gas consists of hydrogen and nitrogen in about 3 1 molar ratio, having residual inerts (CH Ar, sometimes He). The fresh make-up gas is mixed with the loop recycle and compressed to synthesis pressures. AH modern synthesis loops recycle the unreacted gases because of equiUbrium limitations to attain high overall conversions. The loop configurations differ in terms of the pressure used and the point at which ammonia is recovered. 

Product concentration influences the separation cost and the cost of recycling unconverted reactants. Production rate has a strong effect on investment cost for the full synthesis loop. Selectivity sets the raw material... 

In general, for basic petrochemicals that are not much more expensive than fuel (energy) itself, the energy recovery or use is important. Therefore, exothermic reactions should be executed at the highest temperature and endothermic reaction at the lowest, within the range that the reaction permits. In addition, reactors should not be optimized only for their own performance, but also for the optimum economy of the full synthesis loop or the full technology. 

However, not all the hydrogen that is produeed is used in the produetion of methanol. To prevent an exeessive buildup of hydrogen in the reeireulating methanol synthesis loop, a slipstream is drawn off eonsisting of hydrogen and some CO and CO2. It is fed to the boiler as supplemental fuel to the main natural gas fuel supply. The slipstream is at approximately 800 psi pressure and must be redueed to 75 psi before it ean be fed to the boilers. 

Gas recirculation in the anunonia synthesis section is necessary because only 9-30% conversion is obtained per pass over tlie catalyst." There are two tj pes of synthesis loops. One type recovers anunonia product before recycle compression. Inerts entering witli tlie makeup gas are removed with a purge stream. The ammonia is recovered by condensation, which requires refrigeration. Since airliydrous anunonia is readily available, it is normally used as tlie refrigerant. 

Figure 5-5. The ICI low-pressure process for producing methanol " (1) desulfurization, (2) saturator (for producing process steam), (3) synthesis loop circulator, (4) reactor, (5) heat exchanger and separator, (6) column for light ends recovery, (7) column for water removal.

As an example, consider ammonia synthesis. In an ammonia synthesis loop, hydrogen and nitrogen are reacted to ammonia. The reactor effluent is partially condensed to separate ammonia as a liquid. Unreacted gaseous hydrogen and nitrogen are recycled to the reactor. A purge on the... 

Synthesis gas generation routes, for methanol, 16 302-307 Synthesis gas mixture ( syngas ), 73 842, See also Syngas plants Synthesis loop, methanol, 16 307... 

C 0 and H O, unavoidable by-products of alcohols synthesis. Considering chemical reactions of table H, water and carbon dioxide appear as equiva-lentby-products due to shift conversion equilibrium, equation (1). Most other low temperature alcohol synthesis catalysts have a rather high shift activity as well. CO removal fhom reacted syngas of synthesis loop, before recycling to reactor, leads to a significant decrease of water formation which, in turn, results in a lower water content in the raw alcohols, leading to simplified fhactionation-dehydration processes. 

Provided that stable and active catalysts are available, water contents ranging between 2.5-5 wt % can be obtained while operating with reasonable (1.2-1.5) H / CO ratios in the synthesis loop. 

If the makeup gas to the ammonia synthesis loop is absolutely free of catalyst poisons, such as H2O and C02, it can flow directly to the ammonia synthesis converter. This leads to the most favorable arrangement from a minimum energy point of view. This can be accomplished by allowing the gas that leaves the methanation step to pass through beds of molecular sieves to remove water and traces of C02 74... 

Ammonia synthesis is normally carried out at a pressure higher than that for synthesis gas preparation. Therefore the purified synthesis gas that is fed to the ammonia synthesis loop must be compressed to a higher pressure. Synthesis loop pressures employed industrially range from 8 to 45 MPa (80 to 450 bar). However, the great majority of ammonia plants have synthesis loops that operate in the range of 15 to 25 MPa (150 to 250 bar)74. 

Figure 6.1. Schematic flow diagrams of typical ammonia synthesis loops.74 (Reproduced by permission of Wiley-VCH)...

Conventional reforming with methanation as the final purification step produces a synthesis gas that contains inerts (CH4 and argon) in quantities that do not dissolve in the condensed ammonia. Most of the inerts are removed by taking a purge stream out of the synthesis loop. The size of this purge stream controls the level of inerts in the loop at about 10% to 15%. The purge gas is scrubbed with water to remove ammonia and then it can be used as fuel or sent to hydrogen recovery. 

Purification Exit Gas Composition Primary Reforming Secondary Reforming High Temperature CO ShiR Low Temperature CO Shift COj Removal Methanation MUG Ammonia Synthesis Loop... 

The first application in 1992 used a two-bed, hot-wall KAAP reactor that featured a low pressure drop and radial flow. Because of the KAAP catalyst s high activity, thin beds are necessary to keep operating temperatures within the desired range203. In 2002 the KAAP reactor had evolved to a four-bed design. A magnetite catalyst is used in the first bed of the synthesis loop when the ammonia concentration is below 2% of the feed. Then the ruthenium catalyst is used in the next three beds to bring the ammonia level up to 18% or more215. 

The gas that leaves the PSA unit is methanated, cooled and dried. The dried gas enters the ammonia synthesis loop at the circulator suction. In the synthesis loop, gas from the circulator is heated and passed over low pressure ammonia synthesis catalyst to produce ammonia17. 

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