Weekly Topic 10.12.13, Issue 7: What are the causes of imbalanced hydrogen-to-carbon ratios in synthesis, and what are the solutions?

According to the stoichiometric ratio, the H2/CO ratio in the feed gas should be 2; however, in practical plant operations, taking into account the control and removal of reaction heat, the H2/CO ratio in the recycle gas is kept above 5. For copper-based catalysts, CO2 in the feed gas also participates in the synthesis reaction; therefore, control is carried out using H2-CO2/CO+CO2 as an indicator. Generally speaking, if the H2-CO2/CO+CO2 control is set too low, side reactions increase, and carbon deposition reactions can also occur ； An excessively high H2-CO2/CO+CO2 ratio affects yield and leads to increased consumption levels such as energy use. Generally, the H2-CO2/CO+CO2 ratio in fresh gas is controlled at 2.0–2.2, while it is controlled at 4–6 in recycled gas. The reasons for an imbalance in the hydrogen-to-carbon ratio include improper control of the fresh gas ； The circulation volume is too high or too low ； The venting is too large or too small ； Delayed adjustments when increasing or decreasing amounts, etc.

The main reason for the imbalance in the hydrogen-to-carbon ratio is the unsuitable composition of the gas supplied from the previous process stage, while the secondary reasons are instability in the synthesis reaction and operation, as well as excessive or insufficient control of the vent gas. Treatment method: Contact the previous section for adjustment; carry out venting and replacement in this section, or use purge gas for control and adjustment. The hydrogen-to-carbon ratio of the feed gas to the synthesis reactor should be controlled at 2–2.15, while the ratio for the recycle gas varies depending on the tower design and process requirements, typically ranging from 4 to 7. Keep carbon dioxide at 2%-5%; be sure not to let it drop too low.

What is primarily controlled is the hydrogen-to-carbon ratio of the fresh gas, with the target range being 2.05–2.15; since our system pressure is set to automatic mode. When the hydrogen-to-carbon ratio is low, the system’s vent volume increases. As for the hydrogen-to-carbon ratio at the inlet of the synthesis tower, it is primarily determined by the activity of the catalyst, with the influence of the system circulation being a secondary factor. Generally, when the hydrogen-to-carbon ratio differs significantly, we adjust it using a shift converter; an auxiliary adjustment is made by controlling the CO2 content in the low-temperature methanol wash.

I think the hydrogen-to-carbon ratio of 2.05 refers to the value at the inlet of the synthesis tower. Because it is the direct number of reactions inside the tower. Also, I’ve seen that the hydrogen-to-carbon ratio of the gas entering the tower seems to be the same as that of the gas at the outlet of the separation tank. I’m not sure if it’s aligned with the stars or not? I look forward to your advice.

The hydrogen-to-carbon ratio (2.05–2.15) is usually that of fresh gas, while it is higher for the gas entering the tower.

Based on my actual experience, even a change of around 0.03 in the hydrogen-to-carbon ratio of the fresh gas will cause severe fluctuations in the hydrogen-to-carbon ratio at the inlet of the synthesis tower. It’s possible to bring the 62% hydrogen level down to over 70% all at once. An imbalance in the hydrogen-to-carbon ratio is mainly due to imbalances or fluctuations in conversion and low methane (decarbonization) processes. I find that a hydrogen-to-carbon ratio of 1.98–2.01 is the most stable stage for maintaining freshness (online analysis of the structure may show deviations; chromatographic analysis can also lead to inaccuracies). Keeping the fresh gas hydrogen content below 67% ensures stable hydrogen supply to the tower. Different towers, different circulation rates, and inert components can result in variations. It seems that the concentration has decreased; that’s because the inert components have reduced the partial pressure, while the actual amount remains unchanged. The hydrogen-to-carbon ratio entering the tower generally remains stable around 3.

Reply to 4# Shouhou: Is the automatic adjustment of the pressure in your synthesis system achieved through the automatic control of the vent valve?

The hydrogen-to-carbon ratio, when too high or too low, leads to problems: a high ratio prevents an appropriate increase in pressure, while a low ratio prevents a proper decrease in pressure. In other words, with all other conditions remaining constant, a high ratio results in reduced reaction activity in the bed, whereas a low ratio leads to increased reaction activity. Take corresponding measures based on different reasons to stabilize the situation first. Make further active adjustments to the low-temperature washing process, and make changes to bring it back within the operational range. .

Reply to 1# Qingfeng Liushui: Reason 1: Fluctuations in the gas composition occur in the previous stage, such as when the gasification furnace is clogged with slag or when the slag burns through； Reason 2: The transformation section is not adjusted in a timely manner, or the operating range is too large ； Reason three: Fluctuations in the compressor, such as surging ； Reason 4: Large fluctuations in pressure and temperature control within the synthesis system, leading to overheating and carbon buildup in the system ； Reason 5: Issues with online instruments, which require reference manual analysis for calibration. The above are my personal opinions; please point out any shortcomings.