I humbly seek advice from all the experienced colleagues regarding hydrogen production from methanol
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At my workplace many years ago, the methanol-to-hydrogen production unit had a relatively low capacity of 600 NM3/h. I have switched to another company, where a new methanol-to-hydrogen plant with a capacity of 1700 Nm3/h has been installed. There are many differences in the design compared to the previous one, and several aspects seem unreasonable. The manufacturer claims that this is the latest technology; I would appreciate it if experienced colleagues could take a look at it. 1. Description of methanol-to-hydrogen production at the former workplace: The capacity of a single unit is 600 NM3/h; the pressure of the produced hydrogen is 1.0 MPA. The system is ready for operation – the catalyst has been reduced, and nitrogen purging is complete. Start the heat transfer oil circulation, slightly open the steam for heating to raise the temperature, and control the rate of temperature increase until it approaches the operating temperature. There are separate tanks for pure water, methanol, and circulating liquid. Methanol and DW water (with a water-to-methanol ratio of 2:1) are delivered to the heat exchanger using metering pumps (preheated by the thermal energy generated by the converter), and then they proceed to the vaporization superheater for a second round of heating. It is important to monitor the liquid level and outlet temperature in the vaporization superheater; once the temperature exceeds 200 degrees, the converter is activated. The absorption tower consists of 7 towers, a vacuum pump, and a programmable valve assembly. The steps include: adsorption, three-way pressure reduction, reverse expansion, vacuum pumping, three-way pressure increase, and final charging. 2. Methane-to-hydrogen production at the current workplace: the capacity of a single unit is 1700 NM3/h; the pressure of the produced hydrogen is 2.5 MPA. The system is ready for operation – the catalyst has been reduced, and nitrogen purging is complete. There are separate tanks for pure water, methanol, and mixing ingredients. Centrifugal pumps are used to transport pure water and methanol to the mixing tank (with a water-to-methanol ratio of 1.5:1), from where they go to the heat exchanger (preheated using the thermal energy generated by the converter). They then enter the vaporization superheater for a second stage of heating. This vaporization superheater is not equipped with a level gauge (neither on-site glass panels nor remotely). The absorption tower consists of 8 towers and a programmable valve assembly. The steps are: adsorption, four-way equalization downward, flushing, reverse placement, flushing again, four-way equalization upward, and final charging. My questions are as follows: 1. Is it not necessary to install level gauges (on-site and remotely) in the current vaporizer? Thus, it is not possible to see the liquid raw material inside during operation; the manufacturer states that the liquid raw material should keep the tank full (by calculating based on the flow rate of the metering pump), and the outlet temperature of the vaporization superheater should be monitored. In my opinion, it is not reasonable to not install a level gauge in the vaporization superheater and to rely solely on monitoring the outlet temperature, as this increases the operational difficulties. 2. In my previous company, evacuation was carried out using vacuum pumps, while the cleaning process was done using the residual pressure from other towers. My question: Why isn’t a vacuum pump used? Will it cause damage to the packing in the adsorption tower? If the residual pressure in other towers is released after flushing, the hydrogen content is high at that time; is thunderstorm weather a source of danger?
1. In many devices, although level gauges are installed, there is no actual liquid level during operation.
2. PSA does not necessarily require the use of a vacuum pump; using one can increase the hydrogen yield, but whether it is economically viable still needs to be evaluated.
Vacuuming is used for more thorough analysis, and rinsing can also achieve a similar analytical effect.
The original poster does not have a thorough understanding of pressure swing adsorption; moreover, the desorbed gas cannot be discharged directly on-site, so there is no issue during thunderstorm weather. With new technologies, forward flushing and regeneration can also achieve high yields, saving electricity consumption for vacuum pumps; moreover, it is relatively safer under positive pressure. As mentioned above, it is necessary to calculate how many percentage points the hydrogen yield from the vacuum regeneration process can be increased compared to that of the normal flushing regeneration process; furthermore, an analysis should be conducted to determine whether the additional amount of hydrogen produced is more energy-efficient or whether using a vacuum pump consumes less electricity.
The larger the device, the higher its level of sophistication
Thank you for the explanation; it turns out that the company’s vapor superheater does indeed have a liquid level, and its operation is quite stable. I’m just wondering why the level gauge design is being removed now. Is it convenient to control solely based on the outlet temperature and pressure of the pyrolysis gas?
I know it’s not necessary to use vacuuming. But in the previous company and the current workplace, the gas released from the analyzers was directly vented; a flame arrester was installed at the end of the venting pipes, which is why such a question arises. In actual operation, lightning during thunderstorm weather once caused an ignition in the analyzer connected to the venting pipe in the previous company. Since the methanol-to-hydrogen plant is small and serves as a minor unit within the production process, the flow rate of the gas to be separated is low, the distance involved in the process is great, making recovery impractical
With a level sensor in place but no actual level measurement, there should be little difference during normal operation; however, it may make operations more difficult during startup and shutdown.
Does the company not have a gas pipeline network? Are no torch facilities installed? Currently, the exhaust gas is not allowed to be released directly into the atmosphere
This post was last edited by huwei19890613 on 2017-8-23 at 14:46. The author’s questions stem mainly from an insufficient understanding of the equipment involved; pressure swing adsorption does not necessarily require vacuum evacuation for desorption. In PSA hydrogen production, the flushing regeneration process is the most common approach. The main advantage of using vacuum evacuation is a slightly higher yield, but it also presents issues such as negative pressure operation, moving parts, and high electricity consumption, all of which affect the stability and safety of the equipment. Whether a level gauge is installed in the vaporization superheater isn’t really important; it’s just necessary to pay attention to the operational procedures. I have operated four or five ethylene cracking units, and only one of them had a level gauge installed, but even when operating that unit I didn’t check the level indicated by the gauge. Because with the development of manufacturing processes, many things have indeed become unnecessary, and they have been improved or eliminated. Logically, a vaporization superheater is equivalent to an on-line boiler and indeed requires a level gauge, but based on practical engineering experience it fails to serve any practical purpose, so it was ultimately removed. This requires me to explain to you the manufacturing process of this device. Before operation, the catalyst needs to be reduced; neither the methanol-water reduction method nor the nitrogen-hydrogen reduction method can bypass this vaporization superheater. During reduction, the vaporization superheater requires a large heat transfer capacity, so its capacity is quite high – essentially doubled – and there is no issue with uncontrolled liquid level during normal operation. Additionally, no level gauge is installed; theoretically, a thermometer should be placed in the middle and lower sections, and this thermometer can, to some extent, reflect the relative liquid level.