The issue of light hydrocarbon recovery in the coal-to-oil process

This post was last edited by dajun213 on 2009-10-23 09:48. Most lower hydrocarbons are decarburized using potassium hydrogencarbonate and low-temperature methanol washing; the NHD decarburization process (for ammonia synthesis) and the low-temperature methanol washing process were jointly developed by German companies Ruhrchemie and Linde. This method belongs to the category of physical absorption processes. Low-temperature methanol features strong capacity to absorb acidic gases and high gas purification efficiency; at a pressure of 3.0 MPa, 1 m3 of this solution can absorb 160 m3 to 180 m3 of CO2, and the circulation volume of the solution is low. It can remove impurities such as CO2, H2S, and COS simultaneously, featuring strong selective absorption for CO2 and H2S, resulting in a high degree of purification. The CO2 concentration in the purified gas ranges from 10 mg/m3 to 20 mg/m3, the total sulfur content is less than 0.1 mg/m3, and the purity of the CO2 product reaches 99%. In addition, there is the absorbent methanol, which features abundant availability, low cost, low consumption of solution, and low overall energy consumption. Although this method has many advantages, it requires the acquisition of technical patents and related equipment. The patents for this technology are held by a few foreign companies; therefore, to use this technology, it is necessary to purchase patent licenses and key equipment, which results in high investment costs. According to preliminary cost estimates, building a methanol production facility with an annual output of 100,000 tons using the low-temperature methanol washing process requires about 20 million RMB more in investment compared to using the NHD process. The NHD process is a purification method developed by the Nanjing Research Institute of Chemical Technology; it represents a new gas purification technology with independent intellectual property rights. This method belongs to the physical absorption category; polyethylene glycol dimethyl ether is used as the main solvent. The resulting solution is non-toxic, experiences minimal solvent loss, and requires low energy for regeneration. However, its absorption capacity is lower than that of the low-temperature methanol washing process – typically, 1 m3 of this solution can absorb 40–55 m3 of CO2. A large volume of solution is required for use, and it can remove impurities such as CO2 and H2S simultaneously, reducing the CO2 content in the gas to less than 0.1% and the H2S content to 0.1 mg/m3. Yet its absorption capacity for COS is poor; therefore, a COS hydrolysis unit must be installed prior to desulfurization. Since the gases produced by our company have undergone transformation or hydrolysis, all organic sulfur compounds are fully hydrolyzed. Therefore, using NHD for purification is a reasonable approach; it allows for lower investment costs, reduced operating expenses, and better efficiency. Compared to traditional technologies, it offers significant advantages in terms of saving on construction costs and operating expenses.

It seems that there are not many companies engaged in coal liquefaction at the moment. Could you provide detailed information on each of them? A horizontal comparison of the coal liquefaction processes would also be helpful. However, first of all, I would like to ask the original poster and my colleagues working in coal-to-oil conversion: how are the light hydrocarbons generated during the coal-to-oil process? Is it through direct liquefaction or indirect liquefaction, and in which stage or unit do they arise? What is the composition of this fraction of light hydrocarbons? Can the light hydrocarbon recovery process in coal liquefaction systems be directly applied to those used in oil refining plants?

What the original poster is referring to is the recovery of light hydrocarbons in indirect liquefaction. Indirect liquefaction involves a synthesis stage, namely the Fischer-Tropsch synthesis process, during which some low-carbon hydrocarbons are produced. Due to the selectivity issues associated with Fischer-Tropsch synthesis, certain amounts of light hydrocarbons with molecular weights below C5 will inevitably be generated. In the case of low-temperature Fischer-Tropsch synthesis, the weight percentage of light hydrocarbons is around 10%; in high-temperature Fischer-Tropsch synthesis, this proportion is higher, possibly exceeding 30%. The light hydrocarbon composition also varies depending on the process and catalyst, mainly due to differences in the ratios of olefins to alkanes, as well as isomers to paraffins. Generally speaking, the products of Fischer-Tropsch synthesis with a molecular weight of C5 and above have specific uses: those with a molecular weight of C5–C12 are used to produce naphtha and gasoline, while those with a molecular weight of C13–C20 are used to make diesel and aviation kerosene. Products with a molecular weight of C20 and above are used to produce waxes, or they can be subjected to hydrocracking and reforming to yield gasoline and diesel. For compounds with a molecular weight below C5, in the case of methane, after low-temperature separation, if the exhaust gas is recycled, it goes through methane reforming to produce syngas, which is then sent back to the synthesis stage (this is what Sasol and Shell do; they use a large-scale methane reforming cycle). If the gas is not recycled, it is used as natural gas for heating or power generation. Compounds with a molecular weight between C2 and C5 are used as chemical raw materials or as liquefied petroleum gas. I’m not very familiar with direct liquefaction, but based on the principles and processes involved, there must also be some light hydrocarbons present. As for the recovery of light hydrocarbons, I think we can refer to the light hydrocarbon recovery processes used in refineries; there should not be much difference. As for the decarburization of light hydrocarbons mentioned by the original poster, I have no experience with it and am not familiar with it, so it’s difficult for me to answer.

In Section C5, if the quantity is large, cryogenic treatment combined with CO2 stripping can also be employed to increase LPG production.

The recovery and separation of F-T off-gases generally involves cryogenic separation: first, the acidic gases, namely carbon dioxide, are removed, usually using an alkaline scrubber commonly used in the thermal reduction plus ethylene process. After that, cryogenic compression is carried out, during which hydrogen and carbon monoxide are separated first and compressed back to the synthesis reactor. Since these gases contain some nitrogen as well, a small amount of them needs to be released in order to maintain material balance in the system (this gas can typically be used as fuel). The remaining hydrocarbon compounds are separated one by one following the conventional ethylene separation process, thereby yielding ethylene and propylene products of satisfactory purity. Regarding the issue raised by the original poster about the impact of alkali solutions on light hydrocarbons during decarbonization, in the ethylene industry this phenomenon is what we refer to as the formation of \"green oil\"; it actually represents products resulting from the polymerization of certain dienes in the presence of alkali solutions. Since there are very few dienes in F-T off-gases, this issue can be disregarded. Of course, for the sake of safety in engineering design, a green oil tank is often included in the process, though it is likely to be of little use. Another issue to consider is the scale of coal-to-oil production; typically, there is less ethylene and propylene in the exhaust gases (more in high-temperature Fischer-Tropsch processes than in low-temperature ones). Deep cryogenic separation requires a certain scale to be economically viable, and therefore current pilot plants for Fischer-Tropsch processes have not taken this issue into sufficient consideration. However, in order to support future FTO processes on a scale of millions of tons, many industry experts have been conducting in-depth research on the recycling of FTO exhaust gases for many years.

Gaseous hydrocarbons and volatile hydrocarbon compounds in the Fischer-Tropsch synthesis process are present in both the synthesis exhaust gas and the liquid products. Light hydrocarbons in the exhaust gas are generally separated using cryogenic methods, but this depends on whether it is worthwhile to recover these light hydrocarbons, as cryogenic separation techniques are costly. Another approach is to carry out conversion and reforming in the same way as with methane, to convert it back into (CO+H2) for oil production in a cycle, but repeated conversion and reforming result in increased energy consumption. For the Fischer-Tropsch synthesis process, the amount of light hydrocarbons produced is also one of the key indicators for assessing the efficiency of energy recovery and carbon recovery. Reducing the production of light hydrocarbons should also be one of the goals in catalyst development and technological innovation for the Fischer-Tropsch process. Compared to coal-to-natural gas and coal-to-methanol, coal-to-oil has the lowest energy recovery efficiency and carbon recovery efficiency, which is also a drawback of coal-to-oil. Regarding the removal of CO2 from the syngas produced in Fischer-Tropsch synthesis, since Fischer-Tropsch catalysts have poor resistance to toxic substances and require a low total sulfur content in the syngas – generally not exceeding 0.04 ppm – high standards are imposed on desulfurization technologies. The low-temperature methanol washing process is commonly used; it removes CO2 from the syngas as needed, while simultaneously carrying out desulfurization.

That’s the case with 4# fossil-zhang: carbon dioxide is not removed before the recovery of light hydrocarbons; otherwise, it becomes difficult to recover light hydrocarbons at low temperatures

For small-scale applications, direct combustion for heat generation is used; for large-scale applications, capture is considered