Patent Application
20190291048
The present invention allows bulk removal of undesired components from industrial gas streams, e.g.: H2S and/or CO2 from natural gas, separation of CO2 from flue gas, separation of CO2 from the product stream of the water-gas shift reaction, etc.
Currently, a wide range of chemical or physical solvents are used for the removal of undesired components such as H2S and/or CO2 from natural gas streams. The choice of the solvent strongly depends on the individual concentration of the undesired components in the gas streams.
There is a need for safe, reliable and economically feasible technologies to separate undesired components like H2S and/or CO2 from industrial gas mixtures. The natural gas industry is more and more dealing with elevated sour gas concentrations in, for instance, the Bab oil/gas field, where H2S concentrations as high as 35% are met. The current sweetening technology is not dimensioned for such high H2S concentrations.
It is an object of the present invention to provide a method which is a clean technology, i.e. environmentally friendly, for removing undesired components from gas mixtures. It is a further object of the present invention to provide such a method which requires only moderate operating conditions, especially in terms of temperature and pressure. It is another object of the present invention to provide a method which enables bulk removal of undesired components from industrial gas streams, especially the removal of H2S and/or CO2 from natural gas mixtures. It is also an object of the present invention to provide such a method which is versatile and can similarly be used to separate CO2 from gas mixtures, especially from flue gas or from the product stream of the water-gas shift reaction
In order to achieve one or more of the mentioned objects, the present invention provides a method for separating undesired components from gas mixtures comprising the following steps:
providing a gas mixture containing at least two gaseous components, wherein one component is an undesired component,
feeding water to the gas mixture,
forming a hydrate of the undesired component and a remaining gas mixture, wherein the hydrate is formed by spraying a combination comprising the water and the gas mixture, and
separating the hydrate from the remaining gas mixture.
In the above method, it is preferred that the undesired compound is selected from CO2, H2S, C2H6 and C3H8, wherein it is particularly preferred that the undesired compound is H2S. It is further preferred that the undesired compound, especially the preferred undesired compound mentioned above, more preferably H2S, forms 30 vol.-% or more of the gas mixture. In the context of the present invention, percentages are always vol.-%.
According to the present invention, it is particularly advantageous that the hydrate is formed by spraying a combination comprising the water and the gas mixture through a nozzle. In this context it is preferred that the nozzle is operated at a temperature below 6° C. and/or at a pressure below 30 bar. More preferably, the nozzle is operated at a temperature of 5.5° C. and at a pressure of 28 bar.
In an advantageous aspect of the invention, the hydrate is formed by spraying a combination comprising the water, the gas mixture and tetrahydrofuran.
It is further preferred that in the method according to the present invention the molar ratio of water to the gas mixture is in the range of 3:1 to 9:1, more preferably 6:1.
Gas hydrate separation technology offers a clean technology with only water and gas mixtures involved at moderate operating conditions in terms of temperature (T) and pressure (p).
The invention provides new technology preferably for a “model Bab field” gas stream with 70% CH4 and 30% H2S. Basically, the conversion of the feed gas into the hydrate phase causes a significant change in the composition of the gas. Separation of the gas hydrate phase leads to a gas stream with a significantly lower H2S concentration. This aspect of the invention has a thermodynamic basis.
The second aspect of the invention is the kinetics of the hydrate formation. The kinetics of the hydrate formation is a slow process and, therefore, for an industrial application not very appealing. However, application of the advantageous spray technique allows hydrate formation instantaneously.
Both the merger of the thermodynamic and kinetic features are the basis for the invention. Simplification of current methods of removing of large concentrations of impurities from gases especially for example as between H2S and CO2.
The gas hydrate technology according to this invention is specifically suitable for bulk removal of H2S/CO2. The technology according to this invention has a much wider range of applications than H2S removal only; also CO2-removal from flue gas, separation of H2 and CO2, transforming production water from high to low salinity, etc. are contemplated.
The gas hydrate technology of this invention is safe and an excellent precursor for traditional gas sweetening processes.
The present invention will be described with reference to the accompanying drawings of which:
The present invention provides a new technology for e.g. a “model Bab field” gas stream with 70% CH4 and 30% H2S. Basically, the conversion of the feed gas into the hydrate phase causes a significant change in the composition of the gas. Decomposition of the gas hydrate phase leads to a gas stream with a significantly lower H2S concentration. This aspect of the invention has a thermodynamic basis.
The second crucial aspect of the invention of the new technology is the kinetics of the hydrate formation. The kinetics of the hydrate formation is a slow process and, therefore, for an industrial application not very appealing. However, application of a spray technique allows hydrate formation instantaneously.
A hydrate according to this invention is a compound in which water molecules are chemically bound to another compound or an element. Such hydrates are typically crystalline as measurable by X-ray diffraction, for example Powder X-Ray Diffraction (PXRD).
For example, H2S forms hydrates under certain conditions. The known technologies seem to be less useful where H2S is present in high concentrations of e.g. 30% or more. The invention exploits inter alia the fact that H2S is a strong hydrate former. After separating the hydrate phase from the remaining gas phase, the H2S concentration in the desired gaseous product can be significantly reduced, e.g. from 30% or more to 1% or less. In the context of this invention, percentages are always volume percentages (vol.-%).
Further, the hydrate formation is slow which makes it less useful for large scale productions. In order to achieve a rapid hydrate formation, the invention uses a spray technique. The small size of the droplets formed by spraying contributes to a faster hydrate formation.
CH4 was used in some of the experiments related to this invention. The present invention is especially directed at the removal of undesired components from natural gas as the treated gas mixture. Natural gas in the sense of this invention comprises at least one alkane, preferably methane. Major contaminants in raw natural gas that also may form gas hydrate at the same thermodynamic conditions as CO2 and H2S are C2H6 and C3H8. Their common concentration ranges (in volume %) of their presence in raw natural gas are given below. Higher or lower bounds are possible, depending on the location and type of the source:
C2H6: 1.5-7.0
C3H8: 0.1-1.5
Although N2 is a hydrate former as well, the pressure at which this hydrate will be formed is significantly higher than that of CO2 and H2S. Therefore, in the separation process interference by the formation of N2 hydrate will not occur.
Aromatic compounds may form hydrates as well. However, in general their concentration in natural gas is very low and, therefore, interference of the separation process by hydrate formation caused by aromatics is unlikely to occur.
In other experiments related to this invention flue gas was used. Model flue gas is formed by 65% N2 and 35% CO2.
A typical composition of gas-fired flue gas, which is useful in the present invention, is 7.4-7.7% CO2, 14.6% H2O, approx. 4.45% O2, 200-300 ppm CO, 60-70 ppm NO2, and 73-74% N2.
A typical composition of coal-fired flue gas, which is useful in the present invention, is 12.5-12.8% CO2, 6.2% H2O, approx. 4.4% O2, 50 ppm CO, 420 ppm NOx, 420 ppm SO2, and 76-77% N2.
Another gas which can be advantageously treated according to the invention is the water-gas shift reaction product. At the exit of the water-gas-shift process a gas composition of 20% CO2 and 80% H2 is typically found. Minor amounts of CO and CH4 might be present.
Separation of a mixture of 30% H2S+70% CH4 (mimics Bab gas field): this process preferably takes place at a constant pressure of 10 bar. The lowest temperature during the process is 276.40 K (exit) and the highest 284.79 K (entrance).
In addition, other systems for separation are also feasible for the present invention:
i) 70% N2+30% CO2 (flue gas), average temperature over the process is 274K. As can be seen, the separation can take place at relatively low pressures.
ii) 90% CH4+10% CO2 (natural gas), average temperature over the process is 274K. As can be seen, the separation can take place at relatively low pressures.
With respect to the amount of water added for forming the hydrates, it is advantageous that the molar ratio water/gas is in the range of 3:1 to 9:1 and is more preferably stoichiometric, i.e. 6:1 so that all available hydrate cavities are occupied.
After conversion of the raw gas into hydrate, followed by separation of the hydrate phase and decomposition of the hydrate phase, the gas released from the hydrate has a completely different composition compared to the original raw gas. This is the basis of the separation and also shown in the attached FIGS., especially in
One experimental set-up for the spraying step, which is a particularly preferred step of forming the hydrates, includes p, T parameters (pressure, temperature). Specifically T=5.5° C. and p(nozzle)=29.3 bar. This setup proved that the spraying technique is capable to strongly enhance the rapid formation of hydrate. In the experimental setup a nozzle was installed that could produce droplets of 10 μm (average size). That is, the nozzle was specially designed to produce droplets having an average size of 10 μm.
From visual observation it could be seen that with the selected nozzle and pressure the aqueous phase was injected at high speed into the high-pressure vessel and hydrate was formed instantaneously. This was the major breakthrough in this experiment.
In the method of the present invention, a mixture containing H2O and THF (tetrahydrofuran) can preferably be used for forming the hydrates via a spraying step. THF is a so-called hydrate promoter and causes that the hydrate is formed at relatively low pressures. THF is not essential but lower pressures makes the capital investment in this technology much lower.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/052929 | 5/18/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62339231 | May 2016 | US |
