The greenhouse gases (GHS) present the danger for well being of the whole planet. Nowadays, all ecologically aware companies have to remove them as efficiently as possible, for the sake of the planet’s well-being. CO2 is the most abundant GHG, and it gathers a lot of attention in the last years due to pollution and climate change. Besides GHG, there are hazardous gases that are products of the industry. One of those gases is hydrogen sulfide gas. This gas is an easily flammable and highly dangerous gas that can leave severe consequences upon human health. Hydrogen sulfide is a product of the decay of bioorganic matter. Therefore, it is commonly found in all types of crude oil. It is also a standard component in natural gas and liquid propane. Hydrogen sulfide can cause issues in pipelines and other metal parts of the equipment due to its high corrosive effects. When H2s reaches the atmosphere, it is easily spread by the diffusion process in lower atmospheric layers due to its density, which is higher than airs. It irritates the eyes, nose, and throat at concentrations as low as 5 ppm. Concentrations around 30 ppm can cause paralysis of the sense of smell. This gas is lethal for humans when they are exposed to concentrations larger than 1000 ppm. The poisoning happens when gas is inhaled by the respiratory tract. A concentration of H2S gas above 100 ppm is Immediately Dangerous to Life and Health (IDLH). Due to its toxicity, the removal and constant monitoring of its concentration in plants are mandatory. It is essential to mention that hydrogen sulfide gas at higher temperatures burns during the oxidation process and generates toxic gases, such as sulfur dioxide.
It is essential to mention that even one of the fastest-growing renewable sources, geothermal energy, generates a tremendous amount of H2S gas, which has to be removed. Although its concentrations are far less than one found in crude oils and natural gas, the H2S gas represents the product of bio-fuels also. The energy trends are changing in the last years, but the gases mentioned above will remain to be byproducts in this industry for a long time. Thus, there is a need for high efficient removal processes such as amine gas treatment.
Amine gas treating, widely known as amine scrubbing, gas sweetening, and acid gas removal, represents the group of processes where aqueous solutions of various amines are used to remove hydrogen sulfide (H2S) and carbon dioxide (CO2) from gases. The H2S is gaseous acid; therefore, the removal process got the name sweetening.
The amine gas treatment is used as a pretreatment of natural gas. Still, it is also used for the removal of the mentioned components in flue gas, which is the byproduct of the combustion processes in the industry. The process mechanism is quite the same in both situations. The same principles for the mass transfers are included in both cases, but equipment sizing, working parameters, and choice of the amines depends upon various conditions. The recently presented alternative for H2S removal, which is the usage of filters with active carbon, showed many disadvantages. Firstly, this technology has to include chemical impregnation besides the usage of active carbon to be highly efficient. Secondly, it is more likely applicable just for the treatment of flue gasses, because high-pressure drops occur in the membrane and that negatively affects the inlet stream of the crude oil and natural gas. Lastly, equipment cost analysis shows that this technology requires high capital and operating costs. On the other side, amine gas treatment proved itself as an economically beneficial process. Also, aqueous amines that are primary inlet materials can be easily purchased worldwide by affordable prices.
2 Input materials and chemical mechanism of the process
The aqueous amines that are commonly used as inlet material in the industry are alkanol amines. The widespread used alkanolamines are the following: Monoethanolamine (MEA), Diethanolamine (DEA), and Methildiethanolamine (MDEA), Diisopropanolamine (DIPA) and Diglycolamine (DGA).
MEA and DEA are industrially produced by the reaction of aqueous ammonia with ethylene oxide. MEA and DEA are primary and secondary amines. They are highly reactive and can efficiently remove a high volume of CO2 and H2S due to a high reaction rate. The constraint is defined by the stoichiometry, and one mol of amine reacts with 0.5 mols of CO2 or H2S. However, due to stoichiometry, the loading capacity is limited to 0.5 mol CO2 per mole of amine. MEA and DEA also require a large amount of energy to strip the CO2 during regeneration, which can be up to 70% of total operating costs. They are also more corrosive and chemically unstable compared to other amines. Ammonia is one of the most abundantly produced inorganic compounds. It is a highly affordable material, and it has various purposes. Fourteen million tones of ammonia were globally produced in 2019 by Statista analysis. Ammonia has to be converted in an aqueous state, and then it’s ready for amine production. In all production processes, ethylene oxide and ammonia react with each other in a batch process that yields a crude mixture of ethanolamine, diethanolamine, and triethanolamine. The ratio of the products is controlled by the stoichiometry of the reactants. All three products have different evaporation temperatures, so they are easily separated by the process of distillation.
Every alkanol amine has its specific affinity towards CO2 and H2S. Alkanol amine mustn’t have an affinity toward methane and other light alkanes. The absorption as a mass transfer phenomenon can be represented as chemisorption and physisorption. For chemisorption, the solubility of the gas in the liquid significantly increases at low pressures, while at higher pressures, absorption affinity decreases. The alkanol amines are favorably used in industry due to their affinity towards these gases. Most of the other solvents mainly absorb the gas physically, which can’t be efficient as absorption where both transport phenomena take place.
Now, the question that comes up is why the weak base is used for absorption when a much stronger basis can also be efficient. The answer is that alkanol amines are perfectly balanced compounds for absorption and regeneration purposes. Alcohol group is an electron-withdrawing group, and it contributes to the compound’s polarity. Most of the alkanolamines possess pKa values only slightly lower than those of methylamines. This phenomenon is caused by the presence of an alcohol group, which creates the hydrogen bonds to the amine’s proton of the same order. Lone electron pair from oxygen atom forms an intramolecular hydrogen bond with a hydrogen atom from the amine group. So, the lone pair of Nitrogen atom is the only available place for the absorption of the proton. Research proved that intramolecular hydrogen bonds have a crucial role in the determination of the pKa values of alkanolamines along with their temperature dependency. The pKa or alkalinity of the compound is not always a decisive factor for the determination of the kinetic constant, but it undoubtedly plays a significant role in kinetics.
Below are presented reactions of MDEA with CO2 and H2S:
Here, two processes co-occur. The lone pair of Nitrogen atom gains a proton while CO2 and H2 are diluted and transferred into ionic forms. As we may notice, this is a reverse reaction; thus, it is crucial to ensure proper conditions that will favor reaction in the desired direction. Firstly the concentration of a chosen amine has to be at least ten times higher than the concentration of the gases for successful absorption. Secondly, the chemical reaction of absorption is an exothermic process, which means that heat is released during the process. Both reactions present pseudo-first-order kinetics reaction, where rCO2 = k [CO2] and rH2S = k [H2S]. Where kinetic constant is expressed as:
Based on heat and equilibrium law, when the temperature is low, the reactants are abundant, and in this case, the direct reaction will be dominant. When temperatures are higher, the reverse reaction will be favorite, which is extremely important in the regeneration step.
3 Process flow diagram, equipment and description of the process
Common working conditions in the process units:
Absorber: 35 to 50°C and 5 to 205 atm of absolute pressure
Regenerator: 100 to 126°C and 1.4 to 1.7 atm of absolute pressure at tower bottom
It is common engineering praxis to maintain the temperature difference between lean amine and sour gas at least 5°C. If the temperature difference is closer, the condensation of hydrocarbons could occur, which is not acceptable in this process. In the design of this process, most of the sources recommended a steam ratio, defined as the mass flow of steam per volume of amine circulation. The majority of these sources propose the 0.12 kg/l as an optimal value for the steam ratio.
Residual H2S for an MDEA System at feed gas H2S/CO2 ratios of 0.25, 0.5, 1, 1.5, 2, 4, and 10
The absorption process occurs while fluids are flowing in a counter flow direction. The aqueous stream flows downward, while the gaseous stream flows upward. Based on the proposed temperature and pressure suggestion, the optimal number of trays in most cases is 22.
Outlet stream at the top of the absorber presents the stream of natural gas without the presence of CO2 and H2S.
From the physisorption perception of view, the driving force in the absorption process is the partial pressure of the particular gas. Thus, absorption would hardly be feasible under the low pressure without the proper reboiler’s duty. The interesting fact derived from recent process optimization is that higher temperatures than 104 degrees Celsius are not adding any benefits in terms of energy efficiency. At first sight, this fact can sound illogical for an engineer, but when the overall process is integrated in terms of material and energy balance, it is indeed apparent. Below presented graphic clearly shows that a minimum of heat demand is reached at 104 degrees Celsius.
As we may notice from the process flow diagram, there is a recirculation stream of rich amine at the bottom of the absorber. This stream is not a typical recirculation stream, where one part of the stream continuously recirculates to ensure the process efficiency. Every alkanol amine has its saturation level of CO2/H2S at a specific temperature. Thus, the control system of the recirculation stream should be designed in the following way. A control valve that controls the inlet stream of the regenerator should be normally closed. On the other side, the rich amine should recirculate until it reaches the saturation level. When that level is achieved the actuator is opening the outlet valve and reaches amine travels to the regenerator. It is also essential to continually control the flow of the aqueous inlet stream. With tight constraints in terms of the desired inlet flow occurrence of flooding will be avoided. In this way, operation costs of the process are minimized due to proper energy usage.
Another example of efficient energy usage in this process represents the heat exchanger where lean amine transfers heat to the rich amine. It is not possible with this design to achieve that energy from lean to rich amine is perfectly transferred. Hence the reach amine has to be even more cooled by the usage of chilled water, which is a viable solution due to the abundance of water as inlet raw material.
The last step of the amine gas treatment presents the desorption widely known as a gas regeneration. As we said earlier, the alkanolamines are perfect compounds in this case because absorption and desorption can be achieved by proper conditions. Therefore, the pressure of the reach amine is reduced by the usage of the expansion valve. When the pressure of the liquid is reduced the partial pressures of the C02 and H2S are rapidly approaching their saturation pressure. The rise of the temperature to a proper value enables the evaporation of gases in the regenerator. The liquid-vapor equilibrium is formed at every trace. Gases as lighter substances travel upwards and lean amine in the form of liquid travels downwards.
The liquid-vapor equilibrium is achieved in the reflux drum also. The gas from the top of the regenerator is firstly condensed and then enters the reflux drum. This step is involved to ensure that no or minimal amount of alcohol amine exits from the system. That leads us to the conclusion that amine continuously circulates in both process units. It changes its composure, but it is not part of the outlet stream. Pump that is pushing reflux back to the regenerator doesn’t require a lot of energy due to minor pressure demands in the regenerator.
The reboiler heat demands were explained according to the energy needs of the rich amine stream. It is crucial to notice that the reboiler is also a flash drum. As we said earlier, the rich amine stream is saturated with gases, and it is most likely that some percentage of gases would appear in the flash drum. To avoid that, engineers must design the regenerator with a sufficient number of trays. One of the common issues that come with regenerator design is oversizing. Oversizing of the equipment leads to overwhelming investment costs. On the other side, it is essential to keep residual acid gases at the bottom of the regenerator under the permitted level. Otherwise, acidic gases travel at the top of the absorption column, where the majority of them will evaporate. That undesired scenario produces insufficiently clean natural gas and, the H2S concentration in the sweet gas will be too high to meet pipeline specifications.
4 Discussion about design and conclusion
Beside recommended parameters and values, there are many factors that an engineer has to be aware of when design an amine gas treatment facility. The composure of natural gas varies from place to place, and it has to be examined. Based on the natural gas composure and availability of amines, an engineer has to choose proper alkanolamine that will satisfy all criteria. Depending on which particular amine is chosen, there are typical concentrations that are found as optimal for this process.
The cooling and heating utility is water, which is highly affordable material anywhere. The last concern about the design is the proper fuel for heating the water. The best solution for reaching an even more efficient process is using natural gas, which is actually the target compound of exploitation. The combustion of natural gas releases heat that could generate the needed steam. That would be the proper solution for refineries. On the other side, thermal and geothermal plants should use the remained heat of the flue gases to heat the water and transform it into steam. Application of the cogeneration principle, in this case, would be highly beneficial for a significant reduction of operating costs.
The overall efficiency of the process is extremely high. The acidic gases are removed, and that is the main and only purpose. There is still a lot of space for further improvements in terms of energy conservation and process optimization, with a minor increase in investment costs. Thus, the amine gas treatment has a bright future, and it firmly holds the first place in flue gas treatment.
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