University of Groningen Carbon dioxide removal processes by alkanolamines in aqueous organic solvents Hamborg, Espen Steinseth IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2011 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Hamborg, E. S. (2011). Carbon dioxide removal processes by alkanolamines in aqueous organic solvents s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 20-07-2018
Chapter 1 General Introduction 1.1 Gas purification Gas purification involves the removal of one or more gas phase impurities from gas streams. Because of the growing importance of air pollution control and stringent emission standards, gas purification technologies have become increasingly important. The most encountered gas purification processes are listed below. [1] Absorption into a liquid. Absorption refers to the (chemical) transfer of a gaseous component into the liquid phase. The gaseous component dissolves in the liquid phase or reacts with a liquid component to form a reaction product, which can be regenerated. Desorption is the reverse of absorption, the gaseous component is transferred from the liquid phase to the gas phase. Absorption is the single most important operation of gas purification processes, and this thesis solely deals with absorption and desorption processes. Adsorption on a solid. Adsorption refers to the adhesion a gaseous component to the surface of a (microporous) solid. The nature of the attractive forces holding the gaseous components to the surface depends on the different components involved, but can generally be classified as physisorption or chemisorption. The former refers to weaker attractive forces between the gaseous component and the surface, while the latter refers to stronger (chemical) attractive forces. Desorption of the gaseous component from the surface can take place by increasing the temperature or reducing the partial pressure of the component in the gas phase. Permeation through a membrane barrier. Permeation through a membrane refers to the selective transport of (gaseous) components through a physical membrane barrier. The separation of the individual gaseous components is usually based on the different permeation rate of each component through the membrane, and is determined by the characteristics of the migrating components and the membrane material. The driving force through the membrane is based on concentration difference across the membrane. A membrane installation usually consists of several multiple membrane stages in order to archive desired purities of the product stream. Chemical conversion to a product. Chemical conversion to a product refers to (non- )catalytic reactions of the gaseous components. The reactions can take place in the gas phase, or also in a liquid phase or on a solid. The two latter can be considered a case of absorption and adsorption without any form of regeneration. 1
2 CHAPTER 1. GENERAL INTRODUCTION Condensation. Condensation refers to the removal of gaseous components with a relative high boiling point from gaseous components with a lower boiling point by the simple means of cooling the gas stream to a temperature at which some gaseous components condense. The condensate can easily be collected. 1.2 Principles of gas absorption The principles of gas absorption can be generally be classified into the following three different absorption processes. Absorption without chemical reactions. Absorption without chemical reactions, or physical absorption, is a process in which a gaseous component is more soluble in the liquid absorbent than other components of the gas stream. The equilibrium concentration of the gaseous component in the liquid phase is dependent on the partial pressure of the component in the gas phase and the temperature. Absorption with reversible chemical reactions. Absorption with reversible chemical reactions, or chemical absorption, is a process in which a gaseous component reacts with reactant(s) in the liquid absorbent. The reaction is reversible and of finite speed. The equilibrium concentration of the gaseous component in the liquid phase is dependent on the concentration and type of the reactant(s), the partial pressure of the gaseous component in the gas phase, and the temperature. The reactions are usually reversed by increased temperatures or decreased gas phase partial pressure of the gaseous component. Absorption with irreversible chemical reactions. Absorption with irreversible chemical reactions is also a form of chemical absorption, and is a process in which a gaseous component reacts irreversibly with reactant(s) in the liquid absorbent. The reactions are usually very fast and can in some cases be considered as instantaneous. Because of the irreversible nature, the reaction product(s) can not be readily regenerated to release the reactant(s). 1.3 Acid gas removal by absorption Acid gas removal by absorption has been used to separate carbon dioxide (CO 2 ) and other acid gases such as hydrogen sulfide (H 2 S), sulfur dioxide (SO 2 ), carbonyl sulfide (COS), carbon disulfide (CS 2 ), and mercaptans from natural gas, hydrogen, and other gas streams since the 1930s. [1, 2] The basic process covering this application, in which an acid gas is absorbed from a gas stream into an aqueous solution of an (alkanol)amine, was patented as early as 1930 by Bottoms [3] and shown in Figure 1.1. The basic principles of this process are still very much like the ones used for acid gas removal today; the untreated gas stream enters the absorber at the bottom of the column where it is contacted with the solvent at ambient temperatures. The solvent flows from the top countercurrently down the column where it gradually takes up more acid gas by a chemical reaction, until it leaves the bottom of the column as a rich absorbent. The treated gas leaves the top of the absorber to be used for other purposes or released to the atmosphere. The rich solvent is heated in a heat exchanger before it is directed to the top of the desorber column where the (alkanol)amine is regenerated with steam at elevated temperatures, in the range of 100 to 120 C. The acid gas is chemically released from the (alkanol)amine and flows up through the desorber column together with evaporized water. The evaporized water is condensed from the stripping gas in the overhead condenser, thus providing pure acid gas(es) which can be used for other purposes or geologic sequestration. The regenerated and lean
3 Figure 1.1: The acid gas removal process as invented by Bottoms in 1930 [3]. The left column is the absorber unit, the middle part shows pumps, a cooler, and a heat exchanger, and the right column is the desorber unit with an overhead water condenser. alkanol(amine) is directed back to the top of the absorber via a heat exchanger and a cooler to reach ambient temperatures. The aforementioned process has been used for the removal of acid gases from medium to high pressure gas streams since the 1930s primarily to reach a desired (market) gas composition of the treated stream. The treated gas stream would usually be used in a process somewhere else or in the case of natural gas used for energy production in any form. The removed acid gas is used for other (commercial) purposes, vented, or stored as solid waste. An example of commercial use of a removed acid gas is the use of removed CO 2 from natural gas for enhanced oil recovery. The CO 2 is then utilized as pressure support in order to maintain the pressure in the oil reservoir for prolonged crude oil extraction. [1] With increased awareness of the consequences of CO 2 emissions to the atmosphere, the focus of removing CO 2 from low pressure gas streams has gained increased attention the last years. [4] The concept of removing CO 2 from low pressure gas streams, i.e. flue gases, was for the first time evaluated in the early 1990s. [5,6] In particular,
4 CHAPTER 1. GENERAL INTRODUCTION this applies to the removal of CO 2 from the flue gases of fossil-fueled power plants. As of today, there are no operating installations which remove CO 2 at larger scale, i.e. flue gas streams from power plants higher than 50 MW. [2] There are however several pilot plant installations in operation which are connected to larger power plants. The untreated gas stream entering these pilot plants are typically taken as a side stream from the flue gas of the power plants. By such, the pilot plants treat gas under real flue gas conditions. The pilot plant installations are set up primarily to collect the necessary and required information and experience in order to scale the aforementioned process up to the size needed for treatment of the flue gas from a full scale power plant. Triethanolamine (TEA) was the first alkanolamine which became commercially available and was used in early acid gas treating plants. Because of its low capacity, reactivity, and poor stability, TEA has been largely displaced by monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA), and piperazine (PZ) due to commercial interests. Industrial processes have successfully been developed in the past where the absorbent is usually based on (mixtures of) the aforementioned (alkanol)amines. [1] Several research initiatives are currently under way and aimed to develop better and more efficient absorbents and processes for acid gas removal, especially toward the removal of CO 2 from flue gases. Some of these initiatives aim to develop and improve the performance of sophisticated absorbents such as ionic liquids, (in)organic salts of amino acids, polymeric amines, hyperbranched polymers, carbonic anhydrase enzymes, liquid two-phase systems, immobilized activators, etc., while others aim to improve processes involving simpler amines such as ammonia or MEA. 1.4 General introduction to this thesis This thesis is divided into the following Chapters listed below. Chapter 2. This Chapter describes the experimental determination of dissociation constant and thermodynamic properties of several amines, alkanolamines, and amino acids at infinite dilution in pure water at temperatures ranging from from 293 to 353 K. The values determined provide information about the use of these compounds as possible absorbents for acid gas removal, i.e. the basic strength behavior of the absorbent in the absorber and desorber section. Some of the compounds investigated in this Chapter, e.g. MEA, MDEA, PZ, DIPA, etc., are currently commercially used alkanolamines. Chapter 3. This Chapter describes the experimental determination of dissociation constants and thermodynamic properties of MEA and MDEA in aqueous methanol, ethanol, and t-butanol mixtures at infinite dilution with respect to the alkanolamines. The alcohol mole fractions were ranging from 0.2 to 0.95 and the temperatures from 283 to 323 K, 283 to 333 K, and at 298.15 K, respective to the different solvent mixtures. The results of this Chapter provides the same information as Chapter 2 at different solvent compositions, i.e. the varying dielectric constant or polarity of the solvent in which the alkanolamine is dissolved. Chapter 4. This Chapter describes the experimental determination of liquid phase mass transfer coefficients in a controlled environment during gas absorption into a liquid and gas desorption from a liquid. At identical operating conditions, the mass transfer coefficients for absorption and desorption appeared to be the same within the reported experimental uncertainty.
5 Chapter 5. This Chapter describes the experimental determination of chemical enhancement factors in a controlled environment during gas absorption into a liquid and gas desorption from a liquid. At identical operating conditions, the chemical enhancements factors for absorption and desorption appeared to be the same within the reported experimental uncertainty. The results of Chapter 4 were used in this Chapter in order to derive the experimental results. Chapter 6. This Chapter describes the experimental determination of forward and reverse kinetic rate parameters in a controlled environment during gas absorption into a liquid and gas desorption from a liquid. At identical operation conditions, the forward and reverse kinetic rate parameters derived by an analytical relation based on the Higbie penetration theory appeared to be within 25 % of those numerically derived by a system of partial differential equations also based on the Higbie penetration theory. The temperature dependent kinetic rate parameters were, within the uncertainties, shown to remain constant with respect to solute loadings. In addition, the reaction order of the forward reaction in solutions of different CO 2 loadings was shown to be close to unity, and in agreement with the proposed reaction mechanism. Arrhenius type of equations already developed for correlation of forward kinetic rate parameters were further modified in order to sufficiently correlate reverse kinetic rate parameters, and thus form a tool for the correlation and prediction of reverse kinetic rate parameters for engineering purposes. The experimentally determined forward and reverse kinetic rate parameters were accordingly found to be related by an overall temperature dependent chemical equilibrium constant. The results of Chapters 4 and 5 were used in this Chapter in order to derive the experimental results. Chapter 7. This Chapter describes the concepts and fundamentals of a developed process for CO 2 removal using alkanolamines in aqueous organic solvents. The process is based on the results of Chapters 2 and 3 which provide the values of the decreased alkanolamine dissociation at elevated temperatures and reduced solvent dielectric constant (or polarity). The findings of Chapters 4 to 6 relates the forward and reverse kinetic rate constant to the chemical equilibrium constant, which is altered by the use of organic solvents in aqueous alkanolamine solutions according to Chapter 3. In addition, the similarities between absorption and desorption processes proven in Chapters 4 to 6 were also applicable to the process concept described in this Chapter, i.e. the effects of the addition of an organic component before the desorber section. The results of Chapters 2 to 6 are thus applied to this final Chapter and this forms a coherent thesis. The conclusions of Chapter 7 are however open-ended, and warrant further future work.