Process Intensification for Sustainable Energy Conversion

Preface

Process Intensification (PI) is a hot topic in both industrial and academic worlds and is widely considered a key enabling approach to improve the competitiveness of the chemical industry. In essence, Process Intensification aims at the design of innovative (reactor) concepts for significantly smaller, safer, more efficient and cheaper processes.

The need for more efficient processes and more flexible engineering designs with, at the same time, increased safety and decreased environmental footprint is pushing the chemical industry towards novel research in this field. This is also reflected in a large number of funding initiatives focussing on PI worldwide. An example in Europe is the large emphasis of PI in the new research framework Horizon 2020 http://ec.europa.eu/programmes/horizon2020/ and in particular in the subprogram SPIRE http://www.spire2030.eu/. Following the increasing interest in PI, various interesting books have been published discussing new reactor concepts for the chemical and process industries.

Energy is the driver for many developments, and energy demands are foreseen to further increase with increasing population and fast developments in Asian countries. This creates many opportunities for process intensification in the energy sector and, in particular, the reduction of anthropogenic CO2 emissions associated with fossil fuel conversion can help in the long transition period towards renewable energy conversion scenarios. The aim of this book is to compensate for the lack of academic/scientific books concerning the application of process intensification strategies to sustainable energy conversion.

In this book, we have collected, in 11 chapters, information on novel possible intensified methods and reactors for sustainable energy conversion, including – but not limited to – novel concepts for chemical looping combustion, PI concepts for CO2 capture, oxy-fuel and oxygen permeable membranes, blue energy and biomass conversion.

The book has been written for academicians, PhD students, researchers and engineers curious about novel trends of PI applied for a more sustainable energy conversion.

Fausto Gallucci,

Martin van Sint Annaland

List of Contributors

Marco Astolfi,Energy Department, Politecnico di Milano, Italy

Tim Boeltken, Karlsruhe Institute of Technology (KIT), Institute for Micro Process Engineering (IMVT), Germany

Rune Bredesen, SINTEF Materials and Chemistry, Norway

Christoph Brunner, AEE - Institut für Nachhaltige Technologien, Austria

Paolo Chiesa, Energy Department, Politecnico di Milano, Italy

Luca Di Felice, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Roland Dittmeyer, Karlsruhe Institute of Technology (KIT), Institute for Micro Process Engineering (IMVT), Germany

Pier Ugo Foscolo, Department of Industrial Engineering, University of L'Aquila, Italy

Fausto Gallucci, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Katia Gallucci, Department of Industrial Engineering, University of L'Aquila, Italy

Antonio Giuffrida, Energy Department, Politecnico di Milano, Italy

Paul H. Hamers, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Daniel Jansen, Energy & Resources, Copernicus Institute of Sustainable Development, Faculty of Geosciences, Utrecht University, The Netherlands

Giampaolo Manzolini, Energy Department, Politecnico di Milano, Italy

Francesca Micheli, Chemical Engineering Department, University of L'Aquila, Italy

Bart Michielsen, Materials Technology, Flemish Institute for Technological Research, Belgium

Vesna Middelkoop, Materials Technology, Flemish Institute for Technological Research, Belgium

Bettina Muster, AEE - Institut für Nachhaltige Technologien, Austria

Thijs A. Peters, SINTEF Materials and Chemistry, Norway

Martin van Sint Annaland, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Vincenzo Spallina, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Martin Tuinier, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, The Netherlands

Andrew David Wright, Energy Technology, Air Products PLC, UK

Chapter 1

Introduction

Fausto Gallucci and Martin van Sint Annaland

Eindhoven University of Technology, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven, The Netherlands

It is expected that in the current century, the theme energy will become increasingly more important and will pose some serious challenges to our society and our way of living, but it may also create opportunities.

On the one hand, the combination of a rapidly growing world population and increasing energy consumption per capita requires large investments to secure sufficient energy supply at affordable prices. On the other hand, fossil fuel reserves are shrinking, while the transition toward a world economy based on energy supply via sustainable or renewable resources is still in its infancy. According to the World Energy Outlook 2013 of the International Energy Agency (IEA), the world energy demand will increase by more than 30% by 2035 (compared with 2011) and the demand for oil alone will still be more than 57% in 2035. Oil and gas reserves are increasingly concentrated in a few countries that control them through monopoly companies. The dependence of Europe on imported oil and gas is growing: we import 50% of our energy, and it will be 55% by 2035 (Bp Outlook 2035), if we do not act.

The relevance of this issue is even higher when one relates the increase in anthropogenic CO2 emissions by the use of fossil fuels to the evident changes in the Earth's climate. The International Panel on Climate Change (IPCC) has collected results of substantial research efforts to obtain a comprehensive scientific framework describing the evolution of the climate over very long time periods, the observed deviations from this behavior in recent times, the interpretation of both natural and anthropogenic causes and their effect on the increase of the greenhouse effect, the consequences of global warming in the past, present and future and possible solutions to combat further climate changes. In its 2013 Assessment Report, IPCC conclude that (Climate Change 2013: The Physical Science Basis), see Figure 1.1:

Figure 1.1 Detection and attribution signals in some elements of the climate system, at regional scales (top panels) and global scales (bottom four panels). Brown panels are land surface–temperature–time series, green panels are precipitation–time series, blue panels are ocean heat content–time series and white panels are sea ice–time series. Observations are shown on each panel in black or black and shades of grey. Blue shading is the model time series for natural forcing simulations and pink shading is the combined natural and anthropogenic forcings. The dark blue and dark red lines are the ensemble means from the model simulations. All panels show the 5–95% intervals of the natural forcing simulations and the natural and anthropogenic forcing simulations.

(Source: Extracted from the IPCC report 2013)

From up in the stratosphere, down through the troposphere to the surface of the Earth and into the depths of the oceans there are detectable signals of change such that the assessed likelihood of a detectable, and often quantifiable, human contribution ranges from likely to extremely likely for many climate variables.

According to IPCC, the effect of human activities on changes in the climate is very likely to have been dominating natural variations (due to, e.g., variations in solar irradiance) especially in the past 50 years. Since the beginning of the industrial revolution, the concentrations of the relevant greenhouse gases (especially carbon dioxide, methane, nitrous oxide, and halocarbons) have increased substantially and now by far exceed natural ranges encountered in the past 650,000 years [1].

On the short term, significant reductions of carbon dioxide emissions may be attained from energy savings, for example, via efficiency improvements both in power production and consumer products and as a consequence of increased public awareness. However, strong economic growth anticipated in especially the developing countries is expected to impede a net decrease in anthropogenic emissions. On the longer term, the use of fossil fuels for energy supply will need to be phased out not only to stabilize greenhouse gas concentrations but also to avoid shortages in raw materials for the production of, for example, bulk chemicals.

The transition towards a world economy based on energy supply via sustainable sources such as wind-, hydro- and solar energy, or nuclear power (of which fission still suffers from a bad public image caused by concerns over nuclear waste and proliferation, whereas fusion has so far failed to live up to its potential) is therefore expected to be a lengthy process that cannot be expected to be solely responsible for the stabilization of atmospheric greenhouse gas concentrations in this century. Rather, a combination of many of the mitigation alternatives will need to be adopted to significantly curb CO2 emissions.

In this respect, novel concepts based on process intensification can help to reduce CO2 emissions and can lead the transition towards a more sustainable energy scenario. Indeed, according to Ramshaw [2], process intensification is a strategy for making dramatic reductions in the size of a chemical plant so as to reach a given production objective. As such, applying process intensification to the energy sector can result in a dramatic decrease in the production of wastes including greenhouse gas emissions.

According to Stankiewicz and Moulijn [3], the whole field of process intensification can be classified into two main categories:

Process-intensifying equipment:

These include novel reactors and intensive mixing, heat-transfer and mass-transfer devices, and so on.

Process-intensifying methods:

These include new or hybrid separations, integration of reaction and separation, heat exchange, or phase transition (in multifunctional reactors), techniques using alternative energy sources (light, ultrasound, etc.) and new process-control methods (like intentional unsteady-state operation).

Clearly, as also indicated by Stankiewicz and Moulijn, there is a big overlap between the two areas. For instance, membrane reactors are an example of process-intensifying equipment (novel reactor) making use of process-intensifying methods (integration of reaction and separation).

Since the invention of the term process intensification, many articles and books appeared on the same topic. An interested reader is referred to the book of Reay et al. [4] for an overview of the various process intensification methods. In the present book, a selection of different, novel process intensification methods and reactors are presented and discussed with the focus on sustainable energy conversion.

In particular, in Chapter 2 the development of a new cryogenic separation technology based on dynamic operation of packed bed columns is described. When it is possible to exploit the cold available at, for example, LNG regasification stations, this new technology could be used as an efficient post-combustion CO2 capture technology. In the chapter, the technology is described to freeze-out CO2 from flue gases at atmospheric pressures. The dynamic operation and the effects of the operating conditions have been analyzed in detail using modelling and an experimental proof of principle at laboratory scale and small pilot scale is provided. Finally, a techno-economic analysis shows the great potential of the technology over other post-combustion capture processes such as amine scrubbing and membrane separation, when cold duty is available at low prices or when high CO2 capture efficiencies are required. This makes the cryogenic technology also particularly interesting as an auxiliary unit downstream of other post-combustion technologies.

Chapter 3 describes the application of membrane reactors in pre-combustion CO2 capture technologies. Different membrane reactor configurations are described, among which the fluidized bed membrane reactor configuration seems to have the most potential. In this concept, hydrogen perm-selective membranes are submerged in a fluidized suspension. Thus, mass and heat transfer coefficients are much improved compared to packed bed membrane reactor configurations (decreasing problems in heat management and concentration polarization), while maintaining a relatively large amount of catalyst combined with a relatively low pressure drop in comparison with micro-membrane reactors. The chapter also describes a hybrid concept integrating both membrane reactors and chemical looping combustion for autothermal operation with integrated CO2 capture. With this new concept, high hydrogen efficiency can be obtained at lower temperatures compared with other concepts, while the amount of membrane area required is kept to a minimum.

Chapter 4 focuses on the possibility to apply high-temperature oxygen-selective membranes in oxy-fuel power production systems. These perovskite-like or mixed ionic electronic conducting materials present an infinite perm-selectivity for oxygen compared with other gases and can thus be used to separate oxygen from air at high temperatures. The chapter describes the main features of oxygen selective membranes, their production methods and their integration in membrane (reactor) modules. The chapter also reports on the progress of research projects on oxygen selective membranes.

A different kind of oxy-combustion can be achieved by exploiting the air separation through a solid material that is alternating oxidized (with air) and reduced (with a fuel). This solid material is called oxygen carrier and is the catalyst of a new concept called chemical looping combustion. Chapter 5 describes chemical looping combustion (CLC) concepts for power production. The oxidation and reduction stages can be achieved with different reactor technologies. In particular, it is possible to circulate the solid material between two fluidized bed reactors, where one reactor operates as oxidation reactor (air reactor) and a second reactor operates as reduction stage (fuel reactor). Another possibility is to keep the solid in fixed position and to alternately switch the gas feed streams; in this case, the concept is based on dynamically operated packed beds. Finally, the solid can be kept in a fixed position and the reactor can be rotated. The chapter reports all the possible configurations for CLC and compares the efficiencies of different concepts when exploited for power production.

Another interesting concept that can also be used for power production with integrated CO2 capture is sorption-enhanced fuel conversion and is described in Chapter 6. In this concept, a solid sorbent is used together with the catalyst such that the CO2 produced during the reforming (or water-gas shift) of a fuel can be directly captured and separated, while the other products, often hydrogen, can be used for downstream power production. Also with sorption-enhanced processes, the efficiency of the CO2 capture can be significantly increased, because the hydrogen is generally produced at high pressure, whereas in many other concepts, such as the membrane reactor concepts presented in Chapter 3, the hydrogen is produced at lower pressures. On the other hand, the CO2 pressure is higher in membrane processes compared with sorption-enhanced processes. The selection between these two concepts is thus related to many different parameters, and the efficiency should be evaluated separately for each individual case.

Chapter 7 reports on the hydrogen production for fuel cell applications. Also for this application, membrane reactors are described in detail. The need for smart reactor designs in order to reduce or circumvent concentration polarization (or bulk-to-membrane mass transfer resistances) and improve the heat management is pointed out, which places stringent requirements on membrane stability, catalyst activity, sealing technology, support materials and module design. The chapter describes the ongoing research on micro-structured membrane reactors which will be a step forward towards low cost and high productivity units for hydrogen production.

All these chapters describe the smart use of reactor design and process integration to increase the efficiency and reduce the emissions when using fossil fuels as energy source. Of course, intensified systems can also be used for bio-based energy sources. Chapter 8 reports the possibility to convert biomass into substitute natural gas. The chapter describes how both packed beds and fluidized bed reactors can effectively be used to improve the methanation reaction so that the products of biomass gasification can be converted into a more sustainable methane stream.

Chapter 9 describes how to efficiently make use of a salinity gradient for power production. This is a completely CO2-free power production system, so that the concept is often referred to as blue energy. The concept is based on the fact that chemical potential is associated with a difference in salt concentration, so that electric power can be produced by exploiting the salinity gradient between freshwater of rivers and seawater. When exploited at large scale, this concept can supply a large part of the electricity required worldwide. However, the concept is not that easy as conventional hydropower electricity production. The chapter describes the fundamentals of the salinity gradient technology and the attainable energy efficiencies associated with this energy conversion technology.

Chapter 10 describes how process intensification can be applied to efficiently make use of the most abundant energy source: solar energy. The question is addressed whether an intensified process layout can increase the potential of solar process heat and its efficiency. The chapter describes how to make efficient use of solar heat by designing the heat profile in a certain process, where the importance of process modeling in this respect is stressed.

Finally, Chapter 11 reports on intensified processes for biomass utilization. In particular, the authors describe the broad field of power and combined heat and power (CHP) generation from biomass: more specifically, advances in biomass gasification technology aimed at increases in overall conversion and efficiency and hence in a decreased cost of electricity. Poly-generation strategies (for combined heat, power and chemical production applications) are also considered, with particular reference to recent technological innovations in hot gas cleaning and conditioning; these have been developed to achieve the required improvements in syngas quality and have been validated under industrially relevant conditions.

We surely know that many other intensified processes can be designed for efficient power production, or in general for energy conversion. We hope that the content of this book will stimulate the design, implementation and testing of novel integrated reactor concepts for a more sustainable energy future.

References

1. IPCC report 2007: Climate Change 2007: Mitigation of Climate Change

2. Ramshaw, C. (1995) The Incentive for Process Intensification, Proceedings, 1st Intl. Conf. Proc. Intensif. for Chem. Ind., 18, BHR Group, London, p. 1

3. Stankiewicz, A.I. and Moulijn, J.A. (2000) Transforming Chemical Engineering, Chemical Engineering Progress, AiCHE.

4. Reay, D., Ramshaw, C. and Harvey, A. (2013) Process Intensification, Second edn, Butterworth-Heinemann, Oxford. ISBN: 9780080983042

Chapter 2

Cryogenic CO² Capture

M. van Sint Annaland, M. J. Tuinier and F. Gallucci

Eindhoven University of Technology, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, P.O. Box 513, 5612 AZ, Eindhoven, The Netherlands

2.1 Introduction – CCS and Cryogenic Systems

There is a growing worldwide awareness of the fact that the earth's surface temperatures are changing globally. Although the climate of our planet has been altering continuously during its history, the current changes are taking place at an unprecedented pace and are expected to have dramatic consequences on human kind [1]. Since the Industrial Revolution in the late 19th century, fossil fuels started to play an important role in our energy supply for transportation, heating and electricity. The combustion of fossil fuels results in large amounts of CO2, which are emitted into the atmosphere. The increase in CO2 concentrations coincides with the increase in global temperatures. There is a consensus among most scientists that the rise in CO2 concentrations is responsible for the observed increase in temperatures. In 1988, Intergovernmental Panel on Climate Change (IPCC) was established in order to evaluate the risks of climate change caused by human activities.

Based on the evaluations by the IPCC, it is expected that global temperatures will keep on rising in the next century. In order to prevent or at least minimize further temperature increases, it is necessary to reduce anthropogenic CO2 emissions not only for health and safety reasons but also for economic reasons. A study on the economic effects of climate change by Stern [2] states that the costs of mitigating climate change can be limited to around 1% of global GDP per year. Doing nothing (‘business as usual’) and facing the consequences of climate change will be equivalent to losing 5% to possibly 20% of global GDP each year. Therefore, immediate actions to reduce CO2 emissions are essential.

CO2 emission reduction can be achieved in several ways, in the first place by improving efficiencies. Developments in the automobile industry, for example, are leading to more and more economic engines, consuming less fuel. The efficiency of power plants is also increasing, and chemical industry is able to save energy by, for example, heat integration. These developments will contribute to CO2 emission reductions. However, it is not expected that these efficiency improvements will be sufficient to bring down our CO2 emissions to acceptable levels, mainly because of increased energy demands by the developing countries such as China and India. Therefore, more measures are required. A key measure is to switch our energy supply to renewable energy sources such as biomass, solar and wind energy not only to reduce CO2 emissions but also to bring down our dependency on scarce fossil fuels. However, at this point, power supply by renewable energy sources is still under development and is not yet competitive with conventional power generation based on fossil fuels. A third possible route to emission reduction is nuclear power, but safety issues and nuclear waste disposal are causing moral and political concerns. Due to the aforementioned reasons, it is expected that fossil fuels will continue to play a significant role in our energy supply for the next decades.

It is therefore considered to be necessary to introduce carbon capture and storage (CCS) to mitigate anthropogenic CO2 emissions as a midterm solution until a full transition to energy supply by renewables can be realized.

2.1.1 Carbon Capture and Storage

The goal of CCS is to remove CO2 from flue gases and to store it for the long term. This process is schematically represented in Figure 2.1. Fossil fuels are normally combusted using air. The flue gas is therefore composed of a large amount of N2 and 5–20 vol% CO2. Furthermore, it contains H2O and impurities such as sulphur and nitrous oxides, depending on the feedstock and process. Compressing and storing the entire flue gas, including N2, will be too costly. Therefore, it is necessary to obtain CO2 in purified form first, before it can be stored in geological formations. About 75% of the costs involved in CCS are associated with the capture step [4], and, therefore, many research projects focus on the development or optimization of capture technologies.

Figure 2.1 Schematic overview of CCS process [3]

CO2 capture technologies are often classified into oxy-fuel, pre- and post-combustion processes, which are schematically represented in Figure 2.2. In oxy-fuel processes, fossil fuels are combusted using pure oxygen, circumventing dilution of CO2 with N2. The disadvantage is that an energy-intensive air separation unit is required to obtain pure O2, although this could be avoided by using chemical-looping combustion (see the other chapters of this book and a.o. Ishida and Jin [5], Noorman et al. [6]).

Figure 2.2 Pathways to CO2 capture: (a) conventional combustion process without capture, (b) post-combustion, (c) pre-combustion, (d) oxy-fuel

In pre-combustion processes, fossil fuels are first converted into H2 and CO2 via (autothermal) reforming or partial oxidation and water-gas shift, CO2 is subsequently captured and H2 is fed to the combustion chamber or fuel cell. The advantage is that the separation of CO2 and H2 can be carried out at high pressure, resulting in a high driving force for the separation. The disadvantage of pre-combustion is that these processes can only be applied in new plants and not to the many existing operational facilities.

Post-combustion processes are based on capturing CO2 from flue gases from conventional air-fired combustion processes. A disadvantage is that the CO2 is diluted and, at low pressures, reducing the driving force for separation. However, this technology can be retrofitted to already operating power plants and industries. For this reason, post-combustion is considered the most realistic technology on the short term, even though the efficiency of alternatives could be higher [7].

Several post-combustion technologies are under development, such as amine scrubbing, membrane separation, adsorption. Among these technologies, an interesting option is the cryogenic CO2 separation as reported in the following section.

2.1.2 Cryogenic separation

Cryogenic separation is another option for separating CO2 from gas mixtures. The advantages are that no chemical absorbents or adsorbents or large pressure differences are needed and that high-purity products can be obtained. However, cryogenic CO2 capture is not included in most (economic) comparison studies, as it has been considered as an unrealistic candidate for post-combustion CO2 capture in the first place due to expected high cooling costs and because it has been considered as a gas–liquid separation [4, 8]. At atmospheric pressures, CO2 will go directly from its gas phase to its solid phase (desublimation). In order to be able to carry out the CO2 removal from flue gases as a gas–liquid separation, it is necessary to compress the gas to pressures above the triple point of CO2, which is at 5.2 bar and −56.6 °C for pure CO2, as shown in the phase diagram of pure CO2 in Figure 2.3. Compressing flue gases to high pressures for CO2 capture is too energy (and therefore cost) intensive.

Figure 2.3 Phase diagram of pure CO2

On the other hand, expensive refrigeration can possibly be avoided by exploiting the cold duty available at liquefied natural gas (LNG) regasification sites. Currently, LNG is being regasified by using seawater or water baths that are heated by burning a fuel gas [9]. The global LNG market is strongly growing [10]; therefore, integration of LNG regasification with a cryogenic CO2 capture process could be beneficial.

Clodic and Younes [11, 12] have developed a cryogenic CO2 capture process, in which CO2 is desublimated as a solid onto surfaces of heat exchangers, which are cooled by evaporating a refrigerant blend. With calculations and experimental tests, they showed that their process could compete with other post-combustion CO2 capture processes. The main disadvantage of their system is that the water content in the feed stream to the cooling units should be minimal in order to prevent plugging by ice or an unacceptably high rise in pressure drop during operation. Therefore, several costly steps are required to remove all water traces from the flue gas. In addition, the increasing layer of solid CO2 on the heat exchanger surfaces during the capture cycle will adversely affect the heat transfer, thereby reducing the process efficiency. Moreover, the costly heat exchangers have to be switched to regeneration cycles operated at a different temperature, which should be carried out with great care to avoid excessive mechanical stresses. To overcome these disadvantages, a new process concept is developed to separate CO2 from flue gases at atmospheric pressures.

2.2 Cryogenic Packed Bed Process Concept

In this section, a new process concept based on dynamically operated packed beds is described in detail, where the flue gas is represented as a mixture of N2, CO2 and H2O to simplify the description. Continuous separation of these components can be obtained when three packed beds are operated in parallel in three different steps: a capture, recovery and cooling step. These three steps are discussed consecutively in the following sections focusing on the evolution of axial temperature and mass deposition profiles (see Figure 2.4).

Figure 2.4 Schematic axial temperature and corresponding mass deposition profiles for the capture (a), recovery (b) and cooling (c) steps, respectively

2.2.1 Capture Step

When a gas mixture consisting of N2, CO2 and H2O is being fed at a relatively high temperature Tc,in to an initially cryogenically refrigerated packed bed (at T0), an effective separation between these components can be accomplished, due to differences in dew and sublimation points. The gas mixture will cool down and the packing material will heat up, until H2O starts to condense on the packing surface. A certain amount of H2O per volume of the packing material (indicated as c02-math-0001 in Figure 2.4a) will condense, until a local equilibrium is reached (at a temperature c02-math-0002 ). Actually a very small part of the H2O at the front will be frozen to ice, but simulations have revealed that this is a very small part of the H2O and has negligible influence on the resulting axial temperature and mass deposition profiles. The cold energy stored in the packing will be consumed, and a front of condensing H2O will move through the bed towards the outlet of the bed. At the same time, previously condensed H2O will evaporate due to the incoming relatively hot gas mixture. Therefore, two fronts of evaporating and condensing water will move through the bed, with a faster moving condensing front. After all water is condensed, the gas mixture will be cooled further until CO2 starts to change its phase.

At atmospheric pressure, CO2 will desublimate directly from gas to solid, and, therefore, solid CO2 is deposited on the packing surface. Similar to H2O, two CO2 fronts will move through the bed: an evaporation front and a desublimation front. Again an equilibrium is reached, and a certain amount of CO2 ( c02-math-0003 ) is deposited on the packing surface at a temperature of c02-math-0004 . Note that an effective separation between CO2 and H2O can also be accomplished in this way. N2 will not undergo any phase change (as long as T0 is not chosen too low) and will therefore move through the bed unaffected. When the CO2 desublimation front reaches the end of the bed, CO2 may break through and the bed should be switched to a recovery step just before that.

2.2.2 CO2 Recovery Step

The first zone of the bed is heated to Tc,in during the capture step. This heat is used in the recovery step to evaporate the condensed H2O and frozen CO2. A gas flow consisting of pure CO2 is fed to the bed. When feeding a pure CO2 gas flow at a temperature Tr,in to the packed bed, the gas will be heated up to Tc,in and all fronts will move through the bed, as illustrated in Figure 2.4b. However, during the initial period of the recovery step, the ingoing CO2 will deposit on the packing. Due to the increase in CO2 partial pressure compared to that in the capture step, more CO2 is able to desublimate on the packing surface (from c02-math-0005 to c02-math-0006 ), and the bed temperature will slightly increase to c02-math-0007 .

Pure CO2 is obtained at the outlet of the bed after this new equilibrium is reached. Part of the outgoing CO2 should be compressed for transportation and sequestration, while the other part can be recycled to the inlet of the bed at a temperature Tr,in, which is slightly higher than c02-math-0008 due to the heat production associated with compression in the recycle blower and some unavoidable heat leaks. When all CO2 has been recovered, the bed is switched to a step in which H2O is removed and the bed is cooled simultaneously.

Alternatively, the deposited CO2 could be recovered as liquid, avoiding expensive compression costs required for transportation and storage. This could be accomplished by closing the valves connected to the bed and by introducing heat into the bed. CO2 evaporation occurs and pressure builds up until the system reaches the triple point of pure CO2, and liquid CO2 will be formed. The drawbacks of this alternative process are that pressure vessels are required and that heat should be introduced into the bed, for example, by means of internal tubes. Both measures will result in a significant increase in capital costs. Furthermore, not all liquid CO2 might be recovered from the packing due to the static liquid hold up in the bed. This process is not further explored in this chapter.

2.2.3 H2O Recovery and Cooling Step

In the last step, the bed is cooled down by using a gas flow refrigerated before to temperature T0. The cleaned flue gas can be used for this purpose. Cooling can be performed using a cryogenic refrigerator or by evaporating LNG. H2O is evaporated and removed from the bed during the first period of the cooling step. The N2/H2O mixture can be released to the atmosphere, and when all H2O is recovered, the outgoing flow can be recycled to the inlet of the bed, via a cooler. Temperature and mass deposition profiles are shown in Figure 2.4c. It should be noted that it is not required to cool the entire bed to T0. The last zone can be kept at Tr,in, as during the capture step, this last part will be cooled down by the cleaned flue gas.

2.3 Detailed Numerical Model

2.3.1 Model Description

The prevailing heat and mass transfer processes in the periodically operated packed beds have been investigated with a pseudo-homogeneous, one-dimensional plug flow model with superimposed axial dispersion.

The modelling is based on the following main assumptions:

Heat losses to the environment are small (i.e., adiabatic operation) and additionally a uniform velocity profile exists in the absence of radial temperature and concentration gradients, allowing the consideration of the axial temperature and concentration profiles only.

Possible heat transfer limitations between the solid packing and the bulk of the gas phase are accounted for via effective axial heat dispersion (pseudo-homogeneous model).

The rate of mass deposition and sublimation of CO2 is assumed to be proportional to the local deviation from the phase equilibrium, estimating the equilibration time constant (g) at 1 × 10−6 s/m, which is assumed independent of temperature. The rate of sublimation of previously deposited CO2 is assumed to approach a first-order dependency on the mass deposition when this mass deposition approaches zero [13].

The mass and energy conservation equations have been listed in Table 2.1. The constitutive equations for the transport parameters and the mass deposition rate have been summarized in Tables 2.2 and 2.3, respectively. The gas phase (mixture) properties have been computed according to Reid et al. [14], using the pure component data supplied by Daubert and Danner [15]. Uniform initial temperature profiles are taken without any mass deposited on the solid packing, where the gas phase in the bed is initially N2. Furthermore, the usual Danckwerts-type boundary conditions are applied at the inlet and outlet of the beds.

Table 2.1 Model equations for the 1-D pseudo-homogeneous model

Table 2.2 Heat and mass transfer coefficients for a monolith packing

Table 2.3 Mass deposition rate

A system of strongly non-linear, coupled partial differential equations is solved using a very efficient finite volume discretization technique, using a second-order SDIRK (Singly Diagonally Implicit Runge–Kutta) scheme for the accumulation terms, an explicit fifth-order WENO (Weighted Essentially Non-Oscillatory) scheme for the convection terms (with implicit first-order upwind treatment using the deferred correction method), second-order standard implicit central discretization for the dispersion terms and the standard Newton–Raphson technique for the linearly implicit treatment of the source terms. Moreover, time-step adaptation and local grid refinement procedures have been implemented, making effective use of the WENO smoothness indicators and interpolation polynomials [16]. The steep temperature and mass deposition gradients in combination with the strongly non-linear sublimation kinetics require a very efficient and stable numerical implementation using higher order implicit schemes.

2.3.2 Simulation Results

Simulations have been carried out for all three process steps. The bed properties and process conditions used are listed in Tables 2.4 and 2.5, respectively. A stainless steel monolithic structure is chosen as packing material, because axial dispersion and pressure drop are minimal for this type of packing material while the volumetric heat capacity is relatively high. The axial temperature and mass deposition profiles during the capture step are shown in Figure 2.5a and d, respectively.

Table 2.4 Bed properties used in the numerical study

Table 2.5 Conditions used in the numerical study

Figure 2.5 Simulated axial temperature (a–c) and mass deposition (d–f) profiles for the capture, recovery and cooling step. Bed properties and operating conditions can be found in tables 2.4 and 2.5, respectively

At the chosen initial bed temperature of −140 °C, more than 99% of CO2 is recovered. After 600 seconds, the CO2 desublimation front reaches the end of the bed and the capture cycle should be stopped. The conditions at 600 seconds are used as initial conditions for the simulation of the recovery step. Figure 2.5b and e show that during the recovery step extra CO2 will be deposited on the packing surface and that all deposited CO2 is removed after again 600 seconds.

Now the data at the end of the recovery step are used as initial conditions for the cooling step. A refrigerated N2 flow is being fed to the bed and profiles will develop as illustrated in Figure 2.5c and f. Note that not the entire bed is cooled down in this step, the last zone is cooled down during the capture step. The results show that CO2 and H2O capture can be integrated in one single bed.

However, the temperature profile during the recovery step shows that the heat stored in the first zone during the capture step is only sufficient to remove H2O from the bed. The hot zone is moved through the bed, but due to axial heat dispersion this hot zone will be spread out over the bed, which is clearly visible in Figure 2.5b.

When feeding the gas mixture at realistic flue gas temperatures (which are generally lower than 250 °C) during the capture step, insufficient heat is stored in the packing to evaporate previously condensed water again. A possibility would be to introduce extra heat into the bed in the initial period of the recovery step. However, more practical is to carry out the H2O capture step in a separate smaller bed, which can be cooled down to temperatures much higher than the initial bed temperature of the CO2 capture bed.

2.3.3 Simplified Model: Sharp Front Approach

The process concept can be described by the advanced numerical model as detailed in the previous section. However, by assuming that the fronts that are formed during the different process steps are perfectly well defined (sharp), a simplified and relatively easy-to-solve and fast model can be developed, referred to as the ‘sharp front approach’. In the first place, this approach is a very useful tool to quickly investigate the influence of process parameters on process behaviour. Furthermore, it can be used in conceptual design studies. This section describes this sharp front approach and compares the outcomes with the more advanced numerical model. Finally, the influences of several process parameters are studied.

2.3.4 Model Description

2.3.4.1 Capture Step

The model is derived for capturing a component i from a binary gas mixture consisting of components i and j (but could be easily extended to multicomponent mixtures). When feeding this mixture to a refrigerated bed, two fronts are formed: a ‘frost’ and a ‘defrost’ front as depicted in Figure 2.6a.

Figure 2.6 Axial temperature, mass deposition and gas concentration profiles used in the derivation of the sharp front approach for the capture step (a) and recovery step (b)

The defrost front moves from zd,1 to zd,2 during a time period Δt. The mass of i evaporated is equal to the distance the front moved multiplied with the bed cross-sectional area A and the amount of mass deposited per unit of bed volume mi. Due to this evaporation of previously deposited i, the mass flow rate of i after the defrost front ( c02-math-0023 ) is equal to the inlet mass flow rate ( c02-math-0024 ) plus the amount of i evaporated.

This results in the following component mass balance:

in which the front velocity vd is defined as

Due to evaporation of previously frosted i, the mass fraction after the first front (ωi1) will be higher than the inlet mass fraction (ωi2). An overall mass balance can also be formulated. The total mass flow rate after the defrost front (Φ1) is equal to the inlet flow rate (Φ2) plus the amount of component i evaporated:

At the frost front, i is deposited on the packing surface; therefore, the outlet mass flow of component i is equal to the inlet flow minus the amount of i deposited in time Δt, which can be written as follows:

The velocity of this frost front vf is described as

Again, an overall mass balance can be formulated. The total mass flow after the frost front (Φ0) is equal to Φ1 minus the amount of component i deposited on the packing surface:

Energy balances can be formulated for both fronts. At the defrost front, heat is required to heat up the packing material from the saturation temperature (T1) to the inlet temperature (T2). Furthermore, heat is consumed due to the (endothermic) sublimation of component i. The required energy is provided by the feed gas, which is cooled down from T2 to T1. The energy balance results in

At the frost front, the exothermic desublimation is included in the energy balance, resulting in

So, three balances have been derived for each front (component mass, overall mass and energy balances) giving a total of six balances. There are eight unknowns (T1, Φ0, Φ1, ωi0, ωi1, vd, vf and mi). The two mass fractions of component i after the defrost front (ωi1) and at the outlet (ωi0) are related to the temperature by the phase equilibrium:

where M is the average molar weight.

Finally, eight equations and eight unknowns are obtained. The gas and solid heat capacities are dependent on temperature and are normally described by polynomial correlations, and vapor pressures as a function of temperature are normally described by exponential relations. Thus, a system of