GASEOUS COMPOUNDS TO MOVING ADSORBENT PARTICLES

Field of the invention The present invention relates to the field of purification of gas, and more

specifically to a process or system for purification of natural gas.

Background

Raw natural gas contains various amounts of compounds which must be removed or minimized before subsequent processing or use. These compounds include water, heavy hydrocarbons, such as alkanes, iso-alkanes, polycyclic aromatic hydrocarbons (PAHs), and mixtures of benzene, toluene and xylene-isomers (BTX), H ₂ S, C0 ₂ , Hg and heavy metals like V.

The present invention relates to a system for removing dissolved matter or compounds from natural gas. The compounds may for instance be water vapor and/or heavier hydrocarbon molecules, saturated at in-situ pressure and temperature. Part of these compounds needs to be removed to avoid condensation to the liquid phase when the pressure and/or temperature drops, typical in transport systems of natural gas to the end-user. In industry this removal is called water and hydrocarbon dew pointing. Other compounds that need to be removed from natural gas are fractions of mercury, heavy metals, C0 ₂ and H ₂ S depending on conditions in the field. The latter two compounds can be present at rather high concentrations, larger than 10 mol%.

Current gas treatments like dew pointing and removal of contaminants like C0 ₂ and H ₂ S is achieved using glycol absorption methods, molecular sieve adsorption (zeolites) and amine gas absorption, respectively; these processes are highly energy- intensive and the systems have large footprints. Also, amines are corrosive and toxic.

Metal-organic frameworks (MOFs) are a relatively novel class of porous materials having extremely high surface areas and very low crystalline densities. The unique properties of MOFs enable separation processes to be performed cheaper and more efficiently. The performance of MOFs has surpassed standard means to separate and purify natural gas due to their high surface areas, regularity and design flexibility in both structure and properties, such that the described dissolved matter may be removed and the MOF be regenerated. However, due to their small particle size and low mechanical stability and abrasion resistance, non-modified MOFs are not compatible with commonly used separation systems. For use in such systems, the MOFs must be modified into suitably shaped particles, preferably spheres or beads. In the literature, a number of methods for modifying MOFs into materials having properties suitable for use in separation systems are disclosed. WO 2014/118054 Al discloses modified MOF-particles obtained by mixing a MOF compound with a binder, followed by a shaping step to provide the modified MOF- particles as porous beads or spheres. The binder may for instance be a hydrophilic polymer. Modified MOF-particles may also be synthesized by depositing a MOF compound into preformed porous polymer beads, ref. O'Neill et al, J. Mater. Chem., 2010, 20, 5720-5726.

The goal of the present invention is to provide a system and process for removal of undesired compounds from a gas stream, such as a natural gas stream, preferably by use of modified MOF-particles. The system and process avoids or alleviates at least some of the disadvantages of the prior art.

Summary of the invention

The present invention provides a system and process for removal of undesired compounds from a gas stream or flow, for instance a raw natural gas, preferably by use of modified MOF-particles. The undesired compounds are chosen from the group of water, heavy hydrocarbons, such as alkanes, iso-alkanes, polycyclic aromatic hydrocarbons (PAHs), and mixtures of benzene, toluene and xylene- isomers (BTX), H ₂ S, C0 ₂ , Hg and V, especially water.

The present invention is defined in the appended claims and in the following:

In a first aspect, the present invention provides a process for removal of at least one type of compound from a gas flow, comprising the steps of:

a) introducing the gas flow into a fluid inlet of a first mixing device;

b) introducing compound adsorption particles into a particle inlet of the first mixing device to obtain a mixture of the gas flow and the compound adsorption particles exiting an outlet of the first mixing device; and c) separating the gas flow from the compound adsorption particles in a first separator vessel to obtain a compound-depleted gas flow.

The compound adsorption particles in step c) comprises the at least one type of compound. In one embodiment of the process, the first mixing device is a first ejector and the fluid inlet of the first ejector is a driving fluid inlet.

In a further embodiment, the process comprises a step d) of regenerating the compound adsorption particles of step c) and recirculating the regenerated compound adsorption particles for use as the compound adsorption particles introduced in step b).

In a further embodiment, the compound adsorption particles are regenerated in step d) by use of a particle regenerating system, preferably featuring a pressure swing or temperature swing.

In one embodiment of the process the compound adsorption particles are modified metal-organic framework (MOF) particles.

In a further aspect, the present invention provides a gas purification system for removal of at least one type of compound from a gas flow, comprising a first mixing device, a first particle vessel for compound adsorption particles and a first separation vessel, wherein the first mixing device comprises a fluid inlet for the gas flow, a particle inlet for the compound adsorption particles and an outlet for a mixture of the gas flow and the compound adsorption particles;

the first particle vessel comprises an outlet fluidly connected to the particle inlet of the first mixing device, such that the compound adsorption particles may be introduced to the gas flow entering the fluid inlet to obtain a mixture of the gas flow and the compound adsorption particles exiting the outlet of the first mixing device; and

the first separation vessel is for separating the compound adsorption particles from the gas flow and comprises an inlet fluidly connected to the outlet of the first mixing device and at least a gas outlet for providing a compound-depleted gas flow.

In one embodiment of the gas purification system, the outlet of the first mixing device is fluidly connected to the inlet of the first separation vessel by a mixing line, the mixing line being dimensioned such that adequate contact time between the compound adsorption particles and the gas flow may be achieved.

In a further embodiment of the gas purification system, a mixing element is arranged downstream the outlet of the first mixing device and upstream the first separation vessel. The mixing element may be a mixing chamber, a pipe section comprising vanes/swirl generating means, or similar. In a further embodiment of the gas purification system, the first separation vessel comprises a particle outlet for the compound adsorption particles separated from the compound-depleted gas stream, said particle outlet connected to a particle regenerating system for regenerating the compound adsorption particles, and providing the regenerated compound adsorption particles to the first particle vessel.

The particle regenerating system may be based on a pressure swing system or a temperature swing system.

In a further embodiment of the gas purification system, the particle regenerating system comprises a gas recirculation line having a first end and a second end, wherein the first end of the gas recirculation line is connected downstream the gas outlet of the first separation vessel and the second end is connected downstream the particle outlet of the first separation vessel, and the gas recirculation line comprises a compressor and a heater, such that the compound adsorption particles may be regenerated by use of a fraction of the compound-depleted gas flow providing regenerated compound adsorption particles and a compound-containing gas. In a further embodiment of the gas purification system, the particle regenerating system comprises a compound depleting system comprising a cooler for cooling the compound-containing gas and a separator for separating the compound-containing gas into a compound-depleted gas and a compound phase. The compound depleting system is for separating the compound from the compound-containing gas, the compound-containing gas is obtained from the fraction of compound-depleted gas used to regenerate the compound adsorption particles. In a further embodiment of the gas purification system: the cooler of the compound depleting system comprises an inlet and an outlet for the compound-containing gas; and

the separator of the compound depleting system comprises an inlet for a cooled compound-containing gas, an outlet for the compound-depleted gas and an outlet for the compound phase, the outlet for the compound- depleted gas is fluidly connected to the fluid inlet of the first mixing device. In a further embodiment of the gas purification system, the particle regenerating system comprises a second particle vessel for the compound adsorption particles to be regenerated, a second mixing device and a second separation vessel, wherein the second mixing device comprises a fluid inlet connected to the second end of the gas recirculation line, a particle inlet and an outlet; the second particle vessel comprises a particle inlet connected to the particle outlet of the first separation vessel, optionally via a particle accumulation vessel, an outlet fluidly connected to the particle inlet of the second mixing device, such that compound adsorption particles to be regenerated may be introduced to a fraction of compound-depleted gas, entering the fluid inlet of the second mixing device, to obtain a mixture of the fraction of compound-depleted gas flow and the compound adsorption particles to be generated exiting the outlet of the second mixing device during use; and

the second separation vessel is for separating regenerated compound adsorption particles from compound-containing gas and comprises an inlet fluidly connected to the outlet of the second mixing device, a gas outlet for the compound-containing gas, and a particle outlet connected to a particle inlet of the first particle vessel, such that regenerated compound adsorption particles may be provided to the first particle vessel.

In a further embodiment of the gas purification system, the gas outlet of the second separation vessel is fluidly connected to the inlet of the cooler. In a further embodiment of the gas purification system, the second end of the gas recirculation line is connected to the fluid inlet of the first mixing device, and the first particle vessel comprises a particle inlet connected to the particle outlet of the first separation vessel, optionally via a particle accumulation vessel. In a further embodiment of the gas purification system, the gas outlet of the first separation vessel is fluidly connected to the inlet of the cooler.

In a further embodiment of the gas purification system, each of the first separator, the second separator and/or the separator of the compound depleting system is independently chosen from a cyclone separator or a membrane separator.

In a further embodiment of the gas purification system, the first particle vessel and/or the second particle vessel comprises a particle mobilization device having an inlet for a gas flow and an outlet connected to the particle inlet of the first or second mixing device, respectively. In a further embodiment of the gas purification system, the first mixing device is a first ejector, the second mixing device is a second ejector, the fluid inlet of the first ejector is a driving fluid inlet and the fluid inlet of the second ejector is a driving fluid inlet.

In a further embodiment of the process, or the gas purification system, the compound adsorption particles comprises a combination of at least a first type of compound adsorption particles and a second type of compound adsorption particles, the first and the second type of compound adsorption particles able to adsorb a first and a second type of compound, respectively. This feature allows for the

simultaneous removal of at least two types of compounds from the gas flow.

Multiple types of compounds may be removed by using multiple types of compound adsorption particles, each type of compound adsorption particle specific for a corresponding type of compound.

In a further embodiment of the process, or the gas purification system, the compound adsorption particles are modified metal-organic framework (MOF) particles. In a further embodiment of the process, or the gas purification system, the compound adsorption particles are porous beads made in a composite comprising a molecular sieve material and a polymer, the molecular sieve material may be materials such as zeolites, activated charcoal and silica gel, but is preferably a metal-organic framework material.

In a further embodiment of the process, or the gas purification system, the compound is selected from the group consisting of water, higher hydrocarbons, H ₂ S, C0 ₂ , Hg and V. In a further embodiment of the gas purification system, the particle regenerating system operates in a continuous manner or in a batch-wise manner.

The first or second mixing device may advantageously be an ejector, but other mixing devices, for instance wherein the solids inlet of the mixing device is connected to a particle vessel by a screw feeder/conveyor, may also be used.

The term "compound adsorption particles" is intended to encompass any type of molecular sieve compound or composite, the composite comprising a molecular sieve compound and a binder or host, having the required mechanical properties to tolerate being fluidized, circulated and regenerated repeatedly without detrimental abrasion or break down of the particles. The compound adsorption particles are preferably shaped as beads or spheres. The term "regenerating the compound adsorption particles" is intended to describe the process of separating the compound, adsorbed onto/into the compound adsorbing particles, from the compound adsorbing particles.

The term "modified MOF-particles" is in the present disclosure meant to define sphere or bead-shaped particles made in a composite material comprising a MOF material and a binder or host. The binder may for instance be a porous polymer and the host may for instance be a porous polymer bead. The composite material has sufficient mechanical stability and abrasion resistance to be fluidized and circulated in a system according to the invention without detrimental wear.

Short description of the drawings

The present invention is described in more detail by reference to the following drawings:

Fig. 1 is a schematic view of a continuous process according to the invention.

Fig. 2 is a schematic view of a batch process according to the invention.

Detailed description of embodiments of the invention

Two specific embodiments of a gas purification system according to the invention are illustrated in figs. 1 and 2. In both embodiments the specific gas purification is dehydration, i.e. the removal of water from the incoming gas flow.

A gas purification system for a continuous process is shown in fig. l . In use, a gas flow to be purified (i.e. dehydrated in this particular case) is introduced as a driving fluid to the first ejector 1 (i.e. a first mixing device). The first ejector 1 has a driving fluid inlet 4 (i.e. a fluid inlet) for the gas flow, a particle inlet 5 and an outlet 6. The particle inlet 5 is connected to the outlet 7 of the first particle vessel 2 containing particles able to adsorb water from the gas flow (i.e. compound adsorbing particles, wherein the compound is water). The internals of the first particle vessel 2 comprises a particle mobilization device 50 using principles similar to what is disclosed in patent application WO2014180497 (the particle mobilization device 50 may for instance comprise the features of a fluidizing unit as shown in fig. 3 of said patent application), although in the present invention a gas, instead of a liquid, is provided to mobilize the solids (i.e. the particles) in the first particle vessel 2 and a low pressure zone for moving the particles out of the first particle vessel 2 is provided by the first ejector. A valve 48 controls the ratio between the gas flow entering the gas inlet 49 of the first particle vessel 2 for mobilizing the particles and the gas flow entering the driving fluid inlet 4 of the first ejector 1. In the first ejector 1, the particles are entrained in the introduced gas flow and a mixture of fluidized particles and gas exits the first ejector through the outlet 6. The intimate interaction between the gas flow and the particles obtained by use of the ejector provides increased mass transfer efficiency compared to fixed bed systems. The outlet 6 is connected to a particle/gas separator 3 (i.e. a first separator) by the pipe line 12 (i.e. a mixing line). Preferably, the pipe line 12 is dimensioned (i.e. having a required length, diameter etc.) such that the particles have adsorbed the required amount of water from the gas before the mixture enters the gas/particle separator. Optionally, the pipe line 12 comprises a mixing element 13 (i.e. a mixing chamber, a pipe section comprising vanes/swirl generating means or similar) to improve the mixing of the particles/gas and/or to increase the contact time period between particles/gas before entering the particle/gas separator. The particle/gas separator 3 may be any type suitable for separating the particles from the gas, for instance a hydrocyclone or a membrane based separator. An example of a suitable particle/gas separator is disclosed in patent application WO 2007/001174 Al . In the particle/gas separator, the particles (i.e. compound adsorbing particles, wherein water has been adsorbed) are separated from the gas and exits the separator 3 via the particle outlet 10. The particles have adsorbed water from the gas flow, such that a dehydrated gas flow exits the gas outlet 9 of the particle/gas separator 3. The gas outlet 9 is fluidly connected to the purified gas outlet 40.

The particle outlet 10 is connected to a particle regenerating system comprising a particle accumulator 38, wherein the compound adsorbing particles are collected after exiting the outlet 10 of the separator 3. The particle accumulator 38 is arranged to provide the compound adsorbing particles, which are to be generated, to a second particle vessel 14 via the particle inlet 28. The compound adsorbing particles are mobilized in the second particle vessel 14 in the same manner as explained above for the first particle vessel 2. The second particle vessel 14 has a particle outlet 23 connected to the particle inlet 21 of a second ejector 15 (i.e. a mixing device). The second ejector 15 has a driving fluid inlet 20 (i.e. a fluid inlet) and an outlet 22. The driving fluid inlet 20 is connected to the second end 30 of a gas recirculation line 17 comprising a compressor 18 and a heater 19. The gas recirculation line 17 is arranged to heat and pressurize a fraction of the dehydrated gas flow exiting the outlet 9, and to provide said fraction to the driving fluid inlet 20 of the second ejector 15. The fraction of warm dehydrated gas entering the driving fluid inlet 20 and are mixed with compound adsorbing particles provided through particle inlet 21. The mixture of warm dehydrated gas and compound adsorbing particles are connected to the inlet 24 of a second particle/gas separator 16 by the pipe line 47. Preferably, the pipe line 47 (or the second mixing line) is dimensioned such that the warm dehydrated gas have adsorbed the required amount of water from the compound adsorbing particles before the mixture enters the second gas/particle separator 16. Optionally, the pipe line 47 comprises a mixing element as described above. In the second gas/particle separator 16, the mixture entering the inlet 24 is separated into a warm hydrated gas stream exiting the gas outlet 25 and regenerated compound adsorbing particles (i.e. particles wherein the water has been desorbed) exiting the particle outlet 26. The particle outlet 26 is connected to the particle inlet 27 of the first particle vessel 2 to provide said vessel with compound adsorbing particles. The gas outlet 25 is connected to a compound depleting system for removal of water from the warm hydrated gas. The compound depleting system comprises a cooler 31 having an inlet 33 and an outlet 34 for cooling the warm hydrated gas. After being cooled the cooled hydrated gas is introduced to a second separator 32 via the inlet 35 (in this embodiment the separator 32 is a liquid/gas separator, but may in other embodiments be any suitable type of separator able to remove a specific compound adsorbed by the compound adsorbing particles, i.e. compounds such as C0 ₂ or H ₂ S). In the separator most of the water is removed from the cooled hydrated gas and exits the outlet 37 (i.e. an outlet for the compound removed from the gas) of the separator. The dehydrated gas exits the outlet 36 and is led to the driving fluid inlet 4 of the first ejector 1. The valve 41 may regulate the volume of dehydrated gas being recirculated via the gas recirculation line 17 such that the compound adsorbing particles are regenerated continuously or in batches if that is required.

A gas purification system for a batch-wise regeneration of the compound adsorbing particles is shown in fig. 2. The system is based on the same basic principles shown in fig. 1, in that a gas flow to be purified (i.e. dehydrated in this particular case) is introduced to a first process loop 42 as a driving fluid to the ejector 1 (i.e. a first ejector). The ejector 1 has a driving fluid inlet 4 for the gas flow, a particle inlet 5 and an outlet 6. The particle inlet 5 is connected to the outlet 7 (i.e. a particle outlet) of the first particle vessel 2 containing particles able to adsorb water from the gas flow (i.e. compound adsorbing particles, wherein the compound is water). The compound adsorbing particles are mobilized in the same manner as explained above for the first particle vessel 2 in fig. 1. In the ejector 1, the particles are entrained in the introduced gas flow and a mixture of fluidized particles and gas exits the ejector through the outlet 6. The outlet 6 is connected to a particle/gas separator 3 (i.e. a first separator) by the pipe line 12 (i.e. a mixing line). Preferably, the pipe line 12 is dimensioned (i.e. having a required length, diameter etc.) such that the particles have adsorbed the required amount of water from the gas before the mixture enters the gas/particle separator. Optionally, the pipe line 12 comprises a mixing element 13 (i.e. a mixing chamber, a pipe section comprising vanes/swirl generating means or similar) to improve the mixing of the particles/gas and/or to increase the contact time period between particles/gas before entering the particle/gas separator. The particle/gas separator 3 may be any type suitable for separating the particles from the gas, for instance a cyclone separator or a membrane based separator. In the particle/gas separator, the particles (i.e. compound adsorbing particles, wherein water has been adsorbed) are separated from the gas and exits the separator 3 via the particle outlet 10. The particles have adsorbed water from the gas flow, such that a dehydrated gas flow exits the gas outlet 9 of the particle/gas separator 3. The gas outlet 9 is fluidly connected to the purified gas outlet 40.

The particle outlet 10 is connected to a particle regenerating system comprising a particle accumulator 38, wherein the compound adsorbing particles are collected after exiting the outlet 10 of the separator 3. However, contrary to the solution for continuously regenerating the compound adsorbing particles shown in fig. 1, the particle accumulator 38 of the system in fig. 2 is arranged to provide the compound adsorbing particles, which are to be generated, to the first particle vessel 2 (i.e. a second particle vessel) via the particle inlet 27.

To allow for a batch-wise regeneration of the compound adsorbing particles during continuous dehydration of the incoming gas flow, the system comprises a second process loop 39 being identical to, and arranged in parallel with, the first process loop 42.

The gas outlets 9, 9' of the first and the second process loop are connected to the respective driving fluid inlets 4, 4' by a gas recirculation line 17 having a first end 29 and two second ends 30, 30' . The gas recirculation line 17 comprises a compressor 18 and a heater 19, and is arranged to heat and pressurize a fraction of the dehydrated gas flow exiting either the gas outlet 9 of the first process loop 42 or the gas outlet of the second process loop 9' . In use, one of the process loops is used to dehydrate the incoming gas, while the compound adsorbing particles are regenerated in the other process loop. The valves 43, 43' , 44, 44' , 45, 45' are arranged to control the gas flow as required. For example, when the first process loop 42 is used to dehydrate the incoming gas flow, and the compound adsorbing particles are regenerated in the second process loop 39, the valves 43' , 44, 45 and 46' are closed, while the valves 43, 44' , 45' and 46 are open. To regenerate the compound adsorbing particles in the second process loop 39, the particles are first introduced to the first particle vessel 2' from the particle accumulator 38' . The fraction of warm dehydrated gas from the gas recirculation line 17 enters the driving fluid inlet 4' and is mixed with compound adsorbing particles provided through particle inlet 5' . The mixture of warm dehydrated gas and compound adsorbing particles are connected to the inlet 8' of the particle/gas separator 3' by the pipe line 12' . Preferably, the pipe line 12' (or mixing line) is dimensioned such that the warm dehydrated gas have adsorbed the required amount of water from the compound adsorbing particles before the mixture enters the gas/particle separator 3' . Optionally, the pipe line 12' comprises a mixing element as described above. In the gas/particle separator 3' , the mixture entering the inlet 8' is separated into a warm hydrated gas stream exiting the gas outlet 9' and regenerated compound adsorbing particles (i.e. particles wherein the water has been desorbed) exiting the particle outlet 10' . The particle outlet 10' is connected to the particle inlet 27' of the first particle vessel 2' , via the particle accumulator 38' , to provide said vessel with the regenerated compound adsorbing particles. The gas outlet 9' is connected to a compound depleting system for removal of water from the warm hydrated gas. The compound depleting system comprises a cooler 31 having an inlet 33 and an outlet 34 for cooling the warm hydrated gas. After being cooled the cooled hydrated gas is introduced to a separator 32 via the inlet 35 (in this embodiment the separator 32 is a liquid/gas separator, but may in other embodiments be any suitable type of separator able to remove a specific compound adsorbed by the compound adsorbing particles, i.e. compounds such as C0 ₂ or H ₂ S). In the separator 32 most of the water is removed from the cooled hydrated gas and exits the outlet 37 (i.e. an outlet for the compound removed from the gas) of the separator. The dehydrated gas exits the outlet 36 and is led to the driving fluid inlet 4 of the ejector 1 in the first process loop 42. When the compound adsorption capacity of the compound adsorption particles of the first process loop 42 is reached, the valves 43, 43' , 44, 44' , 45, 45' are switched, such that the compound adsorption particles of the first process loop 42 are regenerated and the incoming gas is dehydrated in the second process loop 39. Both embodiments of the invention illustrated by figs. 1 and 2 are described in connection with the removal of water from the gas flow, i.e. dehydration. However, the invention is equally useful for the removal of any other type of compounds from a gas flow by selecting the appropriate type and/or combination of compound adsorbing particles, and adapting the compound depleting system accordingly. Such compounds include C0 ₂ , H ₂ S, pentane and higher hydrocarbons, PAH, sulphates etc.

Conventional molecular sieve beads used in fixed bed dehydration systems are not ideal for use in the present invention as they are commonly too brittle to be fluidized and circulated as required in the present invention. However, the compound adsorbing particles may be any type of bead-shaped molecular sieve compounds having the required mechanical properties to tolerate being fluidized, circulated and regenerated repeatedly without detrimental abrasion or break down of the particles. The preferred compound adsorbing particles are various types of modified metal organic framework (MOF) particles. The separation system according to the invention is designed for use with modified MOF-particles as the compound adsorbing particles, and preferably the modified MOF-particles are shaped as beads or spheres, and have a density of about 500- 1100 kg/m to allow easy separation from the gas flow. Modified MOF-particles may be obtained by several methods as briefly described in the background section. For a review of recent research within the field of MOFs see for example Chemical Society Reviews, 16, 2014. MOFs are a relatively novel class of porous materials having extremely high surface areas and very low crystalline densities. The unique features of MOFs enable separation processes to be performed cheaper and more efficiently. The performance of MOFs has surpassed standard means to separate and purify natural gas due to their high surface areas, regularity and design flexibility in both structure and properties, such that the described dissolved matter may be removed and the MOF be regenerated. Advantageously, MOFs may be designed and synthesized to have a required affinity for a particular type of compound. A particularly interesting group of MOFs is the Zr-based MOFs described in European patent application EP2291384B 1 , which include the specific MOF Zr-UiO-66, however any type of MOF having the required adsorbent properties may be used in the present invention.