Field of Invention

The invention relates to a method and apparatus for carbonation, in particular calcium and magnesium compounds from seawater.

Background

Carbon dioxide emissions due to the burning of fossil fuels is one of the leading sources of global warming. Therefore reducing the amount of carbon dioxide released into the atmosphere through carbon sequestration can help with this problem.

A conventional method for sequestering carbon is the process of mineral carbonation, the most common of which is where carbon dioxide gas is bubbled through an aqueous solution of calcium hydroxide in a reactor to form solid particles of calcium carbonate, a reaction which can be represented as follows:

Thus it will be appreciated that in addition to carbon dioxide, a source of calcium ions is required to feed the above process. Traditionally, calcium oxide (quicklime) is dissolved to create an aqueous feedstock solution of calcium hydroxide containing the required calcium ions. However, production of the calcium oxide starting material usually results in the undesirable release of carbon dioxide.

The US National Energy Technology Laboratory (NETL) developed a direct carbonation process involving grinding of magnesium (or calcium) silicates but disadvantageously this requires very high temperatures of 650°C to activate/remove the oxygen in the silicate, electrolysis of seawater to produce the NaOH for CO2 capture, and results in a mixture of carbonates which are difficult to separate.

Another source of calcium ions is seawater. The mineral ion composition of seawater is around 400ppm Ca ²⁺ , about 1200ppm Mg ²⁺ and approximately 20000ppm Na ⁺ . However, the magnesium ions inhibit natural carbonation of seawater from dissolved CO2 therein - existing methods typically require electrolysis to increase the pH of the solution in order to promote the dissolving of CO2 therein, which makes it unsuitable for large scale production.

An aim of the invention therefore is to provide a method for carbonation which overcomes at least some of the above issues.

Summary of Invention

In an aspect of the invention, there is provided a method of carbonation comprising the steps of: spraying seawater into a carbonation reactor pressurised with carbon dioxide to at least 20 bar, to form a suspension; and collecting the suspension from the bottom of the reactor; characterised in that the seawater is superheated to at least 200°C.

Advantageously a co-precipitate of calcium and magnesium carbonate is formed when the solution of seawater containing calcium and magnesium ions is carbonated and superheated. The spray helps to ensure that the interfacial surface area between the seawater solution and the carbon dioxide is significantly increased, and increases the contact area for the carbon dioxide to dissolve and react with calcium and magnesium ions.

In one embodiment the carbon dioxide in the carbonation reactor has a pressure of at least 40 bar. Typically the carbon dioxide is in a supercritical state.

In one embodiment the seawater is superheated prior to spraying into the carbonation reactor. In a further embodiment the seawater is superheated in the reactor, or in a separate vessel after leaving the reactor.

In one embodiment the temperature of the superheated seawater is in the range of 200°C to 300°C, typically 220°C to 250°C. The optimum temperature was found to be around 220°C. This elevated temperature provides sufficient heat or latent energy for magnesium ions to take part in the carbonation process whereby co-precipitation of magnesium ions and calcium ions in seawater takes place. At lower temperatures, magnesium ions stay in its inert condition or state.

Advantageously the system is scalable as electrolysis is not required.

In one embodiment the seawater is pressurised with carbon dioxide to a pressure of 20- 80 bar, typically around 50 bar. The pressure is maintained at a high enough level to ensure the seawater is liquid phase at the raised temperature i.e. superheated.

In one embodiment carbon dioxide is used to push the seawater to the carbonation reactor.

In one embodiment the superheated seawater is provided to the carbonation reactor continuously.

In one embodiment the suspension is collected in a buffer vessel, which may be separate or integral to the carbonation reactor. Typically the suspension is retained in the buffer vessel for 30-90 minutes. This allows the reaction between the carbon dioxide and the calcium and/or magnesium ions to be completed.

In one embodiment control means in the form of a back pressure regulator or flow control valve is provided to control the length of time that the suspension is retained in the buffer vessel.

Advantageously the suspension forms a barrier to prevent supercritical carbon dioxide from leaking from the reactor. In addition, the continuous flow ensures that any shut down time is minimised.

In one embodiment the control means is adjusted such that the suspension continuously flows out of the buffer vessel while maintaining a predetermined height of suspension therewithin. In one embodiment the suspension is then cooled, typically by passing the same through an air cooler. Typically the air cooler comprises a series of coiled tubes.

In one embodiment the heat recovered from the air cooler may be provided to a boiler used to heat the seawater.

In one embodiment the cooled suspension is filtered and/or collected in a product tank as, a co-precipitation of calcium carbonate and magnesium carbonate.

In one embodiment seawater is used to cool the suspension.

In a further aspect of the invention there is provided apparatus for carbonation of seawater comprising: a carbonation reactor comprising means for introducing pressurised and/or supercritical carbon dioxide thereinto and an injector nozzle for spraying seawater thereinto; the seawater and carbon dioxide forming a suspension; and cooling means for cooling the suspension; characterised in that heating means are provided for superheating the seawater to at least 200°C.

In one embodiment the heating means comprises a separate pressure vessel for superheating the seawater prior to spraying into the carbonation reactor.

In one embodiment the heating means is configured to superheat the seawater in the carbonation reactor or in a separate vessel after leaving the reactor.

In one embodiment a buffer vessel is provided downstream of the carbonation reactor for retaining the suspension for a predetermined time to allow the carbonation reaction to complete.

In one embodiment the carbonation reactor is provided with a source of high pressure carbon dioxide. In one embodiment control means in the form of a back pressure regulator or a flow control valve is provided to control the length of time that the suspension is retained in the carbonation reactor and/or buffer vessel.

In one embodiment heat energy recovered from the cooling means is provided to the heating means.

In a yet further aspect of the invention there is provided a co-precipitation of calcium carbonate and magnesium carbonate made according to the method herein described.

Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible, and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

Figure 1 is a schematic diagram of an apparatus for carbonation of seawater according to an embodiment of the invention.

Figure 2 is a schematic diagram of an apparatus for carbonation of seawater according to a further embodiment of the invention. Figure 3 is a schematic diagram of an apparatus for carbonation of seawater according to a yet further embodiment of the invention.

Detailed Description

With regard to Figure 1, a system for carbonation of seawater according to an embodiment of the invention is schematically illustrated.

In step 1, a carbonation reactor b is pressurised using carbon dioxide to prepare the system for the carbonation reaction. In step 2, real or synthetic seawater is heated to around 250°C in a pressure vessel such as an autoclave a. The minimum pressure to maintain seawater as a liquid at this temperature is 32 bar. The autoclave is pressurised to 50 bar with carbon dioxide to allow the seawater to be superheated.

In step 3, the superheated seawater is directed to the pressurised carbonation reactor b, via a connecting pipe, under control of valves V-l and V-2. The carbonation reactor b contains pressurised carbon dioxide in excess such that when the superheated seawater is injected thereinto in the form of atomised droplets a suspension is formed. The carbon dioxide may be supercritical under pressure of at least 73 bar.

The seawater upon reaction in the carbonator reactor turns slightly blackish due to higher turbidity levels from the co-precipitation of magnesium and calcium carbonates. This colloidal suspension of seawater and mineral carbonate trickles down to the bottom section of the reactor forming a water column or barrier thereby acting as a liquid seal to prevent CO2 gas from flushing out or escaping

In step 4 the suspension may flow into a buffer vessel c which may be separate (as illustrated) or integral with the reactor b. The suspension is retained in the buffer vessel c to allow the carbonation reaction between the carbon dioxide and calcium/magnesium ions from the seawater to be maximised.

As the injection of seawater increases the pressure in the reactor b and buffer vessel c, the suspension is eventually forced out of the buffer vessel bottom of the reactor b under the control of valve V-4, which can be adjusted to suit the requirements i.e. while maintaining a sufficient height of suspension to substantially prevent the carbon dioxide from escaping from the bottom of the reactor b and ensure the appropriate retention time in the buffer vessel e.g. 30-90 minutes.

For example, an 18.2L reactor 50% filled with liquid at 250°C and pressure of 40 bar would have a suspension retention time of 30 minutes and a liquid hourly space velocity (LHSV) of around 2. In step 5 the suspension is transferred to the product tank via a connecting pipe, and cooled by a series of air-containing coiled tubes d around the connecting pipe. In step 6 the product settles in the product tank e. The product is primarily huntite, a co precipitation of calcium carbonate and magnesium carbonate.

Figure 2 illustrates a further embodiment of the invention wherein the seawater is injected into the carbonari on reactor b before being heated.

The reactor b is first pressurised with carbon dioxide up to 20 bar. A pump is provided to pump seawater into the reactor at 25°C at a rate of around 1 to 5 litres per minute.

Control loop 1 comprises a level sensor L and an outlet valve from the reactor and maintains the seawater in the reactor b at a predetermined level.

Once the level of seawater in the reactor is stable, the seawater in the reactor is heated up to 80°C which increases the pressure in the reactor. Once the pressure has stabilised and the safety checks have been conducted the seawater is then further heated up to 250°C, further increasing the pressure in the reactor to around 40-50bar.

Control loop 2 comprises a temperature sensor T and pressure sensor P connected to the seawater heating unit a so that the temperature of the reactor can be maintained at 250°C.

A pressure regulator linked to a carbon dioxide booster compressor and a source of carbon dioxide is adjusted to a level just over that measured in the reactor (around 40 bar at 250°C) to maintain the pressure in the reactor b. Note that for the embodiment where the seawater is superheated in a separate vessel downstream of the reactor, the pressure of the reactor b can be maintained at around 20 bar.

The resulting suspension is directed to a cooling unit d where control loop 3 pumps spent seawater through a cooling tower to cool down the same. The cooled suspension is then directed to a filtration system to separate the spent seawater from the solid carbonates.

Figure 3 illustrates a yet further embodiment of the invention wherein the seawater is sprayed into the carbonation reactor 4 before or after being heated.

The reactor 4 is first pressurised with carbon dioxide up to 20 bar. A pump 1 is provided to pump seawater into the reactor 4 via a superheating package 3 at a rate of around 1 to 5 litres per minute.

The superheating package 3 can either superheat the seawater to 250°C prior to being sprayed into the top of the reactor 4, or direct the seawater at 25°C to the lower section of the reactor where it can be subsequently heated therein.

The pressure of the reactor 4 can be increased and maintained at 40 bar by a carbon dioxide booster 2.

The resulting suspension is directed to a cooling unit 5 where it can be cooled down, and then a filtration unit 6 to separate the spent seawater from the solid carbonates. The spent seawater can be used to help warm the fresh seawater entering the system using a known heat exchange system.

It will be appreciated by persons skilled in the art that the present invention may also include further additional modifications made to the system which does not affect the overall functioning of the system.