GB2504505A

GB2504505A - Apparatus for sequestering gas using a Downflow Gas Contactor

Info

Publication number: GB2504505A
Application number: GB1213583.6A
Authority: GB
Inventors: Sugat Raymahasay
Original assignee: Wrk Design & Services Ltd
Current assignee: Wrk Design & Services Ltd
Priority date: 2012-07-31
Filing date: 2012-07-31
Publication date: 2014-02-05
Anticipated expiration: 2032-07-31
Also published as: GB201213583D0; GB2504505B

Abstract

An apparatus for reducing the relative quantity of a first gas in an input gas mixture (for example carbon dioxide and hydrogen sulphide in biogas), comprises a reactor vessel 110 having a gas inlet 112 for receiving the input gas mixture, a liquid inlet 114 for receiving a reactor liquid, and an output 118 for outputting an output reactor liquid and an output gas mixture. The apparatus also includes a liquid feed vessel 102 for supplying a reactor liquid for sequestering the first gas. A pump 120 is coupled between an outlet 104 of the liquid feed vessel and the inlet of the reactor vessel. The reactor vessel is a Downflow Gas Contactor (DGC) vessel, which comprises a column having an entry section configured to cause sufficient turbulence and shear so as to cause mixing of the reactor liquid and input gas mixture such that the first gas is substantially sequestered by the reactor liquid. The output reactor liquid and output gas mixture are recycled to an inlet 106 of the liquid feed vessel, from which the output gas mixture is recoverable. The output gas mixture comprises a reduced or nil quantity of the first gas.

Description

Apparatus and Method for Sequestering a Gas

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for sequestering a gas or a combination of gases, selectively, from a mixture of gases. More particularly, the present invention relates to apparatus and methods for reducing the relative quantity of a first gas in an input gas mixture, and the apparatus for increasing the calorific value of an input biogas mixture.

BACKGROUND OF THE INVENTION

Alternative energy sources are nowadays being extensively investigated due to the very high energy demand worldwide. Intensive research into fuel sources other than petroleum and fossil fuels is being undertaken, in order to find an alternative fuel that is easily available, economically feasible to produce and non-polluting to the environment.

One of the more attractive and possible viable alternatives is Biogas" which is a gas produced by the biological breakdown of organic matter in the absence of oxygen.

Biogas often originates from biogenic material and is a type of renewable biofuel.

The composition of biogas varies depending upon its origin. However, a typical biogas composition comprises Methane (OH4: 50-75%), Carbon dioxide (GO2: 25-50%), Nitrogen (N2: 0-10%) and Hydrogen sulphide (H2S 0-3%).

However, it is known that the presence of Carbon dioxide and Hydrogen sulphide in the biogas (or any other gas comprising combustible components) reduces the calorific value of the gas per unit volume. Removal of Hydrogen sulphide is also beneficial for use of Biogas in CHA engines as its presence causes corrosion and breakdown of the CHP engines. The improvement of calorific values of Biogas by removing of the 002 content is generally undertaken by using the purely physically effect of water scrubbing where the water is sprayed into the top of a column while the Biogas enters the column through the bottom of the column under pressure and flows counter current to the liquid flow, sometimes with a bed of packing in the middle of the column.

We have therefore appreciated the need to remove such gasses from biogas to improve or increase the calorific value of the biogas and upgrade it into biomethane.

STATEMENT OF THE INVENTION

The present invention provides an apparatus for reducing the relative quantity of a first gas in an input gas mixture, comprising: a reactor vessel having a gas inlet for receiving an input gas mixture comprising a first gas to be sequestered from the input gas mixture, a liquid inlet for receiving a reactor liquid and an output for outputting an output reactor liquid and an output gas mixture; a liquid feed vessel for supplying a reactor liquid for sequestering the first gas, the liquid feed vessel comprising a liquid outlet for outputting the reactor liquid, a recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a gas output for outputting an output gas mixture; a pump coupled between the liquid outlet of the liquid feed vessel and the liquid inlet of the reactor vessel for providing a reactor liquid to the reactor vessel under pressure, wherein the reactor vessel is a Downf low Gas Contactor (DGC) vessel comprising a column having an entry section coupled to the gas inlet and liquid inlet, the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input gas mixture such that the first gas is substantially sequestered by the reactor liquid to generate the output reactor liquid, and wherein the output reactor liquid and output gas mixture are output from the reactor vessel output, and the output gas mixture is recoverable via the liquid feed vessel gas output, the output gas mixture comprising the input gas mixture having a reduced or nil quantity of the first gas.

By using DGC with an entry section configured to provide such high and intense turbulence and shear at an interface between the entry section and the column, advantageously this enables much lower concentrations of active compounds that comprise the reactant liquid to sequester the desired gas or gasses from the input gas mixture.

The input gas mixture may comprise air or biogas. When biogas is used, the biogas comprises methane, carbon dioxide and hydrogen sulphide. In the situation where the input gas mixture is air or biogas, the first gas, that is the gas to be sequestered, is carbon dioxide. Where biogas is used, the biogas may further comprise hydrogen suiphide. In such a situation, the first gas may also comprise hydrogen sulphide.

In embodiments, the reactor liquid comprises solution of water and seasalt. Preferably, the seasalt comprises one or more of Sodium chloride, Magnesium Sulphate, Magnesium Chloride, Calcium chloride Potassium chloride, Sodium bicarbonate and Sodium thiosulphate.

In some embodiments, the reactor liquid comprises a solution of water and Sodium Carbonate. Alternatively, the reactor liquid comprises a solution of water and Sodium Hydroxide. Alternatively, the reactor liquid comprises a solution of water and Methyl eth ylam in e.

In any of the embodiments, one or more constituents of the reactor liquid have a concentration between 0.5M and 1.OM. Such low concentrations (much lower than seen before) are enabled as a result of the entry section being configured to provide high jet energies and liquid inlet powers (in the region of 3 to 30W).

The liquid feed vessel may also comprise a temperature control means coupled to a temperature controller, and wherein the temperature controller is configured to control a temperature of the reactor liquid. With the temperature control means, increasing the temperature of the output reactor liquid causes the first gas to be released from the output reactor liquid via the gas output. As such, the sequestered gasses may be released by controlling the temperature of the liquid feed vessel.

The apparatus may also comprise a liquid flow controller coupled to the pump, the liquid flow controller being configured to control the rate of flow of the reactor liquid.

Similarly, the apparatus may also comprise a gas flow controller configured to control a flow of the input gas mixture. Also, the apparatus may also comprise a compressor coupled to the gas inlet for compressing the input gas mixture.

In embodiments, the apparatus may further comprise a second liquid feed vessel for supplying the reactor liquid for sequestering the first gas, the second liquid feed vessel comprising a second liquid outlet for outputting the reactor liquid, a second recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a second gas output for outputting an output gas mixture. In such embodiments with a second liquid feed vessel, the liquid outlet of the liquid feed vessel is coupled to the second liquid outlet of the second liquid feed vessel, the recycle inlet of the liquid feed vessel is coupled to the second recycle inlet of the second liquid feed vessel and the gas output of the liquid feed vessel is coupled to the second gas output of the second liquid feed vessel. Using a second liquid feed vessel enables a larger volume of reactor liquid to be used, which means that the apparatus can be used for longer times before the reactor liquid needs replenishing.

In embodiments, the entry section is configured to provide a liquid inlet power from about 3 Watts to about 30 Watts, preferably from about 5 Watts to about 25 Watts.

Such liquid inlet powers enable the apparatus to sequester gases even when concentrations of the active compounds in the reactor liquid are relatively low.

The present invention also provides a method of reducing the relative quantity of a first gas in an input gas mixture, the method comprising the steps of: receiving an input gas mixture, the input gas mixture comprising a first gas to be sequestered from the input gas mixture; pumping a reactor liquid for sequestering the first gas from a liquid outlet of a liquid feed vessel into a Downflow Gas Contactor (DGC) reactor vessel via a liquid inlet of the DGC; inputting the input gas mixture into a gas inlet of the DGC, wherein the DGC comprises a column having an entry section coupled to the gas inlet and liquid inlet, and the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input gas mixture such that the first gas is substantially sequestered by the reactor liquid to generate an output reactor liquid and an output gas mixture; pumping the output reactor liquid and output gas mixture from an output of the DCG to a recycle inlet of the liquid feed vessel; recovering the output gas mixture from a gas output of the liquid feed vessel, the output gas mixture comprising the input gas mixture having a reduced or nil quantity of the first gas.

The input gas mixture may comprise air or biogas. When biogas is used, the biogas comprises methane, carbon dioxide and hydrogen sulphide. In the situation where the input gas mixture is air or biogas, the first gas, that is the gas to be sequestered, is carbon dioxide. Where biogas is used, the biogas may further comprise hydrogen sulphide. In such a situation, the first gas may also comprise hydrogen sulphide.

In some embodiments, the reactor liquid comprises a solution of water and Sodium Carbonate. Alternatively, the reactor liquid comprises a solution of water and Sodium Hydroxide. Alternatively, the reactor liquid comprises a solution of water and Methylethylamine.

The method may also comprise controlling the temperature of the reactor liquid.

Increasing the temperature of the output reactor liquid causes the first gas to be released from the output reactor liquid via the gas output. As such, the sequestered gasses may be released by controlling the temperature of the liquid feed vessel.

The method may also comprise the step of controlling the rate of flow of the reactor liquid. The method may also comprise the step of controlling the flow of the input gas mixture. The method may also comprise the step of compressing the input gas mixture.

The present invention also provides a apparatus for increasing a calorific value of an input biogas mixture, comprising: a reactor vessel having a gas inlet for receiving an input biogas mixture comprising a first gas to be sequestered from the input biogas mixture, a liquid inlet for receiving a reactor liquid and an output for outputting an output reactor liquid and an output biogas mixture, the first gas being a gas that reduces the calorific value of the input biogas; a liquid feed vessel for supplying a reactor liquid for sequestering the first gas, the liquid feed vessel comprising a liquid outlet for outputting the reactor liquid, a recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a gas output for outputting an output biogas mixture; a pump coupled between the liquid outlet of the liquid feed vessel and the liquid inlet of the reactor vessel for providing a reactor liquid to the reactor vessel under pressure, wherein the reactor vessel is a Downflow Gas Contactor (DCC) vessel comprising a column having an entry section coupled to the gas inlet and liquid inlet, the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input biogas mixture such that the first gas is substantially sequestered by the reactor liquid to generate the output reactor liquid and an output biogas mixture, and wherein the output reactor liquid and output biogas mixture are output from the reactor vessel output, and the output biogas mixture is recoverable via the liquid vessel gas output, the output biogas mixture comprising the input biogas mixture having a reduced or nil quantity of the first gas, thereby having an increased calorific value.

By using DCC with an entry section configured to provide such high and intense turbulence and shear at an interface between the entry section and the column, advantageously this enables much lower concentrations of active compounds that comprise the reactant liquid to sequester the desired gas or gasses from the input biogas mixture.

In such embodiments, the input biogas mixture comprises methane and carbon dioxide. The first gas mat comprise carbon dioxide. Furthermore, the input biogas mixture may comprise hydrogen sulphide. In such a situation, the first gas comprises hydrogen sulphide.

LIST OF FIGURES

The present invention will now be described, by way of example only and with reference to the drawings, in which: Figure 1 shows a basic system diagram of the present invention.

Figure 2 shows a detailed schematic of a test system according to the invention.

Figures 3a to 3f shows some examples of CO2 absorptions with different solutions of varying salt composition.

Figures 4a and 4b show desorption of CO2 from absorbent reactant solutions and corresponding temperature levels.

Figures 5a to Sf show the results of CO2 and H2S absorption using a variety of reactor liquid compositions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In brief, the present invention injects a gas mixture, for example a biogas or another gas having components that are desired to be removed, into a reactant liquid stream at the inlet of a Downflow Gas Contactor (DGC) Reactor. The composition of the reactant liquid is chosen to absorb or sequester components of the gas that need to be removed and also would otherwise reduce the calorific value of gases like Biogas (for example Carbon dioxide or Hydrogen sulphide, which is a harmful component present in Biogas).

The DGC reactor is very efficient mass transfer device for contacting of liquids and gases. A very high interfacial area is generated in a small containment volume. The intense turbulence and shear at the interface results in efficient gas-liquid mixing and allows the gas to be dissolved in the reactant liquid and approach gas equilibrium solubility levels in relatively very short contact times. This allows sequestration of a specific gas or a combination of gases in the reactant absorbent liquid in the DGC and therefore their removal from the input gas mixture. The unabsorbed and unreacted gases in the gas mixture passes out through the DGC reactor and is degassed from the liquid and the clarified output gas is recaptured as required. As such, the output gas contains a substantially reduced quantity or none of the unwanted gases (like CO2 or H2S), which increases the calorific value of the output gas compared to the input gas, as in Biogas.

Figure 1 shows a basic system diagram of the present invention.

The input gas mixture (for example air or biogas or any other gas mixture where it is desired to absorb or sequester a gas from the mixture) is input to the DGC (110) via a input gas inlet (112) through a specially designed entry section like an orifice or nozzle (116) in the DGC column. An absorbent reactant liquid is pumped from a liquid feed vessel (102) to the DGC (110) by a pump (120). The reactant liquid enters the DGC (110) via a liquid inlet line (114) coupled to the entry section (116). Once the input gas mixture has been mixed in the reactor liquid and the gas or gases to be removed is/are sequestered by the absorbent reactor liquid, the resulting output reactor liquid is output from the DGC (110) via an outlet (118), which is coupled to a recycle inlet (106) of the liquid feed vessel (102).

The clarified output gas mixture, which has not been absorbed or reacted with the reactant liquid passes out with the absorbent liquid and degasses in the feed vessel (102) and flows out through the liquid feed vessel (102) via the gas output (108).Since the gas or gases to be sequestered is/are effectively retained by the reactor liquid, the resulting output gas mixture contains a relatively higher concentration of the desired gas or gases when compared to the input gas mixture. As such, in the example of Carbon dioxide being sequestered from an input biogas mixture, the resulting biogas mixture at the output has substantially reduced or minimum levels of Carbon dioxide, and therefore has a higher calorific value per unit volume.

It has been found that the captured or sequestered gas, like Carbon Dioxide, in the reactor liquid can be recaptured or released from the reactor liquid by heating the reactor liquid. As such, the liquid feed vessel (102) comprises a heating or cooling means (122), which enables the temperature of the reactor liquid either to effect the efficient absorption of a gas (at low temperatures) and so prevent the release of a gas, or to allow the release of the sequestered gas (at high temperatures).

The system of the present invention is built around a Downf low Gas Contactor (DGC) reactor. Downf low Gas Contactor (DGC) reactors are known. In most of these, the liquid flows downwards and the gas flows upwards. However, such reactors would not be suitable for use in the method and apparatus of the present invention.

In DGC reactors suitable for use in the present invention, the liquid and gas are made to flow downwards together at specific rates depending on the specific application.

DGC reactors of this type are disclosed in, for example, GB 1,596,738 A, GB 2,117, 618 A and US 4,834,343. DGC reactors of this type are an efficient mass transfer device for contacting liquids and/or gases.

The present invention uses a downflow co-current device comprising a cylindrical column with a specialised designed orifice or nozzle at its entry section, allowing both liquid and gas inputs into the reactor. The dimensions and configuration of the DGC reactor depend on the application and operating conditions and are designed accordingly.

Liquid enters the top of the fully flooded column via one or more liquid inputs (114) in the form of one or more high velocity liquid streams. No foaming is possible as no free liquid interface is obtained at the inlet. The input gas stream can be fed into the incoming liquid stream immediately prior to the column inlet through the nozzle/orifice concurrently. As the liquid continuous phase expands into the column, part of the kinetic energy imparted to the fluid on its passage through the orifice or nozzle is used in the formation of a very high interfacial area in a gasliquid bubble dispersion. The high velocity liquid jet passing through the specially designed section that generates intense shear and energy and a large interfacial area is generated in a small containment volume without any mechanical aid and a minimum expenditure of energy over than that required for motive power. The interface is subjected to rapid surface renewal through repeated rupture and coalescence, resulting in intense mixing and highly efficient mass transfer to approach equilibrium solubility levels in a very short time of contact. The energy of the jet breaks up the gas into very fine small bubbles and an enormous interfacial area is generated in a small operating volume.This allows the use of much lower concentration absorbing liquids to be used for absorption requirements as in the present absorption of 002 from Air and upgrading of Biogas by removal of 002 and H2S.

It also prevents the formation of a permanent gas space at the top of the column thus maintaining a fully flooded situation. No mechanical aids such as stirrers or baffles are required.

The downflow liquid velocity in the column is maintained at a value below the rise velocity of the gas bubbles so that there is no tendency for the bubbles to be carried downwards. Hence there is no net movement of the gas phase whilst the liquid phase flows downwards through the inter-bubble spaces. The gas-liquid bubble dispersion slowly expands down the fully flooded column and the level of dispersion (and thereby volume of the gas-liquid dispersion) can be controlled by control of the operating conditions (liquid and gas flowrates).

In the lower section of the column as the dispersion proceeds downwards, there is a degree of bubble coalescence since it is no longer within the region of direct inlet steam impingement. This coalescence produces larger bubbles, which rise up the column where they are broken up by the shear of the high velocity inlet liquid jet. If all the gas bubbles are required to be retained in the DOG, the DOG is designed with a conically expanding lower section. The presence of the expanded conical lower section results in disengagement of the bubbles in said section and reduced turbulence in the lower pad of the upper cylindrical portion. As a consequence, the reaction mixture that is ejected from the end of the DGG reactor (liquid and dissolved gas) is bubble free with no escape of undissolved gases. However, the preferred embodiments utilise a DOC reactor without an expanded conical section.

The specific shape, dimensions and configuration of the DOG reactor depend on the application and operating conditions required.

In one example, a test rig was assembled using a DOG reactor having a column of 50 mm internal diameter and 2.0 meter in length. Different diameters of the orifice or nozzle sizes were used varying between 2 mm to 8 mm. Liquid flowrates used, varied between 5 litres/mm and 15 litres/mm.

With regard to suitable orifices or nozzles, the internal diameter chosen is calculated to meet the mixing requirement based on factors such as volume and flow rate.

The orifice can comprise a threaded fitting, the internal diameter of which is chosen according to factors such as flow rate, volume and the like. The orifice is screwed into a hole in a flange fitted at the entry of the DOG reactor shaped to receive said fitting.

Bolt inlets are included at the extremities of the flange. The orifice has a threaded inlet entry point where the liquid and gas are directed.

The nozzle inlet has a substantially cylindrical upper section, a conical lower middle section and finally a much narrower substantially cylindrical distal section. The exact dimensions are chosen depending upon the precise requirements. The nozzle inlet is welded into a suitably configured opening drilled into the centre of a flange at the entry of the DOC reactor. Bolt inlets are included at the extremities of the flange Figure 2 shows a detailed schematic of a test system according to the invention. Figure 2 is effectively figure 1 in greater detail. Features common with figure 1 are shown with like reference numerals.

In addition to a number of valves on each input and output, figure 2 shows a second liquid feed vessel (102b) coupled in parallel with the first liquid feed vessel (102a). A number of Pressure Indicators (Fl) and Temperature Indicators (TI) are present, as well as a Gas analyser (152) to monitor the input and output gas content, and a Dissolved Gas analyser (150) in the return path between the output of the DOC and the recycle inlet of the liquid teed vessel.

The total volume of liquid used per batch during experimental testing varied between and 15 litres. The whole system was operated as a closed loop recycle unit with liquid fed through the DGC reactor and then recycled back into the Feed Receiver Vessel using a Pump. Gas was fed into the DGC reactor at the inlet through a specially designed inlet.

Experiment and Results Operating conditions, were varied, to study the individual effects, as per details below: 1. Experiments of CO2 absorption with AIR a. Variation in liquid flowrates b. Variation in gas flowrates c. Variation in jet energy d. Effect of temperature e. Effect of pressure for absorption f. Use and effect of Variation different liquid compositions 2. Experiments of CO2 and H2S absorption using simulated BIOGAS (Mixture of Methane [OH4] and 002 and also with OH4, 002 and H2S) 1. Experiments of CO2 absorption from AIR Variation in liquid flowrates Liquid flowrates varying from 5 litres/mm to 15 litres/mm were used. It was seen that at higher liquid flowrates the rate of Absorption was greater. This was due to the increased jet energy and increased gas-liquid mass transfer into the system. This increased the rate of 002 absorption per unit time and allowed higher Air flowrate input as well.

Variation in gas flowrates Gas flowrates were used varying from 100-500 cc/mm [at 4 barg] to 8.0 litres/mm [at 4 barg pressure] [32 litres at standard atmospheric pressure]. The input gas flowrates are dependent on the liquid flowrates used and as explained earlier, higher liquid flowrates effect a greater absorption and a higher gas input can be effected. This allows an increased 002 absorption per unit time as a greater amount of 002 is being made available for absorption and the gas absorption rates are higher due to increased gas- liquid mass transfer. In majority of the trials [depending on the operating conditions] 95- 98% of the 002 from Air being put in, was absorbed. (See typical gas absorption from Air Graphs figures 3a to 3f).

Variation in Jet Energy Jet energies to the system were also varied by changing the inlet design specifications.

As seen by variation in liquid flowrates a higher jet energy induced greater absorption due to the increased mass transfer imparted to the system. This was more apparent in the initials rates of absorption obtained.

Effect of temperature Temperatures used varied between 1 5C to 30C. It was seen that a lower temperature appeared to be better for 002 absorption as the equilibrium solubility level of 002 is higher at lower temperatures. Temperatures were only increased to recover the 002 after the absorption was effected.

Effect of pressure for absorption The DGC operating pressures were varied between 7 psig to 60 psi. Increased pressure increases the solubility of 002 in the liquid at a specific temperature and therefore increased 002 absorption. This was also observed during the trials -however as the CO2 absorption process used in the current trails is a combination of gas absorption and chemical reaction the effects of operating pressure cannot be shown significantly from these trials. The effect of higher operating pressures would be -an increased energy requirement [for pumping of the liquid] and this was considered in the total analysis and scale-up design.

Use and effect of Variation different liquid compositions Different solutions were used in the trials: * Water plus Seasalt [: 154gm and 308gm in 10.0 ltrs] * Water plus Sodium Carbonate [154gm and 308 gm in 10.0 ltrs] * Water plus Sodium Hydroxide [0.5 M solution] * Water plus Methylethylamine [0.5 L MEA per batch] The % composition of each of the individual salts of Sodium carbonate, Sodium Hydroxide and Methyl Ethyl Amine used was far lower than has been reported in literature [upto 7 Molar]. This showed that even with these lower salt concentrations the DOC reactor could achieve a much higher rate of absorption of 002 than other comparable reactor systems used. This also showed that if the same concentration of salts were used a higher amount of 002 could be absorbed per unit time in a smaller volume of liquid. In most reported literature upto 60% absorption efficiency is achieved -whereas 100% absorption efficiency could be achieved with the current DCC unit to bring down the 002 levels in Air from an average of 360 ppm to nearly 10-20 ppm in most cases (Figure 3a -3d) -and even 0 ppm (Figure 3e), even with the lower concentration of salts used in the absorbent reactant liquid.

The composition of the absorbent liquid has a major effect on the absorption of 002 as the rate of reaction and absorption is dependent on the component used. Increasing the concentration will increase the concentration driving force and therefore the rate of CO2 absorption if required.

With regards to figure 3d, the air flow rate was as follows: AIR FLOW RATE: START TIME L/MIN [4.0 BARO] 1.0 10.45 2.0 11.20 3.0 12.10 4.0 12.55 5.0 13.30 6.0 14.10 7.0 14.45 8.0 15.15 Starting pH -13.01; END pH -12.03 [BEFORE COOLING OFF] The advantageous effect of using the individual salts and the composition of the absorbing liquid mentioned above and shown in figures 3a to 3e can be seen with the experiment with only water (figure 3f), where there was much lower 002 absorption.

This showed that absorbed 002 was reacting with the salts (when a salt containing absorbent solution was used) and so the driving force for 002 absorption was being maintained.

Recovery of absorbed CO2 The absorbed CO2 was able to be recovered by increasing the temperature of the reactor liquid when the 002 desorbed from the reactant liquid.

Figure 4a shows a typical graph of the amount of desorbed CO2 in the gas outlet (108) vs. time as the temperature of the reactor liquid is increased (figure 4b). The CO2 concentration was measured at the gas outlet. During recovery the concentration of 002 rose to greater than 200 ppm from a concentration of 10 ppm 002. As shown, the temperature profile rose to around 50C over the period of 002 recovery.

CO9 absorption rates from air -calculated * The 002 absorption rates calculated, from the current set of trials varied between 2.0 -3.0 KGCO2/KWH, based on operating conditions used in the trials undertaken.

* It has been evaluated that with minimum piping modifications to the trial unit used and with the use of higher salt concentrations, 002 absorption rates greater between 3.5-4.5 KCCO2/KWH from Air, can be achieved even with the use of the specific DCC reactor used in the trials.

* The absorbed 002 can be recovered from the liquid easily with increase in temperature. The recovered CO2 can be used for other processes as in Algae growth and Biodiesel production, production of chemicals and even for energy (fuel).

2. Experiments of CO2 and H2S absorption -with simulated biogas Trials were done with simulated Biogas). Concentrations used were: * Methane (OH4) 60% -Carbon dioxide (002) 40%.

* Methane (OH4) 70% -Carbon dioxide (002) 30%.

* Methane (OH4) 60% -Carbon dioxide (002) 38% -Hydrogen Sulphide (2%).

Figures 5a to Sf show the results of CO2 absorption using a variety of reactor liquid compositions.

Three different liquid compositions were used with Sea salt, Sodium carbonate and Sodium hydroxide.

Flowrates of the simulated Biogas were varied between of 1.0 -3.0 litres/mm [at 12 psig] and 1.0 -2.0 litres/mm [at 3.0-4.25 barg] were used. Liquid Flowrates were varied between 10.0-15.0 litres/mm. Liquid inlet powers were varied between 6W and 21W.

Operating pressures in the DCC were varied between 1.0 barg to 4.25 barg. For each of the runs 100% absorption of the 002 content was achieved and the outlet OH4 (Methane) concentration increased to greater than 95% and in most cases 100% as shown graphically in Figures 5a to 51 Figure 5a shows 002 absorption from an inlet biogas having 60% CH4 and 40% Co2.

Figure 5b shows 002 absorption from an inlet biogas having 70% CH4 and 30% CO2.

Figures Sc to Sf show 002 and H2S absorption from an inlet biogas having 60% OH4, 38% CO2 and 2% H2S.

The experiments as shown in figures Sc to Sf show the following: * H2S is absorbed to very low concentrations with Seasalt solution [figure 5c] but OH4 concentration only increases to around 80%. Also 02 is not fully absorbed.

It was seen that on addition of NaCH to the solution, OH4 concentration immediately increases to greater than 98%. The concentration of 02 also reduces to very low levels. 002 was completely absorbed.

* Using a mixture of Na2003 and Seasalt solution [figure 5d] the concentration of OH4 increased to around 95%, and complete 002 absorption, but there was still 1-2% 02 and 2-8 ppm H2S in the outlet gas showing that not all the H2S was being absorbed.

* OH4 concentrations in the outlet attained was 100%, with complete absorption of 002 and H2S and very low concentrations (0.5%) of 02 when a mixture of NaCH and Seasalt was used [figure Se]. It was observed that the pH of this solution was higher than the pH of the Na2003 and Seasalt solution used in figure Sd. However on using a smaller inlet section there was complete absorption of all the 002, H2S and 02 content in the simulated Biogas as shown in figure Sf.

* It could be concluded from the above that by using a solution of higher pH, and a mixture of Alkali and Seasalt, and operating at a higher jet velocity (and therefore higher inlet jet energy and liquid inlet power -effected by the smaller inlet section), complete absorption of the 002 and H2S in Biogas can be effected in the DOC reactor.

The experimental results show that the DCC can be used very effectively to absorb all the 002 and H2S content in Biogas and increase the Methane concentration and improve the quality of Biogas and thereby enhance the Calorific value of the Biogas.

Significant cost savings can therefore be made..

Discussion and projection of CO2 capture from air With its existing installation and set up, DGC inlet design and piping there were limitations in the operating conditions -especially with specifications of the pump being used where a liquid flowrate of 10 litres/mm at a differential pressure of 4 barg was the maximum available.

With proper piping installation and change in inlet design of the DGC reactor the same DGC Reactor Unit could be operated at higher liquid flowrates at the required pressure.

This would also allow increase of the jet energy (and therefore liquid inlet power) into the system and allow much higher gas flowrates at the operating pressure depending on the reaction/absorption rate of 002 into the liquid and by change in absorbent reactant liquid characteristics -salt concentrations.

Taking all these into consideration and with slight modifications of the DGC unit that was used, an overall absorption rate of 3.5 -4.5 KGCO2/KWH at least from Air, even with this unit set up and the same salt concentrations can be easily achieved.

The 002 absorption rate is also dependent on the concentration of the salts used as an absorbent in the liquid. In the present experiments, only 10-30 g/litre [0.15-0.45M] of the Salt complex or Na2003 and 1.0 M NaOH solution was used. By increasing the concentration further, the 002 absorption rate per unit time would also increase, as the chemical reaction rate will also increase with increasing salt concentration. Increasing the salt concentration also would increase the concentration driving force for greater 002 absorption per unit time.

Some 002 absorption systems report use of 7.0 M solutions. If same higher concentrations are used in the modified DGC reactor unit, it is predicted that overall CO2 absorption rates much greater than 3.5-4.5 0C02/KWH can be achieved even in this current DGC reactor unit.

Preview of CO, absorption units from air currently available * Much bigger scrubber units than a required DOC reactor unit -higher Capital costs * Contactor efficiency lower, approximately 55%, compared to 100% obtained with a DGC reactor Unit.

* Higher concentration of Absorbent chemicals used than in the current trials with DGC unit.

* Higher operating costs * CO2 recovery system appears to be much more complex than has been seen with use of the DGC -adding to the total overall cost both [Capital and Operating] of the CO2 absorption and reuse process.

Proiection of DGC absorption unit for CO2 capture from air Basis taken: CO2 ABSORPTION REQUIREMENT-10.0 KG/DAY 1 M3 of air= 1230g;W/O of CO2 in air= 0.0607 Required Air flowrate: 610 M3/HR based on the CO2 content in Air Proposed design of scaled up DGC unit for air Diameter of DGC Reactor: 50 cm -65cm Height of Reactor -3.0 M Liquid Flowrates: 150 M3/HR -200 M3/HR Final design would be based on Liquid composition to be used.

Effect of improved biogas Basis: 400 Nm3/hr of Biogas production An evaluation is made of the effect of improvement the Methane (CH4) concentration in Biogas by absorption of the CO2 content on energy and costs. Table 1, shows the compositions of the improved Biogas after the CO2 Absorption process, using the DGC Unit.

Tablel: Improved Composition of Biogas Compound Vol % wt % Methane (CH4) 90-97 95 -98 Carbon Dioxide (002) 2 -8 1 -4 Nitrogen (N2) 1 1 Hydrogen Sulphide 1-2 0.1 -0.2 Water(H20) 1-3 1-3 The above values are conservative values, as in actual trials with simulated Biogas (60% CH4, 38% CO2 and 2% H2S) using the DGC, complete removal of CO2 and H2S was achieved and CH4 concentrations could be increased to 100%.

Biogas quality can be significantly improved and the DGC absorption process produces a product Biogas with the following qualities: -High methane content -Low 002 content -Gas is dry -High calorific value 1 m3 of 94% methane biogas produces 34 MJ/m3 of heat (1 m3 of natural gas produces approximately 38.4 MJ/m3 of heat and the original biogas only produces 20 MJ/m3) If 400 Nm3/hr of Biogas (62 vol% OH4) is available, this is the equivalent of 361 Nm3/hr of 94% CH4 biogas. On the basis of a 24 hours per day of Biogas production on an average of 30 days a month then 259,921 Nm3/month of Biogas would be available. If the ratio of biogas required to give the equivalent heat of natural gas is taken into account, 228,611 Nm3/month of biogas (equivalent to natural gas) would be available per month.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto. C')

0 SEASALT WITH 104 GM IN 1OLWATER r LIQUID FLOWRATE 8 L/MIN; LIQUID INLET POWER 23.71W; AIR FLOWRATE 4 L/MIN; PDGC 15-20 PSI DURING ABSORPTION; C') TEMPERATURE DURING ABSORPTION 16-20°C Figure 3a graph title C') 0 SODIUM HYDROXIDE (O5M) WITH WATER r LIQUID FLOWRATE 10 L/MFN; LIQUID INLET POWER 6W; AIR FLOWRATE 6 LIMIN; PDGC 15-20 PSI DURING ABSORPTION; C') TEMPERATURE DURING ABSORPTION 18-22°C Figure 3b graph title C')

SODIUM CARBONATE WITH WATER

o SODIUM CARBONATE -104 GM IN 10,0 [WATER LIQUID F[OWRATE-10.0 L!MIN; LIQUID INLET POWER 6W; PDGC -15-20 PSI r DURING ABSORPTION; TEMPERATURE-DURING ABSORPTION: 19-21°C

CO

Figure 3c graph title C')

SODIUM CARBONATE WITH WATER

0 SODIUM CARBONATE -308 GM IN 10.0 LWATER r LIQUID FLOWRATE-15.0 L/MIN; LIQUID INLET POWER 9.77W; PDGC-60 PSI DURING ABSORPTION; TEMPERATURE-DURING ABSORPTION: 19-22°C

CO

Figure 3d graph title C') METHYLE ETHYL AMINE (ftSL) IN 1OL WATER 0 LIQUID FLOWRATE 10 L/MIN; LIQUID INLET POWER 6W; r AIR FLOWRATE 4 L/MIN; PDGC 2OPSI DURING ABSORPTION; TEMPERATURE DURING ABSORPTION 18-22°C

CO

Figure 3e graph title C') o WATER ONLY r LIQUID FLOWRATE 6 L/MIN; LIQUID INLET POWER lOW; AIR FLOWRATE 6 L/MIN; PDGC 2OPSI DURING ABSORPTION; TEMPERATURE DURING ABSORPTION 16-20°C Figure 3f graph title C') r LIQUID [SEA SALT-308 gm IN 10 L WATERI; LIQUID FLOWRATE 10.0 L/MIN; LIQUID INLET POWER 6W; BIOGAS FLOWRATE 1.0 L/MIN Co Figure 5a graph title C') LIQUID [Na2COr3O8gm IN 10 L WATER]; LIQUID FLOWRATE 15M L/MIN; 0 LIQUID INLET POWER 20.26W; BIOGAS FLOWRATE 3.0 LIMIN r Figure5b graph title C1) 308 C SEASALT IN 10 L WATER; START pH-83;END pH-77;TEMP-19°C;LARCE 0 ORIFICE; 1 L1MIN BIOGAS AT 3 BARG; LIQUID FLOWRATE 15 L/MIN; LIQUID r INLET POWER 9.77W Co Figure5c graph title C1) 308G NA2CO3-308G SEASALT IN 10 L WATER; START pH-11;END pH-i 0.3; TEMP- 20.5°C; SMALL ORIFICE; BIOGAS 1.2 L/MIN AT 3.5 BARG; LIQUID FLOWRATE 10 0 L/MIN; LIQUID INLET POWER 6W r Co Figure5d graph title C') 4000 NaOH-308G SEASALT IN 10 LWATER; START pH-12.7;END pH-i 2.1; TEMP-O 12°C; LARGE ORIFICE; 2 L/MIN BIOGAS AT 3 BARG; LIQUID FLOWRATE 15 L/MIN; LIQUID INLET POWER 917W r CO Figure5e graph title C1) 400G NaOH-308G SEASALT IN 10 L WATER; START pH-iS; END pH -12.6; TEMP- 17°C; SMALL ORFICE; 1 LIMIN BIOGAS AT 4.25 BARG; 0 LIQUID FLOWRATE 12 L/MIN; LIQUID INLET POWER 10.37W r Co Figure5f graph title

Claims

CLAIMS: 1. Apparatus for reducing the relative quantity of a first gas in an input gas mixture, comprising: a reactor vessel having a gas inlet for receiving an input gas mixture comprising a first gas to be sequestered from the input gas mixture, a liquid inlet for receiving a reactor liquid and an output for outputting an output reactor liquid and an output gas mixture; a liquid feed vessel for supplying a reactor liquid for sequestering the first gas, the liquid feed vessel comprising a liquid outlet for outputting the reactor liquid, a recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a gas output for outputting an output gas mixture; a pump coupled between the liquid outlet of the liquid feed vessel and the liquid inlet of the reactor vessel for providing a reactor liquid to the reactor vessel under pressure, wherein the reactor vessel is a Downflow Gas Contactor (DCC) vessel comprising a column having an entry section coupled to the gas inlet and liquid inlet, the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input gas mixture such that the first gas is substantially sequestered by the reactor liquid to generate the output reactor liquid, and wherein the output reactor liquid and output gas mixture are output from the reactor vessel output, and the output gas mixture is recoverable via the liquid feed vessel gas output, the output gas mixture comprising the input gas mixture having a reduced or nil quantity of the first gas.
2. Apparatus according to claim 1, wherein the input gas mixture comprises air or biogas, the biogas comprising methane, carbon dioxide and hydrogen sulphide.
3. Apparatus according to claim 2, wherein the first gas comprises carbon dioxide.
4. Apparatus according to claim 2 or 3, wherein the biogas further comprises hydrogen sulphide, and the first gas comprises hydrogen sulphide.
5. Apparatus according to any preceding claim, wherein the reactor liquid comprises solution of water and seasalt.
6. Apparatus according to claim 5, wherein the seasalt comprises one or more of Sodium chloride, Magnesium Sulphate, Magnesium Chloride, Calcium chloride Potassium chloride, Sodium bicarbonate and Sodium thiosulphate.
7. Apparatus according to any one of claims 1 to 4, wherein the reactor liquid comprises a solution of water and Sodium Carbonate.
8. Apparatus according to any one of claims 1 to 4, wherein the reactor liquid comprises a solution of water and Sodium Hydroxide.
9. Apparatus according to any one of claims 1 to 4, wherein the reactor liquid comprises a solution of water and Methylethylamine.
10. Apparatus according to any one of claims 5 to 9, wherein one or more constituents of the reactor liquid have a concentration between 0.5M and 1.OM.
11. Apparatus according to any preceding claim, wherein the liquid feed vessel further comprises a temperature control means coupled to a temperature controller, and wherein the temperature controller is configured to control a temperature of the reactor liquid.
12. Apparatus according to claim 11, wherein increasing the temperature of the output reactor liquid causes the first gas to be released from the output reactor liquid via the gas output.
13. Apparatus according to any preceding claim, further comprising a liquid flow controller coupled to the pump, the liquid flow controller being configured to control the rate of flow of the reactor liquid.
14. Apparatus according to any preceding claim, further comprising a gas flow controller configured to control a flow of the input gas mixture.
15. Apparatus according to any preceding claim, further comprising a compressor coupled to the gas inlet for compressing the input gas mixture.
16. Apparatus according to any preceding claim, further comprising a second liquid feed vessel for supplying the reactor liquid for sequestering the first gas, the second liquid feed vessel comprising a second liquid outlet for outputting the reactor liquid, a second recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a second gas output for outputting an output gas mixture
17. Apparatus according to claim 16, wherein the liquid outlet of the liquid feed vessel is coupled to the second liquid outlet of the second liquid feed vessel, the recycle inlet of the liquid feed vessel is coupled to the second recycle inlet of the second liquid feed vessel and the gas output of the liquid feed vessel is coupled to the second gas output of the second liquid feed vessel.
18. Apparatus according to any preceding claim, wherein the entry section is configured to provide a liquid inlet power from about 3 Watts to about 30 Watts, preferably from about 5 Watts to about 25 Watts.
19. A method of reducing the relative quantity of a first gas in an input gas mixture, the method comprising the steps of: receiving an input gas mixture, the input gas mixture comprising a first gas to be sequestered from the input gas mixture; pumping a reactor liquid for sequestering the first gas from a liquid outlet of a liquid feed vessel into a Downf low Gas Contactor (DGC) reactor vessel via a liquid inlet of the DGC; inputting the input gas mixture into a gas inlet of the DGC, wherein the DGC comprises a column having an entry section coupled to the gas inlet and liquid inlet, and the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input gas mixture such that the first gas is substantially sequestered by the reactor liquid to generate an output reactor liquid and an output gas mixture; pumping the output reactor liquid and output gas mixture from an output of the DCG to a recycle inlet of the liquid feed vessel; recovering the output gas mixture from a gas output of the liquid feed vessel, the output gas mixture comprising the input gas mixture having a reduced or nil quantity of the first gas.
20. A method according to claim 19, wherein the input gas mixture comprises air or biogas, the biogas comprising methane, carbon dioxide and hydrogen sulphide.
21. A method according to claim 20, wherein the first gas comprises carbon dioxide.
22. A method according to claim 20 or 21, wherein the biogas further comprises hydrogen sulphide, and the first gas comprises hydrogen sulphide.
23. A method according to any one of claims 19 to 22, wherein the reactor liquid comprises solution of water and seasalt.
24. A method according to claim 23, wherein the seasalt comprises one or more of Sodium chloride, Magnesium Sulphate, Magnesium Chloride, Calcium chloride Potassium chloride, Sodium bicarbonate and Sodium thiosulphate.
25. A method according to any one of claims 19 to 22, wherein the reactor liquid comprises a solution of water and Sodium Carbonate.
26. A method according to any one of claims 19 to 22, wherein the reactor liquid comprises a solution of water and Sodium Hydroxide.
27. A method according to any one of claims 19 to 22, wherein the reactor liquid comprises a solution of water and Methylethylamine.
28. A method according to any one of claims 23 to 27, wherein one or more constituents of the reactor liquid have a concentration between O.5M and 1.OM.
29. A method according to any one of claims 19 to 28, wherein the method further comprises controlling the temperature of the reactor liquid.
30. A method according to claim 29, wherein increasing the temperature of the output reactor liquid causes the first gas to be released from the output reactor liquid via the gas output.
31. A method according to any one of claims 19 to 30, further comprising the step of controlling the rate of flow of the reactor liquid.
32. A method according to any one of claims 19 to 31, further comprising the step of controlling the flow of the input gas mixture.
33. A method according to any one of claims 19 to 32, further comprising the step of compressing the input gas mixture.37. A method according to any one of claims 19 to 33, wherein the entry section is configured to provide a liquid inlet power from about 3 Watts to 30 Watts, preferably from about 5 Watts to about 25 Watts.38. Apparatus for increasing a calorific value of an input biogas mixture, comprising: a reactor vessel having a gas inlet for receiving an input biogas mixture comprising a first gas to be sequestered from the input biogas mixture, a liquid inlet for receiving a reactor liquid and an output for outputting an output reactor liquid and an output biogas mixture, the first gas being a gas that reduces the calorific value of the input biogas; a liquid feed vessel for supplying a reactor liquid for sequestering the first gas, the liquid feed vessel comprising a liquid outlet for outputting the reactor liquid, a recycle inlet coupled to the outlet of the reactor vessel for receiving the output reactor liquid from the reactor vessel, and a gas output for outputting an output biogas mixture; a pump coupled between the liquid outlet of the liquid feed vessel and the liquid inlet of the reactor vessel for providing a reactor liquid to the reactor vessel under pressure, wherein the reactor vessel is a Downflow Gas Contactor (DGC) vessel comprising a column having an entry section coupled to the gas inlet and liquid inlet, the entry section being configured to cause sufficient turbulence and shear at an interface between the entry section and the column so as to cause turbulence and mixing of the reactor liquid and input biogas mixture such that the first gas is substantially sequestered by the reactor liquid to generate the output reactor liquid and an output biogas mixture, and wherein the output reactor liquid and output biogas mixture are output from the reactor vessel output, and the output biogas mixture is recoverable via the liquid vessel gas output, the output biogas mixture comprising the input biogas mixture having a reduced or nil quantity of the first gas, thereby having an increased calorific value.39. Apparatus according to claim 38, wherein the input biogas mixture comprises methane and carbon dioxide, and wherein the first gas comprises carbon dioxide.40. Apparatus according to claim 38 or 39, wherein the input biogas mixture comprises hydrogen sulphide, and the first gas comprises hydrogen sulphide.