GB2489969A

GB2489969A - Apparatus for heating a liquid by using an exothermic chemical reaction

Info

Publication number: GB2489969A
Application number: GB1106269.2A
Authority: GB
Inventors: Mark Collins
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-04-13
Filing date: 2011-04-13
Publication date: 2012-10-17
Anticipated expiration: 2031-04-13
Also published as: GB2489969B; WO2012140170A2; GB201106269D0; WO2012140170A3

Abstract

An apparatus for heating a liquid comprises first and second storage vessels, a reaction vessel, a first heat exchanger, a second heat exchanger, at least one ancillary heat exchanger, a separator and a burner. The first storage vessel contains a metal powder slurry or suspension in water, and the second storage vessel contains a solution of an alkali metal hydroxide. The reaction vessel is in fluid communication with the storage vessels and has at least one inlet through which the metal power slurry/suspension and the alkali metal hydroxide can be introduced so that they react exothermically. First and second outlets are provided for removing liquid and gaseous reaction product streams respectively. The first heat exchanger is located in the reaction vessel so that heat produced by the exothermic reaction is transferred to the liquid. The separator is linked to the second outlet, and collects hydrogen gas formed by the exothermic reaction for supply to the burner, where the second heat exchanger is associated with the burner and transfers heat obtained from combustion of the hydrogen gas to the liquid. An ancillary heat exchanger is located downstream of the first outlet for extracting heat from the liquid reaction product stream.

Description

I

AN APPARATUS FOR GENERATiNG HEAT This invention relates to an apparatus for generating heat for use in a heating system for liquids such as water.

Background of the Invention

It is well known that many chemical reactions are exothermic, i.e. they produce heat, and examples of such reactions include acid-base reactions.

US 4325355 describes a heating system in which an exothermic reaction between a solid metal and a solution takes place in a reactor containing a heat exchanger.

In the specific reaction system described, aluminium pieces are lowered into a solution of sodium hydroxide solution. During the reaction between aluminium and sodium hydroxide solution, the aluminium is converted to aluminium hydroxide with the evolution of hydrogen gas. The aluminium hydroxide reacts with the sodium hydroxide to form sodium aluminate.

DE 3539710 describes a small scale heating system comprising an outer pouch containing an inner pouch partitioned to form two chambers containing reactive chemicals. Pressu rising the pouch (for example by kneading) causes the partition wall to rupture allowing the two reactive chemicals to react to produce heat. The reactive chemicals can be sodium hydroxide and acetic anhyd ride. The heating system of DE 3539710 is described as being particularly useful for warming hands and feet.

GB 2381187 discloses a method and apparatus for cleaning a surface. As part of the cleaning process, a cleaning solution is heated by the mixing of chemicals in an exothermic reaction.

WO 86101880 describes a heating system that can be used for domestic water heating and which involves a multistage process comprising a first heat exchange step in which heat extracted from sea water is used to vapourise a liquefied gas such as ammonia. The ammonia vapour then passes to a second stage where it reacts either with sodium carbonate solution or carbon dioxide in an exothermic process, the heat from which is extracted to heat domestic water.

US 4044821 describes an energy conversion and storage system in which chemical compounds such as ammonia or metal hydrides are decomposed using energy from, for example, a solar energy device. The decomposition products can be recombined in a later step to produce chemical energy.

WO 2004/040645 discloses a microfluidic heat exchanger for providing small scale heating and cooling control using exothermic and endothermic chemical reactions.

The addition of sulphuric acid to water is disclosed as an example of an exothermic heating source.

US 3563226 describes a heating system intended for use underwater or in oxygen-free environments in which an oxidiser such as pure oxygen is reacted with a pyrophoric material such as phosphorus.

US 7381376 discloses steam/vapour generators in which the source of the heat is an exothermic chemical reaction.

DE 3819202 describes a system of heat storage using molten salts.

U54303541 describes latent heat storage devices that make use of saturated solutions of salts. The salts are formed by the reaction of an acid and a base, and there is a passing reference to the possibility that the heat generated in the reaction may be used elsewhere.

My earlier patent application W02008/1 021 64 discloses a method and apparatus for producing a supply of a heated fluid (e.g. water) wherein the method comprises passing the fluid through a heat exchanger unit where it is heated by a heat source which derives its heat from the exothermic reaction of two or more chemical reactants.

The present invention provides an improved apparatus for making use of the heat generated by exothermal chemical reactions to heat liquids such as the water in a water supply.

Summary of the Invention

In a first aspect, the invention provides an apparatus for heating a heatable liquid, which apparatus comprises: (1) a first storage vessel containing (a) a slurry or suspension of a metal powder in water; (ii) a second storage vessel containing (b) a solution of an alkali metal hydroxide; (iii) a reaction vessel in fluid communication with (i) and (ii) and having at least one inlet through which (a) and (b) can be introduced into the reaction vessel so that they react to form reaction products, a first outlet for removal of a liquid reaction product stream containing the reaction products of (a) and (b) and a second outlet for removal of gaseous materials generated by reaction of (a) and (b); (iv) a first heat exchanger disposed in or on the reaction vessel, the heatable liquid being arranged to flow through the first heat exchanger so that heat produced by reaction between (a) and (b) in the reaction vessel is transferred to

the heatable liquid;

(v) at least one ancillary heat exchanger disposed downstream of the first outlet for extracting heat from the liquid reaction product stream, wherein the ancillary heat exchanger is arranged to transfer heat to the heatable liquid or is arranged to pre-heat one or both of (a) and (b) prior to introduction into the reaction vessel; (vi) a separator, linked to the second outlet, for collecting hydrogen gas formed by the reaction of (a) with (b); (vii) a burner for the hydrogen gas; and (viii) a further heat exchanger associated with the burner for transferring heat obtained from combustion of the hydrogen gas to the heatable liquid.

The metal powder is a powdered form of a metal that is reactive with an aqueous solution of an alkaline metal hydroxide to form an oxide or hydroxide form of the said metal and hydrogen gas.

In one embodiment, the metal powder is an aluminium powder.

The alkali metal hydroxide is typically sodium hydroxide or potassium hydroxide, and preferably is sodium hydroxide.

The reaction between aluminium and aqueous sodium sodium hydroxide can be represented as follows: 2A/ + 6H20 + 2NaOH -> 2NaA1(OH)4 + 3H2 (1) NaAJ(OH)4 -> NaOH + Al(OH)3 (2) 2A1 + 6H20 -> 2A1(OH)3 + 3H2 (3) Initially, the H2 production reaction (1) consumes NaOH and produces NaAI(OH)4 which undergoes a decomposition reaction (2) when its concentration exceeds the saturation limit. A crystalline precipitate of AI(OH)3 is produced with the regeneration of the alkali. The overall reaction (3) shows that only Al and H20 are consumed, so that the role of the alkali in this process can be seen as being catalytic.

At least one ancillary heat exchanger is disposed downstream of the liquid waste outlet for extracting heat from the liquid waste stream, wherein the ancillary heat exchanger is arranged to transfer heat to the heatable liquid or is arranged to pre-heat one or both of (a) and (b) prior to introduction into the reaction vessel.

In one embodiment, an ancillary heat exchanger (v) is disposed downstream of the liquid waste outlet for extracting heat from the liquid waste stream and is arranged to transfer heat to the heatable liquid.

A filter or settling tank may be located in-line between the first outlet and the ancillary heat exchanger (v), the filter or settling tank being arranged to remove solid materials from the liquid waste stream prior to the liquid waste stream passing through the ancillary heat exchanger (v). The filter or settling tank may be provided with a waste outlet through which solid waste materials may be removed from the system.

Thus, for example, precipitated crystalline Al(OH)3 (or other precipitated reaction product) may collected by the filter or allowed to settle out in the settling tank before removing through the waste outlet.

The liquid reaction product stream may be recycled back to the reaction vessel after passing through the ancillary heat exchanger (v) and, where present, the filter or settling tank. In this way, Al(OH)3or other precipitated reaction product may be removed from the system and the filtered stream containing the alkali metal hydroxide returned to the reactor vessel.

In another embodiment, an ancillary heat exchanger (v) is disposed downstream of the liquid waste outlet, and is arranged to transfer heat to pre-heat one or both of (a) and (b) prior to introduction into the reaction vessel.

In a particular embodiment, a pair of ancillary heat exchangers (v) is disposed downstream of the liquid waste outlet, the ancillary heat exchangers being arranged to pre-heat both of (a) and (b) prior to introduction into the reaction vessel.

The heatable liquid is preferably water and thus the apparatuses of the invention are particularly useful for heating water.

Accordingly, the apparatus may form part of a domestic water heating system or an industrial or commercial water heating system.

In one embodiment, the apparatus forms part of a water heating system intended to provide water for central heating or sanitation purposes.

In another embodiment, the apparatus forms part of a water heating system for a swimming pool.

In another aspect, the invention provides a method of heating a liquid which method comprises passing the liquid through the heat exchanger of an apparatus as defined herein.

A substantial advantage of the apparatus of the invention is that it provides a very efficient means for heating a liquid such as water whereby heating losses to the external environment are minimised. Heat losses may be minimised still further by insulating the components of the apparatus in conventional fashion.

A further advantage of the apparatus of the invention is that it can be used in locations where mains electricity or mains gas supplies are not available or are restricted. Thus, although electrical power is required to operate the apparatus, the amount of power required is relatively small and can therefore be supplied by renewable resources such as a wind turbine or solar power.

The invention will now be illustrated in more detail (but not limited) by reference to the specific embodiment shown in the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic view of an apparatus according to the invention.

Figure 2 shows the effect on temperature and quantity of evolved hydrogen of varying the mass of aluminium used in a reaction between aluminium and sodium hydroxide.

Figure 3 shows the effect of on H2 temperature and quantity of evolved hydrogen of varying the amount of sodium hydroxide used in a reaction between aluminium and sodium hydroxide.

Detailed Description of the Invention

Figure 1 illustrates two possible embodiments of the heating apparatus of the invention. The first embodiment is denoted in Figure 1 by the features shown as solid lines. The second layout consists of the features shown as dotted lines together with some of the features shown in solid lines as described below.

Embodiment I The apparatus of the first embodiment shown in Figure 1 comprises a first storage vessel T-01 which, in this embodiment, contains a slurry or suspension of aluminium powder. The storage vessel T-01 is connected through pipework and a feed pump P-UI to a heat exchanger E-01 and thence to the inlet I-I of a reaction vessel R-01. On the downstream side of the pump P-UI, a length of pipework allows the slurry or suspension of aluminium to be recirculated back to the storage vessel T-01. Recirculation of the slurry or suspension prevents or reduces settling of the metal particles. As an alternative to a recirculating system, a mechanical stirrer could be mounted inside the storage vessel T-Ul.

A second storage vessel T-02 containing sodium hydroxide or another alkali metal hydroxide is linked via pipework and a pump P-02 to another heat exchanger E-02 and thence to another inlet 1-2 of the reaction vessel R-U1.

The reaction vessel R-0I has a first outlet 0-I for removing liquid reaction products from the vessel. A float valve FV-I is linked to the outlet 0-I and prevents the reaction vessel R-0I from overfilling.

In use, metered amounts of aluminium and sodium are introduced into the reaction S vessel where they react according to the series of reactions shown below.

2A1 + 6H20 + 2NaOH -> 2NaAI(OH)4 + 3H2 (I) NaA1(OH)4 -+ NaOH + A1(OH)3 (2) 2A1 + 6H20 -> 2A1(OH)3 + 3H2 (3) Heat generated by the reaction is transferred via heat exchanger E-04 to water passing through the heat exchangerfrom a water supply WS-1.

At the upper end of the reaction vessel R-0I is a second outlet 0-2 through which gaseous products of reactions inside the reaction vessel R-0I may be vented. The second outlet 0-2 is connected via pipework to a vapour disengagement vessel or separator V-UI which separates hydrogen reaction product from the reaction vessel from water vapour carried out through outlet 0-2 along with the hydrogen. A return pipe provides a gravity feed of condensed water vapour back to the reaction vessel R-0I.

The separator V-UI is linked via a length of pipework containing a tap or valve to a burner H-UI in which hydrogen gas is burnt to produce heat. The heat generated by the combustion of the hydrogen is used to heat water from the water supply WS-I passing through a heat exchanger E-05.

After allowing reaction between the sodium hydroxide and aluminium to take place in the reaction vessel RV-OI, the reaction mixture is allowed to pass out of the outlet 0-2 in the lower end of the reaction vessel.

The outlet 0-I of the reaction vessel R-UI is connected via a length of pipe to a branch point BP-I where the pipeline splits into two branches. A tap or three-way valve may be provided at the branch point. One branch leads to the heat exchanger E-UI where heat from the reaction product stream serves to pre-heat the suspension or slurry of aluminium passing through the heat exchanger en route to the reaction vessel S-Ui. The other branch leads to the heat exchanger E-U2 where heat from the reaction product stream serves to pre-heat the solution of sodium hydroxide or other alkali metal hydroxide passing through the heat exchanger en route to the reaction vessel S-UI.

After passing through the heat exchangers F-UI and F-U2, the two reaction product streams are recombined and directed to the product tank (waste tank) T-U3 where the contents may be allowed to settle and any solids separated from the liquids prior to recycling or disposal of the solids and/or the liquids.

The primary source of heat is the aluminium through its conversion to aluminium hydroxide in an exothermic series of reactions and the generation of hydrogen which is then burnt to produce further heat. The feed rate of aluminium into the reaction vessel is therefore controlled so as to provide a desired constant return temperature of the water from the water supply WS-I.

The sodium hydroxide acts as a catalyst and the rate of reaction of the aluminium in the reaction vessel depends on the pH within the reaction vessel. The feed rate of the sodium hydroxide is therefore controlled so as to maintain a constant pH.

The pressure in the reaction vessel can be controlled by adjusting the rate at which hydrogen is let out of the vapour disengagement vessel V-Ui. This function can be controlled manually or a self-acting pressure regulator can be used for controlling the pressure.

The level of reactants in the reaction vessel S-UI is automatically adjusted using the float valve FV-I. The float valve regulates the rate at which the reaction products leave the reaction vessel, balancing it with the rate at which the reactants are supplied.

The flow balance of the reaction products between the two heat exchangers (pre-heaters) E-Ui and F-U2 is set using individual valves in (not shown) in the reaction product outlets of the two exchangers. In order to prevent simultaneous closure of both valves, thereby stopping all flow through heat exchangers F-UI and F-U2, a mechanical linkage (not shown) is provided between the two valves. The mechanical linkage is typically set so that when one valve is closed, the other valve will be opened. The operator (or when the system is fully automated) the control system will adjust the two valves until the temperatures of the aluminium suspension leaving F-UI and the sodium hydroxide solution leaving F-02 are the same. In general, the valve settings will only need changing if there are changes in the concentrations of aluminium or sodium hydroxide in the system or if the pH of the reaction mixture in the reaction vessel R-0I changes.

Whereas the operation of the apparatus of the invention may be controlled manually, it is preferred that the apparatus comprises a computerised control system. Typically, the control system will comprise a programmable processor by virtue of which a user of the system will be able to select one or more parameters such as the temperature of the heated water supply WS-l emerging from the apparatus and the time periods over which the apparatus should deliver heated water at the required temperature. In order to permit automation of the system, the apparatus is provided with a number of sensors and gauges feeding system information back to the controller so that valves, pumps and other actuating devices forming part of the apparatus can be controlled to achieve the desired outcome. Accordingly, any one or more of the following instruments and monitoring methods may be used in the apparatus: Parameter Monitoring method/instrument Flow of aluminium from T-0l to the reaction Rotameter vessel R-UI Feed flow of sodium hydroxide from T-02 to the Metering Pump Setting reaction vessel R-UI Level of aluminium in T-Ul Sight Glass/Gauge Level of sodium hydroxide in T-02 Sight Glass/Gauge Temperature of aluminium feed at the outlet of F-Multipoint Thermocouple

UI

F-UI differential pressure (both streams) Manometer Temperature of sodium hydroxide feed at the Multipoint Thermocouple outlet of F-U2 F-U2 differential pressure (both streams) Manometer Level of reaction mixture in reaction vessel R-UI Sight Glass/Gauge Pressure in reaction vessel R-01 Bourdon Gauge Temperature in reaction vessel R-01 Multipoint Thermocouple pH in reaction vessel R-01 Sample testing Heat exchanger E-04 outlet water temperature Multipoint Thermocouple Heat exchanger E-04 outlet water flow Rota meter Level in the Vapour Disengagement Vessel V-UI Sight Glass/Gauge Hydrogen flow Rota meter Flue gas temperature in the burner H-UI Multipoint Thermocouple Heat exchanger E-05 outlet water temperature MulUpoint Thermocouple Heat exchanger E-05 outlet water flow Rotameter Reaction product flow to E-01 Rotameter Reaction product flow to E-02 Rota meter Reaction product temperature at outlet of E-UI Multipoint Thermocouple Reaction product temperature at outlet of E-02 Multipoint Thermocouple Product Tank (T-03) level Sight Glass/Gauge Product Tank (T-03) temperature Multipoint Thermocouple External water supply (WS-1) input temperature Multipoint Thermocouple External water supply (WS-1) return temperature Multipoint Thermocouple External water supply flow rate/volume Rotameter Embodiment 2 The apparatus used according to Embodiment 2 in comprises the components shown in solid lines in Figure 1 but with the addition of the components shown by dotted lines and with the omission (or bypassing) of the heat exchangers E-OI and E-02 and the omission of the pipework leading to them from the reaction vessel outlet 0-1. In addition, the float valve arrangement FV-I is omitted.

In this embodiment, a more concentrated metal (e.g. aluminium) suspension or slurry may be stored in the storage vessel T-01. The flow of aluminium suspension

II

from vessel T-0I is then diluted to a required concentration by water pumped from storage tank T-04 by pump P-03 before being introduced into the reaction vessel R-0I.

The apparatus of this embodiment has a recycling system for removing precipitated reaction products (e.g. Al(OH)3) from the reaction product stream and then recycling the reaction product stream back to the reaction vessel. Thus, in this embodiment, the reaction products leaving the outlet 0-1 of the reaction vessel are pumped by pump P-04 to the filter F-UI. The filter removes particulates above a certain size which are then directed to the waste product tank T-03 for disposal.

The filter may be of a self cleaning variety provided with motorised internal scrapers (not shown). Alternatively, or additionally, a backwash arrangement may be provided for removal of solids from the filter.

The filtered reaction product stream then passes through a heat exchanger E-03 where it transfers heat to water from the water supply WS-I before joining the flow of sodium hydroxide from T-02 and re-entering the reaction vessel.

Thus, in this embodiment, residual heat in the reaction products is captured by the ancillary heat exchanger E-03 rather than by the ancillary heat exchangers E-0I and E-02 which have been omitted.

An advantage of the recycling system is that it enables sodium hydroxide to be reused rather than being sent to waste. in addition, by recycling the filtered reaction product stream less of the heat generated in the reaction vessel goes to waste.

As with Embodiment I, in order to permit automation of the system, the apparatus is provided with a number of sensors and gauges feeding system information back to the controller so that valves, pumps and other actuating devices forming part of the apparatus can be controlled to achieve the desired outcome. Accordingly, any one or more of the following instruments and monitoring methods may be used in the apparatus: Parameter Monitoring method/instrument Flow of aluminium from T-01 to the reaction Rotameter vessel R-U1 Feed flow of sodium hydroxide from T-02 to the Metering Pump Setting reaction vessel R-U1 Level of aluminium in T-U1 Sight Glass/Gauge Level of sodium hydroxide in T-02 Sight Glass/Gauge Dilution water flow rate from T-04 Metering pump setting Dilution tank T-04 water level Ball float valve Temperature of aluminium feed to reaction vessel Multipoint Thermocouple

R-O I

Temperature of sodium hydroxide feed to Multipoint Thermocouple reaction vessel R-U1 Level of reaction mixture in reaction vessel R-U1 Sight Glass/Gauge Pressure in reaction vessel R-O1 Bourdon Gauge Temperature in reaction vessel R-U1 Multipoint Thermocouple pH in reaction vessel R-O1 pH meter Heat exchanger E-04 outlet water temperature Multipoint Thermocouple Heat exchanger E-04 outlet water flow Rotameter Level in the Vapour Disengagement Vessel V-UI Sight Glass/Gauge Hydrogen flow Rotameter Flue gas temperature in the burner H-UI Multipoint Thermocouple Heat exchanger E-05 outlet water temperature Multipoint Thermocouple Heat exchanger E-05 outlet water flow Rotameter Reaction product flow to E-O1 Rotameter Reaction product flow to E-02 Rota meter Recycle stream flow Metering pump setting Recycle filter (F-UI) differential pressure (dP) Manometer Filter reject flow to product tank T-03 Rota meter E-03 inlet temperature (hot sides) Multipoint Thermocouple E-03 outlet temperature (hot sides) Multipoint Thermocouple E-03 pressure drop (both sides) Manometer E-03 outlet temperature (cold sides) Multipoint Thermocouple E-03 external water flow Rotameter Product tank 1-03 level Sight glass/gauge Product tank T-03 temperature Multipoint Thermocouple External water supply (WS-1) input temperature Multipoint Thermocouple External water supply (WS-1) return temperature Multipoint Thermocouple External water supply flow rate/volume Rotameter Thus, in each of the two embodiments of the invention, the primary source of heat for the water supply WS-1 is the heat exchanger E-04 in the reaction vessel with additional heat being provided by the combustion of hydrogen gas generated in the reaction vessel.

In the case of Embodiment 1, further heat recovery is effected by circulating the reaction product stream through ancillary heat exchangers E-01 and E-02 before the reaction product stream is sent to waste.

In the case of Embodiment 2, further heat recovery is effected by circulating the reaction product stream through ancillary heat exchanger E-03 before the reaction product stream is recycled to the reaction vessel.

Thus, in each case, heat recovery is maximised.

Investigation of the heat generated and volume of hydrogen produced by reaction between aluminium and sodium hydroxide The following experiments illustrate the effectiveness of aluminium/sodium hydroxide reaction systems as a means for generating heat.

Materials and methods: NaOH pellets (98% purity) and Al powder (laboratory grade, 80% purity) were supplied by Fisher Chemicals. Reagents were used as received without further purification. Deionised water supplied by a Milli-Q water purification system was used to prepare all the aqueous solutions. The different solutions tested in this study were freshly prepared. All experiments were conducted at room temperature (20°C) in triplicate.

Experiments were performed in a 3-neck Pyrex glass beaker containing 75 ml of NaOH aqueous solutions at different concentrations. A manometer was connected to the 3 neck of the Pyrex glass reactor for measuring the pressure of hydrogen generated during the study.

A measured quantity of aluminium powder was added into the sodium hydroxide solution and the time taken for the aluminium to be consumed was recorded. The increase in temperature of the reaction mixture was measured using a thermometer.

After addition of the aluminium, the evolution of hydrogen gas was observed.

Hydrogen emerged from the reactor through a rubber tube of 20 cm length and 3 mm internal diameter. The pressure of hydrogen gas was estimated from the water level changes in the manometer in accordance with standard methods Results and discussion: Hydrogen evolution was observed after a short induction period (c35s) for all the experiments performed. The amount of hydrogen generated is proportional to the pressure as determined from the manometer readings. During the evolution of hydrogen, the formation of a white powder was observed, which partially suspended in the aqueous phase. This was attributed to Al(OH)3 precipitation.

Effect of varying the mass of Al added to the reaction mixture: The relationship between the volume of H2 released and the amount of Al added was studied by fixing the concentration of NaOH to I M and adding varying amounts of Al (from 0.1 to 0.5 g) to the reactor. The results are shown in Figure 2.

As the amount of Al increased, the rate of reaction increased. This can be explained on the basis that the reaction rate of Al should be proportional to its surface which, for a high number of similar small particles, should be proportional to its mass.

The hydrogen gas generated from the Al hydrolysis was collected and passed through rubber tubing to the manometer. The change in the water level in the manometer was recorded and used to calculate the hydrogen pressure in accordance with standard methods.

Table I below shows the experimental conditions used in each individual experiment, the Al consumption time, height of the increased water level in the manometer, the maximum temperature recorded in the experiment and calculated hydrogen pressure values.

Table I

Al Pt-Po NaOH Height Average Run Al (g) consumption T (°C) (M) (cm) Pt (atm) time (mm) a m 1 0.5085 1 733" 4.2 40 412 2 0.5016 1 654" 4.4 40 432 1.00424 3 0.5074 1 707" 4.5 40 441 4 0.3023 1 552" 3.4 32 334 0.3051 1 554" 3.3 32 324 1.00314 6 0.3045 1 629" 3.0 32 294 7 0.2144 1 429" 1.7 28 167 8 0.2168 1 425" 1.7 28 167 1.00168 9 0.2143 1 433" 1.8 28 177 0.1009 1 302" 0.8 24 78 11 0.1035 1 304" 0.6 24 59 1.00071 12 0.1041 1 305" 0.8 25 78 Al NaOH Height ° Average Run Al (g) consumption T (°C) (M) (cm) Pt (atm) time (mm) (atm) 13 0.1125 2 300" 4.7 24 461 14 0.1214 2 314" 5.0 25 491 1.00479 0.1150 2 318" 5.1 25 500 16 0.1018 4 154" 0.6 25 59 17 0.1009 4 117" 0.8 25 78 1.00071 18 0.1026 4 138' 0.8 26 78 The maximum pressure (Pt) of 1.00424 atm was recorded when using 0.5 g Al powder. The temperature change inside the reactor was also recorded. The temperature increased significantly even when using the lowest amount of Al powder in the experiment. The temperature increased to 40°C from room temperature (20°C) when 0.5 g of Al was added to I M NaOH solution.

Effect of NaOH concentration: The results obtained by using 0.1 g of Al powder in NaOH solutions of varying strength are shown in Figure 4. As expected, an increase in NaOH concentration caused an increase of hydrogen production. When the concentration of NaOH increased from I M to 2M, the pressure of H2 was raised to 1.00479 atm. The result is comparable to the one obtained using 0.5 g of Al in I M NaOH solution.

However, when the NaOH concentration was increased further to 4M, the pressure was reduced to 1.00071 atm which is the same as the pressure obtained using I M NaOH. The results suggest that there is an optimum concentration of NaOH in the aluminium hydrolysis reaction. It may be explained by the Al consumption time recorded in Table 1. Al was consumed in around 1.3 minutes when using 4M NaOH solution, and in around 3 mm when using IM or2M NaOH solution, which indicates the concentration of NaOH has an effect on the reaction rate. Although higher concentration of NaOH increased the reaction rate, the heat output is predominantly governed by the mass of Al. As illustrated in Fig. 3, the difference in temperature increase inside the reactor was relatively small when the NaOH concentration was increased.

The volume of hydrogen produced and hydrogen production rate: The volume of hydrogen produced can be estimated from the pressure readings obtained from the manometer by applying the Ideal Gas Law equation: PV = nRT where P is the gas pressure (atm), V is the gas volume (dm3), n is the number of moles of gas (Mole), R is the gas constant (0.08206 L atm/(k mol)), and T is temperature (K).

The results are summarised in Table 2.

Table 2: Volume of H2 generated and the estimated mass balance.

NaOH Pressure Volume of Measured Theoretical Meas/Theo Al (g) (M) (atm) H2 (L) (mole) (mole) (%) 0.5 1 1.00424 0.45 0.0188 0.0222 84.6 0.3 1 1.00314 0.34 0.0142 0.0133 106.4 0.2 1 1.00168 0.21 0.0087 0.0089 98.4 0.1 1 1.00071 0.098 0.0041 0.0044 91.8 0.1 2 1.00479 0.093 0.0039 0.0044 87.4 0.1 4 1.00071 0.101 0.0042 0.0044 94.6 Comparison of the experimental and theoretical heat output: The temperature changes recorded in Table I and Figures 2 and 3 were compared with the theoretical values.

The theoretical heat output was calculated from AH = -415.60 kJ/mol of Al, knowing the number of moles of Al being used. The temperature changes in degree Celsius were converted to Joules using the specific heat of water which is 4.186 Jt(g °C).

In Runs 1, 2 & 3 (see Table 1), 0.5g Al of purity 80% raised the water temperature from 20°C to 40°C.

0.5g Al of having a purity of 80% = 0.5(g)*80%/27(g/mol) = 0.0 148 (mol).

The theoretical heat = 0.0148 (mol)*AH= 0.0148 (mol)*415.60 (kJ/mol) = 6.157 (kJ) In the experiment, the temperature of 75 ml solution was increased by 20°C. The energy needed to increase the temperature of a 75m1 solution from 20°C to 40°C can be calculated as: 4.186 Jt(g °C) * (20) °C * 75g = 6279 J = 6.279 kJ As shown in Table 3, for most experiments, the experimental heat values were similar to those from theoretical calculations. For some experiments, the measured heat values were higher than the theoretical values (runs 13-18), due to the higher NaOH concentrations used. An explanation for this is that at higher NaOH concentrations, the reaction was much faster and the heat generated by the reaction was not evenly distributed. Also, the response time of the thermometer used may not have been fast enough to detect the temperature changes in seconds. Nevertheless, despite the aforementioned minor discrepancies, the measured heat output from the relatively simple experimental set up was generally consistent with predicted values.

Table 3: Comparison of the theoretical values with experimental results for heat generation.

Theoretical heat Experimental result Exp/Theo Run Al (9) (kJ) (kJ) (%) 1-3 0.5 6.157 6.279 102 4-6 0.3 3.694 3.767 102 7-9 0.2 2.463 2.512 102 Theoretical heat Experimental result Expllheo Run Al (g) (kJ) (kJ) (%) 10-12 0.1 1.231 1.256 102 13-15 0.1 1.231 1.350 110 16-18 0.1 1.231 1.664 135 Use of sodium borohyd ride as a third reactant in the reaction system As indicated above, aluminium hydroxide is formed during reaction of sodium hydroxide with aluminium. Aluminium hydroxide reacts with sodium borohydride according to the following reaction (6): 4Al(OH)3 + 3NaBH4 -* 3NaBO2 + 2A1203 + 12H2 (6) molar ratio 4 3 2 12 actual 0.0148 0.0111 0.0074 0.0444 AH = 135.9 x 0.0148 = 2.0 kJ.

H2 combustion generates further heat: 2H2i-02-->2H20 (7) Heat output = 286 kJ/mole x (0.0222 + 0.0444) mole = 19 kJ.

Total heat output = 6.2 + 2.0 + 19 = 27.2 kJ.

Product volume = 0.0111 x65/2.46 + 0.0074 x 102/4 = 0.482 mL Heat/product = 27.2 kJ /0.482 mL = 56 MJ/L (product).

Thus by adding sodium borohydride to the reaction mixture upstream of the heat exchanger, considerable further heat is generated and hydrogen gas produced.

The embodiments described above and illustrated in the accompanying figures and tables are merely illustrative of the invention and are not intended to have any limiting effect. It will readily be apparent that numerous modifications and alterations may be made to the specific embodiments shown without departing from the principles underlying the invention. All such modifications and alterations are intended to be embraced by this application.

Claims

CLAIMS1. An apparatus for heating a heatable liquid, which apparatus comprises: (i) a first storage vessel containing (a) a slurry or suspension of a metal powder in water; S (ii) a second storage vessel containing (b) a solution of an alkali metal hydroxide; (iii) a reaction vessel in fluid communication with (i) and (ii) and having at least one inlet through which (a) and (b) can be introduced into the reaction vessel so that they react to form reaction products, a first outlet for removal of a liquid reaction product stream containing reaction products of (a) and (b) and a second outlet for removal of gaseous materials generated by reaction of (a) and (b); (iv) a first heat exchanger disposed in or on the reaction vessel, the heatable liquid being arranged to flow through the first heat exchanger so that heat produced by reaction between (a) and (b) in the reaction vessel is transferred to the heatable liquid; (v) at least one ancillary heat exchanger disposed downstream of the first outlet for extracting heat from the liquid reaction product stream, wherein the ancillary heat exchanger is arranged to transfer heat to the heatable liquid or is arranged to pre-heat one or both of (a) and (b) prior tointroduction into the reaction vessel;(vi) a separator, linked to the second outlet, for collecting hydrogen gas formed by the reaction of (a) with (b); (vii) a burner for the hydrogen gas; and (viii) a further heat exchanger associated with the burner for transferring heat obtained from combustion of the hydrogen gas to the heatable liquid.
2. Apparatus according to claim I wherein the metal powder is an aluminium powder.
3. Apparatus according to claim I or claim 2 wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.
4. Apparatus according to claim 3 wherein the alkali metal hydroxide is sodium hydroxide.
5. Apparatus according to any one of claims I to 4 wherein an ancillary heat exchanger (v) is disposed downstream of the liquid waste outlet for extracting heat from the liquid waste stream and is arranged to transfer heat to the liquid supply.
6. Apparatus according to claim 5 wherein a filter or settling tank is located in-line between the liquid waste outlet and the ancillary heat exchanger (v), the filter or settling tank being arranged to remove solid materials from the liquid waste stream prior to the liquid waste stream passing through the ancillary heat exchanger (v).
7. Apparatus according to claim 6 which is adapted to enable the liquid waste stream to be recycled to the reaction vessel after passing through the ancillary heat exchanger (v).
8. Apparatus according to any one of claims I to 4 wherein an ancillary heat exchanger (v) is disposed downstream of the liquid waste outlet, and is arranged to transfer heat to pre-heat one or both of (a) and (b) prior tointroduction into the reaction vessel.
9. Apparatus according to claim 8 wherein a pair of ancillary heat exchangers (v) is disposed downstream of the liquid waste outlet, the ancillary heat exchangers being arranged to pre-heat both of (a) and (b) prior tointroduction into the reaction vessel.
10. An apparatus according to any one of the preceding claims wherein the heatable liquid is water.
11. An apparatus according to any one of the preceding claims which forms part of a domestic water heating system or an industrial or commercial water heating system.
12. An apparatus according to claim 11 wherein the water heating system provides water for central heating or sanitation purposes.
13. An apparatus substantially as described herein with reference to the accompanying drawings.
14. A method of heating a liquid comprising the use of an apparatus as defined in any one of the preceding claims.