Patent Application
20050000825
This invention relates to an improvement of a process (IS process) for hydrogen production by the intense thermal energy from thermochemical water splitting using iodine and sulfur dioxide. The steps of reacting the two acids obtained in the process (i.e. hydroiodic acid and sulfuric acid), separating and concentrating them are combined in a membrane reactor such that sulfuric acid and hydrogen iodide which are obtained as the desired intermediate products are concentrated to reduce the amounts of recycling iodine and water.
The invention particularly relates to a technique by which the aqueous solutions of hydrogen iodide and sulfuric acid that are obtained in high concentrations in the method can be effectively separated using electrode portions and a cation exchange membrane.
While a lot of methods have been proposed for producing hydrogen by using intense thermal energy and water rather than fossil fuels, most have been abandoned for various reasons including low reactivity and the only strategies under current review are the UT-3 cycle and the IS process. The IS process is characterized by an improvement of that part of the Norman et al. process (see Gas Research Institute, GRI-80/0105 (1981)) in which an aqueous solution of hydrogen iodide is separated to produce hydrogen.
In Gas Research Institute, GRI-80/0105 (1981), Norman et al. presented an elementary technique for obtaining hydrogen by thermochemical water splitting using iodine and sulfur dioxide. By the IS process, hydrogen could actually be obtained as reported in Kagaku Kogaku Ronbunshu, vol. 24, no. 2, p. 352-355 (1998) [in Japanese].
An outline of the IS process is shown in
As Gas Research Institute, GRI-80/0105 (1981) and Kagaku Kogaku Ronbunshu, vol. 24, no. 2, p. 352-355 (1998) [in Japanese] teach, in the Bunsen reaction in the conventional IS process, water and iodine are added in excess amounts to enhance the reaction while, at the same time, liquid-liquid separation is effected to obtain two phases, one being an aqueous solution of sulfuric acid and the other being an aqueous solution of hydrogen iodide which also contains iodine.
The separated aqueous solution of hydrogen iodide which also contains iodine is heated and distilled to make the vapor of hydrogen iodide (2HI), which is decomposed and heated to yield hydrogen (H2).
The Bunsen reaction is usually carried out with a batch reactor but in Denki Kagaku, vol. 45, no. 3, p. 139-143 (1977) [in Japanese], the use of a cation exchange membrane is proposed for reducing the amounts of the recycling chemical substances.
For efficient operation of the IS process, it is required to reduce the amounts of the recycling chemical substances. In the scheme shown in
However, no attempts have been made to ensure that the aqueous solution containing sulfuric acid as a main component and the one containing hydrogen iodide as a main component are concentrated to higher concentrations than the values listed in Gas Research Institute, GRI-80/0105 (1981) which are respectively (H2SO4+4H2O) and (2HI+8I2+11H2O). The composition shown in Gas Research Institute, GRI-80/0105 (1981) for the aqueous solution containing hydrogen iodide as a main component is approximately equal to the pseudo-azeotrpic composition of hydrogen iodide.
The primary objective of the present invention is to ensure that sulfuric acid and hydrogen iodide as the two acids obtained after the Bunsen reaction have the following respective compositions, H2SO4+aH2O (a<4) and 2HI+bI2+cH2O (b<8 and c<11).
As an alternative to the use of a batch reactor which is a common tool for carrying out the Bunsen reaction, Denki Kagaku, vol. 45, no. 3, p. 139-143 (1977) [in Japanese] proposes the use of a cation exchange membrane. The reactor for implementing this proposal is shown schematically in
In the Bunsen reaction depending upon the liquid-liquid separation between the aqueous solution of sulfuric acid and the aqueous solution of hydrogen iodide which also contains iodine, it is known that if an attempt is made to ensure that the second aqueous solution is obtained separately at the highest possible concentration, the result is an aqueous solution of hydrogen iodide plus iodine which, as shown in
An object, therefore, of the present invention is to provide a process for hydrogen production from sulfur dioxide, iodine and water using unit operations for the Bunsen reaction and separation of the products, by which process an aqueous solution of sulfuric acid and an aqueous solution of hydrogen iodide can be obtained in separated form, the first aqueous solution having high concentrations of sulfuric acid that have been unable to get by liquid-liquid separation and the second aqueous solution having high concentrations of hydrogen iodide in excess of the pseudo-azeotropic composition.
The present invention employs the operating principle proposed in Denki Kagaku, vol. 45, no. 3, p. 139-143 (1977) [in Japanese] and the sulfur-involving reaction and the iodine-involving reaction are performed on opposite sides of a cation exchange membrane, thus reducing the amounts of iodine and water that are conventionally added in excess amounts in the Bunsen reaction.
Briefly, in the present invention, sulfur dioxide (SO2) and water (H2O) are reacted on the positive electrode side of the membrane to make sulfuric acid (H2SO4) whereas on the negative electrode side, iodide ions (2I−) are obtained from iodine (I2), thus eliminating the need for a separating operation; this and other features contribute to reducing the amounts of iodine (I2) and water (H2O) that have heretofore been added in excess amounts and, as a result, the amounts of recyclables are reduced to enable efficient overall operations.
The invention is also characterized in that the interior of the reactor for the Bunsen reaction is partitioned by a cation exchange membrane into a positive electrode portion and a negative electrode portion, with power applied between the two electrode portions such that the reaction of (SO2+2H2O=2H++H2SO4+2e−) proceeds at the positive electrode and the reaction of (I2+2e−=2I−) at the negative electrode, whereby a net chemical change which is the same as the already described Bunsen reaction (2H2O+SO2+I2=2HI+H2SO4) proceeds through the electrode reactions.
This chemical change is due to the permselectivity of the cation exchange membrane and the protons that are evolved at the positive electrode pass through the cation exchange membrane to move to the negative electrode where they combine with iodide ions to make hydrogen iodide. Since hydrogen iodide is generated at the negative electrode, the aqueous solution of hydrogen iodide which also contains iodine is concentrated to have a higher hydrogen iodide content. Since the separation method does not depend upon phase equilibrium, hydrogen iodide can be concentrated without the constraints of pseudo-azeotropism which is a phenomenon unique to phase equilibrium. Similarly, the aqueous solution of sulfuric acid can be concentrated at the positive electrode.
For efficient hydrogen production by the IS process, sulfuric acid and hydrogen iodide must be obtained in high concentrations in the process of reaction starting with sulfur dioxide, iodine and water. In the present invention, the reaction between sulfur dioxide and water is performed on the positive electrode side of the cation exchange membrane, separately from the iodine-involving reaction which is effected on the negative electrode side. As a result, the subsequent separating operations are eliminated, contributing to reducing the iodine and water inputs.
As another advantage, iodine need not necessarily be added in excess amount as in liquid-liquid separation and yet the aqueous solutions of hydrogen iodide and sulfuric acid can be obtained in separate form and it is also possible to ensure that the first aqueous solution has high concentrations of hydrogen iodide that exceed the pseudo-azeotropic composition. As a result, there can be provided a method of hydrogen production that performs the Bunsen reaction using less heat in the subsequent process than is conventionally required to separate the pure, water-free form of hydrogen iodide from the first aqueous solution by distillation. In liquid-liquid separation, the two aqueous solutions can be obtained in separate form but neither can be concentrated. According to the invention, both solutions can be concentrated, so it is possible to reduce the amount of equipment that is required to perform the subsequent essential step of concentrating aqueous solutions.
An embodiment of the invention is described below with reference to
First, an aqueous solution having sulfur dioxide dissolved in water or an aqueous solution having sulfur dioxide dissolved in an aqueous solution of sulfuric acid is fed into the spacer 3 on the positive electrode side. At the positive electrode 1, the reaction of SO2+2H2O=H2SO4+2H++2e−proceeds to produce sulfuric acid 13. Protons permeate the cation exchange membrane 5 to move to the negative electrode 9 whereas electrons pass through the external circuit to migrate to the negative electrode 9. At the negative electrode 9, the reaction of I2+2e−=2I− proceeds. The protons permeating the cation exchange membrane 5 combine with the iodide ions to make hydrogen iodide 18 as the desired intermediate product.
Reference is now made to
At a temperature of 290 K, the aqueous solution containing each of HI and iodine at a concentration of 3.1 mol/Kg-H2O (for easy comparison on
In the next place, reaction was carried out under the passage of current at a temperature of 290 K, with the compositions of the aqueous solution of sulfuric acid and the aqueous solution of hydrogen iodide plus iodine being adjusted to the highest levels that could be achieved by the conventional liquid-liquid separation method shown in
The method of the invention was implemented using Nafion of Du Pont as a cation exchange membrane. A carbon electrode was used as both the positive and negative electrodes. Kalrez (Du Pont) was used as a sealant. Two flasks each having a capacity of 200 ml were provided and one of them was supplied with an aqueous solution containing sulfuric acid at a specified concentration under bubbling of sulfur dioxide gas. The other flask was supplied with an aqueous solution containing hydrogen iodide and iodine at specified concentrations. Using rotary pumps, the two aqueous solutions were flowed to the positive and negative electrodes. A constant current was flowed for a specified period to get the reaction to proceed. The reaction temperature was controlled by heating the system with an external heater and measuring the temperature of the reaction solution at the exit. The concentrations of the respective solutions were measured by titration.
First, a review was made of the decrease in iodine content which is the characterizing part of claim 1. The experiment was conducted at a temperature of 290 K, with the HI content being fixed at 3.1 mol kg-H2O−1. The iodine content was varied between 0.2 and 3.1 mol kg-H2O−1. For easy comparison on
The concentration of sulfuric acid was fixed at 3.7 mol kg-H2O−1. With the iodine content held at 3.1 mol kg-H2O−1, reaction was performed at 0.3 A for 4 hours, whereupon the concentration of hydrogen iodide rose to 3.8 mol kg-H2O−1 and the concentration of sulfuric acid to 4.5 mol kg-H2O−1. It was therefore demonstrated that even under the low iodine condition proposed by the invention, hydrogen iodide and sulfuric acid could be concentrated as the desired intermediate products. The selected concentrations are also reasonable in view of power consumption.
Next, a review was made of the enhancements of the concentrations of hydrogen iodide and sulfuric acid which are the characterizing portions of claims 2 and 3, respectively. The initial concentration of an aqueous solution of sulfuric acid was (H2SO4+4H2O) which is typical in the conventional liquid-liquid separation process, whereas the initial concentration of an aqueous solution of hydrogen iodide was (2HI+2I2+11H2O) which, in terms of the iodine content, was a quarter of the value indicated in
Based on the results of the example, the inventors assessed how much of the cost for the production of hydrogen as the final product would be affected by the decrease in the amount of recycling iodine. The two solutions of interest that needed to be separated had the respective compositions of (H2SO4+4H2O) and (2HI+8I2+11H2O). The heat source for decomposing sulfuric acid had a power of 200 MW to give a temperature of 1123 K and hydrogen was produced in an amount of 23900 Nm3h−1. By calculating the Heat/Mass balance of the overall process, the sizes of various apparatuses were approximated to assess the overall equipment cost. For vaporizing sulfuric acid, four-stage distillation columns of the type described in Proc. 5th World Hydrogen Energy Conf., p. 487-502 (1984), Pargamon Press were employed. For concentrating hydrogen iodide, an electric dialyzer of the type described in J. Membr. Sci., vol. 192, p. 193-199 (2001), Elsevier was employed, and for decomposing hydrogen iodide, a hydrogen permselective membrane reactor of the type described in J. Membr. Sci., vol. 162, nos. 1-2, p. 83-90 (1999), Elsevier was employed.
The cost for hydrogen production was broken down into the fixed cost, ROI (return on investment), electricity, cooling water, heat and chemicals. The fixed cost and ROI were assumed to account for 25% and 8%, respectively, of the construction cost. The prices of heat and electricity were assumed to be 2.29×10−3 yen per kcal and 9.0 yen per kWh, respectively.
By this method, the price of hydrogen was first computed, then the effectiveness of the present invention was assessed. In the example, the amount of recycling iodine was reduced to less than 2% of the previously reported values, so the amount of recycling iodine was ignored in assessing the sizes of various apparatuses. The results are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-271049 | Jul 2003 | JP | national |
