METHOD FOR THE PRODUCTION OF ACRYLIC ACID FROM PROPANE. IN THE ABSENCE OF MOLECULAR OXYGEN The present invention relates to the production of acrylic acid from propane in the absence of molecular oxygen. It is known from European patent application No. EP-A-608838 how to prepare an unsaturated carboxylic acid from an alkane according to a catalytic oxidation reaction in vapour phase in the presence of a catalyst containing a mixed metal oxide comprising as essential components, Mo, V, Te, O, as well as at least one element chosen from the group constituted by niobium, tantalum, tungsten, titanium, aluminium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium, these elements being present in very precise proportions. The uses of such a catalyst devoid of silicon described in the examples of this document lead to good acrylic acid selectivities but they are implemented in the presence of air. Moreover, patents such as the American patents No. 4 606 810, 4 966 681, 4 874 503, 4 830 728, 5 198 590 and 6 287 522 use two or more reactors, called "Risers'", however these patents only relate to applications in the refinery of petroleum cuts. The aim of the invention is therefore to provide a method for the production of acrylic acid from propane and in the absence of molecular oxygen, which allows a high conversion of the propane to be obtained while having a high selectivity. The Applicant has discovered that this aim can be achieved by passing a gaseous mixture of propane and water vapour, and if appropriate, of an inert gas, over a particular catalyst, which acts as a redox system and provides the oxygen necessary for the reaction and by using an apparatus having two reaction zones. The advantages of this novel method are the following: - the limitation of the overoxidation of the products formed which takes place in the presence of molecular oxygen; according to the present invention, due the fact of operating in the absence of molecular oxygen, the formation of COX (carbon monoxide and carbon dioxide), degradation products, is reduced, which allows the acrylic acid selectivity to be increased; the acrylic acid selectivity is maintained at a good level; the conversion is increased without loss of selectivity; the catalyst does not undergo a low reduction and therefore a progressive loss of its activity; it can easily be regenerated by heating in the presence of oxygen or of a gas containing oxygen after a certain period of use; after regeneration, the catalyst regains its initial activity and can be used in another reaction cycle; moreover, the separation of the stages of reduction of the catalyst and of regeneration of the latter allows the partial pressure of propane to be increased, such a partial supply pressure of propane no longer being limited by the existence of an explosive zone created by the propane + oxygen mixture. The subject of the present invention is therefore a method for the production of acrylic acid from propane, in which: a) a gaseous mixture devoid of molecular oxygen and comprising propane, water vapour, as well as, if appropriate, an inert gas, is introduced into a first reactor with a moving catalyst bed, b) at the outlet of the first reactor, the gases are separated from the catalyst, c) the catalyst is returned into a regenerator, d) the gases are introduced into a second reactor with a moving catalyst bed, e) at the outlet of the second reactor, the gases are separated from the catalyst and the acrylic acid contained in the separated gases is recovered, f) the catalyst is returned into the regenerator, g) the regenerated catalyst from the regenerator is reintroduced into the first and second reactors, and in which the catalyst comprises molybdenum, vanadium, tellurium or antimony, oxygen and at least one other element X chosen from niobium, tantalum, tungsten, titanium, aluminium, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium. This method allows an acrylic acid selectivity to be obtained of close to 60% and a high conversion of the propane. Other characteristics and advantages of the invention will now be described in detail in the following description which is given with reference to the single attached figure, which diagrammatically represents an apparatus which is suitable for the implementation of the method according to the invention. DETAILED DESCRIPTION OF THE INVENTION The operation of the method according to the invention can be explained with reference to the attached figure. The gaseous mixture comprising propane, water vapour, as well as, if appropriate, an inert gas, is introduced into a first reactor (Riser 1) containing the moving catalyst bed. Then, at the outlet of the first reactor, the effluents are separated into gases and the moving bed catalyst. The catalyst is sent into a regenerator. The gases are introduced into a second reactor (Riser 2) also containing a moving catalyst bed. At the outlet of the second reactor, the effluents are separated into gases and the catalyst. The catalyst is sent into a regenerator. The gases are treated in a known way, generally by absorption and purification, with a view to recovering the acrylic acid produced. The regenerated catalyst is reintroduced into the first reactor as well as into the second reactor. The method thus operates continuously, the circulation of the catalyst between the reactors and the regenerator is carried out in a regular and generally continuous way. Of course, the single regenerator can be replaced by two or more regenerators. Moreover, it is possible to add, after the second reactor, other reactors which also have a catalyst circulating between each of these reactors and the regenerator or other regenerators. Preferably, the first and second reactors are vertical and the catalyst is transported upwards by the gas flow. As regards the conversion of propane to acrylic acid using the catalyst, it is carried out according to the following redox reaction (1): SOLIDoxidized + PROPANE ? SOLIIDreduced + ACRYLIC ACID (1) Generally, this redox reaction (1) is carried out at a temperature of 200 to 500 C, preferably 250 to 450 C, even more preferably, 350 to 400 C. The pressure in the reactors is generally from l.OlxlO4 to l.OlxlO6 Pa (0.1 to 10 atmospheres), preferably from 5.05xl04 to 5.05x105 Pa (0.5-5 atmospheres). The residence time in each reactor is generally from 0.01 to 90 seconds, preferably, from 0.1 to 30 seconds. The propane/water vapour volume ratio in the gas phase is not critical and can vary within wide limits. Similarly, the proportion of inert gas, which can be helium, krypton, a mixture of these two gases, or nitrogen, carbon dioxide, etc., is also not critical and can also vary within wide limits. As regards the order of magnitude of the proportions of the initial mixture, the following ratio can be mentioned (in volumes): propane/inert (He-Kr)/H20 (vapour): 10-30/40-50/40-50 As regards the catalyst, the proportions of its constituent elements can meet the following conditions: 0.25