MXPA95001398A - An improved procedure for the purification deacidos metacrili - Google Patents

Abstract

The present invention relates to a process for the preparation of a C3-C6 alpha, α-unsaturated carboxylic acid with greater than 98% purity, which combines the procedures of fractional distillation and crystallization of the melt.

Description

AN IMPROVED PROCEDURE FOR PURIFICATION METACRÍLICOS ACIDS BACKGROUND OF THE INVENTION A process for producing methacrylic acid is the catalytic carbonylation of propylene, to produce isobutyric acid, followed by partial oxidative dehydrogenation to supply methacrylic acid. The crude product of this process is a mixture containing water, isobutyric acid, methacrylic acid and other components. The water is removed using azeotropic distillation or solvent extraction, followed by distillation to remove the extraction solvent and produce an anhydrous mixture. Methacrylic acid is traditionally separated from the anhydrous mixture, using distillation or crystallization techniques. In separations involving distillation, the physical and chemical similarities of methacrylic and isobutyric acids typically require a large number of theoretical plates in combination with high reflux ratios to achieve a high purity product. Also, the use of distillation as a separation technique is complicated by the tendency of methacrylic acid to polymerize while distilling.

When crystallization techniques are employed in the separation, the combination of the low melting point for isobutyric acid and the low eutectic temperature for mixtures of isobutyric / methacrylic acids require the use of very low crystallization temperatures. These temperatures make crystallization procedures difficult for your economic practice. Japanese patent Ko ai 62-145044A discloses a distillation method for obtaining purified methacrylic acid by the use of at least four separate distillation towers and an extraction column for removing and purifying an extraction solvent and removing point impurities. boiling both high and low. Japanese Patent Kokai 52-007917A discloses a process for separating methacrylic acid from the crude aqueous product from the gas phase dehydrogenation of isobutyric acid, in which the crude product is first distilled to remove materials with lower boiling points than water, and then extracted with a hydrocarbon solvent to remove methacrylic acid and related materials. The solution of the hydrocarbon solvent, methacrylic acid and related materials is distilled in stages, first to remove the residual water and then to remove the hydrocarbon solvent. This process results in a composition of the methacrylic acid product, which contains approximately 97% by weight of the methacrylic acid. U.S. Patent No. 4,780,568 describes the purification of an anhydrous mixture of methacrylic acid, isobutyric acid and other impurities, using a crystallization separation unit in stages, with 3 to 6 equivalent stages and a secondary recovery section, with a or more steps to supply high purity methacrylic acid. This procedure requires very low crystallization temperatures. Russian patent No. 639,858 describes the purification of acrylic and methacrylic acids using a crystallization process with countercurrent flow, which involves low temperatures. N. ynn in Chemical Engineering Progress, 88 (3). 52-60 (1992) describes the use of distillation and crystallization together, in order to improve the purity of the product. The distillation is used to remove the volume of impurities followed by the crystallization of the melt to obtain a final product of high purity. Because these known methods are often expensive, difficult to control and / or provide a less pure product than desired, there remains a need for improved methods for separating, in an efficient and cost-effective manner, the methacrylic acid of the mixtures containing isobutyric acid.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow diagram of the method of one embodiment of the invention. Figure 2 is a flow chart of the process for a laboratory crystallizer of a melt. Figure 3 is a flow diagram of the method for an alternative embodiment of the invention. COMPENDIUM OF THE INVENTION The present invention relates to a process for the separation of C3-C6 carboxylic acids from, ß-unsaturated mixtures of these C3-C6 a, ß-unsaturated carboxylic acids and their saturated homologs, which comprises combining the processes of fractional distillation and crystallization of the melt, in which a high proportion of the material is recycled. By "high proportion" it is meant that the amount of recycled material exceeds that removed from the process as a product by a factor of at least 1.5 times. Fractional distillation results in two process streams; a higher process stream, in which the ratio of the carboxylic acid to, ß-unsaturated to saturated is about 0.1 by weight, and a stream of the bottoms process, in which the ratio is approximately 15. The current of the bottoms is fed to a melt crystallizer, and results in two additional streams; a product stream of high purity (greater than 98% by weight) of α, β-unsaturated carboxylic acids and a stream of the residue, which is about 80% by weight of α, β-unsaturated carboxylic acids and 20% by weight of saturated carboxylic acids. The top stream of the fractional distillation column is recycled back to the reactor and the stream of the crystallizer residue is recycled back to the fractionation column. Alternatively, the process steps can be reversed, so that crystallization of the melt is employed first to isolate the pure α, β-unsaturated carboxylic acid. The bottom stream of the melt crystallizer is then fractionally distilled to again supply the crystallizer with a stream rich in saturated carboxylic acids, for recirculation to the reactor, and a recycle stream rich in carboxylic acids a, b- unsaturated DETAILED DESCRIPTION OF THE INVENTION One embodiment of the present invention relates to a process for the preparation of C3-C6 a, β-unsaturated carboxylic acids with a purity greater than 98%, this process comprises: a) oxidatively dehydrogenating the saturated carboxylic acids C3-C6, in a reactor, to produce a first product stream, which comprises the α, β-unsaturated carboxylic acids and the saturated carboxylic acids, in which the weight ratio of these α, β-unsaturated carboxylic acids to the saturated is greater than about 0.2; b) fractionally distill the first product stream n a distillation unit, to produce: (1) a higher stream, in which the ratio of the α, β-unsaturated carboxylic acids to the saturates is lower than in the first product stream , preferably less than 0.5, and more preferably less than 0.1, and (2) a bottom stream, in which the weight ratio of the α, β-unsaturated acids to the saturated ones is from about 5 to 200, preferably greater than around 9; c) feeding the upper current into the reactor; d) crystallizing the melt of the bottom stream in a crystallization unit, to produce: (1) a second stream of product, comprising the α, β-unsaturated carboxylic acids, wherein the content of these carboxylic acids a, β-unsaturated is greater than about 90%, preferably 98%, by weight of the second product stream and (2) a mother liquor stream; e) feeding the mother liquor stream into the distillation unit; f) crystallizing the melt of the second product stream in a number of steps, sufficient to obtain a stream of the final product, with the desired content of α, β-unsaturated carboxylic acids, preferably greater than 99.9% by weight; and g) feeding the final mother liquor, coming from the repeated stages of the crystallization of the melt of step f), inside the distillation unit. This method is useful for separating mixtures of α, β-unsaturated and saturated carboxylic acids, including, but not limited to, the methacrylic acid of isobutyric acid, the acrylic acid of propionic acid, the crotonic acid of butanoic acid, and the like . The process is particularly applicable to separations of methacrylic acid from isobutyric acid and acrylic acid from propionic acid, due to its similarity in physical / chemical properties, such as melting points and boiling points, which makes it difficult to obtain efficient separations with the use of standard distillation and crystallization techniques. One embodiment of the present invention, exemplified by the separation of methacrylic acid from iso-butyric acid, is illustrated in Figure 1. The process "of Figure 1 is preferred when the ratio of the α, β-unsaturated carboxylic acids to the saturates, in the first stream of the product, from the dehydrogenation catalytic reactor 1, is less than about 15. The first product stream from the catalytic dehydrogenation reactor can contain up to 40% water.This current can be fed through line 2, directly Within the distillation unit 3 or the intermediate stages, such as filtration, extraction, azeotropic distillation or dehydration, can be performed before feeding into the unit.The configuration of the distillation unit 3 is not critical. a high efficiency fractionation column, More preferred is a fractionation column which supplies 25 to 70 theoretical plates with a reflux ratio of 29 to 355. The operation parameters are also not critical. However, in order to reduce the distillation temperature to a minimum, low pressures are preferred, preferably below 100 mm of mercury. The stream can be fed into the fractionation column at any convenient point. However, for the most efficient operation, the profile of the composition of the column, in a stable state, must be determined and the feed preferably must enter at that point, where the feed composition and the steady state column they are similar. The distillation results in a higher stream enriched in isobutyric acid. The rest of the material in the top stream is methacrylic acid and low boiling materials, which are present in the first product stream. The top stream is recycled back into the catalytic dehydrogenation reactor through line 4, recovering what ordinarily would be the discarded isobutyric acid, which results in an increase in overall performance and lower costs. Another advantage of this method is that neither the catalytic dehydrogenation reaction nor the fractionation need to be operated under conditions that would result in maximizing the amount of the methacrylic acid and minimizing the amount of the isobutyric acid produced. The present invention is effective in reducing losses of isobutyric acid in the reactor, due to secondary reactions and losses of methacrylic acid in the fractionation column, due to polymerization, which again results in higher overall efficiency and costs minor, because there is less waste and, therefore, lower operating costs. This is due to the recycling of these materials back to the reactor. The bottom stream of the distillation unit, which contains at least 90%, preferably at least 95%, by weight of methacrylic acid, up to 10% by weight of isobutyric acid, and minor amounts of high point impurities of boiling, it is fed through line 5 into crystallizer 6 of melts. Although an intermittent type crystallizer may be used, a multi-stage semicontinuous crystallizer, such as that shown schematically in Figure 2, whose operation is described below, or a similar technology, is preferred. Distillations can also be conducted intermittently. However, continuous operation is preferred; the continuous operation, in steady state, is the most preferred. In those cases, where the distillation unit is continuously operated, an element must be provided to accept the flow of the distillation unit bottoms, when this crystallization unit is operating in an intermittent or semi-continuous manner. This can be achieved by the use of a holding tank in line 5, a second crystallization unit or similar elements. This holding tank or similar element can be incorporated into the mother liquor stream from the crystallization unit, which feeds the distillation unit through lines 10 and 11, in order to supply a continuous supply to this unit of distillation. In some cases, the vapor pressure of the α, β-unsaturated carboxylic acid may be higher than that of the saturated carboxylic acid. In such cases, the distillate stream from the distillation unit is fed to the crystallization unit, while the bottom stream is recycled back into the catalytic dehydrogenation reactor. The separation of acrylic acid from propionic acid (see Example 10) is an example of the case. Figure 2 is a flow diagram of a typical melt crystallizer unit 6, from Figures 1 or 2, which can be used with the present invention. The bottom stream of the fractionation column is fed through line 21 (equivalent to line 5 in Figure 1 and lines 32 or 39 in Figure 3, described below) into collection tank 22 of the the melt, where a circulation pump 23 circulates the current through line 24 to a crystallization tube 25 again inside the collection tank. Under the typical crystallization conditions, approximately 50 to 70% by weight, preferably 60 to 65% by weight, of the stream crystallizes in the walls of the tube. The temperatures of the walls of the tube are controlled by the circulation of the hot or cold heat transfer fluid 26, through a cover 27, around the crystallization tube. The desired crystallization temperature will vary with the composition of the stream. For example, a mixture of 0.5% by weight of isobutyric acid in methacrylic acid may require a temperature of 14-152 for crystallization, while a mixture of 6% may require a temperature of 82C in order to crystallize the 60% n weight of the mixture in a period of 1 to 1.5 hours. After the charge has crystallized, the mother liquor is removed from the system through line 28 (equivalent to line 10 in Figure 1 and line 34 in Figure 3) and fed to the fractionation column. Approximately 5 to 50% by weight, preferably 10 to 15% by weight, of the crystallized material is then remelted in a period of 1 to 2 hours, increasing the temperature of the heat transfer fluid. The melted material (second mother liquor stream) is removed from the system and may also be fed to the fractionating column separately or combined with the mother liquor. Finally, the crystalline product is recovered, preferably by melting and collecting in the collection tank for a second stage recrystallization or it is removed from the system as a product, through line 28 (equivalent to line 7 in Figure 1 and the line 33 in Figure 3).

With the conditions described above, crystallizer 6 produces two streams. The first stream is the final product stream, which contains more than 98% by weight of pure methacrylic acid, which is recovered through line 7 (in Figure 1), varying the conditions of crystallization, for example the recrystallization and remelting temperatures and the charge rate, as well as the number of recrystallization steps, a product with more than 99.99% by weight of methacrylic acid can be obtained. The second stream is the final mother liquor stream, which contains more than 80% by weight of the methacrylic acid and up to 20% by weight of the isobutyric acid, as well as other minor amounts of high boiling point impurities. The mother liquor stream can be combined with the first product stream through line 10 and fed back into the fractionation column as a combined charge through line 11. Alternatively, the mother liquor stream and the The final product stream can be fed separately to the fractionation column. Occasionally, the mother liquor stream may require purging through line 9, in order to remove heavy impurities. A major advantage in the present process is that the melt crystallizer does not have to be operated under the conditions designed to remove the maximum amount of methacrylic acid from the bottom stream of the column, because since the current is recycled As the mother liquor stream, any methacrylic acid is recovered. In those cases when the ratio of the α, β-unsaturated carboxylic acids to those saturated in the first product stream is greater than about 15, the alternative procedure, as schematically shown in Figure 3, can be used. Again, with the use of methacrylic acid as an example, in this embodiment, the first product stream is fed through line 32 to crystallizer 6 of the melt, to give a final product stream through the line 33 of essentially pure methacrylic acid (more than 98% by weight of methacrylic acid, preferably more than 99.99% by weight of methacrylic acid). The mother liquor of the crystallizer, which contains at least 80% by weight of the methacrylic acid, is fed through line 34 into fractionation column 3. This fractionation column provides an upper stream rich in isobutyric acid, which is fed to reactor 1 through line 35, and a bottom stream of a mixture of about 95% by weight of methacrylic acid and less than 5% by weight of isobutyric acid, which is combined with the first product stream through lines 36 and 37 and fed back into the melt crystallizer, as a combined charge or fed directly to the crystallizer of the melt. As with the procedure of Figure 1, heavy impurities can be removed through lines 36 and 38. In both variants of the process, neither the fractionation column nor the melt crystallizer are operated separately in a manner which would result in maximum separation of methacrylic acid from the product stream in each unit. Rather, each is operated in a novel and limited manner, so that the combination of fractional distillation and crystallization of the melt, with two recycle streams, in which the volume of the material is recycled, supplies the carboxylic acids. -eos, ß-unsaturated high purity (more than 99.99% by weight), produces very little waste material and operates at reasonable temperatures and pressures. This results in an inexpensive, low cost procedure. The following examples are provided to illustrate the invention, but not to limit its scope. Unless otherwise specified, all percentages in the examples are expressed as one percent by weight.

EXAMPLES The crystallizations of the melts were performed using a laboratory crystallizer of the melt, as described in Figure 2. The material equilibria for the fractional distillation stage are obtained from computerized steady-state simulations, which they use the FLOWTRAN® program (Monsanto Company) to provide simulations of the distillation processes. This program is described in: J. D. Seader, W. D. Seider and A. C. Pauls, FLOWTRAN Simulation -An Introduction. 2nd Edition, Cambridge: CACHE, 1977. In the simulation, an ideal solution is assumed for the coefficients of the liquid activity. The Chao-Seadel correlation is assumed for the fugacity of the liquid and an ideal gas is assumed for the vapor fugacity. In addition, liquid and vapor fugacities are corrected for the association of the organic acid in the vapor phase by the method described by E. Sebastiani and L. Lacquaniti, Chem. Eng. Sci. , 22., 1155 (1967).

Example 1 - Purification by Crystallisation of the Melt Mass of Methacrylic Acid, containing Approximately 0.5% Isobutyric Acid. A solution, containing 0.49% isobutyric acid ("iBuA") in glacial methacrylic acid, was divided into three parts. The first portion was fed to the laboratory crystallizer of the melt over a period of about 1 hour, during which time the temperature decreased from 14.8 to 13.80C for a crystallization of the first stage. Approximately 60% of the load crystallized. The temperature was then increased to a range of 14 to 162 C over a period of 1.5 hours, causing 10% of the crystallized material to melt again. The mother liquor, re-melted material and crystallized material were collected and analyzed separately in their contents in the iBuA. The second and third portions were treated in the same way. Representative results for the crystallizations of the first stage are given in Table 1. The purity of the methacrylic acid ("MAA") and the efficiency of the crystallization were determined by monitoring the content of the iBuA in the crystallized material. Table 1 Crystallization of the First Stage a = Wt (g) = weight in grams b = iBuA = isobutyric acid * Based on the content of iBuA, the crystals had approximately 99.93% pure MAA. The crystallized portions from the crystallizations of the first stage were combined and about one third of them were fed to the laboratory crystallizer of the melt and in a period of 50 minutes the temperature decreased from 17.02C to 16.12C for a crystallization of the second stage. Approximately 64% of the charge crystallized. The temperature then remained at 15.72C, causing 10% of the crystallized material to melt again. Again, the mother liquor, the remelted mass and the crystals were collected and analyzed separately in their iBuA content. The representative results for the crystallizations of the second stage are given in Table 2. Table 2 Crystallization of the Second Stage a = t (g) = weight in grams b = iBuA = isobutyric acid * Based on the content of the iBuA, the crystals are at least 99.99% pure of MAA.

Example 2 - Purification by crystallization of the Melt Mass of Methacrylic Acid Containing Approximately 5% of Isobutyric Acid. Using the procedure of Example 1, a solution, containing approximately 5.8% of iBuA in MAA acid, was fed into a laboratory crystallizer of the melt and in a period of about 80 minutes, the temperature decreased from 13.52C. at 82C. Approximately 62% of the charge crystallized. The temperature was increased to 14 seconds by approximately 4.7 hours, causing 10% of the crystallized material to melt again. The mother liquor, remelted mass and crystallized material were collected and analyzed separately in the isobutyric acid content. Representative results of these analyzes are given in Table 3. Table 3 a = t (g) = weight in grams b = iBuA = isobutyric acid * Based on the content of iBuA, the crystals are at least 99.05% pure MAA.

Example 3 - Purification by Crystallization of the Acidic Acid Melt Mass Which Contains Approximately 0.05% Propionic Acid. Using the procedure of Example 1, a solution containing approximately 0.05% propionic acid ("PA") in acrylic acid ("AA") was divided into four fractions. Each fraction was fed into a melt laboratory crystallizer in a period of 19 to 68 minutes at a temperature of about 15%. About 42% of the charge crystallized. Seven to twenty-seven percent of the crystallized material was remelted in a period of 45 to 232 minutes. The mother liquor, remelted mass and resultant glass fractions were combined and analyzed separately in the PA content. Representative results of these crystallizations are given in Table 4. Table 4 Wt (g) = weight in grams b = iBuA = isobutyric acid * Based on the PA content, the crystals are at least 99.98 pure AA. For the following Examples 4 to 10, and Comparative Example 2, the data from the distillation column was simulated with a steady state process simulator (FLOWTRAN), as described above. The input specifications of the simulator for each current are specified in the examples. The crystallization data of the melt (mother liquor, remelted mass and amount of crystals and purities) were extrapolated using the results presented in Examples 1 to 3 above.

Comparative Example 1 - A fresh filler, containing 80% MAA and 20% iBuA, was fed to a melt crystallizer. The final product specification was adjusted to 0.005% of the iBuA. To meet this specification, 4.6 calculated crystallization steps are required. However, the final mother liquor will contain 70.59% MAA, which was not recycled.

Example 4 - A fresh charge, containing 80% (MAA) and 20% of iBuA, was mixed with the mother liquor stream of the melt crystallizer. The combined stream was fed to the fractionation column. The distillate specification was adjusted to 90% of iBuA, the cash flow specification was adjusted to 0.5% of iBuA and the final product specification was adjusted to 0.005% of the iBuA. To meet these specifications, the required number of stages (theoretical plates) in the fractionation column can be 45 (with a reflux ratio of 234) to 55 (with a reflux ratio of 101). assuming a pressure higher than 30 mm of mercury. The required number of crystallization stages is 2.6. Table 5 details the equilibrium of the material calculated for any of these conditions.

Table 5 Material Balance of Example 4 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 5 - The specifications in Example 5 are identical to those in Example 4, except that the specification for the iBuA in the stream of funds was increased to 5.0%. To meet these specifications, the required number of stages in the fractionation column can be 30 (with a reflux ratio of 194) to 40 (with a reflux ratio of 61), assuming a pressure greater than 30 mm of mercury . The required number of crystallization stages is 3.8. Table 6 details the equilibrium of the material calculated for any of these conditions.

Table 6 Material Balance of Example 5 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 6 - The specifications in Example 6 are identical to those in Example 4, except that the specification for the iBuA in the funds stream increased to 10.0%. To meet these specifications, the required number of stages in the fractionation column is 25 (with a reflux ratio of 197) to 35 (with a reflux ratio of 42), assuming a pressure greater than 30 mm of mercury and The required number of crystallization stages is 4.2. Table 7 details the equilibrium of the material calculated for any of these conditions.

Table 7 Material Balance of Example 6 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 7 - The specifications in Example 7 are identical to those in Example 5, except that the fresh charge was set at 60% MAA and 40% iBuA. To meet these specifications, the number of steps required in the fractionation column can be 30 (with a reflux ratio of 136) to 40 (with a reflux ratio of 40), assuming a pressure greater than 30 mm of mercury . The required number of crystallization stages is 3.8. Table 8 details the equilibrium of the material calculated for any of these conditions.

Table 8 Material Balance of Example 7 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 8 - The specifications in Example 8 are identical to those in Example 7, except that the specification of the iBuA in the fresh charge was decreased to 10.00%. To meet these specifications, the number of stages required in the fractionation column can be 30 (with a reflux ratio of 232) to 40 (with a reflux ratio of 79), assuming a pressure greater than 30 mm of mercury . The required number of crystallization stages is 3.8. Table 8 details the equilibrium of the material calculated for any of these conditions.

Table 9 Equilibrium of the Material of Example 8 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 9 - The specifications in Example 9 are identical to those in Example 7, except that the specification for the iBuA in the fresh charge was decreased to 5.0% and that the specification of the stream of funds for the iBuA was decreased to 0.5 %. Also, the alternative procedure of Figure 3 was used. To meet these specifications, the number of steps required in the fractionation column can be 45 (with a reflux ratio of 355) to 55 (with a reflux ratio of 157), assuming a pressure higher than 30 mm of mercury. The required number of crystallization stages is 3.3. Table 10 details the equilibrium of the calculated material for any of these conditions.

Table 10 Material Balance of Example 9 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph = kilograms per hour.

Example 10 - A fresh charge, containing 80% AA and 20% PA, was mixed with the mother liquor stream from the melt crystallizer. The combined stream was fed to the fractionation column. The distillate specification was adjusted to 9% PA, the fund flow specification was adjusted to 70% and the final product specification was adjusted to 0.005% PA. To meet these specifications, the number of steps required in the fractionation column is 70, with a reflux ratio of 29, assuming a pressure greater than 1 mm of mercury. The required number of crystallization stages is 10.8. Table 11 details the calculated material balance. In this case, since the AA is more volatile than the PA, the bottoms stream was recycled to the reactor and the distillate was fed to the crystallizer.

Table 11 Material Balance of Example 10 PA (%) 20.00 9.66 10.62 9.00 70.00 0.005 * Reference is made to the flow lines of the procedure in Figure 1. ** kgph ^ kilograms per hour.

Comparative Example 2 - A fresh charge, containing 80% MAA and 20 & of iBuA, a fractionation column was fed. The distillation specification was adjusted to 99% of iBuA and the streamflow specification was adjusted to 0.005% of iBuA. To meet these specifications, the calculated number of stages (theoretical plates) in the fractionation column is 150, with a reflux ratio of 50.6, assuming a pressure higher than 30 mm of mercury. These examples show that in the steady state, the amount of the material that is recycled is high compared to the amount of product produced. However, the product produced is of high purity. Since the recycled material is not lost but, rather, is recovered as any product or material returned to the dehydrogenation reactor, the overall process of unexpectedly efficient combinations.

Claims (28)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as property: CLAIMS 1. A process for preparing C3-C6, a, β-unsaturated carboxylic acids, with more than 98% by weight of purity, comprising the steps of: a) oxidative dehydrogenating a C3-C carboxylic acid, in a reactor, to produce a first product stream, comprising a C3-C6 carboxylic acid a, β-unsaturated and the saturated carboxylic acid, where the ratio of the α, β-unsaturated to saturated carboxylic acid is greater than about 0.2; b) fractionally distill the first product stream in a distillation unit, to produce: (i) a higher stream, in which the ratio of the α, β-unsaturated carboxylic acid to the saturated one is lower than in the first product stream and (ii) a stream of bottoms, where the ratio of the α, β-unsaturated to saturated carboxylic acid is greater than in the first product stream; c) feeding the upper current into the reactor; d) feed the streams of funds within the crystallization unit; e) melt crystallizing the stream of funds in the crystallization unit, one or more times, to produce: (i) a second product stream, comprising more than 98% by weight of a, ß-unsaturated carboxylic acid; and (ii) one or more streams of the mother liquor, comprising more than 80% by weight of the α, β-unsaturated carboxylic acid; and f) feeding the one or more streams of the mother liquor into the distillation unit. The method according to claim 1, wherein: a) the ratio of the α, β-unsaturated carboxylic acid to the saturated one in the upper stream is less than about 0.5; and b) the ratio of the α, β-unsaturated carboxylic acid to the saturated in the bottom stream is greater than about 5. 3. The method according to claim 1, wherein: a) the α, β-unsaturated carboxylic acid ratio when saturated in the upper stream, it is less than about 0.1; and b) the ratio of the α, β-unsaturated carboxylic acid to the saturated in the bottom stream is greater than about 9. 4. The method according to claim 1, wherein the step of crystallizing the melt comprises: ) crystallize from about 50 to 70% by weight of the bottom stream; ii) separate the mother liquor stream; iii) melting approximately 5 to 50% by weight of the crystals, to produce a second stream of mother liquor; iv) separating the second stream of the mother liquor from the remaining crystals; v) optionally, melting the remaining crystals and repeating steps i) to iv), in which the remaining crystals melted are used in place of the bottom stream, a sufficient number of times to obtain more than 98% by weight of the carboxylic acid a, β-unsaturated in the remaining crystals. 5. The process according to claim 1, wherein the saturated carboxylic acid is selected from propionic, butanoic and isobutyric acids. 6. The process according to claim 5, wherein the saturated carboxylic acid is isobutyric acid. The method according to claim 4, wherein the first mother liquor and the second mother liquor are combined before being fed to the fractional distillation column. The method according to claim 1, wherein the first stream of the product is dehydrated before fractional distillation. The method according to claim 1, wherein: the content of the α, β-unsaturated carboxylic acid of the second product stream is greater than about 99.9% by weight. 10. A process for preparing a C3-C6 α, β-unsaturated carboxylic acid with more than about 98 wt% purity, which comprises: a) oxidatively dehydrogenating a saturated C3-Cg carboxylic acid in a reactor, for producing a first product stream, comprising a C3-Cg α, β-unsaturated carboxylic acid and the saturated carboxylic acid, in which the ratio of α, β-unsaturated carboxylic acid to saturated is greater than about 15.; b) feeding the first product stream into a crystallization unit; c) melt crystallizing the first product stream in the crystallization unit, one or more times, to produce: i) a second product stream, comprising more than about 98% by weight of the α, β-unsaturated carboxylic acid and ii) one or more streams of the mother liquor, comprising more than about 80% by weight of the α, β-unsaturated carboxylic acid; d) feeding one or more streams of the mother liquor into a distillation unit; e) fractionally distill the one or more streams of the mother liquor in a distillation unit, to produce: (i) a higher stream and (ii) a stream of bottoms; f) feeding the upper current into the reactor; and g) feeding the bottom stream into the melt crystallizer. 11. The method according to claim 10, wherein the melting crystallization step comprises: i) crystallizing about 50 to 70% by weight of the first stream of the product; ii) separate the mother liquor stream; iii) melting about 5 to 50% by weight of the crystals to produce a second stream of the mother liquor; iv) separating the second streams of the mother liquor from the remaining crystals; v) feeding the mother liquor stream and the second mother liquor stream to the fractional distillation column; vi) optionally, melting the remaining crystals and repeating steps i) to v), in which the remaining melted crystals are used in place of the first product stream, a sufficient number of times to obtain more than 98% by weight of the content of the α, β-unsaturated carboxylic acid in the remaining crystals. The process according to claim 10, wherein the ratio of the α, β-unsaturated carboxylic acid to the saturated acid in the upper stream is less than 0.5. 13. The method according to claim 12, wherein the ratio of the α, β-unsaturated carboxylic acid to the saturated one in the upper stream is less than about 0.1. 14. The process according to claim 10, wherein the saturated carboxylic acid is selected from propionic, butanoic and isobutyric acids. 15. The process according to claim 10, wherein the saturated carboxylic acid is isobutyric acid. 16. The method according to claim 11, wherein the mother liquor stream and the second mother liquor stream are combined before feeding to the fractional distillation column. 17. The method according to claim 10, wherein the first product stream is dehydrated prior to the crystallization of the melt. 18. The method according to claim 10, wherein the content of the α, β-unsaturated carboxylic acid of the second product stream is greater than about 99.9% by weight. 19. The method according to claim 1, wherein elements are incorporated to retain the material in the lines connecting the distillation unit and the crystallization unit. The method according to claim 10, wherein the elements for retaining the material are incorporated into the lines connecting the distillation unit and the crystallization unit. 21. A process for preparing the acrylic acid with a purity greater than 98% by weight, which comprises the steps of: a) dehydrogenating the propionic acid oxidatively in a reactor, to produce a first product stream, comprising the acrylic acid and the propionic acid, in which the ratio of acrylic acid to propionic acid is greater than about 0.2; b) fractionally distill the first stream of the product in a distillation unit, under conditions in which the vapor pressure of acrylic acid is greater than the vapor pressure of propionic acid, to produce: (i) a higher stream, in which the ratio of acrylic acid to propionic acid is greater than that in the first product stream and (ii) a bottom stream, in which the ratio of acrylic acid to propionic acid is lower than that in the first product stream; c) feeding the upper current into a crystallization unit; d) feeding the flow of funds inside the reactor; e) crystallizing the melt of the upper stream in the crystallization unit, one or more times, to produce: (i) a second stream of product, comprising more than about 98% by weight of the acrylic acid; and (ii) one or more streams of the mother liquor, comprising more than about 80% by weight of the acrylic acid; and f) feeding the one or more streams of the mother liquor into the distillation column. 22. The method according to claim 21, wherein the first product stream is dehydrated before fractional distillation. 23. The process according to claim 21, wherein the content of the acrylic acid of the second product stream is greater than about 99.9% by weight. 24. A process for preparing the acrylic acid with a purity greater than about 98% by weight, which comprises the steps of: a) dehydrogenating the propionic acid oxidatively in a reactor to produce a first product stream comprising the acid acrylic and propionic acid, where the ratio of acrylic acid to propionic acid is greater than about 15; b) feeding the first stream of the product into a crystallization unit; c) crystallizing the melt of the first product stream in a crystallization unit, one or more times, to produce: (i) a second stream of the product, comprising more than about 98% by weight of the acrylic acid; and (ii) one or more streams of the mother liquor, comprising more than about 80% by weight of the acrylic acid; d) feeding the one or more streams of the mother liquor into a distillation unit; e) fractionally distill the one or more streams of the mother liquor in a distillation unit, with conditions where the vapor pressure of acrylic acid is greater than the vapor pressure of propionic acid, to produce: (i) a higher stream and ( ii) a stream of funds; f) feeding the flow of funds inside the reactor; and g) feeding the upper stream into the crystallizer of the melt. 25. The method according to claim 24, wherein the first stream of the product is dehydrated prior to the crystallization of the melt. 26. The process according to claim 24, wherein the content of the acrylic acid of the second product stream is greater than about 99.9% by weight. 27. The method according to claim 21, wherein the elements for retaining the material are incorporated into the lines connecting the illation unit and the crystallization unit. 28. The method according to claim 24, wherein the elements for retaining the material are incorporated into the lines connecting the illation unit and the crystallization unit.