Patent Application
20210130171
The present invention relates to a process and a device for purifying residual phosphoric acid solutions derived, for example, from industrial or agro-industrial processes and comprising phosphorus in the form of species such as orthophosphate and/or polyphosphate, and also to undesirable volatilizable materials which make their subsequent use difficult, or even impossible. The process of the present invention is particularly efficient and makes it possible to purify residual phosphoric acid solutions of their undesirable volatilizable materials and, where appropriate, to produce phosphoric acid solutions in high concentrations while at the same time limiting the energy consumption required for the production of purified phosphoric acid or polyphosphoric acid solutions. The present invention allows better management of the combustion gases produced by the process before they are discharged into the atmosphere.
Many industrial or agro-industrial processes generate residual aqueous solutions comprising phosphorus. These solutions are termed “residual” since they cannot be used without pretreatment in these same industries. The phosphorus of these solutions may be present in the form of species such as orthophosphate or polyphosphate according to the P2O5 content of these solutions. On account of the presence in these solutions of dissolved undesirable volatilizable materials such as fluorine, sulfur, carbon, etc., these residual solutions generally cannot be recycled in processes for producing materials with increased added value, such as solutions of purified phosphoric acid or polyphosphoric acid (=PPA), or in the production of phosphate salts. For this reason, these volatilizable materials are termed hereinbelow as “undesirable volatilizable materials”. At the present time, these high-potential residual solutions are treated as simple waste with very low value and high polluting power.
Polyphosphoric acid (=PPA) is a viscous liquid which may notably be produced from phosphoric acid. PPA has the general formula HO[P(OH)(O)O]]nH, with n>1. When n=2, PPA is commonly known as pyrophosphoric acid; when n=3, it is known as tripolyphosphoric acid. For n>3, it is known simply as polyphosphoric acid, independently of the value of n. PPA may be produced by dehydration and polycondensation of orthophosphoric acid, H3PO4, according to equation (1). An aqueous polyphosphoric acid solution is thus obtained in which the distribution of the molecular species depends, inter alia, on the polycondensation temperature, Tpc.
Polyphosphoric acid is usually in the form of linear chains. Cyclic forms such as metaphosphoric acid or branched forms may, however, also exist. As illustrated in
The dehydration and polycondensation of a solution of phosphoric acid to polyphosphoric acid requires evaporation of water molecules, which requires heat energy input. Patent EP2411325 B1 reviews a certain number of known processes for producing polyphosphoric acid and describes a novel wet-route process relative to the reviewed processes making it possible to benefit from great energy efficiency and to drastically limit the environmental impact. Said patent describes a device which withstands the very harsh operating conditions for producing polyphosphoric acid, making it possible to limit the maintenance costs and to establish equipment durability and finally to ensure the production of a high-quality polyphosphoric acid without contamination during the manufacturing process.
It would be advantageous to produce a material with high added value such as purified phosphoric acid or polyphosphoric acid from low-value residual solutions. However, the use of such residual solutions in a process for producing purified phosphoric acid or PPA as described in EP2411325 B1 is impossible on account of the presence of undesirable volatilizable compounds in such solutions.
JP2000178014 describes a process for recovering phosphoric acid from recovery solutions, in which a recovery solution comprising phosphorus molecules is incinerated at a temperature of 900 to 1000° C. The combustion gases containing phosphorus molecules are cooled in a cooler and the phosphorus molecules are recovered in the form of phosphoric acid.
The present invention proposes a process that is more efficient than the one described in JP2000178014 for the production of purified phosphoric acid or PPA from residual solutions comprising phosphorus derived generally from industrial processes and which, to date, have simply been treated as waste. The present invention and the advantages thereof are described in greater detail in the following sections.
The present invention is described in the attached independent claims. Preferred variants are defined in the dependent claims. In particular, the present invention relates to a process for purifying an aqueous residual solution comprising phosphorus molecules and undesirable volatilizable materials, comprising the following steps:
The feed solution F0 may comprise a concentration xp0 of between 0.1% and 50%, preferably from 1% to 35%, preferably from 5% to 20% P2O5. In certain cases, the feed solution may also comprise undesirable volatilizable materials, but this is not essential, notably in the case where a residual solution Pr comprising undesirable volatilizable materials is added to the combustion chamber, as explained below. The flow rate, Q0, of the feed solution F0 in the contactor expressed in nominal power units [MW−1] of the combustion chamber is preferably between 100 and 3000 kg/(h MW), preferably between 500 and 2500 kg/(h MW).
The enriched phosphoric acid solution P1 is identical to the recirculated enriched phosphoric acid solution P2 and comprises a concentration xp1 of phosphorus which is preferably greater than or equal to 1%, preferably less than 60%, more preferably between 5% and 50%, preferentially between 10% and 40% P2O5. The total flow rate, Q1=(Qp+O2), of the solution P1 outside the contactor expressed in nominal power units [MW−1] of the combustion chamber is preferably between 600 and 123000 kg/(h MW), preferably between 1000 and 50 000 kg/(h MW). The ratio, Qp/(Qp+Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp+Q2) is preferably less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and in which the ratio Qp/(Qp+Q2) is greater than 0.1%, preferably greater than 0.5%.
The residual solution Pr may comprise a phosphorus concentration xpr of greater than or equal to 2%, preferably at least 5%, more preferably at least 10%, preferably at least 20% P2O5. The residual solution Pr comprises a concentration xpv of undesirable volatilizable materials of at least 5 ppm, preferably at least 10 ppm, preferably at least 100 ppm, preferably at least 1%, preferably at least 5%, preferably at least 10%, more preferably at least 25% by weight relative to the total weight of the solution. The flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber is preferably non-zero and preferably between 5 and 1500 kg/(h MW), preferably between 400 and 1000 kg/(h MW). If the flow rate Qr of the residual solution Pr in the combustion chamber is zero, then the feed solution P0 must comprise a non-zero concentration xpv of undesirable volatilizable materials, for example of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution.
The ratio Qr/(Qr+Q0) may be between 0 and 99%, preferably between 5% and 90%, more preferably between 10% and 80%, or else between 15% and 45%. The mixing stream Fm may comprise a phosphorus concentration xpm preferably greater than 1% P2O5 (xpm>1% P2O5), and comprises undesirable volatilizable materials, originating from the residual solution Pr and/or from the feed solution P0. In the present document, the flow rates Q0, Qp and Qr are mass flow rates of the feed P0, enriched phosphoric acid P1 and residues Pr solutions, respectively.
The mixing solution Pm may comprise a concentration xpm of greater than 2%, preferably greater than 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45% and 60% P2O5. The mixing solution Pm may comprise a concentration xpv of undesirable volatilizable materials preferably of at least 5 ppm, preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, preferably of at least 5%, preferably of at least 10%, more preferably of at least 25% by weight relative to the total weight of the solution. The flow rate Qm of the mixing solution Pm in the combustion chamber expressed as nominal power units [MW−1] of the combustion chamber is preferably between 305 and 3000 kg/(h MW), preferably between 200 and 2000 kg/(h MW).
The combustion solution P3 may comprise a phosphorus concentration xp3 of greater than 1% equivalent of P2O5 units, preferably greater than 10%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30% and 76%. The flow rate Q3 of the combustion solution P3 outside the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber is preferably between 240 and 1500 kg/(h MW), preferably between 600 and 3000 kg/(h MW).
The feed stream F0 and recirculation stream F2 may be either mixed before they are introduced into the gas-acid contactor to form a stream of a mixture of the feed solution P0 and of the recirculated enriched phosphoric acid solution P2, or contacted after having been introduced separately into the gas-acid contactor to form a stream of a mixture of the feed solution F0 and of the recirculated enriched phosphoric acid solution P2.
It is preferable for the residual flow rate Qr of the residual solution Pr to be non-zero. The residual stream Fr and the spraying stream Fp can then be either mixed to form the mixing stream Fm before being sprayed in the flame in the combustion chamber, or sprayed separately in the combustion chamber to form the mixing stream Fm in the flame or just before reaching the flame. The residual solution Pr and, optionally, the feed solution P0 and thus the spraying solution Pp comprise undesirable volatilizable materials. It is also possible that only the spraying solution Pp comprises undesirable volatilizable materials, for example in the case of a flow rate Qr=0. It is preferred, however, for the residual flow rate Qr to be non-zero.
The contact between the feed stream F0 and the recirculation stream F2 and the combustion gases G1 in step (d) may take place co-currentwise or counter-currentwise, preferably co-currentwise by flowing from an upper part to a lower part of the gas-acid contactor. During the contact step (d), the ratio (Qg1/(Q0+Q2)) between a mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor and a total mass flow rate (Q0+Q2) of the contact feed stream F0 and the recirculation feed stream F2 introduced into the gas-acid contactor, is preferably between 0.1% and 50%, preferably between 0.5% and 10%, more preferably between 1% and 7%.
The present invention also relates to a device for producing purified phosphoric acid (P3) following a process according to any one of the preceding claims, comprising:
The residue inlet is preferably in fluidic communication with a source of a residual solution Pr which is aqueous and comprises phosphorus molecules in orthophosphate and/or polyphosphate form and undesirable volatilizable materials.
Various aspects of the present invention are illustration in the following figures.
Since the combustion solution P3 derived from the combustion of the mixing solution Pm obtained at the end of the process of the present invention may comprise purified phosphoric acid alone or as a mixture with polymerized molecules whose respective concentrations vary according to the temperature and the P2O5 concentration of the mixing solution Pm contacting the flame of the combustion chamber (cf.
Unless otherwise mentioned in the present patent, the term “concentration” is used to express mass concentrations (weight percentages, w/o). To define the content in the present case which is of principal concern, when reference is made to a concentration of the solutions of phosphoric acid or the like, this should be understood as the weight content expressed in P2O5 equivalent units, which will be written as “% eq. P2O5” or “% P2O5”. As regards the gas streams, for instance the combustion gases, where several species of interest may coexist as a function of the operating conditions, in gaseous or liquid form (for example by entrainment of droplets), or even optionally in solid form (fumes), the concentration in these streams is also expressed in P2O5 equivalent units (by weight, w/o). The ion dissociation of the species of interest is not considered in the present text. For information, the concentration of a phosphoric acid solution may also occasionally be expressed in H3PO4 equivalent units. The correspondence between the two concentration units is defined by the relationship: 1 eq. P2O5=0.7245 eq. H3PO4.
In the present text, the following expressions are understood as follows:
The process of the present invention comprises the introduction into a gas-acid contactor (1) of the following streams.
The feed stream F0 and the recirculation stream F2 and the combustion gases G1 are thus contacted together in the gas-acid contactor (this is thus referred to as a direct gas-acid contactor) to form, on the one hand, an enriched phosphoric acid solution P1 and, on the other hand, contacted combustion gases G3.
In a preferred variant, illustrated in
In an alternative variant, illustrated in
The contact between the feed stream F0 and the recirculation stream F2 or their mixture (F0+F2) and the combustion gases G1 in the gas-acid contactor may take place by co-current or counter-current stream contact. In particular, the liquid phases flow downward in the direction of gravity, and the gaseous phase rises upward. In a preferred variant, the two or three streams flow co-currentwise from an upper part to a lower part of the gas-acid contactor. In the context of the present invention, the terms “upper” and “lower” are understood according to the direction of the forces of Earth's gravity which extend in the direction of the center of gravity of the Earth. Thus, in the absence of pressure gradients, a liquid naturally flows from the upper part of a reactor to its lower part which is downstream of the upper part following the direction of the Earth's gravity.
It is possible to contact the combustion gases G1 with the phosphoric acid feed and recirculation streams F0&F2 or their mixture (F0+F2) by guiding the combustion gases in a transverse stream relative to that of the phosphoric acids. The placing in contact in co-current streams is, however, preferred.
The contact between the feed stream F0 and the recirculation stream F2 or their mixture (F0+F2) and the combustion gases G1 forms an enriched phosphoric acid solution P1 and contacted combustion gases G3. This contact may take place by percolating the streams through a packing material which withstands the operating conditions. During the contact between the combustion gas G1 and the feed stream F0 and recirculation stream F2, exchanges take place. On the one hand, the phosphorus molecules transported by the combustion gases in the form of droplets and vapor are carried by the streams F0 and F2, allowing the formation of the enriched phosphoric acid solution P1 having a P2O5 content higher than that of each of the streams F0 and F2. On the other hand, an exchange of heat takes place between the hot combustion gases, at a temperature Tg1 between 200 and 600° C., toward the aqueous solutions of the streams F0 and F2 which are at lower temperatures, as indicated in
The enriched phosphoric acid solution P1 and the contacted combustion gases G3 formed following the placing in contact of the phosphoric acid feed and recirculation streams F0&F2 with the combustion gases G1 are then separated by separation means that are well known to those skilled in the art, such as a centrifugal separator or by gravity, a coalescer, a spray eliminator, a mattress, chicanes, etc. The contacted combustion gases G3 are then evacuated from the gas-acid contactor (1) at a temperature substantially below that of the contacted combustion gases G3 introduced into said gas-acid contactor for subsequent treatments. As the majority of the phosphorus molecules contained in the combustion gas stream G1 are transferred into the stream F1 of enriched phosphoric acid solution P1 during the contact of the stream G1 with the streams F0 and F2, the contacted gas stream G3 is much more depleted in P2O5 than the combustion gas stream G1 with contents which may be less than 1% P2O5.
The contacted combustion gases G3 containing the undesirable volatilized materials may also undergo scrubbing after they have left the gas-acid contactor, with an aqueous scrubbing solution in order to dissolve and remove, before releasing the gases into the atmosphere, the undesirable compounds, for instance fluorinated or chlorinated compounds, SO3, etc. Other treatments of the contacted combustion gases G3 are possible, for example including condensation of the gases in an indirect condenser. The enriched phosphoric acid solution P1 is also removed from the gas-acid contactor separately of the contacted combustion gases G3.
The enriched phosphoric acid solution P1 may comprise a concentration xp1 of P2O5 units of greater than or equal to 1%, preferably less than 60%, more preferably between 5% and 50%, preferentially between 10% and 40% P2O5. The P2O5 concentration of the enriched phosphoric acid solution P1 obviously depends on the P2O5 concentration of the feed solution, P0, and on the combustion gas stream, G1. As discussed hereinbelow, the phosphorus concentration of the enriched phosphoric acid solution P1 is generally higher than that of the feed solution, P0.
Before, during or after its evacuation from the gas-acid contactor, the enriched phosphoric acid solution P1 is divided into two separate streams:
The spraying solution Pp and the recirculated enriched phosphoric acid solution P2 are identical to each other in composition and identical to the enriched phosphoric acid solution P1 (P1=Pp=P2) since they have not undergone any alteration between the moment of their formation in the gas-acid contactor and the division into two separate streams, the recirculation stream F2 and the spraying stream Fp. The temperatures Tp and T2 of the solutions Pp and P2 are also substantially identical to the temperature T1 of the solution P1 which may be of the order of 100 to 300° C. The solutions Pp and P2 preferably comprise, in the stationary regime, a higher P2O5 concentration than that of the feed solution P0 of the contactor. For example, the solutions Pp and P2 may comprise phosphorus concentrations of between 1 and 60%, preferentially between 5% and 50% P2O5, preferentially between 10% and 40% P2O5. This is explained by two main reasons.
Firstly, the placing in contact of the phosphoric acid feed and recirculation streams with the combustion gases G1 which are at a higher temperature Tg1 of the order of 300 to 600° C. (cf.
Secondly, as shall be discussed hereinbelow, the combustion gases G1 formed during the combustion of the mixing stream Fm of mixing solution Pm in the combustion chamber comprise phosphoric acid droplets or vapors. The combustion gases G1 may comprise between 0.1% and 15% P2O5, preferably between 0.5% and 13%, preferably between 1% and 10%, preferably between 2% and 5% (cf.
In one variant of the invention, the enriched phosphoric acid solution P1 is divided into two streams, the spraying stream Fp and the recirculation stream F2 at the outlet of the gas-acid contactor in a spraying fluidic connection (3p) and in a recirculation fluidic connection (3r), respectively, as illustrated in
In an alternative variant, the enriched phosphoric acid solution P1 is removed from the gas-acid contactor in a first fluidic connection (3, 3u) which is common and which divides into two at a T-shaped or Y-shaped branching point (5) with, on the one hand, the spraying fluidic connection (3p) which entrains the spraying stream Fp at a flow rate Qp, toward a combustion chamber (2) and, on the other hand, a recirculation fluidic connection (3r) which entrains the recirculation stream F2 at a flow rate Q2 toward the gas-acid contactor, thus forming a recirculation loop. Different variants of this configuration comprising a branching point (5) are illustrated in
The enriched phosphoric acid solution P1 leaves the contactor at a total flow rate Q1=(Qp+Q2). The total flow rate Q1 expressed in nominal power units [MW−1] of the combustion chamber is preferably between 600 and 123 000 kg/(h MW) or between 1000 and 120 000 kg/(h MW), preferably between 12 000 and 100 000 kg/(h MW). As discussed hereinabove, the stream F1 of enriched phosphoric acid (P1) is divided into two streams Fp and F2 each having a spraying flow rate Qp and a recirculation flow rate Q2. The division into two streams may take place before leaving the gas-acid contactor, at the outlet thereof, or after the outlet. The spraying flow rate Qp and recirculation flow rate Q2 must be determined as a function, inter alia, of the capacity of the combustion chamber and of the gas-acid contactor, of the temperature of the combustion gases G1 and of their P2O5 content.
In a stationary production state, the ratio, Qp/(Qp+Q2), between the mass flow rate Qp of the spraying stream Fp and the total mass flow rate (Qp+Q2) of the enriched phosphoric acid stream F1 (which is in fact the sum of the spraying flow rate Qp and the recirculation flow rate Q2) is preferably less than 50%, preferably less than 20% and more preferably less than 10%. In a preferred variant of the invention, the ratio Qp/(Qp+Q2) is less than 5%, preferably less than 4%, more preferably less than 2.5% and even less than 2%. The ratio Qp/(Qp+Q2) is preferably greater than 0.1%, or else greater than 0.2% and preferably greater than 0.5%. Increasing the flow rate Q2 relative to the flow rate Qp makes it possible, on the one hand, to cool the combustion gases G1 to a lower temperature, which is necessary before their evacuation, and, on the other hand, to further enrich the P2O5 content of the enriched phosphoric acid solution Pp.
The ratio, Q2/(Qp+Q2), between the mass flow rate Q2 of the recirculation stream F2 and the total mass flow rate (Qp+Q2) is, needless to say, the remainder of the ratio Qp/(Qp+Q2), the sum of which is equal to 100%. The recirculation flow rate Q2 is thus preferably greater than or equal to the spraying flow rate Qp and, in certain preferred variants, is considerably higher than Qp with a ratio of the flow rates, Qp/Q2, which may range from 0.1/99.9 to 49/51 (=0.1% to 96%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 5/95 (=1% to 5.3%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 4/95 (=1% to 4.2%).
As discussed hereinabove, the flow rate Q2 of the recirculated enriched phosphoric acid solution P2 in the contactor expressed in nominal power units [MW−1] of the combustion chamber may be between 300 and 120 000 kg/(h MW), preferably between 600 and 110 000 kg/(h MW), preferably between 9000 and 100 000 kg/(h MW). Thus, the flow rate Qp of the spraying solution Pp flowing toward the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber may be between 300 and 3000 kg/(h MW), preferably between 600 and 2000 kg/(h MW), preferably between 1000 and 1500 kg/(h MW).
A mixing stream Fm of a mixing solution Pm comprising undesirable volatilizable materials at a non-zero mass concentration xpv and phosphorus at a mass concentration xpm preferably higher than that of the contact feed stream F0 is formed by the spraying stream Fp, optionally mixed with a residual stream Fr of an aqueous residual solution Pr originating from the residues of a prior industrial process. If the feed solution F0 does not comprise any undesirable volatilizable materials, then the mixing of the feed stream P0 with a residual stream Fr is obligatory. Otherwise, it is optional, but preferred. The mixing stream Fm is sprayed through a flame burning in the upper part of a combustion chamber (2) to:
The residual stream Fr comprises:
The residual stream Fr comprises a phosphorus concentration xpr of at least 1% or of at least 5%, preferably at least 10%, more preferably at least 15%, preferably at least 20% P2O5. The flow rate Qr of the residual solution Pr in the combustion chamber expressed in nominal power units [MW−1] of the combustion chamber is preferably non-zero and preferably between 5 and 1500 kg/(h MW), preferably between 400 and 1000 kg/(h MW). The temperature of the residual solution Fr may be between 20 and 200° C., preferably between 40 and 150° C., more preferably between 50 and 100° C. Preheating of the solution Fr is advantageous in terms of efficiency of combustion of the mixing solution Fm in the flame.
The residual solution Pr and, optionally, the feed solution P0 is or preferably contains a solution coming from industry. This solution may be generated by the scrubbing of facilities or during common production or maintenance operations in industries such as the metallurgical, agrifood, pharmaceutical or chemical industries and particularly during the production of phosphate salts or of fertilizers. These solutions are generally difficult to recycle in their native form on account of their contents of various pollutants, notably soluble residues of organic materials and due to their low phosphorus concentrations. They must thus be treated before being subsequently concentrated. The residual solution may also be derived from processes for recovering phosphorus from starting materials known as “secondary” materials and which are in fact solid compounds containing phosphorus other than phosphate ore. Mention may notably be made of bone powder or bone powder ash, sludges or sludge ash from purification stations, pig and poultry manure or manure ash, etc. These residual solutions or residual acid solutions or residual solutions comprise P2O5 but also often undesirable volatilizable materials typically such as carbon, fluorine, chlorine, sulfur, nitrogen in soluble form (ionic or nonionic). The concentrations of the undesirable volatilizable materials obviously depend on the origin of the residual solution. For example, they may be present in the residual solution Fr in concentrations xpv of undesirable volatilizable materials of at least 5 ppm (parts per million), preferably of at least 10 ppm, preferably of at least 100 ppm, preferably of at least 1%, more preferably of at least 5% by weight of total organic carbon relative to the total weight of the solution or else at least 10 ppm of fluorine, at least 100 ppm of fluorine, at least 1% of fluorine. Depending on the applications, these solutions are unusable as such.
In a stationary production state, the ratio, Qp/Q0, between the flow rate Qp of the spraying stream Fp and the flow rate Q0 of the feed stream F0 is preferably between 100% and 250%, preferably between 101% and 140%, preferably between 110% and 115%. This ratio may be higher than 100% since the contact of the streams Q0 and Q2 with the combustion gases in the gas-liquid contactor increases the mass of the stream F1 leaving the gas-liquid contactor. The value of this ratio may decrease as the value of the residual flow rate, Qr, increases.
The flow rate ratio Qr/(Qp+Qr) between the residual flow rate Qr and the sum of the spraying flow rate Qp and the residual flow rate Qr represents the flow rate fraction of the residual solution Pr entering the combustion chamber. The value of this ratio depends inter alia on contents of P2O5 and of undesirable volatilizable materials of the residual solution and/or of the feed solution, which are determining for the content of P2O5 and of undesirable volatilizable materials of the mixing solution Pm. For example, the ratio Qr/(Qp+Qr) may be between 0 and 94%, preferably between 5% and 90%, more preferably between 10% and 80% or else between 15% and 45%.
Independently of the value of the ratio Qr/(Qp+Qr), the mixing solution Pm preferably comprises a phosphorus concentration xpm of greater than 1%, preferably greater than 2% or 5%, more preferably greater than 20%, more preferably greater than 30%, preferably greater than 40%, and more preferably between 45% and 60% P2O5. The flow rate Qm of the solution Pm in the combustion chamber is the sum of the spraying flow rate Qp and of the flow rate of the residual solution Qr. Expressed in nominal power units [MW−1] of the combustion chamber, the mixing flow rate Qm is preferably between 305 and 3000 kg/(h MW), preferably between 900 and 2000 kg/(h MW).
Process—Evaporation of the Undesirable Volatilizable Materials and Concentration of P2O5
A main function of the combustion chamber is to degrade, if necessary, by oxidation and then to vaporize the undesirable volatilizable materials present in the residual solution. A second function of the combustion chamber is to evaporate the water present in the solutions to concentrate the residual and feed solutions. A third (optional) function is polycondensation of the phosphate molecules present into polyphosphoric acid (PPA). The distribution of the species present in the solution thus formed depends on the P2O5 concentration of the mixing solution Fm reaching the combustion flame, and also on the polycondensation temperature. As may be seen in
Combustion in the flame of the mixing solution Pm thus forms, on the one hand, combustion gases G1 formed by the evaporation of the water and, in particular, of the undesirable volatilizable materials and, on the other hand, a combustion solution P3 which is in liquid form and comprising phosphorus and, if the P2O5 concentration and the polycondensation temperature Tpc are sufficient, species polymerized by polycondensation of the phosphoric acid contained in the solution Pm.
The temperature reached by the mixing solution Pm in the flame is an important parameter of the process since it will enable the volatilization of the undesirable volatilizable compounds present in the residual solution and thus in the mixing solution. The P2O5 concentration obtained in the combustion solution P3 is also dependent thereon, as shown by the graph of
The flame is fed with a combustible and a source of oxygen, typically air or, for a higher temperature, oxygen. The flame is preferably a slightly oxidizing flame, preferably comprising between 1% to 5% of excess air. The combustible is preferably natural gas, butane, propane or any other combustible, whether it is gaseous or liquid. In the absence of spraying of the mixing solution Pm, the flame preferably reaches a theoretical temperature of at least 750° C., preferably at least 1000° C., more preferably at least 1700° C., for example 1800° C.±50° C. In the process of the present invention, the temperature increase is instantaneously limited since, on the one hand, the mixing solution Pm is fed at a lower temperature Tm, of the order of 20-300° C. and, on the other hand, because the evaporation of the water molecules from the solution is energy-intensive.
The residual stream Fr and the spraying stream Fp may be mixed to form the mixing stream Fm before being sprayed into the flame in the combustion chamber, as illustrated in
The combustion solution P3 which consists of a purified phosphoric acid solution is thus an aqueous phosphoric acid solution which may contain polymerized species depending on the P2O5 content present in the solution (cf.
If the production of PPA is desired, it is preferable for the mixing solution sprayed into the flame to reach a polycondensation temperature Tpc of at least 400° C., preferably at least 500° C. and even higher than 550° C., of the order of 650° C. or 700° C., for a predefined polycondensation time. A high polycondensation temperature Tpc makes it possible to obtain polyphosphoric acid solutions with a high P2O5 concentration, of the order of 86% and more, with longer chain lengths n (e.g. n≥5 to 12) (cf.
The combustion solution P3 thus formed is then separated from the combustion gases G1 formed during the evaporation of water and from the undesirable volatilizable materials and optionally the polycondensation of polyphosphoric acid in a gas-liquid separator (9). The combustion solution P3 containing phosphoric acid and optionally polyphosphoric acid and virtually free of undesirable volatilizable materials is recovered whereas the combustion gases G1 are transferred into the gas-acid contactor (1) to be placed in contact with the feed stream F0 and the recirculation stream F2, as described hereinabove.
The stream F3 of the combustion solution P3 thus recovered may have a high temperature of the order of 150 to 700° C., preferably 200 to 650° C., preferably from 300 to 500° C. depending on the vaporization temperature of the undesirable volatile materials contained in the mixing solution Pm. Specifically, the temperature required for volatilization of the volatilizable materials present in the mixing solution Pm varies according to the nature of the materials present in the mixing solution Pm. It is preferable to cool the solution P3 in a heat exchanger (11) (cf.
The combustion solution P3 comprising purified phosphoric acid (and optionally polyphosphoric acid) thus formed and recovered comprises a concentration of undesirable volatilizable materials lower than that of the mixing solution Pm. For example, the combustion solution Pm comprises less than 50% of the undesirable volatilizable materials contained in the residual solution Pr, preferably less than 70%, more preferably less than 80% or less than 90%, and ideally less than 95% or 99%. The combustion solution P3 comprises a higher P2O5 concentration than that of the mixing solution Pm. This is explained by the evaporation of a majority of the water from the solution during its passage into the flame. The P2O5 concentration xp3 of the combustion solution P3 is normally greater than 10% P2O5, preferably greater than 15%, preferably greater than 25%, particularly preferably greater than 40%, or is preferably between 30% and 76%.
The flow rate Q3 of the combustion solution P3 out of the combustion chamber is representative of the phosphoric acid purification capacity. Expressed in nominal power units [MW−1] of the combustion chamber, the flow rate Q3 is preferably between 240 and 1500 kg/(h MW), preferably between 500 and 1000 kg/(h MW).
The combustion gases G1 consist mainly, on the one hand, of CO2, O2, H2O, and, on the other hand, of undesirable volatilizable materials, for instance nitrogen oxides (NOx), sulfur oxides, fluorinated or chlorinated compounds or organic materials, and also molecules containing phosphorus, the latter possibly being presents in amounts which may range between 0.1% and 15% by weight of P2O5, depending on the concentration xpm of the mixing solution Pm. In general, the P2O5 content that may be present in the combustion gases G1 ranges between 0.5% and 13% by weight, preferably between 1% and 10%, preferably between 2% and 5% P2O5. The temperature Tg1 of the combustion gases G1 transferred is substantially lower than the temperature which may be reached by the flame since, as discussed hereinabove, the temperature in the combustion unit falls during the polycondensation reaction which requires a substantial amount of energy, mainly to evaporate the water of the polycondensation reaction. The combustion gases enter the acid-gas contactor at a temperature Tg1 which is of the order of the polycondensation temperature Tpc, and is generally between 200 and 600° C., preferably between 400 and 500° C.
As discussed above, a recirculation fraction of the enriched phosphoric acid solution P1 leaving the gas-acid contactor (1) is reintroduced into the gas-acid contactor thus forming a recirculation loop, whereas a spraying fraction Pp is conveyed to the combustion chamber (2). The recirculation fraction is preferably greater than or equal to the spraying fraction and is ideally considerable greater than the spraying fraction, with ratios Qp/Q2 of the spraying flow rate Qp to the recirculation flow rate Q2 which may range from 0.1/99.9 to 49/51 (=0.1% to 96%). Preferably, the flow rate ratio Qp/Q2 is between 1/99 and 5/95 (=1% to 5.3%).
During their introduction into the gas-acid contactor, the feed stream F0 and the recirculation stream F2 may be mixed before being introduced into the gas-acid contactor to form a stream of a mixture of the contact feed solution P0 and of the recirculated enriched phosphoric acid solution P2, as illustrated in
The recirculation loop is an important element of the present invention. The main consequence of introducing such a recirculation loop is that the ratio (Qg1/(Q0+Q2)) between the mass flow rate Qg1 of the combustion gas G1 introduced into the gas-acid contactor (1) and the total mass flow rate (Q0+Q2) of the feed stream F0 and the recirculation stream F2 introduced into the gas-acid contactor (1), is much lower than in the absence of such a recirculation loop. The ratio (Qg1/(Q0+Q2)) according to the present invention is preferably between 0.1% and 50%, more preferably between 0.5% and 20% or less than 10%, and is ideally between 1% and 7%. In the absence of such a recirculation loop (i.e. Qg1>0, Q0>0 and Q2=0), the ratio (Qg1/Q0) is considerably greater, with values of greater than 60%, in general greater than 100%, indicative of a combustion gas flow rate Qg1 which is higher than the feed flow rate Q0 of feed solution P0 of the contactor.
The recirculation loop thus makes it possible to control the ratio between the flow rate of combustion gas G1 and the total flow rate (Q0+Q2) of phosphoric acid feed solutions P0 and of recirculated enriched phosphoric acid solution P2. In particular, it makes it possible to considerably increase the mass of phosphoric acid solution contacted with the combustion gas. This has several advantages.
On the one hand, the transfer of the P2O5 droplets and vapors contained in the combustion gases G1 to the stream of the mixture of solutions P0 and P2 is much more pronounced. The P2O5 concentration of the spraying solution formed during contact with the combustion gases is thus higher than if the flow rate ratio Qg1/(Q0+Q2) had been higher. The better gas/liquid contact thus obtained allows better recovery by the enriched phosphoric acid solution P1 of the P2O5 contained in the combustion gases G1. Furthermore, the combustion gases G3 after contact with the streams F0 and F2 are thus purified of their P2O5 content, lightening their treatment before they are released into the atmosphere.
On the other hand, with such flow rate ratios, the temperature Tg3 of the combustion gases G3 after their contact with the streams F0 and F2 is reduced much more efficiently than in the process described in EP2411325 B1, thus not requiring any other heat exchanger (or at least a heat exchanger of lower capacity), which is essential in the process of EP2411325 B1 to lower the temperature of the combustion gases to a value that is acceptable for their evacuation into the atmosphere.
The process of the present invention may be implemented in a device comprising a combustion chamber (2), a gas-acid contactor (1) and various fluidic connections between the combustion chamber and the gas-acid contactor. It is clear that the device may comprise several combustion chambers and/or several gas-acid contactors positioned in parallel or in series.
The combustion chamber (2) makes it possible to perform the combustion of the mixing solution Pm, by spraying it into the flame. The mixing solution Pm is formed from the spraying solution Pp mixed with the residual solution Pr to form a combustion solution P3 comprising phosphoric acid (and possibly polyphosphoric acid) purified of the undesirable volatilizable materials. The walls of the combustion chamber must withstand the corrosive nature of the spraying solution Pp and of the residual solution Pr and the high temperatures prevailing therein. It is preferable for the walls to be made of silicon carbide or of amorphous carbon. It is possible to use jackets with a neutral gas or the combustion gases circulating between the two walls, which may have advantages in terms of temperature of the walls, and impermeability of said walls to the (poly)phosphoric acid solutions.
The combustion chamber (2) has one or more spraying inlets (2pu) into the combustion chamber for introducing a spraying solution Pp at a flow rate Qp, or a mixing solution Pm at a flow rate (Qp+Qr), in sprayed form into a combustion unit located in an upper part of the combustion chamber (cf.
The combustion chamber (2) comprises a combustion unit (2c) arranged in the upper part of the combustion chamber, and capable of forming a flame having a temperature of at least 1000° C., preferably at least 1500° C., and even at least 1700° C., preferably 1800° C.±50° C., by combustion of a combustible in the presence of oxygen. The temperature of the flame may be controlled by varying the flow rate of oxygen feeding the flame. The combustion unit comprises:
The combustion chamber (2) is equipped with a gas-liquid separator (9) to separate the combustion solution P3 thus formed from the combustion gases G1. For example, the combustion gases may be separated from the combustion solution by enlarging the flow rate transverse surface area, the consequence of which is to reduce the flow speed and thus the kinetic energy of the gas stream G1 and the combustion stream F3. As the streams flow from the top downward, by lowering their kinetic energy, the gases will slow down and can be diverted toward a deflector which guides them to the combustion gas outlet. By means of their higher density, the purified phosphoric acid and optionally polyphosphoric acid droplets of the combustion solution P3 continue their flow downward by gravity.
The combustion chamber (2) has a combustion outlet (2pd) from the combustion chamber for recovering a purified (poly)phosphoric acid liquid phase, and arranged downstream of the combustion unit, which is itself arranged downstream of the mixing or spraying, and residue inlet. The term “downstream” is expressed relative to the direction of flow of the spraying solution Pp and the polyphosphoric acid solution P3 in the combustion chamber. As explained hereinabove, the direction of flow is preferably from the top downward following the direction of gravity. The device may thus be equipped with a tank for storing the phosphoric acid thus produced (not illustrated). Preferably, the device comprises a heat exchanger (11) arranged between the combustion outlet (2pd) and the storage tank, in order to cool the combustion solution P3 from a temperature of between about 200 and 650° C. to a temperature of the order of 100 to 150° C. when it reaches the storage tank.
Finally, the combustion chamber (2) is equipped with an outlet for evacuating the combustion gas G1 obtained from the flame. These combustion gases are charged with droplets and vapors of P2O5 and of undesirable volatilized materials. They have a temperature Tg1 and do not need to be cooled before being introduced into the gas-acid contactor.
The gas-acid contactor (1) makes it possible to heat and to increase the equivalent concentration of P2O5 units of the feed solution introduced into the contactor, before it enters the combustion chamber (2) so as to optimize the phosphoric acid purification yield and the energy consumption of the polycondensation reaction.
The gas-acid contactor (1) has a feed inlet (1pu) connected to a source of a feed solution P0 of the contactor or of a mixture of feed solution P0 of the contactor and of enriched phosphoric acid solution P2. As discussed hereinabove, the feed solution P0 of the contactor comprises between 0 and 54% P2O5, preferably from 0.1% to 50%, preferably from 1% to 35%, more preferably between 15% and 20% P2O5. The feed inlet (1pu) must be dimensioned to allow the introduction of the feed solution P0 of the contactor at a feed flow rate Q0 or the introduction of the mixture of feed solution P0 of the contactor and of recirculated enriched phosphoric acid solution P2 at a flow rate (Q0+Q2). The recirculated enriched phosphoric acid solution, P2, may also be introduced into an inlet (1pru) for recirculated enriched phosphoric acid, P2, separate from the feed inlet (1pu).
The gas-acid contactor (1) is preferably a direct contactor. It comprises a combustion gas inlet (1gu) for introducing into the gas-acid contactor combustion gases G1 coming from the outlet for evacuating the combustion gases G1. The feed inlet (1gu) must be dimensioned to allow the introduction of the combustion gases G1 at a flow rate Qg1. As discussed hereinabove, the combustion gases G1 placed in contact with the feed solution P0 of the contactor make it possible (a) to increase the temperature of the feed solution P0 of the contactor, (b) to evaporate part of the water from the feed solution P0 of the contactor and (c) to exchange with the solution P0 the P2O5 droplets and vapors contained in the combustion gas G1.
The gas-acid contactor (1) is equipped with a recirculation inlet (1pru), for introducing a solution of recirculated enriched phosphoric acid P2. In one variant of the invention, the streams F0 and F2 are mixed before being introduced into the gas-acid contactor and the recirculation inlet is then the same as the feed inlet (1pu). In an alternative variant, the feed inlet (1pu) and recirculation inlet (1pru) are separate. The recirculation inlet must be dimensioned to allow the introduction of the recirculated enriched phosphoric acid solution P2 at a feed flow rate Q2.
The gas inlet (1gu), the feed inlet (1pu) and, if it is separate therefrom, the recirculation inlet (1pru) are arranged to allow, on the one hand,
The gas inlet (1gu) is preferably arranged so that the combustion gases G1 (which are named G2 during the contact) flow co-currentwise with the phosphoric acid feed stream F0 and recirculation stream F2. However, it is possible to arrange the gas inlet so that the combustion gases flow counter-currentwise relative to the streams F0 and F2.
The gas-acid contactor preferably comprises a packing material, through which percolate the feed stream F0 and the recirculation stream F2 of phosphoric acid solutions. The packing material is preferably arranged on a perforated support, for example a support grate.
The gas-acid contactor (1) comprises one or more enriched phosphoric acid outlets (1pd, 1prd), the enriched phosphoric acid outlet(s) (1pd, 1prd) are positioned downstream of the gas inlet (1gu), which is itself positioned downstream of the feed inlet (1pu) and, if it is separate therefrom, the recirculation inlet (1pru). The term “downstream” is expressed relative to the direction of flow of the feed stream and the recirculation stream of the phosphoric acid feed solution and the recirculated enriched phosphoric acid feed solution P2 in the gas-acid contactor. The enriched phosphoric acid outlet(s) (1pd, 1prd) make it possible to remove the enriched phosphoric acid solution P1 formed in the gas-acid contactor formed by contact between the streams F0 and F2 and the combustion gases G1.
The gas-acid contactor (1) comprises a gas-liquid separator for separating the liquids from the gases after contact between the combustion gases G1 and the solutions P0 and P1. For example, the gas-acid contactor may comprise a demister for recovering any droplets of liquid present in the contacted combustion gas G3 before it leaves via the gas outlet (1gd).
The gas-acid contactor (1) also comprises a combustion gas outlet (1gd), for evacuating from the gas-acid contactor the contacted combustion gases G3 after their contact with the mixture of solutions P0 and P2. The device may be followed by a tower for scrubbing the contacted combustion gases G3 located downstream of the combustion gas outlet (1gd) of the gas-acid contactor, for removing any fluorinated and sulfur-based compounds that the gases contain before releasing them into the atmosphere.
The device is equipped with a combustion gas fluidic connection (6) connecting an end (6u) coupled to the combustion gas evacuation outlet of the combustion chamber (2), to an end (6d) coupled to the combustion gas inlet (1gu) in the gas-acid contactor (1). The temperature in this fluidic connection (6) should preferably be maintained as high as possible so that, at the inlet (1gu) in the gas-acid contactor, the combustion gases G1 have a temperature that is as close as possible to the temperature Tg1 that they have at the combustion chamber outlet, i.e. about 200 to 600° C.
The device is equipped with a fluidic connection (3, 3p) connecting an upstream end (3u) coupled to the enriched phosphoric acid outlet (1pd) of the gas-acid contactor (1), to a downstream end (3d) coupled to the spraying inlet (2pu) of the combustion chamber (2). As the enriched phosphoric acid solution P1 has recovered the majority of the phosphorus molecules carried by the combustion gases G1, the fluidic connection (3, 3p) makes it possible to reinject these molecules into the combustion chamber so as to obtain a combustion solution P3 which is as rich as possible in P2O5. The enriched phosphoric acid solution P1 has a temperature above that of the feed solution P0 of the contactor, which allows better management of the heat energy of the process by injecting into the combustion chamber a solution that is already at a relatively high temperature. In the case where PPA production is desired, the higher concentration of P2O5 and the higher temperature of the enriched phosphoric acid solution P1 make it possible to increase the concentration yield in the combustion chamber.
This improvement in the transfer of phosphoric acid molecules and in the concentration yield is made possible by means of the recirculation loop for reintroducing into the gas-acid contactor part of the phosphoric acid stream P1 taken from the same gas-acid contactor. Thus, the device also comprises a recirculation fluidic connection (3r) connecting an upstream end coupled either
The device is equipped with means for controlling and maintaining a ratio, Qp/(Qp+Q2), between a spraying mass flow rate Qp flowing in the first fluidic connection (3) and a total mass flow rate (Qp+Q2) defined as the sum of the spraying mass flow rate Qp and of a recirculation mass flow rate Q2 flowing in the recirculation fluidic connection (3r) at a value of less than 50%, preferably less than 10%, preferably less than 5%, more preferably less than 2.5% and the ratio Qp/(Qp+Q2) has a value of greater than 0.1%, preferably greater than 0.5%.
As illustrated in
In an alternative variant illustrated in
In a first variant illustrated in
In a second variant illustrated in
In a third variant (not shown), the means for ensuring the ratio Qp/(Qp+Q2) comprise a pump (4) arranged on the first fluidic connection (3) upstream of the branching point (5) and having a capacity for pumping a liquid at a main flow rate (Qp+Q2) and pipes forming the spraying (3p) and recirculation (3r) fluidic connections dimensioned so as to obtain the desired ratio Qp/(Qp+Q2). This solution is less flexible than the first two in the sense that once the pipes have been dimensioned, the ratio Qp/(Qp+Q2) cannot easily be varied, which is not necessarily a problem if the ratio does not need to vary during the lifetime of the device.
In a fourth variant (not shown), the means for ensuring the ratio Qp/(Qp+Q2) comprise a pump (4) arranged on the first fluidic connection (3) upstream of the branching point (5) and having a capacity for pumping a liquid at a main flow rate (Qp+Q2) and forming the spraying (3p) and recirculation (3r) fluidic connections and also valves adjusted so as to obtain the desired ratio Qp/(Qp+Q2).
Table 2 lists a series of ranges of values of the various parameters adapted for performing the process of the present invention.
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indicates data missing or illegible when filed
| Number | Date | Country | Kind |
|---|---|---|---|
| 20185218 | Mar 2018 | BE | national |
| 20185916 | Dec 2018 | BE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/057706 | 3/27/2019 | WO | 00 |
