Process for the preparation of calcium dipropionate

Abstract

The present invention relates to a process for the preparation of calcium dipropionate, comprising the steps a) introduction of a solid mixture of 0% to 100% calcium oxide and 100% to 0% calcium hydroxide into a pressure-resistant solid mixing reactor, b) Start of the moderation of the internal pressure of the pressure-resistant solid-mixing reactor by introducing inert gas, c) Addition of propionic acid to the solid mixture, d) chemical conversion of the solid mixture, f) Obtaining solid calcium dipropionate after the end of the chemical reaction, characterized in that the chemical reaction takes place in an inert gas atmosphere with which the reaction pressure is constantly regulated between 2 bar and 6 bar during the chemical reaction.

Description

[0001] The present invention relates to a process, in particular a solid process, for producing calcium dipropionate from calcium oxide / calcium hydroxide and propionic acid.

[0002] Propionic acid is the trivial name of propanoic acid (IUPAC), the propionates are systematically referred to as propanoates.

[0003] Calcium dipropionate, colloquially abbreviated to "calcium propionate," is the colorless calcium salt of propionic acid (CAS no. 4075-81-4 or EC no. 223-795-8). Calcium dipropionate is produced on an industrial scale. The salt is used, among other things, in the food and feed sectors as a preservative (E282).

[0004] In addition to anhydrous calcium dipropionate (Ca(CH3CH2COO)2) (anhydrate), calcium dipropionate monohydrate (Ca(CH3CH2COO)2·H2O) exists as a hydrate form. Salts with a significantly predominant anhydrate content are of commercial importance.

[0005] Calcium dipropionate is produced by neutralizing propionic acid with alkaline calcium salts or solutions. Neutralization reactions are usually exothermic and release water as a byproduct. The amount of water released depends on the raw materials used. Typical calcium salts used as raw materials are calcium hydroxide, calcium oxide, and calcium carbonate.

[0006] The most commonly used production method for calcium dipropionate involves the neutralization of propionic acid with calcium hydroxide in an aqueous mixture followed by spray drying (see Ullmann's Encyclopedia, 7th edition online, Volume A22, Wiley-VCH Verlag). The enormous energy required to dry a ≤ 30 wt.% calcium propionate solution in water is considered a disadvantage.

[0007] Another option is crystallization from a saturated calcium dipropionate solution. Exceeding the solubility limit of approximately 30 wt.% leads to crystallization and enables solid / liquid separation. The raw crystals must be adjusted to the desired moisture content through further drying steps. This process is also energy-intensive.

[0008] EP 0 093 317 A1 describes a continuous process with at least two combined reaction lines for the production of calcium dipropionate. The reaction waste heat and offgases from the first reaction line are transferred to the second reaction line as a 20 wt. % to 30 wt. % propionic acid vapor mixture. The reaction product in both reaction lines is a calcium dipropionate suspension. The free-flowing solid can be separated by solid / liquid separation, e.g., a centrifuge. This solid is then dried and ground.

[0009] EP 1 072 581 A2 discloses how an aqueous suspension of calcium dipropionate is prepared from the raw materials propionic acid and a calcium hydroxide solution. This suspension, with a calcium dipropionate content of > 30 wt.%, for example, 40 wt.% to 55 wt.%, is homogeneously dispersed and dried by injection into a convection dryer.

[0010] These well-known manufacturing processes use water as a carrier. Water serves as a solvent, a heat carrier / moderator, and a transport medium between combined process equipment.

[0011] EP 3 319 931 B1 describes, for the first time, a solid-state process that differs from previous publications by omitting water as a carrier. The reaction of the starting materials propionic acid with calcium hydroxide, calcium oxide, and calcium carbonate takes place in a pressure-resistant reactor. The pressurized operation is intended to retain the reaction energy in the closed system. The reaction water also remains in the system. Following the reaction, the reactor is depressurized, and the stored energy is to be used proportionally for drying.

[0012] EP 3 319 931 B1 describes an experiment using a raw material mixture of calcium oxide and calcium hydroxide to neutralize propionic acid. The reaction of the described raw material mixture is specified as 180 °C and a corresponding pressure of 5 bar gauge. The reaction product is subsequently dried by reheating.

[0013] One conclusion from this state of the art is that the water is distributed between the gas phase and the dispersed solid / liquid phase in a closed system, depending on the prevailing system pressure. High pressures adversely shift the molar ratio of gaseous water to non-gaseous water. After the reaction is complete, the reactor is depressurized, allowing water to escape as vapor. Due to the evaporation of the largely liquid reaction water, the reactor cools rapidly, and the reaction product must be further dried using additional energy, according to the experimental description in EP 3 319 931 B1.

[0014] A disadvantage of the process according to EP 3 319 931 B1 is the high internal reactor pressure due to the closed pressure mode. To enable the storage of reaction energy in the reactor, temperatures of up to 180 °C and an overpressure of 10 bar are described. The overpressure essentially corresponds to the water vapor pressure at the set temperature.

[0015] Deviating from the process description according to EP 3 319 931 B1, for the use of raw material mixtures of calcium oxide / calcium hydroxide containing ≥ 20% calcium hydroxide, vapor pressures greater than 10 bar are theoretically required to store the reaction energy. Implementation with conventional solid-state mixers / reactors is not possible. The overpressure described in the process of up to 10 bar, or theoretically greater than or equal to 10 bar, leads to safety risks and thus to limitations with conventional solid-state mixers / reactors.

[0016] Due to this technical limitation of conventional reactors, the storage of reaction energy through pressurized operation described in EP 3 319 931 B1 is only possible to a fraction of it; the remainder must be released unused, for example, by opening the reactor. This results in a significant amount of external heat being introduced into the system for drying, contrary to the economically viable use of the released reaction energy.

[0017] The present invention is therefore based on the object of providing a novel process for the production of calcium dipropionate, which operates at more moderate pressures and efficiently utilizes the resulting reaction heat.

[0018] This object is achieved in a first aspect of the present invention by a process for the preparation of calcium dipropionate, comprising the steps a) introduction of a solid mixture of 0% to 100% calcium oxide and 100% to 0% calcium hydroxide into a pressure-resistant solid mixing reactor, b) Start of the moderation of the internal pressure of the pressure-resistant solid-mixing reactor by introducing inert gas, c) Addition of propionic acid to the solid mixture, d) chemical conversion of the solid mixture, f) Obtaining solid calcium dipropionate after the end of the chemical reaction, characterized in that the chemical reaction takes place in an inert gas atmosphere with which the reaction pressure is constantly regulated between 2 bar and 6 bar during the chemical reaction.

[0019] The term "pressure-resistant solids mixing reactor" in the context of the present invention refers to a design of a solids mixer. In addition to the basic operation of mixing, this process apparatus produces products discontinuously from raw materials through chemical reactions. The solids mixing reactor is technically sealed within a defined pressure range. Pressure regulation of the solids mixing reactor is possible through connected fittings and technical devices. The solids mixing reactor is equipped with a double jacket and insulation. Fluids can be passed through the double jacket. In the following description, the solids mixing reactor is also referred to as the "reactor" for short.

[0020] Any suitable inert gas can be used for the inert gas atmosphere, although nitrogen is preferred for the present invention.

[0021] The process according to the invention is carried out in particular as a discontinuous batch process.

[0022] The present invention relates to the process according to the invention for the production of calcium dipropionate salts, particularly as a solid-state process, under constantly controlled pressure conditions between 2 bar and 6 bar. In the process according to the invention, a portion of the reaction heat released during the reaction is advantageously utilized for the continuous evaporation and extraction of the water formed in the reaction. Another portion of the latent reaction heat is stored in the system during the reaction. This leads to a reduction in the evaporation energy required for drying after the reaction and to a more stable and reproducible process.

[0023] By controlling the reaction pressure between 2 bar and 6 bar according to the invention, a significant disadvantage of the aforementioned prior art is overcome, in that pressure-resistant solids mixing reactors do not have to be designed for excessive pressures of 10 bar and more. The design for lower pressures is not only less complex and material-intensive, but also more cost-effective and environmentally friendly.

[0024] The pressures used in this description are absolute pressures.

[0025] In a further development of the process according to the invention, the temperature during the chemical conversion is adjusted to a range of 100 °C to 160 °C. The temperature, as an expression of the heat in the reactor, depends on the quantity and mixing ratio of the raw materials calcium oxide / calcium hydroxide and on the dosing rate of the propionic acid.

[0026] By limiting the temperature to a maximum of 160 °C, the system used for the process can be designed with less technical effort and thus constructed significantly more cheaply.

[0027] To increase energy efficiency compared to the prior art, it has proven advantageous in the process according to the invention if excess reaction heat is transferred to a heat transfer medium. This heat transfer medium can be, for example, a thermal oil.

[0028] This also ensures that neither the temperature nor the pressure in the system reaches a critical value.

[0029] In a further training, the chemical conversion is carried out in 0.5 h to 2.0 h.

[0030] An advantageous embodiment of the process according to the invention provides that, following the chemical reaction in step d), the reaction pressure is reduced in a step e) using a defined gradient over a period of 15 to 90 minutes. The gradient runs exponentially, quasi-asymptotically, against the ambient pressure.

[0031] By reducing the pressure, water is evaporated from the reaction product in a controlled manner, so that on the one hand it is dried and on the other hand it is further moderately cooled.

[0032] In another further development, the excess reaction heat transferred to the heat transfer medium is recoupled parallel to or after step e).

[0033] Once the pressure reduction has progressed or been completed, the reaction heat previously removed from the system can now be re-injected for further drying of the reaction product without requiring significant additional external energy. This further increases the energy efficiency of the process according to the invention.

[0034] It is preferred if the resulting solid calcium dipropionate, after the pressure reduction in step e) to 1.2 bar to 1.0 bar, has a residual moisture content of 2% to 5%, which is present essentially as water of crystallization. At this residual moisture content, the resulting solid calcium dipropionate is free-flowing.

[0035] For further drying of the reaction product, it has proven advantageous in a further development if, following the pressure reduction in step e), the solids mixing reactor is placed under vacuum, whereby the resulting solid calcium dipropionate has a residual moisture content of 1% to 4%. The vacuum has a negative pressure of up to 0.3 bar.

[0036] This also serves to extract further energy from the reaction product in order to prevent unwanted condensation in the downstream processes.

[0037] The process according to the invention prefers an energy balance in which the heat released during the reaction is greater than the heat required for water evaporation and plant-specific losses and limitations, so that after step e), the pressure reduction to 1.2 bar to 1.0 bar, a residual moisture content of 2% to 5% results in the resulting solid calcium dipropionate. For this purpose, the molar ratio of calcium oxide to calcium hydroxide in step a) must be adjusted to the overall energy balance and plant-specific losses and limitations.

[0038] The adjustment of the molar ratio of calcium oxide to calcium hydroxide in step a) can be carried out in particular by separate gravimetric weighing of the required batch size.

[0039] In addition, the setting of the molar ratio depends on other factors that cannot be specified in general terms. These factors include the weather, as air pressure and humidity, for example, have an influence, and the geometry of the reactor used, which affects the mass conversion.

[0040] Further objects, features, advantages, and possible applications will become apparent from the following description of non-limiting embodiments of the invention. All described features, individually or in any combination, constitute the subject matter of the invention, regardless of their summary in the claims or their reference to each other.

[0041] The process according to the invention describes the production of calcium dipropionate as neutralization of propionic acid with alkaline calcium salts (calcium hydroxide (Ca(OH)2) and calcium oxide (CaO)) presented as a moderated, discontinuous solid-liquid reaction.

[0042] The reaction is carried out in a pressure-resistant solid-state mixed reactor. This reactor is equipped with a temperature-controlled heat transfer medium for heat storage and heat transport. The storable heat output is adjusted to the amount of reaction heat.

[0043] A mixture of calcium hydroxide and calcium oxide is placed in the solid-state mixing reactor. By adjusting the molar ratio of calcium hydroxide to calcium oxide, the total reaction heat released in the reactor can be controlled. The amount of propionic acid to be added is stoichiometrically adjusted to the total amount of calcium in the reactor.

[0044] Calcium hydroxide and calcium oxide react differently with propionic acid. Calcium oxide reacts in a preliminary reaction [Reaction 1] to form calcium hydroxide. This reaction, also known as slaking, is highly exothermic.

[0045] Calcium hydroxide reacts exothermically with propionic acid [Reaction 2]. The overall reaction of calcium oxide differs from the reaction of calcium hydroxide in the reaction energy and the amount of water released as a byproduct. CaO + H2O → Ca(OH)2 Reaction 1 Ca(OH)2 + 2 CH3CH2COOH → Ca(CH3CH2COO)2 + 2 H2O Reaction 2 Ca(CH3CH2COO)2 + H2O ↔ [Ca(CH3CH2COO)2 · H2O] Reaction 3

[0046] The mixture is homogeneously mixed throughout the entire residence time in the reactor. This promotes the conversion of propionic acid to calcium dipropionate.

[0047] The pressure in the reactor is adjusted before the start of the reaction using an inert gas, e.g., nitrogen. The inert gas content in the reactor atmosphere allows the reactor pressure to be kept constant even as the reaction begins. The reactor pressure should be set between 2 bar and 6 bar so that the vapor pressure of the propionic acid and any possible evaporation during the reaction are negligible.

[0048] Propionic acid is added to the solid mixture. At the beginning of the addition, the overall reaction consisting of [Reaction 1] and [Reaction 2] predominates, since the water byproduct of [Reaction 2] is also a starting material for [Reaction 1]. The simultaneous occurrence of both reactions releases large amounts of reaction heat.

[0049] To control the heat flow of the reactions, the dosing rate and dosing time of the propionic acid can be adjusted depending on the available reactor volume (contact area to the heat transfer medium) of the amounts of substance used.

[0050] Depending on the molar ratio of calcium oxide and calcium hydroxide used and the progress of the propionic acid dosage, the slaking reaction [Reaction 1] ends. Further released water is in thermodynamic equilibrium with the subsequent reaction [Reaction 3] between the forward and reverse reactions. Elevated temperatures above 100 °C promote the reverse reaction, i.e., the decomposition of calcium dipropionate monohydrate into calcium dipropionate and free water.

[0051] The reaction heat is transferred to the entire reactor contents and the reactor itself. The resulting temperature increase can cause the reactor pressure to increase due to the vapor pressure of the released water.

[0052] The reaction heat can simultaneously be transferred to the heat transfer medium for storage or transport until it has absorbed the necessary amount of heat for final drying.

[0053] The reactor pressure is set to a value lower than the resulting vapor pressure. This leads to continuous evaporation and release of water vapor from the reactor, thus resulting in water reduction. A controlled pressure between 2 bar and less than or equal to 6 bar is recommended. At the same time, the open reactor system maintains a moderate temperature between 100 °C and 160 °C.

[0054] The open reactor system is preferably ensured by an adjustable control valve on the reactor, which opens when the maximum adjustable internal pressure is reached and releases, in particular, water vapor. This not only achieves the necessary water reduction in the system but also prevents an undesirable or impermissible pressure increase.

[0055] After the chemical conversion is complete, no more latent heat of reaction is released. The heat storage can then be used to maintain water evaporation. The evaporation cools the reactor with the reaction product. To maintain the release of water vapor, the reactor's pressure is gradually reduced. The pressure reduction occurs via a defined gradient down to a pressure of 1.2 bar to 1 bar.

[0056] Due to evaporation during the reaction and pressure reduction, the residual moisture can be reduced to a level of less than or equal to 5 wt.%.

[0057] To reduce the residual moisture content to a defined value between 1 wt.% and 4 wt.%, the pressure reduction gradient is extended into the negative pressure range. The reactor system is briefly placed under vacuum, preferably with a negative pressure of up to 0.3 bar, while maintaining the temperature.

[0058] At the end of the process according to the invention, the reaction product calcium dipropionate is present as a free-flowing solid with a defined residual moisture content of less than or equal to 4 wt.%.

Claims

[1] Process for the production of calcium dipropionate, comprising the steps a) Placement of a solid mixture of 0% to 100% calcium oxide and 100% to 0% calcium hydroxide in a pressure-resistant solid mixing reactor, b) Initiation of the moderation of the internal pressure of the pressure-resistant solid-state mixed reactor by supplying inert gas, c) Addition of propionic acid to the solid mixture, d) chemical reaction of the solid mixture, f) Obtaining solid calcium dipropionate after the end of the chemical reaction, characterized by , that the chemical reaction takes place in an inert gas atmosphere, with which the reaction pressure is constantly regulated between 2 bar and 6 bar during the chemical reaction. [2] The method of claim 1, wherein the temperature during the chemical reaction is set to a range of 100 °C to 160 °C. [3] Method according to claim 1 or 2, wherein excess reaction heat is transferred to a heat transfer medium. [4] Method according to any one of claims 1 to 3, wherein the chemical reaction is carried out in 0.5 h to 2.0 h. [5] Method according to any one of claims 1 to 4, wherein following the chemical reaction in step d) the reaction pressure is reduced in step e) by means of a defined gradient over a period of 15 min to 90 min. [6] Method according to any one of claims 1 to 5, wherein in parallel to or after step e) the excess reaction heat transferred to the heat carrier is re-coupled. [7] Method according to any one of claims 1 to 6, wherein the solid calcium dipropionate obtained after pressure reduction in step e) to 1.2 bar to 1.0 bar has a residual moisture content of 2% to 5%. [8] Method according to any one of claims 1 to 7, wherein following the pressure reduction in step e) the solid mixing reactor is placed under vacuum, whereby the solid calcium dipropionate obtained has a residual moisture content of 1% to 4%. [9] Method according to any one of claims 1 to 8, wherein the stoichiometric ratio of calcium oxide to calcium hydroxide in step a) is defined by the overall energy balance in which the heat of reaction released is greater than the sum of the necessary heat of vaporization of the water and plant-specific losses, such that after step e), the pressure reduction to 1.2 bar to 1.0 bar, a residual moisture content of 2% to 5% is obtained in the solid calcium dipropionate obtained.