MXPA01004603A - Process for production of esters - Google Patents

Abstract

A process for producing an ester comprises the steps of:(a) feeding to a first vessel a feed that comprises organic acid, alcohol, and water, whereby organic acid and alcohol react to form monomeric ester and water, and whereby a first liquid effluent is produced that comprises as its components at least some ester, alcohol, and water, the components of the first liquid effluent being substantially in reaction equilibrium;and (b) feeding thefirst liquid effluent to a second vessel, whereby a vapor product stream and a second liquid effluent stream are produced, the vapour stream comprising ester, alcohol, and water, wherein the second vessel is maintained substantially at vapor-liquid equilibrium but not substantially at reaction equilibrium.

Description

PROCESS FOR SHORT STEREO PRODUCTION DESCRIPTION OF THE INVENTION The present invention relates to processes for producing esters from the reaction of an organic acid in an alcohol. Esters are organic chemicals of significant industrial importance, for example for use as solvents and as reagents. One way to form esters is by reacting an organic acid as an alcohol to form an ester and water, as shown in reaction (1) Ri-COOH + R2-CH2-OH < ~ > R! -COO-CH2R2 + H20 Reaction (1) organic acid + alcohol < "ester + water Many processes or systems of esterification focus on the removal of water to boost yield or conversion.The removal of water diverts the equilibrium towards the products shown on the right side of Equation (1). for esterification it has been applied in a wide range of organic acids and alcohols, for example, long-chain alcohols that form heterogeneous azeotropes with water can be used for the removal of water in the vapors from the reaction. It is easy to use a higher alcohol that is sufficiently high boiling and slightly soluble in water, and excess alcohol can be used to boost the reaction.

Alternatively, an agent that forms added azeotrope can be used to remove water in the case of esterification using lower alcohols such as ethanol and methanol. An example of an agent that forms a suitable azeotrope is benzene. However, this proposal for removal of water to drive the reaction has been found less successfully with systems wherein one or both of the reactive components tend to form dimers, oligomers, polymers or secondary reaction products when dehydrated. The esterification of lactic acid is such an example. In these cases, progress has been constrained by the problem that while water removal is necessary to boost the reaction equilibrium (1), this removal of the water at the same time also produces undesirable secondary dehydration reactions such as of various dimers and oligomers or lactic acid. This leads to loss of performance. The problem of oligomer formation has been partially overcome in the past by the addition of significant excess of alcohol, which tends to suppress the reactions that lead to the formation of dimers and oligomers. However, this proposal has not been completely successful and has also led to higher costs for product recovery due to high levels of excess alcohol that must be withdrawn. Another proposal that has been used is a simple reactor wherein a batch of lactic acid or other acid is dehydrated, or is initially present at high concentration and is heated in such a way that when the alcohol is introduced into the container, after the ester, the excess of alcohol, and the water formed in the reaction are suddenly evaporated from the reaction vessel. An example of lactic acid esterification is given by Gabriel et al (US Patent 1,668,806) who prepared 1-butyl lactate by dehydrating 70% lactic acid with excess 1-butanol at 117 ° C, followed by catalyst addition HCl, followed by reflux and esterification with addition of excess 1-butanol and extracting at the steam outlet an azeotrope of water of 1-butanol. The process involves the dehydration of the system and the removal of the water before the esterification stage. Bannister (US Patent 2,029,694) discloses a method for producing esters having boiling points of at least 120 ° C. The lactic acid and the acid catalyst are charged to a reactor and heated to the boiling point of the ester or not less than 20 ° C below this temperature. The alcohol is introduced into the reactor below the surface of the hot partially dehydrated acid. The ester, water of reaction, and excess of alcohol are separated at the outlet of steam. For example, methyl lactate is formed at temperatures of 130 to 140 ° C by introducing methanol into partially dehydrated lactic acid. The distillate from the steam outlet is 8-10% water, 42-42% methanol, and 50% methyl lactate by weight. For every 4.8 moles of methyl lactate produced in the system a total of 17.9 moles of methanol are fed into the system. Most or all of the water captured at the steam outlet (5.0 moles) is produced by the esterification reaction. The effective feed water level is 0.2 moles. This means that the feed streams are essentially free of water. Weisberg, Sti pson and Miller (US Patent 2,465,772) mix lactic acid substantially free of water with 3 to 20 parts by weight of aliphatic alcohol of 1 to 3 carbon atoms, by reacting the mixture below the boiling point and then suddenly evaporating the mix at a high temperature. For example, in the case of the formation of methyl lactate, the mol ratio of methanol to lactic acid is at least 8.5: 1. It can be as high as 56 moles of methanol per mole of lactic acid. Filachione and Fisher (Industrial Engineering and Chemistry, Volume 38, page 228, 1946) present another example of such technology. Its scheme involves bubbling excess hot alcohol, such as methanol vapor, through a solution of partially dehydrogenated lactic acid heated to a temperature above the boiling point of the alcohol, whereby the produced lactate ester is removed with the vapors of alcohol and any water produced from the reaction. Approximately 9 moles of methanol are required per mole of lactic acid from an 82% solution. Dramatically larger amounts of methanol are required for more dilute lactic acid feed solutions. These methods described above typically require excess alcohol and also dehydrated lactic acid. They are not energy efficient and they also require equipment that is large and expensive. In cases where dehydrated lactic acid is used, the reaction temperature is typically close to that of the boiling point of the ester. In each case the reaction is conducted at the same temperature as the boiling or mass transfer. An alternative proposal is to try the reaction without dehydrating the lactic acid first. The following two references illustrate previous attempts that have been made to effectively use that proposal. Wenker (U.S. Patent 2,334,524) describes a process wherein an esterification reactor and an adjacent hydrolysis reactor feed steam to a common distillation column. The alcohol is removed from the top of this column and returned to the esterification reactor. The liquid product at the bottom of the column comprises mainly water and ester is fed to the hydrolysis reactor. The ester is continuously hydrolyzed in that reactor to form free acid. The organic acid is a concentration of 70 to 85% and approximately 1.5 moles of alcohol are used per mole of organic acid. The reaction times are 12 to 16 hours for this system. The fractionation column is located immediately on top of the two reactors. This process seems to use a relatively low level of alcohol, but this is really the initial load to the system in an unstable state. The system is only suitable for batch esterification. At the end of the batch, the ratio of methanol to lactic acid in the esterification reactor will be very high, as much as 20: 1 or more. This is because during the batch the acid is gradually removed from the esterification reactor while the alcohol is continuously returned during the run. The process will be quite energy intensive since there will be a need to take large amounts of methanol or alcohol into the steam outlets towards the middle and end of the batch. This process is not suitable for efficient continuous operation, on a large scale. The effective average ratio of methanol to lactic acid would be close to 10: 1 if this process were run in multiple containers in such a consideration as to allow continuous operation. The vast effective excess of methanol is needed to obtain high performance in this configuration of equipment and process. Franke, Gabsch and Thieme, (East German Patent 206 373, January 25, 1984) describes a similar process for the formation of lactic acid esters of Cl, C2 and C3 alcohols with the reaction and evaporation occurring under reduced pressure and temperature in an empty evaporator and modified recirculation. They use vacuum to operate the equipment at temperatures below the boiling point of alcohol and use high levels of sulfuric acid as the catalyst. For example, in their example 2, they load 2.0 of concentrated sulfuric acid in 15 liters of 80% unrefined lactic acid at 50 Torr of pressure. This is about 15% w / w sulfuric acid in the initial lactic acid load. The system is heated to 60 ° C under vacuum, and then 15 to 20 liters of methanol are added slowly and continuously below the surface. As soon as the methanol comes in contact with the lactic acid, it reacts with the lactic acid and tends to evaporate and bring the mixture of hot alcohol acid ester into the portion of the reactor heating tube and to the part of the reactor that evaporates suddenly. . The system is mixed and operated in such a way that the three portions of the reactor - the heating zone, the evaporation zone, and the supply zone - are essentially at the same pressure, and in such a way that the temperatures in each area are similar or perhaps the temperature in the area of sudden evaporation is greater than that in the container due to the consumption of heat. The liquid in the container is drawn upwards by convection through the heat exchanger inside the steam chamber. The liquid flows by gravity back to the container. The chamber produces the liquid that drains back into the container and the vapor. The resulting vapor output condensate is 50% by weight methyl lactate. Since the liquid runs back by gravity, so the evaporation chamber and the container must be at similar pressures. If the evaporation chamber was at reduced pressure, then the liquid would not run back into the container. Consequently, the system operates essentially identically to a simple heating reactor. Other such systems can be contemplated, for example heat could be applied in an external recirculation loop, or with internal coils, or with a jacket. In each case the steam would be extracted from the steam outlet as shown in this patent. This invention is limited to C1-C3 alcohols, low pressures, and low reaction temperatures. These low reaction temperatures require the aforementioned high levels of sulfuric acid to maintain the reactor volume at an economical size. However high levels of sulfuric acid, even at low temperatures, could lead to secondary reaction products of dehydration such as dialkyl ethers derived from alcohols, and also various products of the degradation of lactic acid in the presence of sulfuric acid. The relative extent of these secondary reactions to these low temperatures is not known, but they will be smaller since the yield reported in the patent is 96-98% for a prolonged run. For the ethyl lactate example, the system temperature would not exceed 78 ° C. this system is quite similar to the early references where the alcohol mixture is introduced below the surface into the mixture of lactic acid and a mixture of water, ester and alcohol removed in the steam from the steam outlet. In their claim 2 they note that system pressures of 15 to 50mm Hg and temperatures of 40 to 65 ° C are favored. Their claims 3, 4 and 5 present how a modified vacuum-recirculation evaporator can be used to overcome the operation of its process in a semi-continuous (ie semi-batch) mode. This involves loading the system with a batch of lactic acid, then operating the system with a continuous alcohol feed. After a certain amount of run time, the lactic acid is exhausted from the reactor device and a new batch is loaded. This equipment involves a thermosiphon heat exchanger (2) that extracts the liquid from the reactor inside a vapor-liquid separation chamber (3) from which the liquid drains back into the reactor (1). A valve (9) regulates the flow through the heat exchanger. No mention is made of the need to balance the temperaturepressure, concentration, retention time and catalyst concentration or the need for sufficient water to ensure successful operation. The mol ratio of methanol to lactic acid in this process can be calculated from a typical example. A feed of 15 to 20 liters of methanol is contacted with 80% w / w lactic acid. Taking into account the densities of these feeds, this represents a molar ratio of alcohol to acid from 3.3: 1.0 to 2.5: 1.0. The molar ratio of water feed to lactic acid is approximately 1.24: 1. It is important to note that the methanol feed for the Franke process is not inside the reactor just inside the inside of the riser tube that feeds the evaporator / heat exchanger. The methanol will come into contact with the hot lactic acid and tends to evaporate suddenly, bringing the hot mixture of methanol and lactic acid vapor and liquid, together with water and methyl lactate up through the heat exchanger to the chamber. instant evaporation. Thus the reaction with the methanol occurs mainly as the methanol is added to evaporate and bring the hot boiling mixture to the reaction line through the heat exchanger to the flash evaporation area. The residence time and the temperature of the flash unit and the reactor unit will be similar. The key difference between the Franke process and those of Filachione and Wenker is this equipment and the way of introducing the methanol in such a way that the liquid and the boiling vapors are carried upwards into the heat exchanger. Their devices do not allow operation at temperatures above the normal boiling point of alcohol, they have no way to separate the catalyst, and they use an unusually high level of catalyst 15% w / w sulfuric acid. Steam from the steam outlet is 50% w / w on the basis of lactic acid in the form of methyl lactate. This is a high level of methyl lactate in the steam from the steam outlet. The process however is limited in the scope of operation. Datta and Tsai (US Patent 5,723,639) present a more modern proposal that uses pervaporation or steam permeation to achieve dehydration. A three-stage system comprising a reactor, a water permeation system and an ester permeation system is used. A pervaporation membrane is used to permeate the water formed in the reactor. The water permeates within a vapor space of reduced pressure. A second separation membrane system is used to selectively remove the ester formed in the reaction. As in previous technologies, the reaction is driven by a dehydration step. The question that is not effectively addressed in the above patents is how to drive the reaction to high conversion levels while minimizing (1) the production of unwanted by-products such as dimers, oligomers, and polymers, (2) minimizing the use of energy, and (3) effectively allowing continuous operation with low-cost capital equipment. There is a long-term need for improved processes to produce esters of organic acids. The current invention concerns the separation of time scales. The discovery shows that under selected conditions, the reaction is minimized in the equipment where the mass transfer occurs, and thus the formation of undesired secondary reaction products such as dimers and oligomers is avoided. One aspect of the present invention is a process for producing an ester, comprising the steps of: (a) feeding a first feed containing organic acid, alcohol, and water into a first container, whereby the organic acid and the alcohol react to form monomeric ester and water, and thereby produce a first tributary liquid comprising as its components at least one ester, alcohol and water, the components of the first tributary liquid being substantially in reaction equilibrium; and (b) feeding the first tributary liquid to a second container, whereby a vapor product stream and a second tributary liquid stream are produced, comprising the vapor stream, ester, alcohol and water, wherein the second container it is substantially maintained in vapor-liquid equilibrium but not substantially in reaction equilibrium. In one embodiment of the invention, the first container is operated at a pressure Pi and the second container is operated at a pressure P2, where Pi and P2 are substantially the same, and the average residence time of the feed in the first container it is at least 10 times longer than the average residence time of the first tributary liquid in the second container. In another embodiment, the first container is operated at a temperature Ti and the second container is operated at a second temperature T2 that is sufficiently lower than Ti so that the contents of the second container are not substantially close to the reaction equilibrium.

In yet another embodiment, the first container is operated at a pressure P which is about 30-500 psig and the second container is operated at a pressure P2 which is about 1-14 psia. In yet another embodiment, catalyst is added to the first vessel in an amount sufficient to catalyze the formation of the ester, and wherein at least some of the catalyst is removed from the first tributary liquid before it enters the second vessel, so that the contents of the second container are not substantially close to the reaction equilibrium. In some more specific versions of this embodiment of the process, the catalyst is heterogeneous in the first container and is not substantially present in the second container. Alternatively, the catalyst is homogeneous in the first container and is removed substantially before the second container by means of washing. Characteristics of various process modalities that can achieve the necessary separation of the time scales may include: the temperature and pressure in the first container that are greater than in the second container; the second container is operated under a higher pressure than atmospheric but with a short residence time (for example approximately 1 to 10 minutes); the first vessel is operated at a liquid temperature of 150 to 220 ° C and the second vessel is operated with a steam outlet temperature of 30 to 100 ° C; the first vessel is operated under pressure greater than atmospheric pressure; or the first vessel is operated substantially in a liquid vessel with pressures sufficient to substantially suppress vaporization at temperatures up to 220 ° C without any added catalyst. In these latter systems, the organic acid (eg, lactic acid) itself acts as the catalyst for the reaction. In a preferred embodiment, the organic acid is selected from the group consisting of mono- and tricarboxylic acids having 3-8 carbon atoms. In another preferred embodiment, the organic acid is selected from the group consisting of acetic acid, succinic acid, citric acid, malic acid, lactic acid, hydroxyacetic acid, pyruvic acid, itaconic acid, formic acid, oxalic acid, propionic acid, beta-acid. hydroxybutyric, and mixtures thereof. The alcohol is preferably an aliphatic alcohol having 1-20 carbon atoms, more preferably an aliphatic alcohol having 1 to 12 carbon atoms. Alcohols that are presently especially preferred include i-butanol, t-butane, n-butanol, i-propanol, n-propanol, ethanol, and methane. The feed for the first container can be created by feeding a simple mixing stream to the container. Alternatively, a plurality of streams may be fed, each containing one or more components for the feed mixture. For example, a feed stream may consist essentially of water. Preferably the feed comprises sufficient water to substantially suppress the formation of lactic acid oligomers and oligomers of ethyl lactate in the first tributary liquid and the second tributary fluid. In one embodiment of the invention, the first tributary liquid is substantially liquid. Preferably the vapor product stream of the second container comprises at least 5% by weight each of ethyl lactate, ethanol and water. The second tributary liquid can optionally be recycled to the first container. Food can take a variety of forms. For example, the feed may contain lactic acid, oligomers of lactic acid and ethanol. As another example, the feed mixture may comprise one or more of polylactic acid, polylactide, polyhydroxybutyrate, or one or more other polyesters based substantially on alpha or beta hydroxy acids, pure or mixed, and may additionally comprise water. As yet another example, the feed may comprise more than one organic acid alcohol, and as a result, mixed esters are formed. However, in this mode the boiling points of alcohols, esters, and water preferably does not have a range of more than 110 ° C from the lowest boiling point to the highest boiling point. The present invention is quite useful in conjunction with fermentation processes, so that the feed may comprise unrefined or partially purified broths derived from the fermentation of sugars which has been treated to form an acid pH stream. In that situation the feed mixture will also typically comprise one or more impurities selected from the group consisting of inorganic salts, protein fragments, sugar residues, ketones and metal ions. In a specific embodiment, the feed mixture comprises lactic acid which is substantially optically pure, which is at least 90% optically pure and ideally 99% or more optically pure. Instead of using a first single container and a second single container, the process may employ a plurality of either of the two, operated in series. For example, in one embodiment, the second container is divided into various sub-vessels operated in series, each with temperature, pressure, catalyst, and average residence time such that they will operate with liquid vapor exit streams that are not substantially in reaction equilibrium, such that the vapor product stream of each sub-vessel is richer in alcohol and water than the vapor product stream of the subsequent sub-vessel. If the feed comprises polylactic acid, the feed may optionally be pre-treated with hot water at temperatures of about 240 ° C and high pressures of 500 psig before entering the first container. It will normally be desirable to include in the process the steps of dehydrating and purifying the vapor product stream and separating the ester and alcohol stream therefrom. The process and equipment of the present invention are efficient for reaction systems wherein the acid or alcohol feed or both tend to form unwanted dimers, oligomers, and polymers when dehydrated which reduce the yield of the desired ester product. A two-stage process and equipment is used to achieve the separation of the time scales for the equilibrium of the reaction and for the equilibrium of the mass transfer while maintaining high levels of water needed to suppress the formation of unwanted oligomers. . This invention is advantageous where the ester product is more volatile than the organic acid feed. A suitable recirculation reactor system can include two containers, or groups of containers. The first container is operated to give a product that is substantially close to the reaction equilibrium. The second container involves vapor-liquid equilibrium and phase change and produces a liquid reaction effluent that is substantially far from the reaction equilibrium. The vapors removed from the second container or containers include water, ester, and alcohol. A recirculation stream can be passed from the second group of containers back to the first container, device or group. For successful operation the conditions of temperature, pressure, retention time, reagent concentration, and catalyst levels in the first vessel or group should be such that they approximate the reaction equilibrium, while the conditions in the second vessel or group they must be such that the reaction equilibrium in the liquid affluent does not substantially approach. For this invention, sufficient water is required as part of the feed for the esterification reaction. If the water content of the alcohol and acid used as feed of raw materials to the system is too low, then water must be added. This is unusual since esterification reactions are normally driven by water withdrawalIn addition to requiring water in the feed, the appropriate balance of alcohol and acid feed rates is necessary for the best operation. The process and equipment of the present invention can operate at high water levels in the reaction system and with good energy efficiency and good performance. This invention is of particular value for esters made by reacting methanol, ethanol, propanol, isopropanol, isobutanol, tert-butanol, 1-butanol or pentanols with any of the following organic acids: • aliphatic alpha hydroxy monocarboxylic acids containing 2 to 10 carbon atoms, such as hydroxyacetic acid, lactic acid and alpha hydroxybutyric acid; • beta hydroxy monocarboxylic aliphatic acids containing from 2 to 10 carbon atoms, such as β-hydroxy propionic acid and 2-hydroxy butyric acid, • gamma and delta hydroxy acids; • other polyfunctional compounds that include acid and alcohol groups. The present invention differs from the processes of the prior art in various forms, such as the ability to use a wider range of alcohols, higher temperatures, less alcohol, and more water. The present invention makes use of the separation of containers in an esterification system to allow independent control of temperature, retention time, recirculation rate, pressure, vapor fraction, catalyst level and feed ratios. A feed dehydration step in the present invention is not required. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a process flow diagram for one embodiment of the present invention. The following definitions are used in this patent: Reaction equilibrium: the condition wherein the chemical species are in reaction equilibrium with a given simple phase, such as a liquid phase. Steam-liquid balance: the condition where there are two phases, one of vapor and one of liquid and there is a physical balance between them. Lactic acid: monomeric free lactic acid as it is commonly found is diluted in aqueous solutions. "88% lactic acid" and "commercial lactic acid" refers to a typical commercially available lactic acid which is currently a mixture of monomeric lactic acid, linear dimeric lactic acid or lactic lactic acid, short chain lactic acid oligomers , water, and also a small amount of cyclic dimer lactic acid or lactide. When this lactic acid is diluted in a large excess of water, it will hydrolyze or slowly convert to any monomeric form of lactic acid. Lactoyl ethyl lactate: This species is formed when the ethyl lactate reacts with lactic acid by means of an esterification reaction with the elimination of a single molecule of water, ethyl lactoyl lactate has three functional groups comprising two ester groups and a secondary alcohol group. Lactoyl ethyl lactate species can also be formed by the esterification reaction of lactoyl lactic acid with ethanol and the removal of water. Ethyl lactate: This ester is formed by the reaction of lactic acid and ethanol. Lactoyl-lactic acid: This species is formed when two molecules of lactic acid react by means of an esterification reaction with the elimination of a single molecule of water. Lactoyl lactic acid has three functional groups - a secondary alcohol, an ester and an organic acid. Lactate oligomers: short chain polyesters based on lactic acid. The smallest molecule referred to herein is lactoyl-lactoyl-lactic acid, which is a linear trimeric ester of lactic acid. The next in the series is lactoyl lactoyl lactoyl lactic acid which is a linear tetramer of lactic acid. Each molecule of lactic acid is linked to the next in the chain by an ester bond. All lactate oligomers have a terminal free organic acid, a free secondary alcohol group, and two or more ester linkages. Each time an additional lactic acid molecule is added to the polyester chain, one molecule of water is removed. Thus, the conditions that favor the withdrawal of water will promote the formation of these oligomers. Reciprocally, the addition of water will tend to drive the reaction equilibrium back to the monomeric lactic acid. Ethyl lactate oligomers: These refer to short chain polyesters similar to lactate oligomers, but where the free organic acid group has reacted with ethanol to form an additional ester linkage. Thus the ethyl lactate oligomers do not have any free organic acid group. Ethyl lactate oligomers can also be formed by the successive reaction of lactic acid with shorter chain ethyl lactate oligomers or with ethyl lactate. The key of this invention is the hitherto not considered concept that the time scale separation can not be used for further advantage in the esterification or mixtures that have to react to form oligomers or polymers upon dehydration. To achieve said time scale separation, the equipment in which the processes of equilibrium of reaction and vapor-liquid balance must be separated physically. The reaction equilibrium occurs in one container or group of containers while the vapor-liquid equilibrium occurs in another container or group of containers. The first container or group of containers may involve the vapor-liquid equilibrium as well as the reaction equilibrium. However, the second container or group of containers will have a liquid tributary that is not substantially in reaction equilibrium. This liquid tributary of the second container or set of containers can be recycled to the first container or set of containers. The separation of time scales from the two vessels can be achieved in a variety of ways. In one embodiment of the invention, the first and second containers are operated at the same pressure but the residence time in the first container is only just enough to reach 95% approximation to the reaction equilibrium and the residence time in the second container It is not more than a tenth of that in the first container. In another embodiment of the invention, the separation of time scales is achieved by lowering the pressure in the second vessel such that the temperature of the liquid tributary is sufficiently low so that the retransmission time of that liquid stream is not a substantial approximation to the temperature of the liquid stream. balance in the second container. This embodiment is normally most favored if the first container is operated under substantial pressure, such as 30 to 500 psig mixtures, such that the reaction temperature in the first container can be kept high and the reaction rate in the first container is high. The second container is then operated at pressures for example from 1 to 14 psia. This case leads to some other efficiencies in the use of energy. Still another embodiment of the invention is to achieve time scale separation by use of different levels of catalyst in the first and second containers. There must be sufficient catalyst in the first vessel to substantially approximate the reaction equilibrium. The catalyst used in this vessel may be in the form of a heterogeneous catalyst such as an ion exchange resin either located within the vessel or associated with the first vessel in a recirculation loop. The solid phase catalyst is then removed before feeding the reaction liquor to the second reactor. Alternatively, a homogeneous catalyst can be used such as sulfuric acid in the first container and then this catalyst can be removed by precipitation or ion exchange before feeding the second container. These proposals are contrary to a common proposal used for esterification, so-called catalytic distillation, where the mass transfer of the reaction are combined in a simple distillation column. Yet another embodiment of this invention involves combining two or more of the above proposals for retention time, temperature or pressure, and catalyst level to achieve the desired separation of the time scales. One embodiment of the present invention is shown in Figure 1, and includes a first container 4 and a second container 10 arranged in series. The reaction can be catalyzed by one or more of a number of methods. A heterogeneous catalyst can be stopped in an external recirculation loop comprising extraction pipe 31, pump 32, stream 33, heterogeneous catalyst bed 34 and return pipe 35. The homogeneous catalyst can be fed to the first container 4 by means of the feed pipe 36 or by means of the recirculation pipe 14. The reaction can also autocatalyze due to the acidity of one of the reagent feeds, organic acid feed 1. Other feed streams include alcohol 2 and optionally water 3. Note that enough water may need to be added if the feeds are essentially dehydrated. Also note that for optimal operation, the feed ratio 1 of organic acid, feed 2 of alcohol, feed 3 of water, and stream 14 of recirculation need to be adjusted correctly to balance the reaction conditions in the first vessel 4 and the conditions of mass transfer in the second container 10. The pressure and temperature in the first container 4 are preferably observed with pressure gauges 41 indicating the liquid temperature, vapor pressure, and optionally also steam temperature. The liquid level 6 in the first container 4 is up at the point at which the reaction product 5 is withdrawn from the liquid tributary of the container. The reaction product 5 of the first container 4 passes into a second container 10 by means of the final catalyst removal unit 37 and the optional valve 7. The catalyst removal unit 37 may be a filter or centrifuge or other solid-liquid or solid-vapor separation device in the case of heterogeneous catalysts. For homogeneous catalysis, the catalyst removal device can be a weakly basic anion exchange bed or a cold water washing step or another step to remove the homogeneous catalyst. In the preferred embodiment of the invention, such a device is not used since the time scale separation is achieved by the use of variables of temperature, pressure and retention time. The stream 38 passes to the pressure relief valve 7 and the reduced pressure stream 8 passes to a second container 10. Here the level 18 of liquid or amount of liquid present in this container is optimally maintained at a low level to minimize the holding time. The reaction in the first container 4 which is mainly in the liquid phase, then heat can be added to the liquid in the second container 10 by any means known in the art. By way of example, a recirculation loop is shown in which the liquid stream 11 passes to a pump 12 and the stream 13 is divided into the stream 14 and stream 15. The stream 15 is heated in a heat exchanger 16 and the stream 17 heated returns to a second container 10. Any other means for adding heat to the liquid in the container 10 readily known in the art can be used. The second container 10 also preferably includes devices 42 for measuring temperature and pressure. If the reaction in the first container 4 is mainly in the vapor phase, then the second container 10 may involve partial condensation instead of partial evaporation. The removal of heat from the liquid stream in the container 10 may be by any means known in the art. It is not illustrated in Figure 1. The steam removed from the second container 10 may be one or several different vapor streams. These steam streams 19 contain the ester product formed, water, and any excess alcohol. The vapors pass to any of a variety of separation process stages such as vapor phase permeation, partial condensation, absorption, adsorption, distillation, extractive distillation, or condensation and pervaporation. The purpose of these various processes 20 is to separate the ester product formed from the alcohol and water streams. one of the streams 21 of either of these processes will pass to some type of optional condensation system 22, possibly a receiver vessel. A product stream 27 is taken from the receiver 25 and eventually to a vacuum or pressure control system 28. This system 28 will be important because it will control the consequent pressure in the container 10. The control of the pressure in the container 10 is important to control the temperature and consequently the reaction rate in the container 10.

The receiver vessel 25 preferably includes devices 43 for measuring temperature and pressure. A background current can also be extracted from the receiver 25. As discussed above, the time scale separation requires that the reaction rate in the second container 10 be sufficiently low for mass transfer to occur without a substantial approach to equilibrium. reaction of the liquid phase tributary 14 of the container 10. Note that in the event that the system runs continuously, one or more of the feed streams may contain salts, sugars, or other non-volatile impurities. As the recirculation loop between containers 4 and 10 is operated over time, the concentration of these impurities in the system will rise at a point where they need to be removed by some method in process such as absorption, precipitation, filtration, etc. or alternatively as a purge stream 51. Although shown with a first single container 4 and a second single container 10, the process may alternatively use a plurality of said containers. For example, there may be a plurality of first containers arranged in series, with the liquid tributary of the latter in that series feeding the first in a series of second containers, also arranged in series.

Other arrangements that use multiple containers are also possible. The containers may for example have a continuous stirred reactor (CSTR), or pipe reactors or piston-type expense (PFR), or any other common container or groups of containers suitable for undertaking liquid phase or vapor reactions. The present invention can be further understood from the following examples: Example 1: Steam-Liquid Equilibrium in Container 2. A glass vessel was charged with a mixture of lactic acid, water, ethyl lactate and ethanol as shown in Table 1 It heated to the boil and stirred. The vapors from the steam outlet were extracted by means of an outlet steam pipe. A mixture of ethanol, ethyl lactate and water as shown in Table 1 was fed continuously for a period of 5 hours and 44 minutes by means of a liquid addition line below the surface. The heat consumption rate of the container was adjusted to have a constant level in the container. The liquid temperature in the kettle was approximately 105 ° C and the steam 101-103 ° C. Three samples of exit vapor taken during the run were analyzed and showed similar concentrations. The composition of the second said sample is shown in Table 1. The final kettle liquor is shown in Table 1 as well. Note that in this experiment the feed below the surface does not contain lactic acid. This test was to examine the vapor-liquid equilibrium conditions and to determine if an exit vapor rich in ethyl lactate could be removed from a broth that reacts slowly or does not react. The molar ratio of total ethyl groups to total lactyl groups in the exit vapor is in this case approximately 6: 1. The liquor is in reaction equilibrium K of 2.80, where K is the molar concentration ratio K = [ethyl lactate] * [water] / ([ethanol] * [lactic acid]). Here [] means mol / liter. This represents the limit of previously reported technology, where a relatively high ratio of ethanol to lactic acid should be used if the vapor-liquid equilibrium and the reaction equilibrium occur in the same container. To obtain the most efficient use of ethanol and lower steam and energy costs, a more effective method is needed to conduct the vapor-liquid reaction and equilibration process. Note that neither in this example nor in Example 2 do they represent a process that could probably be used commercially. This is because the feed in both examples is a mixture containing only ethyl lactate, ethanol and water. This steam is fed to the vessel as part of the study of vapor-liquid equilibrium conditions. The results of these studies are then used in calculation models of the processes that combine the reaction equilibrium in vessel 1 and the vapor-liquid equilibrium in vessel 2. Table 1 Example 2: Liquid Vapor Equilibrium in Container 2 This example differs from the case of Example 1 in that the container was run warmer and that the feed mixture was fed at a higher speed. The total time elapsed for the experiment was 3 hours and 2 minutes. No catalyst was added to this mixture, simulating with this a process where this reaction mixture is prepared using a heterogeneous catalyst in the reaction vessel. The analytical results for the samples of this reaction are shown in Table 2. The bottoms had a K ratio of 4.74, showing that in a non-reaction equilibrium, the liquid can be obtained in the bottoms which is in vapor-liquid equilibrium with the steam outlet. The average kettle temperature was 117 ° C and the average outlet steam was 110 ° C. The ratio of the total ethyl groups to the total lactyl groups in the exit vapor was 5.05: 1.0 in this case. Again, this example shows vapor-liquid equilibrium but does not show an operable process, because the feed mixture, as in Example 1, was a mixture of only ethyl lactate, ethanol, and water and did not contain acid lactic. Table 2 This example indicates that a more favorable ratio of ethyl groups to lactyl groups in the exit vapor can be obtained if the liquid in the final vessel is not in reaction equilibrium. Example 3: Simulated process with recirculation and two vessels. The vapor-liquid balance data was used to create a simulation model of two vessels connected with recirculation. The first vessel was operated in the simulation at sufficient temperature, pressure, retention time and catalyst level such that the liquid tributary is substantially in reaction equilibrium. The second vessel was operated at lower levels of one or more of temperature, retention time or catalyst level than the first vessel, such that the liquid tributary of vessel two did not approach the reaction equilibrium. In a 68,229 lb / hr feed of a wet ethanol stream containing 90% ethanol and 10% water was fed to vessel 1, along with 28,120 lb / hr of solution that was nominally 80% lactic acid and 20% of water. Either or both of the streams may contain substantial impurities, although they are not modeled with such in this example. In addition to the vessel 1, a recirculating stream of the container 2 was fed. The current may be in any of a range of flow rates as shown in the following example. For this example, the flow rate was selected at 59,500 lb / hr. The three feeding currents were combined. The K ratio of the molar reaction for the combined feed stream was 1.96. This was less than the equilibrium value of 2.85 and thus the reaction could proceed for the formation of ethyl lactate in the first container. The vessel 1 acted as a plug flow reactor and reached the reaction equilibrium. Ethanol, ethyl lactate, lactic acid and water were in reaction equilibrium. The equilibrium constant depends on the level of ethanol, the temperature, and the water level. Under these conditions, for purposes of illustration, a value of 2.85 was selected for the molar reaction equilibrium constant. The liquid tributary of vessel 1 comprises, approximately, the following: ethanol 52.908 lb / hr; water 18.446 lb / hr; ethyl lactate 69.494 lb / hr, and lactic acid 15,000 lb / hr. This illustrative purpose does not consider dimers or oligomers of lactic acid or ethyl lactate. However, in later examples we will see that this does not materially affect the general nature of the result. This example uses calculated estimates of liquid vapor equilibrium. The tributary of the container 1 was fed to the container 2. Sufficient heat and / or vacuum is applied to the container 2 to remove an exit vapor of the following composition: ethanol 49,900 lb / hr; water 16,900 lb / hr; ethyl lactate 29,500 lb / hr, and a small amount of lactic acid. The liquid residue in the container 2 comprises the equilibrium of the material: ethanol 3,000 lb / hr; water 1,500 lb / hr; ethyl lactate 40,000 lb / hr, and lactic acid 15,000 lb / hr. This liquid waste forms the 59,500 lb / hr that is recirculated to the first container. The molar reaction K ratio for current is 59.37. This was well above the equilibrium value 2.85 and shows that vessel 2 was not close to the reaction equilibrium. The liquid vapor equilibrium process that occurs in vessel 2 was roughly modeled here, and so this example was a close but not exact representation of the actual behavior. The K values of molar liquid vapor equilibrium in the model were as follows: ethyl lactate 0.5, water 1.0, and ethanol 3.8. Under similar conditions in Example 1 above, real experimental K-values of ethyl lactate of 0.49, water 1.05, and ethanol 3.80 were observed. Example 4: Varying the recirculation rate An example similar to Example 3 was modeled with variation of recirculation velocity. The ethanol feed was 15 ton / hour of 90% methanol 10% water. The lactic acid feed was 5.6 ton / hour of 80% nominal lactic acid with 20% nominal water, on a weight basis. The recirculation stream from vessel 2 to vessel 1 was varied from 5 ton / hour to as much as 1,000 ton / hour. It was found that with a recirculation rate of less than 11 tons / hour, there was insufficient lactic acid in the container 1 for the forward reaction to occur. So if the recirculation is insufficient, then the reaction can not proceed. At a recirculation rate of 10 ton / hour, the equilibrium constant of reaction in vessel 1 would need to be 3.091 for the reaction to proceed in some way. This is higher than the true value of 2.56 to 2.85 for this system. As the recirculation rate was increased to 80 tons / hour, the reaction ratio K of the reagents feeding container 1 dropped to 2061. In this proportion, the average reaction speed in vessel 1 will be larger. It was found that with recirculation speeds above 80 tons / hour, the K ratio began to increase again. Thus, in terms of average reaction speed, the recirculation speed of 80 ton / hour represents an optimum. Note, however, that increasing the recirculation ratio tends to increase the rate of mass flow through the reactor. The optimum reactor size is determined by a combination of the effect of the recirculation ratio on the velocity and the effect of the recirculation ratio on the velocity of the mass. The optimum for this set of conditions will be between 20 and 50 ton / hour. This represents a recirculation rate equal to 0.97 to 2.43 pounds per hour of recirculation per pound per hour of total feed streams. Example 5 A mixture of lactic acid, ethyl lactate, ethanol, and water was prepared by refluxing a mixture of ethyl lactate, lactic acid, water and ethanol overnight (for 12 hours) with 0.1% w / w of sulfuric acid at atmospheric pressure. A sample of this mixture was analyzed containing the following: ethanol 19.53% w / w; water 9.68% p / p; free lactic acid monomer 16.4% w / w; and ethyl lactate 48.95% w / w. A small sample of steam collected during equilibration on this sample contained the following: ethanol 80.41% w / w; water 19.56% p / p; free lactic acid monomer 0.018% w / w; and ethyl lactate 5.34% w / w. This shows that the vapor on a boiling reaction equilibrium mixture under these conditions contains relatively low levels of ethyl lactate. This reflux represents the reaction vessel 1. The liquid mixture is fed at 10 grams / minute to a cleaned film evaporator operating at 260 mm Hg absolute pressure. The heating element was adjusted to obtain a mass ratio 1: 1.98 of condensates of vapor outputs collected for liquor liquor. This represents a recirculation rate equal to 1.98 pounds per hour of recirculation per pound per hour of total feed streams. The cleaned film evaporator in this case is container 2. Note that the liquid tributary was not in reaction equilibrium for the formation of ethyl lactate. Also note that 2 different constants calculated for the formation of lactic acid dimers and for the formation of ethyl lactyl lactate oligomers that have increased dramatically at the bottoms of the cleaned film evaporator. This shows that the system was not in equilibrium reaction as it leaves this container 2. Note that the catalyst was not removed from the liquid. A cleaning speed of 120 rpm was used in the fixed film cleaning equipment. Table 3 Example 6 An experiment similar to that of the previous example was undertaken, but the bottoms were successively recirculated 4 times, identified here as Runs 1, 2, 3, and 4, of the container 2, the liquid vapor separation device or the film evaporator. cleaned, to vessel 1, the reaction equilibrium vessel. At each recirculation, fresh ethanol and 88% lactic acid were added to the bottoms before heating and reaction in vessel 1. Catalyst was added for the first cycle but not for subsequent cycles. The total mass of material processed in the reactor of vessel 1 in each cycle was, for runs 1 to 4 respectively, 3368 grams, 3083 grams, 2987 grams, and 2830 grams. This material was then fed to container 2. The quantity of exit vapor collected in each case was 1524, 1445, 1357, and 1349 grams. This represents a total of approximately 20 hours of operation for the four recirculation tests. Again, the value of K for the sample liquid in the final container 2 can be calculated and found to be very far from the reaction equilibrium. The exit vapors in each case are rich in ethyl lactate, the desired product. Table 4 The above description of the specific embodiments of the present invention is not intended to be a complete list of each possible embodiment of the invention. Those skilled in the art will recognize that modifications can be made to the specific embodiments described herein that would be within the scope of the present invention.

Claims (33)

1. CLAIMS 1. A process for producing an ester, characterized in that it comprises the steps of: a. feeding a first container a feed comprising organic acid, alcohol, and water, whereby the organic acid and the alcohol react to form monomeric ester and water and, therefore, a first liquid tributary is produced which comprises as its components the less any ester, alcohol, and water, the components of the first liquid tributary being substantially in reaction equilibrium; and b. feeding the first liquid tributary to a second container, whereby a vapor product stream and a second liquid tributary stream are produced, the vapor stream comprising ester, alcohol, and water, wherein the second container is substantially maintained in vapor-liquid equilibrium but not substantially in reaction equilibrium. The process according to claim 1, characterized in that the first container is operated at a pressure Pi and the second container is operated at a pressure P2, where Pi and P2 are substantially the same, and the average residence time of the feed in the first container is at least 10 times longer than the average residence time of the first liquid tributary in the second container. The process according to claim 1, characterized in that the first container is operated at a temperature Ti and the second container is operated at a second temperature T2 which is sufficiently lower than the Tx so that the contents of the second container are not they are substantially close to the reaction equilibrium. The process according to claim 3, characterized in that the first container operates a pressure Pi which is approximately 30-500 psig and the second container is operated at a pressure of P2 which is approximately 1-14 psia. 5. The process according to claim 1, characterized in that the catalyst is added to the first vessel in an amount sufficient to catalyze the formation of the ester, and where at least some of the catalyst is removed from the first liquid tributary before the second enters. container, so that the contents of the second container are not substantially close to the reaction equilibrium. The process according to claim 5, characterized in that the catalyst is heterogeneous in the first container and is not substantially present in the second container. The process according to claim 5, characterized in that the catalyst is homogeneous in the first container and is substantially eliminated before the second container by means of washing. The process of claim 1, characterized in that the temperature and pressure in the first container are greater than in the second container. 9. The process according to claim 1, characterized in that the second container is operated under a higher pressure than atmospheric but with short residence time. The process according to claim 1, characterized in that the first container is operated under a pressure greater than atmospheric pressure. The process according to claim 1, characterized in that the first container is operated substantially in a liquid phase with pressures sufficient to substantially suppress vaporization and at temperatures of up to 220 ° C without any added catalyst. The process of claim 1, characterized in that the first container is operated at a liquid temperature of 150 to 220 ° C and the second container is operated with a steam outlet temperature of 30 to 100 ° C. The process of claim 1, characterized in that the organic acid is selected from the group consisting of mono-, di-, and tricarboxylic acids having 3-8 carbon atoms. The process according to claim 1, characterized in that the organic acid is selected from the group consisting of: acetic acid, succinic acid, citric acid, malic acid, lactic acid, hydroxy acetic acid, pyruvic acid, itaconic acid, formic acid , oxalic acid, propionic acid, beta-hydroxybutyric acid, and mixture thereof. 15. The process in accordance with the claim 1, characterized in that the alcohol is an aliphatic alcohol having 1-20 carbon atoms. 16. The process according to claim 8, characterized in that the aliphatic alcohol has 1-12 carbon atoms. 17. The process according to claim 1, characterized in that the alcohol is selected from the group consisting of i-butanol, t-butanol, n-butanol, i-propanol, n-propanol, ethanol, and methanol. 18. The process in accordance with the claim 1, characterized in that more than one feed stream is fed to the first container, and one of the feed streams consists essentially of water. 19. The process according to claim 1, characterized in that the first liquid tributary is substantially liquid. 20. The process according to claim 1, characterized in that the organic acid is lactic acid. The process according to claim 20, characterized in that the vapor product stream of the second container comprises at least 5% by weight of each of the ethyl lactate, ethanol, and water. 22. The process according to claim 21, characterized in that the feed comprises enough water to substantially suppress the formation of lactic acid oligomers and ethyl lactate oligomers in the first liquid tributary and the second liquid tributary. 23. The process according to claim 1, characterized in that at least part of the second liquid tributary is recirculated to the first container. 24. The process according to claim 1, characterized in that the feed contains lactic acid, oligomers of lactic acid, and ethanol. 25. The process according to claim 1, characterized in that the feed comprises unrefined or partially purified broths derived from the fermentation of sugars that have been treated to form an acid pH stream. 26. The process according to claim 1, characterized in that the feed comprises one or more impurities selected from the group consisting of inorganic salts, protein fragments, sugar residues, ketones and metal ions. 27. The process according to claim 1, characterized in that the feed comprises lactic acid that is substantially optically pure. 28. The process according to claim 1, characterized in that the second container is divided into several sub-vessels operated in series, each with temperature, pressure, catalyst, and average residence time such that it operates with steam and outlet currents. of liquid that are not substantially in reaction equilibrium, and such that the vapor product stream of each sub-vessel is richer in alcohol and water than the vapor product stream of the subsequent sub-vessel. 29. The process according to claim 1, characterized in that the feed comprises one or more of the polylactic acid, polylactide, polyhydroxybutyrate, and wherein the feed additionally comprises water. 30. The process in accordance with the claim 1, characterized in that the feed comprises one or more polyesters substantially based on alpha or beta hydroxy acids, pure or mixed, and wherein the feed additionally comprises water. 31. The process according to claim 1, characterized in that the feed comprises polylactic acid and is pre-treated with hot water at temperatures of about 240 ° C and pressures of up to 500 psig before entering the first container. 32. The process in accordance with the claim 1, characterized in that it additionally comprises the steps of dehydrating and purifying the vapor product stream and separating the ester and alcohol stream therefrom. 33. The process according to claim 1, characterized in that the feed comprises more than one alcohol or organic acid, and mixed esters are formed, with the proviso that the boiling points of the alcohols, esters, and water do not have a range of more than 110 ° C from the lowest boiling point to the highest boiling point.