Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
1998-11-19
2003-12-16
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S231000, C560S098000, C560S205000, C560S179000
Reexamination Certificate
active
06664413
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to processes for producing esters from the reaction of an organic acid and 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 with an alcohol to form an ester and water, as shown in reaction (1)
R
1
—COOH+R
2
—CH
2
OH⇄R
1
—COO—CH
2
R
2
+H
2
O Reaction (1)
organic acid+alcohol⇄ester+water
Many esterification processes or systems focus on removal of water to drive the yield or conversion. Removal of water biases the equilibrium towards the products shown on the right hand side of Equation (1). This approach to esterification has been applied to a wide range of organic acids and alcohols.
For example, long chain alcohols that form heterogencous azeotropes with water can be used for water removal in the reaction's overhead vapors. Water removal can be easily carried out by using a higher alcohol that is sufficiently high boiling and slightly soluble in water. In addition, excess alcohol can be used to drive reaction.
Alternatively, an added azeotroping agent may be used to remove water in the case of esterification using lower alcohols such as ethanol and methanol. One example of a suitable azeotroping agent is benzene.
However, this approach to water removal to drive the reaction has met with less success with systems wherein one or both of the reagent components tend to form dimers, oligomers, polymers or side reaction products when they are dehydrated. Esterification of lactic acid is one such example. In these cases, progress has been restricted by the problem that while water removal is necessary to drive the equilibrium of reaction (1), this water removal at the same time also produces unwanted dehydration side reactions such as formation of various dimers and oligomers or lactic acid. These lead to yield loss.
The problem of oligomer formation has in the past been partially overcome by addition of significant excess alcohol, which tends to suppress the reactions that lead to formation of dimers and oligomers. However, this approach has not been entirely successful and also it has lead to higher costs for recovery of the product due to high levels of excess alcohol that must be removed.
Another approach that has been used is a single reactor wherein a batch of lactic acid or other acid is dehydrated, or is present initially at high concentration and is heated such that when the alcohol is introduced into the vessel, then ester, excess alcohol, and water formed in the reaction are flashed out of the reaction vessel.
One example of the esterification of lactic acid is given by Gabriel et al (U.S. Pat. No. 1,668,806) who prepared 1-butyl lactate by dehydrating 70% lactic acid with excess 1-butanol at 117° C., followed by addition of HCI catalyst, followed by refluxing and esterification with addition of excess 1-butanol and drawing a 1-butanol water azeotrope overhead. The process involved dehydration of the system and removal of water prior to the esterification step.
Bannister (U.S. Pat. No. 2,029,694) describes a method for producing esters that have boiling points of at least 120° C. The lactic acid and acidic 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 alcohol are taken off overhead. For example, methyl lactate is formed at temperatures from 130 to 140° C. by introducing methanol into partially dehydrated lactic acid. The overhead distillate 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 is fed to the system. Most or all of the water taken overhead (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 water free.
Weisberg, Stimpson, and Miller (U.S. Pat. No. 2,465,772) mix substantially water-free lactic acid with 3 to 20 parts by weight of aliphatic alcohol of 1 to 3 carbon atoms, reacting the mixture below the boiling point and then flashing the mixture at a higher temperature. For example, for the case of formation of methyl-lactate, the mole ratio of methanol to lactic acid is at a minimum 8.5:1. It may 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. Their scheme involves bubbling excess hot alcohol, such as methanol vapor, through a hot partially dehydrated lactic acid solution at a temperature above the alcohol boiling point, whereby the lactate ester produced is removed with the alcohol vapors 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 quantities of methanol are required for more dilute lactic acid feed solutions.
These methods described above typically require both excess alcohol and also dehydrated lactic acid. They are not energy efficient and also they require equipment that is large and expensive. In the 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 alternate approach is to attempt the reaction without first dehydrating the lactic acid. The following two references illustrate previous attempts that have been made to effectively utilize such an approach.
Wenker (U.S. Pat. No. 2,334,524) describes a process wherein both an esterification reactor and an adjacent hydrolysis reactor feed vapor to a common distillation column. Alcohol is removed from the top of this column and returned to the esterification reactor. The liquid product from the bottom of the column comprising largely water and ester are fed to the hydrolysis reactor. The ester is continuously hydrolyzed in that reactor to form free acid. The organic acid is 70 to 85% concentration and about 1.5 moles of alcohol are used per mole of organic acid. Reaction times are 12 to 16 hours for this system. The fractionating column is located immediately on top of the two reactors. This process appears to use a relatively low level of alcohol, but this is really the initial charge to the unsteady state system. 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 as towards the middle and end of the batch there will be a need to take large amounts of methanol or alcohol into the overheads. This process is thus not suitable for large scale, continuous efficient operation. The effective average ratio of methanol to lactic acid would be near to 10:1 if this process were to be run in multiple vessels in a configuration such as might allow continuous operation. The effective vast excess of methanol is needed to obtain high yield in this equipment and process configuration.
Franke, Gabsch, and Thieme, (East German Patent 206 373, Jan. 25, 1984) describe a similar process for formation of lactic acid esters of C1, C2 and C3 alcohols with the reaction and evaporation both occurring at reduced pressure and temperature in a modified vacuum-recirculation-evaporator. They use vacuum to operate the equipment at temperatures below the boiling point of the alcohol and use high levels of sulfuric acid as the catalyst. For example, in their example 2, they charge 2.0 liter of concentrated sulfuric acid into 15 l
A. E. Staley Manufacturing Co.
Oh Taylor V
Rotman Alan L.
Williams Morgan & Amerson P.C.
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