Glycerin was far from meeting people’s needs and the price was very expensive in the past. As a result, many epoxy-making devices around the world co-produce glycerol, or “synthetic glycerol”, by acidifying and hydrolyzing dichloropropanol or pure epichlorohydrin. In recent years, with the rising international oil prices, biodiesel as alternative energy is favored. For every 10 tons of biodiesel produced, there’s a ton of glycerin. With the rapid growth of biodiesel production, the glycerol market continued to produce excessive supply, the glycerol price fell. However, the price of epichlorohydrin has been rising all the way, which has had a great impact on the traditional glycerin co-production device by high-temperature chlorination method, which has led to the closure of many co-production devices were closed, such as DOW chemical company in the United States, closed in the end of January 2006.
Early Study on Synthesis of Dichloropropanol from Glycerol
In fact, glycerin synthesis of dichloropropanol reaction was used in the initial discovery of epichlorohydrin. In 1854, epichlorohydrin was first synthesized by treating crude glycerin with hydrochloric acid and then hydrolyzing it with lye. Several years later, Reboul proposed that epichlorohydrin could be made directly by reacting dichloropropanol with sodium hydroxide solution. Subsequently, the direct synthesis of dichloropropanol from glycerin has received increasing attention, including the development of catalysts to improve the glycerol conversion rate and the use of appropriate extraction solvents to increase the yield of dichloropropanol (the amount of dichloropropanol as a percentage of the amount of raw glycerol).
(1). Catalyst development
Under the action of no catalyst, glycerol and hydrogen chloride gas can also react independently, but the reaction rate is very slow, at 130℃ for 10 hours, the main product is still 3-chloropropanediol, the production of dichloropropanediol is very small. Reboul noticed earlier that adding acetic acid of the same volume to glycerol significantly improved the reaction rate, but acetic acid was used as a thinner at that time. Later, it was found that in this reaction process, acetic acid was actually a good catalyst, and the amount of catalyst could also be reduced, which saved the cost economically.
On the basis of acetic acid catalysts, formic acid, propionic acid, succinic acid, and Aromatic shuttle acid (such as phenylacetic acid) catalysts have also been developed. Under the action of these catalysts, glycerol can not only react with HCI gas but also with hydrochloric acid solution to obtain relatively pure dichloropropanol. In 1908, the catalysis of some organic acid vinegar (such as glycerol acetate) and inorganic acid was also explored.
In addition, the amount of catalyst has a great influence on the selectivity of reaction products. The dosage of acid catalyst, catalyst quality relative to the quality of all the reaction mixture percentage) tend to form a two chlorine propanol formic acid (5%), acetic acid (1%), acetic acid (5%), acrylic acid (1%), acrylic acid (5%), butyric acid (5%) and acetic acid glyceride (10%) and the dosage of the catalyst is more inclined to form chlorine 3 – propanediol, including formic acid (1%), butyric acid (1%) and chloride (1%), ethyl acetate acid (1%), oxalic acid (5%), succinic acid (5%), tartaric acid (6%) and acetic acid glyceride (2%), etc. However, if the temperature is too high, the reaction tends to produce condensates, regardless of the catalyst. This is due to the dehydration of two light bases between glycerol and glycerol molecules and between each reaction product. Therefore, the reaction temperature control is generally less than 120℃.
(2). Latest Developments in the Synthesis of Dichloropropanol from Glycerin
Early studies on the synthesis of dichloropropanol with glycerol were mostly batch intermittent reactions with relatively long reaction time. In addition to the high price of glycerol at that time, this study did not have many practical applications. As a result, from the second world war until the end of the last century, the synthesis of dichloropropanol by glycerol was almost completely unknown. Until recent years, due to market changes, the synthesis of dichloropropanol by glycerol has developed rapidly.
During the experiment, DOW found that the yield of dichloropropanol was significantly increased under higher reaction pressure.HCI of a certain pressure was injected into a pressure vessel containing 30 grams of glycerin, and acetic acid was used as a catalyst to seal the reaction at 90℃. According to the results of several experiments, when the pressure increased to 689.4kpa (surface pressure), the total yield of the two isomers of dichloropropanol reached 90%.
However, at higher pressures, the water and dichloropropanol products are not easily extracted by gas. At present, the continuous production technology with air extract water has been developed to shorten the reaction time as far as possible under the premise of meeting the total yield of dichloropropanol, so as to meet the needs of actual production.
In 2004, the Czech republic developed a reactor with an external circulation. Glycerol and catalyst are added continuously, but the water and dichloropropanol products are not easily extracted by gas at higher pressures. At present, the continuous production technology with air extract water has been developed to shorten the reaction time as far as possible under the premise of meeting the total yield of dichloropropanol, so as to meet the needs of actual production.
Acetic acid as a traditional catalyst, although the catalytic activity is very high, but compared with propionic acid, malonic acid, succinic acid, and other substances, the practical application of acetic acid is not ideal. This is because the boiling point of acetic acid is relatively low (117℃). At reaction temperature of more than 100℃, the catalyst is subject to the gas extraction action of HCI and is easily carried out, which reduces the content of the catalyst in the reaction mixture and is not conducive to the reaction. The concentration of the catalyst in the reaction mixture (the amount of catalyst material contained in the reaction mixture per unit mass) is usually higher than 0.1mol/kg, and 1 ~ 4mol/k is more suitable. Suitable catalyst concentration can not only reduce the reactor volume but also minimize the generation of by-products. Therefore, in industrial applications, in order to make up for the loss of acetic acid catalyst, acetic acid must be added continuously to ensure the normal reaction. Therefore, the catalyst boiling point at atmospheric pressure is better than 200℃ (such as glutaric acid, adipic acid and acid mixture, etc.), so that the reaction can be carried out at a higher temperature (above 130℃), most of the low-volatility catalysts remain in the reactor, and the purity of dichloropropanol is easily guaranteed. At the same time, in the follow-up treatment, it is also convenient to use crystallization and other means to recover the catalyst.