January 19, 2020

In and the design of the reactor can be

In the era of modern life there is an increasing demand for phenol since it plays a key role in the manufacture of polycarbonates which are used in the various industrial branches like healthcare, electricity and automotive. The predominant method of producing phenol is the cumene process which roots back to 1944 when was discovered by Heinrich Hock. The Hock process is based on using low cost reagents, oxygen and sulfuric acid which makes the cumene process to be the most economical method of phenol production. Over the recent years, the cumene process has become even more popular due to applying more environmentally friendly zeolite to produce cumene instead of acidic catalysts. In 2008 approximately 95% of worldwide phenol was produced by the cumene method. In the future, the production of phenol is predicted to increase rapidly due to rising demand for Bisphenol A and phenol resins, especially in Asia where need for these products is huge. (Hwang and Chen, 2010) The main objective of this report is to provide the detailed description of the nowadays phenol production. The cumene process consists of two main stages in which cumene is oxidised to its hydroperoxide and then due to influence of acid it decompose to phenol and acetone. The production of cumene, which is based on alkylation of benzene with propylene, is usually an integral part of phenol production plant, therefore, this process is necessary to be included. The figure 1 shows that the phenol production starts with alkylation process of the raw materials which are benzene and propylene in the presence of zeolite catalyst. As a result, the cumene is created and undergoes the oxidation process with participation of sulfuric acid which results in production of cumene hydroperoxide. The final step involves the decomposition of cumene hydroperoxide to phenol and by-product named acetone. This chemical plant is designed to produce 400,000t of phenol, 2500,000t of acetone and 550,000 of cumene per year. The process of producing cumene is based on using two raw materials namely, benzene and propylene. The propylene is passed to reactor (1) in which catalyst zeolite-Y is present. Before benzene is introduced to reactor (1), it goes through the separator (2) where the propane and overabundance of water is removed. Therefore, in the reactor benzene is in molar excess to propylene which helps to avoid the formation of unwanted by-products. The temperature maintained in the reactor is relatively low which means that less energy is used and the design of the reactor can be smaller. Hence, this method is also less expensive. The temperature at the bottom part of reactor is 249°C while the top temperature is 198°C. To provide only liquid phase reaction, the pressure should be sufficiently high which is around 15 atm. The mixture is cooled down to 124°C between the beds where the side stream heat exchangers are. Then, it is passed to the cumene column (4) where the bottom and top temperatures are 172°C and 110°C, respectively. The pressure is around 0.3 atm. The cumene is obtained from the top part of the cumene column, while the bottom product is diisopropylbenzene. The DIPB is introduced to DIPB column (5) where is recycled and returned back to transalkylation reactor (3), in the meantime heavy aromatics are discharged from bottom part of DIPB column and mixed into fuel oil. In transalkylation reactor where the temperature estimates around 140-150°C benzene and DIPB is mixed and more cumene is produced. Figure 3 (Grzywa and Molenda, 2000) shows the phenol production by cumene method. The process of cumene oxidation is carried out in the reactor (2a) with the design of a shelf column in which each of the shelves has a coil. When process starts, steam is applied and the coil is powered, whereas when the heat of reaction is received, coils are cooled with water. The tank (3) contains cumene which is used in the reaction. The vapor is passed to the coils in reactor and heated up to the temperature of 110°C. Then the cumene is heated to the temperature of 120°C in preheaters (4) and (1b) and introduced to the reactor.  The air is applied as soon as the reactor shelves are heated to the suitable temperature and the reactor is filled with the cumene. When the oxidation process begins, it is important to constantly provide the reactor with fed of cumene and in the meantime post-reaction mixture ought to be discharged. The described process is exothermic which means that the large amount of heat is created. Thus, to prevent the reactor from failing, the coils need to be cooled with water. Conducting the process with the right temperature range and maintaining the desired distribution of temperature on the shelves is necessary to provide proper course of oxidation process and safety. Therefore, the maximum temperature on the top shelf should be 120°C, whereas the lower shelves must be applied with the temperature not greater than 110°C. To maintain these parameters, the pressure should be constant at around 5 atm and the use of adequate amount of water is necessary for cooling. Otherwise, exceeded temperature may cause an explosion. To prevent this from happening each system is equipped with warning lights and sounds. In addition, the fed of cumene and air is automatically stopped and valves supplying cooling water are opened. Next step involves withdrawing post-reaction mixture from the bottom of the reactor. The mixture contains only 25% of cumene hydroperoxide, therefore, the mixture is pumped into a vacuum distillation column (9a) and concentrated. At the bottom part of distillation column, the temperature is maintained at 90°C, whereas the temperature at the upper part is equal 60°C. These conditions allow to obtain mixture containing roughly 90% of cumene hydroperoxide and this mixture returns back to output tank (3). As soon as the mixture cools down, it is accumulated in the tank of the small capacity (10). The size of this tank is really important since storing large amount of CPO is dangerous due to possibility of explosion. Because of this, the temperature should not be higher than 120°C and any dangerous conditions are signalized by sound and light and right vacuum and temperature is adjusted.3.3 Decomposition of cumene hydroperoxide to phenol and acetone.The reactor (2b) is filled with the hydroperoxide cumene from the tank (10) and the mixture consisting of a recycled degradation product is passed to the reactor. The degradation undergoes at the temperature of 60°C. The higher temperature should be avoided because it causes unwanted side reactions which reduce the efficiency of phenol and acetone production. The amount of concentrated sulfuric acid (VI) which is added to the reactor is no greater than 0.1% of reaction mixture’s mass. This mixture should contain relatively small amount of hydroperoxide and the mass ratio of decomposition product to cumene hydroperoxide ought to be 30:1. Otherwise, the exceeded amount of cumene hydroperoxide cause the rapid decomposition releasing tremendous amount of heat which cannot be eliminated by the cooling system. Consequently, the temperature will rise to above 120°C which may contribute to an explosion of the system.  The safe and efficient decomposition is provided by the suitable temperature, concentration of hydroperoxide and the amount of sulfuric acid (VI). The post-reaction mixture coming from the reactant (2b) is cooled down in a condenser (5e) to 40°C. Then it is divided into two streams. One of them is responsible for getting rid of the heat and hydroperoxide destined for decomposition is diluted. Whereas the second stream undergoes the process of neutralization by sodium hydroxide in mixer (11). The obtained mixture consists of 35% of acetone, 55% of phenol and insignificant amount of cumene and tars. Then the mixture is introduced to the column (9b) where distillation process is applied starting with separation of acetone. In column (9c) the large amount of ?-methylstyrene is distilled off and reduced with hydrogen to produce cumene which is then transported back to the cumene column (3). The phenol is directly obtained from the rectification column (9d). The remains consist of infinitesimal amount of phenol tars which are difficult to reuse.In conclusion, there is no doubt that modern way of producing phenol is more efficient than early ways of phenol production. They were based on distillation of coal tars which required a lot of energy and the sulphonation of benzene having low atom economy. The efficiency of modern phenol production is obtained by the use of zeolite in the first step. Not only does it not produce acidic emissions but it is also cheap and contributes to higher purity of cumene which results in obtaining greater amount of phenol. However, the cumene method produces also a large amount of acetone which is not in such a high demand like phenol. Therefore, it may be in oversupply and become a waste. As a result, there is a lot of research made to improve phenol production. One of the alternative is to find a catalyst to reduce the energy intake in the phenol production by oxidising toluene. The oxidation process is also being improved by inventing microreactors which will fit benzene, oxygen and hydrogen in one vessel. Another possible route is extracting phenol from the biomass like grass.

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