SK280103B6 - Preparation method of vapour-gaseous mixture benzene-hydrogen at a cyclohexane production - Google Patents

Abstract

Benzene-to-hydrogen evaporation has two stages, whilst in a first stage as much as 95 percent of benzene, relative to the total amount thereof necessary for the preparation of the mixture, is evaporated at adiabatic conditions. In a second stage, benzene is evaporated at isothermal conditions. A ratio of hydrogen overflow-to-benzene overflow, fed in the adiabatic evaporation stage, is from 0.007 to 0.018 kg/kg, whilst a flow of both the phases is counter.

Description

Technical field

The invention belongs to the field of chemical technology. It solves a process for the preparation of a steam-gas mixture of benzene-hydrogen in the production of cyclohexane by catalytic hydrogenation of benzene in the gas phase.

BACKGROUND OF THE INVENTION

The only industrially important source of cyclohexane is the hydrogenation of benzene. The reaction is exothermic and low temperature. Since benzene and cyclohexane have the same boiling points and, moreover, form an azeotrope, hydrogenation must be complete. It is operated in the vapor or liquid phase. Temperatures are up to 200 ° C and pressures up to 4 MPa. Heterogeneous nickel or platinum catalysts and more recently very active homogeneous nickel or lithium catalysts are used. The necessary rapid heat dissipation is provided in the bubbled reactors by recirculation and external cooling of part of the batch in the coolers, in the deck reactors by spraying the coolant between the catalyst layers and in the tubular reactors by boiling water with steam production.

In processes where the hydrogenation of benzene takes place in the gas phase, the first production step is the preparation of a steam-gas mixture benzene-hydrogen. This is prepared by bubbling hydrogen through liquid benzene in an evaporator. Since the system pressure is given by the catalyst used and its activity, the only variable that can control the composition of the benzene-hydrogen gas / steam mixture is the evaporator temperature. The heat necessary for the evaporation of benzene is supplied in an indirect heat exchanger, most often by condensing water vapor. The heat of the reaction products is used to preheat the raw materials.

For example, in the CYCLOPOL process, the hydrogenation of benzene takes place in two reaction stages. The first reaction stage is carried out under an excess of benzene at 0.3 to 0.5 MPa and in the second reaction stage under an excess of hydrogen at a pressure of 0.8 to 1.0 MPa. The unreacted hydrogen from the second reaction stage is utilized in the first stage. The reaction products from the first reaction stage are used to preheat the hydrogen entering the process. After subsequent cooling of the reaction products and condensation of benzene and cyclohexane and separation of the inert and unreacted hydrogen, the liquid semi-product is injected into the second stage, where it is preheated by the heat of the reaction products from the second stage and re-evaporated. The reaction products from the second stage after preheating the feed to this stage are still used to preheat the benzene entering the process. A disadvantage of this preparation method is the consumption of steam to evaporate benzene into the hydrogen stream, which is higher than 0.12 kg per 1 kg of benzene in the first stage and higher than 0.10 kg per 1 kg of the intermediate product in the second stage.

SUMMARY OF THE INVENTION

The process of the present invention characterized by the combination of adiabatic and isothermal evaporation of benzene into a stream of hydrogen solves the problem of preparing a benzene-hydrogen steam-gas mixture in the production of cyclohexane by catalytic gas-phase hydrogenation. By combining adiabatic and isothermal evaporation of benzene into the hydrogen stream, it is possible not only to prepare a gas-gas mixture of benzene-hydrogen of the desired composition, but also to cool the benzene to a temperature that allows waste heat from the process to heat the benzene. The heated benzene is then injected into the adiabatic and isothermal evaporation stages. By utilizing the waste heat from the process to heat the cooled benzene, it is possible to reduce the steam consumption for the preparation of the benzene-hydrogen mixture and the consumption of cooling water to remove the waste heat from the process. The benzene is evaporated in an adiabatic step into a stream of hydrogen at temperatures below the boiling point at a given pressure. Evaporation takes place as a result of various partial pressures of benzene in hydrogen and at the surface of liquid benzene. Evaporative heat is used to evaporate the benzene, thereby cooling the benzene. Some of the heat needed to evaporate the benzene is covered by cooling hydrogen. In this way, it is possible to cool the benzene to a temperature that allows it to be subsequently heated by the heat of the reaction products. The steam-gas mixture exiting from the adiabatic stage is fed to an isothermal stage where the evaporation of the benzene takes place at a constant temperature which is determined by the desired composition of the benzene-hydrogen mixture. The heat required to evaporate the benzene is supplied by condensing steam in an indirect heat exchanger. The advantages of the method are apparent from the following examples.

DETAILED DESCRIPTION OF THE INVENTION

Example 1 (comparative).

In FIG. 1 shows the original apparatus connection. The steam-gas mixture of benzene-hydrogen is prepared in one isothermal stage carried out by evaporator 1 at a temperature of 88 ° C and a pressure of 0.35 MPa. The evaporator 1 is charged with hydrogen 4 in an amount of 705 kg.h ^-1 at a temperature of 125 ° C with a composition of 98 mol%. hydrogen and 2% mol. nitrogen and benzene 5 in an amount of 13 010 kg.h- ¹ preheated to 104 ° C in exchanger 2 by the condensation heat of reaction mixture 8 from the second reaction stage of the hydrogenation process. The heat output of the exchanger 2 is 571 kW. The reaction mixture 9 exiting the exchanger is then cooled with water to condense the cyclohexane. The heat required to evaporate the benzene is supplied to the evaporator 1 by steam 6 at a pressure of 1.4 MPa in an amount of 1850 kg.h ^-1 in an indirect heat exchanger 3. The evaporator 1 exits a steam-gas mixture of benzene-hydrogen 7 at 13 715 kg.h ' ¹ . Steam consumption per kg of benzene is 0.142 kg ·

Example 2.

In FIG. 2 shows an arrangement of devices for taking advantage of the process according to the invention. The steam-gas mixture of benzene-hydrogen is prepared in two stages. In the first stage, with respect to the flow direction of hydrogen 14, which is the carbonating column 1, both phases are contacted under adiabatic conditions on five theoretical levels. In the second step, evaporator 6, evaporation is carried out under isothermal conditions. Benzene 8 with a flow rate of 13 010 kg.h ^-1 and a temperature of 20 ° C is injected into the tank 5 where it is mixed with benzene 9 cooled during its adiabatic evaporation. The temperature of benzene at the bottom of the column is 57 ° C and the tank is 50 ° C. From tank 5, benzene is injected via pump 3 through heat exchangers 4 (new) and 2 (original) back to column head 1 and evaporator 6. 35,000 kg / h of benzene 10 flows through heat exchanger 4 and is heated to 92 ° C. the condensation heat of the reaction products 18 from the first reaction stage of the hydrogenation process. The heat output of this exchanger is 797 kW. The reaction mixture 19 exiting the exchanger is then cooled with water to condense the cyclohexane and benzene with cooling water. The heat output of the water cooler is 797 kW lower, ie the heat output of the heat exchanger 4. Water consumption for cooling the reaction products in the first reaction stage is 68.6 m ³ h ⁴ lower. 32 476.5 kg / h of benzene 11a flows through the exchanger 2 and is heated to 93 ° C by the condensation heat of reaction mixture 20 from the second reaction stage of the hydrogenation process. The heat output of this exchanger is 751 kW. Compared to the original solution, the thermal output of this apparatus is higher by 180 kW. The reaction mixture 21 exiting the exchanger is then cooled with water to condense the cyclohexane with cooling water. The heat output of the radiator is reduced by 180 kW and the water consumption for cooling is reduced by

15.5 m ³ h ⁴ . From the exchanger 2, a portion of the heated benzene 12 in an amount of 2,476.5 kg.h ⁴ flows into the evaporator 6. A second portion 13 in an amount of 30,000 kg.h ⁴ flows to the saturator column 1. with 35,000 kg.h ^{4 of} benzene 10 from exchanger 4. In column 1, adiabatic evaporation of benzene into the hydrogen stream occurs, whereby benzene is cooled from 92.5 ° C to 57 ° C. Hydrogen 14 in an amount of 705 kg.h ⁴ with a temperature of 125 ° C with a composition of 98% mol. hydrogen and 2% mol. The nitrogen is fed to the bottom of column 1. In this column, it is saturated with benzene and it is said to have a temperature of 83 ° C at the outlet of the column. Pre-saturated gas 15 flows from column 1 to evaporator 6. The flow of this stream is 11 238.5 kg.h ⁴ and contains 93.72 wt. benzene. In evaporator 6, it is saturated to the desired composition at 88 ° C and 0.350 MPa. The prepared benzene-hydrogen 16 steam-gas mixture (13,715 kg.h ⁴ ) emerges from the evaporator (6 ^). The heat necessary for the evaporation of benzene is supplied to the evaporator (6) with a pressure of 1.4 MPa at 544 kg.h ⁴ . which represents 0.042 kg per kg of benzene.

Example 3.

The steam-gas mixture is prepared as in Example 2, except that in the carbonation column both phases are contacted on two theoretical levels. The temperature of benzene at the bottom of column 1 is 67 ° C and in tank 5 the temperature is 58 ° C. The heat output of the heat exchanger 2 is 649 kW and the consumption of the coolant water for cooling the reaction products 21 from the second reaction stage is 6.8 m ³ h ⁴ lower than the original state. The heat output of the exchanger 4 is 651 kW and the cooling water consumption for the cooling of the reaction products 19 from the first stage is 56 m ³ h ⁴ lower than the original state. At the outlet of column 1, the benzene-hydrogen mixture 15 has a temperature of 80 ° C. The flow rate of this mixture is 9 480 kg.h ⁴ and contains

92.6 wt. % benzene. In evaporator 6, it is saturated to the desired composition at 88 ° C and 0.350 MPa. Prepared steam-gas mixture of benzene-hydrogen 16 in the amount of 13 715 kg.h ⁴ emerges from the evaporator 6. The heat necessary to evaporate the benzene is supplied to the evaporator 6 by steam 17 at a pressure of 1.4 MPa in an amount of 850 kg.h ⁴ in an indirect heat exchanger 7, which represents 0.065 kg per 1 kg of benzene.

Claims (3)

PATENT CLAIMS Process for the preparation of a steam-gas mixture of benzene-hydrogen in the production of cyclohexane by catalytic hydrogenation of benzene in the gas phase, characterized in that the evaporation of benzene to hydrogen is carried out in two stages. 95% of the total amount of benzene needed to prepare the mixture and in the second stage the benzene is evaporated under isothermal conditions. Process according to claim 1, characterized in that the liquid benzene from the adiabatic evaporation stage is mixed with the benzene injected into the process in a ratio of 3.5 to 7.0 kg / kg, subsequently heated by the heat of the reaction products to 80 to 100 ° C and injected. to the adiabatic and isothermal degree of evaporation. Method according to claim 1, characterized in that the ratio of hydrogen and benzene flows injected into the adiabatic evaporation stage is 0.007 to 0.018 kg / kg, the flow of both phases through the adiabatic stage being countercurrent.