The beneficial using the heat of the exhaust gases of the furnaces of the technological unit for the ethylene oxide production

The solution to the issues of increasing energy efficiency and optimizing the work of enterprises in the petrochemical industry is due to the constant increase in prices for primary energy resources. In this article, the technological site for the production of ethylene oxide, which is characterized by high energy intensity, is considered as an object of study. The largest consumer of energy in this area are technologically furnaces. In the framework of the study, an analysis of the operation of the furnaces was carried out, a balance of the heat-technological flows of the site was compiled, the potential for energy conservation in the utilization of the exhaust gases of the process furnaces was calculated, and the possibility of the beneficial use of the obtained heat was calculated. The economic efficiency and payback period of the proposed method of utilization of flue gases is determined.


Introduction
Modern competitive market conditions motivate industrial facilities to change approaches to the use of energy resources in technological processes. The petrochemical industry is one of the major consumers of energy resources. One of the promising directions of energy saving policy at the petrochemical plants is the development of energy technological combination methods, allowing to reduce the consumption of fuel and energy resources without significant change of the entire processing line and to provide the specified indicators of output product [1][2][3]. Considering that the enterprises of basic organic synthesis is one of the leading ones in terms of production volumes in the petrochemical industry, the production of ethylene oxide is considered as the object of research. Ethylene oxide and its derivatives are the largest petrochemical synthesis products [4][5][6][7][8][9]. The working purpose is to increase the efficiency of this production.

Methods
Consumers of fuel gas of the process section are furnaces Н-1, Н-2, Н-3. Data on operating parameters of furnaces Н-1, Н-2 are given in table 1. All expenditure data of the balance scheme (figure 1) are accepted according to the process regulations of furnace operation.
Calculation of the amount of heat produced in furnaces by burning fuel is made taking into account the chemical composition of fuel gas corresponding to the calorie coefficient of 1.814. Heat losses to the environment are taken equal to 5%.
The heat coming out of the furnace with combustion products is calculated on the composition and volume of contact gases and vapor RES (renewable energy resources) [10]. where Gok.et that is equal to 4800 tons per month (6670 t/h) is ethylene oxide production; GT1 that equals 760 kg/h is fuel consumption for ethylene oxide production; qT that is equal to 206.85 kgoe/ton of ethylene oxide is specific fuel consumption per ton of ethylene oxide; Qy.t that equals 7000 kcal/kg is the amount of heat released during burning 1 kgoe.

Results
It is proposed to use heat of stack gases to obtain 2.16 t/h steam with pressure that is possible to use in heat point [11][12][13].
Calculation of the theoretically required amount of air for the combustion of 1 m 3 fuel, taking into account the composition of the fuel gas, is carried out according to the expression:  where G0 that equals 760 kg/h is total fuel consumption at the furnaces Н-1 and Н-2; GSG that is equal to is 22.26 kg is stack gas consumption per 1 kg of fuel.
To determine the potential of heat utilization, it is taken the temperature of the exhaust gases equal to 150°C (it is limit value to which the stack gases can be cooled from sulfuric acid corrosion conditions [14,15]), then: where G = 16918 kg/h is stack gas flow rate; ct350 = 0.285 kcal/kg·°C is stack gas heat capacity at 350 °C; ct150 = 0.285 kcal/kg·°C is stack gas heat capacity at 350 °C; The amount of generated steam Gsteam is calculated by the follow formula: Gsteam = (Qsg /(i1 -i2))· ƞbu = 1556 kg/h, where ƞ = 0.9 is an efficiency of the boiler-utilizer; i1 = 663 kcal/kg is steam enthalpy at P = 10 kgf/cm 2 ; i2 = 99.8 kcal/kg is enthalpy of feed water at t = 98 °C; Annual savings of thermal energy is: where 460 rub./Gcal and 120 rub./Gcal are cost of steam 13 from the external heat source and steam secondary energy.
Estimated costs for equipment and construction and construction and installation works is 12.6 million rubles. Thus, the payback period of the event is: T = 12 600/2740.85 = 4.6 years

Discussion
The technical and economic analysis of the efficiency of the proposed event showed that technological steam with total energy potential of 7831 Gcal/year is produced as a result of the boiler-utilizer operation.