Improving the energy efficiency of the distillation unit for the production of ethylbenzene



Optimization of the technological mode of operation of the distillation columns and improving the energy efficiency of the rectification process in the technology of alkylation of benzene with ethylene by replacing the used steam with a pressure of 4 MPa (40 kg/cm²) with a pair of 2 MPa (20 kg/cm²).

Ethylbenzene is used as an intermediate in the production of styrene and synthetic rubber [2]. The rectification section for obtaining from the reaction mass ethylbenzene — rectification and dried benzene (Figure 1) consists of three distillation columns K-52, K-62 and K-72: column K-52 is intended for distilling off the return benzene from alkylate, column pos. K-62 is intended for the separation of ethylbenzene — rectified and column pos. K-72 is designed to separate the PUB from the resin [3].

Figure 1. Technological scheme of ethylbenzene rectification

Table 1 shows the structural characteristics of existing columns.

Table 1

Design parameters of distillation columns

Column parameters	К-52	К-62	К-72
Internal diameter, mm	2000	2200	1400
Number of plates	60	80	37
Interlopel distance	450	450	400

Distillation columns K-62 and K-72 consume heat, which is supplied through steam T-63 and T-73 by steam P = 28 kgf/cm², chopped from steam P = 40 kgf/cm², heat is supplied to distillation column K-52 through boiler T-53 steam P = 20 kgf/cm². According to the operation mode of these distillation columns, water vapor with a temperature of about 200 °C is required. According to the passport, the CHP generates steam with parameters P = 20 kgf/cm² T = 300 °C and P = 38 kgf/cm² T = 340 °C, but due to the large length of the pipeline (about 10 km), the parameters of steam P = 40 at the installation are significantly reduced.

A reasonable option to optimize the mode is to reduce the temperature of the cube, which will allow you to create the temperature difference required for using steam 20 (This option does not require significant reconstruction). The lower limit of the temperature of the coolant — a pair of 20 is determined by the value of 194 ° C. Reducing the temperature of the cube is possible by reducing the pressure in the column, which in turn will lead to a decrease in the boiling points of the components of the mixture to be separated and the liquid evaporated in the boiler.

Calculations were carried out to determine the optimal operating mode of the technological scheme in order to increase the efficiency of the ethylbenzene distillation process and to establish the possibility of lowering the temperature of the bottom of the columns [6].

The Aspen HYSYS modeling environment toolkit allows you to select the optimal parameters for the operation of distillation columns and immersion heaters in compliance with the requirements of production technology and to evaluate energy efficiency.

As a result of technological calculations using computer modeling, the parameters of the technological regime, the material balances of the columns, and the composition of product and raw material flows were obtained. Next, a calibration calculation was performed for boilers, the results are shown in Table 2.

Table 2

Results of the calibration calculation of boilers

	Temperature, C	Pressure, kPa	Flow rate, kg/h	Technical characteristics
Flow	Heater T-53
Hot vapor P=20	194.0	1300.0	3800.0	A=182m2, D=1470 mm, H=4516 m, Ptube-side=10 кгс/см2 Рshell-side=14 кгс/см2
Cooled vapor P=20	154.8	1250.0	3800.0
PABs	151.0	130.0	22420.1
Hot PABs	178.0	90.0	22420.1
	Heater T-63
Hot vapor P=20	196.0	1300.0	2500.0	A=182м2, D=1000мм, Н=4805мм, Ptube-side=0.55 MPa, Рshell-side =3.5 MPa
Cooled vapor P=20	180.4	1250.0	2500.0
Ethylbenzene	174.0	100.0	15539.6
Hot ethylbenzene	191.2	90.0	15539.6
	Heater T-73
Hot vapor P=20	195.0	1300.0	435.0	F=61м2, Д=630мм, L=4830мм, Ptube-side =4 MPa, Рshell-side =4 MPa
Cooled vapor P=20	134.5	1250.0	435.0
Resin	132.7	12.0	2396.9
Hot resin	189.2	2.0	2396.9

According to calculations, under the calculated conditions, the boiling point of the bottom liquid of the K-62 column is about 174 °C, which makes it possible to use steam as a heat carrier in the column boiler.

Column K-72 is operated under vacuum. The boiling point of the bottom liquid, under the calculated conditions, is 132.7 °C. Thus, steam 20 can also be used to preheat the bottom fluid.

Conclusion: Based on the analysis of the research results, we can conclude that the proposed option to optimize the operation of the rectification unit is effective, because when implementing the proposed technical solution, it will be possible to achieve the use of steam 20 with an increase in the process efficiency, namely reducing steam consumption. This option does not require significant reconstruction (replacement of equipment, contact devices, etc.), which makes it cheap.

In the Aspen HYSYS environment, the simulation of the upgraded scheme of the separation unit for rectification products was performed. Calculations made using the developed computer model showed the possibility of lowering the working temperatures of the K-53, K-62 and K-72 columns to the parameters supplied to the boiler for heating the cube of the R-20 vapor. In this regard, the modernization will allow abandoning the steam P-40 and transferring the installation to steam P-20 and achieve a reduction in the consumption of steam consumed.

References:

1. Энергосберегающая модернизация ректификационной установки выделения фенола/ Башаров М. М.// Нефтегазовое дело — 2011. — № 2. — с. 136-
2. Ульянов Б. А., Бадеников В. Я., Ликучёв В. Г. Процессы и аппараты химической технологии. Ангарск: изд-во АГТА, 2006.
3. Разработка моделей аппаратов химической технологии в системе компьютерного моделирования HYSYS: учебное пособие/ Лисицын Н. В., Федоров В. И.// СПб, СПБГТИ (ТУ). — 2005.- 30 с.
4. Теоретические основы и расчет аппаратов разделения гомогенных смесей: учебное пособие/ Лаптев А. Г., Конахин А. М., Минеев Н. Г.// М: Теплотехник, 2011. — 424 с.
5. Методы энергосбережения при ректификации смесей органических веществ/ Тимошенко А. В., Анохина Е. А.// Ползуновский вестник — 2010. — № 3.– с.134–136.
6. Synthesis of ethylbenzene by alkylation of benzene with diethyl oxalate over HZSM-5/ Li Y., Xue В., Yang Y.// Fuel Proc. Tech. 2009. Vol. 90. № 10. P. 122– 125