Optimization of Thermal Conditions of Heat Recovery Boilers with Regenerative Heating in the High-Temperature Section of Isoamylene Dehydrogenation

Improving the efficiency of use of energy resources at large-capacity energy-consuming enterprises in the petrochemical industry in conditions of high internal and external competition is the priority for the development of the fuel and energy industry. This is confirmed by various legislative acts, including the energy strategy of the Russian Federation for the period up to 2035. This research focuses on a high-temperature section of dehydrogenation of isoamylenes into isoprene, the production of which relates (isoprene production relates to large petrochemical enterprises that consume a huge amount of energy resources) to large-capacity energy-consuming industries. To increase the thermodynamic efficiency of the research object, regenerative feedwater heating for heat recovery boilers is proposed due to deeper cooling of fuel and contact gas (the term "contact gas" is used in the technological regulations of an isoprene production company), which are secondary thermal energy resources in this technology. In accordance with the industry’s technology regulations, a block diagram of the initial and improved high-temperature section with the indication of material flows (The term "material flow" refers to the type of substances that are used in the high-temperature dehydrogenation stage of isoamylenes) was developed. The balance equations of the section under consideration are provided, and the thermal efficiency and exergy efficiency for systems utilizing fuel and contact gas are determined. The estimated economic effect was determined in physical terms; it was found to be 2008.58 toe/h. An exergy flow diagram is also provided to show how the system utilizes contact gas.

Energy efficiency; Energy technological combination; Heat recovery boiler; Regenerative heating

The petrochemical industry is characterized by high energy intensity. Thermal energy costs reach 30–40%, making it difficult for enterprises in this industry to save energy. This is particularly problematic given the rapid increase in fuel prices currently observed both in Russia and around the world (García-Olivares, 2015; Shkrabets and Berdnyk, 2016).

One of the most promising areas of energy savings in the industry is the organization and improvement of energy technology complexes, where building energy in main technological processes can significantly reduce fuel and energy consumption without changing the structure and parameters of the processing line or affecting established product indicators (Balzamov and Konakhina, 2010).

The application of the principle of energy technological combination (This principle implies the joint elaboration of a technological product and an energy resource) (ETC) becomes an indispensable prerequisite when designing new productions in the petrochemical industry. The ETC principle can be implemented at existing enterprises by having systems utilize secondary energy resources (SERs) not used in the main production processes (Patrascu and Minciuc, 2012; Kosasih and Ruhyat, 2016; Ketoeva et al., 2019). A promising direction for fuel and energy optimization at industry is the introduction of regenerative heating in heat transfer agents utilizing the heat rejected into the environment (Kusumah et al., 2019; Kusrini and Kartohardjono, 2019).

Organic synthesis enterprises have many SERs, and their utilization can significantly reduce fuel consumption. At present, their actual use in relation to their potential use is currently about 40%. The beneficial use of SERs at Russian enterprises is about 40% (Nazmeen and Konakhina, 2002). This is because most SERs produced at petrochemical enterprises are low-temperature thermal SERs, which cannot be used in high-temperature heat technologies. Therefore, the implementation of low-potential SERs in energy balance in petrochemical industries is a topical issue.

Isoamylenes undergo dehydrogenation to produce isoprene, in accordance with the accepted classification (Nazmeen and Konakhina, 2002) has a classification of technological processes by temperature regime. These processes are subdivided into high-temperature, medium-temperature and low-temperature (Nazmeen and Konakhina, 2002), is related to the high-temperature stage of isoamylenes, since the temperature of the main technological process exceeds 800°C.  Dehydrogenation of isoamylenes into isoprene is related to this stage. Figure 1 shows a diagram of the high-temperature section of the dehydrogenation stage; the numbers label the material flow connecting the elements of the section. Flow parameters are presented in Table 1.

The process of dehydrogenation is as follows. The feed stock is isoamylene. Before entering the furnace, isoamylene undergoes the previous heating stage at the evaporation station, which includes the elements ES1-ES3. Fuel supplied to the furnace burners is a mixture of natural gas and absorption gas (i.e., SER in the main production process); it is also preheated in the fuel heater (FH) and absorption gas heater (AGH) heat exchangers, respectively.

The raw material is evaporated in the oven and overheated to a temperature of 530°C. Then, the raw material mixed with water vapor is fed to reactor R, where the contact gas is formed in the catalyst layer. The contact gas is sent to the next stages of production for cooling and treatment.

As can be seen from Table 1, during the intermediate stage, the process ovens and contact gas make the flue gases’ temperatures sufficiently high, which allows the system to generate water vapor (of required parameters in Table 1) from the heat contained in the gases.

Figure 1 High-temperature section of the dehydrogenation stage of isoamylene to isoprene: I – raw material evaporation station; II – fuel and absorption gas heating station; III – main process unit; ES1, ES2, ES3 – heat exchangers at the evaporation station; FH – fuel heater; AGH – absorption gas heater; OB – oven burners; SH – steam heater; OH – raw material overheater; R –reactor

Table 1 Material flow of the high-temperature section of the dehydrogenation stage of isoamylene into isoprene

Thus, at production, it is proposed to organize two heat utilization units that have the same structure and purpose, but differ in load and temperature conditions. Thus, it is necessary to create two heat recovery units at the production site that have the same design and purpose, but differ in load and temperature conditions.

The proposed schemes give the opportunity (to the industries) to save a significant amount of heat energy (i.e., 58730.25 kW/h) in the production of isoprene and achieve a reduction in its cost. The efficiency of energy technological schemes was evaluated using thermal and exergetic efficiencies, the values of which were high , respectively.

The suggested option of energy technological combination based on SERs can be extended not only to the production of isoprene, but also to other stages of petrochemical production characterized by a high yield of high-temperature SERs. Regenerative feedwater heating can also be quite effective in other industries such as the production of ethylene, butadiene, ethanol and other organic products.

The study was carried out within the framework of a scientific project of the Russian Science Foundation (RSF) No 18-79-10136.

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