Energy-saving technologies for distillation processes

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Abstract: This paper summarizes the main energy-saving technologies for distillation processes. It classifies these technologies based on whether the process is altered and whether process technologies are utilized, into categories such as energy-saving through process technologies, special distillation techniques, and high-efficiency materials for distillation columns. The main energy-saving methods, advantages, disadvantages, and application scopes of each category are reviewed, and the current status and trends of energy savings in distillation processes in China are presented. Keywords: distillation process ； Energy saving ； thermocouple distillation ； Heat pump distillation ； In the chemical production process, separation is a very important process unit; it directly determines the quality and yield of the final product. The separation method that plays a dominant role in industrial production is distillation. Distillation is an operation unit that utilizes the differences in volatility among the various components in a mixture, along with energy, to achieve separation, and it possesses unique advantages. It is estimated that 40% to 70% of the energy consumption in chemical processes is used for separation, with distillation accounting for 95% of that amount. Therefore, as the world faces an increasing shortage of energy, the distillation process has always been a focal point for researchers seeking ways to save energy; every advancement in this area brings significant economic benefits. Over the years, various methods and approaches have been employed to study ways of saving energy and reducing consumption in distillation processes. These can be classified into three categories based on whether the process flow is altered and whether process technologies are utilized: ① Utilizing process technologies to optimize the operating conditions of the distillation column in order to reduce the energy consumed by it, such as preheating the feed with product stream, increasing the number of trays, reducing the reflux ratio, and adding intermediate reboilers and condensers ； ② Many efficient and energy-saving special distillation processes have been developed, such as heat pump distillation, thermocouple distillation, multi-effect distillation, etc ； ③Improve the insulation materials of distillation towers and develop efficient types of trays and packing. Since Category 3 does not utilize process technologies, this paper only provides a review of the first two types of energy-saving methods. 1 Energy Saving in Process Technology 1.1 Energy Saving through Optimization of Operating Conditions This method involves using simulation software to study the operating conditions of distillation processes. The main operating conditions of a distillation column include operating pressure, operating temperature, tray pressure drop, feed location and temperature, number of theoretical plates, reflux ratio and reflux temperature, amounts of product taken from the top and bottom of the column, the degree of separation between key components, as well as the heat loads at the top and bottom of the column. With the exception of the operating pressure of the column, which is usually fixed (except in the case of two-effect processes), all other parameters can be considered as operating variables. By means of sensitivity analysis, design specifications, or optimization techniques, the optimal values that meet the separation requirements can be determined, thereby minimizing the condensation load and the reboiler heat load and reducing the energy consumption of the distillation column. At present, due to the increased commercialization of general-purpose simulation software, the operations of large chemical companies are generally carried out under optimal conditions; however, the distillation processes in some smaller and medium-sized chemical enterprises still need further optimization. 1.2 Energy savings through intermediate heat exchange: In distillation columns where there is a large difference between the temperatures at the top and bottom of the column, it is possible to save or recover heat (or cooling capacity) by adding intermediate heat exchangers. There are two methods of intermediate heat exchange: intermediate condensers and intermediate reboilers. For the bottom reboiler (taking the bottom reboiler as the reference), the intermediate condenser is used to recover heat, while the intermediate reboiler is used to save heat ； As for the top condenser (taking the top condenser as the reference), the intermediate condenser saves cooling capacity, while the intermediate reboiler recovers cooling capacity. l. If the load on the intermediate condenser and intermediate reboiler is high, the heat loads on the top condenser and bottom reboiler will decrease. This leads to a reduction in the reflux ratio in the distillation section and the vapor ratio (gas-phase reflux ratio) in the stripping section. To maintain the purity of the product, it is necessary to increase the number of tray levels as a result of this reduced reflux ratio, which in turn increases the cost of equipment investment. Figure 1 shows a schematic diagram of these two intermediate heat exchange methods: if there is a significant variation in the temperature distribution below the distillation column, an intermediate reboiler can be used with a low-grade heat source to reduce the consumption of high-grade heat from the main reboiler; however, this reduces the separation capacity of the trays below the intermediate reboiler ； When there are significant variations in the temperature distribution in the upper part of the tower, an intermediate condenser can be installed, using a refrigerant of lower quality to reduce the amount of high-quality refrigerant required in the main condenser and thereby cut energy consumption; however, the separation efficiency of the trays above it decreases. The intermediate heat exchange method is classified as an energy-saving technique in process engineering because the original distillation tower remains unchanged; it is merely the addition of intermediate heat exchange that alters the slope of the operating line, thereby making use of low-grade energy. Given a fixed separation task, the heat supply to the reboiler at the bottom of a conventional distillation column is equal to the sum of the heat supplies to the reboiler at the bottom of the distillation column and the intermediate reboiler in a column equipped with an intermediate reboiler. The total heat load required remains unchanged before and after installing the intermediate reboiler. Only after installing an intermediate reboiler can part of the heat be supplied by inexpensive waste steam at a lower temperature than that from the bottom reboiler, thereby reducing the thermal efficiency of the tower and improving overall thermal efficiency. For a given distillation column, by properly setting up and using intermediate reboilers, the highest thermal efficiency can be achieved, resulting in the greatest energy savings. 2 Energy savings in special distillation processes 2.1 Heat pump distillation Heat pump distillation relies on the compensation or utilization of mechanical work to transfer heat from the low-temperature area at the top of the distillation column to the high-temperature area at the bottom of the column, thereby allowing the low-temperature vapor at the top of the column to be used as a heat source for the reboiler at the bottom. 2.1.1 Types of heat pump distillation: Depending on the external energy consumed by the heat pump, heat pump distillation can be divided into two types: the steam-pressure type and the absorption type. There are two types of heat pump distillation using steam pressure: the steam compressor method and the steam injection method. In the steam compressor approach, taking into account the balance of heat loads on the condenser and reboiler as well as ease of control, additional coolers and heaters are often included in the process. Depending on the process, the steam compressor method can be further divided into four types of processes: indirect type, direct compression of overhead gas type, split type, and liquid flash reboiling at the tower bottom type. Among them, the indirect heat pump distillation process operates using a working fluid (refrigerant) in a separate closed cycle, while the direct compression type based on the top gas is a heat pump that uses the gas at the top of the tower as its working fluid ； The split-type heat pump distillation process consists of two columns: the upper column functions similarly to a conventional heat pump distillation unit, with the addition of an inlet ； The lower column is similar to the stripping section of conventional distillation, that is, the evaporator column (or stripping column); the feed comes from the bottom liquid of the upper column, while the vapor product enters the bottom of the upper column ； Flash reboiling is a variant of heat pumps; it uses the liquid outlet from the bottom of the tower as the refrigerant, which is then sent to the top of the tower after throttling to exchange heat, absorb heat and evaporate into a gas, and after being compressed to increase pressure and temperature, it returns to the bottom of the tower. The steam compressor approach is suitable for the following systems: ① In cases where the temperature difference between the top and bottom of the tower is small; as long as this difference is less than 36°C, good economic results can be achieved ； ②Situations where the boiling points of the substances to be separated are close to each other, making separation difficult; a high reflux ratio is required, thus a large amount of steam is needed ； ③ During low-pressure operation, a refrigerant must be used for condensation; whereas to use cooling water or air as the condensing medium, it is necessary to separate certain volatile substances at higher tower pressures. A steam-jet heat pump is a specialized device for increasing the pressure of low-pressure steam. Its principle involves using the high-speed steam flow generated by the injection of high-pressure steam (driving steam) to raise the pressure and temperature of the low-pressure steam, while simultaneously reducing the pressure and temperature of the high-pressure steam. The pressure and temperature of the low-pressure steam are increased to levels suitable for use in the process, thereby achieving energy savings. 2.1.2 Several issues to consider when applying heat pump technology. Heat pump technology is widely used due to its efficient and energy-saving properties, but it is not suitable for use under all conditions. Feasibility should be assessed from the following aspects: (1) Whether there is a high-quality heat source – typically, such a source should have a high temperature, stable supply, be located close to the heat pump’s installation site, and not be corrosive or prone to scaling, thereby minimizing wear on the equipment ； (2) To determine whether there is an appropriate heat demand, the suitable heating temperature must be established based on the type of heat pump used, in order to ensure good economic efficiency for the heat pump system ； (3) Are the operating costs low? Due to the change in heating methods, other types of consumption increase accordingly; it is necessary to evaluate whether this approach is economically viable. Generally, a heat pump can achieve an energy savings rate of over 30% in order to result in lower costs compared to boiler-based heating ； (4) It is also necessary to pay attention to whether the adoption of heat pump technology has other effects on the original system, such as resilience to unexpected failures, adaptability to load changes, and the overall heat balance of the system. 2.2 Thermocouple Distillation Thermocouple distillation has attracted widespread attention due to its energy efficiency and reduced equipment costs. The earliest form of thermocouple distillation was proposed 50 years ago by Petlyuk. Studies showed that thermocouple distillation can save at least 30% more energy compared to conventional distillation processes, but it was difficult to implement on an industrial scale due to the technical limitations of that time. In recent years, with increasing demands for energy efficiency and advancements in control technology, research on thermocouple distillation has become more active again, and some large companies have industrialized the partition wall distillation columns developed in this field. This article mainly introduces the characteristics of thermocouple distillation, as well as various processes and current application status, in order to promote research and application of this technology in China. Thermocouple distillation is mainly used for the separation of three-component mixtures or for dividing such mixtures into three separate products. It can take the following forms: ① A side-stream distillation column, which consists of a main column and a side-stream distillation column ； ②The side-line stripping column consists of a main column and a side-line stripping column. ③ Complete thermocouple distillation was first proposed by Petlyuk; hence it is also known as the Petlyuk distillation column. It is made up of a main column and a pre-distillation column. The function of the pre-distillation column is to carry out a preliminary separation of the mixture – the light components are all removed from the top of the column, while the heavy components are completely extracted from the bottom. The intermediate components are distributed between the top and bottom of the column. The role of the main column is to further separate the materials from the top and bottom of the pre-distillation column in order to obtain products that meet the required standards ； ④In a vertical baffle column, vertical baffles are used inside the column to divide it into two sections; this structure can essentially be considered as combining the main column and the pre-separation column of a Petlyuk distillation column within a single column. For a given material, tray tower distillation requires a lower reflux ratio compared to conventional distillation processes, which increases operational capacity; energy savings of over 60% can be achieved, and equipment investment can be reduced by 30%. Partitioned tower distillation units can be widely used in petroleum refining, petrochemicals, chemicals, and gas purification. There are two main reasons for the energy savings in thermocouple distillation columns: ① Thermocouple distillation columns effectively address the issue of remixing of intermediate components within the column ； ② The material entering the main column from the thermocouple distillation pre-column has a composition that can match well with that of the feed tray in the main column, satisfying the requirements for an optimal feed tray. The thermocouple distillation process is not suitable for all chemical separation processes, and its application has certain limitations. This is because, although such columns possess the most ideal system structure from a thermodynamic perspective, they rely primarily on the \"reuse\" of heat supplied to the distillation column. 2.3 Multiple-effect distillation: Multiple-effect distillation uses multiple columns in place of a single column; the columns have different energy levels, and the energy from the columns with higher energy levels is utilized by those with lower energy levels, thereby achieving energy savings. The process flow of multi-effect distillation can be divided into co-current, counter-current, and mixed-flow types, depending on the flow directions of the heating steam and the material ； Based on the number of effects, they can be classified as two-effect (double effect), three-effect, four-effect, etc., with the two-effect (double effect) type being the most common. Double-effect distillation involves dividing a single distillation column into two columns that operate at different pressures; the difference in pressure between these two effects allows the condenser of the first effect to work in conjunction with the reboiler of the second effect. Compared with distillation in conventional distillation columns, double-effect distillation can make full use of the inherent temperature difference between the cold and hot fluids, reduce the irreversibility of heat transfer, and cut down on the consumption of utility resources; however, it increases the cost of equipment. The 4 basic types of double-effect distillation are: (1) Countercurrent type: The feed is divided into two roughly equal streams, which are fed into the high-pressure and low-pressure columns respectively; the steam from the top of the high-pressure column supplies heat to the bottom of the low-pressure column, with products being extracted from both columns at their tops and bottoms. (2) LGH type: The process starts with feeding into the high-pressure tower; the product from the top of the high-pressure tower serves as the feed for the low-pressure tower. Products are obtained from the bottoms of both towers, while all the products from the tops of the towers come from the top of the low-pressure tower. (3) Flow HGL type: This process also takes feed only from the high-pressure tower; the product at the bottom of the high-pressure tower serves as the feed for the low-pressure tower, with products being obtained from the tops of both towers. The product at the bottom of the tower is taken only from the low-pressure tower. (4) Counter-current type: All feedstocks enter the low-pressure column, with the product obtained from its bottom being sent as feedstock to the high-pressure column. Products are obtained from the tops of both columns, while only the high-pressure column has a product output at its bottom. 3 Current Status and Trends in Energy Saving for Distillation Processes in Our Country In recent years, due to energy shortages, there has been vigorous research and development in technologies for energy saving in distillation processes. On the one hand, with the development of computer technology and software, more and more large-scale chemical engineering software is being commercialized. Static simulation software such as Aspen and ProII has become essential tools for chemical engineers in design and optimization tasks. Dynamic simulation software such as gPORMS, as well as CFD software used to study the flow properties of substances, are also gaining popularity to a certain extent. All of these have contributed to a better understanding of the patterns and fundamentals underlying distillation processes, which in turn facilitates research on energy savings in such processes. On the other hand, the technologies related to various special distillation processes are becoming increasingly mature, and they are now being put into practical use in industrial processes. For example, heat pump distillation is widely used in the treatment of propylene-propane systems and ethylbenzene-p-xylene systems, while the use of thermocouple distillation in butadiene systems has also yielded good energy-saving results. There is a significant gap between our country and foreign countries in terms of theoretical research on energy conservation in distillation processes, as well as in technology development and application. Abroad, research on energy-saving methods that combine various approaches is already underway, but no such reports exist in China; the gap is particularly evident in practical industrial applications. This is also related to the overall relatively backward level of process technology in China’s industrial production, such as in the area of study on split-wall distillation columns used in thermocouple distillation. So far. At least 40 sets of partition wall distillation columns are in commercial operation, most of them belonging to the German company BASF, and they are primarily used for separating ternary mixtures with a high content of intermediate products. To date, there are no reports of industrial applications of partitioned towers in our country. Therefore, our country should strengthen research on the application of energy-saving technologies, especially research on the application of partitioned distillation columns, which are currently the most promising. 4 Conclusions The distillation process attracts considerable attention among chemical unit operations due to its high energy consumption and significant potential for energy savings. This paper summarizes the main energy-saving techniques for distillation processes, providing an overview of the types, advantages, disadvantages, and application ranges of these techniques. It offers guidance for chemical engineers in the design and optimization of distillation processes, and also identifies the key areas for further research and development in energy-saving technologies.