Optimization of distillation tower using the concept of internal thermal integration and exergy analysis

Master's Thesis in Chemical Engineering Process Design

Abstract:

In this thesis, a method for thermodynamic analysis and evaluation of energy consumption of a distillation tower by using side cooling is introduced. The research done in the past was aimed at the distribution of the heat diagram and checking the possibility of adding or extracting heat from all the different parts of the tower. The main problem of these types of diagrams is that they are referred to reversible towers and the goals obtained from these diagrams cannot be effectively used for modifications in the real tower. One of the main features of the proposed method is to obtain the minimum driving force by applying the defined conditions to achieve the desired goals in real towers. The distribution of exergy loss in real towers is by using side converters in the tower to optimize energy, which leads to significant energy savings. In addition, in order to achieve the goals, a new approach has been designed by determining the best place to place the side transducer, and using this design, improvements are made in this tower. In this method, a normal tower can be simulated using industrial simulations and the desired goals can be implemented on it.

In this research, the result of the simulation on a multi-component system using this approach and the feasibility of the desired goals are presented and examined. From the comparison of this method with the previous methods, we come to the conclusion that there are no problems such as temperature changes and errors caused in the thermal load after replacing the welder and side cooling that were created in the previous methods, and there will be no need for additional simulations and more detailed investigations. Drive

-1- Introduction

One ??of the most important parts of industrial units is the distillation unit. Distillation operation towers are one of the most common unit operations in process industries, and at the same time, they are the most expensive unit in terms of energy consumption. This process has taken a significant share of energy consumption in the industry. Therefore, today, due to the excessive increase in the consumption of energy carriers, as well as global energy prices, efforts to find ways to save energy in distillation operations have become doubly important. Reducing energy consumption in distillation operations today can also be very effective in reducing the finished price of products. Therefore, optimal energy consumption and comparison with a standard criterion and analysis and interpretation of the deviation from the standard state are essential in the stages. For this reason, providing methods to reduce energy consumption compared to standard criteria is of particular importance and is receiving attention. To improve the efficiency of energy consumption in distillation towers, many ideas are available in relevant references. The modification and optimization of the distillation tower for better energy efficiency is a very complex task]1[.

1-2- Investigating energy consumption in the distillation tower

The necessary energy in a distillation process is provided through the boiling point. Due to the availability and economy of the process, sources of heat supply for the welder generally include water vapor, hot oil, or furnaces. The light components evaporate and enter the vapor phase, while the heavy components condense and are transferred from the vapor phase to the liquid phase [2]. The hot vapors inside the tower move upwards and come into contact with the liquids moving downwards in several stages on the trays and reach equilibrium if there is enough time. Vapors condense in the upper part, which is rich in light components. A part of these liquids is returned to the tower and a part is removed from the top of the tower as a product. Figure 1-1 shows the general view of a distillation tower [2]. The heat from the condensate is often transferred to air or water or both, and sometimes it is used as a preheater for the feed stream or in other cases. In fact, the amount of this energy is considerable, and this makes recycling it more attractive.But due to its lower temperature level compared to other stages of the tower, especially the boiling one, it is not possible to use it usefully for heating. In addition, a part of it is also transferred to the environment by radiation and movement from the body of the tower [2]. The thermal energy injected in the welder is used to evaporate the liquid rich in heavy components, and on the other hand, the aforementioned energy is discarded to provide the external return flow. However, this energy cannot be recovered and used due to the low temperature of the condenser compared to other stages [2]. 1-3- Expression of methods to improve the efficiency of energy consumption in the distillation tower

A number of different situations have been proposed to check the status of energy optimization of the tower in the past, which include:

- Checking the status of the number of trays or Return flow speed

- Determining the input feed tray

- Using the welder and side cooling in the design of combined problems

- Internal thermal integration

One ??of the newest methods used to improve energy consumption efficiency is internal thermal integration.

In a conventional distillation tower?, the temperature of the absorption section is lower than that of the rejection section, and therefore the boiling energy cannot be used in the condenser. Now, if the absorption part has a more significant pressure than the rejection part, then it is possible to reach a higher temperature in the absorption part and save the boiling energy. Another approach that is proposed is to investigate the improvement of the tower efficiency by considering the internal thermal integration of the tower and the effect of the boiling heat load and the side cooling on the tower efficiency at the same time, which has not yet been properly addressed. By using a suitable method and a new design, the best amount of heat load and the best location for the side exchanger can be achieved, which helps to improve the energy efficiency of the tower. In this thesis, this new approach has been investigated]2[.

1-4- Necessity of conducting research

Optimization through systematic simulations for the entire existing settings? requires a long time in terms of time and calculations and is very difficult and difficult, and it is also not economically viable. The common method of improving the energy efficiency in the distillation tower is to use the minimum thermodynamic conditions of the columns based on the reversible tower model. In the past, countless magazines and publications have studied the use of reversible towers for binary systems and its importance [3 and 4]. ABSTRACT In this study a methodology for thermodynamic analysis and distillation column 'targeting' is presented, with emphasis on the use of side condensers. Research in the past has been towards the establishment of a heat distribution curve, showing the way in which heat can be added or extracted across the different column sections. One major disadvantage of these profiles is that they refer to reversible columns, and cannot be used effectively to target for modifications in a real column. The main feature of the proposed methodology is the introduction of a minimum driving force, defined in terms of exergy loss distribution of the existing column, to set realistic targets for side condensing in real columns, resulting in considerable energy savings. In addition to providing realizable targets, the new approach also provides the design engineer with information about the best location to place a side exchanger, and the required additional column modifications. The methodology can be applied using conventional column models in commercial process simulation programs, but can be significantly simplified by using reboiled and refluxed absorber models in a bespoke program. Simulation results for modified designs set by the new approach, for multicomponent separations, verify the feasibility of the targets.