Abstract: aiming at the problem that the electric heater of conventional air separation plant is used as a heat source to heat sewage nitrogen and conduct molecular sieve analytic regeneration, this paper puts forward that the steam heater is connected in series in the system and the heat energy of part of the surplus steam in the steel mill is utilized, the design idea of energy saving transformation of molecular sieve system. Through the comparison of technological process before and after the transformation, the description of technical advantages and economic benefits after the transformation, and from the perspective of energy conservation and environmental protection, the article expounds the advancement and necessity of technological process transformation, the work focus and direction of the gas operation company in a certain period in the future have been clarified.
Preface
Heater is widely used in air separation equipment, petrochemical industry, food industry and metallurgical industry. It mainly consists of electric heater and steam heater. In the air separation equipment, it mainly uses electric energy or thermal energy of steam condensation to heat the polluted nitrogen. When the polluted nitrogen is heated to a certain temperature, it can be used for the regeneration of molecular sieve system.
The traditional air separation plant mainly focuses on electric heaters, but electric heaters need a large amount of electricity. With the increasing shortage of energy, the country attaches great importance to energy conservation and emission reduction. In the investment of air separation plant, as long as there is a steam condition, it is an inevitable trend to replace the electric heater with the steam heater.
This paper discusses the energy-saving transformation of the air separation plant which has been put into operation and used electric heater, using part of the surplus steam in the steel mill to connect a steam heater in series in front of the electric heater, the polluted nitrogen is preheated by the steam heater first and then heated by the electric heater, so as to achieve the purpose of saving power consumption, energy saving and emission reduction.
1. Technological process
1.1. The purification system process of molecular sieve with conventional electric heating (Figure 1)
MS1/2 --- molecular sieve adsorber EH1/2 --- electric heater SL1 --- silencer
After the sewage nitrogen is heated by electric heater, the molecular sieve is desorbed and regenerated. Take a set of "20,005" air separation plant as an example, the power of each electric heater needs 1500kw.
1.2. The process of the modified molecular sieve purification system (Figure 2)
2. Type selection of steam heater
The steam heater can choose two structural types: high-wing composite pipe type or low-wing in-line type.
2.1. High-wing composite pipe steam heater (figure 3 ):
The steam heater has no tube plate, but adopts the form of collector tube. One collector tube corresponds to a set of heat exchange tubes respectively. Each steam heater is divided into N groups, and the collector tube at the bottom can float freely in the barrel, the heat exchange pipe adopts the form of rolling stainless steel coat aluminum tube into high-threaded tube. The steam heater product is relatively mature, stable and reliable in operation.
2.2. Low-wing direct discharge steam heater (Figure 4)
The steam heater is designed to be a staggered type of steam and process gas. The gas is swept across the bundle at one time, and the inlet and outlet tubes of the gas are at the same elevation, which is convenient to install, thus greatly reducing the floor space of the whole equipment. The heat exchange pipe adopts carbon steel or stainless steel integral rolling-fin tube, which has high heat transfer efficiency. Therefore, the overall structure is compact, the volume is small and the weight is light.
2.3. Both of the two types of steam heaters introduced above can meet the technical conditions of transformation. As this article discusses the transformation of electric heaters of the air separation plant which has been put into operation, the situation of each project is different, the layout of equipment and pipeline is also different, therefore, reasonable selection should be made according to the site space and the actual situation of on-site pipeline.
3. Transformation advantages
This article discusses how to use part of the steam in the steel mill to reform the electric heater of the air separation plant that has been operated, and connect a steam heater in series in front of the electric heater, the polluted nitrogen is preheated and then heated by electric heater to achieve the purpose of saving electricity consumption, energy saving and emission reduction. This transformation plan is simple and feasible, with less investment and high return. It is an effective technical scheme to reduce operation cost, save energy and reduce consumption of air separation plant.
Take a set of "20,005" level of air separation as an example, the power of each electric heater is 1500kw, using the steel mill's about 0.4mpa steam source, connecting a steam heater in series, the power is 900kw, after the sewage nitrogen is heated by the steam heater, the outlet temperature can reach 120℃, and then the sewage nitrogen of 120℃ is heated by the electric heater. The power of the electric heater is only 500kw, which is calculated by 12 hours a day, then the annual power consumption can be reduced (1500-500) * 12*365 = 4.38 million kw · h, calculated by 0.67 yuan/kw · h, the annual saving of electricity fee is about 3 million yuan.
Steam heater power is 900kw, steam consumption is about 1.6 t/h, then the annual steam consumption is 1.6*12*365 = 7000 t, calculated by 110 yuan/t, then the annual steam demand is about 800,000 yuan.
According to the above calculation, after the implementation of the project, each set of "20,005" level of air separation, can achieve the annual saving of operating costs about 2.2 million yuan.
The project needs to add a new steam heater and matching valves, meters, pipelines and so on. The total investment of the project is expected to be 1 million yuan. According to the above analysis, the cost can be recovered in half a year, the economic benefits are very obvious.
4. Conclusion
According to the requirements of national and local relevant standards, specifications and energy development policies, make use of the surplus saturated steam in the steel mill pipeline section and make full use of the project heat allowance to connect a steam heater in series in front of the electric heater, to implement the transformation of electric heater, so as to achieve the purpose of reducing power consumption and saving energy.
The first domestic reconstruction project has been applied in Hebei Qianjin steel plant with obvious benefits.
This technology can be widely applied in similar air separation units, especially in many Hangyang gas companies, which can create great economic and social benefits for manufacturing and operating enterprises.
