Development of all low-voltage small air separation equipment

In the 1990s, with the deepening of reforms and technological progress, large-scale air separation and liquefaction equipment provided the ability and means to transport liquid products. Tank trucks for liquid oxygen, liquid nitrogen and liquid argon were sold all over the country. Due to the good quality, reasonable price, and easy storage of liquid products, many small oxygen plants have ceased production and turned into filling stations and distribution points for large liquid sellers. Even many small oxygen plants for self-production have stopped production and switched to purchases. liquid.

   Such a change is undoubtedly a severe test for small air separation equipment manufacturers. The main reason is that large-scale air separation equipment has introduced technology and products in recent years, and technology has advanced rapidly, which has greatly reduced production costs. Although the technology of small air separation equipment has also been improved, it has not made much progress. The technical gap with large air separation equipment is getting wider and wider, and the technical indicators of small air separation equipment are seriously out of date.

   This raises a question for us: how to retain or even develop the living space of small air separation equipment through technological progress? After 3 to 4 years of painstaking research, we have proposed an energy-saving approach for small air separation products-to achieve a full low pressure process. We believe that in order to compete with liquid supply for small air separation equipment, the first step is to reduce energy consumption and improve reliability. The way to achieve these two points is to use a full low pressure process.

  1. Determination of all low pressure small air separation process

  At that time, the smallest all-low-pressure small air separation equipment developed by domestic industry factories was 350N3/h (standard state, the same below), using a positive flow expansion process [1] [2]. For the two smaller air separation plants of 80m3/h and 170m3/h that we plan to develop, there are two options for forward flow expansion and backflow expansion. The process flow is shown in Figure 1 and Figure 2. We made a serious and in-depth analysis of their advantages and disadvantages.

  1.1 Product extraction rate

   Due to the large unit cold loss of small air separation equipment, the use of positive flow expansion will inevitably prevent a larger proportion of expanded air from entering the tower, which will significantly reduce the product extraction rate. Although the backflow expansion will affect the extraction rate due to the increase in the pressure of the upper and lower towers, the decrease is not large.

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Figure 1 Schematic diagram of the process flow of forward flow expansion Figure 2 Schematic diagram of the process flow of back flow expansion

  1.2 Air compressor discharge pressure

   Backflow expansion requires a certain pressure before expansion to ensure the cooling capacity, so the air compressor discharge pressure is required to be 0.65~0.75MPa. The positive flow expansion can theoretically be 0.5MPa, but because of the use of a gas bearing turbo expander, it is necessary to ensure that the supply pressure of the bearing gas forces the air compressor to increase the discharge pressure. Although the expansion volume can be reduced by increasing the pressure in front of the expander, most of the processing air can only be throttled and pressure-reduced into the lower tower, and the pressure energy is obviously not fully utilized.

  1.3 Starting time

Starting time is a problem that users of small air separation equipment are very concerned about. Long starting time and increased user costs, but the most important thing is to affect the intermittent starting performance of the equipment. Generally speaking, the starting time is long and the intermittent starting time is also long. The worse the adaptability. The backflow expansion process can increase the inlet pressure during the starting phase, thereby increasing the expansion ratio and intake air volume, and shortening the starting time. Although the positive flow expansion process can also increase the inlet pressure of the expander during the start-up phase, the effect is not obvious due to the small proportion of expansion.

  Through the above demonstration, we believe that the reflux expansion process for air separation plants below 300m3/h is reasonable, and the overall technical performance is better than the forward flow expansion process.

  2. Design of unit equipment

  Unit equipment is the foundation and guarantee of the entire air separation equipment. To improve the performance of the air separation equipment, we must start with each unit:

  2.1 Air compressor

   Because of the low-pressure process, it has the conditions to choose the finalized oil-free lubrication air compressor from professional domestic and foreign manufacturers, which improves the reliability of the equipment, enhances the versatility, and reduces the cost of use. In order to select the finalized air compressor, the parameters of the air separation equipment can only be determined according to the capacity of the finalized air compressor.

  2.2 Precooler

   In the design of the pre-cooler, we have introduced the design concept of large-scale air separation equipment and adopted a nitrogen water pre-cooling system. The needs of users of small air separation equipment are usually oxygen as the main component and nitrogen as a supplement. When the amount of nitrogen is not large, nitrogen is relatively rich, which provides conditions for the use of nitrogen-water pre-cooling systems. Compared with the use of freon pre-coolers, this solution not only reduces energy consumption, but more importantly, reduces the number of moving parts, and the maintenance of freon refrigerators has always been a problem for users. The nitrogen water pre-cooling system has only a high-pressure circulating water pump, which reduces maintenance requirements and costs, and improves reliability.

  2.3 Purifier

   By adopting a double bed design, the regeneration temperature is reduced; at the same time, the heat of the air compressor is used to preheat the regeneration gas, and the power consumption of the electric heater is significantly reduced.

  2.3 Turbo expander

  The backflow expansion process provides good working conditions for the gas bearing turboexpander: one is the low speed and reliable operation; the second is the bearing gas pressure is guaranteed; the third is the expansion ratio is small and the efficiency is high. In the five years since the first equipment was put into operation, no expander has been repaired.

  2.5 Fractionating Tower

   By adopting a sealed cold box and improving the cold retention performance, the cold loss is reduced and the separation efficiency is improved. The cold box adopts an all-aluminum structure to reduce the possibility of leakage. The necessary technical measures have also been adopted to shorten the start-up time of the equipment.

  3. Security and other functions

  ⑴ Safety is an issue that must be paid attention to in the design, manufacture and operation of air separation equipment. In the design of small all-low-voltage air separation equipment, considering that the equipment has an operating cycle of 12 months, during which there are many intermittent shutdowns, if there are no safety measures, major accidents may occur. For this reason, after careful consideration, a liquid oxygen safety vaporization device is set up to ensure the safe operation of the equipment.

  ⑵ Quick start is of great concern to most oxygen stations. Due to various reasons such as gas sales, moving equipment maintenance, and power outages, the time of intermittent operation is often uncontrollable. If the downtime is short and the liquid evaporates less, the target can be reached soon after restarting. For a longer shutdown, we use liquid oxygen recharge to speed up the start (it can also be used in the normal start process). Our refilling process not only uses cold energy, but also recovers oxygen, and compresses and fills the bottle with an oxygen compressor.

  ⑶ Variable load operation. In order to ensure the economic operation of the equipment and enhance the user's adaptability and adaptability to the market, in addition to enhancing the equipment's ability to respond to short-term shutdowns, we also achieve changes in production load through changes in the air compressor supply. To enable the air separation plant to operate in a drastically changed working condition, each unit equipment must also adapt to this load change, which undoubtedly puts forward high requirements on the design.

  ⑷ Instrumentation and control system. Generally speaking, the instrument control of small air separation equipment is relatively simple, and the equipment of instrument control is directly related to the long-term stable operation of the air separation equipment. In the design, we use a differential pressure transmitter to measure liquid level, product flow, etc., which are displayed by a digital display. Recently, we have improved the technical level of the instrument control system by one step. The pre-cooler chilled water circulating pump adopts frequency conversion control technology, while the purifier, fractionator, expander and other parts adopt PLC control and LCD touch screen display, and strive to make the air separation equipment instrument control The level has been significantly improved.

  4. Product technical parameters and operating conditions

   Carrying out product development according to the above-mentioned design concept, we first started the development of KDON-80/40 air separation equipment in 1997, and successfully commissioned it in early 1998. All performance indicators are better than design requirements and contract indicators. In the next two years, we have carried out a full range of follow-up tests on the whole set of equipment to understand the condition of the equipment in the long-term operation process, and carried out various tests on the equipment to fully grasp the first-hand of all low-voltage small air separation equipment Data, and summarize and improve. On this basis, in 2001, we designed and developed the KDON-170/100 full low pressure air separation equipment. The performances are better than the design requirements and contract values, and of course it is better than the KDON-80/40 full low pressure air separation equipment. equipment. The main technical parameters of KDON-80/40 and KDON-170/100 all low pressure air separation plants are shown in Table 1.

Table 1 Main technical parameters of KDON-80/40 and KDON-170/100 all low pressure air separation equipment

Table 1 Main technical parameters of KDON-80/40 and KDON-170/100 all low pressure air separation plants project KDON-80/40型 KDON-170/100型 Design specifications Measured index※ Design specifications Measured index※ Oxygen production/m3/h 80 88.5 170 193 Oxygen purity/% 99.5 99.6 99.6 99.6 Oxygen outlet cold box pressure/MPa(A) 0. 2 0.2 0.18 0.18 Nitrogen production/m3/h 40 41 100 107.4 Nitrogen purity/PPmO2 ≤3 ≤2 ≤3 ≤2 Pressure of nitrogen out of cold box/MPa(A) 0.7 0.7 0.65 0.65 Starting time/hour 16 16 14 14 Equipment operating cycle/month 12 12 12 12 Unit oxygen consumption/KW.h/m3 0.75 0.72 0.65 0.63 Total installed power/KW 110 110 222 222 The test report of the Machinery Industry Gas Separation and Liquefaction Equipment Product Supervision and Inspection Center.

In terms of other performance:

a) Disconnect the car in time: After actual testing of the KDON-170/100 full low pressure air separation equipment, after 8 hours of parking, oxygen can be released in 20 minutes after starting, and all design indicators can be reached in 40 minutes (no liquid oxygen perfusion)

b) Quick start: The shutdown time is longer, especially the liquid oxygen perfusion solution during hot start, which can shorten the start time by 6 hours.

c) Variable load operation: Through the combination of different air compressors, the KDON-170/100 air separation plant has a load adjustment range of 75%-100%-125%, that is, the oxygen output is 130m3/h~170m3/h- 230m3/h has segmental load regulation, and the unit energy consumption has little change.

5. Development experience

The advent of the KDON-80/40 and KDON-170/100 all-low-voltage air separation equipment not only created the record of the smallest domestic all-low-voltage process energy-saving air separation equipment, but also proved that small air separation equipment is energy-saving. The full low pressure process is completely feasible. At the same time, it also gives us an important enlightenment: In today's rapid technological progress, small air separation equipment design and production companies must closely follow the technological momentum of large air separation plants and strive to be at the forefront of the world.