6 factors reduce the cost of supercritical CO2 fractionation

The energy consumption of the supercritical CO2 fractionation process mainly depends on the amount of heating steam (or other heat carrier) in the bottom reboiler and the cooling water (or other coolant) in the top condenser, both of which are related to the rising steam in the tower. Quantity related. When the reflux rate of the top product is constant, the amount of rising steam is proportional to (R+1), so increasing the reflux ratio R will increase the amount of heating steam and cooling water consumed by the supercritical CO2 fractionation column, and the operating cost will increase accordingly. Read More: Supercritical carbon dioxide
It can be seen that to reduce the heat required at the bottom of the fractionation column, the amount of gas-liquid must be reduced, and the reflux ratio must be reduced to reduce the amount of gas-liquid, and there are many factors that affect the reflux ratio.

Factors 1#: Product purity

In the operation of supercritical CO2 fractionation, the minimum reflux ratio is the lower limit of the reflux ratio of the supercritical CO2 fractionation column. If the actual reflux ratio is lower than this value, no matter how many columns are used, the mixture cannot be separated to the desired product purity. . Relevant data show that the annual cost of equipment accounts for less than 3% of the total cost, while the operating cost accounts for more than 97% of the total cost, of which the heat cost accounts for more than 69% of the operating cost, resulting in the lowest point of the total cost tending to the minimum reflux ratio. Therefore, in the design process of supercritical CO2 fractionation, the operating cost, especially the heat cost, should be taken as the primary basis for selecting the economical reflux ratio. Under the conditions of meeting the separation requirements, the reflux ratio should be as small as possible.

Factors 2#: Number of CO2 Fractionation Column Plates

Under the premise of keeping the product purity unchanged, appropriately increasing the number of column plates can reduce the reflux ratio. According to reports, increasing the number of columns and plates by 10% to 20% can save a lot of heat; but if the number of columns and plates continues to increase, it is of little significance to reduce the reflux ratio, and the energy saving effect is not significant. On the contrary, the cost of equipment will be increased. . Therefore, in the design of a supercritical CO2 fractionation column, it is necessary to balance the cost of increasing the number of columns and the benefit of energy saving. Due to the different methods of increasing the number of columns and plates, the increase in equipment costs is also different. For a plate tower, increasing the number of columns and plates can be achieved by increasing the height of the tower body or reducing the plate spacing, and the cost of increasing the height of the tower body is less than that of reducing the plate spacing, and reducing the plate spacing may cause mist entrainment and reduce the column plate efficiency. With the development of modern technology, various high-efficiency packings have emerged one after another, providing designers with a new method to increase the number of column plates.

Factors 3#: Operating pressure

In the supercritical CO2 fractionation process, the operating pressure affects the average relative volatility of the components, and the average relative volatility affects the minimum reflux ratio. When the operating pressure of the tower is reduced, the average relative volatility can be increased, the reflux ratio can be reduced, thereby reducing the gas-liquid mass flow rate, but at the same time the gas density can be reduced, and the gas volume flow rate can be increased. Therefore, when designing a supercritical CO2 fractionation column , the operating pressure can only be determined according to the simulation calculation of the tower. Generally speaking, for towers that are already high-pressure systems, lowering the operating pressure will increase the relative volatility and increase the processing capacity of the tower; The relative volatility will be reduced, the reflux ratio and the gas will be increased, but the gas flow rate will be reduced more.

Factors 4#: Feed temperature

The simulation of the fractionation column shows that the separation power of the supercritical CO2 fractionation column comes from the heat of the reboiler at the bottom of the column, and the load on the upper and lower sides of the full fractionation column is not uniform, which is related to the working conditions of side draw and material selection.

Generally speaking, the gas-liquid volume at the bottom of the fractionation column is larger than that at the top of the fractionation column, and the liquid flooding of the fractionation column starts from the bottom of the fractionation column.

When the feed is cold, the reboiler at the bottom of the fractionation column is required to provide more volume, which also increases the amount of gas-liquid at the bottom of the fractionation column. For the supercritical CO2 fractionation column in operation, when the number of column plates is constant, when the reflux ratio and the distillate at the top of the fractionation column are constant, the feed temperature has no effect on the heat load of the top condenser, but it is different from that of the bottom reboiler. The heat load of the reboiler is inversely proportional to the relationship. The higher the feed temperature, the greater the molar enthalpy of the feed, and the smaller the heat load of the reboiler. For any system with a given separation requirement, the heat required by the supercritical CO2 fractionation system is basically the same, and the increase of the feed temperature can reduce the heat load of the bottom reboiler, so as to achieve the purpose of reducing energy consumption.

In conclusion

Therefore, when conditions permit, try to choose a higher temperature feed, which can save energy consumption. However, when designing a supercritical CO2 fractionation column, the change of feed temperature will change the position of the operation line in the stripping section, which will lead to a change in the number of theoretical column plates required to complete the same separation task. Realizing the reduction of energy consumption is at the cost of increasing the equipment cost of the tower, how to save energy and reduce consumption requires a total economic calculation.

Factors 5#: Feed status

If the feed state is changed from liquid state to gas state, under the same temperature, the enthalpy of the feed increases, the heat load of the reboiler decreases, and the energy consumption of the supercritical CO2 fractionation system decreases. Therefore, the material feed with larger feed enthalpy can be selected. However, the feed feed increases and the feed state parameters decrease. Under the same reflux ratio, compared with the hot feed, the cold feed can reduce the number of required column plates and reduce the equipment cost of the tower. In the design process of the Jingpian column, we should not only consider the influence of the feed state on the equipment cost, but also fully consider the influence of the feed state on the following parameters, namely the reflux ratio, the bottom heater, and the top coolant. The influence of consumption, gas-liquid load and the number of theoretical plates. For the supercritical CO2 fractionation column in operation, since the number of column plates remains unchanged, the hot feed can save the amount of heating agent. In addition, the feed state will also affect the diameter of the column, so the choice of the best feed state should be determined on the basis of weighing economic benefits.

Factors 6#: Feed position

The determination of the feed position of a general supercritical CO2 fractionation column is mainly determined according to the feed composition. The feed position should be selected on the column plate whose composition of the column plate is close to that of the feed composition. In this case, the non-optimal feed position will increase the reflux ratio, resulting in an increase in the heat load at the bottom of the column, and also cause back-mixing due to the large composition difference, resulting in a decrease in the efficiency of the column plate. Usually, when designing a supercritical CO2 fractionation column, several feed ports are reserved. During operation, a suitable feed position is selected according to the feed composition and product requirements.

Summarize

Theoretically, when the fractionation column has several feeds, the materials of different compositions enter different positions, the separation effect is the best, and the energy saving is also significant, because the mixing of materials of different compositions is the process of effective energy loss. In recent years, due to the successful application of high-efficiency corrugated packing and vertical sieve plates in supercritical CO2 fractionation columns, the superiority of new packing and new column plates has been proved. When designing a supercritical CO2 fractionation column, attention should be paid to the use of new technology, which will produce greater economic and social benefits in terms of energy saving and consumption reduction.