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Piston chamber continuous CO2 extraction vessel

Supercritical CO2 fluid extraction technology is a new separation technology developed in recent years. Its principle is to extract specific components according to the difference in solubility of the substances to be separated in a supercritical fluid, so as to achieve a certain separation and purification purpose.

Supercritical CO2 Fluid Extraction

CO2 extraction machine

The application of supercritical CO2 fluid extraction in various industries has developed rapidly, and it has been widely used in food, medicine, tobacco, plants, and petrochemical fields. Since the critical temperature (Tc=31.26°C) of carbon dioxide (CO2) is close to room temperature, and the critical pressure (Pc=7.38MPa) is not high, it has good diffusion performance, low surface tension, non-toxic, tasteless, Non-flammable, easy to obtain and recyclable, so supercritical CO2 fluid has been widely used. Read More: CO2 Extraction Vs Cold Pressed

Supercritical CO2 Continuous Extraction

Although a lot of progress has been made in the research on supercritical CO2 extraction technology in the world, the research on supercritical CO2 extraction equipment is relatively lagging behind. An extraction requires loading and unloading operations, which is an intermittent process. To achieve continuous extraction, we should start with the CO2 extraction process and equipment for in-depth research.

At present, this research has become an emerging topic at home and abroad, mainly focusing on the use of multiple CO2 extraction vessels in parallel to achieve continuous extraction in the process, with the following progress:

  • The continuous CO2 extraction process has been further improved by switching the mechanical valve in the closed-circuit pipeline
  • CO2 extraction equipment fed by continuous screw conveyor: double-screw structure or ball-type continuous CO2 extraction equipment realizes continuous operation of process equipment and promotes industrial applications.
CO2 extraction vessels of platform arrangement
CO2 extraction vessels of platform arrangement

The existing intermittent supercritical CO2 extraction vessel with a quick-opening structure has problems such as long loading and unloading time, energy loss during opening and closing, and safety accidents that are likely to occur once the self-locking of the quick-opening structure fails.

The above-mentioned continuous CO2 extraction equipment still has problems such as complex process and equipment structure, easy blockage of the feeder and unloader of the screw conveyer during the transmission process, and serious friction damage to the extract, which limits the further promotion of engineering applications.

Piston chamber continuous CO2 extraction vessel

In order to solve the above problems, there are laboratories in China that are researching piston chamber-type extraction devices with feasible processes and simple equipment manufacturing. Continuous feeding and unloading methods are used to achieve continuous extraction and improve extraction efficiency.

In the process of research, they solved the problems of reciprocating seal, seal leakage, and sealing body wear of the piston chamber type CO2 extraction vessel under high extraction pressure: according to the needs of the continuous extraction process, by setting multiple piston chambers, the extracted materials were put into the piston chamber to form a chain of piston chambers to realize continuous CO2 extraction. According to the test analysis, the structural design of the piston silo chain continuous CO2 extraction vessel is carried out.

Influence of sealing method on sealing dynamic pressure

The piston silo chain-type continuous extractor is composed of multiple piston silo extraction vessels. The movement of a single-piston silo in the cylinder must meet the requirements of dynamic sealing. Refer to the dynamic sealing structure of the reciprocating compressor. Both sides of the piston silo O-rings are set and multi-channel sealing is used for sealing. Since the liquid phase continuous feeding method is easier to realize than the solid phase, this design is based on solid phase extraction. Since the solid powder is in the process of reciprocating motion The middle-to-seal ring will cause severe wear, so a V-shaped dust stop ring is proposed to reduce the wear effect on the sealing material and improve the service life of the piston silo.

Piston test signal and seal structure
Piston test signal and seal structure: End cap 2. Seal groove 3. Barrel 4. o-ring

Design the piston seal groove structure shown in Figure 1(a) for the test, the end covers on both sides play the role of the piston, and this structure belongs to the radial seal. Design piston outer diameter φd9=60mm, groove width b=4.8mm, groove depth t=2.85mm, O-ring section diameter 3.55mm, cylinder φd4, and piston φd9 adopt H8/h7 fit, piston 3 Both sealing grooves can be installed with O-rings. The left and right grooves can also be equipped with V-shaped dust-blocking rings, and the end cover is threadedly connected with the sealing grooves.

Effect of sealing material on dynamic sealing pressure

Nitrile rubber, fluoro rubber, and polyurethane are selected as the O-ring rubber ring materials for the reciprocating dynamic seal pressure test. Since the fluid temperature has little influence on the dynamic seal pressure, the test is measured at the common extraction temperature range of 40-60°C. Put three O-shaped rubber rings of different materials in the double-channel seals in the grooves on both sides [see Figure 1(b)], and measure the dynamic seal pressure and wear stroke with supercritical CO2 fluid.

The dynamic sealing pressure of the O-ring tends to be stable with the increase of the wear stroke, and the sealing pressure gradually decreases. The dynamic sealing pressure of polyurethane is the highest, stable at about 12MPa, followed by fluorine rubber, at about 10.6MPa, and the lowest of nitrile rubber is about 8. 8MPa, and the fluorine rubber test has a serious swelling phenomenon, while the polyurethane has no obvious change. Generally speaking, polyurethane should be selected as the dynamic sealing material.

Influence of the number of sealing rings on dynamic sealing pressure

Then use a single-channel polyurethane O-ring to measure the reciprocating dynamic sealing pressure [see Figure 1 (c)]. Compared with the double-channel seal, the curve of sealing pressure and wear stroke is obtained.

The sealing pressure of the single-channel polyurethane O-ring has a similar effect to that of the double-channel seal, that is, the initial sealing pressure is higher, and as the wear stroke increases, the sealing pressure tends to be stable (about 8.3MPa), and its stable sealing pressure is higher than that of the double-channel O-ring. The road seal is about 3.7MPa lower.

Influence of V-shape Dust Retaining Ring on Dynamic Sealing Pressure

Piston Chamber Continuous CO2 Extraction Vessel
Figure 4    V-shaped dust barrier

The test shows that the polyurethane material has a high degree of wear resistance. However, after long-term use, solid particles will still wear it and affect the dynamic sealing pressure. A V-shaped dust-blocking ring is designed, as shown in Figure 4.

Structure of O ring and V-shaped dust
barrier seal
Structure of O ring and V-shaped dust barrier seal: End cap 2. Seal groove 3. Barrel 4. O-ring 5. V-shaped dust stop ring

Use this V-shaped dust barrier ring with a single-track polyurethane O-ring, see Figure 5.

Measure its reciprocating dynamic sealing pressure to obtain the relationship curve between sealing pressure and wear stroke.

The V-shaped dust-blocking ring can improve the dynamic sealing pressure to a certain extent, but the improvement is not obvious. From the test results, the solid powder is concentrated on the upper part and recess of the V-shaped dust-blocking ring, but there are few sealing grooves and O-rings, which shows that it can effectively block solid powder and is beneficial to prolong the O-ring. ring life.

Influence of Combination Mode on Seal Leakage

Adding a V-shaped dust-blocking ring can effectively block the wear damage of the O-ring by solid powder, which is beneficial to improve the service life, and has a certain effect on improving the pressure of the dynamic seal. Two different sealing methods of O-ring plus V-shaped dust blocking ring and double O-ring plus V-shaped dust blocking ring were compared and studied. Since the supercritical fluid extraction temperature is mostly between 40 and 60°C, the research found that after CO2 reaches the supercritical state, the temperature change has little effect on the leakage, so the test was carried out at 60°C to measure the leakage of the seal.

When the dynamic sealing pressure is 5~8MPa for a single O-ring plus a V-shaped dust-blocking ring, the leakage increase is not obvious, less than 0.5mL/h;
When the sealing pressure exceeds 8MPa, the leakage will increase sharply, failing the seal. Therefore, the effective sealing pressure of a single O-ring plus a V-shaped dust-blocking ring is 8MPa.

The double-channel O-ring plus V-shaped dust-blocking ring is similar. When the sealing pressure is 5.6~12.2MPa, the leakage rate does not exceed 0.5mL/h; when the sealing pressure exceeds 12.2MPa, the leakage rate changes rapidly. Large, the seal fails, so the effective sealing pressure of the double O-ring plus the V-shaped dust-blocking ring is 12.2MPa.

Wear stroke test analysis

The effective wear stroke of the O-ring can be measured by using a micrometer to measure the number of turns of the lead screw that drives the piston. According to the previous test results, the polyurethane O-ring test is used to obtain the wear amount and wear stroke curve in the 8m stroke.

The wear amount under 0.08mm is the initial wear, and 0.08~0.10mm is equivalent to normal wear. This curve is consistent with the initial wear and everyday wear. Since the pressure changes continuously during the reciprocating motion, to obtain an effective wear Stroke, the wear stroke under 11.0~12.2MPa is taken as the effective wear stroke, the effects wear stroke of the test is converted to 1.7m, and the effective wear stroke of the initial wear amount (0.00~0.08mm) is 0. About 13m.

Analysis of Contact Reliability of O-ring

The principle that the O-ring can achieve a reliable seal is that the elastic body of the sealing material deforms and squeezes the groove to generate contact pressure. As long as the contact pressure is greater than the internal pressure, an effective seal can be achieved, which is a self-tight seal. When extracting the mixture, the CO2 extraction vessel works under static conditions. At this time, the seal is static. After the extraction is completed, moving the extraction vessel is a dynamic seal. To ensure that the extractor can work reliably, in addition to the requirements for dynamic sealing, There are also requirements for static sealing.

Calculation results

When the pressure difference on both sides of the O-ring is 0~25, 0~26, 0~27MPa, the working state of the O-ring is simulated, and the results show that:

When the pressure difference on both sides is 0~25MPa, the maximum contact pressure is about 25.65MPa, when it is 0~26MPa, it is about 26.28MPa, and when it is 0~27MPa, it is about 26.7MPa. It can be seen that when the pressure difference on both sides is 27MPa, the O-ring can no longer achieve reliable sealing, and its maximum static sealing pressure difference is 26MPa.
In the test model, the piston and cylinder will produce certain deformation under the action of internal pressure, and the H8/h7 fit is adopted. When the cylinder fits with the maximum tolerance of 0.089mm, leakage is most likely to occur.

In the process of pressure increase, the deformation of the cylinder is also increasing, so the cylinder cannot be regarded as a rigid body in the calculation.

When the pressure difference on both sides is 10MPa: In the case of a constant pressure difference, the maximum contact pressure of the O-ring is greater than the internal pressure, which can meet the sealing requirements.

O-ring Effective Sealing Stroke Calculation

In the wear stroke test, the sharp wear critical point of the O-ring has not been correspondingly measured, and its service life can be obtained through simulation analysis. It is found that the maximum contact pressure is about 27MPa when the stable wear value is 0.01mm, which is higher than the maximum internal pressure at this time. (26MPa) is large, which can realize effective sealing. When the wear amount gradually increases to 0.14mm, the contact pressure of the O-ring is about 26MPa, which is almost equal to the maximum internal pressure. Therefore, it can be considered that the polyurethane O-ring wears rapidly. The critical value is 0.14mm. It can be seen that the effective sealing stroke of the O-ring is 11.02m.

Read More: co2 extraction vs cold pressed

Final Words

In order to improve the efficiency of the intermittent operation of supercritical CO2 fluid extraction, a continuous CO2 extraction vessel of the piston silo chain type is designed, and the feeding and unloading operations are moved out of the extraction process to realize continuous extraction. The effects of different combinations of single and double O-rings and V-shaped dust-blocking sealing rings on dynamic sealing pressure, leakage, and wear stroke are studied through experiments, and different relationship curves are obtained. Conduct simulation analysis on O-ring contact reliability to determine seal effectiveness and wear travel. The force of the piston chamber was analyzed and the structural design and verification were carried out. A continuous CO2 extraction vessel for fractional extraction of different component extracts was designed, and the realization method of the specific process was given.

Read More: Supercritical carbon dioxide