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CO2 Extraction Vessels: 4 Basics, 6 special requirements, 7 Important Parts, 4 Key Design Requirements

The supercritical CO2 fluid extraction vessel is one of the keys to the development of supercritical fluid extraction technology. It needs to solve the problems of machinery, heat exchange, fluid transportation, and safety assurance that the process is under high pressure (generally 8-35Mpa or higher). It should have the characteristics of safety, easy operation, strong versatility, and continuous operation under the premise of meeting the production purpose.

Top 4 Basic Knowledge

CO2 extraction vessels
CO2 extraction vessels

How to choose a CO2 extraction vessel?

Key reading: An important parameter of the extraction kettle structure is the aspect ratio. For solid materials, the aspect ratio is between 1:4 and 1:5. For liquid materials, the aspect ratio is about 1:10.

According to the regulations on pressure vessels, supercritical CO2 extraction vessels are high-pressure equipment, and different CO2 extraction vessels should be selected for different forms of materials. An important parameter of the structure of the CO2 extraction container is the aspect ratio. For solid materials, the aspect ratio is about 1:4~1:5, and for liquid materials, the aspect ratio is about 1:10. Due to the high pressure of the CO2 extraction vessel, the study of high-strength special steel can reduce the wall thickness of the CO2 extraction vessel and save materials and costs.

The CO2 extraction and separation vessel can be set up with one-stage or multi-stage separation according to the separation purpose and can be combined with processes such as rectification and adsorption to achieve the purpose of extraction, separation, and purification.

The capacity of the CO2 extraction equipment

Laboratory supercritical CO2 fluid extraction vessel, its volume is generally less than 5000mL, simple structure, (some particularly small tabletop CO2 extractors are not equipped with CO2 circulation system), the pressure capacity can reach 70Mpa, suitable for laboratory exploratory work. The smallest tabletop CO2 extractor has a capacity of 2mL, which can be used directly with analytical instruments. It is mainly used to prepare supercritical CO2 extractors for analytical samples.

The capacity of the CO2 extraction vessel of the pilot-scale supercritical CO2 extraction equipment is generally 10~20L, its compatibility is good, and the CO2 can be recycled.

The volume of the CO2 extraction vessel of the industrial production unit is generally 50L to several cubic meters, and BIT has been able to reach the industrialized CO2 extraction equipment from 50 to 1000L.

Three levels of standard capacities of the CO2 extraction vessel

The following are the extraction vessels that can be used in the stand-mounted double extraction vessel structure CO2 extraction machine:

  • Small CO2 extraction machine: 100ml, 200ml, 500ml, 1000ml.
  • Pilot level CO2 extractor: 2L, 5L, 10L
  • Industrial CO2 extraction machines: 20L, 25L, 30L, 50L, 100L, 150L.

Note: When there are three or more extraction machines with 100L and 150L CO2 extraction vessels, the platform structure must be used.

Two working combinations of the CO2 extraction vessel

When the object to be processed by supercritical CO2 fluid extraction is a solid material, batch operation is adopted.

Single CO2 extraction vessel: If a single CO2 extraction vessel is used, it will take more than one hour to heat up, pressurize and cool, and it will take several hours to circulate.
More CO2 extraction vessels: In order to improve production efficiency, a semi-continuous operation mode with two or more CO2 extraction vessels in parallel can be used, that is, when one extraction kettle is working, the other can be unloaded and loaded, and the extraction kettle cover needs to adopt a quick-opening structure to save energy. The auxiliary operation time can stabilize the composition of the extraction stream, so as to achieve the purpose of improving the product quality.

How to arrange the CO2 extraction vessel of the CO2 extractor?

There are two types to arrange the CO2 extraction vessel of the supercritical CO2 extractor, the rack type, and the platform type.

The extraction vessels of the double extractor CO2 extractor are arranged on the vessel holder. Under normal circumstances, the CO2 extraction vessels will be arranged side by side and horizontally. When the vessel is very large for some special reason, it can also be arranged vertically on the right side of the CO2 extractor (such as special aspect ratio, or special high pressure such as 600bar extraction pressure will be resulting in a large vessel diameter).

CO2 extraction vessels of platform arrangement
CO2 extraction vessels of platform arrangement

Generally, the capacity of supercritical CO2 extraction machines using vessel rack arrangement will not be larger than 300L (150L+150L). For CO2 extractors larger than 300L, we will design them as platform types to provide a more stable operating space.

Extraction vessel for CO2 extraction machine
CO2 extraction vessel in rack arrangement

Reading Highlights: What volume of CO2 extraction vessels use a platform arrangement?

Capacity Combinations of CO2 Extraction Vessels

Usually, the capacity of the CO2 extraction vessels is the same, which facilitates the simultaneous extraction of the extraction work.
Of course, there are also special configurations, such as a small laboratory CO2 extraction machine with a capacity of 6L, which consists of 1L and 5L CO2 extraction vessels, 1L is used for extraction parameter experiments, and 5L is used for extraction process scale-up experiments.

Choice of the separation process in CO2 extraction

According to the characteristics of difficult separation and collection of the materials to be extracted, an appropriate separation method must be selected. Generally, in order to improve the separation efficiency and yield, a separation method combining two separation vessels and a fractionation column can be used, so that the extract can be effectively separated.

Six Special Requirements for CO2 Extraction Vessels

CO2 extraction vessels
CO2 extraction vessels

The CO2 extraction vessel is the key equipment in the supercritical CO2 extraction process technology. Compared with the equipment used in other separation methods, its manufacturing cost is greatly increased due to the high operating pressure. The new type of CO2 extraction vessel that is easy to use, safe, and low-cost is of great significance to speed up the industrialization of supercritical fluid extraction technology.

According to the characteristics of the extraction process, the design and calculation of the supercritical CO2 fluid extraction device for solid materials can be divided into two parts:

  1. Design the quick-opening sealing device according to the basic structure of the CO2 extraction vessel;
  2. Carry out the strength calculation of the CO2 extraction vessel cylinder.

The operating pressure of supercritical fluid extraction is generally 7-35Mpa, and some can reach more than 100 Mpa, but it is often subjected to alternating loads. Therefore, in addition to meeting the requirements of general high-pressure vessels, high-pressure vessels for supercritical fluid extraction of solid materials also the following 6 special requirements must be met.

Special Requirements 1#: Meet Health Regulations

Extracting solutes in solids with supercritical fluids is often used in food, medicine, cosmetics, and other industries. The equipment should meet various health regulations for food and medicine production.

Special Requirements 2#: With Quick Switch Cover Device

The intermittently operated supercritical CO2 fluid extraction vessel needs frequent material replacement operations, and it takes a lot of time to open and close the top cover of the vessel. Therefore, the use of a quick switch device can reduce the operation time and improve production efficiency.
The quick-opening device is mostly used in medium and low-pressure containers, and the supercritical CO2 fluid extraction requires that the quick-opening device can be used in high-pressure or even ultra-high-pressure containers. At present, many high-pressure containers in China have adopted quick-opening devices. For example, the food puffing kettle adopts a toothed structure, so the supercritical CO2 fluid extraction container adopts the clamp structure and so on.

Special Requirements 3#: Good Fatigue Resistance

When the pressure vessel is subjected to alternating loads, fatigue failure is likely to occur in the high-strain regions where plastic strain occurs, such as the root of the nozzle. my country’s fatigue design code stipulates that when the whole part of the container bears the alternating load no more than 1000 times, and the non-integral part does not exceed 400 times, fatigue analysis is not required. Intermittent operation of supercritical CO2 fluid extraction container, each extraction cycle is generally 2~3h, one year is calculated as 300 days, and it needs to be boosted and depressurized 2400 times, such as 15 years of life, CO2 extraction container is subjected to alternating loads It is 36,000 times, which has already exceeded the above regulations. Therefore, the CO2 extraction container is required to have good fatigue resistance.

Special Requirements 4#: Easy Temperature Control

The operating temperature has a great influence on the solubility of supercritical fluid, and endothermic or exothermic phenomena often occur during the extraction process, which destroys the temperature balance in the CO2 extraction vessel. Therefore, good temperature control is required in supercritical fluid extraction. Currently, in most cases the
The method of introducing circulating water into the sleeve to ensure the temperature in the CO2 extraction container is constant, this method is simple and easy but not effective; This method can reduce the heat transfer resistance and improve the temperature control effect in the CO2 extraction container, but the structure is complex, easy to block and the flow of circulating water is limited; it can also be controlled by placing a bend such as a coil in the CO2 extraction container. The purpose of temperature, but is limited by the operation process, and this method cannot be used on many occasions.

Special Requirements 5#: Compact Structure and Low Cost

When designing a CO2 extraction container, the container should be compact in structure, simple in manufacture, and low in production cost under the premise of satisfying the strength. If the hemispherical head and the cylinder are connected at the same thickness, it is not only a waste of material but also cumbersome. If the unequal thickness connection is used, the steel can be saved and the manufacturing cost can be reduced, but the problem of stress concentration caused by the discontinuity of the structure at the connection needs to be solved.

Special Requirements 6#: Security Device

Accidents caused by misoperation can be prevented.

7 Important Parts Introduction

Vessel closures

These CO2 extraction vessels are breech lock vessel closures on top and plug and screw closures on the bottom.
This closer mechanism uses a nut to hold down a plug to allow the CO2 extraction vessel to pressurize: seals on this CO2 extraction vessel O-rings that create a seal on the sidewall of the extraction vessel.

Read more: Quick-Open Construction of Supercritical CO2 Extraction Vessels

Loaded and unloaded

Extraction vessels are loaded and unloaded from the top of the vessel. When loading, unloading and cleaning these vessels be sure to keep the face surface and threads clean. Failure to keep these surfaces clean can cause the CO2 extraction vessel not to seal correctly.

Locking Vessel Isolation Valves

Each CO2 extractor has two locking isolation valves that should be utilized whenever the operator is working in any vessel. These valves are intended to valve off the inlet and outlet of the CO2 extraction vessel to protect the user from any release of pressure.

WARNING: Failure to use these valves can lead to injury.

Temperature Control Heat Exchangers

The temperature control heat exchangers regulate the temperature of the CO2 just before it enters the CO2 extractor vessels and is in a cooling loop along with the extractor vessel water jackets and the extractor flow switch.

Extractor Flow Switch

The flow switch continually monitors for water flow in the CO2 extractor loop and will stop the system if the flow is lost.

Vent Valves

Each CO2 extraction vessel has its own vent valve. These valves will open automatically or manually to vent the extractor so they are safe to open. These valves should be vented to the vent valve manifold (located on the Separator Stand) or outside.

Pressure Gauges

CO2 extraction pumps
CO2 extraction pumps

Extractor A, Extractor B, and CO2 extraction pump outlet are located on the extractor operation panel(they are digital displays, but also can be pointer meter displays if the customer insists we will adopt such a scheme) and correlate to the pressures on the HMI (Human Machine Interface) for each portion.

Four Key Requirements for CO2 Extraction Vessel Design

CO2 Extraction Vessel Design
CO2 Extraction Vessel Design: Stress Testing

The CO2 extraction vessel should be designed according to the original data and process requirements provided by the design task, that is, to meet the process design conditions first.

The design conditions of CO2 extraction vessels are usually represented by design condition diagrams, which mainly include schematic diagrams, design requirements, and takeover tables. The schematic diagram schematically shows the container body, the shape of the main internal parts, the main structural dimensions, the position of the nozzle, the form of the support, and other contents that need to be expressed.
Design requirements mainly include the following:

  1. Working medium: medium scientific name or molecular formula, main components, relative density, and hazard, etc.
  2. Pressure and temperature: working pressure, working temperature, ambient temperature, etc.
  3. Operation method and requirements: indicate continuous operation or intermittent operation, and whether the pressure and temperature are stable; when there is fluctuation in pressure and temperature, the frequency and range of change should be indicated; for containers that are opened and stopped frequently, indicate the annual start-up , the number of parking;
  4. Others: It should also indicate volume, material, corrosion rate, design life, whether it has safety accessories, whether it is insulated, etc.
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