As there is no competition for adsorption, this process easily dehydrates ethanol to the desired level of purity so that it can be considered fuel grade. The size of the pores of both Type A and Type X molecular sieves is closely controlled during the manufacturing process. Sodium, calcium, and potassium ions can be exchanged with one another in the molecule to regulate the size of the pore opening.
This allows for preferential adsorption of gas and liquid molecules. As hard as you may try, you are not going to get your vehicle into the garage. She has a master's and bachelor's degree in Chemical Engineering and Petroleum Refining from the Colorado School of Mines, and over 22 years of experience in the oil and gas industry. She has published 9 technical papers at international conferences and is a coinventor on two technology patents in the gas processing industry.
About the Presenter MR. To check if the sieves are active, you can hold them in your hand while wearing gloves and add water to them. If they are completely active, then the temperature rises significantly, and you will not be able to hold them even while wearing gloves. The use of safety equipment like PPE kits, gloves, and safety glasses is recommended as the process of activation of the molecular sieves involves dealing with high temperatures and chemicals, and the associated risks.
Contact Us today for more information. Molecular Sieve vs. Silica Gel Silica gel can also be used as a moisture removing desiccant but is very different from a molecular sieve. The key differences between a molecular sieve and silica gel are: The rate of adsorption of a molecular sieve is greater than that of silica gel. This is because the sieve is a fast-drying agent. A molecular sieve functions better than silica gel in high temperatures, as it has a more uniform structure that binds water strongly.
At low Relative Humidity, the capacity of a molecular sieve is far better than that of silica gel. The structure of a molecular sieve is defined and has uniform pores, while the structure of silica gel is amorphous and multiple irregular pores.
How to Activate Molecular Sieves To activate molecular sieves, the basic requirement is exposure to super-high temperatures, and heat should be high enough for the adsorbate to vaporize. Activated Alumina. Furthermore, in contrast to molecular sieves, alumina and silica gel are large-pore adsorbents and, therefore, more co-adsorption of hydrocarbons will occur. Similar to molecular sieves, water will displace the coadsorbed components since it adsorbs stronger to the adsorbent; however, co-adsorption will also influence the length of the MTZ.
More importantly, when regenerating the adsorbents, these co-adsorbed species will leave the adsorbent as peaks. If the regeneration gas must be treated, then the treating unit must be sized such that it can process these peaks. This factor significantly increases the cost of such a unit. The majority of topics discussed to this point are related to the chemistry involved in the process.
However, other factors are equally important from a design point of view—more specifically, those determined by process engineering and economics. In gas processing, a plant is typically designed and operated with a single purpose: to process natural gas so that the maximum amount of condensates can be recovered, and so that produced gas adheres to certain specifications. In general, the gas is produced for use in the natural gas grid or for liquefaction and transport to markets.
The economic aim for such a lineup is to minimize the number and size of vessels, process steps and plot space. The total amount of gas and liquids to be processed is the economic basis of such a project.
The design of the vessel must adhere to maximum flow and minimum flow criteria. In gas phase systems, gas flow during adsorption moves downward to prevent fluidization of the bed under abnormal flow conditions.
The diameter of the bed can be calculated once the superficial velocity is determined. Some of these criteria are of importance to the main process flow, but they should also apply to the regeneration gas flow in each stage of the temperature profile , which, for gas processing vessels, is typically upflow.
Apart from these criteria, the regeneration gas flow must carry enough heat into the system to ensure that the beds are properly regenerated. Sufficient flow will ensure the stripping efficiency of the desorbed molecules, and the beds should be cooled within the time frame available. Particularly when the regeneration gas is sent back to the feed, a minimum regeneration flow ensures minimum-size regeneration loop equipment and, even more importantly, helps reduce the size of the adsorber vessels.
For lineups such as those shown in Fig. As the regeneration flow is upflow, another important design constraint is to avoid fluidization of the bed.
The length L and diameter D of the beds are limited mainly for practical and economic reasons. One important limitation is the strength of the adsorbent particles, especially those at the bottom of the bed. These should be sufficiently strong to withstand the sum of the pressure drop over the bed, the weight of the bed above them when saturated with water, and the weight of the pressurized gas.
A more practical observation is that bed heights of more than 10 m are rarely observed. In such cases, the adsorber vessels tend to become the highest units at the site. Minimizing the diameter is an important economic constraint.
A larger diameter is associated with greater wall thickness, which will increase the cost of the vessel. A sketch of a typical adsorber vessel is shown in Fig.
The flow is reversed during regeneration, and the same requirement applies to the bottom portion of the bed. A careful observer will notice that the difference in size between the ceramic ball layers never exceeds a factor of 2. This ratio is preserved to lower the probability that smaller particles will migrate between larger particles. Sketch of a regenerable adsorber vessel. Note: Sketch is not to scale. Dump ports, manholes and probes are not included.
This depletion will ultimately result in an early breakthrough of the bed. As the capacity of a molecular sieve bed is limited and declines as a function of the number of regeneration cycles, an important design criterion is the required lifetime i. That depends somewhat on the application; in general, molecular sieve beds are changed out during a major shutdown of the plant.
Major turnarounds are planned every 2 yr—3 yr, as various pieces of kit require maintenance. For an LNG site, turbine and compressor maintenance needs determine when a major shutdown occurs. For most sites, the timing is every 4 yr. Although this timing provides the process engineer with a time frame for designing a molecular sieve bed, a translation to the number of cycles that fits into that period is required because the beds must retain sufficient water uptake capacity at the end of the 4-yr period.
An adsorption cycle comprises several steps. The major steps are adsorption and regeneration, whereby the regeneration step can be subdivided in heating, cooling and standby steps.
Although regeneration can take place at lower temperatures, the consequence is that more water will remain on the molecular sieve, thereby reducing the effective water uptake capacity. A typical regeneration profile is shown in Fig. The step during ramp-up is inserted to prevent an overly rapid heating, which can result in hot water formation.
0コメント