The regeneration of molecular sieves is the most crucial step in temperature swing adsorption TSA processes. It determines the performance and operating cost of molecular sieves over a given lifetime. It is well known that molecular sieves age with each regeneration cycle. The adsorption process on the molecular sieve is a complex phenomenon and often poses challenges for the design and operation of the units, due to a number of parameters that govern the efficiency of such processes. Poor design decisions and deviations from design conditions can cause a wide range of operating issues in a molecular sieve unit.
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Molecular sieve - Wikipedia
The regeneration of molecular sieves is the most crucial step in temperature swing adsorption TSA processes. It determines the performance and operating cost of molecular sieves over a given lifetime. It is well known that molecular sieves age with each regeneration cycle. The adsorption process on the molecular sieve is a complex phenomenon and often poses challenges for the design and operation of the units, due to a number of parameters that govern the efficiency of such processes.
Poor design decisions and deviations from design conditions can cause a wide range of operating issues in a molecular sieve unit. Most of the damage from these upsets or deviations from design conditions usually occurs during the regeneration step. If not addressed properly, these upsets can result in reduced lifetimes, increased pressure drops across beds, loss of product quality, formation of hydrates, increased system corrosion or increased plant downtime—all of which have cost implications.
It is, therefore, important to properly design and operate the regeneration phase of molecular sieves. The main design principles and recommendations for the regeneration step in TSA processes are presented here, along with ways to address key issues concerning troubleshooting, optimizing and adapting plant operations under different constraints.
Part 1 focuses on the general design philosophy and key operating parameters, and Part 2 will concentrate on the regeneration gas and contaminants.
Industrial adsorption on zeolite molecular sieves. The principles of industrial adsorption on the zeolite molecular sieve have been well documented in the academic and industrial worlds. They are made of a three-dimensional succession of SiO 4 and AlO 4 tetrahedra sodalite cages in the presence of metal ions to balance the negative charges created by the trivalency of aluminum.
This results in a final network of repeated crystals with channels and uniform size cavities known as porosity that exhibit a large and electronically active surface area. In the separation application, for an impurity to be efficiently adsorbed, the molecule of the impurity should be polar and of an appropriate size that corresponds to the pores present in a given type of zeolite.
The diameters of the pores and cavities depend on the type of zeolite and on the nature and size of the charge-compensating cations, which are situated around the pores. The zeolite crystals are compounded with a binding clay to form beads or extrudates. Zeolites, also known as molecular sieves, are used industrially in the fixed beds to purify a gas or a liquid stream. Depending on the application and molecules to be captured, an appropriate type of zeolite is chosen.
The adsorption capacity of a molecular sieve is governed by isotherms function of temperature and pressure , and once the molecular sieve is saturated with impurity molecules, the equilibrium conditions need to be changed either temperature or pressure or both to desorb the impurity and reuse the molecular sieve for adsorption.
Adsorption with molecular sieves is usually performed in systems having at least two vertical absorbers, one in adsorption and one in regeneration. For high flowrates, several adsorbers can be used in parallel, as shown in Fig. The regeneration stage features cooling and heating in series, for the case where the regeneration gas flowrate must be reduced.
During adsorption, a molecular sieve bed is modeled by a three-zone system, as illustrated in Fig. Close to the inlet is the equilibrium zone EZ , where the adsorbent is in equilibrium with the process fluid—i. The MTZ shows a gradient of the impurity concentration, which decreases to a required outlet specification and can be defined as the length required for the adsorbent to bring the impurities from their initial concentration to the final specification.
The third area is made of fresh adsorbent that, for a given adsorption time, has not been in contact with the impurities. Sieve design. A number of parameters influence the design and operation of a molecular sieve unit. The operating temperature and partial pressure of an impurity, along with the type of molecular sieve 3A, 4A, 5A, 13X defines the adsorption capacity of that impurity. Flowrate and pressure drop constraints, coupled with the adsorption capacity, are important to select the appropriate flow regime, mass transfer kinetics and, by extension, the design of vessels.
An adapted pore size of molecular sieve, along with an appropriate bed configuration based on adsorbent size and composed of big particles, small particles or a split bed , also influence the size of vessels. The choice of an appropriate diameter-to-height ratio will influence the pressure drop and flow distribution, as well as the regeneration duty requirement. Regeneration of molecular sieves is normally performed in either the pressure swing adsorption PSA processes or in the TSA processes.
In the first case, the change in the adsorption equilibrium is obtained by decreasing the pressure. PSA processes, which are usually used in non-aggressive applications with less stringent outlet specifications i.
TSA processes, usually used in applications with stringent outlet specifications 0. A majority of natural gas, petrochemical and refinery units are TSA based. The design for these units is provided by molecular sieve suppliers specializing in the know-how of these applications. The PSA process is usually designed by technology licensors, and the molecular sieve supplier role provides appropriate product and as-needed operational assistance, so that a unit is less susceptible to deviations or problems.
TSA-based regeneration processes are discussed here. Regeneration phase design. It is well known that molecular sieves age over time and with each regeneration cycle.
The speed and the way the sieves age determine the lifetime of a unit. Therefore, it is important to properly design and operate the regeneration phase. An incorrect pressure change rate can lead to sub-zero temperatures, resulting in ice formation that will damage molecular sieve structure. Channeling or lifting of the bed can occur if an appropriate pressure drop per length is not respected.
The heating step during regeneration is critical for several reasons: The gas must be clean enough to avoid side effects which are usually exacerbated at high temperature , and it must carefully address the required regeneration energy duty i. Correct temperature ramps should be provided to avoid damage of the molecular sieves by thermal stress. An intermediate heating step is often recommended in the drying applications to avoid the phenomenon of hydrothermal damaging of the molecular sieve.
An appropriate temperature profile that is adapted to the specific type of molecular sieve should be provided, along with sufficient heating time, to attain a stable outlet temperature plateau to mark the completion of regeneration. The type of regeneration gas e. After heating, a cooling phase is needed before switching back to adsorption; this helps avoid a temperature peak that might disturb the downstream process. A wide range of operating issues in the molecular sieve units can be caused by poor design decisions, as well as deviations from the design conditions like change of process conditions, presence of contaminants, 2,9 emergency shutdowns, etc.
A major part of the damage from these upsets usually happens during the regeneration step at high temperatures. If not addressed properly, these upsets can result in reduced lifetime, increased pressure drop across beds, loss of product quality, hydrate formation, increased system corrosion or increased plant downtime—all of which have cost implications.
Over the course of this two-part article, recommendations for the design of the regeneration step in TSA processes will be presented, along with how these recommendations can be adapted for several key issues that industrial operators may face while operating a molecular sieves unit, and which EPC companies may face when designing a new unit. Regeneration of molecular sieve. The equilibrium adsorption capacity of an impurity on a given molecular sieve is determined by using isotherms.
As an example, Fig. At the end of the adsorption phase, the molecular sieve is saturated and no more impurity can be adsorbed. However, the adsorption is reversible. The adsorbed impurity can be desorbed and eliminated from the molecular sieve by shifting the adsorption equilibrium and modifying the operating conditions—or, more commonly, by increasing the temperature, by decreasing the pressure, or by doing both.
At the end of regeneration, the molecular sieve can recover most of its original adsorption capacity—i. Thermal regeneration is a widely used method in the oil and gas industry. To regenerate the molecular sieve, several steps are generally involved:. The regeneration stage often starts with a depressurization step. The depressurization is required if the regeneration is performed at a pressure lower than that of the adsorption phase.
This may arise for different reasons, as the regeneration gas is not the product gas, but rather another gas available at a lower pressure, such as N 2 , H 2 , CH 4 and fuel gas. The lower pressure can also be chosen to optimize the regeneration energy requirement. In liquid applications, the depressurization step can be followed by a draining step. During the draining step, all liquid must be evacuated from the adsorber before the introduction of a heating gas.
The liquid is generally drained by gravity flow. At the end of the draining step, almost all of the liquid is drained, except for a very small portion that is trapped in the molecular sieve and on the surface of the adsorber vessel. This liquid is removed during the purging step, where cold regeneration gas is introduced for 1 hr—2 hr to strip off this remaining liquid.
Following this step, the heating step is started by introducing hot regeneration gas into the vessel. The temperature is slowly increased to the heating temperature to avoid thermal stress.
The energy needed to obtain suitable regeneration is achieved through the sum of four components:. A hot gas with a certain flowrate, a specific temperature and a given flow time supplies the heating energy necessary for the regeneration.
As can be seen from the isotherms, the temperature rise causes a decrease of the impurity loading on the molecular sieve.
Passing a hot gas stream has two effects: first, desorption of the impurity due to the temperature rise, and second, transport of the desorbed impurity out of the bed. This means that a certain quantity of heat at a minimum temperature should be brought into the adsorber to desorb the impurity. Furthermore, if the regeneration gas flowrate or duration is too small, then an accumulation will occur on the top layers of the molecular sieves due to a lack of desorption energy.
In the case of laminar flow or channeling, the gas does not pass through the entire bed due to the preferential path of the flow, but rather through only part of the bed. This may lead to localized accumulation in the bed. At the end of the heating step, the molecular sieve is regenerated.
However, the impurities are not completely desorbed. A certain residual content of impurity always remains in the molecular sieve, corresponding to the equilibrium conditions at the end of heating and the thermal profile observed by the molecular sieve. The value of this residual impurity content depends on the quality of regeneration i. The result is a reduced adsorption time for the next adsorption cycle.
Molecular sieve suppliers often design the unit based on the end of lifetime adsorption time. This means that, initially, the adsorption time and, therefore, the adsorption capacity of the molecular sieve is higher when the molecular sieve is fresh. Throughout its lifetime, this adsorption time decreases due to aging. These factors mean that the molecular sieve unit can be operated in two ways:. After heating, a cooling phase is needed before switching back to the adsorption; this helps avoid a temperature peak that would usually cause significant disturbance to the downstream process.
The energy needed to obtain suitable cooling is the sum of three components:. A cool gas with a certain flowrate, a specific temperature and a given flow time supplies the energy necessary for cooling. At the end of the cooling step, the vessel is repressurized to the adsorption pressure, and also refilled with liquid in case of liquid applications, and then put on standby before switching to the adsorption phase of the next cycle.
The standard recommendations for several of these parameters, and how they can be adapted in the case of process deviations, will be presented in Part 2.