Dehydration: Molecular Sieve Bed To Remove H2O

 

ADSORPTION Molecular Sieve Bed to Remove H2O

Lando Deardo Siringoringo 2002322012

Molecular Sieve

 

Types Ty pes of Molecular Sieve Sie ve Basic Principles

Outlines

Mechanism of the Process & Technology Applications Factors Affecting Adsorp Adsorption tion

Advantages & Disadvantages

 

Overview Prevention of hydrate formation

ehydration methods : a. Absorbtion Absorbtion (Glycol drying) b. Adsorption Adsorption (Silica  (Silica gels, Activated Alumina, and Molecular Sieve) Sieve) c. Condensation (Cooling with injection of hydrate hy drates s inhibitors inhibi tors (glycols or methanol)). d. Membran processes. e. Chemical method hygroscopic salts. There are two main reasons why the  process needs to be done dehydration:

Avoid the corrosion

 

Definitions

Adsorption The afinity for a molecule molecule to adhere to the surfaceof surfa ceof a solid.

Gas dehydration : Adsorption • Silica Gel -65 C (-85 F) • Activated Alumina -80 C (-112 F) • Mole Molecular cular sieve < 0.1 ppmv

Adsorption

 

 

 

Adsorbent

Adsorbent

The solid solid that th at holds h olds the molecule on on its surface.

Adsorbate

Adsorbate

The molecule molecule is being being held h eld onto the th e surface.

 

Molecular  Molecular Sieve Molecular Sieves are synthehtic zeolite ma terials engineered with pores Molecular of precise and u niform structure and size. This allows them to preventially adsorbt gas and liquids based on  molecular size and polarity. Small pellets of inorganic materials with extremely small pores (usually 3-5 Ang stroms). Most Most organic solvents are too small to fit into these pores, but water can. Adsorption units are capa ble of reaching extremely low specifications, which makes them viable pieces of equipment for incorporation into a process lineup. A major advantage of molecular sieves is that they can be regenerated, which reduces the required required amount am ount of molecular sieve to economically feasible quantities..

Types  of of  Molecular Molecular  Sieve Types

 

3A The 3A molecular sieve is an alkali metal aluminosilicate with a pore opening of approximately 3 angstroms. angstroms. The 3A molecular sieve will exclude most m ost molecules except exce pt water, making it very selective. This type of bead also has advantages in crush strength, durability and high rate of adsorption.

4A The 4A molecular sieve is an alkali metal aluminosilicate with an effective pore opening of approximately 4 angstroms. Type 4A beads bead s can be used to adsorb water, ammonia, methanol, ethanol and carbon dioxide. This type of molecular sieve is often used to remove moisture from gas and liquid streams, where co-adsorption of sulfur compounds and carbon dioxide is not a concern.

5A The 5A molecular sieve is an alkali metal aluminosilicate with an effective pore opening of 5 angstroms and is the calciumexchanged form of the type A zeolite. This product is also effective for the bulk separation of normal and iso-paraffin hydrocarbons. This type of molecular m olecular sieve has a high rate of adsorption and desorption, a higher rate of contamination resistance and a high crush strength.  

13X The 13X molecular sieve is the sodium form of zeolite X and has a much larger pore opening than the type A crystals, with an effective effe ctive pore diameter of 10 angstroms. Type 13X offers enhanced adsorption performance over the type A zeolite, and it can remove impurities too large to fit into the type Atozeolite crystal cages. It isthe a lso used separate nitrogen from airoften to produce a high-purity oxygen stream.

Source : Leading Molecular Sieve Manufacturer & www.molecularsievedesiccant.com

 

Commercially Available Types  

Natural gas dehydration dehy dration when COS minimi minimization zation and/or MeOH is present.

Type 3A

H  S + CO 

COS + H  O. Less Le ss water capacity and not as rugged as Type 4A.

 

Workshore of the industry. Used in most natural gas drying applications. Will adsorb H S on active sites.

Type 4A

 

 

Removal H S and normal RSH RSH from natural gas. gas. Less water capacity and less COS COS  

Type 5A

formati formation on compared to type 4A. Coadsorbs propane propane and higher normal hydrocarbons.   Type 13X

Removal of H S, nRSH, and iRSH from natural gas and LPG. Higher Higher water capacity than type 4A.  

Basic Principles

 

IV

Regenerator Column When heated the impurities are vaporized

III

source : ChemSurvival ChemSurvival entreprises, LLC - 2 018

Adsorber Column

and released by the adsorbents.

Impurities enter the pore space and are held on the surface inside of the voids.

II

Solid Soli d Adsorbents Adsorbent s Very porous solids with hug e surface areas.

It

 

The effecti e ffectivene veness ss fo forr wate waterr removal is based on two important characteristics:   • The channel diameter diamete r acting as a filter, which limits the number of species that can co-adsorb.   • The polar environment created in the channels, which creates an environment env ironment in which preferentially polar molecules are adsorbed.

 

How  It How It  Works?

 

Mechanism Mechan ism of the Process An adsorption adsorption unit used for water removal is called called a dehydration unit (DHU). A DHU often consists of two or more vessels, filled with molecul molecular ar sieves, that adsorb water during an adsorption period and are subsequently regenerated using a heated stream of treated gas.  

• • •

  A & B (adsorber) C (regenerator)  

to adsorb the wat water er.. to regenerate the adsorbent.

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Process in Adsorption Vessel

In the adsorbing bed, the capacity-limited portion is situated at the top. It is saturated with water under the feed conditions of temperature, pressure and water concentration, and is referred to as the saturation zone (SZ). The part of the bed below the SZ that is engaged in dehydrating the gas from feed water concentration (wet) to effluent water concentration (dry) (dr y) is called called the mass-transfer mass-tran sfer zone (MTZ) (MTZ) . During adsorpt ion, there the MTZ migrates migrates from of tthe he bed bottomadsorption, of the bed, by lengthening length ening thethe SZ.top Once MTZto the leaves the bed, bed, breakthrough breakth rough occurs and a nd the bed must be taken offline for regeneration.

t he vessel Feed gas enters the

Diffusion of water through

through the the inletwater stream. Adsorbtion onto the pore wall (Equilibrium zone).

gas into the th e pores (MTZ). (MTZ).

The cylical nature nature of the process result in an active

The product exits through the outlet stream.

zone.

 

During adsorption, adsorption, a molecular sieve bed is modeled by a three-zone system. syste m. Close to the inlet is the equilibr equilibrium ium zone (EZ), where whe re the adsorbent is iin n equilibr equilibrium ium with the process proc ess fluid saturated w with ith impuri impurity ty a att the partial pressure and temperature t emperature cond conditio itions. ns. is follo fo llowed wed byof the mass transfer zone (MTZ), (M TZ), The EZthe where dynamics ads adsorpti orption on take place. The M TZ MTZ shows a gradient of the impuri impurity ty conc concentration, entration, which w hich decreases to a required outlet specification 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 adsorb adsorbent ent that, for a given adsorption adsorption time, has not be been en in contact with the impurities.  

 

Typical Deactivation and  Molecular Molecular  Sieve Changeout Profile important parameter Another important paramete r to take into account is that the capacity of the adsorb adsorbent ent ove time as a function functi on ofdecreases the number numbeover r ofrregeneration cycles; therefore, end-of-run (EOR) capacity must be used when cons considering idering the required amount of adsorbent. adsorbent. When the capacity of the adsorbent falls below the level where all a ll water in the feed can be adsorbed during the minimum adsorption time, then the adsorbent must be replaced.

 

Structure of Adsorber Bed

 

Regeneration Regeneration Molecular Sieve shoul should d be in typical cycli cyclic c systems consti removal ofand the purging adsorb adsorbate ate from the gas. molecular molecul arconstitutes sievetutes be bed d by he heating ating with a carrier   Sufficient Suffici ent heat hea t must be applied to raise raise the temperature te mperature of the adsorbate, adsorb ate, the adsorbent and the vessel to vaporize the liquid and offset offset the heat of wetting we tting the molecular-sieve molecular-sieve surface. The bed temperature tempe rature is critical critical in regeneration. regene ration.   The high temperature during regeneration c causes auses wate waterr to desorb from from the molecul molecular ar sieve, a process call called ed te temperature mperature swing adsorption (TSA).   After regeneration, regene ration, a cooling cooling period is necessary to reduce the molecular molecul ar sieve temperature tempe rature to within 15℃ of the temperature of the stream to be proc processed. essed.   For optimum regeneration, gas flow should be counterc countercurrent urrent to adsorption adsorption during the he heat at up cycle a and nd conc concurrent urrent (relative to the process stream) during cooling.  

 

Regeneration In classical applications, TSA heater realized as an ordinary burner or as athe shell and tube is heat exchanger warmed by steam or by hot oil. The regeneration gas warms in the heater and flows into the column. In the column passes through the adsorbent andgas. the water desorbs into the regeneration   The water saturated regeneration gas then flows into the cooler. cooler. The cooler usually uses cold air to to decrease temperature of the regeneration gas. When the the water saturated regeneration gas is cooled, partial condensation of the water wat er occurs. The regeneration gas is led further into the separator, where the condensed water is removed.  

 

Regeneration  Temperatures Typical Regeneration

 

Factors Affecting Adsorption

Characteristics of adsorbent

Temperature

Polarity of the Substance The polar environment created in the channels, which Absorbate creates an Molecule Size environment in which preferentially polar molecules are . The adsorbate molecules that can adsorbed are adsorbed.

molecules whose diameter is smaller or equal to the pore diameter of the adsorbent.

When the temperature is low, the ability adsorption increases so so tha t the adsorbate increases.

pH Organic acids are more easily adsorbed at low pH, whereas organic base adsorption is effective at high pH.

The pore size, surface area of the adsorbent., and the purify of adsorbent are chara cterist cteristic ic important adsorbent

Contact Time Contact time is very important because the process reaches

Agitation The adsorption rate is controlled by both film and pore diffusion. depending on the level of agitation in the system.

the time of adsorption, equilibrium and is equilibrium economical.

 

Upstream of a LNG Plant Upstream of Cryogenic Gas Plants

Upstream of NGL Extraction Plants

Dehydration for LPG and NGL streams.

etc.. etc

 Applications

in Industry

Advantages

 

Very low dew point and water content can be obtained.

Best suited for large volumes of gas under very high pressure .

Dehydration of very small quantities of natural gas at low cost.

Insensitive to moderate change in gas temperature, flow rate, and pressure.

Some types can be used for simultaneous simul taneous dehydration and sweetening.

They are relatively free from problems of corrosion, corrosion, foaming,, etc. foaming

Disadvantages

 

The most expensive adsorbent.

The regeneration temperature is very high (operating cost).

Pressure drop is too high.

High space space and weight required.

Mechanical breaking and contamination contaminat ion of liquid, oil, and glycol are possible.

 

“Love what you do; Do what you love” love” Wayne W. Dyer

 

Reference Kidnay, Arthur J., William R. Parrish, and Daniel G. McCartney. Fundamentals of natural gas processing. CRC press, 2019.   Malino, H .M., http://www.jmcampbell.com/tip-of-the-month/2015/05/ http://www.jmcampbell.com/tip-of-the-month/2015/05/ benefits-of-standby-time-in-adsorption-dehydration-process/, PetroSkills – John M. Campbell, 2015   Hawes, P., “Molecular sieves in natural gas processing,” GPA Europe Young Professional Training Day, Manchester, UK, February 11, 2016.   Gas Processing Processing & LNG ; Gulf Publishin g Holding s LLC LLC   http://www.chemxin.com/news/html/?486.html   https://www.molecularsievedesiccants.com   http://m.pm-chem.com/desiccant/molecular-sie http://m.pm-chem.com/des iccant/molecular-sieve-adsorbents/ ve-adsorbents/ molecular-sieve-type-5a.html   https://www.youtube.com/watch?v=4iWdidLLklc  

 

Thank You Any Questio Questions? ns?