separation co2 avec psa

7/30/2019 Separation CO2 Avec PSA

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Mini Project (RDD-750)Project Title:

Design of Pressure Swing Adsorption system for

removal of CO2 and H2S from biogas using

zeolites.

______________________

Guided By : Submitted By:

Prof. V. K. Vijay Vikas Tiwari

C.R.D.T. Dept. of Chemical EngineeringIIT-Delhi IIT- Delhi


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Table of Contents

Table of contents...1

ABSTRACT..2

1. Objectives of the project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2.Introduction to PSA system....3

3. Advantages of PSA over conventional methods to remove CO2. . . . . . . . . . . . .4

4.Zeolite used for CO2 and H2S removal and its properties.........4

5. Selection of adsorbent on the basis of nature of adsorbent ................................5

6. Zeolite used for CO2 and H2S removal and its properties:................................5

7. Principles of adsorption...................................6

8. Design requirements............................................................................................79. Design Calculations.............................................................................................8

10. Conclusion.......................................................................................................12

11. References........................................................................................................13


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ABSTRACT

This project concentrates on the design of pressure swing adsorptionsystem for the removal of CO2 and H2S gases from biogas produced.

There is large amount of CO2 is present in biogas, removal of which

increases the calorific value of the biogas to extent of natural gas.

Project also illustrates the potential of different zeolites to adsorb

certain gases specifically. So we will be using Zeolite 13X for the

removal of CO2 and H2S in the pressure swing adsorption system.


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1. Objectives of the project :Main objectives of the project can be specified as:

Reading and understanding of the pressure swing adsorption system for the selectiveadsorption of gases from mixture of gases.

Design of pressure swing adsorption system for removal of H2S and CO2 from biogasusing zeolite 13X as adsorbant for volumetric flow rate of 20m3/hr of biogas.

2. Need of removal of CO2 and H2S from biogas:Typical composition of biogas consists around 40% CO 2 and 1000ppm of H2S . So to

improve calorific value of biogas to significant amount we need to remove this CO2 from thebiogas and increase percentage of methane to around 95%. H2S increases corrosion and foul

smeel in the gas so we have to remove H2S also. A typical composition of biogas is given intable 1.

CH4 vol % 50-60%

CO2 vol % 35-40%

H2S conc. 1000 ppm

Table 1. Biogas Composition in terms of CO2 and H2S

And the extent we want to purify this biogas is composition of natural gas so we can use biogasin different applications. Typical composition of natural gas is given in table 2:

CH4 vol % 87-95%

CO2 vol % 2-4%

H2S conc. 25 ppm

Table 2. Natural gas Composition in terms of CO2 and H2S

3. Introduction of pressure swing adsorption:Pressure swing adsorption (PSA) is a technology used to separate some gas species from

a mixture of gases under pressure according to the species' molecular characteristics and affinityfor an adsorbent material. Special adsorptive materials (e.g. zeolites) are used as a molecularsieve, preferentially adsorbing the target gas species at high pressure. The process then swings tolow pressure to desorb the adsorbent material [1].
http://en.wikipedia.org/wiki/Adsorbenthttp://en.wikipedia.org/wiki/Zeoliteshttp://en.wikipedia.org/wiki/Molecular_sievehttp://en.wikipedia.org/wiki/Molecular_sievehttp://en.wikipedia.org/wiki/Molecular_sievehttp://en.wikipedia.org/wiki/Molecular_sievehttp://en.wikipedia.org/wiki/Zeoliteshttp://en.wikipedia.org/wiki/Adsorbent


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Pressure swing adsorption processes rely on the fact that under pressure, gases tend to beattracted to solid surfaces, or "adsorbed". The higher the pressure, the more gas is adsorbed;when the pressure is reduced, the gas is released, or desorbed. PSA processes can be used toseparate gases in a mixture because different gases tend to be attracted to different solid surfacesmore or less strongly. If a gas mixture such as air, for example, is passed under pressure through

a vessel containing an adsorbent bed that attracts nitrogen more strongly than it does oxygen,part or all of the nitrogen will stay in the bed, and the gas coming out of the vessel will beenriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen, it can beregenerated by reducing the pressure, thereby releasing the adsorbed nitrogen. It is then ready foranother cycle of producing oxygen enriched air.

Adsorbents for PSA systems are usually very porous materials chosen because of theirlarge surface areas. Typical adsorbents are activated carbon, silica gel, alumina and zeolite.Zeolite utilize their molecular sieve characteristics to exclude some gas molecules from theirstructure based on the size of the molecules, thereby restricting the ability of the larger moleculesto be adsorbed. Schematic diagram of a pressure swing adsorption system is shown in fig. 1 :

Fig.1 Schematic diagram of pressure swing adsorption system

4. Advantages of PSA over conventional methods to remove CO2:Major advantages of PSA over conventional methods of CO2 removal like high pressure

water scrubbing and chemical absorption are:

A high CH4-enrichment Low power demand Low level of emission Higher purity at a lower capital cost Zeolites are size specific and can be reused.
http://en.wikipedia.org/wiki/Airhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Activated_carbonhttp://en.wikipedia.org/wiki/Silica_gelhttp://en.wikipedia.org/wiki/Aluminahttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Zeolitehttp://en.wikipedia.org/wiki/Aluminahttp://en.wikipedia.org/wiki/Silica_gelhttp://en.wikipedia.org/wiki/Activated_carbonhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Nitrogenhttp://en.wikipedia.org/wiki/Air


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5. Selection of adsorbent on the basis of nature of adsorbent:Adsorbent material is the most important part of the adsorption column. It provides the

surface over which the gas flows, presenting a large area for mass transfer to occur. The

following factors provide a general guide for selecting adsorbent materials [2]:

Adsorptive ability

Adsorbent should have specificly high adsorptiom capacity for aspecific compound from the mixture for effective seperation.

Low pressure drop - Pressure drop is a function of the volume of void space in a tower

when filled with material. Generally, the larger the packing size for a given bed size, the smaller

the pressure drop becomes.

Large specific area - A large surface area per cubic foot of material, m2/m3 (ft2/ft3), is

desirable for mass transfer.

Structural strength -Packing must be strong enough to withstand normal loads duringinstallation, service, physical handling, and thermal fluctuations.

6. Zeolite used for CO2 and H2S removal and its properties:Zeolite molecular sieves are crystalline, highly porous materials, which belong to the class of

aluminosilicates. These crystals are characterised by a three-dimensional pore system, with poresof precisely defined diameter. The corresponding crystallographic structure is formed bytetrahedras of (AlO4) and (SiO4). These tetrahedras are the basic building blocks for variouszeolite structures, such as zeolites A and X, the most common commercial adsorbents. The poreopening of the sodium form of zeolite X (13X) is approximately 8 ngstrom [3].

Fig. 2: Structure of A and X type of zeolite.


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Table 3. Properties of Zeolite 13X: [3]

7. Principles of adsorption:Concentration patterns in fixed beds: In fixed bed adsorption concentration in the fluid

phase and in the solid phase change with time as well as position in the bed. Initially mass

transfer takes place near the inlet. As time passes bed near the inlet start to get saturated.So theconcentration in the fluid goes down to zero before coming out of the bed. This concentrationprofile of the fluid can be shown using following curves [2] :

Fig.3 (a) Concentration profile and (b) breakthrough curve for adsorption in a fixed bed.

Breakthrough curve: When concentration reaches some finite permissible value, or break

point , the flow is stopped or diverted to a fresh adsorbent bed. The break point is often taken asa relative concentration of 0.05 or 0.10 , and since only the last portion of fluid processed hasthis high a concentration, the average fraction of solute removed from the start to the break pointis often 0.99 or higher.

For a unit area of bed cross section, the solute feed rate is the product of the superficialvelocity and the concentration:

Property zeolite 13X

bulk density (kg/m3

) 689BET surface area (m

2/g) 726

micro pore area (m2/g) 678

micro pore volume (cm3/g) 0.25


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FA= uoco

For an ideal breakthrough curve , all the solute fed in time t is adsorbed, and the concentration onthe solid has increased from the initial value W

oto the equilibrium or saturation value W

sat.

Thus:

uocot = L b (WsatWo)

where L and b are the length and bulk density of the bed, respectively. For fresh adsorbent orcompletely regenerated adsorbent, Wo=0, but complete regeneration is often too costly.

8. Design requirements:In this project, a laboratory pressure swing adsorption column is designed whose designrequirements are given below:

Volumetric flow rate of feed= 20 m3/hr

Inlet concentration of CO2 = 40% vol/vol

Outlet concentration of CO2 = 4% vol/vol

Inlet concentration of H2S = 1000 ppm

Outlet concentration of H2S= 25 ppm

9. Design Calculations:Assumptions:

Usually the cross sectional area is calculated to give a superficial velocity of 0.5 to 1.5ft/s, which results in a pressure drop of few inches of water per foot.

So we assumed superficial velocity to be equal to:

uo = 1ft/s

Also either we can determine adsorber bed length for a desired cycle time or we candetermine cycle time for desired bed length. As we are working on laboratory scale, we assumedour adsorber column length to be 1m [5] .

L = 1m.Porosity of bed was taken = 0.3

And bulk density of the bed was taken = 756 kg/m3

No. of moles in mixture = 807.24 moles ( using PV=nRT)

So moles of CO2= 323 moles and of CH4= 444 moles

co = 16.15 M


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Area of cross section of the bed:

As volumetric flow rate = 20 m3/hr

Superficial velocity = 1ft/s = 0.3048 m/s

So area of cross section = 20/(0.3048 x 3600) = 182.25 cm2

3.14x r2 = 182.25

r = 7.61 cm

So diameter of the bed = 15.22 cm.

Saturation and working capacity of the bed:

Saturation capacity (Wsat) = moles of CO2 adsorbed per kg of zeolite 13X after adsorption

Working capacity = Wsat - Wowhere Wo = moles of CO2 adsorbed per kg of zeolite 13X after desorption.


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Fig. 4: CO2 adsorption equilibrium on zeolite 13X at temperatures 25o

C , 35o

C , 50o

C [3]

So we are using 1.6 atm pressure for adsorption in the bed and desorption will take place 0.12atm. So Wsat = 5 mol/kg and Wo= 1 mol/kg so working capacity is 4 mol/kg.

So cycle time can be given as:

t= L b (Wsat-Wo)/ uoco

t= 1x 756 x 4/ (0.3048 x 16.15) = 10.23 mins.

Length of unused bed: As we do not take bed height to be used fully because after some time ofusage its efficiency start to decrease. So we generally leave some bed length to be unused

because if we use the whole bed length its result will not be consistent. So to compensate theeffect of this decreased efficiency we leave some bed length unused, this is called length ofunused bed (LUB). For our purpose we take length of unused bed to be equal to half of the initialbed length. So cycle time decreases.

LUB = L/2 = 0.5 m.

So Leff= 0.5 m.

So cycle time for the process ( tc )= (Leff/L)xt = 0.5 x 10.23= 5.12 mins.

Pressure drop calculations in the bed:

Ergun equation was used to calculate pressure drop in the fixed bed:


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where

Fluid viscosity= 18x10-6 Pa.

Fluid density= 1.18 kg/m3

Void space = 0.3

Particle diameter = 1.6 mm.

So pressure drop comes out to be:

P = 8.9 kPa per meter length of the bed

which is satisfactory.

Mass of adsorbent required:

mads= 1x 3.14x(.15/2)2 x756x2= 26.5 kg.

Design calculations for H2S:

For the given cycle time H2S will require a very less bed length for complete adsorptionas its concentration is very low.

Moles of H2S in the coming gas = 0.815 moles

So length of bed required for its complete adsorption in a cycle time of 5 mins.

LH2S= tc x uo x co/(b x (WsatWo))

Wsat= 3.23 moles of H2S/ kg of 13X

Wo = 0.73 moles of H2S/ kg of 13X

So LH2S = 1.9 mm.


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Fig.5 : Adsorption isotherm and dielectric constatnt of H2S on zeolite 13X [4]

Life of the Zeolite material:

General life of zeolite 13X is more than 2 years but due to high flow rates an continuoususe it start breaking. So life of zeolite in the bed is generally 2 years.

Loss of methane:

Zeolite 13X also adsorb methane on this pressure so there would be some loss of methanealso, But as there is very less amount which is adsorbed because methane adsorbance on zeolite13X is very less compared to CO2 or H2S. As it will be very difficult to evaluate its adsorbedamount, we evaluate this amount using comparison of their adsorbance on zeolite 13X. As CO2to methane adsorbance ratio at room temperature is 16:1 and total moles of CO2 adsorbed in onecycle are 326 moles. So moles of methane adsorbed can be calculated as:

Loss of methane = (1/16)*326=20 molesWheight loss of methane per cycle = 20*16= 320 gm.


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10. Conclusions:I have presented the detailed design of the pressure swing adsortion system for the removal

of CO2 and H2S gases from the biogas. I have presented the data of adsorbent used, data of

packed bed height and packed bed diameter with calculations. Also, I have presented the detailedspecifications of pressure swing adsorption system. We have seen that use of PSA can

significantly improve the quality of the biogas since the process removes most of the CO2 andH2S. Future work involves fabrication of the column and verification of the obtained data from

literature. We can also obtain further more precise data from experiments.


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11. References:1. Sircar, S. Ind. Eng. Chem. Res. 2002, 41, 1389-1392.2. Unit Operations Of Chemical Engineering, 5th Ed, McCabe And Smith3. Cavenati,S.;Grande,C.; Rodrigues,A. J. Chem. Eng. Data 2004, 49, 1095-1101.4. Staudt R.; Rave H.; Keller J.; Adsorption 5, 159167 (1999)5. J. Me rel, M. Clausse, and F. Meunier. Published online 28 November 2006 in

Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10166

separation co2 avec psa

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