Separacion de Metano Del Carbon i

7/28/2019 Separacion de Metano Del Carbon i

1/9

02638762/03/$23.50+0.00# Institution of Chemical Engineers

www.ingentaselect.com=titles=02638762.htm Trans IChemE, Vol 81, Part A, April 2003

METHANE SEPARATION FROM COAL MINE METHANE GAS

BY VACUUM PRESSURE SWING ADSORPTION

A. OLAJOSSY1

, A. GAWDZIK2

, Z. BUDNER2

and J. DULA2

1University of Mining and Metallurgy, Cracow, Poland2Institute o f Heavy Organic Syn thesis Blachownia, Kedzierzyn-Kozle, Poland

The ndings were presented from laboratory investigationsand computer calculations forthe VPSA (vacuum pressure swing adsorption) process employed to recover methanefrom coal mine methane gas. The separation process of coal methane gas was carried

out at temperature of 278 K under adsorption pressure equal 300 kPa abs., and desorptionpressure equal 25 kPa abs. Coal mine methane gas with the CH4 content of 55.2vol% wasenriched into methane-rich gas which contained 9698 vol% CH4. The methane recovery

efciency reached 8691% with the ratio for re-circulated methane to that introduced with thefeed falling within P=F 1.82.12. The methane content in the adsorption efuent gas was1115 vol% under those conditions while nitrogen and oxygen were removed at 9698% fromthe methane-rich gas. That separation level of the coal mine gas can be achieved when themethane sorption wave is brought to the outlet point of the sorption bed at the adsorptionphase, and then the methane sorption wave is brought to the entry point of the sorption bed atthe counter-current purge phase.

Keywords: vacuum pressure swing adsorption; coal mine methane gas; methane separation;nitrogen rejection; computer process simulation; activated carbon.

INTRODUCTION

Methane contained in coal mine gas and released from coalbeds when the beds are subjected to preconditioning beforeexploitation makes a valuable energy carrierits value isconsiderable but still not fully utilized. Its emission toatmosphere would contribute to the greenhouse effect.Coal mine gas is most frequently utilized as a low-energy-value fuel gas intended for captive use in local heat-generating plants or thermal-electric power stations. It isalso known to be used in gas turbines. An alternative way ofutilizing coal mine gas is to convert it into methane-rich gas,containing at least 96 vol% methane, to be then delivered tolocal natural gas supply systems. Among the known meth-ods for separation of gas mixtures, the PSA (pressure swingadsorption) method can be used in practice to recovermethane from coal mine gas. This method has been emp-loyed to reject nitrogen from small streams of nitrogen-richnatural gas DAmico et al., 1993; Buras and Mitariten,1994; Shirley et al., 1996). Interesting embodiments of thatmethod have been presented in patent specications (Davies

and Gray, 1992; Reinhold and Knaebel, 1998). The rst, andso far the only, pilot plant for the recovery of methane fromcoal mine gas was built in 1980s in Germany (Pilarczyk andKnoblauch, 1987). A few papers can be found on modellingthe PSA process when it is employed to separate CH4N2mixtures (Baksh et al., 1990; Warmuzinski et al., 1990;Bays et al., 1994).

Typical composition of coal mine methane gas is notfavourable for the gas separation methods since it is not easyto separate methane from nitrogen and oxygen and to keepthe operating costs reasonably low at the same time. It isalso for that reason that any discussion on those problemscan hardly be found in the papers published so far.

When selecting from among the known separation meth-ods for gases, one appears at a conclusion that the pressureswing adsorption technique makes it possible to efcientlyrecover methane from coal mine gas. Modications of theclassical PSA process become important since the gasesconsidered are hard-to-separate components; nitrogen andoxygen have to be separated from methane precisely andboth of them at the same time, and the product gas(methane-rich gas) has to contain 96 vol% CH4 at least.The VPSA (vacuum pressure swing adsorption) methodwith carefully selected sorption materials makes a morepromising separation process than the classical PSA processowing to its superior efciency and productivity. It does notmake a process which would be applicable in effectiverecovery of methane from coal mine gas.

In the suggested VPSA method, CO2 is removed fromcoal mine gas with help of solid potassium hydroxide. Thenthe gas undergoes compression and is dried with wide-poresilica gel and activated alumina. The resulting mixture ofmethane, nitrogen and oxygen is subjected to separation inthe VPSA facility under a pressure of about 300kPa abs.The process comprises the following stages: adsorption;

474


2/9

counter-current purge; desorption of methane down to thenal pressure of about 25 kPa abs.; and lling. From thequality viewpoint of the separation process of the CH4N2,O2mixture, the processes should be controlled chiey bythe diffusion mechanisms for the gas molecules in thetransport pores of the sorption material and by the adsorp-

tion equilibrium conditions. Hence, the most importantcriteria within process engineering for selecting the adsorp-tion agent should be high selectivity at equilibrium formethane against nitrogen and oxygen, as well as high andnot much diversied values of the diffusion coefcients forthose gases in the transport pores of the sorption material.These conditions can be satised by suitable activatedcarbons which are characterized by well-developed transportpores and high capacity of micropores.

The purpose of this study is to determine experimentallythe basic parameters for the VPSA process employed torecover methane from coal mine methane gas. The activatedcarbon selected for that purpose should make it possible toconvert coal mine gas into methane-rich gas with CH4content of 96 vol% at least. Also, the study is intended toverify experimentally the computer calculation software.The ndings from laboratory tests and from computercalculations would be utilized to design a pilot plant.

MATHEMATICAL MODEL

Coal mine methane gas contains typically 4555 vol%CH4, 78vol% O2, 23 vol% CO2 and balance nitrogen.Before entering a VPSA unit, coal mine gas is subjected to

drying and carbon dioxide is removed. Since adsorption ofoxygen in the selected activated carbon is similar to that ofnitrogen, the computer calculations for the VPSA processcan be simplied when the feed gas is assumed to be a two-component mixture only: methane and nitrogen.

In order to simplify the laboratory-scale process in whichthermal effects are negligible, the VPSA-based methanerecovery process was assumed to be described by meansof an isothermal, non-equilibrium model of two compo-nents: methane and nitrogen.

However, the thermal effects have to be taken intoconsideration when modelling a large-scale process. Theseeffects are revealed by, among other things, the existence ofthe heat decit zone and lower temperature at the feed inlet.This decreases methane desorption from this zone and henceit lowers the efciency of methanenitrogen separation byVPSA.

The VPSA process for methanenitrogen separation isdescribed by the following equations:

equation for transport of i-component of gas mixture

DL@2

@z2(Cyi)

@

@z(_qq yi) eb

@

@t(C yi) Ri 0

(1)

overall balance equation

@_qqi

@z eb

@C

@t

X2j1

Rj 0 (2)

where molar gas velocity is referred to the internal free areaof the adsorber vessel and it is described as:

_qq C v (3)

and gas satises the perfect gas equation

C PRg T

(4)

Gluckauf equation for adsorption kinetics

Ri rN@aai

@t rNki(ai

aai) (5)

Langmuir isotherms

ai ai

1bici

1 P2

j1 bjcj(6)

where

bi bi0 exp( DHi)

Rg(T1 T0

1)

(7)

and ai1 constant.

The mass transfer is limited by the molecular diffusion (DM)and the Knudsen diffusion (DKi) in transport pores ofactivated carbon granules (Yang, 1987; Ruthven et al.,1993). The diffusion coefcient for the diffusion in transportpores of the adsorbent is described by the equation:

1

Dpi t

1

DKi

P

P0

1

DM

(8)

The equation dening the transport coefcients for the gascomponents in the activated carbon has the following form:

ki OepDpi

Rp2(1 ep)

c0i

a0i

1 e

rN

(9)

The molecular diffusion coefcient DMcan be calculatedfrom the Arnold Gilliland equation while the Knudsendiffusion coefcient can be calculated from the followingformula (Yang, 1987):

DKi 97:0v

p

T

Mi

1=2

(10)

The relation between individual types of adsorbent porosityis given by the equation:

e eb (1 eb)ep (11)

The pressure proles in the VPSA process, for the opera-tions carried out under changing pressure, can be describedwith the equations:

P(t)l PP,lPK,l PP,l

1 1 t

tl

jl(12)

for the operations with rising pressure, and

P(t)l PP,l

PK,l PP,l 1

t

tl

jl(13)

for the operations with decreasing pressure, where jl ispressure history experimental shape factor.

Trans IChemE, Vol 81, Part A, April 2003

METHANE SEPARATION FROM GAS 475


3/9

COMPUTER SOFTWARE

The system of equations (1)(6) was solved by the niteelement method. The adsorbent bed was divided into 50elements of the same volumes. The number of time intervalswas calculated separately for each process operation, underequal residence times for unit volumes of gas in unitvolumes of the bed. The nal parameters of a unit volumeof bed were calculated for the dened boundary and initialconditions, and for a given unit volume of the bed in a giventime interval. The iterative calculation method was used andthe unit volume under investigation was considered to makea unit reactor with perfect gas mixing. The nal parametersfound for a unit volume of bed make the boundary para-meters for the next unit volume of bed, while the parametersfor the previous time interval make the initial parameters.The calculations are made for all the bed volumes and all thetime intervals within the process operation. Then the calcu-lations are repeated for the next process operation, and then

for the next cycle in the VPSA process. The calculations arerepeated again and again until stable parameters are arrivedat for the gaseous phase, adsorbent phase and the samebalance of components in two subsequent cycles.

The software for the calculations of the VPSA process,which in fact is a computer simulation of the VPSA process,was given a modular structure. There are modules whichdescribe the adsorption process, modules which characterizethe individual process operations, and a supervisoryprogram which provides the initial data for calculationsand denes the structures of the VPSA cycle.

EXPERIMENTAL PLANT

The tests were run at a bench-scale stand as shown inFigure 1. Specications for adsorber vessels have beenprovided in Table 1. CO2 was removed from coal minemethane gas in a column lled with solid potassium hydro-xide (not shown in the diagram) until its content droppeddown to the level of 0.1 vol%. The product gas wascompressed in a membrane compressor (K-01) to thepressure of 300320 kPa abs. and dried in a drier (D-02)lled with wide-pore silica gel and activated alumina. Drygas containing 55.2 vol% CH4was transferred to a buffervessel (V-03). Excess gas was recycled by a pressure controlvalve KV(PC-1) to the compressor inlet line. Compressedgas from the buffer vessel (V-03) was passed to the VPSAmodule composed of adsorbers (D-04 A, B, C) and solenoidvalves. Adsorbers were packed with a properly selectedactivated carbon, which was employed in the VPSA processto separate the feed gas into nitrogen, oxygen and methane-

rich gas. The nitrogenoxygen mixture (with low methane

Figure 1. Process diagram for experimental plant. K-01, compressor of coal mine gas; D-02, drier; V-03, buffer vessel; D-04, PSA module; A, B, C,adsorbers; V-05, nitrogen buffer vessel; P-06, vacuum pu mp; V-07, intermediate buffer vessel; K-08, methane compressor; V-09, methane buffer vessel; KV,solenoid valves; PC, pressure controller; QI, owmeter; FI, rotameter; PI, manometer; A, sampling point; CMM gas, coal mine methane gas; Z, manualcontrol valve.

Table 1. Specication of adsorber vessels in experimental plant.

Internal diameter 0.069 mHeight of sorbent bed [G2X7=12] 2.4 mTotal height of adsorber vessel 2.5 mAmount of activated carbon in one adsorber vessel 3.9kgAmount of alumina in one adsorber vessel:

Bottom layer 0.4 kgTop layer 0.15 kg


476 OLAJOSSY et al.


4/9

content) was transferred from adsorbers to a buffer vessel(V-05), then it was passed through a volumetric gas owmeter and released to atmosphere. Some part of thenitrogenoxygen mixture was recycled to adsorbers at there-pressurization step (RR). At the desorption step (D),methane-rich gas was extracted from the adsorbers by

means of a membrane vacuum pump (P-06) down to thepressure of about 25 kPa abs. Thus recovered methane wastransferred to a buffer vessel (V-07), then it was compressedin a membrane compressor (K-08) to the pressure of about350 kPa abs. and passed to another buffer vessel (V-09).Some volumes of methane-rich gas were recycled from thatvessel to the VPSA module to purge nitrogen and oxygenaway. The ow rate of recycled gas was measured by arotameter (FI). Excess methane-rich gas was carried away bya pressure control valve KV(PC-2) installed in the buffervessel (V-09). The amount of produced gas was measuredby a volumetric meter. Gas samples were taken for analysisfrom sampling points (A). Individual components ofgas samples were analysed by gas chromatography. Theamount of CH4 in gas was measured with a Perkin-Elmerchromatograph with ame ionization detector (FID), usinga chromatographic column which was loaded withdimethylsulfolane onto Chromosorb P.

The amounts of N2, O2 and CO2were measured with aPerkin-Elmer chromatograph with thermal conductivitydetector (TCD), using a column loaded with Carbosiv Gactivated carbon. The VPSA process was carried out byopening and closing solenoid valves (KV 112), which wereoperated by a programmable controller.

DESCRIPTION OF VPSA PROCESS

The VPSA process cycle is composed of the followingunit operations:

(A) Adsorption stage, duration of 320s, pressure of

300kPa abs.feed coal mine methane gas is passed

through the adsorber vessel upwards, methane is selec-

tively adsorbed in the bed of activated carbon, and the

mixture of nitrogen and oxygen (with small percentage of

methane) leaves the adsorber vessel and is sent to the

buffer vessel (V-05) from where it is continuously

released to the atmosphere. (R) Counter-current rinse, duration of 320 s, pressure of

305 kPa abs.methane-rich gas produced from the deso-

rption phase (D) is subjected to compression and is

passed through an adsorber vessel downwards to rinse

nitrogen and oxygen out; the efuent gas is mixed with

the feed stream. (D) Desorption phase, duration of 380 smethane-rich

gas is evacuated from the adsorber vessel, a vacuum

pump is employed for that purpose, the adsorber vessel

pressure is lowered down to 25 kPa abs., methane-rich gas

is then compressed to 350 kPa abs. and some part of thatgas is recycled to the process.

(RR) Re-pressurization phase, duration of 20 sthe adsor-

ber vessel pressure is increased to 200 kPa abs., the mixture

of nitrogen and oxygen from the buffer vessel (V-05) is

charged to the adsorber vessel and it ows in counter-

current to the gas ow direction at the adsorption phase.

(AR) Pressurization-with-feed phase, duration of 60 sthe adsorber vessel pressure is increased to 300 kPa abs.,the feed gas is charged to the adsorber vessel from itsbottom and at the same time the mixture of nitrogen andoxygen is let out of the other side of the adsorber vesseland is sent to the buffer vessel.

The complete cycle for the VPSA process comprisesthree sequences while each sequence takes 400s. Eachsequence is composed of all the unit operations as describedabovesequences refer to different adsorber vessels. Theseparation experiments were carried out at the averagetemperature of 278 K.

The ow diagram for gas streams in the VPSA cycle ispresented in Figure 2, while the time schedule for the VPSAcycle is shown in Figure 3. Pressure proles for the adsorbervessel (A) can be found in Figure 4.

ACTIVATED CARBON

The following criteria were considered when selecting asuitable activated carbon grade:

high selectivity for methane adsorption versus nitrogen

and oxygen; high adsorption capacity for methane;

low difference between possibly high values of diffusion

coefcients for methane and nitrogen oxygen in trans-

port pores of the sorption material; price of the activated carbon.

A few adsorbents were tested and the activated carbonG2X7=12 from Takeda Industries (Japan) was selected.

Figure 2. Flow diagram for gas streams in VPSA cycle. A, adsorption; R,counter-current rinse; D, desorption; RR, re-pressurization; AR, pressuriza-tion with feed; V-05, buffer vessel for N2 O2; P-06, vacuum pump; V-07,intermediate buffer vessel; K-08, methane compressor; V-09, methanevessel; CH4, methane-rich gas; N2 O2, N2 O2mixture after methanewas separated therefrom; CMM gas, coal mine methane gas.

Figure 3. Time schedule for VPSA process. D-04 A, B, C, adsorbers; A,adsorption; R, counter-current rinse; D, desorption; RR, re-pressurization;AR, pressurization with feed; , pause in the process.




5/9

Table 2 presents its specication. Adsorption isotherms formethane and nitrogen were found with the use of thestandard volumetric method, at temperatures of 273 and298 K. The proles for those isotherms have been presentedin Figure 5. The values for ai

1, bi0 and (DHi) werecalculated from experimental data, with the use of equations(6) and (7). The employed activated carbon offers highadsorption selectivity for methane versus nitrogen whatmakes it possible to separate efciently methane fromnitrogen and oxygen in the VPSA process.

MASS TRANSFER COEFFICIENT ki

Taking advantage of equations (8), (9) and (10), thevalues of mass transfer coefcients ki were calculated;these were average values for individual process operations.The data for the calculations can be found in Table 3. Thevalue of tortuosity factor for activated carbon was assumedto be t 10, as per the data available in literature (Yang,et al., 1985). The value ofO was assumed a priori to beequal to O 15 for all process operations. Table 4 providesthe calculated values of ki coefcients for methane andnitrogen as well as the values ofjl coefcients for indivi-dual process operations. The values of ki coefcients were

calculated for the average pressure and the feed compositionat a given process operation. As results from the calcula-tions, the share of the Knudsen diffusion component 1=DKiin the resistance experienced in the mass transport isconsiderable and it affects the transport processes in everyoperation, and in particular in the desorption operation.

DISCUSSION OF RESULTS AND COMPUTERCALCULATIONS

The study made use of coal mine methane gas which,after purication, contained: 55.2 vol% CH4, 8.6vol% O2,

Figure 4. Pressure history for adsorber D-04 A in PSA cycle. A, adsorption;R, counter-current rinse; D, desorption; RR, re-pressurization; AR,pressurization with feed.

Table 2. Specication of activated carbon G2X7=12.

Pore surface as found by BET=N2method 1007.2 m2g1

Pore surface as per Langmuir isotherm 1343.3m2g1

Pore volume 0.4964 cm3g1

Average pore diameter 1.471nmDiameter of dominant pores 3.00 nmDiameter of transport pores 1090 nmSaturation constant a1(273 K) 3.32 mol kg1

Saturation constant a1(298 K) 3.30 mol kg1

Afnity constant (dm3mol1):bCH4 (273 K) 25.36bCH

4(298 K) 12.99

bN2

(273 K) 4.11bN2

(298 K) 2.97KCH

4=N

2(273 K) 6.17

KCH4=N

2(298 K) 4.38

DHCH4

18,100J mol1

DHN2

8800Jmol1

The values for the parameters: a1, biand (DHi) were calculatedon the basis of adsorption isotherms which had been foundexperimentally.

Figure 5. Adsorption isotherms for methane and nitrogen at temperatures of273 and 298 K. d, experimental points. Solid lines, calculated by equations(6) and (7) with the use of data from Table 2.

Table 3. Data for calculation of transport coefcients ki.

DM(278 K, 101.3 kPa) 0.164104m2s1

DKCH4

(278 K) 0.081104m2s1

DKN2

(278 K) 0.061104m2s1

rp 20109m

t 10O 15ep 0.33eb 0.63e 0.75rN 500kgm

3

Table 4. Values of calculated transport coefcients ki for individual processoperations and values ofjl factors [equations (12) and (13)].

Name and symbol ofoperation kCH

4(s1) kN

2(s1)

jl

Adsorption (A) 0.047 0.094 P(t)constantRinse (R) 0.047 0.094 P(t)constantDesorption (D) 0.075 0.140 3Re-pressurization

(RR)0.082 0.151 1

Pressurizationwith feed (AR)

0.052 0.104 2

Values of jl factors were calculated from time proles for pressures atindividual process operations in the VPSA cycle as carried out at theexperimental plant.


478 OLAJOSSY et al.


6/9

0.1vol% CO2and balance nitrogen. Stable operating condi-tions for a test stand were obtained after 34 h on-line. Gassamples for analysis were taken 56h after starting anexperiment. At nearly constant gas ow rate from adsorp-tion, falling within QIN 49.750.3 dm

3(STP)=S*, we chan-ged the amount of methane recycled to the system. The ratio

of recycled methane and methane charged in the feed, P=F,was modied in that way. The coal mine gas ow rate at thetest plant inlet was adjusted automatically by the pressurecontrol system KV(PC-1) (Figure 1) and it fell withinQF 97.6121.9dm

3(STP)=S*, depending on the volumeof methane recycled to the process. Pressure controlprovided by the control system KV(PC-2) (Figure 1)changed the ow rate for methane-rich gas QCH4, whichwas the difference between the volume of methane producedin the desorption operation (D) and that recycled to theprocess in the counter-current rinse phase (R). The experi-mental ndings and computer calculation results arepresented graphically in Figures 611. The experimentwas carried out at the plant output QCH4 44.772.5dm3(STP)=S*, and the ratio of recycled methane tofeed methane was changed within P=F 2.121.12(Figure 6). At the plant output of QCH

4 45

55dm3(STP)=S*, i.e. with the P=F ratio of 1.82.12(Figure 6), methane-rich gas was produced which containedYCH

4 9698 vol% CH4 (Figure 7). Efciency of methane

separation under such process conditions falls withinZCH4

8691% (Figure 8), while the inert content(N2 O2) in the adsorption efuent gas is within 8589%(Figure 9). At the same time, the rejection degree fornitrogen oxygen from the feed gas is high, i.e.

ZIN 9698% (Figure 10). Figure 11 presents the inter-

Figure 6. Recycle ratio for methane P=F vs plant capacityQCH

4[dm3(STP)=S*]. , Experimental points; solid line, calculated prole;

S*, 400 s (one sequence in PSA cycle); P=F, recycleratio QR YCH4=QF YCH4

F.

Figure 7. Product purity YCH4

(vol%) vs plant capacity QCH4

[dm3(STP)=S*]. , Experimental points; solid line, calculated prole; S*,400 s (one sequence in PSA cycle).

Figure 8. Efciency of methane recovery ZCH4

(%) vs plant capacity QCH4

[dm3(STP)=S*]. , Experimental points; solid line, calculated prole; S*,400 s (one sequence in PSA cycle).




7/9

Figure 9. Content of inerts (N2 O2) in the adsorption efuent gas YIN(vol%) vs plant capacity QCH

4[dm3(STP)=S*]. , Experimental points;

broken line, extrapolation of experimental ndings; solid line, calculatedprole.

Figure 10. Rejection degree for nitrogen and oxygen ZIN (%) vs plantcapacity QCH

4[dm3(STP)=S*]. , Experimental points; solid line, calculated

prole; S*, 400s (one sequence in PSA cycle).

Figure 11. Content of inerts (N2 O2) in the adsorption efuent gas YIN(vol%) vs methane content YCH

4(vol%) in the desorption efuent gas. ,

Experimental points; broken line, extrapolation of experimental ndings;solid line, calculated prole.

Figure 12. Diagram for optimum charging the sorbent material withmethane in VPSA process. A, adsorption; R, counter-current rinse; D,desorption. The lines present charging the sorbent material with methane atthe end of: A, adsorption operation; R, rinse operation; and D, desorptionoperation.


480 OLAJOSSY et al.


8/9

dependence of the inert (N2 O2) content in the adsorptionefuent gas (YIN) and the methane content in the methane-rich gas (YCH

4). When the plant produces the methane-rich

gas with the methane content of YCH4 9698 vol% CH4,

the methane content in the methane-lean gas dischargedfrom the plant at the adsorption stage falls within

1115vol% (YIN 8589 vol%). When the methane-rich gas is obtained from the plant with a lower methanecontent, below 94 vol% CH4, the methane content in themethane-lean gas stabilises at the level of 10vol%(YIN 90%).

That optimum separation of coal mine methane gas wasfound possible when the methane sorption wave wasbrought to the outlet point of the sorption bed at theadsorption phase, and then the methane sorption wave wasbrought to the entry point of the sorption bed at the counter-current purge phase. That was a precondition for optimumseparation of methane and nitrogen oxygen. Those condi-tions of running the separation process are presented inFigure 12. The methane content in the adsorption efuentgas is principally dependent on the desorption step nalpressure as well as on the kinetics of methane adsorption inthe activated carbon bed and on the sorption capacity of thesieve for methane. The dynamics of the VPSA process isinferior here. Reduced volume of methane recycled to theprocess, and hence reduced P=F ratio, improves the ow ofproduct QCH

4. Thus, the methane content in the methane-

rich gas becomes reduced, YCH4 8694 vol% (Figure 7),efciency of methane recovery becomes improved to thelevel of ZCH

4 9093%, and the methane content in the

adsorption efuent gas becomes reduced to 10 vol% CH4

(YIN 90 vol%). Resulting from lower purity of the productgas obtained, elimination of inerts from methane-rich gas isconsiderably worse (Figure 10). When the calculation results(solid lines in Figures 611) are compared with experimen-tal ndings, good conformity is observed for those two setsof data. The experimental data points in Figure 7 lie 12%below the prole for calculation. The experimental datapoints in Figure 8 are placed 13% below the prole forcorresponding calculation. The differences are higher incase of Figure 9, which stems from the simplicationadopted for the mathematical model. Moreover, when themethane sorption wave is brought to the outlet point of thesorption bed at the adsorption phase, and then when themethane sorption wave is brought to the entry point of thesorption bed at the counter-current purge phase, the gasseparation process becomes very sensitive to changes in thegas ow rate in individual operations. Even a slight increasein the ow rate of the methane-lean gas, and hence aslight increase in the ow rate of feed gas, results inmoving the methane sorption wave beyond the bed. Thus,the methane content will be increased in the plant efuentgas at the adsorption phase. Similarly, a slight increase inthe ow rate of the methane-rich gas which is recycledto the process in the purge operation will result in movingthe methane sorption wave beyond the bed. In effect,

higher doses of methane will be introduced to the processin its adsorption phase and this in turn will movethe methane sorption wave away from the bed. As aconsequence, the methane content will become excessivein the methane-lean gas which leaves the plant at theadsorption phase.The ndings from the research forthe VPSA methane recovery process from coal mine

gas were employed in designing a pilot plant with capacity2025 m3(STP)=h of coal mine gas. The pilot plant was builtin 2001 and it was used for systematic research of the VPSAprocess, achieving good results.

SUMMARY

The ndings from computer calculations for the VPSAmethane recovery process from coal mine methane gascorrespond satisfactorily to the experimental data obtainedfrom a laboratory. When coal mine gas was used forseparation which contained 55.2 vol% CH4, and when theratio for re-circulated methane to that introduced in the feedwas adopted to fall within P=F 1.82.12, methane-rich gaswas produced which contained 9698 vol% CH4. The ef-ciency gure for methane recovery was 8691%, while themethane content in methane-lean gas released from theadsorption facility was 1115 vol% CH4. The degree fornitrogen and oxygen rejection from methane-rich gas washigh and it reached 9698%. A precondition for so goodseparation of coal mine methane gas is bringing the methanesorption wave to the outlet point of the sorption bed at theadsorption phase, and then bringing the methane sorptionwave to the entry point of the sorption bed at the counter-current purge phase. Under optimum conditions, the VPSAprocess employed to separate coal mine methane gasbecomes very sensitive to changing ow rates of feed gasand recycle (purge) gas.

NOMENCLATURE

ai adsorbed phase concentration for component i, molkg1

ai equilibrium adsorbed phase concentration for component

i, molkg1

ai1 saturation constant in Langmuir isotherm, molk g

1

a0i equilibrium adsorbed phase concentration for component i

at inlet of adsorber vessel (z 0), to initial gas phaseconcentration (c0i), molkg

1

bi afnity constant in Langmuir isotherm for component i,dm3mol

1, m3mol1

bi0 afnity constant at basic temperature T0, dm3mol

1,m3mol1

C total concentration of gas phaseP=Rg T, moldm3,molm

3

ciCyi gas phase concentration for component i, moldm3,

molm3

c0i gas phase concentration for component i at inlet (z 0),moldm3, molm3

DL axial diffusion coefcient, m2s1

DM molecular diffusion coefcient at 273K, 101.3kPa,cm2s

1, m2s1

DKi Knudsen diffusion coefcient for component i, cm2s1,

m2s1

Dpi pore diffusion coefcient for component i, cm2s1,

m2s1

(DHi) isosteric heat of adsorption for component i, Jmol1

ki overall mass transfer rate, s1

KCH4=N2 aCH4yN2 =aN2

yCH4 , methane=nitrogen equilibrium

selectivity_qq molar ow rate, moldm

2s1, mol m

2s1

QF feed ow rate, dm3S*1(at 273K, 101.3kPa), (STP)

QCH4

product ow rate, dm3S*1(at 273K, 101.3kPa), (STP)

QIN inerts ow rate, from adsorption, dm3S*1 (at 273K,

101.3kPa), (STP)QR recycle ow rate, dm

3S*1 (at 273K, 101.3kPa), (STP)

L adsorbent bed height, mn number of components, n 2; 1, N2, 2, CH4P pressure, kPa, atmP0 standard pressure 101.3kPaP(t) pressure history, equations (12) and (13), kPa, atm




9/9

PK,l nal pressure in operation l, kPa, atmPP,l initial pressure in operation l, kPa, atmP=F recycle ratio QR YCH4 =QF YCH4

F

Rg universal gas constant 8.314JK1mol

1

tp mean transport pore radius of adsorbent, mS* sequence of cycle, here 400 sT temperature, KT0

basic temperature, here 273Kt time, stl time of cycle steps, s lA, R, D, RR, ARv intersitial gas velocity, dm s

1

yi mol fraction for component i in gas phaseYCH

4methane concentration in product, vol%

YCH4F methane concentration in feed, vol%

YIN N2 O2concentration in gas from adsorption, vol%z axial distance, m

Greek symbolsjl pressure history experimental shape factor for operation l,

equations (12) and (13)rN adsorbent bulk d ensity, kg m

3, kgdm3

ep adsorbent particle porosityeb

voidage of adsorbent bede total porosityt tortuosity factorZCH4 methane recovery efciency, %ZIN N2 O2, inerts rejection efciency, %O Gluckauf parameter, in equation (9)

REFERENCES

Baksh, M.S.A., Kapoor, A. and Yang, R.T., 1990, A new composite sorbentfor methane nitrogen separation by adsorption, Sep Sci Technol, 25(78): 845868.

Bays, M., Buczek, B. and Zietkiewicz, J., 1994, Modelling study of PSAprocess for methane recovery from mine gas, Presented at the Silesian

International Conference on Coalbed Methan e Utilisation, Katowice.

Buras, R.J. and Mitariten, M.J., 1994, Nitrogen rejection with pressure swingadsorption. Principles design and remote control using expert system, inProc. 44th Laurence Reid Gas Conditioning Conference, pp 93103.

DAmico, J., Reinhold, H. and Gamez, J., 1993, A PSA p rocess for nitrogenrejection from natural gas. Presented at 1993 Laurence Reid Gas Condi-tioning Conference, Norman, OK.

Davies, M.M. and Gray, R.L., 1992, Process for the purication o f naturalgas, United States Patent no. 5,174,796.

Pilarczyk, E. and Knoblauch, K., 1987, Separation technology, in Proc. Eng.Found. Conference, Schloss Elmau.

Reinhold, H.E. and Knaebel, K.S., 1998, Separation of gases by pressureswing adsorption, United States P atent no. 5,792,239.

Ruthven, D.M., Farooq, S. and Knaebel, K.S., 1993, Pressure SwingAdsorption (VCH, Canada).

Shirley, A., Porto, J., Ramachandran, R. and Kleinberg, W., 1996, Non-cryogenic nitrogen rejection from natural g as, in Proceedings of theSeventy-Fifth GPA Annual Convention, Oas Process Association,pp 6266.

Warmuzinsk i, K., Buzek, L., et al., 1998, Separacja mieszanin metanu iazotu na weglowym sicie molekularnym metoda adsorpcji zmienno cis-nieniowej. In_zzynieria Chemiczna i Procesowa, 19.4: 915928.

Yang, R.T., 1987, Gas Separation by Adsorption Processes (Butherworths,Oxford, UK).

Yang, R.T. and Doong, S.J., 1985, Gas separation by pressure swingadsorption. A pore diffusion model for bulk separation., AIChE J, 31:18291841.

ADDRESS

Correspondence concerning this paper should be addressed to ProfessorA. Olajossy, Akademia Gorniczo-Hutnicza im. S tanisawa Staszica, Al.Adama Mickiewicza 30 , 30-59 Krakow, PolandE-mail: [email protected]

The manuscript was received 17 September 2001 and accepted forpublication a fter revision 30 October 2 002.


482 OLAJOSSY et al.

Separacion de Metano Del Carbon i

Documents

Transcript of Separacion de Metano Del Carbon i