Ind. Eng. Chem. Res. 2009, 48, 6405–6416
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Hybrid Cycles to Purify Concentrated Feeds Containing a Strongly Adsorbed Impurity with a Nonlinear Isotherm: The PSA-TSA Supercycle Pradeep K. Sharma and Phillip C. Wankat* School of Chemical Engineering, Purdue UniVersity, Forney Hall of Chemical Engineering, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907-1283
A new adsorption process is developed to remove a strongly adsorbing gaseous impurity that has a very nonlinear isotherm from a non- or weakly adsorbing gas. This process, a number of pressure-swing cycles with low purge ratios followed by thermal regeneration, is called a PSA-TSA supercycle because it consists of thermal cycles on top of PSA cycles. The PSA cycles never reach cyclic steady state by themselves, although the supercycle does reach cyclic steady state. A water/nitrogen mixture with zeolite 13X is taken as model system because water exhibits strongly nonlinear adsorption characteristics. At low feed concentrations (1000 ppm water), PSA works well and is the preferred method. At intermediate feed concentrations (4000 ppm water), TSA is the preferred method. The PSA-TSA supercycle is better than PSA and TSA in handling high solute concentrations in the feed (40 000 ppm water) and producing high-purity product (1 ppm water). PSA fails to meet this purity, and TSA has low productivity. The new hybrid cycle is promising for highly nonlinear systems with high solute concentrations in the feed. regeneration temperatures, lower regeneration times are required; however, higher temperatures increase energy costs.9
Introduction Separation is an important step in the production of chemical products. Gas separations exploit cryogenic distillation, pressureand temperature-swing adsorption, and membrane technology. Pressure-swing adsorption1 (PSA) uses a variation in the column pressure followed by a low-pressure purge to carry out separation, whereas temperature-swing adsorption2-4 (TSA) uses a temperature increase and purge. PSA is well-known for air drying, air separation, and hydrogen purification. The majority of PSA processes are equilibrium-driven instead of kinetically driven.1 PSA is an efficient process for mixtures with linear adsorption isotherms, because a moderate pressure drop also results in a moderate amount of desorption. Tondeur and Wankat5 referenced early work of Montgareuil, Domine, and Brunel and reported that linear isotherms are more favorable to approach a reversible operation than nonlinear ones. With highly nonlinear isotherms, the strongly adsorbing species requires a large amount of purge gas to remove it from the adsorbent, resulting in poor PSA recoveries at reasonable purities. The use of a subatmospheric purge is often advantageous because isotherms are closer to linear at lower partial pressures. Alternatively, a high vacuum can be incorporated to attain reasonable desorption in vacuum-swing adsorption (VSA), but this process can be uneconomical. TSA is the oldest cyclic adsorption process3 and is commonly used for the removal of volatile organic compounds (VOCs), emitted in processes such as drying, gluing, and coating.6 Typically, TSA has a long adsorption step followed by column regeneration using a countercurrent hot purge of the product with an optional cooling step.7 The regeneration step is often the time-limiting step in the TSA cycle. For strongly adsorbed species with low feed concentrations, the cooling step can often be omitted because the thermal (cold) wave should move ahead of the solute wave during adsorption.8 Also, with higher * To whom correspondence should be addressed. Tel.: 765-494-0814, Fax: 765-494-0805. E-mail:
[email protected].
PSA and TSA Cycles One of the first PSA cycles developed, the Skarstrom cycle,10,11 shown in Figure 1, has four steps: Bed pressurization with highpressure feed, adsorption at high pressure and production of product, depressurization to low pressure (blowdown), and finally purge at low pressure. Product gas is generally used for the purge. Usually, steps 1 and 3 are short and equal in time, whereas steps 2 and 4 are long and equal in time. The general TSA cycle studied (Figure 2) could have the following steps: (1) long adsorption step, (2) countercurrent hot feed purge, (3) countercurrent hot product purge, (4) countercurrent cold product cooling, and (5) cocurrent cold product cooling. Product is collected in steps 1 and 5. Step 2 uses hot feed instead of hot product (which is typically used)7 to increase the recovery of the nonadsorbed gas. We show later that, for our examples, heating with hot product does not improve product purity compared to heating with hot feed. Step 3 is an optional hot product purge, used to keep a gap between the hot feed and cold product in the column. This step avoids contacting the hot feed with the cold adsorbent. Step 4 cools the bed from the product end and also
Figure 1. Skarstrom cycle.
10.1021/ie801661w CCC: $40.75 2009 American Chemical Society Published on Web 06/08/2009
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Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009 Table 1. Initial and Boundary Conditions for Mass Balancea During blowdown in PSA (not applicable in TSA), the governing boundary conditions for concentration and velocity are1
∂ci ) 0 and ∂z z)0
∂ci )0 ∂z z)L
(1.i)
Vf z)L ) 0 and
∂Vf )0 ∂z z)0
(1.ii)
During the pressurization (not applicable in TSA), adsorption, and purge steps in PSA, the appropriate boundary conditions on concentration are, respectively1
∂ci )0 ∂z z)L
(1.iii)
∂ci ) 0 and ci z)0 ) cfeed ∂z z)L
(1.iv)
ci z)0 ) cfeed and
Figure 2. Temperature-swing adsorption (TSA) showing possible steps in cycle.
pushes the hot thermal wave from steps 2 and 3 farther toward the feed end. Step 5 partially cools the feed end of the bed and is used if the cold thermal wave from step 4 does not achieve breakthrough. Other types of heat exchange such as with a jacket around the adsorber,6 with electric coils installed inside the adsorber,12 or with channels or tubes carrying hot/cold fluid13 are not studied here because scale-up and packing of the adsorbent are likely to be more challenging.
(| )
)
ci
z)L purge
(| )
PL c PH i z)L
and
adsorption
∂ci )0 ∂z z)0 (1.v)
For the adsorption and purge steps, respectively, in TSA24
∂ci ) 0 and ci z)0 ) cfeed ∂z z)L
Theory The equations for nonisothermal fixed-bed adsorption are the mass and energy balances, the mass- and energy-transfer equations, and the equilibrium isotherm. For the mass balance, it is assumed that radial gradients are negligible, no chemical reactions occur, and mass transfer follows a linear-driving-force model14 ∂qji ∂ci ∂cji,pore + Kd,i(1 - εe)εp + Fs(1 - εe)(1 - εp) + εe ∂t ∂t ∂t ∂(Vfci) ∂2ci - εeEz 2 ) 0 (1) εe ∂z ∂z Table 1 summarizes the appropriate initial and boundary conditions for the mass balance. The model system studied in the present work is the adsorption of water vapor (from a mixture of water vapor and nitrogen) on zeolite 13X. The adsorption beds were initially charged with 100% N2. Of course, this application of the supercycle process might not be economical, whereas other applications might be. The assumptions for the energy balance are negligible radial gradients, a linear driving force for heat transfer, and an adiabatic bed. The energy balance is14 FfCp,fεe
∂T ∂T* + FfCp,fεp(1 - εe) + FsCp,s(1 - εp)(1 ∂t ∂t js ∂T ∂(VT) ∂2T + FfCp,fεe - Ez,TFfCp,fεe 2 ) 0 εe) ∂t ∂z ∂z
ci
) cfeed or 0 and
z)L purge
Vf z)L ) 0 and
(1.vii)
∂Vf )0 ∂z z)0
(1.viii)
Vf z)0 ) Vfeed and
∂Vf )0 ∂z z)L
(1.ix)
Vf z)L ) Vpurge and
∂Vf )0 ∂z z)0
(1.x)
Boundary conditions on velocity for the adsorption and purge steps, respectively, in TSA are24
Vf | z)0 ) Vfeed and
(2)
(3)
∂ci )0 ∂z z)0
Boundary conditions on velocity for the pressurization, adsorption, and purge steps, respectively, in PSA are1
∂Vf )0 ∂z z)L
(1.xi)
∂Vf ∂z z)0
(1.xii)
Vf z)L ) Vpurge and
Initial and boundary conditions for the energy balance are in Table 2. The mass- and heat-transfer equations assume a lineardriving-force (LDF) model in terms of the solid phase14,15 ∂q¯i ) kMTCsolid(q*j - qji) ∂t
(| )
(1.vi)
The initial conditions for a clean bed and a saturated bed, respectively, are1
a
ci(z, 0) ) 0 and qji(z, 0) ) 0
(1.xiii)
ci(z, 0) ) c0i and qji(z, 0) ) q0i
(1.xiv)
Note that i refers to water vapor in the current study.
Ind. Eng. Chem. Res., Vol. 48, No. 13, 2009
∆Hads ∂q ∂Ts ap ) hHTC (T - Ts) ∂t Cp,fFp f Cp,f ∂t
(4)
The LDF coefficient, kMTCsolid, includes several different masstransfer resistances, and hHTC is the overall heat-transfer coefficient. Analytically, under the assumption of very small crystals, kMTCsolid can be calculated from the equation1 1 kMTCsolid
Rp Rp ) + 3kf 15εpDp
(
kf ) (2.0 + 1.1Sc1/3Re0.6)
(5)
)
(6)
In eq 6, Dk is the Knudsen diffusivity
MT
Dk ) 97rpore
(7)
kMTCsolid )
()
Dm T n p0 ) Dm0 T0 p
(8)
For water vapor diffusing in air, n ) 1.81, Dm0 ) 2.26 × -5 m2 s-1, p0 ) 100 kPa, and T0 ) 273.15 K.16 Table 2. Initial and Boundary Conditions for Energy Balancea During PSA blowdown, the appropriate boundary conditions on temperature are1
∂T )0 ∂z z)L
T| z)0 ) Tfeed and
∂T )0 ∂z z)L
(2.ii)
T| z)0 ) Tfeed and
∂T )0 ∂z z)L
(2.iii)
For the TSA adsorption and purge steps, respectively
T| z)0 ) Tfeed and
∂T )0 ∂z z)L
(2.v)
T| z)L ) Tpurge and
∂T )0 ∂z z)0
(2.vi)
The initial condition for temperature is1
In these simulations, Tinitial ) Tfeed.
q*i
(10)
(2.vii)
15 E D exp 2 s0 RT Rp
( )
(11)
(12)
qmbPin mol ) kg 1 + bPin
( )
(13)
where qm ) k1 + k2T, b ) k3 exp(k4/T), and n ) k5 + k6/T. Pi is measured in kilopascals, and T is in kelvin. This LangmuirFreundlich isotherm does not allow calculations in the partper-million H2O range because Henry’s law is not defined for this model. Hence, this Langmuir-Freundlich model was fit by a temperature-dependent Langmuir model that was used in all simulations q*i
24
a
(9)
where j ) 1.66Re-0.51 for Re < 190 and j ) 0.983Re-0.41 otherwise. In eq 12, Cp,f ) yN2Cp,N2 + yH2OCp,H2O, with Cp [kJ/ (mol · °C)] ) a + bT + cT2 + dT3, where T is in degrees Celsius.21 In this expression, for N2, a ) 2.9 × 10-2, b ) 2.199 × 10-6, c ) 5.723 × 10-9, and d ) -2.871 × 10-12, and for H2O, a ) 3.346 × 10-2, b ) 6.88 × 10-6, c ) 7.604 × 10-9, and d ) -3.593 × 10-12. The equilibrium adsorption data for water adsorption on zeolite 13X was correlated with an extended Langmuir-Freundlich isotherm in the “no-capillary-condensation” regime18
∂T )0 ∂z z)0 (2.iv)
T(z, 0) ) Tinitial
2FfVfrp µ
Re )
hHTC ) jCp,fVfFfPr-2/3
(2.i)
During the PSA pressurization, adsorption, and purge steps, respectively, the necessary boundary conditions on temperature are1
(T| z)L)purge ) (T| z)L)adsorption and
for 3
