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Technology Separation and Purification Technology 15 ( 1999) 3 140

A new PSA process as an extension of the Petlyuk distillation concept

Fei Dong a, Hongmei Lou a, Motonobu Goto b, Tsutomu Hirose bv* a Graduate School of Science and Technology, Kumamoto University, 2-39-2 Kurokami, Kumamoto 860, Japan

b Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-l Kurokami, Kumamoto 860, Japan

Received 22 February 1998; received in revised form 20 April 1998; accepted 27 April 1998

Abstract

A new PSA process called the Petlyuk PSA process, which consists of two pairs of adsorption columns, an intermediate feed inlet and a side stream outlet, is proposed by extending the concept of Petlyuk distillation. By a simulation of the process with a simple mathematical model, the enrichment of a dilute ternary mixture of CO,-CH,-N, on activated carbon was taken as an example to investigate the performance of this PSA process numerically. It was found that both CO, and CH, could apparently be enriched in the top and side stream products, while both CO1 and CH4 were greatly removed from the bottom gas product (N,). Some comments concerning process control, feasible applications and future work on the proposed Petlyuk PSA process are also described. It can be concluded that this process, with the advantages of low capital investment, low energy consumption and wide applicability, is a promising candidate for the simultaneous separation of ternary gas mixtures. 0 1999 Elsevier Science B.V. All rights reserved.

Keywords: Adsorption; Gas separation; Petlyuk distillation; Pressure swing adsorption; Ternary gas mixture

1. Introduction

Since Skarstrom first proposed a two-column pressure swing adsorption (PSA) cycle for the purpose of air drying, PSA has attracted growing interest in industrial separation, and a great variety of complicated PSA processes have been developed and commercialized because of its low energy requirement as well as the low capital investment involved. In almost all PSA processes, only one component in the feed mixture, generally the

* Corresponding author: Tel. : + 8 1 96 3423665; Fax: + 81 96 3423679; e-mail: hirose@chem.kumamoto-u.ac.jp

weakly adsorbed component, is the desired pro- duct. However, from the point of view of econom- ics, it is very important to recover all the components in the feed mixture.

For the separation of a binary mixture, Diagne et al. [ 1,2] proposed a PSA process with an intermediate feed inlet position, analogous to a conventional distillation column (see Fig. 1). Its performance was tested experimentally by applica- tion to the simultaneous removal and enrichment of carbon dioxide, and a very good ability was shown for both dense and dilute mixture systems.

For a ternary gas mixture or a multicomponent mixture, there are practical applications from the

1383-5866/99/$ -see front matter 0 1999 Elsevier Science B.V. All rights reserved. PZI S1383-5866(99)00081-l

32 F. Dong et al. / Separation and Purification Technology 15 (1999) 31-40

Condenser

R&oiler B

(a) Distillation

Feed_ (A+B)

Recttfying tetlux

Compression

(b) t’s.4

Fig. 1. Analogy between distillation and the PSA process with an intermediate feed inlet position.

point of view of energy recycling and environmen- tal protection, such as the separation of fermenta- tion gas (CO,-air-CH3 and the separation of the effluent gas from ethylene oxidization with air (CO,-N,-H,), etc. However, the separation of multicomponent mixtures has not attracted so much interest: a few exceptions are noted below.

Nataraj and Wankat [3] suggested a PSA unit for the separation of a ternary mixture which consisted of three packed adsorption columns operating cyclically 120” out of phase with each other. They predicted the performance of this three-column process by a characteristic method.

For bulk separation of a ternary mixture of H,CH,-H,S, Cen et al. [4,5] conducted an exper- imental and theoretical study by employing a PSA process which operated with four steps in each cycle, i.e. (1) pressurization, (2) adsorption, (3) concurrent blowdown, and (4) countercurrent evacuation. From their results, three products i.e. clean H, (over 99% H, and below 0.01% H,S), clean CH4 (over 95% CH4 and below 0.01% H2S) and H,S (over 10% H,S), were obtained from a feed mixture containing 49.5/49.5/1.0% H2/CH4H2S.

Sircar [ 61 also proposed a multistep PSA process

for the direct production of an ammonia synthesis gas (HZ/N, mixture in a mole ratio of 3: 1) and pure CO, from a reformer off-gas which typically contained 20.0% COZ, 1 .O% CO, 4.0% CH4 and 75% Hz at 200-300 psi (gauge). This process con- sisted of six adsorption columns, one compressor and one vacuum pump. The author indicated that this process was extremely efficient, producing an ammonia synthesis gas with a H, recovery of about 95.0% from the feed reformer off-gas while producing high-purity CO2 (99.4%) with a high recovery (94.0%), and approximately 75% of the external Nz used in the process was recovered in the primary ammonia synthesis gas product.

In nature, the three processes cited above all employ a three pressure-swing step strategy, i.e. the process pressure swings between a high pres- sure stage, a low pressure stage and a vacuum pressure stage, respectively.

On the other hand, integrated three-product (Petlyuk) distillation, the concept diagram of which is shown in the left-hand side of Fig. 3, was proposed as an attractive and promising alterna- tive method in the field of distillation separation by Wright about 50 years ago for separating a ternary feed [7]. This configuration is usually denoted as a Petlyuk column after Petlyuk et al., who later studied the scheme theoretically [S]. In fact, the design of Petlyuk distillation consists of an ordinary column shell with the feed and the side stream product draw divided by a vertical wall through a set of trays. Many authors have since predicted considerable savings in energy capi- tal cost with such a design, but few of these integrated columns have been built. One reason for this fact was that the Petlyuk column, as compared with an ordinary distlllation column, had many more degrees of freedom in both opera- tion and design, and this made the design of both the column and its control system more complex.

In the present paper, we try to apply the Petlyuk distillation concept to the PSA field, with the aim of proposing a more widely applicable process for the simultaneous separation of ternary gas mix- tures. This process has only a single PSA unit, in which the pressure swings between two pressure stages, and there is no difference to the pressure swing of a traditional PSA process, while low

F. Dong et al. 1 Separation and Purljiration Technology 15 (1999) 31-40

energy consumption and low capital investment are expected. A ternary dilute gas mixture of CO,-CH,-N, was taken as an example, and the performance of the proposed process was investi- gated numerically using an isothermal model and a linear driving force model.

2. PSA configuration

Distillation and PSA processes can be operated on analogous underlying principles. Conventional distillation and a PSA process with dual refluxes are shown in schematic flow sheets in Fig. 1 to emphasize the analogy between them. The low- pressure side in PSA corresponds to the vapor phase in the distillation process, in which the lighter component (i.e. the more volatile or more adsorbable component) is concentrated in the ligh- ter phase. Compression in PSA is equivalent to a condenser since they play the common role of rectifying reflux, by which the lighter component, once transferred into the heavy phase, is released into the light phase (vapor phase or low-pressure column) in a much higher concentration. Also, the depressurization of PSA is similar to a reboiler in distillation.

A primary set of PSA for separating a ternary mixture can be a combined process consisting of three separate PSA units, as shown in Fig. 2. The integrated three-product distillation (Petlyuk dis- tillation) as shown in the left-hand side of Fig. 3 is equipped with a single condenser and reboiler set instead of three separate distillation columns. By imitating the Petlyuk distillation concept, the original three units of the PSA system in Fig. 2 can be simplified into a new PSA separation pro- cess as shown in the right-hand side of Fig. 3. This PSA process consists of two pairs of physical adsorption columns (Al, A2, Bl, B2), in which column Al and A2 play a role which is similar to the prefractionator in the Petlyuk distillation cycle. During the first half cycle, columns Al and Bl connected each other constitute the high-pressure side or the adsorption side, while columns A2 and B2, also connected each other, constitute the low- pressure side or the desorption side. During the next half cycle, their roles are reversed. The four

CH4 Intermediate

Product CH4

Fig. 2. Combined process by three PSA units with an intermedi- ate feed.

columns are all packed with adsorbent (i.e. acti- vated carbon). Feed is introduced periodically and alternatively at the middle of column Al or A2, and a side stream is drawn out periodically and alternatively at the middle of columns Bl and B2. Therefore, three product fluxes are taken out, respectively, from the top of the low-pressure side columns Bl and B2, the bottom of the high- pressure columns B2 and Bl, and the side stream exit of the low-pressure side columns.

For an easier and clearer understanding and analysis, the four-column PSA process shown in Fig. 3 could be dismantled and transformed to a 12-bed process, as shown in Fig. 4, in which the feed inlet and side stream outlet act as the connect- ing points for columns 1 and 11, columns 2 and 12 and columns 10 and 3 in series. Beds 1, 2, 3, 10, 11 and 12 and 4, 5, 6, 7, 8 and 9 work at the same high or low pressure, respectively. Part of the gas produced at the top of bed 3 is recycled to the top of bed 4 after compression, and part of product gas produced at the bottom of bed 9 is

34 F. Dong et al. /Separation and Purification Technology 15 (1999) 31-40

Feed -

A+B+S

81 Bi

Reboiler ’ (Deprrssluizalion) stripping renur

(a) Petlyuk distillation (b) Petlyuk PSA

Fig. 3. Analogy between three-product integrated distillation (Petlyuk distillation) and the Petlyuk PSA process.

Top product (CO,)

Bottom product (Nz)

Fig. 4. Conceptual diagram of the Petlyuk PSA process for pro- cess simulation.

recycled to the bottom of bed 10 after depressuriza- tion, resulting in adjustable reflux ratios (RI, R2, R3, R4) by a series of valves. The reflux ratio is defined as the ratio of the reflux gas to the feed gas. The positions of the feed inlet and the side stream outlet were changed horizontally and periodically by a series of automatical program- controlled solenoid valves; hence, the positions of beds 1, 2, 3, 10, 11 and 12 were transferred horizontally to those of beds 5, 6, 4, 9, 7 and 8.

3. Mathematical model

For mathematical simplicity, a dilute ternary mixture of a light component (CO& an intermedi- ate component (CHJ and a heavy component (N,) as the inert component is assumed, and further approximations are also made to simplfy this system: (1) A constant mole flux of inert gas through

the bed, (2) Ideal plug flow, in the sense that axial and

radial dispersion are not considered, (3) A low density of the gas phase,

F. Dong et al. /Separation and PuriJcation Technology 15 (1999) 31-40 35

(4) (5)

(6)

(7)

(8)

The bulk gas phase follows ideal gas behavior, The pressure drop through the bed is negligible, The mass transfer rate is expressed by linear driving force assumptions, Mass transfer coefficients are constant all through the columns, The adsorption equilibrium is expressed by a Langmuir isotherm.

The material balance is formulated in terms of the mole ratio of the adsorbate (Y) relative to the inert gas instead of the mole fraction (X) or mole concentration.

The mole ratio (Y) and the mole fraction are simply related by:

x Y ys -__ and X= ~

(1 -X) (l+Y)

Therefore, with the above approximation, the material balance is given by Eq. (2) i.e. :

-*pp, 014i =o a(G yi)

az at (2)

where r is the mole flux of the inert gas, pc is the bulk density, and the subscript ‘i’ refers to the bed number.

The mass transfer rate is given by:

aYi PC - = kKia(4i-qT)

at

where Kia is the overall volumetric mass transfer coefficient and qr is the amount adsorbed at equilibrium.

The Langmuir isotherm is:

(4)

where k is the Langmuir constant reciprocal of a general expression, and the adsorbate concen- tration Ci is related to the mole fraction Xi through:

yi Ci = C~‘, Xi = CT, ___

C1 + yi)

(5)

in which

c =pi T, RT

(6)

When the column length L, the half cycle time t,, the mole ratio in the feed gas Y, and the amount adsorbed ql in equilibrium with the feed gas are introduced as reference quantities, the above equa- tions are non-dimensionalized as follows:

dhi - =Mi(h; -hi) aT

and

ayi d, = 1Ni(h,* -hi) (8)

in which

Yi = yi/yO

hi = qi/ql

t = t/t,

z = Z/Li

The parameter Mi is defined by:

(9)

K;at, M;=-

PC

Ni is the number of mass transfer units, defined by:

KiaLifi Ni= G (11)

in which p is the adsorption coefficient for the feed gas, and is given by:

(12)

fl is the reduced velocity of the inert gas which is converted under the feed pressure, and is defined by:

(13)

?6 F. Dong et al. / Separation and Purification Technology 15 (1999) 31-40

The Langmuir isotherm is given by:

h= C/CO

r+(l -r)C/ClJ (14)

in which r is the Langmuir parameter, defined by:

k r=-

k+C, (15)

The value of C/C, is related to the reduced mole ratio y by:

c @Tit1 + yO)

c,= t1 + yOYi) (16)

in which M. is the pressure ratio to the feed gas. The above simultaneous partial differential

equations were solved with the improved Euler method under the following initial and boundary conditions.

Y1.1 =(Yo +YllMJw(1. +&I, Y2.1 =y,,.,;

~3.1= [YI.M( 1. + RI) +~2.&2 - Q,>l/ (l.+&+R2-Q,>

y4.M =y3.M; y5.M =y6.M =y4.1; y6.1 =y8.M;

Y7.1 =YS.M; Y9.M = (Y8.1 rr, +Y7.1 u7)/(“8 + u7);

Y9.1 ‘YlO.1; Y1l.l =Y12.1 =YlO.M (17)

3.1. Initial conditions

4. Casestudy

4.1. Comments on the model and simulation

By extending the concept of Petlyuk distillation to PSA gas separation, this research was performed with the aim of proposing a promising process for the simultaneous separation of a ternary gas mix- ture. With the help of numerical process simula- tion, we could obtain information about the

emichiment of the key components in the mixture gases all through the adsorption columns, and then analyzed the performance of the Petlyuk PSA process to determine its feasibility in practical simultaneous separation of a ternary gas mixture. From this point of view, and for mathematical simplicity, a dilute ternary mixture system con- sisting of two adsorbable components (CO2 and CH,) and an inert gas (N,) was adopted. The interaction between the three components could be neglected, and an isothermal model for a binary mixture system was employed for this ternary mixture system.

Activated carbon was selected as the adsorbent. The physical parameters and operational parame- ters shown in Tables 1 and 2 were used for the simulation. The simulation results are shown in Figs. 5 and 7, in which the concentration ratio of the product gas to the feed gas (y) was employed as the enrichment index to indicate the perfor- mance of the Petlyuk PSA process.

4.2. Performance of the petlyuk PSA process

In order to gain a better understanding of the performance of the Petlyuk PSA process, the con- centration ratios (y) of the components CO2 and CH4 with the column number (1-12) are plotted in Fig. 5. When the process reaches steady state, in the first half cycle time, the feed under low pressure is introduced between column 1 and column 11, and the side stream is drawn out between column 2 and column 12. Since CO2 is more strongly adsorbed on activated carbon than CH,, it is apparent that the change of component CO2 through the columns is much larger than that of CH,. At the top of column 3, the concentration ratios of CO2 and CH4 are both much higher than 1 (i.e. higher than the concentration of the feed) and both are concentrated there, but the concen- tration of CO2 is much higher than that of CH4. At the bottom of column 9, the concentration ratios of CO2 and CH, are both much lower than 1 (i.e. lower than the concentration of the feed) and both components are removed in large amounts. At the side stream between column 2 and column 12, the concentration ratio of CO2 is much lower than that of CH,. The above results

F. Dong et al. /Separation and Purljication Technology 15 (1999) 31-40 37

Table 1 Physical parameters for simulation

Particle diameter Particle density Bed voidage Langmuir constant Amount adsorbed at saturation Mass transfer coefficient

Or=3 x 10-3m r=483 kg mm3 E=0.60 k ,o,=33.5molm-3,kc,,=118molm-3 qco, =3.07 mol kg-r, qcu, =3.31 mol kg-’ Kco, =K,-+ = 1.325 kg m-3 s-r

Table 2 Operation parameters for simulation

Temperature Half cycle time Adsorption pressure Desorption pressure Flow rate distribution Column length Reflux ratio

293 K 60 s 4.0 atm (gauge pressure) 0.0 atm (gauge pressure) Qr=Q,=O.l, Qs=O.SQ,, L1=L2=L3=L,=Lg=L6=L7=L8=0.3 m, L,=L,0=L,,=L,Z=0.3m R1=0.2, R,=0.3, R,=0.2, R,=0.3

Side swam outlet

Fig. 5. Concentration distribution of the Petlyuk process,

show that using the Petlyuk PSA process, COz and CH, are preferentially concentrated in the top gas product and in the side stream gas product, respectively, and nearly pure Nz can be obtained as the bottom gas product.

We should point out that the calculation was only accomplished for an example by using a set of operation parameters. If the parameters such as the relative position of the side stream outlet, the length of the columns, the reflux ratios, the half cycle time, etc., are changed, better results may be obtained, but the tendency should not be altered too greatly.

4.3. Comparison with the combined process

If we separate a ternary gas mixture by a tradi- tional PSA process, three separate PSA units with an intermediate feed have to be used, and the process shown in Fig. 2 should be adopted. To make a comparison of the performance between the combined process and the Petlyuk process, process simulation was also completed according to Fig. 6. The operational and physical parameters were chosen as the same as those used for the Petlyuk process. The results are shown in Fig. 7. For the top product recovering COz, the concen- tration ratio of CO, to CH, seems better in the combined process, while for the side-stream pro- duct enriching CH4, the results for the Petluyk process are better than those for the combined process. It is apparent that the combined process is too complicated and is difficult to operate from the point of view of capital investment and opera- tional control. The Petluyk PSA process seems to work similarly to the combined process while being lower in capital investment and easier to operate, since it employs only four columns and one com- pressor. On the other hand, although running costs or energy consumption are difficult to discuss, they are related to the throughput rate of the gas and the concentration of the feed mixture, and it is

38 F. Dong et al. / Separation and PuriJication Technology 15 (1999) 31-40

Compressor Top product 4.4. Process control and application D * co2

I 22 B

Unit 2

23

Fig. 6. Conceptual diagram of the combined process for simulation.

12 “‘(“‘!“‘!“‘!“““’ i i -Y-C02

14 21 22 23 24 31 32 33 34

Unit I’S ColumM Uatt 2’s Columns unit 3’S Cohmns

Fig. 7. Concentration distribution of the combined process.

difficult to find a common standard to evaluate the energy consumption of the process. Compared with the process of the three pressure-stage swing strategy, the Petlyuk PSA process takes a two pressure-stage swing strategy, and therefore it should be lower in energy consumption.

As the Petlyuk PSA process is a simple two pressure-stage swing process in general, controlling it should be much simpler. Furthermore, not only the relative position of the side stream, the length of all the adsorption columns, the half cycle time, etc., but also modification of the reflux ratios and the flow rate as operational parameters can be easily realized when running this process by a series of valves, and it is very flexible in treating gas mixture with a larger concentration range. This is possibly a great advantage of this process as compared to the original Petlyuk distillation concept, and perhaps this is why the original Petlyuk distillation concept has not yet been real- ized practically.

If all the operation parameters can be chosen, the process can be compacted, and this leads to a double-layer column process, as shown in Fig. 8, in which the inner columns have changeable ori- fices for modifying the relative reflux ratio at the top and the bottom. The feed is introduced into

CH4

strippine r&u

Fig. 8. Compacted Petlyk PSA process.

E Dong et al. J Separation and Purzjication Technology 15 (1999) 31-40 39

the inner column and the side stream product is drawn out from the outer column. Superficially, the control and operation of such a process is no different to that of a single PSA unit.

For a process in which three pressure-stage swing strategy is adopted, application is confined to a mixture in which the adsorption properties of each component differ sufficiently largely, such as a mixture of CO,-CH4-H,S. The Petlyuk PSA process takes a two pressure-stage swing strategy, and this may be another advantage for wider application. Furthermore, for the mixture C02-CH,-Nz, if we could select other two adsor- bents on which CO, and CH, are favorably adsorbed and N2 is nearly unadsorbed, it can be predicted that the performance of the process would be greatly improved by exchanging acti- vated carbon in the columns 2 and 3, 4 and 6, 8 and 9, and 10 and 12. Therefore, the key compo- nent CH, in the side stream product will reach a relatively higher concentration. Alhough a pure product of CH, is difficult to obtain, we also can see very valuable applications.

5. Conclusion

By extending the Petluyk distillation concept to the field of PSA gas separation, a PSA process with an intermediate feed inlet position and a side- streamed outlet position (called the Petlyuk PSA process) is proposed for the simultaneous separa- tion of a ternary gas mixture. The performance of this process was investigated numerically for a dilute mixture of CO,-CH,-N, as an example with a simple isothermal model. From the simula- tion results, it was found that under the given conditions, the concentration of the lighter compo- nent (CO,) could be enriched by more than nine times, the concentration of the intermediate com- ponent (CH,) could be enriched by more than three times, and both CO, and CH, could be removed in the bottom product gas. As this Petluyk PSA process has the advantage of a lower capital investment, a lower energy consumption and easier control, it is a promising process for the simultaneous separation of ternary gas mix- tures, and will be more widely applicable to the

separation of a gas mixture in which the adsorption properties of each component differ distinctly. However, a lot of work, such as experiment tests, numerical investigations using a more complicated mathematical model for bulk separation and oper- ations with multiple adsorbents, are expected to be necessary before the process can be applied industrially.

Acknowledgment

This research was supported by a Grant-in-Aid for Scientific Research (No. 8455368) from the Ministry of Education, Science, Culture and Sports, Japan.

Appendix

Nomenclature

c h k K Li M N P 4 q* Q

x Ri t tc Tll TC T u* x Y Y

k ci r PC 5 B

Concentration of the adsorbate (mol m-j) Dimensionless amount adsorbed, q/q*,, Langmuir constant (mol me3) Mass transfer coefficient (kg m-’ s- ‘) Length of each column (m) Capacity ratio (Eq. (10)) Number of mass transfer units ( Eq. ( 11)) Column pressure (atm) Amount adsorbed (mol kg- ‘) Amount adsorbed at equilibrium (mol kg ‘) Product flow rate of inert gas relative to the feed Langmuir parameter (Eq. ( 15)) Gas constant (J mol-’ I(-‘) Reflux ratio Time(s) Half cycle time (s) Beginning of each half cycle time End of each half cycle time Temperature (K) Reduced superficial velocity (m s - ‘, Eq. ( 13)) Mole fraction of adsorbate Reduced mole ratio, r/Y, Mole ratio of adsorbate to inert gas Dimentionless axial distance, Z/L, Axial distance (m) Pressure ratio Superficial mole flux of inert gas (mol m-’ s-r) Bulk density (kg rnm3) Dimensionless time t/t, Adsorption coefftcient (m3 kg-‘, Eq. (12))

40 F. Dong et al. / Separation and Purification Technology 1.5 (1999) 31-40

Subscripts

0 Reference to the feed gas B.S.T Bottom side and top stream

M Column number Top of each column

L Bottom of each column

References

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[4] P.L. Cen, W.N. Chen, R.T. Yang, Ternary gas mixture sepa- ration by PSA: A combined hydrogen-methane separation and acid gas removal process, Ind. Eng. Chem. Process Des. Dev. 24 (1985) 1201-1208.

[5] P.L. Cen, R.T. Yang, Separation of a five-component gas mixture by PSA, Sep. Sci. Technol. 20 (9) (1985) 725-747.

[6] S. Sircar, Recent trends in PSA: Production of multiple products from a multicomponent feed gas, Gas Sep. Purif. 7 (2) (1993) 69-73.

[7] R.O. Wright, US Patent 2 471 134, 1949. [8] E.A. Wolff, S. Skogestad, Operation of integrated three-

product (Petlyuk) distillation columns, Ind. Eng. Chem. Res. 34 (1995) 2094-2210.