Ind. Eng. Chem. Res. 1995,34, 3947-3955
3947
Production of Ultrahigh-Purity Oxygen: A Distillation Method for the Coproduction of the Heavy Key Component Stream Free of Heavier Impurities Rakesh Agrawal Air Products and Chemicals, 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195-1501
Use of a side stripper for the coproduction of the heavy key component stream free of undesirable heavier impurities has been suggested. A side stream containing the heavy key component is withdrawn from a column at a location that is sufficiently above the feed such that it is essentially free of the undesired impurities. This side stream is then distilled in a side stripper to remove lighter components. From a given feed, this method is capable of coproducing a significant fraction of the heavy key component with extremely high purities and with minimal additional energy usage. Coproduction of ultrahigh-purity oxygen, containing only ppb level impurities, along with standard-grade oxygen (99.5%) is presented. In comparison with earlier techniques, this method of distillation is simpler and more efficient and uses less equipment while maintaining a high recovery of oxygen from air.
Introduction
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I
Many gases are used in the production of semiconductor devices. On-going trends toward device miniaturization, and the need for high production yields, are leading to gas specifications with ultrahigh purities (Sugiyama et al., 1989). Typical cryogenic processes can produce gases such as oxygen, nitrogen, and argon with parts-per-million by volume (ppm) impurities. However, ppm impurity concentration levels have become a thing of the past for the semiconductor industry. By about 1988, the level of impurity allowed in the process gas for integrated circuit manufacturing was about 100 ppb, and by 1990, the acceptable level had already dropped below 10 ppb (Hope et al., 1990;Parlier and Noel, 1990). It is widely felt that this push t o decrease the impurity level will continue as the devices continue to advance, and gases with parts-per-trillion impurity concentrations will shortly be needed. Oxygen is one of the ultrahigh-purity process gases needed by the electronics industry. Oxygen with high purity can be produced by electrolysis of water. However, this method of oxygen production is energy intensive and is generally not used for large-scale requirements. Oxygen is produced on commercial scale by air separation. Besides main constituents nitrogen (78.1%), argon (0.9%),and oxygen (20.9%),air has trace quantities of other components. Isalski (1989)tabulated a list of all major trace components and their typical and maximum concentrations in air. While contaminants such as water, carbon dioxide, acetylene, ethylene, and some of the other hydrocarbons can be easily removed by adsorption technology, it is not possible to remove argon, neon, hydrogen, krypton, methane, etc., t o extremely low concentrations by adsorption methods. As a result, recently developed pressure swing andor vacuum swing adsorption processes are not suitable for the production of ultrahigh-purity oxygen from air. The separation of air to produce oxygen by cryogenic methods is a well-established industrial process (Isalski, 1989; Latimer, 1967; Thorogood, 1986; Timmerhaus, 1989). A conventional double distillation column process to produce standard-grade oxygen of about 99.5% purity is shown in Figure 1 (Agrawal et al., 1990). In this cryogenic process, feed air is compressed to about 6 atm and is passed through a bed of 13X molecular
MOLE SIEVES
N2
I
REFRIGERATION
I
CRUDE LOX
Figure 1. A conventional process for oxygen production.
sieves. Contaminants such as water, carbon dioxide k 3 6 0 ppm), unsaturated hydrocarbons-acetylene (0.1- 1 ppm), ethylene (0.01-2 ppm), propylene (0-0.2 ppm), and C4 and heavier hydrocarbons are totally adsorbed on the molecular sieves. Even though some ethane (0.02-0.1 ppm) and propane (0-0.1 ppm) are retained by the molecular sieve bed, most of these and the methane (2.0-10 ppm) flow with the feed air t o the cryogenic process. Other contaminants present in the feed air are helium ( ~ ppm), 5 hydrogen (0.5-10 ppm), neon ( ~ 1 ppm), 8 krypton ( ~ 1 .ppm), 1 xenon ( ~ ~ 0 . 0 8 ppm), carbon monoxide (0.01-5 ppm), and nitrous oxide (0.01-0.5 ppm). This feed air is cooled in a main heat exchanger against the returning cold product nitrogen and oxygen streams. Nitrogen and oxygen are separated by distillation of the cold air in a two-stage distillation process. In the first stage, the lower column, which operates at a higher pressure, separates the feed air into a nitrogen vapor stream and an oxygen-enriched liquid stream. This oxygen-enriched liquid stream, often called crude LOX, is fed to the upper column. The upper column, which operates close to ambient pressure, produces standard-grade oxygen and nitrogen streams. Liquid nitrogen reflux for both columns is generated at the top of the lower column. Nitrogen vapor at the top of the lower column is condensed against the liquid oxygen at the bottom of
Q888-5885/95/2634-3947$09.QQ/Q 0 1995 American Chemical Society
3948 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
the upper column by heat exchange between the two streams in a reboiler-condenser. The pressure difference between the two columns ensures a proper temperature difference between the condensing and the boiling fluids. A portion of the nitrogen vapor from the top of the lower column is warmed and expanded in a turboexpander to provide the needed refrigeration for the plant. Of the three primary elements in air, nitrogen is the most volatile, oxygen is the least volatile, and argon has intermediate volatility. All the contaminants in the feed air t o the lower column that are much more volatile than oxygen, such as helium, hydrogen, neon, and carbon monoxide, leave in the nitrogen stream. Whereas, all the other trace components such as krypton, xenon, nitrous oxide, methane, ethane, and propane, which are less volatile than oxygen, concentrate in the oxygen product. Some argon also remains in the oxygen product. Giacobbe (1989) tabulated the typical concentration ranges of impurities found in oxygen produced from conventional plants. The conventional standardgrade oxygen is composed of about 99.5% oxygen, 0.5% argon, 10 ppm methane, 0.5 ppm other hydrocarbons, 5 ppm krypton, 0.4 ppm xenon, 0.1 ppm nitrous oxide, and essentially no nitrogen. While this standard-grade oxygen is suitable for a range of applications including fabrication and cutting of metals, steel industry, chemicals manufacturing and medical uses; it is definitely not suitable for the electronic industry. Production of Ultrahigh-Purity Oxygen from Standard-Grade Oxygen. Several processes have been described for the production of ultrahigh-purity oxygen from the standard-grade oxygen supplied from the conventional cryogenic plant. These processes can be grouped in two broad classes. In one class are the on-site purifiers, which upgrade the purity of oxygen by catalytic combustion (Giacobbe, 1989). Even though this method is successful in removing hydrocarbons and carbon dioxide, it does not reduce the concentration of other rare gases, such as argon, krypton, and xenon. It has been stated that some heavy components such as krypton are especially detrimental to the quality of the products associated with the electronic industry (Cheung, 1985). Furthermore, it is possible that the particulates generated from the granulated catalyst and adsorbent beds could contaminate the final oxygen product, thereby making the use of a conventional catalytic combustion method unsuitable (Cheung, 1988B). The second class of methods uses cryogenic distillation techniques to obtain ultrahigh-purity oxygen from a standard-grade oxygen stream (Cheung, 1988B; McGuinness and Cilen, 1989; Eyre, 1989; Nagamura, 1989).All these patented processes are basically similar to wellknown distillation schemes for ternary mixtures (Minderman and Tedder, 1982; King, 1980). For this purpose, it can be regarded that standard-grade oxygen is to be separated into three streams: one containing all the light components, which is mainly argon (stream A), an ultrahigh-purity oxygen product (stream B), and a third stream containing all the heavy components such as methane and other hydrocarbons, krypton, xenon, and nitrous oxide (stream C). Of these heavy components, methane is the most volatile and its low concentration in the ultrahigh-purity oxygen ensures even lower concentrations of the other heavy components. All the patented processes can be readily derived from the configurations discussed by King (1980). Separation schemes of Cheung (1988B) and Nagamura (1989) are essentially similar to "direct sequence configuration 2".
Similarly, the distillation scheme of Eyre (1989) may be derived from "inverted sequence configuration 1".The distillation scheme of McGuinness and Cilen (1989) is embodied in "configuration 7". All these processes for upgrading standard-grade oxygen using known distillation schemes require two distillation columns and multiple boilers and condensers. To provide boiling and condensing duties, a heat pump is used. Typically, nitrogen is used as the heat pump fluid. Even though a great deal of work has been done to produce ultrahigh-purity oxygen from standard-grade oxygen, coproduction of ultrahigh-purity oxygen from cryogenic air separation plants producing standardgrade oxygen product has not been explored in detail. Such coproduction processes provide unique opportunities. The present work was undertaken to explore process simplifications and energy reduction which are provided by coproduction processes. Coproduction of Ultrahigh-purityOxygen. Some attempts have been made in the patent literature to coproduce ultrahigh-purity oxygen by modifying the conventional process for oxygen production shown in Figure 1. Cheung (1988A) and Grenier and Mazieres (1990) have proposed processes where oxygen vapor from the bottom of the upper column is withdrawn and distilled in a two-column system. Figure 2 shows Cheung's process. In this figure, and in the subsequent process figures, only distillation columns, boiler, and condensers are shown for simplicity. Other appropriate heat exchangers were used in the process simulations but are not shown. In Figure 2, oxygen vapor of standard-grade purity from the bottom of the upper column is fed a t the bottom of a rectifying column. The condensing duty at the top of the column is provided by vaporizing a portion of the crude LOX. The function of this rectifying column is to remove all the heavy components from the vapor ascending this column. The liquid from the bottom, which contains all the heavy components,is returned to the upper column. A portion of the liquid from the condenser at the top of the rectifying column, which is free of heavy components and contains only oxygen and argon, is fed at the top of a stripping column. The boilup at the bottom of this stripping column is provided by condensing a portion of the high-pressure nitrogen from the top of the lower column. In this column, argon is separated from oxygen and ultrahigh-purity oxygen is produced from the bottom. An oxygen stream with argon concentration similar to the standard-grade purity is produced from the top. Computer simulations were done for the process of Figure 2 to estimate the impact of ultrahigh-purity oxygen production on the recovery of oxygen from the feed air. In these calculations, about 12% of oxygen in the feed air was produced as liquid a t ultrahigh purity and the rest as gaseous product at standard-grade purity of about 99.5%. Attempts were made to maximize the recovery of the standard-grade oxygen from the feed air. The results from one set of calculations are summarized in Table 1. Thirty theoretical stages of separation were used in the rectifying column and 36 for the stripping column. The liquid feed to the stripping column contained 99.6% oxygen, 0.4% argon, and 1 ppb methane and its flow rate was 13% of the feed air flow. Since methane is lightest of all the heavy impurities, the concentration of other heavy components is well below that of methane. The total oxygen recovery for this process is 78.9%, distributed as follows: 50.1% of the oxygen in the feed air is recovered from the top of the stripping column with 99.5%purity,
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3949
N2
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I 02
b TO
COLUMN
ULTRA-HIGH
PURITY 02
Figure 2. Coproduction of ultrahigh-purity oxygen from a conventional oxygen process (Cheung, 1988A). Table 1. Process Conditions for Key Streams in Figure 2a ~~
~
~
concentration stream no. lb 10 11 12 14 15 16 20 (1
state gasfliq gas gas liq liq gas liq liq
flow (mol) 100 3.5 22.0 9.0 13.0 10.5 2.5 0.1
pressure (kPa) 570 149 149 149 133 133 152 149
temp (K) 99.7 93.9 93.9 93.9 92.7 92.7 94.1 93.9
0 2
(%I
21.0 99.7 99.7 99.8 99.6 99.5 100.0 99.7
Ar (ppm) 9300 3000 3000 1951 3726 4613 0.6 1950
CH4 (ppm) 2 48 48 117 0.001
Kr (ppm) 1.1 21 21 51
0.004 161
315
Xe (ppm) 0.1 0.3 0.3 0.6
91
Basis: 100 mol of feed air. While other streams in this table have no nitrogen, stream 1 has 78.1% nitrogen.
12%of the oxygen is recovered from the bottom of this column as ultrahigh-purity oxygen containing 0.6 ppm argon and 4 ppb methane, and 16.7% of the oxygen in the feed air is recovered with 99.7% purity from the bottom of the upper column. Calculations were also done for the process of Figure 2 with all the process conditions similar to that for Table 1,except that no ultrahigh-purity oxygen was produced. The number of theoretical stages of separation in both the lower and upper columns was unchanged. In order to make minimal changes in the process conditions, 2.5 mol of oxygen400 mol of feed air was produced as liquid from the bottom of the upper column. Essentially the conventional process of Figure 1 was simulated t o produce standard-grade oxygen of 99.5% purity. It was found that 99% of the oxygen contained in the air was recovered. This is about 20%more than the process of Figure 2, reflecting a heavy penalty for the coproduction of ultrahigh-purity oxygen. In order to understand the cause for this severe drop in oxygen recovery, it is useful to examine the pseudo McCabe-Thiele diagram of the upper column for the process of Figure 1 shown in Figure 3. In the bottom section of the upper column, when oxygen concentration is greater than 85%,the distance between the equilibrium curve and the operating line is small and separation is quite difficult. In this section of the column, argon is being separated from oxygen and its relative volatility is only about 1.5. The liquid-to-vapor molar flow rate ratio (W) for this bottom section is 1.41. For
1 .o
B 0.8 c
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e
I 8
0.6
E
8
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-.-s
.u
0
0.4
E
c
P u
O2
0.0
0.0
0.2
0.4
0.6
0.8
1.o
mole fractlon 01 llght component in liquid
Figure 3. Pseudo McCabe-Thiele diagram of the upper column for the process of Figure 1.
the processes in both the figures, the amount of highpressure nitrogen to be condensed at the top of the lower column is the same. This means that total boilup available is also the same. For 100 mol of feed air t o the lower column, the total amount of liquid oxygen that can be vaporized for the example calculations was 69.5
3950 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
Y1
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CRUDE LOX
I
"
i
ULTRA-HIGH
PURITY 02
Figure 4. Coproduction of ultrahigh-purity oxygen from a conventional oxygen process [from Grenier and Mazieres (199011.
mol. However, for the process of Figure 2, this boilup is distributed between the three columns. In Table 1, the boilup need for the rectifying column is 9 mol and for the stripping column 10.5 mol. This decreases the boilup available at the bottom of the upper column t o only 50 mol. This reduced boilup is the cause for the sharp decline in total oxygen recovery. In Table 1,the LN in the bottom section of the upper column increased to 1.5, and only 78.9% of the oxygen in the feed air could be recovered. For the process of Figure 2, as the production of ultrahigh-purity oxygen is increased, the vapor flow in the bottom section of the upper column will decrease. This will reduce the recovery of standard-grade oxygen (stream 10) from the bottom of the upper column. Eventually this flow will reduce to zero, giving maximum flow of ultrahigh-purity oxygen. For the process conditions used, this maximum recovery of ultrahighpurity oxygen was about 14% of the oxygen in the feed air and the total oxygen recovery for this case was 74.4%. Grenier and Mazieres (1990)suggested a modification to Cheung's process which is shown in Figure 4. They added a rectifying section and a condenser on the top of the stripping column, making it a complete dtrahighpurity oxygen distillation column. Since only a small flow rich in argon is taken from the top of this column and returned to the upper column, the amount of feed needed for this column is substantially reduced. This also decreases the amount of feed to the rectifying column. As a result, the boilup available for the bottom of the upper column is increased, leading t o better overall oxygen recovery. A quick calculation for the conditions shown in Table 1indicated that the modified process can increase the total oxygen recovery to about 90%. This is a substantial increase in oxygen recovery but it comes at the cost of another condenser and a rectifylng section. A better solution is obtained by examining the concentration profiles of various components in the lower and upper distillation columns. In Figure 1, as the vapor ascends in the lower column, all the heavy components are removed by the descending liquid
*Ot
'\I L
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0
BOITOM
I
40
20
THEORETICALSTAGES
UPUlD
6C
TOP
Figure 6. Concentration profile for oxygen and methane in the liquid phase of the upper column for a conventional oxygen plant of Figure 1.
within a few stages of bottom. The crude LOX stream from the bottom of this column contains all the heavy components of the feed air. The nitrogen reflux from the top of this column has no heavy component. These two streams constitute the feeds to the upper column. The concentration profiles for oxygen and methane in the liquid phase for the upper column are shown in Figure 5 . It is observed that the concentration of methane drops to zero ppm within five trays above the crude LOX feed tray location, while the concentration of oxygen is still significantly high at about 50%. This is due to the fact that at this location of the upper column the relative volatility of nitrogen with respect to methane (c$Q is about 14.2 while with respect to oxygen it is only 3.9. The oxygen-methane relative volatility of 3.6 provides an opportunity to withdraw a vapor or a liquid stream from the upper column at a location such that concentration of all the heavy components has dropped to negligible levels and then distill it in an ultrahigh-purity column to produce the desired oxygen product. Figure 6 shows a scheme based on liquid withdrawal from the upper column. A liquid stream free of heavies but containing oxygen and the lighter components is withdrawn a couple of trays above the crude LOX feed and is fed at the top of an ultrahigh-purity oxygen column. This column is a stripping column where ultrahigh-purity oxygen is produced from the bottom
(e) (eH4)
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3961
UPPER
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TO EXPANDER
CRUDE LOX
>
ULTRA-HIGH
PURITY 02
Figure 6. A new process for the coproduction of ultrahigh-purity oxygen from a conventional oxygen process. Table 2. Process Conditions for Key Streams in Figure 6'" concentration stream no.
state
flow (mol)
pressure (kPa)
temp (K)
Nz (%I
1 10 14 15 16 20
gadiq gas liq gas liq liq
100 17.8 12.0 9.5 2.5 0.1
569 149 127 127 148 149
100.1 93.9 84.5 87.1 93.9 93.9
78.1
a
0 2
(%)
21.0 99.5 53.7 41.5 100.0 99.6
40.1 50.6
Ar (ppm) 9300 4999 62217 78589 0.6 3254
CH4 (ppm) 2 11
