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Analysis of Refinery Hydrogen Distribution Systems Joao J. Alves† and Gavin P. Towler*,‡ University of Manchester Institute of Science and Technology (UMIST), P.O. Box 88, Manchester M60 1QD, U.K.
Increasingly strict environmental and product-quality regulations, the shrinking market for highsulfur fuels, and the price benefit of processing heavier and more sour crude oils has pushed oil refiners to increase their hydrocracking and hydrotreating capacities. The resulting increase in hydrogen consumption and limited or even decreased generation are creating tight hydrogen balances in many refineries throughout the world. The efficient use of hydrogen is a necessity, with refineries facing eroded margins due to constrained refinery operations or the need for significant investments in hydrogen generation and purification. This paper addresses the problem of refinery hydrogen distribution. A systematic method for the analysis of hydrogen distribution systems based on the concept of hydrogen surplus is proposed. This method sets targets for the minimum flow rate of fresh hydrogen required by the refinery before any system design. The analysis method is used to provide quantitative insights and to identify the existence of bottlenecks in the hydrogen distribution system. 1. Introduction The oil refining industry uses increasingly large quantities of hydrogen. The shrinking market for heavy fuel oil and the move to heavier crude oils are forcing refiners to increase their use of hydrocracking as a means of upgrading heavy oils to more valuable products. Continuous reduction of the allowed sulfur content in fuels throughout the world has increased the need for hydrotreating, while low-aromatics gasoline has reduced the scope for hydrogen production as a byproduct of catalytic reforming. Hydrocracking and hydrotreating processes are not the only consumers of hydrogen in refineries. Other hydrogen-consuming processes include lubricant plants, isomerization processes, and petrochemical processes that can be integrated with the refinery hydrogen network. The refinery might also export hydrogen across the fence, which can be considered as an additional amount consumed. Figure 1 shows a flow diagram of a high-conversion refinery, highlighting the hydrogenconsuming processes. In addition to the hydrogen-consuming processes, two other elements comprise the hydrogen system: the hydrogen-producing processes and the purification units. The interactions between these elements define the behavior of the refinery hydrogen distribution network and establish the hydrogen demand. Hydrogen is usually regarded as a utility in oil refining. The supply of hydrogen, which represents a cost to the refinery, must be maintained to avoid constraints on the refinery operations. If the demand for hydrogen exceeds the available supply, then the incremental demand must be met by debottlenecking the hydrogen distribution system, adding hydrogen * To whom correspondence should be addressed. † Currently at UOP Limited, Liongate, “Ladymead”, Guildford GU1 1AT, U.K. E-mail
[email protected] ‡ Currently with UOP LCC, 25 East Algonquin Road, Des Plaines, IL 60017. E-mail
[email protected].
Figure 1. Simplified flow diagram of a high-conversion refinery.14 The hydrogen-consuming processes are shown in bold.
production capacity, or purchasing additional hydrogen from an external source. Both in the design of a new hydrogen distribution system and in the debottlenecking of an existing one, it is very important to know the minimum hydrogen supply required by the system. If the system is operating at or near the minimum supply, then the network is operating at the maximum efficiency possible without violating any of the constraints set by the requirements of the hydrogen-consuming processes. It is the objective of this paper to propose a systematic method for establishing a target for the minimum supply to a hydrogen distribution system. 2. Background The design and operation of refinery hydrogen distribution systems usually begins with a consideration of the available sources of hydrogen. The hydrogen used in the refinery can have different origins. Catalytic reforming is the preferred source of hydrogen in most refineries. This process increases the octane number of
10.1021/ie010558v CCC: $22.00 © 2002 American Chemical Society Published on Web 10/15/2002
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heavy naphtha by cyclization and dehydrogenation of aliphatic hydrocarbon molecules into aromatic compounds1 and at the same time generates large amounts of hydrogen at 70-90% purity that can be used elsewhere in the refinery.2 If additional hydrogen is needed, which is the case for hydrocracker refineries or refineries that achieve deep conversion of heavy oils, then this hydrogen is supplied by the steam reforming of natural gas, refinery off-gases, or naphtha. Partial oxidation of hydrocarbons and the import of hydrogen from a neighboring production facility or hydrogen pipeline, if available, are possible alternatives to steam reforming. The integration of steam reforming or partial oxidation facilities with pressure-swing adsorption (PSA) separation technology is a common practice in new designs, allowing for the production of gas above 99% hydrogen purity.3 In most refineries, the supply of hydrogen from producing processes is supplemented with hydrogen recovered from the off-gases of the hydrogen-consuming processes. These off-gases usually contain significant amounts of hydrogen, together with light hydrocarbon compounds such as methane, ethane, and propane. If the off-gas hydrogen concentration is high enough, then the off-gas can be used directly as a hydrogen source. If the hydrogen concentration is very low (typically, below 40 mol %), then the off-gas is usually vented into the refinery fuel system. For intermediate hydrogen concentrations, the hydrogen can be recovered by purification if this is economical. Three purification technologies are used industrially for hydrogen recovery: PSA, membrane permeation, and cryogenic separation. In general, for high flow rates, cryogenic separation or PSA are most economical, whereas membrane separation is preferred for low-flow-rate, high-pressure feeds. If a very high purity product is required, then PSA is the best option. Abrardo and Khurana,4 Wilcher et al.,5 Spillman,6 and Miller and Stoecker2 describe the factors influencing the choice between these three hydrogen purification technologies. Absorption7,8 and selective surface flow permeation9 have also been proposed for use in hydrogen purification, but these techniqueshave not yet seen widespread commercial application in oil refining. Most of the analyses of hydrogen distribution in oil refineries have been carried out internally by oil processing companies, and very little has been published on the subject. Much of what is published addresses the application of specific technologies rather than the overall problem of hydrogen distribution. For example, Kramer et al.10 discussed the design of flexible hydrogen plants, and Hiller et al.11 addressed alternative sources for hydrocracker hydrogen makeup. Bollinger et al.12 and Pacalowska et al.13 focused mainly on refinery hydrogen purification technologies. Although improvements in the hydrogen system can be achieved by modifying individual units or processes, it is the interactions between the different units that ultimately define the performance of the system as a whole. Towler et al.4 proposed analysis of the hydrogen distribution system through the cost of recovering hydrogen, using value composite curves.14 This approach can provide insight into economic tradeoffs that affect the hydrogen management problem; however, it does not account for the physical constraints that influence the design of the hydrogen network.
Figure 2. Simplified flow diagram of a hydrogen-consuming process.
This paper proposes a systematic method for setting a target for the minimum supply of fresh hydrogen to a hydrogen distribution system. The method defines sources and sinks of hydrogen that allow effective extraction of the important data of the hydrogen distribution problem. The target that is calculated is independent of the distribution system design. 3. Theory The first step in developing an analysis method for establishing the minimum flow rate of fresh hydrogen required by a hydrogen distribution system is to identify the sinks and sources of hydrogen in the system. In hydrogen distribution, a sink is a stream that takes hydrogen from the hydrogen network, whereas a source is a stream that makes hydrogen available to the network. The operational constraints of the network elements can be captured by careful extraction of the source and sink data. The choice of appropriate sources and sinks requires insight into each of the network elements: hydrogen-consuming processes, hydrogenproducing processes, and purification units. 3.1. Sinks and Sources Framework. Figure 2 shows a simplified flow diagram of a typical hydrogenconsuming process. A liquid hydrocarbon feed stream is mixed with hydrogen-rich gas, heated, and fed to a reactor. Part of the hydrogen is consumed by reaction with the feed. Light hydrocarbon compounds (methane, ethane, and propane), H2S, and NH3 are usually formed as part of the reaction products. The effluent from the reactor is cooled and sent to a high-pressure flash separator. The gas released in the separator is often treated in an amine scrubber to remove H2S. Part of the gas is vented from the process through a highpressure purge to prevent the buildup of hydrocarbons in the recycle. The remaining hydrogen-rich gas is recompressed and returned to the reactor, together with a fresh hydrogen makeup stream. The liquid stream removed from the bottom of the high-pressure separator contains some hydrogen, light hydrocarbon gases, and H2S in solution, which are lost from the hydrogen system. This liquid stream is often sent to a lowpressure separator, from which an off-gas is taken and typically sent to a flare or to the fuel gas system. The reactor operates with a large excess of hydrogen. The excess hydrogen ensures that the hydrogen partial pressure is high enough throughout the reactor to maintain the reaction rate and prevent coking of the catalyst.15,16 A high throughput of hydrogen also serves to sweep away reaction products that can act as catalyst inhibitors.15,17 As explained by McCulloch and Roeder, the hydrogen partial pressure at the outlet of the reactor is a very important variable in hydrotreating applica-
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tions. In this method, this partial pressure is assumed to be constant.18 The chemical hydrogen consumption, hydrogen lost by solubility in the liquid products, process yield, process pressure, and high-pressure separator temperature are also assumed to be constant. Except for constant pressure, these are the assumptions used by Hiller et al. for the economic assessment of different sources of hydrogen for a hydrocracker.11 The effect of these assumptions is to maintain constant operation of the hydrogen-consuming processes and focus attention only on changes to the distribution system. Under the above assumptions, a constant flow rate and hydrogen purity are imposed on the reactor gas inlet stream to keep the partial pressure of hydrogen constant at the outlet of the reactor. This sets the minimum recycle rate that is required by the reactor, at any known makeup flow rate. It can be shown that if the proper makeup flow rate is chosen as a function of the makeup purity, then the high-pressure purge purity will not be affected by changes to the makeup purity.19 In addition, if the recycle flow rate is adjusted to keep the reactor gas inlet flow rate constant, then the purity of the reactor inlet gas is also constant, as demonstrated by Alves,19 resulting in constant hydrogen partial pressure at the outlet of the reactor. If the process does not have a gas recycle stream, then the only way of keeping the hydrogen partial pressure constant is by tightly controlling both the flow rate and the purity of the makeup stream. Notice that the requirement of a constant hydrogen partial pressure at the reactor outlet is more restrictive than the specification of a fixed recycle (or purge) hydrogen purity. The specification of a required flow rate and hydrogen concentration for the reactor gas inlet stream captures the constraints imposed by the process on the hydrogen supply to ensure process performance. The reactor gas inlet stream is therefore chosen to represent the hydrogen sink associated with a hydrogen-consuming process. Under the assumptions made for hydrogen system targeting and design, the specification of a required gas flow rate and hydrogen concentration in the reactor gas inlet stream results in a constant separator off-gas stream flow rate and hydrogen purity. The high-pressure separator off-gas stream contains the gas made available by the reactor and separator section of the process. This gas can be compressed and recycled to the reactor inlet, or else it can be sent elsewhere in the hydrogen system (as a high-pressure purge). The separator off-gas stream is therefore chosen to represent the hydrogen source associated with a hydrogen-consuming process. The data for the high-pressure hydrogen sink and source of a hydrogen-consuming process are usually not measured directly. They must be calculated from data on the makeup, high-pressure purge, and recycle streams. The equations for the reactor gas inlet flow rate (FIN) and purity (yIN) are
FIN ) FM + FR yIN )
FMyM + FRyP FIN
(1) (2)
where FM and FR are the flow rates of makeup and gas recycle, respectively, and yM and yP are the makeup and purge hydrogen mole fractions, respectively. The equa-
Figure 3. Main hydrogen streams in the refinery hydrogen system of Example 3.1. The numbers represent the total gas flow rate (mol/s) and hydrogen purity (mol % H2).
tions for the high-pressure separator off-gas flow rate (FOUT) and purity (yOUT) are
FOUT ) FP + FR
(3)
yOUT ) yP
(4)
where FP is the high-pressure purge flow rate. When the hydrogen-consuming process has a lowpressure separator, then there is another stream that can be integrated in the hydrogen distribution system: the low-pressure separator off-gas. This stream can be counted as an additional hydrogen source. In the other classes of network elements, the sinks and sources are easier to identify than in the case of the hydrogen-consuming processes. A hydrogen-producing process is represented as a stream that supplies hydrogen to the distribution system. This stream is a hydrogen source. A purification unit has one hydrogen sink: the feed stream. The purified product and the lowpurity residue stream are both hydrogen sources. A hydrogen-containing stream that is exported from the refinery (for example, to a pipeline or to an adjacent chemical plant) is a hydrogen sink. It has a demand for hydrogen with fixed flow rate and purity requirements that are imposed by a supply contract instead of by process constraints. 3.1.1. Example 1. This example illustrates the calculation of the hydrogen sources and sinks for the fictitious refinery hydrogen distribution system shown in Figure 3. The network is supplied with hydrogen from the catalytic reformer (CRU), the steam reformer (SRU), and the import from a nearby hydrogen facility. The hydrogen-consuming processes are a hydrocracker (HCU), a straight-run naphtha hydrotreater (NHT), a cracked naphtha hydrotreater (CNHT), and a diesel hydrotreater (DHT). The stream data for these processes and the main hydrogen sink and source streams of each
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Table 1. Stream Data and Results of the Sink and Source Calculations for the Hydrogen-Consuming Processes in Example 1 processes variable
units
flow rate purity
mol/s mol % H2
flow rate purity flow rate
HCU
NHT
CNHT
DHT
makeup 762.4 138.6 93.36 80.00
304.9 82.14
277.2 82.14
mol/s mol % H2
purge 69.3 75.00
97.0 75.00
41.6 70.00
69.3 73.00
mol/s
recycle 1732.6
41.6
415.8
277.2
flow rate purity
mol/s mol % H2
sink 2495.0 80.61
180.2 78.85
720.7 75.14
554.4 77.57
flow rate purity
mol/s mol % H2
source 1801.9 75.00
138.6 75.00
457.4 70.00
346.5 73.00
Table 2. Source Data for the Hydrogen-Producing Processes in Example 1 flow rate source
maximum (mol/s)
minimum (mol/s)
current (mol/s)
H2 purity mol % H2
SRU CRU import
623.8 415.8 346.5
0.0 415.8 0.0
623.8 415.8 277.2
93.00 80.00 95.00
hydrogen-consuming process, which were calculated using eqs 1-4, are summarized in Table 1. Lowpressure separator off-gas streams are not considered for reuse in this example. Table 2 summarizes the data for the hydrogen sources that are not related to hydrogenconsuming processes. This information will be used later to calculate the minimum supply of fresh hydrogen to the distribution system. 3.2. Hydrogen System Analysis Principle. The process constraints (in the form of the purity and flow available from each hydrogen source and the requirements of the hydrogen sinks) and the material balance constraints define the minimum hydrogen supply that is required by the hydrogen system. This minimum flow rate can be used as a target for an actual design. The degree to which the target is approached is indicative of how much excess hydrogen is supplied to the system above the minimum required. This target is calculated assuming that any source can supply any sink; therefore, network constraints from any existing hydrogen distribution system are not considered. The pressure at which the sinks require the hydrogen might be higher than the pressure at which it is available from the hydrogen sources. Such pressure constraints are overcome using a compressor. Because the pressure constraints do not affect the mass balances (assuming constant pressure requirements), they do not affect the target; however, they affect the operating and investment costs of the hydrogen system, and therefore, they must be taken into account in the design of the hydrogen network. The material balance on total gas captures the flow rate constraints imposed on the hydrogen distribution system by the hydrogen sinks; however, it does not account for the purity constraints that are also imposed by the hydrogen sinks. The strategy used to account for the purity constraints acknowledges that the flow rate of total gas and the purity and flow rate of hydrogen are not independent variables. A set of constraints on
Figure 4. Example of a purity profile showing the hydrogen sources and sinks in the distribution system.
the flow rate of total gas and the hydrogen purity imposed on the hydrogen supply by sink j is equivalent to a set of constraints on the flow rate of total gas and the flow rate of hydrogen imposed on the hydrogen supply by the same hydrogen sink. A formulation of constraints using the flow rates of total gas and hydrogen is convenient because both constraints affecting each hydrogen sink are taken into account using a combination of two independent conservation principles: the material balance on total gas and the material balance on hydrogen. These two material balances and the appropriate selection of sources and sinks are the basis of the targeting method. The graphical and analytical tools required to implement the method are described in the following sections. 3.2.1. Purity Profile. The material balance on total gas for each of the hydrogen sinks is conveniently represented in a two-dimensional plot with flow rate of total gas on the horizontal axis and purity on the vertical axis, as shown in Figure 4. This purity profile contains the hydrogen sinks plotted in order of decreasing purity as a composite sink profile. The same procedure is used to plot the hydrogen sources in order of decreasing purity as a composite source profile. The position in the vertical axis and the length of the first step in the sink curve corresponds to the purity and flow rate of the hydrogen sink with highest purity. The next step corresponds to the hydrogen sink with the secondhighest purity, etc. The construction of the curve continues until the lowest-purity hydrogen sink is represented. The source curve also starts at zero flow rate. Its construction follows the same procedure used for the sink curve, representing the highest-purity hydrogen source first and the lowest-purity source last. The purity profile diagram does not require any information on connections within the hydrogen network and is based solely upon the source and sink data. In the purity profile diagram, the projection of both curves onto the horizontal axis represents the material balance on total gas. The first necessary condition for the feasibility of the hydrogen distribution system is that the amount of gas available from the sources must equal or exceed the amount of gas required by the sinks. The amount of gas supplied in excess of the demand is flared or sent to the fuel gas system. If the source curve is shorter than the sink curve, then the material balance on total gas is violated for at least one of the sinks, rendering the system unfeasible. The solution is to add more gas to the system or to operate one or more hydrogen-consuming processes with reduced liquid throughput (which reduces the amount of gas required
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Figure 5. Purity profile highlighting the pockets between the source and sink curves with hydrogen excess (+) and deficit (-).
by the corresponding hydrogen sink). The material balance on total gas over the whole refinery is represented analytically by the equation nSR
nSK
FSR,i ) ∑FSK,j + FF ∑ i)1 j)1
(5)
where FSR,i is the flow rate available from source i; FSK,j is the flow rate required by sink j; FF is the amount of gas available for fuel; and nSR and nSK are the numbers of sources and sinks, respectively, in the hydrogen distribution system. The total flow rate available from the sources (FSR) and the total flow rate required by the sinks (FSK) are nSR
FSR )
FSR,i ∑ i)1
(6)
nSK
FSK )
FSK,j ∑ j)1
(7)
The supply available from the sources exceeding the demand from the sinks is not a sufficient condition for feasibility of the hydrogen distribution problem. The supply also must comply with the constraints on the purity of hydrogen imposed by each sink. A new variable, hydrogen surplus (H′), is introduced to describe the availability of hydrogen at different purity levels. The hydrogen surplus is developed from the information contained in the hydrogen purity profiles, although it can be calculated independently of the profiles. It can be represented graphically in a hydrogen surplus diagram and is described in more detail in the next section. 3.2.2. Hydrogen Surplus Diagram. The hydrogen surplus can be calculated from the purity profile. The area underneath each segment of the sink curve is equal to the flow rate of pure hydrogen required by the hydrogen sink represented by that segment. The area underneath the whole sink curve is the flow rate of pure hydrogen that the system must provide to all of the sinks. The area underneath the source curve is the total amount of pure hydrogen available from the sources. When both curves are put together to build the purity profile, there are regions where the source curve lies above the sink curve and regions where it lies below the sink curve, as illustrated in Figure 5. If the source curve is above the sink curve for a given range of hydrogen purity, then in this range of purity, the sources provide more hydrogen than is required by the
sinks. Here, the purity profile has an excess of hydrogen that amounts to the area of the space between the curves. Because this amount of hydrogen is in excess to that needed in this purity range, it constitutes a surplus and can be used to compensate for a deficit in hydrogen supply at a lower purity. If the source curve is below the sink curve, then in this region, the sources do not provide enough hydrogen to the sinks; here, the purity profile has a deficit of hydrogen. The amount of deficit of hydrogen is equal to the area between the curves. This deficit can be compensated by use of surplus hydrogen of a higher purity, but not by use of hydrogen of lower purity. The physical meaning of a deficit region is that the sink process requires hydrogen of greater purity than that available from the corresponding source. If surplus hydrogen is available at a higher purity, then it can be mixed with the lower-purity source to raise its purity until the sink purity requirement is satisfied. Overall, the purity profile is divided into several regions with alternating excess and deficit of hydrogen. We call the net cumulative excess of hydrogen in the purity profile at a given flow rate F the hydrogen surplus (H′). Its analytical definition is
H′ )
∫0F(ySR - ySK) dF
(8)
where F is the flow rate of total gas and ySR and ySK are the values of the source and the sink curve purities, respectively, at the value of flow rate F. If the hydrogen surplus is negative at any value of flow rate between 0 and FSR, then the system is not receiving sufficient hydrogen at adequate purity. At least one of the constraints on hydrogen flow rate imposed by the sinks cannot be satisfied. More hydrogen or higher-purity hydrogen is required to make the system feasible. The second necessary condition for feasibility of the hydrogen distribution system can then be stated as
∀ F ∈ [0,FSR]:
∫0F(ySR - ySK) dF g 0
(9)
The H′ function can be represented as a hydrogen surplus diagram, as shown in Figure 6. The curve in the diagram is generated by plotting whichever is the lower of ySR and ySK versus H′ for each value of F between 0 and FSR. It is necessary to use the lower of ySR and ySK, as the surplus or deficit of hydrogen changes depending on whether the source or the sink curve is lowermost. If the sink curve is lowermost, then there is an excess of hydrogen, and H′ will increase. In this case the surplus is plotted at a purity of ySK. If the source curve is lowermost, then there is a deficit of hydrogen. In this case, H′ will decrease and is plotted at a purity of ySR. If the whole hydrogen surplus curve lies at or above zero hydrogen surplus, then the second necessary condition for feasibility is satisfied. If both the first and the second necessary conditions are satisfied, then the network design problem has at least one feasible solution. The hydrogen supply target is defined when the system is constrained on hydrogen. This occurs if there is at least one place in the hydrogen surplus diagram where the hydrogen surplus is 0 and any reduction in the supply creates a negative hydrogen surplus, making the distribution problem unfeasible. When the con-
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Figure 6. (a) Purity profile and (b) corresponding hydrogen surplus diagram, showing the relation between the area in the pockets and the horizontal sections in the hydrogen surplus diagram.
Figure 7. Identification of the pinch in the (b) hydrogen surplus diagram at zero H′, corresponding to the end of a pocket with hydrogen deficit in the (a) purity profile.
straint is just satisfied, then the hydrogen surplus diagram appears to be pinched where the surplus is 0. The hydrogen pinch thus sets a target for the minimum hydrogen consumption. 3.2.3. Hydrogen Pinch. The pinch occurs at the end of a range in the purity profile where the hydrogen is in deficit. It corresponds to a discontinuity in the sink line where a hydrogen sink that is above the source line ends and another sink, below the source line, starts. Figure 7 shows the purity profile and the corresponding hydrogen surplus diagram of a hydrogen distribution system with a pinch. Because of the discontinuity in the sink profile, a vertical segment at zero H′ follows the pinch. This segment is located between the values of purity of the hydrogen source at the pinch and sink after the pinch. It represents the fact that the pinch divides the overall distribution system into a subsystem with net zero hydrogen surplus (above the pinch) and a subsystem with net hydrogen surplus (below the pinch). The pinch purity (yPINCH) is the purity associated with the deficit that causes the pinch, i.e., the purity of the hydrogen source at the pinch. In Figure 7, the curve in the hydrogen surplus diagram has zero H′ at the pinch purity and also at the point of highest purity. This point at the highest purity is the value of H′ at zero flow rate, which is always 0 (according to eq 8). Because this point can never have negative H′, regardless of the hydrogen supply, it is not
a pinch; however, a pinch point might occur at the top of the network. In this case, the hydrogen surplus diagram shows a segment at zero H′ from the purity of the first source to the purity of the sink underneath. The analytical condition for the existence of a pinch is
∃ FPINCH ∈[0,FSK]:H′ )
∫0F
(ySR - ySK) dF ) 0 (10)
PINCH
where FPINCH is the flow rate at which the integral is 0, resulting in the pinch. The subsystem above the pinch includes the pinch recycle, which is the portion of the hydrogen source at the pinch purity that belongs to the region above the pinch. The pinch recycle flow rate is represented by FPR in Figure 7. This variable tells us how much gas from the source at the pinch must be reused by the sinks above the pinch to meet the hydrogen supply target. If an additional amount of gas from below the pinch is reused above the pinch, then an identical amount of gas originally above the pinch, at higher purity, must be sent to the sinks below the pinch to maintain material balance. This reduces the hydrogen surplus above the pinch, and a penalty in the supply must be paid to keep the system feasible. Thus, a simple rule for designing a hydrogen system at the minimum supply is that gas should never be exchanged across the pinch.
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Figure 8. Targeting procedure, including (a) purity profile and (b) hydrogen surplus diagram. If the hydrogen surplus diagram (b) does not have a pinch, then the flow rate of utility supplied to the system can still be reduced.
This is the main design guideline provided by the hydrogen targeting method. It corresponds closely to the pinch design rule used in the analysis of heat exchanger networks. 3.3. Finding the Target. The procedure for calculating the supply target requires that the flow rate of gas supplied to the system be varied until a hydrogen pinch is found. Few sources of hydrogen in the refinery have variable flow rates. The sources from hydrogen-consuming processes or from processes that generate hydrogen as a secondary product, such as catalytic reformers and other dehydrogenation plants, have flow rates that will be determined by the normal operation of the processes. These flow rates will be determined by optimization of the refinery and can be assumed to be fixed for the hydrogen network design problem. Purification units usually have low operating costs, and once installed, they should be operated at maximum capacity. Their outlet gas streams are therefore also fixed sources of hydrogen. The hydrogen sources that are flexible with respect to flow rate are thus imports from external suppliers and processes that generate hydrogen as the main product (steam reformers or partial oxidation units). These sources act as hydrogen utilities. The targeting procedure is the same for a feasible system initially operating at low efficiency or for an initially unfeasible set of sinks and sources: start with maximum flow rate for all of the utilities, and minimize the most costly one. If this utility is eliminated before the system is at the minimum supply, then the minimization proceeds with the next utility in decreasing order of cost. Figure 8 illustrates the reduction in the utility flow rate required by the system until a pinch appears in the hydrogen surplus diagram. The reduction in the utility flow rate moves the remainder of the source curve toward the vertical axis. Consequently, there is a change in the overlap of the purity profiles. All of the areas of hydrogen excess are reduced, and all of the areas of hydrogen deficit are increased. The utility target is found when the reduction in the hydrogen surplus creates a pinch in the hydrogen surplus diagram. If the system is not feasible with all of the utilities at maximum capacity, then there are three debottlenecking options available at the network level to make it feasible: add more hydrogen production, increase (or start using) hydrogen imports, or add purification capacity. On the hydrogen-consuming process side,
reducing the throughput in at least one of the processes above the unfeasible region is a costly short-term option that will also make the system feasible. 3.3.1. Example 2. The use of the targeting method is illustrated using the hydrogen system of Figure 3, previously described. The cost of utility hydrogen, based on total gas, is 2.0 US$/kmol for the imported hydrogen and 0.8 US$/kmol for the hydrogen from the SRU. Without a targeting method for the hydrogen supply, it is difficult to know how well the distribution system is performing. From the data in Figure 3, we see that the gas sent to fuel totals 110.9 mol/s with an average hydrogen content of 71.9% (mol/mol). If no gas were sent to fuel, then the 95% hydrogen import could be reduced from 277.2 to 193.3 mol/s, saving the refinery 4.8 × 106 US$ per year. This value is large enough to justify putting time and effort into identifying changes to the network topology that permit saving this amount of hydrogen; however, not all of these savings are achievable. Using the targeting method, the maximum achievable savings are revealed before any effort is put into changing the network design. Figure 9 shows the purity profile and the hydrogen surplus diagram for this example. The total flow rate from the sources exceeds the flow rate required by the sinks, so the system obeys the first necessary condition of feasibility. The amount of fuel generated by the hydrogen system is FF ) 110.9 mol/s. The hydrogen surplus diagram shows that the hydrogen surplus is always positive; therefore, there is scope to reduce the hydrogen utility. The supply target is calculated as described above. The purity profile and the hydrogen surplus diagram for the hydrogen distribution system using the targeted amount of hydrogen import are shown in Figure 10. The purity profile remains almost unchanged. The hydrogen surplus diagram now shows a pinch at a hydrogen mole fraction of 0.70 mol of H2/mol. The import target is 268.8 mol/s, corresponding to a reduction of only 8.4 mol/s and a savings of 0.5 × 106 US$/year. This means that the current system is already operating near the maximum efficiency. In this case, the savings that can be achieved by changing only the network topology might not justify the effort required; however, further utility savings can still be achieved by using other debottlenecking techniques. 3.4. Purity/Flow Rate Tradeoff. One of the options available for debottlenecking the hydrogen distribution
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Figure 9. (a) Purity profile and (b) hydrogen surplus diagram for the current supply to the hydrogen system in example 4.1.
Figure 10. (a) Purity profile and (b) hydrogen surplus diagram for the minimum fresh hydrogen supply to the system in example 4.1.
Figure 11. Changing the purity of a hydrogen source on a system with a single hydrogen-consuming process.
system is to increase the purity of the gas from one or more hydrogen sources. This takes advantage of the fact that, if two streams have the same flow rate of hydrogen, the one with higher purity will provide the hydrogen system with more hydrogen surplus per unit of flow rate. This additional hydrogen surplus can be used to increase the reuse of hydrogen within the system and reduce the need for fresh hydrogen utility. Figure 11 illustrates the effect of increasing the purity of a source of hydrogen in a single-process system. The higher the source purity, the lower the makeup flow rate required to guarantee the required hydrogen consumption and partial pressure.
Figure 12 shows the effect that changing the purity of a source stream above the pinch has on the purity profile diagram and on the hydrogen surplus diagram of a typical hydrogen system. In this figure, the first step of the source curve on the purity profile corresponds to the hydrogen utility. Increasing the purity of this stream from the base case value (dotted line) to a higher value (solid line), as indicated by the arrow in the purity profile, introduces an additional amount of hydrogen surplus into the system. This additional hydrogen surplus is equal to the area between the initial line and the new line of the modified utility. The resulting effect on the hydrogen surplus diagram (Figure 12b) is an
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Figure 12. Effects on the (a) purity profile and (b) hydrogen surplus diagram of increasing the utility purity.
increase in the length of the line representing the area between the utility and the sinks underneath in the purity profile (Figure 12a). Because the hydrogen surplus is an integral, the entire curve below that line is shifted by the amount of surplus added with the modification. The system initially pinched (dotted line) becomes unconstrained (solid line) with the increase in the utility purity. The additional hydrogen surplus thus created can be used to reduce the need for hydrogen utility, resulting in a lower target. It is important to notice that a change in the purity of a source stream only affects the target if it involves a source that is above the pinch after the purity change. The analytical interpretation of the tradeoff between the purity of the sources above the pinch and that of the target is based on eq 10, which states that the region above the pinch has zero hydrogen surplus. The purity of the sinks does not change. Consequently, its integral is a constant, and eq 10 simplifies to
∫0F
ySR dF )
PINCH
∫0F
ySK dF ) constant (11)
PINCH
The form of ySR in the purity profile is a step function. The integral of such a function can be represented as a sum. For that purpose, we assume that the sources are numbered according to their purity, with the highest purity first. If m is the number of the source at the pinch, then the integral is equivalent to the following sum
∫0F
m-1
ySR dF )
PINCH
(FSR,iySR,i) + FPRyPINCH ∑ i)1
(12)
where FPR is the flow rate of the portion of source m that belongs to the subsystem above the pinch (the pinch recycle) and yPINCH is the pinch purity (which is equivalent to the purity of source m). The combination of eqs 11 and 12 results in m-1
(FSR,iySR,i) + FPRyPINCH ) constant ∑ i)1
(13)
Equation 13 means that, when the purity of one or more sources changes, the system remains pinched only if the sum on the left-hand of this equation remains constant.
Equation 13 is valid only if the number of sources above the pinch (m) is constant. The material balance on total gas above the pinch shows that the total flow rate supplied to this subsystem must be constant and equal to the flow rate at the pinch m-1
(FSR,i) + FPR ) FPINCH ∑ i)1
(14)
Equations 13 and 14 can be used as the basis of an analytical procedure to estimate the change in the utility target resulting from a change in the purity of a hydrogen source. The most useful application of this analytical procedure is to estimate the change in the hydrogen supply target as a result of using a hydrogen utility with a different purity. In this case, the only values of flow rate that change are those of the utility (one of the m - 1 sources in the sum) and the pinch recycle. All of the terms from constant sources can be added to the constants in eqs 13 and 14. The resulting equations relating the system at minimum supply before the change in the utility purity and after this change are then
FUyU + FPRyPINCH ) F/U y/U + F/PR yPINCH
(15)
FU + FPR ) F/U + F/PR
(16)
where the superscript * represents the variables before the change to the system; FU and yU are the utility flow rate and purity, respectively, after the change. The combination of eqs 15 and 16 gives the following result
y/U - yPINCH FU ) F/U yU - yPINCH
(17)
Equation 17 shows that increasing the purity of the hydrogen utility results in utility savings and that the higher the pinch purity, the larger the savings. Equation 17 is valid only at constant pinch purity. If the changes in utility purity result in a change in the pinch purity, then eq 13 is not valid, and it becomes less practical to assess the modifications analytically. The targeting method presented in section 3.3 should be used instead. 3.4.1. Example 3. This example illustrates the effect of the source purity on the target. It uses the same base
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Figure 13. Target for hydrogen import as a function of the import purity. The solid line was obtained using eq 17, and the points were generated using the graphical targeting method.
case refinery as example 2. The most costly utility in the hydrogen system is the import, which has a purity of 95% hydrogen. We want to know the target for the import stream as a function of its purity, in the range between 90 and 99.99% purity. We also want to assess the effect on the hydrogen supply target of increasing the purity of the steam reformer production from 93 to 95% at the current hydrogen import purity. The results are shown in Figure 13. The points on the curve were obtained using the graphical targeting method. The solid line represents the target calculated using eq 17. The perfect agreement between the points and the curve in this example is due to the pinch purity remaining constant in the purity range of the analysis. These results show that importing gas at a purity higher than 95% can save up to a maximum of 44.7 mol/s of imported hydrogen (for a purity of 99.99%), which corresponds to saving 2.6 × 106 US$ per year. The solid point underneath the curve at 95% import purity is the result of increasing the purity of the steam reformer production from 93 to 95%. This allows a reduction of 49.9 mol/s in the imports, giving savings of 2.9 × 106 US$ per year. Example 3 shows that a system that is operating at the minimum hydrogen supply can produce significant savings by manipulating the purity of the source streams. The targeting method quantifies the effect that process modifications resulting in changes of hydrogen source purity have on the performance of the hydrogen system; however, process changes are not the only option available for changing the purity of a hydrogen stream. An alternative is to increase the purity by means of a purification unit. In this case, the change in purity can be very large, and consequently, there is potential to achieve major savings in the operation of the hydrogen distribution system. Alves and Towler cover the subject of hydrogen purification.19
adding new ones, can be estimated using this method. This requires only that the sink and source data be updated to incorporate the process modifications. The use of the proposed method for the analysis and specification of hydrogen purification equipment will be covered in a future paper. The hydrogen surplus diagram gives the number of pinch points and the purities at which they occur. The hydrogen surplus function is an integral that depends on the flow rates and purities of sources and sinks. The hydrogen surplus values in the diagram are not the actual flows of hydrogen in the network. The main assumptions on which the targeting method is based are that the operation of the hydrogen-consuming processes is constant and that the refinery gases can be treated as a binary mixture. The assumption of constant process operation is necessary to establish a constant framework of hydrogen sinks and sources for the targeting method. By separating the process operation from the hydrogen distribution problem, it is possible to establish a common basis for comparing the benefits of different network modifications. The assumption that the gas can be treated as a mixture of hydrogen and methane deserves deeper scrutiny. Although methane is the most common impurity, other compounds are often present in the gas mixture. This simplification is adequate if the modifications introduced in the hydrogen system do not substantially modify the distribution of impurity compounds in the makeup of major hydrogen-consuming processes. This is usually the case because these processes tend to be supplied with high-purity hydrogen in which methane is the major impurity. 5. Conclusions This paper presents a method for estimating the minimum fresh hydrogen supply to a hydrogen distribution system. The incorporation of partial pressure constraints into the targeting method is possible by using the source and sink formulation. The targeting method is adequate to deal with extensive gas recycle, which is a characteristic of hydrogen distribution systems. The concept of hydrogen surplus is used to identify bottlenecks in the hydrogen distribution system. The targeting method can then be used to screen debottlenecking options before modifying the distribution system. The concept of hydrogen surplus provides valuable theoretical insights into the impact of hydrogen purification on the performance of a hydrogen distribution system. The method is easily coded into software, making the procedure very fast. The potential savings in the network operation resulting from changes to the hydrogen system can be large, as illustrated by the examples given.
4. Discussion In this paper, an analysis method based on the novel concept of hydrogen surplus is proposed for establishing the minimum flow rate of fresh hydrogen required by a hydrogen distribution system. This information can be used to benchmark the operation of the hydrogen system and to estimate the potential benefits of different debottlenecking options. The analysis method is useful for setting the minimum size of a hydrogen import or hydrogen plant required to debottleneck the hydrogen system. The effect on the hydrogen system of changing the refining scenario, revamping existing processes or
Acknowledgment J. Alves acknowledges the financial support of JNICT through the program Praxis XXI. Nomenclature F ) total gas flow rate (mol/s) FF ) flow rate of gas flared or sent to the fuel gas system (mol/s) FIN ) flow rate of gas supplied to the reactor (mol/s) FM ) flow rate of makeup gas (mol/s)
Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5769 FOUT ) flow rate of high-pressure separator off-gas after amine scrubbing (mol/s) FP ) flow rate of high-pressure purge (mol/s) FPINCH ) flow rate at which the pinch occurs in the purity profile (mol/s) FPR ) pinch recycle flow rate (mol/s) FR ) recycle gas flow rate (mol/s) FSK ) total flow rate required by the hydrogen sinks (mol/ s) FSK,j ) flow rate required by sink j (mol/s) FSR ) total flow rate available from the hydrogen sources (mol/s) FSR,i ) flow rate from source i (mol/s) FU ) hydrogen utility flow rate (mol/s) H′ ) hydrogen surplus (mol/s) m ) number of sources above the pinch nSK ) total number of sinks nSR ) total number of sources yIN ) purity of the gas supplied to the reactor (mol % H2) yM ) purity of makeup gas (mol % H2) yOUT ) purity of high-pressure separator off-gas after amine scrubbing (mol % H2) yP ) purity of high-pressure purge gas (mol % H2) yPINCH ) pinch purity (mol % H2) yR ) purification unit residue purity (mol % H2) ySK ) purity of the sink curve in the purity profile (mol % H2) ySK,j ) purity of sink j (mol % H2) ySR ) purity of the source curve in the purity profile (mol % H2) ySR,i ) purity of source i (mol % H2) yU ) purity of the hydrogen utility (mol % H2) Indices i ) source j ) sink Superscripts * ) value of the variable before a system modification Abbreviations CDU ) crude oil distillation unit CNHT ) cracked naphtha hydrotreating unit CRU ) catalytic reforming unit DHT ) diesel hydrotreating unit FCC ) fluid catalytic cracking unit H-cracker ) hydrocracking unit HCU ) hydrocracking unit H-treat ) hydrotreating unit LPG ) liquefied petroleum gases NHT ) naphtha hydrotreating unit PSA ) pressure-swing adsorption SRU ) steam reformer unit VDU ) vacuum distillation unit
Literature Cited (1) Dachos, N.; Kelly, A.; Felch, Reis; E. Meyers, R. A. Handbook of Petroleum Refining Processes, 2nd ed.; Meyers, R. A., Ed.; McGraw-Hill: New York, 1997.
(2) Miller, G. Q.; Stoecker, J. Selection of a hydrogen separation process. Presented at the NPRA Annual Meeting, San Antonio, TX, Mar 19-21, 1989. (3) Philcox, J.; Fenner, G. W. A Texas project illustrates the benefits of integrated gasification. Oil Gas J. 1997, 95, 41-46. (4) Abrardo, J. M.; Khurana, V. Hydrogen technologies to meet refiner’s future needs. Hydrocarbon Process. 1995, 74, 43-49. (5) Wilcher, F. P.; Miller, G. Q.; Mitariten, M. J. Technologies for the separation and recovery of hydrogen from refinery streams. In PreprintssAmerican Chemical Society, Division of Petroleum Chemistry; Haines, W. E., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 40, pp 720-723. (6) Spillman, W. R. Economics of gas separation membranes. Chem. Eng. Prog. 1989, 85, 41-62. (7) Peramanu, S.; Pruden, B. B. Solubility study for the purification of hydrogen from high-pressure hydrocracker off-gas by an absorption-stripping process. Can. J. Chem. Eng. 1997, 75, 535-543. (8) Che, S. C.; Bredehoft, R. L. Hydrogen Production Technologies. In PreprintssAmerican Chemical Society, Division of Petroleum Chemistry; Haines, W. E., Ed.; American Chemical Society: Washington, DC, 1995; Vol. 40, pp 713-718. (9) Anand, M.; Langsan, M.; Rao, M. B.; Sircar, S. Multicomponent gas separation by selective surface flow (SSF) and polytrimethylsilylpropyne (PTMSP) membranes. J. Membr. Sci. 1997, 123, 17-25. (10) Kramer, K. A.; Patel, N. M.; Sekhri, S.; Brown, M. G. Flexible hydrogen plant utilizing multiple hydrocarbon streams. KTI Newsl. 1996, Fall, 15-20. (11) Hiller, M. H.; Lacatena, J. J.; Miller, G. Q. Hydrogen for hydroprocessing operations. Presented at the NPRA Annual Meeting, San Antonio, TX, Mar 29-31, 1987. (12) Bollinger, W. A.; Long, S. P.; Metzger, T. R. Optimizing hydrocracker hydrogen. Chem. Eng. Prog. 1984, 80, 51-57. (13) Pacalowska, B.; Whysall, M.; Narasimham, M. V. Improve hydrogen recovery from refinery offgases. Hydrocarbon Process. 1996, 75, 55-59. (14) Towler, G. P.; Mann, R.; Serriere, A. J-L.; Gabaude, C. M. D. Refinery hydrogen management: Cost analysis of chemically integrated facilities. Ind. Eng. Chem. Res. 1996, 35 (7), 23782388. (15) Jones, D. S. Elements of Petroleum Processing; John Wiley & Sons Ltd: Chichester, U.K., 1995. (16) Gary, J. H.; Handwerk, G. E. Petroleum Refining: Technology and Economics, 3rd ed.; Marcel Decker: New York, 1994. (17) Landau, M. V. Deep hydrotreating of middle distillates from crude and shale oils. Catal. Today 1997, 36, 393-429. (18) McCulloch, D. C.; Roeder, R. A. Find hydrogen partial pressure. Hydrocarbon Process. 1976, 55, 81-86. (19) Alves, J. J. Analysis and Design of Refinery Hydrogen Distribution Systems. Ph.D. Thesis, UMIST, Manchester, U.K., 1999.
Received for review June 28, 2001 Accepted May 14, 2002 IE010558V
