Effect of Natural Gas Composition on the Design of Natural GasLiquid DemethanizersWilliam L. Luyben*

Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States

ABSTRACT: The hydrocarbon composition of natural gas varies fairly significantly from location to location. The relativeamounts of methane (C1) and ethane (C2) have a profound effect on the cryogenic high-pressure distillation column used torecover the C2 and heavier components as a bottoms product called “natural gas liquid” (NGL). This “demethanizer” column isa complex system that uses compression and expansion to achieve the required low temperatures (190 K) for achieving liquids atthe operating pressure of 25 atm. Side-reboilers are used to cool the ambient-temperature natural-gas feed stream and to partiallyreboil the column. A multiunit heat exchanger network, multiple flash drums, and an expander are used to produce four differentliquid and vapor feed streams to the column. This paper studies the effect of feed composition on the design of the demethanizercolumn. Natural gas feeds with C2 compositions ranging from 5 to 15 mol % are considered. Significant modifications in thedesign are required as the amount of NGL varies. For example, bypassing some of the feed around the side-reboilers is requiredwhen the C2 composition of the feed is high. High C2 feed compositions require more external refrigerant, and less power isgenerated in the expander. More NGL and less product gas is produced, so product compression work is lower.

1. INTRODUCTION

The processing of natural gas has grown in importance inrecent years due to the increase in production from sourcessuch as the Marcellus and Utica natural gas supply areas. Theprocessing of natural gas as it comes from wells has many steps,but one of the most important is distillation. Most of thenatural gas is methane (C1). The remainder is heavierhydrocarbons: ethane (C2), propane (C3), isobutane (iC4),n-butane (nC4), and small amounts heavier components downto C7s. The separation of C1 from the other components isaccomplished in a high-pressure cryogenic distillation columncalled a demethanizer. The overhead product is mostly C1 witha small amount of C2, whose amount is dictated by the desiredC2 recovery. The bottoms product is called natural gas liquid(NGL). It contains most of the C2 and essentially all of theheavier components.The composition of natural gas varies from source to source.

The amount of C1 relative to C2 and heavier imposes differentloads on the demethanizer. Different authors assume differentfeed gas concentrations. Lee et al.1 assumed a feed with 86 mol% C1 and 6.5 mol % C2 in a study of natural gas processing onfloating offshore platforms. Nawaz et al.2 assumed a feed with78 mol % C1 and 13 mol % C2 in a design optimization study.The C2 content varies in these two examples by a factor of 2.The purpose of this paper is to study the impact of natural

gas feed composition on the design of the demethanizer. Thebasic structure of the process is fixed (number and types ofunits: heat exchangers, flash tanks, expanders, etc.), but theeffect of natural gas feed composition on the operatingconditions and the size of equipment is explored.

2. DEMETHANIZER FLOWSHEET

The study is based on the configuration developed by Nawaz etal.2 These authors used simulated annealing optimizationmethods to develop the optimum equipment sizes and

optimum operating conditions using profit as the economicobjective function. They studied only one feed composition.

2.1. Design Basis. For a given feed composition, the designis based on the following specifications and assumptions:

1. The recovery of C2 in the column bottoms NGL streamis set at 80%. This is achieved by varying the exittemperature from the externally cooled heat exchanger.Lower temperatures increase C2 recovery but increaserefrigeration duty.

2. The ratio of C1 to the sum of C1 and C2 in the bottomsstream is specified to be 2 mol % C1. Thus the C1concentration is on a C3-and-heavier-free basis. Since allof the C1 in the NGL will appear in the ethane productproduced in a downstream de-ethanizer distillationcolumn, it is important to not drop too much C1 outthe bottom of the demethanizer.

3. The column has 31 stages with four feed streams fed ondifferent trays depending on their C1 compositions.

4. The two side reboilers have liquid pumparound flowrates of 2000 kmol/h and are located at stages 24 and 28.

5. The pressure at the top of the column is 25 atm andstage pressure drop is 0.01 atm per tray.

6. Minimum approach temperature differentials in the heatexchangers are 2 K.

7. The assumed overall hear-transfer coefficients for sizinggas−gas systems and gas−liquid systems are 0.17 kW K−1

m−2 and 0.28 kW K−1 m−2, respectively.8. The fraction of the gas from the first flash tank that is

sent to the expander is optimized to minimize the net

Received: February 23, 2013Revised: April 12, 2013Accepted: April 17, 2013Published: April 17, 2013

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energy requirements (reboiler + cooler − expanderwork). This optimization is discussed later in the paper.

9. Higher C2 feed compositions require bypassing some ofthe feed gas around the side-reboilers to prevent the dutyof the steam-heated reboiler at the base of the columnfrom becoming too small and presenting potentialhydraulic problems. The minimum reboiler duty isassumed to be 100 MW (compared to the design of352 MW for the 5 mol % C2 feed). The fraction ofbypassing is adjusted to maintain this minimum reboilerduty.

2.2. Flowsheet Description. Figure 1 shows the flowsheetdeveloped for a 5 mol % C2 natural gas feed. The feed flow rateis 5269 kmol/h with the composition given on the figure. TheNGL produced from the bottom of the column is 507.1 kmol/hwith a composition of 0.85 mol % C1 and 41.56 mol % C2, theremainder being essentially all the C3 and heavier components.The product gas at 4762 kmol/h is 1.7 mol % N2, 97.14 mol %C1, and 1.11 mol % C2 with a trace of C3.The feed is compressed to 61 atm and enters two side-

reboiler in series, which circulate cold liquid from stages 28 and24. The feed is cooled from 313 to 293.4 K. The heat added tothe column is 360.8 kW and 1101 kW in the two side-reboilers.The duty of the steam-heated reboiler at the bottom of thecolumn (352.3 kW) is significantly smaller than that of the side-reboilers.The feed flows to the next heat exchanger in which the cold

methane product stream is used to further cool the feed. The

cold stream to this feed precooler is at 257.7 K with a flow rateof 4762 kmol/h. It is heated to 291.4 K by the feed stream,which is cooled from 293.4 to 273.7 K. The heat-transfer rate is1717 kW. The feed then goes to a cooler that uses an externalrefrigerant to further cool the feed to 261.0 K, requiring 1178kW of refrigeration duty.At 261.0 K and 60 atm, some liquid is formed. The feed

stream goes to the first flash drum. The liquid phase at 178.7kmol/h is flashed through valve V1 to the operating pressure ofthe column (25 atm) and fed onto stage 18 of the 31-stagecolumn. The gas phase from the first flash drum is split into twostreams. One is fed to an expander (3156 kmol/h) thatproduces 929.3 kW of power, which is used to help drive themain product compressor. The temperature of the expanderexit is 221.6 K. The stream leaving the expander goes to asecond flash drum. The vapor from this drum (3035 kmol/h) isfed on stage 5. The liquid (120.8 kmol/h) is fed on stage 10.The other portion of the gas stream from the first flash drum

(1934 kmol/h) goes to a heat exchanger in which it is cooledand condensed at 180.6 K by the overhead vapor stream fromthe top of the column at 178.6 K. The heat exchanger transfers4437 kW. The stream leaving this heat exchanger is all liquid. Itflashes through valve V2 from 60 to 25 atm, which drops thetemperature to 173.9 K and produces a vapor fraction of0.0887.The overhead vapor from the column flows through the

reflux heat exchanger and the feed preheater before beingcompressed to 60 atm in the product gas compressor whose

Figure 1. Demethanizer flowsheet (5 mol % C2; expander split = 0.62).

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energy duty is 3855 MW. The product gas flow rate is 4762kmol/h. The liquid bottoms NGL product from the column hasa flow rate of 507.1 kmol/h.2.3. Split Ratio. An important design optimization variable

is how the vapor stream leaving the first flash tank is split. Someis sent to the expander and some is sent to the heat exchangerto be cooled by the overhead vapor. In the flowsheet given inFigure 1, the stream fed to the expander is 62% of the gasstream.The optimum split is determined by looking at the effects of

the split on the recovered expander power, the reboiler duty,the refrigerated cooler duty, and the NGL production rate.Table 1 gives this information. The C2 recovery is keptconstant at 80% by adjusting the refrigerated cooler temper-ature. The composition of the NGL is kept constant at 2 mol %C1 on a C3-and-heavier-free basis. These specifications areachieved by using an Aspen Design spec/vary function in theRadFrac column block that adjusts the bottoms flow rate togive the molar flow rate of C1 that gives a 2 mol % compositionof C1 relative to the total C1 and C2 molar flow rates in thebottoms. Then the temperature of the refrigerated cooler Tcool

is adjusted to give a molar flow rate of C2 in the bottoms thatcorresponds to 80% recovery of C2 in the feed.Table 1 results show that the refrigerated cooler temperature

Tcool initially increases as the fraction fed to the expander isincreased. This decreases the refrigeration duty Qcool. Reboilerduty QR also initially decreases. The power recovered in theexpander initially increases, and there is a small increase in thevaluable NGL product. However for values above about 60%,the required cooler temperature begins to decrease while thereboiler duty starts to increase. The minimum in the net energy

(cooler + reboiler − expander) occurs a 62%, so this is selectedas the operating point for this specific feed composition.

3. FEED COMPOSITION EFFECTS

The flowsheet described above is with a 5 mol % C2 natural gasfeed composition. A range of feed compositions is explored tosee their impact on the demethanizer design. Results are givenin Figure 2. When the C2 composition is increased, the C1composition is decreased by the same amount. The recovery ofC2 is kept constant at 80% and the C1 composition of theNGL is kept constant at 2 mol % (on a C3-and-heavier-freebasis).As C2 feed composition increases, we naturally produce

more NGL and less product gas. The required temperatureTcool in the heat exchanger using external refrigerant decreases,and the refrigeration requirement increases.The lower right graph shows that the duty of the steam-

heated reboiler QR decreases as C2 feed composition increases.If there is no bypassing of the side-reboilers, the duty goes tozero with a feed composition of 10 mol % C2. We assume theminimum operable reboiler duty is 100 MW. The fraction ofbypassing Fby is adjusted to maintain this minimum reboilerduty at the higher feed compositions.At 15 mol % C2 feed composition, the fraction of feed gas

bypassed is 0.21. Figure 3 gives the flowsheet under theseconditions. The optimum fraction of the gas fed to theexpander is now 67%, as shown in Table 2. Comparing Table 1with Table 2 and Figure 1 with Figure 3 shows that higher C2feed compositions require lower cooler temperatures, use moreexternal refrigerant, and produce less expander work. The liquidflow rates in the two flash drums must increase to produce the

Table 1. Expander/Heat-Exchanger Split; 5 mol % C2 Feed

fraction = Fexp/(Fexp +FHX) Tcool (K) Qcool (MW) QR (MW) Wexp (MW) NGL (kmol/h) net energy (MW)

0.50 256.7 1426 354.0 720.3 506.2 1059.70.58 261.0 1243 351.0 869.4 506.8 724.60.60 262.1 1197 350.6 908.1 507.0 639.50.62 261.0 1178 352.3 929.3 506.5 601.00.65 253.4 1252 358.2 906.5 507.1 703.7

Figure 2. Effect of feed composition.

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larger liquid NGL bottoms product. The NGL product is more

valuable than the gas product, so the increase in refrigeration

cost is more than justified.If the C2 content of the natural gas feed is always going to be

high, it may be possible to eliminate the steam-heated reboiler

in the column base and only use reboilers that are heated by the

warm feed gas. However, an auxiliary steam-heated reboiler

may be required for startup.

4. CONCLUSION

The composition of the natural gas has a profound effect on the

design of the first distillation column that separates the

methane from the ethane and heavier hydrocarbons. Richer

natural gas (higher ethane concentration) requires more

refrigeration but produces more valuable liquid NGL product.

Side-reboiler bypassing is required with rich natural gas.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: WLL0@Lehigh.edu; Tel.: 610-758-4256; Fax: 610-758-5057.NotesThe authors declare no competing financial interest.

■ REFERENCES(1) Lee, S.; Long, N. V. D.; Lee, M. Design and optimization ofnatural gas liquefaction and recovery processes for offshore floatingliquefied natural gas plants. Ind. Eng. Chem. Res. 2012, 51, 10021−10030.(2) Nawaz, M., Jobson, M., Finn, A. Synthesis and optimization ofdemethanizer flowsheets for NGL recovery. Spring 2012 AIChEmeeting, Houston, TX.

Figure 3. Flowsheet (15 mol % C2; expander split = 0.67; bypass split = 0.21).

Table 2. Expander/Heat-Exchanger Split; 15 mol % C2 Feed

fraction = Fexp/(Fexp +FHX) fraction bypass Tcool (K) Qcool (MW) QR (MW) Wexp (MW) NGL (kmol/h) net energy (MW)

0.63 0.215 241.2 3039 99.9 650.6 937.5 24880.65 0.216 241.5 3011 99.4 675.3 937.8 24360.67 0.210 240.5 3016 100.2 682.3 937.5 24330.70 0.183 236.5 3114 100.4 652.0 838.1 2562

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