{"id":2633,"date":"2019-02-01T14:44:42","date_gmt":"2019-02-01T20:44:42","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=2633"},"modified":"2019-02-01T14:54:52","modified_gmt":"2019-02-01T20:54:52","slug":"impact-of-heavy-end-on-the-performance-of-a-mechanical-refrigeration-plant-with-meg-injection","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2019\/02\/impact-of-heavy-end-on-the-performance-of-a-mechanical-refrigeration-plant-with-meg-injection\/","title":{"rendered":"Impact of Heavy End on the Performance of a Mechanical Refrigeration Plant with MEG Injection"},"content":{"rendered":"

Continuing the\u00a0January 2019<\/a>\u00a0[1] Tip of The Month (TOTM), this tip investigates the impact the heavy end characterizations on the performance of a mechanical refrigeration plant with\u00a0mono-ethylene\u00a0glycol (EG or MEG) injection for hydrocarbon dew point (HCDP) control. Specifically, the impact of heavy end characterization on the gas-gas heat exchanger and chiller duties,\u00a0the mechanical refrigeration system, and the liquid propane recovery will be investigated and reported. The details of a mechanical refrigeration plant with MEG injection and regeneration system are given in Chapters 6 and 15 of the Gas Conditioning and Processing, Volumes 1 and 2 [2, 3], respectively.<\/p>\n

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Figure 1 presents the process flow diagrams for a typical mechanical refrigeration plant with MEG injection system. In this tip, all simulations were performed with UniSim Design R443 software [4] using the Peng-Robinson equation of state.<\/p>\n

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Figure 1.<\/strong>\u00a0Process flow diagrams for a mechanical refrigeration plant using a sub-cool economizer and MEG Injection system<\/em><\/p>\n

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CASE STUDY:<\/strong><\/p>\n

Let\u2019s consider a rich gas with the compositions and conditions presented in Table 1. Based on the reported molecular weight and relative density for the C7+ fraction, Table 2 presents the estimated normal boiling point (NBP), critical properties and acentric factor which are needed by the equation of state. The objective is to meet a hydrocarbon dew point specification of \u00a0-20 \u00b0C [-4\u00b0F] at about 4000 kPa (580\u00a0psia) for the sales gas by removing heat in the \u201cGas\/Gas\u201d heat exchanger (HX) with a hot end approach temperature of 5\u00b0C [9\u00b0F] \u00a0and in a propane chiller and rejecting it to the environment by a propane condenser (\u201cE-103\u201d) at 37.8\u00b0C [100\u00b0F]. Pure propane is used as the working fluid in the simulation. The pressure drops in the \u201cGas\/Gas\u201d HX and the propane chiller are assumed to be 34.5 kPa (5 psi).<\/p>\n

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Table 1.<\/strong>\u00a0Rich feed gas compositions and conditions<\/p>\n

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Table 2.<\/strong>\u00a0Estimated C7+ properties [4]<\/p>\n

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The feed gas is flashed in the \u201cInlet Separator\u201d at 30 \u00b0C (86 \u00b0F) and 4000 kPa (580\u00a0psia) to remove any condensate. The \u201cInlet Separator\u201d vapor (stream \u201c2\u201d) is saturated with water by the \u201cSaturate -100\u201d to form stream \u201c2 Wet\u201d upstream of mixing with MEG hydrate inhibitor, stream \u201cEG1\u201d and the recycle stream \u201c18A\u201d from the\u00a0deethanizer\u00a0overhead vapor (located at the right hand side of Fig. 1). The estimated hydrate formation temperature of streams \u201c2 Wet\u201d is 14.7 \u00b0C (58.4 \u00b0F). The hydrate inhibitor is injected at the inlet of \u201cGas\/Gas\u201d HX by stream \u201cEG1\u201d and at the inlet of the \u201cChiller\u201d by stream \u201cEG2\u201d. Stream \u201c5\u201d cools to about -8 \u00b0C (17.6 \u00b0F) and stream \u201c7\u201d cools down to the specified temperature of -20 \u00b0C (-4 \u00b0F) which are below the hydrate formation temperature (HFT) of 14.7 \u00b0C (58.4 \u00b0F). The injection rates of streams \u201cEG1\u201d and \u201cEG2\u201d for 80 weight % lean MEG and water solution are estimated by the Adjust tool of UniSim. A design margin of 1.1 \u00b0C (2 \u00b0F) HFT below the cold temperature for streams \u201c5\u201d and \u201c7\u201d were assumed.\u00a0 Table 3 presents the estimated hydrate inhibition injection rates.<\/p>\n

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Table 3.<\/strong>\u00a0Estimated 80 weight % lean MEG hydrate inhibition injection rates<\/p>\n

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Assuming an approach temperature of 5\u00b0C\u00a0 (9\u00b0F) and a 6.9 kPa (1 psi) pressure drop in the propane chiller (\u201cRefChiller\u201d) shell side, the pressure of saturated propane vapor leaving the chiller is 203.3 kPa (29.5psia), and at a temperature of -25\u00b0C (-13\u00b0F).\u00a0 Assuming no frictional losses in the suction line to the propane compressor \u201cK-101\u201d, the resulting suction pressure is 203.3 kPa (29.5\u00a0psia).<\/p>\n

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The condensing propane pressure at the specified condenser temperature of 37.8 \u00b0C (100 \u00b0F) is 1303 kPa (189 psi). The condenser \u201cE-103\u201d frictional losses, plus the frictional losses in the piping from the compressor discharge to the condenser was assumed to be 34.5 kPa (5 psi); therefore, the compressor discharge pressure is 1338 kPa (194\u00a0psia). The propane compressor adiabatic efficiency was assumed to be 75%.<\/p>\n

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External Sub-Cool Economizer:<\/strong><\/p>\n

The cold Stream 7 is flashed in the 3-phase separator \u201cV-102\u201d at -20 \u00b0C (-\u00b04F) and 3931 kPa (570\u00a0psia). The vapor stream \u201c4\u201d from this cold separator is used to cool down the incoming warm feed gas in the \u201cGas\/Gas\u201d HX. The heavy liquid stream \u201c8B\u201d (rich MEG solution) from the cold separator is regenerated in the regeneration unit (not shown in Fig. 1) and the lean 80 weight % MEG is recycled and used in streams \u201cEG1\u201d and \u201cEG2\u201d. The cold NGL stream \u201c8\u201d (light liquid phase) from the cold separator, \u201cV-102\u201d, is combined with the plant \u201cInlet Separator\u201d condensate (stream \u201c3\u201d) in the mixer \u201cMix-101\u201d to form stream \u201c9\u201d at about 5 \u00b0C (41 \u00b0F) and 3945 kPa (572.2\u00a0psia). To prepare the liquid to be fed to the\u00a0deethanizer, the process specification is to raise the temperature of the NGL product stream \u201c9A\u201d from about -4\u00b0C (25\u00b0F) and 1535 kPa (222.6\u00a0psia) to 20 \u00b0C (68 \u00b0F) and 1500 kPa (217.6\u00a0psia) in \u201cE-102\u201d HX. The required heat duty will be supplied from a propane refrigerant sub-cool economizer \u201cE-104\u201d HX. The process duty and the temperature of the NGL product stream\u00a0is\u00a0set by the\u00a0deethanizer\u00a0process requirements, thus the sub-cool economizer duty is fixed.<\/p>\n

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The sub-cool economizer cools the condensed propane (refrigerant stream \u201cR4\u201d) from 37.8\u00b0C (100 \u00b0F) at 1303 kPa (189\u00a0psia) to a cooler temperature at 1269 kPa (184\u00a0psia), depending upon the specified propane refrigerant flow rate (stream \u201cR5\u201d). The pressure drops in \u201cE-102\u201d and \u201cE-104\u201d HXs are 35 kPa (5 psi); respectively. The heat removed by the sub-cool economizer is fixed by the process duty required to heat the NGL process stream \u201c9A\u201d.<\/p>\n

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Deethanizer Specifications and Performance:<\/strong><\/p>\n

The\u00a0deethanizer\u00a0column specifications are:<\/p>\n

\u25baTo recover 90 mole\u00a0percent of propane of the feed in the bottom product and<\/p>\n

\u25baEthane to propane mole ratio equal to 5 % in the bottoms product<\/p>\n

\u25baTop and bottom pressures are 1450 and 1500 kPa (210.3 and 217.6 psia); respectively<\/p>\n

\u25baNumber of theoretical stages 12 plus the condenser and reboiler (determined by the material balance and column shortcut calculations)<\/p>\n

The\u00a0deethanizer\u00a0simulation results are summarized in Table 4.<\/p>\n

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Table 4.\u00a0<\/strong>Summary of\u00a0deethanizer\u00a0\u00a0key design parameters for C7+<\/p>\n

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Impact of Heavy End Characterization:<\/strong><\/p>\n

Figure 2 presents the phase envelopes for the key streams of feed (\u201cDry Feed\u201d), inlet separator vapor (stream \u201c2\u201d) and sales gas (stream \u201c4\u201d). All phase envelopes are generated on\u00a0the dry\u00a0basis. As expected the bubble point curves are very close to each other but large deviations are observed for the dewpoint curves. Similar diagrams for the nC7 and nC8 as the heavy\u00a0end \u00a0are\u00a0presented in the Appendix in Figures 1A and 2A; respectively.<\/p>\n

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Figure 2.<\/strong>\u00a0Phase diagrams for the key streams for the case of C7+ as the heavy end<\/em><\/p>\n

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Figures 3, 4, and 5 present the impact of heavy ends on the phase envelope of the key streams of feed, inlet separator vapor (stream \u201c2\u201d) and the sales gas (stream \u201c4\u201d), respectively. These figures indicate that as the heavy components are removed in the \u201cInlet Separator\u201d and cold separator (\u201cV-102\u201d) from the process streams, the impact of heavy end characterization on the phase envelope reduces and vanishes almost completely for sales gas (stream \u201c4\u201d) in Figure 5.<\/p>\n

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Figure\u00a03.\u00a0<\/strong>The impact of\u00a0<\/em>heavy<\/em>\u00a0end on the phase envelope of the feed stream<\/em><\/p>\n

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Figure\u00a04.\u00a0<\/strong>The impact of\u00a0<\/em>heavy<\/em>\u00a0end on the phase envelope of the inlet separator vapor stream<\/em><\/p>\n

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Figure\u00a05.\u00a0<\/strong>The impact of\u00a0<\/em>heavy<\/em>\u00a0end on the phase envelope of the sales gas (Stream 4)<\/em><\/p>\n

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Table 5 presents the impact of heavy end characterization on the \u201cGas\/Gas\u201d HX\u00a0 and \u201cChiller\u201d duties. Note that the \u201cGas\/Gas\u201d HX duty is controlled by stream \u201c4\u201d composition and rate. Based on the phase envelopes in Figure 5, the sales gas composition is almost independent of heavy ends because they are removed from the sales gas but the heavy ends have more impact on the composition of streams \u201c2\u201d.<\/p>\n

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Table 5.<\/strong>\u00a0Impact of\u00a0heavy\u00a0end on the Gas\/Gas HX\u00a0 and Chiller duties<\/p>\n

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Table 5 indicates that as the heavy ends become heavier,<\/p>\n

\u25bastream \u201c2\u201d flow rate decreases because there is more liquid leaving the \u201cInlet Separator\u201d.<\/p>\n

\u25bastream \u201c4\u201d rate increases by about 0.27% (nC7 to C7+) because most of the C7+ has been removed.<\/p>\n

\u25ba\u201cGas\/Gas\u201d HX \u00a0duty is set by stream \u201c4\u201d rate and fixed \u0394T=25-(-20) =45 \u00b0C (81 \u00b0F) because\u00a0Q = m\u0394(HSalesgas<\/sub>\u00a0\u2013 H4<\/sub>).<\/p>\n

\u25ba\u201cGas\/Gas\u201d HX \u00a0duty increases slightly, less than 0.8 %,\u00a0because\u00a0stream \u201c4\u201d rate increases by about 0.27%<\/p>\n

\u25bastream \u201c2A\u201d rate decreases, \u201cGas\/Gas\u201d HX\u00a0 duty increases, stream \u201c5A\u201d gets colder, chiller \u0394T decreases; therefore, \u201cChiller\u201d duty decreases<\/p>\n

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Assume the\u00a0design-heavy\u00a0end was nC8 and feed gas heavy end is C7+, not the design nC8. More liquids would leave the \u201cGas\/Gas\u201d HX so the chiller duty would decrease by about 38%.\u00a0But the additional duty to condense the liquids in the \u201cGas\/Gas\u201d HX\u00a0 has to come from somewhere. If the \u201cGas\/Gas\u201d HX has\u00a0excessarea to accommodate the additional duty requirements, then there would indeed be a decrease in chiller duty.\u00a0 If it does not, the duty of the chiller may actually increase.<\/p>\n

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If the feed gas got lighter, and the heavy end is nC7, not the design nC8, then more gas would go to the chiller (less\u00a0liquids leaving the \u201cGas\/Gas\u201d HX) and the chiller duty would increase by about 20%. \u00a0Here, the chiller would have to have excess capacity.<\/p>\n

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This indicates that a change in feed gas characterization would have an effect on the ability of a refrigeration unit to make spec. For easier reference of the stream, see Figure 6.<\/p>\n

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Figure 6.\u00a0<\/strong>Simplified schematic of the front end segment of the process flow diagram<\/em><\/p>\n

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Table 6 presents the impact of heavy end characterization on the refrigeration systems. This table indicates that the rate, compressor power, condenser\u00a0and\u00a0the sub-cool economizer duties decrease as the heavy end becomes heavier. Table 6 also indicates that the rate, compressor power and condenser duty for the sub-cool economizer refrigeration system are lower compared to the simple refrigeration system. Because the chiller duty decreases, the refrigeration systems become smaller; therefore, the OPEX and CPEX\u00a0decrease.<\/p>\n

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Table 6.<\/strong>\u00a0Impact of\u00a0heavy\u00a0end on the refrigeration systems key parameters<\/p>\n

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The heat removed by the sub-cool economizer \u201cE-104\u201d is used to heat stream \u201c9A\u201d in \u201cE-102\u201d HX. Location of \u201cE-102\u201d HX and streams \u201c9A\u201d and \u201c9B\u201d are shown in Figure 7.<\/p>\n

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Figure 7.<\/strong>\u00a0Simplified schematic of the back end of the process flow diagram<\/em><\/p>\n

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Table 7 presents the impact of heavy ends on the rates and the molecular weights for stream \u201c3\u201d from the \u201cInlet Separator\u201d and stream \u201c8\u201d from the cold separator (\u201cV-102\u201d) and the combined NGL stream \u201c9\u201d. This table indicates that as the heavy end becomes heavier,\u00a0rate\u00a0of stream \u201c3\u201d increases but the rate of stream \u201c8\u201d decreases. Because the rate of heavy ends entering the \u201cGas-Gas\u201d HX and \u201cChiller\u201d decrease, the chiller duty decreases and condensation of components decrease\u00a0resulting\u00a0lower streams \u201c8\u201d and \u201c9\u201d rates. Table 1A in the Appendix present components flow rates for streams \u201c3\u201d and \u201c8\u201d.<\/p>\n

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Table 7.<\/strong>\u00a0Impact of the heavy end on streams \u201c3\u201d and \u201c8\u201d and the combined NGL stream \u201c9\u201d rates and molecular weight<\/p>\n

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Using \u201cVLV-100\u201d stream 9 pressure is reduced from 3945 kPa (572.2\u00a0psia) to 1535 kPa (222.6\u00a0psia) in stream \u201c9A\u201d. Table 8 presents the combined NGL streams \u201c9A\u201d and \u201c9B\u201d (see Figure 7) properties. This table indicates that as heavy end becomes heavier, the \u201cE-102\u201d HX duty decreases\u00a0because\u00a0combined NGL stream rate decreases. The required heat for this HX is supplied by the sub-cool HX (\u201cE-104\u201d) of the refrigeration system.<\/p>\n

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Table 9 presents the impact of\u00a0heavy\u00a0end on the plant overall material balance. This table indicates that as the heavy end becomes heavier,<\/p>\n

\u25bathe sales gas rate increases (stream 4)<\/p>\n

\u25bathe deethanizer\u00a0feed (combined NGL stream, 9 ) rate decreases because the sales gas rate (stream 6) has increased<\/p>\n

\u25bathe overhead vapor temperature from the deethanizer\u00a0top remains almost constant because the overhead composition does not significantly change<\/p>\n

\u25bathe overhead vapor rate from the deethanizer\u00a0top decreases because the\u00a0deethanizer\u00a0feed rate decreased<\/p>\n

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Table 8.<\/strong>\u00a0Impact of the heavy end on the combined NGL streams \u201c9A\u201d and \u201c9B\u201d properties<\/p>\n

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Table 9.<\/strong>\u00a0Impact of the heavy end on liquid propane recovery<\/p>\n

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The overhead vapor of\u00a0deethanizer\u00a0is compressed from 1450 kPa (210.3\u00a0psia) to the feed gas inlet pressure of 4000 kPa (580\u00a0psia) by the recycle compressor (\u201cK-100\u201d) and cooled down to the inlet feed gas temperature of 30 \u00b0C (86 \u00b0F) in the \u201cE-101\u201d HX. The liquid from compressor suction scrubber is recycled and combined with\u00a0deethanizer\u00a0feed by the recycle pump. Table 10 presents the compressor and pump power and the \u201cE-101\u201d HX duty requirements. Table 10 indicates that as the heavy end becomes heavier, the recycle compressor and pump power and the cooler duty decrease because the recycle stream rates decrease.<\/p>\n

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Table 10.<\/strong>\u00a0Impact of the heavy end on the recycle compressor, pump, and cooler<\/p>\n

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SUMMARY:<\/strong><\/p>\n

The feed analysis and \/or heavy end characterization in natural gas play an important role in the equipment sizing and process design. Feed analysis may change when different wells of slightly different composition are brought to the production facility. This tip demonstrated the impact of heavy end characterization in the feed gas on the process streams rates, phase behavior, the equipment sizes and the refrigeration requirement by replacing, the C7+ with n-heptane (nC7) and n-octane (nC8). All other specifications and operating conditions were kept the same.<\/p>\n

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As demonstrated in this tip, it would be a good practice to size the equipment with a design margin of 1.2 to 1.3 to take into account the changes in feed gas heavy end composition and characterizations.<\/p>\n

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To learn more about similar cases and how to minimize operational problems, we suggest attending ourG4 (Gas Conditioning and Processing)<\/a>,<\/strong>\u00a0G5 (Practical Computer Simulation Applications in Gas Processing)<\/a>\u00a0<\/strong>and\u00a0G6 (Gas Treating and Sulfur Recovery)<\/a><\/strong>\u00a0courses.<\/p>\n

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By: Dr. Mahmood Moshfeghian<\/em><\/p>\n

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