{"id":2783,"date":"2020-03-03T10:20:29","date_gmt":"2020-03-03T16:20:29","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=2783"},"modified":"2020-03-03T10:29:24","modified_gmt":"2020-03-03T16:29:24","slug":"impact-of-process-gas-pressure-on-the-performance-of-a-mechanical-refrigeration-plant","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2020\/03\/impact-of-process-gas-pressure-on-the-performance-of-a-mechanical-refrigeration-plant\/","title":{"rendered":"Impact of Process Gas Pressure on the Performance of a Mechanical Refrigeration Plant"},"content":{"rendered":"
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The process gas pressure has a significant effect on the choice of hydrocarbon dewpoint (HCDP) control process. If the process inlet pressure is less than about 8000 kPa (1160 psia) and the sales gas pressure is 6900 kPa (1000 psia) or higher, the most common process choice is mechanical refrigeration. There are exceptions to this, particularly for very lean gases where a silica gel adsorption system may be a better choice [1].<\/p>\n

If the sales gas pressure is low,\u00a0e.g.<\/em>, gas turbine fuel, 3500-4100 kPa (500-600 psia), an expansion process is often more economic due to the availability of \u201cfree\u201d pressure drop [1].<\/p>\n

Expansion processes are the preferred alternative when the inlet gas pressure is at least 20-30% greater than the sales gas pressure. This is particularly true at pressure above about 8000 kPa (1160 psia) because mechanical refrigeration systems are generally not effective at high pressure due to the retrograde phase behavior of the gas. If the inlet pressure is provided by the reservoir and no inlet compression is required, the valve expansion process might be more economical when inlet pressure to sales gas pressure ratio exceeds about 1.5 [1].<\/p>\n

If the inlet pressure is provided by compression or when the ratio of inlet to sales gas pressure falls to about 1.1 to 1.5 an expander process is often preferred. Table 15.5 of reference [1] provides a comparison between mechanical refrigeration, valve expansions, and expander\/compressor processes.<\/p>\n

The details of a mechanical refrigeration, silica gel, valve expansion, and expander processes are given in Chapters 6, 15 and 18 of the Gas Conditioning and Processing, Volumes 1 and 2 [1, 2].<\/p>\n

Continuing the\u00a0June 2019 [3] Tip of The Month<\/a>\u00a0(TOTM), this tip investigates the impact of\u00a0process gas pressure on the performance of a mechanical refrigeration plant with mono ethylene glycol (EG or MEG) injection for hydrocarbon dew point (HCDP) control. Specifically, how\u00a0process gas pressure impacts the gas-gas heat exchanger and chiller duties.\u00a0The operation of the mechanical refrigeration system will be investigated and reported. In addition, the effect of process gas pressure on the compressor power and cooling duty requirements for gas recycling and pipeline transportation and liquid recovery will be presented.<\/p>\n

Figure 1 presents the process flow diagrams for a typical HCDP control plant using mechanical refrigeration with MEG injection system. This figure is similar to the June 2019 TOTM [3] which utilizes a flash tank economizer with two stages of compresssion. 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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Click to enlarge<\/em><\/figcaption><\/figure>\n
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Figure 1.<\/strong>\u00a0Process flow diagrams for a HCDP plant using mechanical refrigeration with a flash tank economizer and MEG injection system<\/em><\/p>\n<\/div>\n

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Case Study:<\/strong><\/p>\n

Let\u2019s consider the same case presented in June 2019 TOTM [3] for a rich gas with the compositions and conditions presented in Table 1 [3]. 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 at the cricondentherm of \u00a0-19.5 \u00b0C (-3 \u00b0F) at about 4708 kPa (682 psia) for the sales gas by removing heat in the \u201cGas\/Gas\u201d heat exchanger (HX) with a hot end temperature approach (TA) of 3\u00b0C (5.4\u00b0F) \u00a0and in a propane chiller with TA of 5 \u00b0C (9 \u00b0F). The heat will be rejected to the environment by a propane condenser AC-100 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 on the process side of the propane chiller are assumed to be 34.5 kPa (5 psi). The tip will investigate the impact of inlet pressure from 2000 to 7837 kPa (290 \u2013 1136 psia) on the performance of mechanical refrigeration presented in Figure 1.<\/p>\n

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

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

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The phase envelope for the feed gas and the desired sales gas specification with a cricondentherm of -19.5 \u00b0C (-3 \u00b0F) at about 4700 kPa (682 psia) are presented in Figure 2. In addition, Table 3 presents the inlet pressure range, cold separator pressure and the corresponding cold separator temperature to meet the sales gas specifications. The process gas temperatures at the outlet of Gas\/Gas HX (Stream 5) are also presented.<\/p>\n

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Figure 2.<\/strong>\u00a0Feed gas and sales gas-phase envelopes<\/em><\/p>\n

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Table 3. Inlet pressure, cold separator pressure, the required cold separator temperature and stream 5 temperature to meet sales gas specification<\/strong><\/p>\n

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\u00a0Cricondentherm Pressure\u00a0\u00a0\u00a0\u00a0 \u00a0and ** \u00a0\u00a0Cricondenbar Pressure<\/em><\/p>\n

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The feed gas is flashed in the \u201cInlet Separator\u201d at 30 \u00b0C (86 \u00b0F) and the process gas inlet pressure to remove any condensate. The \u201cInlet Separator\u201d vapor (stream 2) 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 deethanizer overhead vapor (located at the right-hand side of Figure 1).<\/p>\n

The hydrate formation temperature (HFT) of streams 5 and 7 are estimated. 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. Streams \u201c5\u201d and \u201c7\u201d cool down below their respective HFTs. The injection rates of streams \u201cEG1\u201d and \u201cEG2\u201d for 80-weight % lean MEG and water solution are estimated manually or by the Adjust tool of UniSim. A design margin of 1 \u00b0C (1.8 \u00b0F) HFT below the cold temperature for streams \u201c5\u201d and \u201c7\u201d were assumed.<\/p>\n

Assuming a TA of 5\u00b0C (9\u00b0F) and a 6.9 kPa (1 psi) pressure drop in the propane \u201cChiller\u201d shell side, the pressure of saturated propane vapor leaving the chiller depends on process gas pressure and is in the range of 117 to 200 kPa (17 to 29 psia). Assuming no frictional losses in the suction line to the propane compressor \u201cK-101\u201d, the resulting suction pressure is 117 to 200 kPa (17 to 29 psia).<\/p>\n

The condensing propane pressure at the specified condenser temperature of 37.8 \u00b0C (100 \u00b0F) is 1303 kPa (189 psi). The condenser \u201cAC-100\u201d frictional losses, plus the frictional losses in the piping from the compressor discharge to the condenser were assumed to be 34.5 kPa (5 psi); therefore, the discharge pressure of compressor \u201cK-102\u201d is 1338 kPa (194 psia). The compressors inter stage pressure was determined by equalizing the power for \u201cK-101\u201d and \u201cK-102\u201d. The compressors adiabatic efficiency was assumed to be 75%.<\/p>\n

The cold Stream 7 is flashed in the 3-phase separator \u201cV-102\u201d at the separator pressure and corresponding temperature (see Table 3). Assuming this cold separator has no liquid carry over, 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. To prepare the liquid to be fed to the deethanizer, the process specification is to raise the cold temperature of the NGL product stream \u201c9A\u201d to 20 \u00b0C (68 \u00b0F) and 1500 kPa (217.6 psia) in \u201cE-102\u201d HX, regardless of plant inlet pressure. The process duty and the temperature of the NGL product stream is set by the deethanizer process requirements. The pressure drops in \u201cE-102\u201d HX is 35 kPa (5 psi). Sometimes the heat duty of \u201cE-102\u201d is used as a cooling medium for the deethanizer overhead condenser.<\/p>\n

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

Like the June 2019 TOTM [3], regardless of inlet pressure, the deethanizer specifications are:<\/p>\n

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

\u25baEthane to propane mole ratio equal to 5 % in the bottom\u2019s product<\/p>\n

\u25baFeed pressure 1500 kPa (217.6 psia) and temperature of 20 \u00b0C (68 \u00b0F)<\/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

\u25baFeed tray from top is 4<\/p>\n

The deethanizer simulation results for feed gas inlet pressure 4700 kPa (682 psia) are summarized in Table 4.<\/p>\n

Table 4. Deethanizer key design parameters for feed gas inlet pressure of 4700 kPa (682 psia)<\/strong><\/p>\n

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Impact of Inlet Feed Gas Pressure:<\/strong><\/p>\n

The Gas\/Gas HX utilizes the cold temperature of stream 4 to cool down stream 2A and reduce the chiller duty. The specified temperature approach (TA) 3 \u00b0C (5.4 \u00b0F) sets the sales gas temperature to 30 – 3 = 27 \u00b0C (86 – 5.4 = 80.6 \u00b0F). Table 3 indicates that increasing the inlet pressure, up to the cricondentherm pressure of about 4708 kPa (682 psia), increases the required cold separator temperature then the cold separator temperature decreases to a minimum at -32.4 \u00b0C (-26.3 \u00b0F) at sales gas cricondenbar pressure. The \u201cGas\/Gas\u201d HX duty is a function of stream 4 rate and temperature (cold separator temperature). Note that the colder separator temperature results in more liquid drop out and less vapor (stream 4) rate. This may result in lower \u201cGas\/Gas\u201d HX duty and higher stream 5 temperature. The cold separator temperature has a big impact on \u201cGas\/Gas\u201d HX duty and stream 5 temperature. Therefore, streams 5 (Gas\/Gas HX outlet) and 7 (Chiller outlet) temperature and their HFT (less the margin) determine the MEG injection rate.<\/p>\n

Figure 3 presents the impact of inlet pressure (and the corresponding cold separator temperature) on the MEG injection rate of streams \u201cEG1\u201d and \u201cEG2\u201d upstream of \u201cGas\/Gas\u201d HX and the Chiller, respectively. As the inlet pressure increases, stream 7 temperature initially increases and then decrease. The required MEG injection rate of stream \u201cEG1\u201d remain relatively constant but reduces at the maximum pressure (minimum cold separator temperature) at which stream 5 temperature is 2.7 \u00b0C (37 \u00b0F). Stream \u201cEG2\u201d rate initially decreases and then increases at higher pressure. The total MEG injection (EG1 + EG2) rate stays the same. The calculated \u201cEG1\u201d and \u201cEG2\u201d were summed up and presented in Figure 3.<\/p>\n

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Figure 3.<\/strong>\u00a0Impact of the inlet feed gas pressure on the MEG injection rate upstream of \u201cGas\/Gas\u201d HX (EG1) and Chiller (EG2) (See an alternative figure in the Appendix)<\/em><\/p>\n

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\u201cGas\/Gas\u201d HX cools stream 2A temperature to stream 5 and reduces the required chiller duty. Except for high inlet pressure, typically the \u201cGas\/Gas\u201d HX removes about 70% of the total required cooling duty to meet the specified sales gas dewpoint temperature. Therefore, the chiller duty decreases resulting in lower compressor power, decreased propane refrigerant circulation rate, and reduced propane refrigerant condenser duty requirements.<\/p>\n

Both Chiller and \u201cGas\/Gas\u201d HX duties depend on the cold separator temperature. The \u201cGas\/Gas\u201d HX duty also depends on vapor stream 4 rate which decreases with the cold separator temperature. The net effect is an increase in the Chiller and \u201cGas\/Gas\u201d HX duties with the inlet pressure increase. As the chiller duty increases, the compressor power and condenser duty increase, too. Figures 4 A and B illustrate the impact of inlet pressure on the compressor power (K-101 and K-102), \u201cGas\/Gas\u201d HX, Chiller, and condenser (AC-100) duties in SI and FPS system of units, respectively.<\/p>\n

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Figure 4A.\u00a0<\/strong>Impact of the inlet feed gas pressure on the Gas\/Gas HX, chiller and condenser duty, and compressor power (See an alternative figure in the Appendix)<\/em><\/p>\n

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Figure 4B.\u00a0<\/strong>Impact of the inlet pressure on the Gas\/Gas HX, chiller and condenser duty, and compressor power (See an alternative figure in the Appendix)<\/em><\/p>\n

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To meet pipeline inlet specifications, the sales gas was compressed to 10,000 kPa (1450 psia) in K-103 and cooled to 40 \u00b0C (104 \u00b0F) in E-100 (located at the lower left corner of Figure 1). Similarly, the deethanizer overhead gas was compressed in K-100 and cooled in E-101 (located at the top of Figure 1) to the stream \u201c2 Wet\u201d pressure and temperature. Figure 5 A (SI) and B (FPS) present the impact of inlet pressure on the required pipeline and recycle compressors powers and the cooler duties for the recycle and pipeline gases.<\/p>\n

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Figure\u00a05A.\u00a0<\/strong>Impact of the inlet pressure on the pipeline and recycle gas compressors powers and coolers duties<\/em><\/p>\n

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Figure\u00a05B.<\/strong>\u00a0Impact of the inlet pressure on the pipeline and recycle gas compressors powers and coolers duties.<\/em><\/p>\n

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Figure 6 presents the impact of inlet pressure on the liquid propane and total liquid rate from deethanizer bottom. Figure 7 presents the impact of inlet pressure on the liquid propane percent recovery (ratio of propane rate in deethanizer bottom to total propane rate in the plant inlet feed gas). It looks like one would want to operate below about 7000 kPa (1015 psia), which is about 10% less than the cricondenbar.<\/p>\n

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Figure\u00a06.<\/strong>\u00a0Impact of the inlet pressure on the liquid propane total liquid rate from deethanizer bottom<\/em><\/p>\n

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Figure\u00a07.<\/strong>\u00a0Impact of the inlet pressure on the liquid propane percent recovery (rate of liquid propane\/rate of propane in inlet gas)<\/em><\/p>\n

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

This tip demonstrated the impact of inlet pressure on the performance of mechanical refrigeration for a HCDP control plant and the following observations were made.<\/p>\n

\u25baThe cold separator temperature (chilling temperature) is the key design variable and has a significant impact on the performance of mechanical refrigeration.<\/p>\n

\u25baThe relation between the inlet pressure and the required cold separator temperature to meet the required sales gas specification is nonlinear and complex; therefore, the knowledge of sales gas phase envelope is essential (see Figure 2 and Table 3).<\/p>\n

\u25baTo minimize the cost, typically the optimum inlet pressure for a mechanical refrigeration process is close to the cricondentherm pressure of sales gas. However, the sales gas pressure and the recycled deethanizer overhead gas compression should be considered.<\/p>\n

\u25baThe inlet pressure to the HCDP facility cannot exceed the cricondenbar of the sales gas phase envelope.<\/p>\n

\u25baAbove 7837 kPa [1136 psia] this gas could not be processed in a mechanical refrigeration plant to meet the HCDP specification. This is because at higher pressures you cannot condense enough liquid to meet the specification. One may want to operate below about 7000 kPa (1015 psia), which is about 10% less than the cricondenbar.<\/p>\n

To learn more about similar cases and how to minimize operational problems, we suggest attending our\u00a0G4 (<\/strong>Gas Conditioning and Processing)<\/strong><\/a>,<\/strong>\u00a0G5<\/strong>\u00a0(Practical Computer Simulation,\u00a0<\/strong>and Applications in Gas Processing)<\/a>\u00a0courses.<\/p>\n

By: Mahmood Moshfeghian, Ph.D.<\/em><\/p>\n

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