{"id":1415,"date":"2012-08-01T06:00:48","date_gmt":"2012-08-01T11:00:48","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=1415"},"modified":"2012-10-23T08:13:59","modified_gmt":"2012-10-23T13:13:59","slug":"transportation-of-natural-gas-in-dense-phase","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2012\/08\/transportation-of-natural-gas-in-dense-phase\/","title":{"rendered":"Transportation of Natural Gas in Dense Phase"},"content":{"rendered":"

When a pure compound, in gaseous or liquid state, is heated and compressed above the critical temperature and pressure, it becomes a dense, highly compressed fluid that demonstrates properties of both liquid and gas. For a pure compound above critical pressure and critical temperature, the system is oftentimes referred to as a \u201cdense fluid\u201d or \u201csuper critical fluid\u201d to distinguish it from normal vapor and liquid (see Figure 1 in December 2009 Tip Of The Mont (TOTM<\/a>) [1] for carbon dioxide and in January 2010 TOTM<\/a> [2] for a typical natural gas). Dense phase is a fourth (Solid, Liquid, Gas, Dense) phase that cannot be described by the senses. The word \u201cfluid\u201d refers to anything that will flow and applies equally well to gas and liquid. Pure compounds in the dense phase or supercritical fluid state normally have better dissolving ability than do the same substances in the liquid state. The dense phase has a viscosity similar to that of a gas, but a density closer to that of a liquid.<\/em> Dense phase is a favorable condition for transporting CO2<\/sub> and natural gas as well as carbon dioxide injection into crude oil reservoir for enhanced oil recovery.<\/p>\n

Pipelines have been built to transport CO2<\/sub> and natural gas [3] in the dense phase region due to its higher density, and this also provides the added benefit of no liquids formation in the pipeline.<\/span><\/p>\n

Recently (January through April 2012 TOTMs<\/a>) we discussed several aspects of transportation of carbon dioxide (CO2<\/sub>) in the dense phase. We illustrated how thermophysical properties change in the dense phase and their impacts on pressure drop calculations. The pressure drop calculation utilizing the liquid phase and vapor phase equations were compared. In this TOTM, we will discuss the dense phase transportation of natural gas. The application of dense phase in the oil and gas industry will be discussed briefly.<\/p>\n

 <\/p>\n

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

For the purpose of illustration, we will consider transporting 244 MMSCFD (6.9×106<\/sup> Sm3<\/sup>\/d) of a natural gas with composition presented in Table 1. The corresponding mass flow rate is 154.95 lbm<\/sub>\/sec (70.28 kg\/s). For simplicity, the calculations and subsequent discussion will be done on the dry basis. The 100 miles (160.9 km) long pipeline with inside diameters of 14 to 24 inches with an increment of 2 inches \u00a0(356 to 610 mm with an increment of 51 mm) have been considered. The inlet conditions are 580 psia (4000 kPa) and 86\u02daF (30\u02daC). The following assumptions and correlations are\/used:<\/p>\n

    \n
  1. Dry basis, ignoring water.<\/li>\n
  2. C7+<\/sub> considered as nC8<\/sub>.<\/li>\n
  3. Steady state<\/li>\n
  4. Delivery pressure is 580 psia (4000 kPa).<\/li>\n
  5. Pressure drop in each heat exchanger is 5 psi (35 kPa).<\/li>\n
  6. No pressure drop in separators.<\/li>\n
  7. Horizontal pipeline, no elevation change.<\/li>\n
  8. Inside surface absolute roughness is 0.0018 in (0.046 mm)<\/li>\n
  9. Multiphase flow correlation: Beggs and Brill.<\/li>\n
  10. Single Phase Friction Factor: Colebrook<\/li>\n
  11. Number of Length Increments: 10 (Each segment is 10 mile (16.09 km)<\/li>\n
  12. Overall Heat Transfer Coefficient: 0.25 Btu\/hr-ft2<\/sup>-\u02daF (1.42 W\/m2<\/sup>-\u02daC).<\/li>\n
  13. Simulation software: ProMax [4]<\/li>\n
  14. Equation of State: Soave-Redelich-Kwong (SRK).<\/li>\n<\/ol>\n

     <\/p>\n

    Table 1. Composition of the feed gas<\/p>\n

    \"\"<\/a><\/p>\n

    Two cases for transportation of this natural gas as shown in Figure 1 are considered and each is explained briefly in the proceeding section.Note this is a rich gas stream. A lean gas stream more typical of transmission pipelines would be: 93-97 % CH4<\/sub>, 1-2 % N2<\/sub>\/CO2<\/sub>, balance C2+<\/sub> (mole basis).<\/p>\n

     <\/p>\n

    Case 1: Two Phase (Gas and Liquid)<\/strong><\/p>\n

    The gas from the inlet separator (see Figure 1) is compressed to a sufficiently high pressure in two stages to just meet the delivery pressure of 580 psia (4000 kPa) at the destination. The compressed gas after each stage of compression is cooled to 100\u02daF (37.8\u02daC). The interstage pressure was set to give equal compression ratio in each stage and approximated the same power requirement for each stage. The outlet separator removes any condensed liquid.<\/p>\n

    \u00a0<\/strong><\/p>\n

    Case 2: Dense Phase<\/strong><\/p>\n

    The process flow diagram (PFD) for this case is exactly the same as for case 1 with two exceptions:<\/p>\n

      \n
    1. The pipeline outlet pressure must end up in the dense gas phase region.<\/li>\n
    2. A Joule-Thompson (JT) valve is added at the outlet of pipeline to reduce pressure to the specified delivery pressure of 580 psia (4000 kPa) and extract natural gas liquid (NGL).<\/li>\n<\/ol>\n

       <\/p>\n

      \"\"<\/a><\/p>\n

      Figure 1. Process flow diagrams (PFD) for cases 1 and 2<\/p>\n

       <\/p>\n

      Simulation Results and Discussions:<\/strong><\/p>\n

      The two PFDs shown in Figure 1 are simulated using ProMax [4] for the six different inside diameters. Table 2 presents a summary of simulation results for the six inside diameters considered in the field (FPS, foot, pound and second) and SI (System International) sets of units.<\/p>\n

      Figure 2 presents the variation of linear pressure drop per length as a function of inside diameter. As expected, the pressure drop decreases as diameter increases. This figure also shows that as diameter decreases, the pressure drop for the Two Phase case increases at a lower rate than the Dense Phase (ratio of pressure drop for small diameter\/large diameter \u2248 6.6\/1 for Two Phase and \u2248 15\/1 for Dense Phase).<\/p>\n

      Figure 3 presents the variation of compressor power and cooling duty requirement as a function of inside diameter. Figure 3 indicates that as diameter decreases the energy requirement increases but the difference between the corresponding compressor power and cooling duty for the two cases decreases with smaller size pipelines.<\/p>\n

      Since no separation takes place between the inlet and outlet separators, the total composition remains the same as the gas passes through the compressors, coolers, pipeline and JT valve. The flow rate through the equipment between the separators remain constant at 238.73 MMSCFD (6.76×106<\/sup> Sm3<\/sup>\/d). The corresponding mass flow rate is 142.67 lbm<\/sub>\/sec (64.71 kg\/s). \u00a0\u00a0 Figure 4 presents the phase envelope, the required compression and cooling stages and pipeline pressure-temperature profile for the inside diameter of 14 in (356 mm). This figure shows that for the Two Phase case, the pipeline outlet condition after passing through the retrograde region ends up in the two phase region with a liquid fraction of 1.81 %. For Dense Phase case, the pipeline outlet condition ends up above the dew point curve. After passing through the JT valve and reducing the pressure to the specified delivery pressure of 580 psia (4000 kPa), the produced liquid fraction is 3.15 %. In this case the NGL extraction (liquid condensed) is about 74% higher than the Two Phase case. However the Dense Phase case requires more compression power and cooling duties. Table 2 also indicates that:<\/p>\n