Exergoeconomic Evaluation of an Integrated Nitrogen Rejection Unit with LNG and NGL Co-Production Processes Based on the MFC and Absorbtion Refrigeration Systems

Natural gas contains heavy ingredients and nitrogen. According to negative influence of nitrogen on fuel heating value, it is necessary to remove the nitrogen of more than 4% concentration from natural gas streams. Liquefied natural gas plants are increasing in number because of the growing demand for natural gas (Alabdulkarem, Mortazavi, Hwang, Radermacher, & Rogers, 2011).  Gas is widely recognized as a clean and economical energy source because of its low carbon intensity and relatively low price in comparison with other fossil fuel types (Wang, Khalilpour, & Abbas, 2014). The LNG facilities are equipped with an intermediate pressure distillation column for recovery ethane and heavier components from the processed natural gas stream in a manner where surges operational and capital costs remain unchanged. (Ransbarger, 2006).Value of the SPE changes from 0.2 to 0.6 kW/kg LNG and at large scale liquefaction processes it ranges from 0.4 to 0.25. However increasing this value decreases the operating cost of the plant, while the process configuration and number of the equipment is another important factor which can affect the capital costs of the plant significantly (Ghorbani, Hamedi, & Amidpour, 2016; Ghorbani, Hamedi, Amidpour, & Mehrpooya, 2016). Separation of Methane, Ethane, Propane, and natural gas liquids (NGL) from the natural gas is generally carried out through one of the following alternativ processes: i) external refrigeration (ER), ii) turbo expansion TE), iii) JouleeThompson expansion, and iv) absorption. In many procedures, a combination of these processes is applied to improve the energy efficiency or obtain greater recoveries (Amidpour et al., 2015; Ghorbani, Mafi, Shirmohammadi, Hamedi, & Amidpour, 2014; Ghorbani, Salehi, Ghaemmaleki, Amidpour, & Hamedi, 2012; Shirmohammadi, Ghorbani, Hamedi, Hamedi, & Romeo, 2015).

New methods in energy saving have led to the development of analyses techniques based on the second law of thermodynamics, particularly, the concept of exergy. In exergoeconomic analysis the quality of energy (exergy) in allocating the production costs of a process to its products is considered. A general methodology for this kind of analysis is presented by Electric Power Research Institute (Bejan & Tsatsaronis, 1996; Fazelpour, 2015), known as the Total Revenue Requirement method (Total Revenue Requirement method).

A systematic method based on a combination of mathematical methods and thermodynamic viewpoints are adopted to acquire optimal design configuration through non-linear programming techniques (Ghorbani, Mafi, Amidpour, Nayenian, & Salehi, 2013; Lashkajani, Ghorbani, Amidpour, & Hamedi, 2016; Mafi, Ghorbani, Salehi, Amidpour, & Nayenian). A superstructure optimization is applied for a separation system which includes distillation column units, heat exchangers and heat integration (Lashkajani et al., 2016). LNG production technologies of compression cooling cycles are applied for cooling. Compression cooling cycles are widely applied in various fields, especially in oil and gas industry and there exist many studies regarding how to enhance their return. Where theim proved performance of vapor compression cooling cycles are applied. In this field the operational characteristics like pressure, flow rate cooling and combined cooling are optimized through studies run by (Ghorbani, Hamedi, Shirmohammadi, Mehrpooya, & Hamedi, 2016; Ghorbani, Ziabasharhagh, & Amidpour, 2014; Salehi et al., 2012).

When the design and integration of the processes take place in a simultaneous manner, the number of required equipment and energy consumption decrease (Ghorbani, Mafi, et al., 2013; Ghorbani, Maleki, Salehi, Salehi, & Amidpour, 2013; Ghorbani, Salehi, Amidpour, & Hamedi, 2012; B Ghorbani, GR Salehi, H Ghaemmaleki, et al., 2012; Khan & Lee, 2013; Lashkajani, Ghorbani, Salehi, & Amidpour, 2013a, 2013b). In order to integrate these units, Conoco Phillips, APCI and Ortloff Company have introduced new plants. Some smaller companies have addressed several patented design limitations in this respect. In design plants based on ConocoPhillips cascade cycle], for better separation in the column is used lean liquid ethane in recycle. Pre-cooling cycle runs through by pure propane or a mixture of propane and ethane . The APCI Company has introduced a method for the production of these two products where the kettle type heat exchanger, absorption tower, and separation and external refrigeration cycle are applied (Roberts & Brostow, 2005). Ortloff Company has designed a process based on recovery NGL process liquefied natural gas (GSP). Lee et al designed and patented an integrated process for the production of LNG and NGL. This process used two columns that work in different pressure in the NGL recovery. [After initial cooling gas enter into this section and the liquid recycle, that must be rich from heavy hydrocarbons, produce from condensing of gas exited from second column or liquefaction section. M.S. Khan et al. analyzed a new process of KSMR system for the simultaneous production of natural gas condensate and NGL. Integrated nitrogen rejection unit for producing LNG is considered using dedicated reinjection circuit (Chen, Liu, Krishnamurthy, Ott, & Roberts, 2015a). In addition, the integrated nitrogen rejection process for the production of LNG is carried out through the intermediate feed gas separator (Ott, Krishnamurthy, Chen, Liu, & Roberts, 2015) . Integrated nitrogen removal in LNG production is assessed through the refrigerated heat pump (Ott, Krishnamurthy, Chen, Liu, & Roberts, 2015)..As an alternative approach to improving energy return cycles of absorption cooling units, LNG can be used as the cooling agent. Taking advantage of the energy dissipation properties of absorption cooling cycles from different parts of the unit is possible in this cycles (Ghorbani, Salehi, Esnaashary, & Amidpour, (2012); Mafi, Ghorbani, Amidpour, & Naynian, (2013); Salarian, Ghorbani, Amidpour, & Salehi, (2014)).

Ghorbani et al. developed two integrated NGL-LNG with nitrogen rejection by C3MR and MFC refrigeration cycle. This introduced cycle indicates that, integration due to the reduced in equipment and increase the efficiency.

In this paper, exergy and exergoeconomic analyses are applied to recently alternatives integrated processes for cogeneration of LNG, NGL and NRU with reasonable energy consumption and high ethane recovery. Exergy efficiency and exergy destruction of the process components are calculated. Next all of the equipment are sized and cost of them are calculated with a suitable cost function. Mathematical modeling of the process is done in order to finding the exergoeconomic factors. Exergoeconomic variables, exergy destruction cost, relative cost difference, exergoeconomic factor are computed and thermoeconomic analysis of the process is investigated and discussed.

2. Process Description {Vatani, 2013 #16}

2.1. Conceptual Design

The MFC process consists of three pure refrigerants of methane, ethylene, and propane with different boiling temperatures, . First, the natural gas is cooled up to -25 ºC in the propane cycle, next up to -86 ºC in the ethylene cycle; finally, it is liquefied to -160 ºC in the methane cycle. The MFC process is highly efficient due to the low shaft power consumption of the three MRC compressors. A structure of integrated process consisting of recycling natural gas liquids, natural gas liquefaction, and nitrogen removal from an absorption refrigeration cycle for precooling and two refrigeration cycle, (multi-component refrigerant) for cooling and liquefaction are proposed.

The absorption refrigeration cycle replaced by compression refrigeration cycle (cycle 400) in the integrated process of LNG, NGL, and NRU with the objective to reduce energy consumption is shown in Fig. (1). Natural gas feed enters at about 37 ºC and 63.09 bar with a 14000 kg.mol/h flow rate. The Liquid produced through this process is categorized into three: Natural gas liquids (1578.8 kgmole/h), liquefied natural gas (11559.3 kg.mol/h) and Removing nitrogen (620.7 kg.mol/h). A propane refrigeration cycle is applied to supply both the required cooling for NGL recovery and precooling for the LNG process.

Figure 1. Cascade refrigeration (combined refrigerant and absorption refrigeration) integrated in the NGL-LNG-NRU consumption structure.

2.2. Basic Design

The relation between different equipment of the integrated process of natural gas liquids, natural gas liquefaction, and denitrification is shown in Fig. (2). How the alternative absorption refrigeration system is replaced by precooling compression cycle (Cycle 400) is clearly shown in this figure. Inlet feed stream at 37  and 63.09 bar enters the multi-stream heat exchangers HX1 and HX2 and is cooled up to 8  and -27 ,respectively. The next outlet stream, 103, is cooled -41 . A part of the required cooling is provided by a three stage propane refrigeration cycle. The outlet gas from the NGL recovery unit, 102, is channeled into the D3 separator. The required heating for the tower is supplied using three side streams; side1, side2 and side3 at approximately 14.5  ,6.8  and -7.4 , respectively. These streams exit the column and enter the multi-stream heat exchangers. Side1 enters the HX1 heat exchanger and exits at 35 . Side1R is named as the backflow and is marked on the PFD. Side2 and side3 are channeled into the HX2 and HX3 respectively and exit the heat exchangers at 0  and -20 , respectively. The outlet streams from the heat exchanger return to the column.  De-ethanizertop gas is channelled to HX4 through V-5 expansion valve where its temperature reaches -119 . Next the stream is ready to enter the nitrogen rejection unit. Stream 118 containing methane and nitrogen with a standard volume of about 1.2% from bottom of the column T200 as the liquid product enters HX-3 from the bottom. Stream 128 is channeled to D2 flash drum and the gas product of this separator returns to the HX-4. Line 123 is branched into two: branch one, (88%), stream 126, enters HX4 multi-stream heat exchanger and its temperature decreases up to -132  and turns to liquid. This cooled liquid passes through an expansion valve where its pressure and the temperature reach up to about 25 bar and -131 . Then it is channeled to the top of the nitrogen removal column as washing liquid. Stream 124, containing 12% of gas output from D2, enters HX3 as well and is cooled up to -75 . Next stream 125 is channeled to column. After eliminating the ethane plus hydrocarbons and nitrogen from the gas stream, it is channeled to the liquefaction section. Stream 129 is compressed up to 63 bar and enters the HX4 heat exchanger. This stream is first heated up to -99  when passing through HX4 and it is then super-cooled by HX4 super-cooling heat exchanger up to 63 bar and at -164  and eventually becomes the final product after passing through V8 where the pressure is reduced to atmospheric pressure, and enters phase D1. The final product is named LNG stream which exits from the separator as gas stream containing a high percentage of nitrogen in which is channeled back into the unit. To meet the cooling requirements, pre-cooling and liquefaction systems are carried out by applying an absorption refrigeration cycle for precooling and two refrigeration cycle (multi-component refrigerant).

Figure 2. Schematic of block flow diagram of the first stage for typical NGL-LNG-NRU plants

Table 1. Thermodynamic data for configuration process of material streams

(Continuation)

3. Exergy Analysis

Exergy is the maximum available work when some forms of energy are transferred in a reverse manner to a reference system, which is in thermodynamic equilibrium with the surroundings, and is disable. Exergy is a measure of distance of a system from global equilibrium; with respect to the state variables of temperature, pressure, and composition of the system approaching surroundings]. therefore, the reference state is named the dead state. The total exergy of multicomponent streams is the sum of its three contributions: change due to mixing, chemical exergy, and physical exergy. The exergy of mixing results from the isothermal and isobaric mixing of streams at the actual process conditions. The chemical exergy is the difference between the process and reference components in their environmental concentration, temperature, and pressure in chemical potentials. The physical exergy is the maximum obtainable amount of shaft work (electrical energy) when a stream is brought from process conditions (T, P) to an equilibrium at ambient temperature by a reversible heat exchange. In general practice the exergy analysis is based on the overall thermodynamic efficiency, the ratio of the lost work to the ideal work required for separation. The overall exergy efficiency for distillation is the product of external and internal exergy efficiencies. (B Ghorbani, GR Salehi, M Amidpour, et al., 2012; B Ghorbani, GR Salehi, H Ghaemmaleki, et al., 2012; Meratizaman, Amidpour, Jazayeri, & Naghizadeh, 2010; Morosuk & Tsatsaronis, 2008; Romero Gómez, Romero Gómez, López-González, & López-Ochoa, 2016; Sheikhi, Ghorbani, Shirmohammadi, & Hamedi, 2014, 2015). The exergy of the process material streams is tabulated in Table (2). In this study , and , are defined as the fuel exergy rate, the product exergy and the exergy destruction rate, respectively.

The exergy balance over the kth component is

where, , and  are the exergy rates of fuel, product and destruction, respectively. and  y_k is defined as the exergy destruction ratio:

The streams operating conditions and exergy analysis of refrigeration cascade structure are tabulated in Table2.  The definitions applied for calculation of exergy efficiency of the process components and the exergy efficiency of the process components are tabulated in this table.

Table2.  Definitions for exergy efficiencies of the process components

4. Exergoeconomic Analysis

All the costs associated with a project, including a minimum required return on investment is calculated through this method. Based on the estimated total capital investment and assumptions made in economical, financial, operating, and market input parameters, the total revenue requirement is calculated on a year-by-year basis. Finally, the non-uniform annual monetary values associated with the investment, operating (excluding fuel), maintenance, and the fuel costs of the system subject to analysis are levelizedو that is, they are converted into an equivalent series of constant payments (Fazelpour & Morosuk, 2014; B Ghorbani, GR Salehi, M Amidpour, et al., 2012).

4.1. Economic Model

Total Revenue Requirement method is applied in this study for economic analysis. The detailed descriptions on the economic model and its terms are presented in Table 3. Economic constants and assumptions are tabulated in Table 3.

The levelized annual total revenue requirement  is calculated as follows through the Capital Recovery Factor:

where, TRR_j is the total revenue requirement in j^th year of system operation, BL is economic life cycle of the system (yr) and i_eff is the average annual rate of effective devaluation. Capital recovery factor (CRF) is calculated as follows:

TRR_j is the sum of four annual terms: minimum return on investment (ROI), total capital recovery (TCR), operation and maintenance costs (OMC) and fuel costs (FC)(Fazelpour & Morosuk, 2014; Wang et al., 2014; Yang, Wei, & Chengzhi, 2009).  More explanation about the economic terms and analysis are found in:

where, FC₀ is the fuel cost at the beginning  point, the  year and calculated as follows:

where:

= total annual time (in hours) that is 7300 h year^-1

C_w = unit cost of fuel (0.071 $ kWh^-1)

= power (kW)

Cost of electricity during the j^th year is computed as follows:

The constant escalation levelization factor (CELF) for the fuel is obtained through:

where:

r_FC is the average annual escalation rate of fuel cost. The levelized annual operating and maintenance costs OMC_L are calculated as follows:

where,

r_OMC is the annual escalation rate for the operating and maintenance costs. The levelized carrying charges CC_L is calculated as follows:

Capital investment  and operating and maintenance costs  of the total plant are gained based on the process components purchased cost.

where,  and PEC_k are the total annual hours of plant operation and the purchased-equipment cost of the k^th component, respectively. Symbol  is the cost rate associated with the capital investment and operating and maintenance costs:

Rate of levelized costs is calculated as follows:

The cost functions used for calculation of the process equipment cost and the purchased equipment and investment costs are tabulated, respectively in Tables 4 and 5.

Table 3. Economic constants and assumptions.

Table 4. Equations regarding the cost of the process components

Table 5. Purchased equipment and investment costs for configuration process components

4.2. Cost Balance Equations

The exergy cost of the streams is gained by writing the cost balance over each component. The cost balance terms are: outlet streams cost, inlet streams cost, primary investment cost, , and the operation and maintenance  costs.

For the components which have more than one output, some auxiliary equations are written ; therefore, based on the cost balances and auxiliary equations for all components a set of linear equations is yield as follows:

where, , and  are exergy rate matrix, costs per unit of exergy vector and coefficient vector for , respectively. The cost balance and auxiliary equations for the process components are tabulated in table 6.

Table 6. Main equations and auxiliary equations for the process components

4.3. Exergoeconomic variables

Based on the fuel/product concept for a component,  and are the fuel and product exergy rates, respectively. Accordingly  and are defined as fuel cost and product cost rates, respectively. For the k^th component of a system ( ) is the average cost per unit of exergy of fuel:

 is the product average cost per unit of exergy:

 is the cost of exergy destruction for the k^th component.

Relative cost difference is defined as follows:

Exergoeconomic factor is the ratio of investments’ cost to the total investment plus exergy destruction costs, calculated as follows:

Table7 show the exergy unit cost for each stream is obtained of the processes.

Table 7. Unit exergy cost of configuration process streams

5. Results and Discussion

5.1. Exergy analysis

The highest rates for exergy destruction occurs in the Modified MFC, in AC3 by 16.9%, in V11 by amount of 16.52% and in AC2 by of 13.23%, respectively, Table 8. The least amount of exergy destruction in the Modified MFC structure occurs  at 0.00001% in V10, 0.00002% in HX7, and 0.00031% in V5 rates, respectively. 

the Modified MFC structure exergetic efficiency of expansion valves are less than in comparison with the other equipment, while their irreversibilities are low, Fig. (2). This fact indicates that the equipment performance in energy consumption must be analyzed in terms of irreversibility and exergy efficiency. The structure of Modified MFC exchanger HX7 with efficiency of 98.24% has the most exergetic efficiency and HX4 with efficiency of 86.35% has the lowest exergetic efficiency. The contribution of each heat exchanger in the total exergy losses is shown in a pie chart Fig. (3), wherethe most exergy loss occurs in the heat exchangers HX4 and HX5 which are responsible for more than 39.23% of the total exergy loss among the heat exchangers.Among the heat exchangers HX4 by 21784.68 kW has the highest Exergy loss and HX7 with 4.6 kW has the least amount of exergy loss.

Table 8. Results of exergy and exergoeconomic analysis of the process

Figure3. Exergy destruction of hrat exchangers in the integrated structure of LNG-NGL-NRU

Figure 4.  Breakdown of compressors, expander and pump exergy losses in the integrated structure of LNG-NGL-NRU

The contribution of the expander, compressors and pump in the total exergy loss are shown in a pie chart in Fig. (4). Among compressors, structure of Modified MFC, the C5 by the efficiency of 78.54% has the highest and C3 by efficiency of 72.61% has the lowest efficiency. Among compressors C1 and C2 have the highest and lowest amount of Exergy destruction equal to 12661.34 kW and 1157.33 kW, respectively. Among the throttle valves in Modified MFC, V3 with efficiency of 80.8% and throttle valve V8 with an output efficiency of 6.2% have the highest and lowest efficiencies, respectively. Among the throttle valves V11 and V10 with exergy loss of 33475.23 kW and 2.184406kW have the maximum and minimum amount of exergy loss, respectively.

The contribution of air coolers in the total exergy loss are shown in Fig. (5). Among the air coolers in the Modified MFC, AC4 with efficiency of 98.9% and AC2 with the efficiency of 97.12% have the highest and lowest efficiency, respectively. Among air coolers AC3 with 34241.78 kW and AC1 with 283.72 kW have the highest and lowest amounts of exergy loss, respectively. Fig.6 demonstrates the contribution of the towers in the total exergy loss.

Figure 5.  Breakdown of air coolers exergy losses in the integrated structure of LNG-NGL-NRU

Figure 6.   Breakdown of towers’ exergy losses in the integrated structure of LNG-NGL-NRU

5.2. Exergoeconomic analysis

In TRR method, the capital cost of the system is estimated first. Next, some economical techniques are applied to calculate the revenue requirement of the system in $/h and then, the cost balance equations are written to calculate the unit cost of exergy for each stream. The exergoeconomic factor is defined and comments are made on the balance between the capital investment and operating costs of the system inflicted by the exergy loss which have to be compensated by more fuel consumption (Ghorbani et al (2016))

There exists a distinct algorithm able to obtain the abovementioned results, where:

1- All the components are put in descending order based on their importance, known from the magnitude of the sum  ,hence, the manner in modeling components based on their importance. C4 column with 5751872  has the greatest value. Accordingly, to improve performance , the components with high value of cost should be considered.

2- The exergoeconomic factor is applied in finding the prominent factor of the cost infliction as follows:

a) if the value of f is large, we should check whether it is economically justified to decrease the capital cost of the equipments, because it is assumed that the capital cost is so high that it has lost its economical justification. The exergoeconomic factor(s) of the process components are illustrated, where, HX8 has the greatest value of 89.49%.

b) if f is small, attempt should be made to increase the efficiency even if it yields higher capital cost, since it is assumed that low efficiency of the system inflicts a high expenditure on the system.

Table 9. Components, based on their f values

6. Sensitivity Analyses

In order to choose the appropriate decision variables of the system it is necessary to capture determine the behavior of the objective function with respect to decision variables. Since much of the electrical energy consumption of the plant the compressors, optimizing and reducing their exergy loss will lead to a more economical approach of the entire system.

The cost of exergy loss and exergoeconomic factor of C4, HX2, HX3 and HX3 versus pressure ratio of compressor C4, respectively are shown in Figs. (7 and 10). Cost of exergy destruction of of C4, HX2, HX3 and HX3 is subject to pressure ratio; while, the decrease in C4, HX2, HX3 and HX3 exergoeconomic factors is subject pressure ratio. The C4 exergy destruction rate increases with a higher rate compared to the other components because with an increase in the C4 pressure ratio its power increases directly and the cost of exergy destruction increases at a higher rate. The cost of exergy loss and exergoeconomic factor of C1, AC2 and HX4versus pressure ratio of compressor C1, are shown in Figs. (11 and 12), respectively. The cost of exergy destruction of  C1 decreases and cost of exergy destruction of AC2 and HX4 increases as pressure drops. The exergoeconomic factor of AC2 and HX4 decrease while the exergoeconomic factor of C1 increases with pressure ratio.

Figure 7. Variation of exergy loss and exergoeconomic factor of C4 with respect to compression ratio in C4 compressor

Figure 8.Variation of exergy destruction and exergoeconomic factor of HX2 with respect to compression ratio in C4 compressor

Figure 9. Variation of exergy loss and exergoeconomic factor of HX3 with respect to compression ratio in C4 compressor

Figure 10.Variation of exergy destruction and exergoeconomic factor of HX4 with respect to compression ratio in C4 compressor

Figure11.Variation of exergy loss and exergoeconomic factor of C1 with respect to compression ratio in C1 compressor

Figure 12.Variation of exergy destruction and exergoeconomic factor of AC2 with respect to compression ratio in C1 compressor.

Figure 13. Variation of exergy loss and exergoeconomic factor of HX4 with respect to compression ratio in C1 compressor