Comprehensive pinch-exergy analyzes of the NGL recovery process

1- Introduction

The amount of natural energy resources is decreasing while human's need for energy has increased, especially for industries with high energy demand, such as oil, gas, and natural gas liquids (NGL) refineries (Dong, Xu, Li, Quan, & Wen, 2018). The rising global need for energy sources, especially in industries like oil and gas leads to an increase in the production of natural gas. Natural gas is expected to supply 30 % of the world's supply of fossil fuels by 2030 (Tesch, Morosuk, & Tsatsaronis, 2016). On the other hand, considering high greenhouse gas emissions, and the existence of many pollutants, policies and environmental controls are forcing oil and gas industries to reduce the impacts of fossil fuel CO₂ emissions(Wang et al., 2017). For solving this problem, many countries have sufficiently tried toward controlling the rise in global temperature and preventing climate change. Due to limitations caused by the environmental effect of CO₂ emission, natural gas is applied as the cleanest fossil fuel, and its consumption is growing rapidly (B Ghorbani, Salehi, Ghaemmaleki, Amidpour, & Hamedi, 2012). According to the 2018 statistical report of BP World Energy Magazine, Iran with 16.2% of the proven natural gas reserves is ranked as the world's second nation with the highest natural gas reserves (Petroleum, 2019). This shows a promising future for its natural gas and NGL recovery industries. In this regard, Iran's natural gas production has increased rapidly over the past two decades, from 0.9 Tcf in 1991 to 1127.7 Tcf in 2018 (Petroleum, 2019).

It is worth mentioning that, energy consumption in each country is one of the important indicators of economic development. But, energy quality is more important than energy consumption, influencing the economic growth of that country. In designing a plant, the designer's main goal is determining the optimal state of energy consumption in relation to environmental and operating conditions, which can be done through exergy analysis (Khoshgoftar Manesh, Amidpour, & Hamedi, 2009). In this regard, many studies have been done on the evaluation of oil, gas, and chemical plants.

 Feyzi et al.,(Feyzi, Beheshti, & Intensification, 2017) performed exergy analysis on the performance of reactive distillation column in a plant of acetic acid production. Song et al.,(Song, Lin, & Wu, 2019) employed extended exergy analysis to investigate a typical cement production chain in China. Vilarinho et al.,(Vilarinho, Campos, & Pinho, 2017) appraised exergy and energy analysis for a pre-distillation unit (Un-0100) of an aromatics plant from a Portuguese refinery. Navarro et al.,(Leal-Navarro, Mestre-Escudero, Puerta-Arana, León-Pulido, & González-Delgado, 2019) evaluated exergetic performance of the Amine Treatment Refinery Unit in Colombia. Feyzi et al.,(Feyzi, Beheshti, & Kharaji, 2017) considered conventional exergy analysis(CEA) for assessing the CO₂ removal process from syngas using methyl diethanolamine activated by piperazine (a-MDEA).

NGL is also extracted for petrochemical companies as their primary feed. NGL recovery is mostly among cryogenic processes in Iran and the industrial propane cooling cycle is the main part of these plants. High energy consumption is the most important problem of NGL production technologies, especially in the refrigeration cycle(Bahram Ghorbani, Shirmohammadi, & Mehrpooya, 2018). Exergy analysis in such plants allows determining the most inefficient parts of a process where energy is wasted(Safarvand, Aliazdeh, Samipour Giri, & Jafarnejad, 2015). Raising the quality level of energy consumption is logical to improve the efficiency of these plants (Ansarinasab & Mehrpooya, 2017).

Throughout the last decades, many researchers have performed the CEA method on NGL plants to evaluate improvement priorities(Ansarinasab & Mehrpooya, 2017). In this regard, Mehrpooya et al.,(Mehrpooya, Gharagheizi, & Vatani, 2009) considered the CEA method in NGL1300, one of the biggest NGL recovery units in southern Iran. Jiang et al.,(Jiang, Zhang, Jing, & Zhu, 2019) performed the CEA method on China’s ethane recovery processes based on rich gas. Hu et al.,(Hu et al., 2019) studied the NGL plant equipment and found that air cooler contributed to the highest exergy destruction. Meanwhile, a new analysis method called advanced exergy analysis (AEA) has been employed in recent years to provide useful information for the identification of system behavior(Anvari, Saray, & Bahlouli, 2015). Tsatsaronis in a study performed the AEA method for the first time (Tsatsaronis, 1999). This method has been used for chemical and non-chemical industries.

Acikkalp et al.,(Açıkkalp, Yucer, Hepbasli, & Karakoc, 2014) performed the AEA method on milk processing facilities and they found that the evaporator had the highest avoidable exergy destruction. In another study, results of performing the AEA method on the Kalina cycle showed that the cycle had a great potential for improvement by increasing performance efficiency of condenser, turbine, and evaporator, respectively (Fallah, Mahmoudi, Yari, & Ghiasi, 2016). Jiang et al.,(Zhang et al., 2020) studied advanced exergy destruction in three improved schemes based on the recycle split vapor( RSV) ethane recovery process with the gas feed in a Chinese large-scale ethane recovery plant. They showed that improving compressor efficiency is the most effective measure to reduce process exergy destruction because of its high proportion of Ėx_D^AV,EN. Acikalp et al.,(Açıkkalp, Aras, & Hepbasli, 2014) considered the AEA method for analyzing electricity generation plant in Turkey's Industrial Zone. Their results showed that performance of gas turbine and combustion chamber should be improved to reduce their exergy destruction.

Furthermore, Feng et al.(Feng & Zhu, 1997) introduced combined pinch and exergy analysis to complete the analysis. This combined analysis was used in many chemical industries which require simultaneous analysis of heat and power. Kalantar et al.,(Kalantar-Neyestanaki, Mafi, & Ashrafizadeh, 2017) used the CPEA method to optimize existing multi-stage cooling cycles in a gas refinery by considering component performance limitations and interactions between the cooling cycle and the core process. This new scheme resulted in a 15.4% reduction in specific power consumption. Ataei et al.,(Ataei & Yoo, 2010) used this CPEA to study the Olefin plant and its cooling cycles. By reducing the temperature values of the refrigeration cycle of an Olefin plant, he reduced the production of the refrigeration cycle by 2,553 kW.

Furthermore, this technology is used for the refrigeration system of NGL plants that, in addition to thermal energy, also deals with the power or axial work of compressors. However, not much research was done to optimize energy consumption and reduce the workload of NGL plants. Ghorbani et al.,(B Ghorbani et al., 2012) examined the NGL plant and its propane refrigeration cycle using a CPEA method. As a result of this study, the work of the refrigeration cycle compressor was reduced by 170 kW. Moreover, the replacement of the refrigerant from the propane to the R-600a resulted in an 11.5 % reduction in the refrigerant flow and a reduction in the axial performance of compressors by 570 kW. Mehrpooya et al.,(Mehrpooya, Jarrahian, & Pishvaie, 2006) have investigated the behavior of an Iranian NGL plant 1300 and its propane cooling cycle by the CPEA method. The efficiency of the cryogenic cycle was set at 27%, showing significant opportunities for improvement. The results showed that refrigerant coolers and chillers have the lowest exergy efficiency among other refrigeration cycle components. Mehdizadeh et al.,(Mehdizadeh-Fard & Pourfayaz, 2019) used the AEA method to identify the advanced exergy destruction of HEN in a Gas Refinery Complex on an Iranian gas field in South Pars. Their study results indicated that the exergy destruction of the system was avoidable, and optimization methods could modify it. Hence, the potential to improve the operating costs at this plant is high. Hackl et al.,(Hackl & Harvey, 2013) studied the CPEA method for the refrigeration cycle of an NGL plant on the west coast of Sweden. The results revealed that by performing the first scenario of energy-saving optimization, 1.5 MW in the axial operation of the compressors would be saved. Furthermore, in the second scenario, it is possible to save by cooling the cooler from two other places outside the plant will save an additional 2.5 MW of axial work. The economic evaluation of proposed scenarios represents a payback period of about 4 years. Raei(Raei, 2011) aimed to minimize the amount of axial work of the NGL plant refrigeration cycle. After optimizing by the CPEA method, the refrigeration and fuel consumption decreased by 24.38 GJ/h. Besides, the profit from this optimization of the economy was estimated at 1,174,534 $ per year. Mehdizadeh-Fard et al.,(Mehdizadeh-Fard, Pourfayaz, Mehrpooya, & Kasaeian, 2018) suggested a new approach of Heat Exchanging networks using the AEA method and considering ∆T_min= 0 °C in the ECC diagram For a Natural Gas refinery complex. Considering avoidable exergy destruction in the retrofitted HEN, it was shown that CPEA was shown to lead to an 88% efficiency output which is more than 78% higher than the existing network.

This study considers the energy quality of NGL plant No. 800 from National Iranian South Oil Company (NISOC) with a production capacity of 120,000 NGL barrels per day located in Ahvaz, Koreit Industrial Zone as an actual case study in different methods. In this regard, CEA, AEA, CPEA, and as an innovation, CPAEA were performed on the current state of NGL plant No.800. As an innovation, CPAEA was used to simultaneously show the energy consumption rate and the advanced exergy destruction of the heat exchanging network of the current state of NGL plant No.800.

 

Process description

Fig. 1 shows PFD for the current operating condition of the NGL plant.

 

 

 

 

Fig. 1 PFD of NGL plant

 

 

According to the process flow diagram, the NGL plant 800, located in the industrial city of Ahvaz, has one input feed and two output productions including NGL and sales gas. The feed stream enters demethanizer column after cooling down to -23.3 ˚C by a triple heat exchanger (E101, E102 in the cryogenic cycle and E100 refluxed feed stream). After extraction in the demethanizer column, the sale gas and exchanged gas in the heat exchanger (E-100) will be sent to pressure-boosting units. The NGL from the bottom of the demethanizer column will be sent to petrochemical companies at 48 °C and 63 psi for other uses. In this plant, the propane cryogenic cycle completely separated from the production process is used for the procession and cooling of the NGL product. Its streams can be seen in Fig. 1 marked with the letter "P". This cycle is pressurized up to 23.84 bars by a low-pressure compressor (K-101), medium-pressure compressor (K-102), and high-pressure compressor (K-103). Economizer towers (V-102, V-103, and V-104) separate propane gas (to return to compression system) from liquid propane, which continues heat exchanging in the cryogenic cycle. Inlet feed streams and outlet product streams will exchange heat with liquid propane by heat exchangers (E-101, E-102, and E-103).

Processing is completed by the condenser (E-105) and cooler (E-104). Cooler provides the required heat for reboiler of the demethanizer column and condenser cools down the pressurized propane to 65.55 °C.

The data were collected in accordance with the Iranian Petroleum Standards (IPS-E- PR-170) ("ENGINEERING STANDARD FOR PROCESS FLOW DIAGRAM- IPS-E-PR-170," 1996). Peng-Robinson equation of state (PR EOS) was selected for determining the thermodynamic properties of the NGL plant. This state equation has been used in the previous simulations of the NGL plants (B Ghorbani et al., 2012). For simulating in Aspen HYSYS software, the operating conditions of the NGL plant are listed in Table 1.

The following assumptions were used to simulate the NGL plant:

• Peng Robinson equation of state is applied, to predict governing equations and calculate the balances in the simulation are considered.
• The NGL simulation is performed in a steady state.
• Pipelines are assumed to be insulated in the HEN and do not have heat loss.

Feed and product specifications and simulation deviations from operating conditions are also shown in Table 2. The error rate in this simulation indicates that the simulation  complies with operating conditions.

 

 

Table 1: The operating conditions and specifications of the NGL plant components.

Rotary machines
	Operating type			Isentropic efficiency. (%)		Power (kW)			ΔP (kPa)	P ratio (-)			Head [m]
K-101	Centrifugal			75		568.92			237.15	2.48			4273
K-102	Centrifugal			75		1393.59			384.85	1.97			3330
K-103	Centrifugal			75		2447.37			1601.13	3.046			5719
P-100	Centrifugal			75		260.97			4136.85	2.88			849
Heat Exchangers
	Type			Minimum approach (ºC)		LMTD (ºC)			Power (kW)	Cold Pinch Temp. (ºK)			Heat exchanging area(m2)
E-100	Shell and tube Horizontal-AEL			10.645		28.80			1550.55	308.62			66.1
E-101	Shell and tube Horizontal-AEL			5.556		18.26			6548.00	267.57			61.9
E-102	Shell and tube Horizontal-AEL			7.791		16.84			3663.56	242.04			61.6
E-103	Shell and tube Horizontal-AEL			5.555		10.11			411.62	315.67			71.6
Cooler and heaters
		Type			ΔT(ºC)			ΔP (kPa)				Power (kW)	Heat exchanging area(m2)
E-104		Cooler			-50.44			0				4259.92	532
E-105		Cooler			-22.76			0				9949.96	2761
Column
		Type	Number of stages				Feed stage		Tray/Packed Space (m)		Tray/Packed Volume (m3)		Tower diameter (m)
T-100		demethanizer	10				1		0.5		0.883		1.5
		Type	orientation				ΔP (kPa)		Volume (m3)		Diameter (m)			phases
V-100		Separator	horizontal				13.79		2.86		1.066		3
V-101		Separator	vertical				0		12.95		1.67		2
V-102		Separator	vertical				0		26.42		1.82		2
V-103		Separator	vertical				0		47.80		2.59		2

 

Table 2: The operation conditions of the NGL plant.

Stream No.	Temperature [ºC]	Pressure [bar]	Molar Flow [kmol/s]
15(Feed stream)	46.1	24.9	0.15
16	37.2	24.5	0.61
17	0	24.2	0
18	-23.3	23.9	0.76
19	-23.4	23.7	0.76
20	-23.5	23.7	0.03
21	-23.4	23.7	0.74
22	-24.5	22.1	0.46
23	-23.6	23.6	0.28
24	-23.6	23.6	0.46
25	35.5	22.5	0.28
26	-24.6	21.9	0.46
27(Sale gas)	25.4	21.9	0.24
28	37.9	22.1	0.24
29	42.5	63.4	0.24
30(NGL product)	48.9	63.4	0.5
P1	65.6	23.8	0
P2	65.6	23.8	0.24
P3	65.6	23.8	0.24
P4	48.1	23.8	0.74
P5	15.1	23.8	0.74
P6	88.3	23.8	0.76
P7	22.1	23.8	0.76
P8	17.5	7.8	0.76
P9	17.5	7.8	0.15
P10	17.5	7.8	0.61
P11	17.5	7.8	0
P12	17.5	7.8	0.76
P13	-5.6	4	0.76
P14	-5.6	4	0.03
P15	-5.6	4	0.74
P16	-5.6	4	0.46
P17	-31.1	1.6	0.28
P18	-31.1	1.6	0.46
P19	-5.6	4	0.28
P20	-31.1	1.6	0.46
P21	-31.1	1.6	0.24
P22	8.4	4	0.24
P23	-1.1	4	0.24
P24	30.4	7.8	0.5
P25	30	7.8	0
P26	88.3	23.8	0.24
P27	88.3	23.8	0.24

 

Table 3: Specifications of the plant's production streams, operational conditions data and error analysis of simulator output.

Stream number		Sale gas (27)						NGL Product (30)
		Simulation			Real		Error (%)		Simulation		Real		Error (%)
Temperature (°C)		25.44			25.49		0.19		48.89		49.01		0.24
Pressure (bar)		21.86			21.81		0.22		63.43		63.49		0.09
Mass flow (kg/s)		13.92			13.91		0.07		23.51		23.53		0.08
Composition(mole fraction)
H2O	0			0			0		0		0		0
H2S	0			0			0		0		0		0
CO2	0.006			0.006			0.2		0.003		0.003		0.846
Methane	0.774			0.774			0.013		0.025		0.026		0.407
Ethane	0.17			0.169			0.177		0.278		0.273		1.756
Propane	0.042			0.043			1.408		0.334		0.333		0.24
i-Butane	0.003			0.003			0.793		0.064		0.065		0.619
n-Butane	0.005			0.005			0		0		0		0
i-Pentane	0.001			0.001			0		0.18		0.18		0
n-Pentane	0			0			0		0.044		0.044		0
n-Hexane	0			0			0		0.031		0.032		0.317
n-Heptane	0			0			0		0.024		0.024		0.043

 

 

 

• Conventional exergy analysis

The exergy analysis method is a key issue for a better understanding of the locations, causes, and magnitudes of the process inefficiencies (Safarvand et al., 2015). CEA is a useful technique for evaluating the performance of chemical processes (Bahram Ghorbani, Hamedi, Amidpour, & Engineering, 2016). The main purpose of the designer in designing a plant is to determine the optimal state of energy consumption in relation to the environmental and operating conditions of the plant. CEA determines the most inefficient equipment and shows where the energy is being wasted in operating conditions (Khoshgoftar Manesh et al., 2009). Therefore, it is important to determine ambient conditions for conducting exergy analysis. As a real-life case study, NGL plant 800 from National Iranian South Oil Company (NISOC) with the production capacity of 120,000 NGL barrels per day located in Koreit Industrial Zone(Ahvaz City, Khuzestan Province, Iran)was chosen. Average ambient conditions in Ahvaz City were assumed as T₀ = 25 ˚C and P₀ = 101.325 kPa ("Ahvaz municipality official web site ").

According to Equation (1), the total exergy of the system for the material stream is split into four parts, namely kinetic (Ėx_ke), potential (Ėx_po), physical (Ėx_ph), and chemical (Ėx_ch) exergies (Ansarinasab, Mehrpooya, & Parivazh, 2017). The potential and kinetic exergies are neglected (Ansarinasab, Mehrpooya, & Mohammadi, 2017).

(1)

So, the material stream exergy rate is defined as the sum of chemical and physical parts (Ali Vatani, Mehdi Mehrpooya, & Ali  Palizdar, 2014b).

(2)

Physical and chemical exergy are defined according to Eqs. (3) and (4) (Ansarinasab, Mehrpooya, & Mohammadi, 2017):

	(3)
	(4)

Where, "0" subscription refers to an ambient condition in the above equations. T₀, h₀, and s₀ are the reference ambient temperature, specific enthalpy, and specific entropy, respectively, in Eq. (3). and G_i are the standard chemical exergy and Gibbs free energy for chemical exergy, respectively, in Eq.(4). (Ansarinasab, Mehrpooya, & Mohammadi, 2017).

After obtaining these parameters, exergy destruction and exergy efficiency are two main parameters of the process, which are required to be defined in exergy analysis (Bahram Ghorbani, Hamedi, & Amidpour, 2016). These essential parameters are investigated and discussed for the kth component of the process components by equations (5) and (6).

	(5)
	(6)

Where, P, D, and F represent the product, destruction, and fuel in these equations, respectively. According to the fuel-product methodology, Table 4 presents exergy calculation formulas in the main component of the NGL plant. Table 5 shows the result of conventional exergy. Also, the Grassmann diagram is a graphical illustration of the exergy flows of a system. In this diagram, the quantitative and qualitative evaluation of the exergy loss of equipment is made according to the first and second laws of thermodynamics (Carrero, De Paepe, Bram, Parente, & Contino, 2017). And it helps the reader to easily identify where the system's highest exergy destruction is located (Jankowiak, Jonkman, Rossier-Miranda, van der Goot, & Boom, 2014). The width of flow arrows represented the amount of streams exergy. According to fig. 2, the NGL plant exergy flow rate and exergy destruction of the equipment are modeled in e! Sankey Pro.

Validation of exergy analysis calculations is based on its main parameters (temperature, pressure, composition). This validation in relation to the operating condition is given in Table 3.

According to table 6, Fig. 3 shows the Grassmann flash diagrams for conventional exergy destruction of the main components on the NGL plant. The highest exergy destruction rate belonged to compressors K103 and heat exchanger E-101 with 510 and 629 kW respectively. Exergy destruction percentage of other equipment was at the least level to be considered for improvement. Also, the pie chart regarding the result of assessing the exergy destruction rate of the main components is in Fig. 4.

 

 

 

 

 

Table 4: Exergy calculation formulas in the main component of the NGL plant.

Compressor (Mehrpooya & Shafaei, 2016)	Exergy destruction
	Exergy efficiency
Heat exchanger(B Ghorbani et al., 2012)	Exergy destruction
	Exergy efficiency
Column/Tee/ Mixer(Bahram Ghorbani, Mehrpooya, Hamedi, & Amidpour, 2017)	Exergy destruction
	Exergy efficiency
Cooler(Bahram Ghorbani et al., 2018)	Exergy destruction
	Exergy efficiency
Pumps(Ali Vatani, Mehdi Mehrpooya, & Ali   Palizdar, 2014a)	Exergy destruction
	Exergy efficiency
Expansion valves(Vatani et al., 2014a)	Exergy destruction
	Exergy efficiency

Table 5:Summarized conventional exergy for process and cryogenic cycle.

Stream No.	Physical exergy(kW)	Chemical exergy(kW)	Total exergy(kW)
15(Feed stream)	8672.3	1834696	1843368
16	8559.3	1834696	1843255
17	8678.5	1834696	1843375
18	9170.4	1834696	1843867
19	4536.2	589253	593789.2
20	3863.6	1246656	1250520
21	1.8	34.1	35.9
22	3854	1246656	1250510
23	0.1	20.4	20.4
24	4529	589232.6	593761.6
25	4358.7	589232.6	593591.3
26	855.6	114293.6	115149.2
27(Sale gas)	5145.2	703526.2	708671.4
28	2334.7	1132771	1135105
29	2526.5	1132771	1135297
30(NGL product)	2553.2	1132771	1135324
P1	4338.2	1650154	1654493
P2	853.2	324531.5	325384.7
P3	3485	1325623	1329108
P4	813.2	324531.5	325344.7
P5	3264	1325623	1328887
P6	0	0	0
P7	4049.3	1650154	1654204
P8	3942.2	1650154	1654096
P9	125.6	56791.3	56916.9
P10	3816.6	1593363	1597180
P11	2371.3	989974.3	992345.6
P12	1445.3	603388.7	604834
P13	2300.2	989974.3	992274.5
P14	1402	603388.7	604790.6
P15	1551.9	989974.3	991526.2
P16	1254.2	509105.2	510359.4
P17	1210.6	509105.2	510315.8
P18	361.3	509105.2	509466.5
P19	1699.7	1084258	1085958
P20	0	0	0
P21	361.3	509105.2	509466.5
P22	777.3	509105.2	509882.5
P23	2472.5	1593363	1595835
P24	3520.6	1593363	1596884
P25	3645.6	1650154	1653800
P26	5583	1650154	1655737
P27	5583	1650154	1655737

 

 

Fig 2: Grassmann diagram of the of the components exergy flow rate.

 

Table 6: Conventional exergy results of main equipment.

Exergy efficiency (%)	Exergy destruction (kW)	Component
		Rotary machines
73.49	69.19	P100
73.13	152.85	K101
75.21	345.48	K102
79.16	509.99	K103
		Heat exchangers
66.31	283.25	E100
15.93	629.04	E101
57.91	357.46	E102
66.88	13.24	E103
90.33	420.65	E104
84.5	228.39	E105
		Columns
80.01	455.39	T100

 

 

Fig 3.Grassmann diagram of the main component exergy destruction.

 

 

Fig. 4 Exergy destruction pie charts of the main component.

 

• Advanced exergy analysis

The inefficiency of a system can be defined by the CEA method. While irreversible resources and real potentials for system improvement can only be identified by the AEA method. It is possible to better identify values of exergy destruction and ways to improve it by splitting the concept of exergy. This is possible only using the AEA method (Balli, 2017). This analysis splits conventional exergy destruction into two exogenous and endogenous parts according to the origin, and also into unavoidable and avoidable parts according to the ability to remove and modifications.

The endogenous exergy destruction is based on the irreversibility rates occurring within the kth component when all other components operate without irreversibility rates (theoretically).

According to the exogenous and endogenous exergy destruction definitions, exergy destruction of the kth component can be formulized as below:

(7)

Where  represents endogenous exergy destruction and can be calculated by two methods. Including the engineering method and an approach based on the thermodynamic cycle (Salehzadeh, Saray, & JalaliVahid, 2013). In this paper, an engineering approach was used. For using the engineering approach, Fig. 5 provides more details about for each process component.

These schematics represent the destruction of the total plants҆ exergy due to exergy destruction in other components, except for component k. In this method, since  of the component depends on the component's exergy efficiency, the exergy efficiency of component k must be constant, whilst exergy destruction varies in other components, and the graph should have a straight line and not a curve (Kelly, Tsatsaronis, & Morosuk, 2009). The intersection of this diagram with the vertical axis shows the endogenous exergy destruction value of a component k.

 

Fig. 5 Plot obtained from the engineering approach to calculate endogenous exergy destruction(Fallah, Siyahi, et al., 2016).

 

Table 7 shows assumptions of actual, theoretical, and unavoidable conditions to calculate  and  for the main equipment. Theoretical operation conditions should be in accordance with the assumptions (Ėx_D = min or Ėx_D = 0). Whilst, simulation of unavoidable operating conditions depends on the manufacturer's experience and knowledge. It should be noted that technical and economic constraints (manufacturing methods, production costs, and material characteristics) prevent the achievement of ideal equipment conditions. In this study, computations of the advanced and conventional exergy and simulations of all the needed basic conditions and the system assumptions were carried out in Aspen HYSYS, Microsoft Excel, and MATLAB software.

 

 

Table 7:Assumptions for calculating endogenous and unavoidable exergy destruction the actual, theoretical and unavoidable conditions (Kelly, 2008).

Components, k	Actual conditions	Theoretical conditions	Unavoidable conditions
Pump	= 75%	= 100%	= 90%
Compressor	= 75%	= 100%	= 90%
Heat exchanger	∆Tmin=real	∆Tmin=0 K	∆Tmin=0.5 K
	∆P=real	∆P=0	∆P=real

 

 

The following equation calculates exogenous exergy destruction by measuring endogenous exergy destruction value.

(8)

Figs. 6 and 7 show the results of measuring endogenous advanced exergy of rotating and heat exchanging equipment.

 

 

Fig. 6 Calculation of the endogenous exergy destruction for heat exchanging equipment.

 

 

Fig. 7 Calculation of the endogenous exergy destruction for pump and compressors.

 

 

Based on the possibility of eliminating the irreversibility of the equipment and achieving a realistic measure of improvement potential, the total exergy destruction of the equipment k is split into two parts, unavoidable and avoidable. The exergy destruction rate that is not reducible due to technical constraints, such as material quality, production methods, and design parameters is considered as an unavoidable part of the exergy destruction and is avoidable exergy destruction that can be avoided. These definitions are formalized as follows:

	(9)
	(10)

These splitting are combined to provide a better understanding of their effect on the system and options for improvement of the overall system efficiency and consequently, we will be able to determine which part of the inefficiencies caused by interactions between components, and which part can be prevented by improving plants҆ technology (Liu, He, & Saeed, 2016). Therefore, exergy destruction is divided into four main groups including (i) avoidable-endogenous exergy destructions, (ii) unavoidable-endogenous exergy destructions, (iii) avoidable-exogenous exergy destructions, and (iv) unavoidable-exogenous exergy destructions. The algorithm for the division of exergy destruction into four main groups is shown in Fig. 8 for a better explanation. These new exergy destruction terms can be illustrated as follows:

(11)

Fig. 8 summarizes a flow diagram for a better understanding and comparison of CEA and AEA applied for the exergy destruction rate in the kth component.

 

 

 

Fig. 8.The conventional and AEA flow chart.

 

 

The is the unreduced part of exergy destruction due to technical and economic constraints of the kth component, and is formulized as Eq. (12) (Liu et al., 2016).

(12)

Similarly, the unavoidable exergy of equipment k, which is unreduced because of economic and technical limitations of other components of the process is called as the and can be formalized by the following equation (Mehrpooya & Shafaei, 2016).

(13)

Part of the avoidable exergy destruction that will be reduced by improving the performance of the k component is called avoidable endogenous exergy destruction and is formalized as equation 14:

(14)

Similarly, a reducible part of the avoidable exergy destruction by improving the efficiency of other process components is called the . And is shown as below:

(15)

Finally, the results of the AEA method applied for the main equipment of the NGL 800 plant can be detailed in Table 8 and the bar chart presented in Fig. 9.

 

 

Table 8:Detailed results of the advanced exergy analysis for main equipment.

Advanced exergy analysis (kW)								Conventional exergy analysis (kW)
ĖXD UN,EN	ĖXD UN,EX	ĖXD AV,EN	ĖXD AV,EX	ĖXD EX	ĖXD EN	ĖXD AV	ĖXD UN	ĖXD
Heat exchangers
167.14	28.49	73.69	13.93	42.42	240.83	87.62	195.63	283.25	E-100
529.58	2.41	76.26	20.79	23.2	605.84	97.05	531.99	629.04	E-101
126.6	1.49	213.81	15.56	17.05	340.41	229.37	128.09	357.46	E-102
10.5	0	0.94	1.8	1.8	11.44	2.74	10.5	13.24	E-103
Rotary machines
50.66	0.8	85.27	16.12	16.92	135.93	101.39	51.46	152.85	K-101
115.9	0	200.74	28.84	28.84	316.64	229.58	115.9	345.48	K-102
171.01	0.17	294.4	44.41	44.58	465.41	338.81	171.18	509.99	K-103
27	1.11	35.08	6	7.11	62.08	41.08	28.11	69.19	P-100

 

 

 

Fig 9. Detailed pie chart of the AEA for main equipment.

 

 

• Exergetic parameters

In the AEA, some exergetic parameters are used for evaluating the system, namely modified exergy efficiency, exergetic improvement potential ratio, and exergetic rehabilitation ratio (Chen, Zhu, Huang, Chen, & Luo, 2017).

	(16)
	(17)
	(18)

Where ɛ_modified, EIP_k and ERR_k are the modified exergy efficiency, exergetic improvement potential ratio, and exergetic rehabilitation ratio of the kth component respectively. The results of the exergetic parameters on the main equipment can be seen in Table 9.

 

 

Table 9: Exergetic parameters results of the main equipment.

EIPk	ERRk	εconv,k	εmodified,k
Rotary machines
2.83	2.21	73.13	82.99	K-101
10.39	5.2	75.21	83.93	K-102
8.81	7.63	79.16	84.97	K-103
13.07	0.91	73.49	84.54	P-100
Heat exchangers
123.05	1.91	66.31	29.73	E-100
85.27	1.98	15.93	9.92	E-101
29.21	5.54	57.91	69.7	E-102
30.78	0.02	66.88	96.62	E-103

 

 

According to Table 9, in equipment where the efficiency of exergy destruction in the conventional analysis is less than the efficiency in the advanced study. The efficiency of such equipment can be improved up to a greater value which is its actual value. For example, the ε for K-103 can be improved up to 85% denoted by ε_modified. Furthermore, ERR_E-102, ERR_K-102, and ERR_K-103 have the highest values, which suggests that this equipment has the top priorities for improvement in another way. Also, EIP_E-101 and EIP_E-102 have the highest value, because their ε_modified are the lowest among all the components.

Also, the strategy for improving the exergy destruction, in NGL plant equipment is expressed in Table 10. According to Table 10, the unavoidable conditions isentropic efficiency is assumed as 90% for compressors. This assumption is due to the limitations of compressors Construction technologies.

 

 

Table 10: Realistic strategies for reducing exergy destruction.

	Conventional exergy analysis(kW)		Advanced Exergy destruction categories (kW)			The part should be focused	Possible strategies to reduce exergy destruction
	ĖXD		ĖXD AV	ĖXDAV,EN	ĖXDAV,EX		Strategy A a	Strategy B b
Heat exchangers
E-100		283.25	87.62	73.69	13.93	EN,EX	√	√
E-101		629.04	97.05	76.26	20.79	EN,EX	√	√
E-102		357.46	229.37	213.81	15.56	EN,	√
E-103		13.24	2.74	0.94	1.8	EN,EX	√	√
Rotary machines
K-101		152.85	101.39	85.27	16.12	EN,EX	√
K-102		345.48	229.58	200.74	28.84	EN,	√
K-103		509.99	338.81	294.4	44.41	EN,	√
P-100		69.19	41.08	35.08	6	EN,EX	√	√
a Strategy A: Improve kth component efficiency, or replace the component with efficient devices. b Strategy B: Improving the efficiency of the remaining components.

 

 

Among the compressors, the highest exergy destruction belonged to the K103, with 503 kW (endogenous exergy of 461 kW). In this regard, for analyzing equipment improvement and reducing energy destruction, technical limitations were considered again. Fig. 10 displays change in the compressor's isentropic efficiency with endogenous exergy destruction

As can be seen, increasing compressors҆ efficiency increased the ĖX_D^AV,EN, and decreased the ĖX_D^UN,EN, showing that more attention shall be focused on improving the compressor's efficiency to reduce exergy destruction.

 

 

Fig. 10 Effect of isotropic efficiency on the exergy destruction within the compressor (K103).

 

 

• Combined pinch and exergy analysis

Pinch technology is widely used today, but the limitation of this technology is that pinch analysis only analyzes the thermal energy targeting systems. In other words, this technology is not used for systems such as refrigeration cycles of the NGL plants, which in addition to thermal energy, deals with power. In this way, with the proper combination of pinch and exergy analyses, a practical and useful solution can be achieved for such systems (Raei, 2011). This graphical analysis is suitable to show the energy consumptions and exergy destruction of the heat exchanging network.

In applying the pinch method design for HEN,  the correct stream must match with the pinch rules in order to achieve the minimum energy targeting. The main scope of pinch analysis is decomposing the HEN into above and below pinch points after identifying the pinch point, and considering pinch rules in two separated networks.

There are three main rules in the pinch analysis. These rules must comply to achieve the minimum energy target for the HEN.

1- Heat should not be transferred across the pinch points.

2-No external cooling above the pinch.

3-No external heating below the pinch.

Violating these three rules will result in increased energy requirements(March 1998).

There are also two sub-rules. For stream matching, the outgoing CP of the streams must be higher than incoming streams, and outgoing streams must be higher than incoming ones (Liew, Alwi, Klemešb, Varbanov, & Manan, 2014).

(3)
(4)

Thus, the results of inequality in CP rule and stream number for two regions of above and below the pinch are as follows (Liew et al., 2014):

	(5)
	(6)
	(7)
	(8)

 

If these two rules are not satisfied with the streams, then stream splitting is required as shown in Fig. 11 (Gundersen, 2013). Also, if needed, stream splitting is done for dividing an existing stream between two heat exchangers and using them more efficiently. Fig. 11 shows a brief description of the pinch rules procedure in the HEN.

 

 

Fig 11. A systematic approach for pinch rules(Ebrahimi, Ghorbani, & Ziabasharhagh, 2020).

The key tool for the combined pinch and exergy analysis technique is the ECC diagram which is accessible using the pinch analysis tool (CC diagram). The exergy composite curve graph is capable of displaying the exergy destruction of HEN. To achieve this graph, the vertical temperature axis of CC is converted to the Carnot factor to generate the exergy composite curve (ECC), as illustrated in Figs. 12 (Njoku, Egbuhuzor, Eke, Enibe, & Akinlabi, 2019).

 

The Carnot factor η_c , is obtained with E.q 29.

(19)

Where T₀ is the ambient temperature.

 

 

 

Fig.12 Exergy destruction in exergy composite curves(ECC) (Mehdizadeh-Fard et al., 2018).

 

 

Fig 13.Composite Curve (CC) chart for utilities, cold and hot streams of the NGL plant.

 

Fig. 14 The Grid Diagram of the HEN.

 

 

In this regard, NGL plant simulation is exported from Aspen Hysys to Aspen Energy Analyzer, and CC and Grid Diagrams are then obtained. Based on Fig. 13, the minimum hot and cold required utilities are equal to 411 and 10,211 kW, respectively. The pinch temperature difference is 10.645 °C. This parameter was calculated by introducing process and needed utility streams to Aspen Energy Analyzer.

The composite curve in Fig. 13 shows all involved utility and process streams in HEN. Utility streams are displayed as dash lines. (P17-P18), and (P13-P15) streams are the cold utilities and (P2-P4) is the hot utility used in HEN. This figure shows 4 heat exchangers in HEN involved through process flows or utility streams. The technical specifications of these heat exchangers are shown in Table 11.

The current state of HEN is represented in Grid Diagram in Fig. 14. This diagram simply shows the HENs. It also shows thermal utilities and process heat recovery duties, target and supply temperatures (in °C) of streams, and pinch hot and cold temperatures. Moreover, it considers the main rules of pinch analysis.

As shown in Fig. 14, the cold and hot pinch temperatures of the current HEN are equal to 35.46 and 46.11 °C, respectively. There is no cross-pinch stream. Also, there is no hot utility below the pinch and no cold utility above the pinch. All of the main pinch rules are met in the current state, and there is no need for any correction in retrofit Pinch Analysis.

Due to heat exchangers' technical specifications in the real condition, the ECC diagram was plotted as Figure 15. According to previous explanations, the enclosed area among the composite curves (process or utility) illustrates the exergy destruction value. The technical specifications of heat exchangers and their conventional exergy destruction can be seen in Tables 1 and 11, respectively. Therefore, the amount of hot and cold utility consumption is 410 and 10210 kW, respectively, in Figure 15. The result of these analyzes shows that the heat exchanger (E-101) has the highest exergy destruction.

 

 

 

 

Fig 15.Exergy Composite Curve (ECC) chart for utilities, cold and hot streams of the NGL plant.

 

 

• Combined pinch and advanced exergy analysis

Due to defining the ability of modifications in these heat exchangers and according to the assumption table, heat exchanger exergy destruction is split into unavoidable and avoidable parts. As a new graphical concept, avoidable and unavoidable exergy destruction is introduced in the heat load vs. Carnot factor diagram at assumed ΔTmin=0.5 °C (according to table 7). Based on this method, the graphical exergy destruction splits into two avoidable and unavoidable parts based on the ability to remove and modifications. This graph is named Combined Pinch and Advanced Exergy Analysis (CPAEA). This method graphically illustrates the exergy destruction of HEN heat exchangers in unavoidable and avoidable parts. In this regard, the CPAEA form is presented as in Fig.16. In this figure, ĖX_D^AV and ĖX_D^UN of each heat exchanger, along with the energy consumption of HEN, are displayed.

The graphical form of AEA is based on the distribution of inlet and outlet temperatures of the heat exchanger according to ΔTmin characteristics. For each heat exchanger, all enclosed areas are based on the real ΔTmin, and the hatched area is based on the assumed ΔT_min=0.5 °C. Figure 16 shows that the percentage of ĖX_D^UN in the E-102 heat exchanger is higher than other heat exchangers. Based on the results of Table 8 analyses, the E-102 exchanger has a better improvement potential than other exchangers. In this regard, to analyze equipment improvement and reduce energy destruction, technical limitations were considered again. Fig. 17 shows the heat exchangers' endogenous exergy destruction variation with ΔTmin.

It could be seen that increasing ΔTmin efficiency increases Ėx_D^AV,EN, and decreases the Ėx_D^UN,EN exergy destruction part. It shows that more attention shall be concentrated on improving the heat exchangers' performance due to reducing the exergy destruction.

 

Conclusion

In the current study, the results of applying two practical ways of exergy analysis including CEA, AEA, CPEA, and CPAEA, methods were presented. These methods were used for a better understanding of locations and causes of inefficiencies and ways for their improvement in a typical, large-scale NGL recovery plant located the southwest of Iran. Summary of exergy analyses performed for this plant and real potentials for improvement are given in the following.

• The results of CEA illustrated that the highest amount of exergy destruction belonged to compressors (K103) and heat exchangers (E-101) with 510 and 629 kW, respectively, showing that more attention shall be focused on improving this equipment to reduce exergy destruction.
• CPEA diagram can simultaneously show the energy consumption rate and the exergy destruction of the heat exchanging network. According to the ECC diagram, there is no cross-pinch stream. Hence, there is no hot utility below the pinch and no cold utility above the pinch. Thus, all main pinch rules are met, and despite the plants' life (more than 40 years), there is no need for any modification in Pinch Analysis. The plant's minimum hot and cold required utilities are equal to 411 and 10,211 kW, respectively, for ΔTmin of 10.645 °C. Also, this diagram shows that the exergy destruction of the heat exchanger (E-101) is higher than the other heat exchangers.
• To have a better understanding of exergy destruction of main equipment, advanced exergy analysis was performed. The exergy destruction of the compressor (K103) and heat exchanger (E-102) will reduce by improving the performance of these components. However, for (E-101) heat exchanger, despite having the highest rate of exergy destruction, is not in the priority of modification due to its low level of avoidable exergy destruction.
• As a new graphical concept, avoidable and unavoidable exergy destruction can be shown in the CPAEA diagram (at assumed ΔTmin=0.5 °C). Based on this method, the graphical exergy destruction splits into two Ėx_D^AV and Ėx_D^UN, based on the ability to remove and The results show that the heat exchanger (E-102) has the highest priorities for improvement. Moreover, Ėx_D^AV,EN will decrease with a concentration on improving the heat exchangers performance.

 

 

 

Fig 16.CPAEA chart for utilities, cold and hot streams of the NGL plant.

Fig. 17 Effect of ΔTmin on the exergy destruction within the heat exchanger (E-102).

Table 11:Specification of the performance of heat exchangers.

E-100				E-101	E-102	E-103
Tema type			Shell and Tube - AEL
Orientation			Horizontal
	Power	kW	1551	6548	3663	411
Real condition	ΔTmin	°C	10.645	5.557	7.791	0
	Hot Pinch Temp.	°C	46.111	-0.022	-23.32	0
		ηc	0.066	-0.092	-0.193	0.072
	Cold Pinch Temp.	°C	35.465	-5.579	-31.111	0
		ηc	0.034	-0.114	-0.232	0.056
Unavoidable condition	ΔTmin	°C	0.5
	Hot Pinch Temp.	°C	46.111	37.222	-30.611	0
		ηc	0.066	0.039	-0.229	0.057
	Cold Pinch Temp.	°C	45.611	36.722	-31.111	0
		ηc	0.065	0.038	-0.232	0.056

 

Abbreviations

Nomenclature

Subscripts

AEA

Advanced exergy analysis

Ke

Kinetic

°C

Temperature in Celsius

Po

Potential

CC

Composite curves

Ph

Physical

CP

Specific heat at constant pressure (kJ/kg-C)

Ch

Chemical

CEA

Conventional exergy analysis

0

Ambient condition

CPAEA

Combined Pinch and advanced exergy analysis

D

Destruction

CPEA

Combined pinch and exergy analysis

P

Product

Standard chemical exergy

F

Fuel

Ėx

Exergy destruction (kW)

L

Loss

Ė

Exergy destruction (kW)

i

Inlet

e

Exergy destruction rate (J/kmol)

in

Inlet

ECC

Exergy composite curves

is

Isentropic

EIP

Exergetic improvement potential ratio

o

Outlet

ERR

Exergetic rehabilitation ratio

out

Outlet

Gi

Gibbs free energy

C

Cold

h

Specific enthalpy (kJ/kmol)

H

Hot

HEN

Heat Exchanging Network

Superscript

K

Temperature in Kelvin

EX

Exogenous

kPa

Pressure in Kilopascal

EN

Endogenous

kW

Power in Kilowatt

AV

Avoidable

Molar Flow rate (kmol/s)

UN

Unavoidable

N

Number of streams

Abbreviations used for process flow diagrams:

MW

Power in Megawatt

E

Heat exchanger, Reboiler and condenser

NGL

Natural Gas Liquid

K

Compressor

PFD

Process flow diagram

MIX

Mixer

P

Propane

P

Pump

P

Pressure in Kilopascal

T

Demethanizer column

Q

Heat load (kW)

TEE

Tee

s

Specific entropy (kJ/kmol.K)

V

Separators and Flash drum

T

Temperature (K)

VLV

Throttle valve

Tcf

Trillion cubic feet

W

Work (kW)

Yi

Mole fraction of component i

Greek letters

ɛ

Exergy efficiency (%)

ɛmodified

Modified exergy efficiency (%)

εconv

Conventional exergy efficiency (%)

Ƞis

Isotropic efficiency(%)

ΔTmin

Minimum approach temperature (°C)

ηc

Carnot factor

 

Acknowledgments

The author is grateful to NISOC, for permission to publish this work.

 

Highlights

• The real-life case study NGL plant 800 has been analyzed by Advanced Exergy Analysis.
• Equipment was identified based on the priority of modification by Advanced Exergy Analysis.
• The effects of performance improvement were considered for the highest priority equipment.
• A new graphical method called, Combined Pinch and Advanced Exergy was performed on the NGL plant.

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