A simulation study of n-butane absorption refrigeration system using commercial hydrocarbons as absorbents

International Journal of Refrigeration 112 (2020) 110–124

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A simulation study of n-butane absorption refrigeration system using commercial hydrocarbons as absorbents Muhammad Arafia∗, Ahmed Soliman, Ayat Ossama Chemical Engineering Department, Faculty of Engineering, Cairo University, Giza 12613, Egypt

a r t i c l e

i n f o

Article history: Received 18 August 2019 Revised 4 January 2020 Accepted 6 January 2020 Available online 9 January 2020 Keywords: Absorption Refrigeration Chiller Butane Hydrocarbon Simulation

a b s t r a c t A heat-powered single-effect absorption refrigeration system operating with commercial n-butane as a refrigerant and six commercial hydrocarbons (stabilized condensate, light naphtha, heavy naphtha, reformate, light reformate and heavy reformate) as absorbents was studied. The main target of the study was actuating these systems to the available waste heat. Simulation models were created using Aspen HYSYS process simulator. The main parameters of an absorption refrigeration system, using the proposed working fluids, were compared at different evaporator, condenser and absorber temperatures to select the best working fluid. Moreover, a focused interpretation was given to the behavior of the best system at different operating conditions. Furthermore, the effect of both butane recovery and purity, in the generator, on the performance of the best system was investigated. Results revealed that, among the proposed working fluids, the best performance was achieved by butane/heavy naphtha. It could achieve a Coefficient of Performance (COP) of 0.78 at evaporator temperature of 20 °C and both condenser and absorber temperatures of 30 °C. The COP passed through a different maximum value at certain recovery for each evaporator temperature and condenser and absorber temperatures and similar behavior was observed for purity. Low reboiler temperatures were feasible at low butane recoveries with almost the same COP of the high butane recoveries. Generally, Butane/heavy naphtha system could compete effectively with the commercial absorption refrigeration systems, within their range of operating conditions, without experiencing their limitations. © 2020 Elsevier Ltd and IIR. All rights reserved.

Étude de simulation d’un système frigorifique à absorption au n-butane utilisant des hydrocarbures commerciaux comme absorbants Mots-clés: Absorption; Froid; Refroidisseur; Butane; Hydrocarbure; Simulation

1. Introduction

Abbreviations: API, american petroleum institute; ARS, absorption refrigeration system; C4, butane; COP, coefficient of performance; CR, circulation ratio; D-86, ASTM D-86; FBP, final boiling point; GWP, global warming potential; HC, hydrocarbon; HN, heavy naphtha; HR, heavy reformate; IBP, initial boiling point; LN, light naphtha; LR, light reformate; ODP, ozone depleting potential; PRV, pressurereducing valve; QE , evaporator duty; QR , reboiler duty; R, reformate; SC, stabilized condensate; TBP, true boiling point; TC , condenser and/or absorber temperature; TE , evaporator temperature; UOPK, characterization factor; VCRS, vapor-compression refrigeration system; PP , pump power. ∗ Corresponding author. E-mail address: muhammad.arafi[email protected] (M. Arafia). https://doi.org/10.1016/j.ijrefrig.2020.01.004 0140-7007/© 2020 Elsevier Ltd and IIR. All rights reserved.

Due to the quick rise in worldwide demand for energy and the requirement to decrease greenhouse gas emissions, the interest in finding new efficient methods of utilizing energy is growing. Refrigeration technologies are of high economic, energetic and environmental importance (Mansouri et al., 2015; Shukla et al., 2015; Raghuvanshi and Maheshwari, 2011). The refrigeration system used nowadays is Vapor-Compression Refrigeration System (VCRS), which demands high-grade energy for its operation. Moreover, for many years, the main refrigerants for VCRS have been the chlorofluorocarbons. This group of materials, not only have high

M. Arafia, A. Soliman and A. Ossama / International Journal of Refrigeration 112 (2020) 110–124

Global Warming Potential (GWP) but also high Ozone Depleting Potential (ODP) because of their chlorine content, which devastates the protective ozone layer of the stratosphere when released. Recently, the sale of chlorofluorocarbons, has already been banned in several countries, and even hydrochlorofluorocarbons, which were found to be less harmful, are being phased out because of their containment of chlorine. The most favorable fluids, called hydrofluorocarbons; however, they do contribute to the GWP in a way similar to, and in some conditions to a quite greater extent than carbon dioxide (Moran et al., 2014; Borgnakke and Sonntag, 2013; Balmer, 2011; Çengel and Boles, 2015; Dincer and Kanoglu, 2010). Latterly, hydrofluoroolefins has been presented as a group of new refrigerants having zero ODP and a very low GWP, and hence considered an environmentally friendly replacement. The promising advantages of the hydrofluoroolefins have attracted a rising number of development studies and some of them became commercially available few years ago (Liu et al., 2014; Wu et al., 2017). Priority should be given for investigating and using alternative driving energy sources instead of electricity for cooling applications and for using eco-friendly working fluids. Therefore, the interest became focused on Absorption Refrigeration System (ARS), also known as absorption chiller, as it is an attractive alternative and sustainable solution to replace VCRS. The ARS harnesses inexpensive waste heat, solar, biomass or geothermal energy sources for which the cost is negligible in many cases. Furthermore, the working fluids of the ARS are eco-friendly (Herold et al., 2016; Srikhirin et al., 2001; Kaynakli and Kilic, 2007; Jaruwongwittaya and Chen, 2010). Performance of ARS and its limiting operating conditions are closely related to the refrigerant/absorbent working fluid (Dardour et al., 2015). The most common working fluids are water/lithium bromide (H2 O/LiBr) and ammonia/water (NH3 /H2 O) (Al-Zubaydi, 2011; Anusha and Chaitanya, 2017). Despite their many advantages, these working fluids present serious drawbacks that limit their applications. The water freezing point is 0 °C, therefore, the H2 O/LiBr system, cannot work at evaporation temperature less than 0 °C, making it useless for subfreezing refrigeration (Karamangil et al., 2010; Muthu et al., 2008). Moreover, crystallization of the H2 O/LiBr solution is very common, particularly at high absorption temperature or relatively low evaporation temperature (Izquierdo et al., 2004; Wang et al., 2011). In addition, maintaining high vacuum conditions in the H2 O/LiBr system is necessary for effective operation, otherwise, its performance will greatly degrade (Wu et al., 2014). The NH3 /H2 O system does not present these problems but needs rather high generator temperatures. The major drawback of the NH3 /H2 O system is the relatively difficult refrigerant/absorbent separation that requires a rectifier to guarantee the purity of the refrigerant vapor leaving the generator, otherwise, the performance will greatly degrade (Kalogirou, 2008). Furthermore, the NH3 /H2 O system has additional drawbacks like its high pressure, toxicity and corrosive action to copper and copper alloy (Srikhirin et al., 2001; Ware and Tiwari, 2015). Regarding these limitations and drawbacks, the search for alternative working fluids is largely justified. Hydrocarbons (HCs) as refrigerants are chemically stable over a wide temperature range, nontoxic and environment-friendly with extremely low GWP and zero ODP (Dincer and Kanoglu, 2010; Granryd, 2001). They offer good thermodynamic and transport properties (Palm, 2008; Mansouri et al., 2015): low viscosity and high thermal conductivity, which lead to good performance of the condenser and the evaporator. They are also recognized for their compatibility with copper, the material of choice for such equipment. The only real problem with the application of HCs as working fluids in refrigeration is their flammability. However, this could be avoided by considering some special precautions during installation and handling (Dardour et al., 2015).

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Using HCs as refrigerants, in VCRS, have been researched and implemented in many applications. However, research concerning their use in ARS are rare and thus more research are still needed. A unique advantage of using commercial HCs in ARS is that they are commonly available in the oil and gas industry, which is considered one of the major producers of waste heat, and they are also familiar for those working in that industry. Chekir et al. (2006) has tested some binary mixtures of light n-alkanes as working fluids and has discovered ten mixtures that are theoretically possible for being used in the air-conditioning by chilled water. Considering the n-butane/n-octane as working mixture for the single-effect ARS, the simulation results have led to interesting performance compared to that of the NH3 /H2 O system (Chekir and Bellagi, 2010). In a latter study (Chekir and Bellagi, 2011), a modification has been proposed to improve the performance of the n-butane/n-octane absorption chiller through recuperating part of the energy discharged out of the rectifier by the strong solution entering the column. Mansouri et al. (2015) has studied ARS operating with binary mixtures of alkanes as refrigerant and has found that the binary mixture propane/nhexane achieves the best performance. Marghany (2015) presented simulation results of ARS using four pure refrigerants with several other pure HCs of different structures as absorbents. The main objective of this study is to determine the best commercial butane/hydrocarbon absorbent (stabilized condensate, light naphtha, heavy naphtha, reformate, light reformate or heavy reformate) working fluid for using in an absorption refrigeration system at different evaporator, condenser and absorber temperatures. The parameters of the system using the best working fluid pair were investigated with respect to the operating conditions and both butane recovery and purity. Eventually, a comparison with commercial absorption chillers is also established. The proposed working fluids are readily available in refineries where surplus amounts of waste heat are available, and hence they can be easily utilized in an absorption chiller in those refineries. 2. Process description The modeled single-effect ARS is shown in Fig. 1. Its major components are generator (distillation column with condenser and reboiler), evaporator, absorber section (mixer-cooler cascade), two throttling valves, solution pump and solution heat exchanger. The refrigerant strong solution leaving the absorber section is pumped and transferred to the generator whilst passing through the solution heat exchanger, where it is preheated prior to entering the generator. The rich solution supplying the generator receives amount of heat in the reboiler at the temperature, which separates the refrigerant dissolved in this solution. At the top of the generator, almost pure refrigerant vapor (recovered refrigerant) is obtained and at the bottom of the column, the lean solution (recovered absorbent). The lean solution exiting the generator preheats the strong solution when flowing through the solution heat exchanger. The Pressure-Reducing Valve (PRV) decreases the pressure of the lean solution going back to the absorber section to maintain the absorber-generator pressure difference. The refrigerant vapor leaving top of the generator is liquefied, at its saturation conditions, in the condenser. This process involves rejection of amount of heat to the cooling medium (water or air). The condensed liquid refrigerant is throttled when passing through the expansion valve to gain the desired evaporator temperature. After expansion, which decreases its pressure, the refrigerant is vaporized, at its saturation conditions, in the evaporator by extracting amount of heat from the medium to be cooled. Finally, the refrigerant in the saturated vapor state from the evaporator goes into the absorber section, finishing the cycle, where it dissolves in the lean solution from the generator. This absorption phenomenon is

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Fig. 1. Single-effect ARS process in HYSYS.

Table 1 Commercial butane composition (Muhammad et al., 2011).

exothermic and is accompanied by heat release, therefore, the absorber is cooled to limit the temperature rise and aid in absorption. 3. Methodology Aspen HYSYS process simulation software was employed to predict the thermodynamic properties and perform flash calculations on the desired process streams under steady-state cases. The Peng-Robinson equation of state was selected as the property package. This study procedures were as follows: 1. A single-effect ARS model was developed using commercial butane as refrigerant and different commercial HCs as absorbents at specified evaporator, condenser and absorber temperatures to determine the best working fluid combination that allows obtaining the best performance. 2. The main parameters of the best system were investigated further for a better analysis of the system. 3. The effect of butane recovery and butane purity, in the generator, on the best system parameters was studied. 4. The generator column was optimized considering the best working fluid pair. 5. The performance of the best system was compared to that of H2 O/LiBr and NH3 /H2 O systems. 3.1. Working fluids Six commercial HCs were used as absorbents in ARS operating with commercial butane (C4) as refrigerant. These absorbents are Stabilized Condensate (SC), Light Naphtha (LN), Heavy Naphtha (HN), Reformate (R), Light Reformate (LR) and Heavy Reformate (HR). The proposed HCs are ordinarily found in oil and gas facilities, especially in natural gas and petroleum refinery plants, which are considered major producers of waste heat. Compositions and specifications of the commercial butane and the commercial HC absorbents are given in Table 1–4, which were used as basis for the simulation. Since the proposed absorbents comprise a quite large number of components, stabilized condensate, light naphtha and heavy naphtha were provided from assays (TOTAL, 2015; ExxonMobil, 2016), whereas the compositions of reformate, light reformate and heavy reformate were reduced

Components

Mass (%)

i-Butane n-Butane Isobutene 1-Butene Neopentane i-Pentane n-Pentane

0.30 98.12 0.08 0.10 0.17 1.12 0.11

into fewer components for the purposes of this simulation study (Dejanovic´ et al., 2011). The commonly found light ends in light naphtha and heavy naphtha were assumed and were set to be auto calculated by the Oil Manager (in HYSYS) for a more accurate simulation. However, the provided compositions and specifications are arbitrary as they strongly depend on the HC source. The criteria used for selecting the best working fluid pair were the maximum Coefficient of Performance (COP), the minimum Circulation Ratio (CR) and a readily available reboiler temperature. 3.2. Absorption refrigeration system model Specifications and equations used in the ARS simulation model are given in the following subsections. 3.2.1. Model specifications The process components of the proposed ARS were modeled with logic operations as presented in Fig. 2. These components were specified as follows: Absorber: •

•

The absorber is generally modeled using mixer-cooler cascade scheme. However, in this case, a flash vessel with energy stream for cooling was used to model the absorber to facilitate the simulation. The absorbent flow rate was adjusted for complete refrigerant absorption. This was done using the Adjust logic operation, which keeps increasing the absorbent flow rate until there is no vapor in the absorber (i.e. the solution leaving the absorber is a boiling liquid).

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Fig. 2. Single-effect ARS model with logic operations in Aspen HYSYS.

•

Table 2 Stabilized condensate specifications (TOTAL, 2015).

•

Bulk Properties API Gravity UOPK Viscosity at 20 °C (cSt) Viscosity at 50 °C (cSt)

62.47 12.08 0.6791 0.5242

Solution Pump: • •

Distillation Type Volume (%)

TBP Temperature ( °C)

37.57 45.10 53.20 64.04 72.95 80.84 86.31 90.11 92.92 95.00 96.55 97.70 98.53

80 90 100 120 140 160 180 200 220 240 260 280 300

The absorber product temperature was specified according to the temperature of the available cooling medium. The absorber pressure is controlled by the desired evaporator temperature.

The pump was modeled with 75% adiabatic efficiency. The outlet pressure was calculated based on the distillation overhead pressure, which depends on the overhead product temperature and thus the available cooling medium temperature in the condenser. This was achieved by using a Set logic operation, which also accounted for the pressure drops in the distillation tower and the solution heat exchanger. Solution Heat Exchanger:

• •

•

Light Ends

The specified pressure drop per side was 20 kPa. The temperature of the tube side outlet was adjusted to achieve a minimum approach temperature of 10 °C using the Adjust logic operation. The simple weighted model was selected for the heat exchanger. Generator:

Components

Mass (%)

Methane Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Cyclopentane Benzene Toluene

0.0000 0.0000 0.0049 0.0407 1.0407 7.7919 8.9980 0.7504 2.3694 3.4597

•

•

The generator was modeled as distillation column with total condenser. In order to simplify the simulation, the total condenser was used instead of using partial condenser (for the column) followed by refrigerant condenser. Hence, the condenser of the distillation column is also the condenser of the ARS. The distillation column was considered a debutanizer as it separates butane from heavier hydrocarbons. A typical debutanizer has a number of actual trays in the range of 25–35 with tray efficiency in the range of 85–95% (Gas Processors Suppliers Association 2012). The minimum number of actual trays was

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Table 3 Light naphtha and heavy naphtha specifications (ExxonMobil, 2016). Bulk Properties Cut

Light Naphtha

Heavy Naphtha

API Gravity Viscosity at 40 °C (cSt) Viscosity at 100 °C (cSt)

88.25 0.4693 0.3093

59.69 0.7017 0.4145

•

Evaporator: •

Distillation Cut

Light Naphtha

Heavy Naphtha

Type

D-86

D-86

Volume (%)

Temperature ( °C)

0.5 (IBP) 5 10 20 30 40 50 60 70 80 90 95 99.5 (FBP)

34.06 34.75 35.23 37.26 41.08 42.64 44.21 48.67 53.66 56.87 59.97 64.14 68.33

•

98.16 100.02 101.26 105.56 109.75 115.34 120.82 126.75 132.59 138.72 144.80 149.96 155.72

Methane; Ethane; Propane; i-Butane; n-Butane; i-Pentane; n-Pentane; 2,2-Dimethylbutane; Cyclopentane; 2,3-Dimethylbutane; 2-Methylpentane; 3-Methylpentane; n-Hexane; Methylcyclopentane; Benzene and Cyclohexane.

•

•

•

Reformate

Light Reformate

Heavy Reformate

1.90 6.41 4.50 8.01 4.30 8.61 2.00 24.72 3.50 4.20 12.21 5.51 4.70 7.71 1.70

5.75 19.00 13.45 23.82 12.65 24.50 0.83 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.50 2.60 37.34 5.31 6.41 18.32 8.31 7.11 11.61 2.50

•

•

•

•

•

considered, which is 25. However, the HYSYS default stage efficiency (100%) was used, which makes them act as theoretical stages. This overestimation makes the column operates as if it has more than 25 actual trays in order to guarantee that the highest performance is achieved, with respect to the number of stages. Moreover, considering tray efficiency of 85–95%, the number of actual trays should be in the range of 27–30, which is in the range of the number of actual trays for a typical debutanizer. The middle stage (13th) was selected as the inlet stage as a starting point. The specifications used were 98.12% butane purity by mass (as in the butane commercial composition of Table 1) and 99% butane recovery in the condenser. Usually the available cooling medium used in the absorber cooling is used in the condenser cooling as well. Therefore, the

The Recycle logic operations were used to recycle the refrigerant and the absorbent back to the absorber to complete the cycle.

The operating conditions have a great effect on the system performance. For example, it depends on the available cooling medium temperature, which decides the condenser and absorber temperatures, and the desired temperature for the external stream to be refrigerated, which determines the evaporator temperature and consequently the absorber pressure. Hence, the proposed system was operated at the following conditions: •

Mass (%)

The valve was used to lower the absorbent pressure to that of the absorber, which equals the evaporator pressure, using a Set logic operation. Recycles:

Table 4 Reformate, light reformate and heavy reformate compositions (Dejanovic´ et al., 2011).

n-Butane i-Pentane n-Pentane 2-Methylpentane n-Hexane Benzene 3-Methylhexane Toluene Ethylbenzene p-Xylene m-Xylene o-Xylene 3-Ethyltoluene 1,3,5-Trimethylbenzene 1,4-Diethylbenzene

The evaporator feed is the partially vaporized refrigerant from the expansion valve and the product was specified as saturated vapor (vapor fraction = 1) with the required cooling temperature from the refrigeration system. The pressure drop was 10 kPa in the evaporator. PRV:

Light Ends Components (Assumed)

Components

pressure of the condenser was adjusted to give condenser temperature equal to that of the absorber using the Adjust logic operation. A Set logic operation was used to calculate the reboiler pressure from the column pressure drop, which is 25 kPa.

The condenser and the absorber were adjusted to operate at same temperature (TC ) in the range of 30–50 °C according to the available cooling medium, which could be either water or air depending on both location and season. The evaporator was operated at an outlet temperature (TE ) in the range of 2.5–20 °C. Lower temperatures are not favorable since the boiling point of pure butane is −0.5 °C (Granryd, 2001) and the commercial one used is predicted to be slightly higher. Furthermore, going below the boiling point will expose the system to vacuum conditions that should be avoided to prevent the possibility of air leakage into the system, which can form a hazardous explosive mixture with the HC working fluid. The butane recovery in the generator was changed in the range of 50–99% to study its effect on the system parameters. The butane purity in the generator was changed in the range of 90–99.25% to study its effect on the system parameters. The evaporator duty was assumed constant (3516.85 kW).

Butane as a refrigerant could be utilized in many applications for the above-mentioned range of evaporator temperature, especially in oil and gas plants operating in hot weather and tropical regions. For example, it can provide cold water for quench water coolers and lean amine coolers of Sulphur recovery units in sour gas plants. Usually, the water is cooled to around 25 °C by having the evaporator temperature around 18 °C, depending on the design approach. Moreover, it can be utilized in cooling the inlet-air to the compressor of a gas turbine, and thus raise its net power or reduce its fuel consumption at high ambient temperature, while making use of the available waste heat energy in the turbine exhaust (Herold et al., 2016). Furthermore, it can be applied in removing the heat of reaction in the alkylation process, in refineries, and maintain the reaction at about 27 °C (Jones and Pujadó, 2006) or lower if needed. In this case, the evaporator maybe operated at

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Table 5 Heat of vaporization for all absorbents at atmospheric pressure. Working Fluid

Heat of Vaporization (kJ.kmol−1 )

Stabilized Condensate Reformate Heavy Reformate Heavy Naphtha Light Reformate Light Naphtha

61145.61 41620.77 37786.37 37330.51 29517.12 28219.90

4. Results and discussion When simulations were performed for the six proposed C4/HC absorbent systems, the following limitations were found. •

Fig. 3. Short-cut column model in Aspen HYSYS.

about 20 °C or lower. In addition, it can used in air-conditioning for plant control rooms. Determining the number of theoretical and actual trays and the optimal feed stage location is important to optimize the column operation. Setting the feed stage too high can lead to high condenser duty, and hence high reflux ratio, whereas, setting it too low can lead to high reboiler duty. In order to determine the number of theoretical trays, for the best system, a short-cut column was used, as shown in Fig. 3, with the following parameters. • •

•

•

The feed stream was the same feed to the generator column. The light key in bottoms and heavy key in distillate mole fractions were very close to the generator outlet streams. The condenser and reboiler pressures were set to be the same as the condenser and reboiler pressures of the generator using Set logic operations. The external reflux ratio was manipulated until it became 1.3 times the minimum reflux ratio, which is a common approach (Gas Processors Suppliers Association 2012).

The short-cut column gives a rough estimation of the optimal feed stage location. Therefore, it was followed by a rigorous stageby-stage method to verify the optimal feed stage location by lowering both condenser and reboiler duties. 3.2.2. Equations used In order to determine the COP of the ARS, Eq. (1) was used. The COP is defined as the energy sought, the evaporator duty, to the energy that costs, the reboiler duty and pump power; i.e. (Balmer, 2011; Srikhirin et al., 2001)

COPARS =

E vaportor duty QE = Reboiler duty + P ump power QR + PP

(1)

The work input for the pump is negligible relative to the heat input at the reboiler; thus, the pump power was discarded for the purposes of the analysis; i.e. (Srikhirin et al., 2001)

QE COPARS ∼ = QR

(2)

In order to determine the circulation ratio of the ARS, Eq. (3) was used. The CR is described as the ratio of the mass flow of the rich solution, going from the absorber to the generator, to the mass flow of the recovered refrigerant; i.e. (Herold et al., 2016)

CR ∼ =

Solution mass f low rate Recovered re f rigerant mass f low rate

(3)

•

Below an evaporator temperature of 4 °C in both C4/HN and C4/HR systems, the saturation pressure was lower than 110 kPa. The escape of some heavy components with the recovered refrigerant from the generator lowers the refrigerant saturation pressure. It is a safe precaution to avoid vacuum conditions as mentioned earlier in Section 3.2.1. Therefore, the evaporator was not operated at a temperature lower than 4 °C in these systems; i.e. the range used for evaporator temperature was changed to 4–20 °C instead of 2.5–20 °C. In both C4/LN and C4/LR systems, when going below evaporator temperatures of 5 °C and 9 °C, at absorber temperatures of 45 °C and 50 °C respectively, the absorption operation became extremely difficult. Accordingly, the minimum evaporator temperature was raised for these systems, at absorber temperatures of 45 °C and 50 °C; i.e. the range used for evaporator temperature was changed to 5–20 °C and 9–20 °C instead of 2.5–20 °C, at absorber temperatures of 45 °C and 50 °C, respectively.

4.1. Choice of the best system The objective of this section is to find the best working fluid based on the following results. 4.1.1. Coefficient of performance Comparison of the COP values for the proposed systems is given in Fig. 4. Systems using C4/HN and C4/HR showed the maximum and almost similar COP values at all conditions. Moderate COP values were achieved by C4/SC and C4/R systems, whereas the minimum COP values were achieved by C4/LN and C4/LR systems. All systems were simulated at the same constant evaporator duty, and hence the reboiler duty is the dominant factor, which is responsible for increasing or decreasing the COP, according to Eq. (1). Generally, the reboiler duty depends on the absorbent heat of vaporization, reflux ratio and feed flow rate that are illustrated as follows: •

Heat of vaporization: Large heat duty for the reboiler can result from a component with large heat of vaporization being enriched in the column bottom. Table 5 gives the heat of vaporization for the proposed absorbents. It can be realized that heavy naphtha and heavy reformate have lower heat of vaporization than stabilized condensate and reformate and thus higher COP according to Fig. 4. Moreover, the heat of vaporization of heavy naphtha and heavy reformate are almost similar, which leads to nearly similar COP values as well. However, absorbent like light naphtha has the lowest heat of vaporization but corresponds to the minimum COP and absorbent like stabilized condensate has the highest heat of vaporization but corresponds to a moderate COP. Absorbent heat of vaporization is a function of absorbent amount, and hence it cannot be adequate to justify the differences in COP alone.

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Fig. 4. Effect of TE on COP at (a) TC =30 °C, (b) TC =40 °C and (c) TC =50 °C.

Table 6 Reflux ratio for all systems at TC =30 °C and TE =20 °C.

•

•

Working Fluid

Reflux Ratio

Light Naphtha Light Reformate Stabilized Condensate Reformate Heavy Naphtha Heavy Reformate

1.0093 0.8221 0.6193 0.5396 0.1753 0.1580

Reflux ratio: High reflux means high reboiler duty and thus low COP. Table 6 gives the reflux ratio for each system at both condenser and absorber temperatures of 30 °C and evaporator temperature of 20 °C as an example. It can be realized that heavier absorbent systems show lower reflux ratio (i.e. higher COP) while lighter absorbent systems show higher reflux ratio (i.e. lower COP). The more light components present in the absorbent, the higher the reflux ratio. Hence, the reflux ratio can be considered one of the reasons behind the COP results in Fig. 4. Feed flow rate: Each absorbent requires a certain flow rate to achieve complete absorption of the same refrigerant at different evaporator temperature (i.e. absorber pressure) and absorber

temperature. In other words, all absorbents have different absorption capacities. Higher absorbent flow rates contribute to higher feed flow rates to the reboiler and thus higher heat duties (i.e. higher COP values). This will be clearly shown in Section 4.1.2. 4.1.2. Circulation ratio Comparison of the circulation ratio for the proposed systems is given in Fig. 5. System using C4/LN showed the maximum circulation ratio at almost all conditions followed by C4/LR, but the difference between them is large. Systems using C4/HN and C4/HR showed the maximum circulation ratio at the highest evaporator temperature and the lowest condenser and absorber temperatures, however, they showed the minimum circulation ratio at almost all conditions followed by C4/SC and C4/R systems that have almost the same circulation ratios at all conditions. According to Eq. (3), the circulation ratio increases when either raising the solution flow rate or reducing the refrigerant flow rate. Since all systems were simulated at the same constant evaporator duty and at the same butane purity and recovery, in the recovered refrigerant stream from the generator, the recovered refrigerant flow rate is almost constant for all systems working at the same evaporator, condenser and absorber temperatures. There-

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Fig. 5. Effect of TE on CR at (a) TC =30 °C, (b) TC =40 °C and (c) TC =50 °C.

fore, the solution flow rate and consequently the absorbent flow rate, can be considered the controlling factor of the circulation ratio. Absorbents have certain absorption capacities at different conditions of evaporator and absorber temperatures. The higher the absorption capacity, the lower is the absorbent flow rate (i.e. lower solution flow rate). In addition, the increase in the circulation ratio (i.e. increase in the solution flow rate) contributes to lower COP values, due to higher feed flow rates to the reboiler, which justifies the results regarding the COP in the Section 4.1.1. System using C4/LR did not correspond to a circulation ratio as high as C4/LN system because light reformate contains a considerable amount of butane. In order to attain the same recovered refrigerant flow rate, the refrigerant feed to the absorber was decreased and consequently the absorbent required for complete absorption decreased. In other words, the butane contained in light reformate was used to compensate the decrease in the refrigerant feed to obtain the same recovered refrigerant flow rate. Systems using C4/HN and C4/HR showed the maximum circulation ratio at the highest evaporator temperature and the lowest condenser and absorber temperatures due to the escape of some heavy components from the absorbent with the recovered

refrigerant. These heavy components reduce the refrigerant saturation pressure at the same evaporator temperature, and hence the absorber pressure is reduced (i.e. harder absorption) and consequently more absorbent flow rates are required to absorb the refrigerant completely. When decreasing the evaporator temperature (i.e. decreasing the absorber pressure) or increasing the absorber temperature, the absorption capacity of the absorbent becomes a stronger dominator than the absorber pressure. 4.1.3. Reboiler temperature Comparison of the reboiler temperature for the proposed systems is given in Fig. 6. The evaporator and absorber temperatures showed insignificant effect on the reboiler temperature and thus only the effect of condenser temperature was studied. Systems using C4/HN and C4/HR showed the maximum reboiler temperatures. Moderate reboiler temperatures were obtained by using C4/SC and C4/R while the minimum reboiler temperatures were obtained by using C4/LN and C4/LR. The rich solution needs to be heated to certain temperature to boil-off and separate the refrigerant. Therefore, absorbents with higher bubble point need higher reboiler temperatures. Fig. 7 gives the bubble point for each absorbent. Despite that light naphtha and light reformate have almost the same bubble point, they have

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Fig. 6. Effect of TC on reboiler temperature.

Fig. 8. Effect of evaporator temperature on absorber pressure.

•

•

In terms of reboiler temperature, it required high reboiler temperatures but considerably lower than that achieved by C4/HR despite having almost the same COP values. The reboiler temperature is not considered a limitation if the system is applied in the oil and gas industry due to the availability of waste heat with temperature that can easily cover the operating range of C4/HN reboiler temperature. The C4/HN working fluid provides another advantage as heavy naphtha is commonly available in petroleum refineries and at a lower cost of production than the reforming process products (reformate, light reformate and heavy reformate).

4.2. Performance of the best system

Fig. 7. Bubble point for all absorbents at atmospheric pressure.

different reboiler temperatures because most of the butane originally contained in the light reformate is retrieved within the recovered refrigerant in the generator. After reaching steady-state condition, the remaining portion of the light reformate is mostly heavier components than butane, which increases its bubble point. 4.1.4. Concluding remarks Applying the criteria for choosing the best working fluid in Section 3.1, the best working fluid was found to be C4/HN because of the following reasons: •

•

In terms of COP, it achieves almost the maximum COP values at all conditions. Despite that C4/HN comes second to C4/HR, the differences are very small and can be neglected for the sake of a much lower reboiler temperatures than those achieved by C4/HR. In terms of CR, it shows a very low CR at all conditions and the minimum CR as conditions get worse. Compared to C4/HR, in particular, it mostly had lower or equal CR, depending on the operating conditions.

The variations of the COP with the operating temperatures are shown in Fig. 4. The COP values obtained in C4/HN system were 0.23–0.78. Absorption favors high pressures and low temperatures. When the evaporator saturation temperature increases, its saturation pressure increases (i.e. higher absorber pressures) as displayed in Fig. 8. Accordingly, lower absorbent flow rates are required to absorb the vapor refrigerant completely. This leads to lower solution flow rates (i.e. lower circulation ratios as shown in Fig. 5). Hence, lower feed flow rates go into the generator and reduce the required reboiler duties (i.e. higher COP values). Lower absorber temperatures lead to higher COP values for the same reason. Furthermore, when increasing the evaporator temperature, the enthalpy of the evaporator outlet rises, and when decreasing the condenser temperature, the enthalpy of the condenser outlet falls. The throttling in the expansion valve is isenthalpic process, and hence the enthalpy of the evaporator inlet (i.e. expansion valve outlet) is equal to the enthalpy of the condenser outlet. As a result, higher evaporator duties can be achieved due to the higher difference in enthalpy across the evaporator. However, to operate the system at constant evaporator duty, the refrigerant flow rate is decreased to compensate for the increase in the evaporator duty. The absorbent and consequently the solution flow rates decrease (i.e. lower circulation ratios as shown in Fig. 5), and hence lower reboiler duties can be achieved (i.e. higher COP values). The variations of the condenser pressure and reboiler temperature with the condenser temperature are given in Fig. 9. When the condenser temperature increases, its saturation pressure in-

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Fig. 9. Effect of condenser temperature on condenser pressure and reboiler temperature.

creases. Thus, the reboiler saturation pressure and temperature increase. The C4/HN system is driven by reboiler temperature in the range of 144–174 °C, at condenser temperature in the range of 30– 50 °C. This reboiler temperature range can be supplied by waste heat available in the chemical and process industries. Exhaust gas temperatures of gas turbines are usually at 450–550 °C and are considered a valuable clean energy in the form of exhaust heat, which can be recovered (Baakeem et al., 2017). The flue gas directly exhausted from natural gas-fired boilers is normally at temperatures of 150–200 °C (Qu et al., 2014), which is also considered a waste heat energy source. However, this temperature range can be lowered as will be shown in Section 4.3.1.

4.3. Generator column specifications The aim of this section is to study the column specifications, which are the purity and recovery of butane, considering an ARS operating with the best working fluid.

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Fig. 11. Effect of butane recovery on CR.

4.3.1. Butane recovery effect The effect of butane recovery on the COP at different evaporator temperatures and same condenser and absorber temperatures at best conditions (TE =20 °C and TC =30 °C) and worst conditions (TE =4 °C and TC =50 °C) was investigated. Fig. 10 gives the effect of butane recovery on the COP. As the butane recovery in the generator increases, the system COP reaches a maximum value prior to decreasing again at high butane recoveries. Before reaching the maximum COP value, increasing the butane recovery increases the amount of recovered refrigerant and thus higher evaporator duties are achieved. However, to operate the system at constant evaporator duty, the refrigerant flow rate is decreased to compensate for the increase in the evaporator duty. As a result, the absorbent and consequently the solution flow rates decrease and lower reboiler duties can be achieved (i.e. higher COP values). After reaching the maximum COP value, the separation becomes more difficult and the reflux ratio increases. As a result, the reboiler duty increases (i.e. lower COP values). It was also noticed that for each evaporator temperature and both condenser and absorber temperatures, the maximum COP value corresponds to a different recovery. However,

Fig. 10. Effect of butane recovery on COP at (a) TE =20 °C and TC =30 °C and (b) TE =4 °C and TC =50 °C.

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decrease in the reboiler temperature can be achieved for a small decrease in the recovery, especially at high evaporator temperatures. For example, operating the system at 90% butane recovery, 20 °C evaporator temperature and 30 °C condenser and absorber temperatures requires 98 °C reboiler temperature instead of 140 °C at 99% butane recovery while showing almost the same COP and slightly higher circulation ratio. Thus, this system can be powered by low temperature heat energy, such as solar energy or residual waste heat utilized in other applications, at lower recoveries with good performance. Briefly summarized, the choice of the operating conditions, with respect to the butane recovery, is based on the available process conditions. In some cases, the heat source temperature is the restricting condition, which requires operating at low recovery to reduce the reboiler temperature at the expense of higher fixed cost. In other cases, the restricting condition may be the amount of available heat energy, which requires operating near the maximum COP value.

Fig. 12. Effect of butane recovery on reboiler temperature.

changing the butane recovery did not have a significant effect on the COP. The effect of butane recovery on the circulation ratio is given in Fig. 11. With the decrease in the butane recovery, the circulation ratio rises. When the butane recovery decreases, the non-recovered refrigerant amounts are lost to the recovered absorbent stream. Thus, the refrigerant flow rate grows less while the absorbent flow rate grows more. However, the evaporator duty decreases when reducing the refrigerant flow rate. Since the system is operated at constant evaporator duty, the refrigerant flow rate is increased to compensate for the decrease in the evaporator duty. The absorbent and consequently the solution flow rates increase, and hence the circulation ratio increases. This increases the equipment sizes and escalates the fixed investment of the system. The effect of butane recovery on the reboiler temperature is given in Fig. 12. With the decrease in the butane recovery, the reboiler temperature decreases. When the butane recovery decreases, the butane concentration in the reboiler increases, and hence reducing the reboiler saturation temperature. A much

4.3.2. Butane purity effect The effect of butane purity on the main parameters of the best system was investigated at evaporator temperatures of 2.5 °C, 4 °C and 20 °C and condenser and absorber temperatures of 30 °C and 50 °C, which considers all best and worst conditions. The evaporator temperature of 2.5 °C was considered as it might be reached, while avoiding vacuum, at higher purity. Fig. 13 gives the effect of butane purity on the COP. The system shows a maximum COP value at butane purity equal to or higher than 98%. After reaching the maximum COP value, the separation becomes more difficult. As a result, the reboiler duty increases (i.e. lower COP values). Before the maximum COP value, decreasing the butane purity increases the associated heavier hydrocarbons, which should have higher heat of vaporization, and thus the evaporator duty should increase (i.e. higher COP values). However, it seems that this is not the case. The effect of the heat of vaporization is weaker than the effect of the circulation ratio. The effect of butane purity on the circulation ratio is given in Fig. 14. With the decrease in the butane purity, the circulation ratio increases. As the circulation ratio increases, the feed flow rate to the generator increases, which leads to higher reboiler duties (i.e. lower COP values).

Fig. 13. Effect of butane purity on COP at (a) TC =30 °C and (b) TC =50 °C.

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Fig. 14. Effect of butane purity on CR at (a) TC =30 °C and (b) TC =50 °C.

temperature. However, in the studied range, this reduction is very small.

4.4. Generator column optimization

Fig. 15. Effect of butane purity on absorber pressure.

When the butane purity decreases, the evaporator pressure decreases, and hence the absorber pressure decreases as shown in Fig. 15. At low absorber pressures, the absorption becomes more difficult and higher absorbent flow rate is required to completely absorb the refrigerant, which leads to an increase in the circulation ratio, as presented in Fig. 14. As previously mentioned, it is recommended to not operate the system at absorber pressure lower than 110 kPa to avoid getting close to vacuum conditions. The vacuum and recommended conditions are shown in Fig. 15. For evaporator temperature of 2.5 °C and 4 °C, the purity shall not be less than 98.5% and 98%, respectively. The effect of butane purity on both condenser pressure and reboiler temperature is given in Fig. 16. With the decrease in the butane purity, the condenser pressure and reboiler temperature decrease. When the butane purity decreases, the distillation pressure decreases for the same condenser temperature due to the presence of the higher boiling components. This reduction in pressure is accompanied by a decrease in amount of the recovered absorbent in the bottom product, which leads to a reduction in the reboiler

The distillation column was optimized at best conditions (TE =20 °C and TC =30 °C) and worst conditions (TE =4 °C and TC =50 °C) in order to determine the number of theoretical and actual trays and the optimal feed tray required to produce the same specification. The short-cut column was used to estimate the number of theoretical trays. Table 7 shows the values of the main inputs and results of the short-cut column model. The short-cut calculations show that the required number of theoretical trays is 13 and 15 for best and worst conditions respectively, while having the 8th stage as the optimal feed stage for both conditions. The number of theoretical stages is less than 25 because of the high difference in volatilities between top and bottom products. Considering an efficiency of 85–95%, the number of actual trays should be in the ranges of 14–16 and 16–18 for best and worst conditions, respectively. However, since the short-cut column gives a rough estimation of the feed stage location, a rigorous stage-by-stage method was used to verify the optimal feed stage location by lowering both condenser and reboiler duties. The number of theoretical trays in the generator column were changed to 13 and 15 for the best and worst conditions, respectively. Then, different feed stage locations were selected and the stage giving the minimum condenser and reboiler duties was chosen as the optimal one. Fig. 17 presents the variation of both condenser and reboiler duties with feed stage location. The optimal feed stage location is found to be stage 8 for both best and worst conditions, which verifies the results of the short-cut calculations. From the above results, it can also be estimated that for the C4/HN system at the proposed operating range of evaporator, condenser and absorber temperatures (TE =4–20 °C and TC =30–50 °C), the generator column will likely have 13–15 theoretical trays with the 8th stage as the feed one. Moreover, they show that the column was oversized, and hence increasing the number of trays above 25 would not have improved the performances in the previous sections. Furthermore, changing the feed stage location around the middle has a very small effect on the condenser and reboiler duties, which validates the assumption of having the feed stage in the middle.

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TE 20 °C and TC 30 °C

TE 4 °C and TC 50 °C

Inputs

Light Key in Bottoms, n-Butane (Mole Fraction) Heavy Key in Distillate, Benzene (Mole Fraction) Condenser Pressure (kPa) Reboiler Pressure (kPa) External Reflux Ratio

0.0215 0.0014 280.1 305.1 0.214

0.0026 0.0026 490.8 515.8 1.169

Results

Minimum Reflux Ratio Minimum Number of Trays Theoretical Number of Trays Optimal Feed Stage

0.165 4.26 12.85 7.72

0.899 6.39 14.70 7.91

Fig. 16. Effect of butane purity on (a) condenser pressure and (b) reboiler temperature.

Fig. 17. Feed stage location versus condenser and reboiler duties at (a) TE =20 °C and TC =30 °C and (b) TE =4 °C and TC =50 °C.

4.5. Comparison with the commercial absorption refrigeration systems The proposed best system using C4/HN working fluid was compared to the widely used H2 O/LiBr and NH3 /H2 O systems to

measure its competence. The main point of comparison was the system COP. The COP results from C4/HN system were roughly compared to that of H2 O/LiBr system used in a comparison by Marghany (2015) and to that of NH3 /H2 O simulated by Le Lostec et al. (2013) as shown in Table 8. Generally, the C4/HN system can

M. Arafia, A. Soliman and A. Ossama / International Journal of Refrigeration 112 (2020) 110–124 Table 8 Comparison of COP values with commercial absorption systems.

Acknowledgments

Working Fluid

COP

TE (°C)

TC (°C)

Water/Lithium Bromide Ammonia/Water Butane/Heavy Naphtha

0.40–0.70 0.44–0.60 0.34–0.78 0.41–0.67

5–20

30–40 35 30–40 35

compete effectively with the commercial chillers, with respect to the COP, while having the following advantages over them: •

• •

•

• • •

There is no risk of solution crystallization Compared to the H2 O/LiBr system. Vacuum conditions are not needed as in the H2 O/LiBr system. It is operated at low pressure compared to the NH3 /H2 O system. There is no probability for ice formation in the evaporator (i.e. evaporator clogging) as in the NH3 /H2 O system. It is not toxic like the NH3 /H2 O system. Special materials for construction are not required. The working fluids are familiar and readily available in oil and gas plants.

5. Conclusions In this study, an ARS operating with different commercial C4/HC absorbent working fluids was simulated and the results were compared to choose the best working fluid and further study the effect of operating conditions and butane recovery on the system performance. In addition, comparison with the commercial absorption chillers was carried out. The conclusions of the work conducted in this study are as follows: •

•

•

•

•

•

•

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Among the six proposed absorbents, the best absorbent for an absorption chiller operating with commercial butane as a refrigerant was found to be heavy naphtha. The use of commercial C4/HC absorption chillers is promising, especially at high evaporator temperatures and low condenser and absorber temperatures, where COP is high and circulation ratio and reboiler temperature are low. A system using C4/HN as working fluid achieved a COP values of 0.23–0.78 at evaporator temperatures of 4–20 °C and both condenser and absorber temperatures of 30–50 °C. However, the required reboiler temperature were in the range of 144– 174 °C, which is considerably high. The COP passed through a maximum value with the increase in butane recovery, however, the variations were insignificant. Moreover, a small decrease in the butane recovery contributed to a large decrease in the reboiler temperature, especially at high evaporator temperatures. A maximum COP value can be obtained at butane purity equal to or higher than 98 Mass%. Furthermore, decreasing the butane purity may expose the system to vacuum at low evaporator temperatures. The C4/HN system is likely to have generator column with 13– 15 theoretical trays, 14–18 actual trays with efficiency 85–95% and the middle stage as the feed tray. The COP values of C4/HN absorption chiller were comparable to that of H2 O/LiBr and NH3 /H2 O chillers within their operating range while having the advantage of operating without the troubles related to these systems.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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