The research was aimed at providing water for cooling recycled cyclohexane in polyethylene plant to 15°C. This research could be redounding to the benefit of polyethylene plants that are using solution based polymerization technique. A chiller unit of 630 T, which has compressor power input of 378.33 kW and can provide chilled water capable of cooling recycle cyclohexane to 15°C, was designed using Aspen Hysys version 7.1. Four different refrigerants were tested to know the best fit refrigerant for the design and the best among them was R134a. The designed chiller has a coefficient of performance of 6.3 and a capacity greater than that of the defective chiller (550 TR). Unlike the defective chiller, with the increased cooling capacity and corresponding increase in compressor power from 296 kW to 378.6 kW, it could discharge chilled water capable of cooling recycle cyclohexane from 38°C to 15°C without tripping of the unit. With this design, ethylene absorption rate could increase to 40 T/h. Evaporator and condenser were designed and duties were 2166.39 KJ/sec and 2544.72 KJ/sec respectively. A thermostatic expansion valve with flow coefficient of 46.17 gpm was designed. The designed suction and discharge pressure were 414 kpa and 1053 kpa respectively while condenser temperature of 40°C was used for the design. The cooling of recycle cyclohexane from 38°C to 15°C using the chilled water supplied by the designed chiller was simulated using Aspen Hysys.
Polyethylene production can be achieved using solution based polymerization technology [
Ethylene absorption is highly exothermic and the temperature of the recycle cyclohexane affects the equilibrium absorption temperature [
The aim of this research was to design a chiller capable of supplying water for cooling the recycle cyclohexane to 15˚C. A chiller is one of the heat transfer equipment and it employs the principles of vapour compression cycle.
The objectives of the project include: simulation of the cooling of cyclohexane from 38˚C to 15˚C, using Hysys, to get the chiller water requirements, test with different refrigerants to get the best fit refrigerant for the design, development of algorithm for the design and design of the proposed chiller based on the optimum chiller water parameter requirements and best fit refrigerant.
The materials used for this report includes P-h diagram for R134a, R125, R152a & R1270 respectively; ASHRAE handbooks, Handbook of Air Conditioning & Refrigeration; Process Technology manual, software (Aspen Hysys, version 7.1) and input data. The input data, which were gotten from the Plant, includes cyclohexane flow rate, temperature & pressure; cooling water pressure and temperature.
To achieve the aim of this research, Aspen Hysys software, version 7.1, which used the principles of mass & energy conservations, principles of heat transfer and Thermodynamics to generate results, was used. A design tolerance of 10% was used in this work and a steady state simulation and design procedure was applied. The following assumptions were made:
1) Potential and kinetic energy effect were negligible
2) Evaporator and condenser were at constant pressure
3) Evaporator outlet was saturated vapour while condenser outlet was saturated liquid
4) The vapour compression cycle was at steady state.
5) Adiabatic process occurs at the expansion valve
6) The chiller was considered to be an ideal vapour compression refrigeration cycle and thus, the compression process was isentropic.
The process of cooling 200 T/hr of cyclohexane from 38˚C to 15˚C was simulated using Aspen Hysys to determine the volume of water and the temperature of water going to chiller as shown in
Q s = m s C p s ( t s o − t s 1 ) (1)
where, Qs is the rate of heat loss by the hot cyclohexane, ms is the mass flow rate of cyclohexane, Cps is the specific heat capacity of cyclohexane at mean temperature, tso is the temperature of cyclohexane after cooling and tsi is the inlet temperature of cyclohexane.
In other to estimate the water requirement, energy balance on the plate heat exchanger was done:
Q s = m w C p w ( t w o − t w i ) (2)
where, two is the temperature of the water returned to the chiller evaporator after cyclohexane cooling, which is to be determined (as shown in
Simulation of the vapour compression cycle and cyclohexane cooling were done at evaporator temperature of 5˚C, 8˚C, 10˚C, 12˚C, & 13˚C to respectively to determine the refrigerant that will require least energy input in the compressor. The refrigerants used were 1,1,1,2-tetraflouroethane (R134a), 1,1,1,2,2- pentaflouroethane (R125), propylene (R1270) & 1,1-diflouroethane (R152a). The volume of the refrigerant required, compressor power requirement and temperature of water returned to evaporator were noted.
Using the cyclohexane cooling simulation results, the cooling load of the chiller was calculated using Equation (3) [
Q e v a p = V ρ C p w ( T i n − T o u t ) (3)
The cooling load was converted to chiller capacity in refrigeration tons (TR) using a conversion factor of 1 TR is 3.5 kW [
The ideal P-h diagram for the chiller is shown in
1) Compressor Design
Since the compression process is isentropic, the discharge temperature was calculated using the temperature variation between stages in isentropic process.
T d = T S [ r ( γ − 1 γ ) ] (4)
In this work, the discharge temperature does not exceed 107˚C because the discharge temperature should be less than the critical temperature of refrigerant [
Using
W c = h 2 − h 1 (5)
From the definition of enthalpy, Equation (5) could be transformed as [
W c = m ˙ C Pr ( T 2 − T 1 ) (6)
To determine the isentropic efficiency of the designed compressor, isentropic head was estimated as [
W i s e n = C p R ( T 2 r e v − T 1 ) (7)
To get Trev, the compression was assumed reversible and the entropy of isentropic process was zero [
Δ S = C p ln ( T 2 r e v T 1 ) − R ln ( P 2 / P 1 ) = 0 (8)
The isentropic efficiency of the compressor was calculated using Equation (9) while the compressor power was estimated using Equation (10) [
η i s e n = W i s e n W s (9)
Power = m ˙ C Pr ( T 2 − T 1 ) (10)
In this study, an isentropic efficiency range of 70% - 85% was used [
2) Evaporator and Condenser Design
The chiller was designed using a condenser temperature of 40˚C and evaporating temperature of 10˚C. The type of evaporator designed was flooded water-cooled shell and tube heat exchanger [
Taking energy balance on the evaporator, the heat transfer rate can be estimated as shown on
Q e v a p = m ( h 1 − h 4 ) (11)
The area of evaporator required for heat transfer was estimated using Equation (12) while the number of tubes was estimated using Equation (14) [
Q e v a p = U A Δ T (12)
The range of values of “U’’ in Equation (12) in this study was 1080 - 3600 kJ/h m2・C (300 - 1000 W/m2・K) for evaporator [
The log mean temperature difference (LMTD) was evaluated as shown below [
Δ T = T D 1 − T D 2 ln ( T D 1 T D 2 ) (13)
N t = A π d o 2 (14)
Using
Q c o n d = m ( h 2 − h 3 ) (15)
3) Metering Device
At the valve, the refrigerant exists as saturated liquid (f) and saturated vapour (g) due to the flashing action. The dryness fraction of the refrigerant was denoted by “X”. To get the dryness fraction of the refrigerant at the valve outlet, the enthalpy of the refrigerant was obtained as [
h 4 = h f + x 4 ( h g − h f ) (16)
where, hf and hg were gotten from chart (16)
From
C O P = h 1 − h 4 h 2 − h 1 (17)
For this study, the value of COP was considered to be greater than 5.7 since the Carnot COP of the chiller was 5.7.
The chiller was designed using Aspen Hysys simulator. The following algorithm was developed as solution techniques (
The results of the simulation of the cooling of the cyclohexane are as shown in
Increased in evaporator temperature from 5˚C through 13˚C reduced the compressor power input for all the refrigerants due to drop in compression ratio which reduced the compressor work as shown in
This effect was maximum when R134a was used; thus, R134a is best fit for the design.
Evaporating temperature of less than 5˚C was not considered to avoid freezing of the chilled water inside the tubes of evaporator; while evaporating temperature greater than 13˚C was not considered to avoid temperature cross.
Parameter | Existing | Simulation |
---|---|---|
Water Flow-rate (m3/h) | 103 | 120.00 |
Chilled-water temperature (˚C) | 20.00 | 15.00 |
Water returned temperature (˚C) | 27.30 | 31.65 |
Cyclohexane flow (T/h) | 200.00 | 200.00 |
Cyclohexane temperature (˚C) | 35.00 | 38.00 |
The summary of the results of chiller designed is shown in
The type of compressor is centrifugal because of the high volume of refrigerant that is needed in the vapour compression cycle.
The area of heat transfer was 288 m2 as compared to that of defective chiller which is 244 m2 (from vendor diagram) and the evaporator had 764 tubes. This area increase was as a result of the rise in heat duty.
The overall heat transfer coefficient of the condenser was 3796 kJ/h-m2 C and the area of heat transfer was 332.5 m2 as compared to that of the defective chiller that was 319 m2. The total number of tubes of the designed condenser was 882.
To avert the effect of corrosion, carbon steel was used for shell design.
Designed Parameters and their Units | Values |
---|---|
Chiller Capacity(TR) | 630 |
Cooling Load(kJ/h) | 7,799,004 |
Power(kW) | 378 |
Refrigerant flow(m3/h) | 42.2 |
Suction Temperature(˚C) | 10 |
Discharge Temperature(˚C) | 49.53 |
Refrigerant | R134a |
Type | Centrifugal |
Parameter | Value |
---|---|
Power consumed (kW) | 378.60 |
Adiabatic efficiency (%) | 75.00 |
Adiabatic head (m) | 1995.00 |
Suction pressure (kPa) | 414.20 |
Discharge pressure (kPa) | 1053.00 |
Flow rate (m3/h) | 42.070 |
Suction temperature (˚C) | 10.00 |
Discharge temperature (˚C) | 49.53 |
Compressor type/stages | Centrifugal/single stage |
Parameter | Value |
---|---|
Evaporator duty (kJ/h) | 7,799,004 |
Overall coefficient of heat | 2495.00 |
Transfer (kJ/h-m2-C) | |
Area of heat transfer (m2) | 288.00 |
Number of tubes | 764.00 |
Tube internal diameter (mm) | 16.00 |
Tube external diameter (mm) | 20.00 |
Tube length (m) | 6.00 |
Tube thickness (mm) | 2.00 |
Number of shell passes | 1 |
Number of tube passes | 2 |
Shell diameter (mm) | 774mm |
Tube layout angle | Triangular (30 degrees) |
Tube pitch (mm) | 25.00 |
Baffle spacing (mm) | 309.00 |
Tube side inlet temperature (˚C) | 31.48˚C |
Tube side outlet temperature (˚C) | 15.00˚C |
Tube side pressure (kPa) | 350.00 |
Shell side pressure (kPa) | 421.2 |
Water flow rate (m3/h) | 120.0 |
Refrigerant flow rate (m3/h) | 42.1 |
Material for tube construction | Copper |
Parameter | Value |
---|---|
Condenser duty (kJ/h) | 9,167,040 |
Overall coefficient foe hat transfer (kJ/h-m2-C) | 3796.00 |
Area of heat transfer (m2) | 332.50 |
Number of tubes | 882 |
Tube internal diameter (mm) | 16.00 |
Tube external diameter (mm) | 20.00 |
Tube material | Copper |
Number of shell passes | 1 |
Number of tube passes | 2 |
Shell diameter (mm) | 820 |
Material for shell construction | Carbon steel |
Tube layout angle | Triangular (30 degrees) |
Tube pitch (mm) | 50.00 |
Baffle spacing (mm) | 331.00 |
Tube length (m) | 6 |
Shell side pressure (kPa) | 1053 |
Tube side pressure (kPa) | 350 |
Shell side inlet temperature (˚C) | 49.53 |
Shell side outlet temperature (˚C) | 40.00 |
Cooling water inlet temperature (˚C) | 30.00 |
Cooling water outlet temperature (˚C) | 36.51 |
Cooling water flow (m3/h) | 327.2 |
The flow rate through the valve was 5.22 × 104 kg/h while the pressure drop of the valve was 597.1 kPa.
Valve sizing parameter and units | Value |
---|---|
Inlet pressure (kPa) | 1018.00 |
Valve opening (%) | 50.00 |
ΔP (kPa) | 597.1 |
Flow rate (kg/h) | 5.224 × 10−4 |
Cv(gpm) | 46.17 |
Vapourfraction of refrigerant | 0.2265 |
Element/Parameters | Present Chiller | Propose Designed chiller |
---|---|---|
Chiller Capacity (Tons) | 550.00 | 630.00 |
Compressor Duty (kJ/hr) | 1,065,600.00 | 1,361,988.00 |
Evaporator duty (kJ/hr) | 5,245,200.00 | 7,799,004.00 |
Condenser duty (kJ/h) | 6,310,800.00 | 9,167,040.00 |
Refrigerant flow (m3/h) | 33.00 | 42.2 |
Suction temperature (˚C) | 10.00 | 10.00 |
Discharge temperature (˚C) | 29.26 | 49.53 |
Chilled water flow (m3/h) | 103 | 120 |
Condenser cooling water flow(m3/h) | 257 | 372 |
Valve Coefficient(gpm) | 26.7 | 46.17 |
The discharge temperature of the designed chiller increased from 29.26˚C to 49.53˚C due to rise in compressor work that occurred as a result of increase in refrigerant flow rate from 33 m3/h to 42.2 m3/h. Similarly, the condenser cooling water flow increased from 257 to 372 m3/h because of the increased in the rate of heat rejection in the condenser from 6,310,800 kJ/h to 9,197,040 kJ/h.
The compressor power input in the new designed chiller was greater than that of the old chiller because of increased in compression ratio which was as a result of the rise in discharge temperature from 29.3˚C to 49˚C.
The presence of the chiller caused a temperature drop of up to 23˚C in the cyclohexane used in ethylene absorption when cyclohexane flow was raised to 200 T/h. Cyclohexane was discharged to the ethylene absorption unit at 15˚C. This was because greater heat energy was removed from the cyclohexane by chilled water, which came from the chiller at a lower temperature of 15˚C.
A centrifugal water-cooled chiller of 630 tons of refrigeration was designed. The chilled water designed flow rate was 120 m3/h at 15˚C. This chiller is capable of cooling 200 T/h of cyclohexane at 7 Kg/cm2 from 38˚C to 15˚C at the same pressure.
Out of the four refrigerants (R134a, R125, R152a and R1270) used, R134a was the best fit refrigerant for the design. R134a had the least compressor power input requirement at 10˚C evaporating temperature and thus, gave a chiller with the highest coefficient of performance. R152a had a low compressor input but it is not suitable for evaporation temperature less than 8˚C; a blend of the R134a and R152a could be used in further studies to see the cooling effect.
The designed chiller had a coefficient of performance of 6.3 and a capacity greater than that of the defective chiller which was 550 TR. With the increased cooling capacity and corresponding increased in compressor power from 296 kW to 378.6 kW, it could discharge chilled water capable of cooling recycle cyclohexane from 38˚C to 15˚C without tripping of the unit. Optimization of the designed chiller could be done to further reduce the energy consumption in subsequent studies.
The 630 TR chiller needed a single stage centrifugal compressor with power input of 378.33 kW. The design suction pressure was 414.2 kPa, and discharge pressure was 1053 kPa, and temperature lift of the chiller was 39.53˚C, while the suction temperature was 10˚C. 42.2 m3/h of R134a at 10˚C was needed to absorb 7,799,004 kJ/hr heat from 120 m3/h chilled water at 31.48˚C. Similarly, 327.2 m3/h of cooling water at 30˚C was needed to eject 9,167,040 kJ/hr heat at condenser with a condensing temperature of 40˚C. The maximum condenser water returned temperature was 36˚C. The heat transfer area requirement for the evaporator was 288 m2 while that of condenser was 332.5 m2 (as compared to that of the present defective chiller which is 244 m2 and 319 m2 respectively).
Algorithm was developed for the design procedure and this algorithm was self explanatory and easy to work with.
Finally, although the designed chiller could have cost implications, the chiller would increase the rate of ethylene absorption to 40 T/h thereby leading to increase in plant through-put and revenue. Further study can be done to cool the chilled water further and study the effects on the cyclohexane cooling and ethylene absorption rate.
The authors declare no conflicts of interest regarding the publication of this paper.
Akpa, J.G., Dagde, K.K. and Inyang, N.B. (2019) Design of Industrial Water Cooled Chiller for Recycle Cyclohexane in Polyethylene Plant. Advances in Chemical Engineering and Science, 9, 143-158. https://doi.org/10.4236/aces.2019.92011