Simulation of a Plant for the Production of Polyethylene

The simulation of a 270 KTA capacity polyethylene plant was performed using Aspen Hysys version 8.8. A Hysys model of the polyethylene was devel-oped using the polyethylene plant layout of Indorama Eleme Petrochemical Company. A material and energy balance for the various components of the plant was performed manually and with Hysys for comparison. The design of the various components of the Hysys model was performed. The polyethylene reactor was simulated to study the effect of process functional parameters such as reactor dimensions, temperature and pressure. The effect of reactor size and number on polyethylene output was studied by simulating the plant with five continuous stirred tank reactors (CSTRs) in series and a single reactor. The results of the material and energy balance of the various components of the plant were performed manually and with Hysys which showed a maximum deviation of 0.8%. The design results of the sizing parameters for the Multiple and single CSTRs were compared in terms of Volume, Diameter, Height, Spacetime, Space Velocity, and Volumetric flowrate respectively. At 90% Conversion, the multiple CSTRs gave 600 dm 3 , 0.7668 m, 1.198 m, 0.052 hr, 195.83 hr −1 , and 117.5 m 3 /h for the above listed parameters, while the single CSTR gave 6000 dm 3 , 1.721 m, 2.581 m, 0.056 hr, 17.867 hr −1 and 107.2 m 3 /h for the same conversion. The sizing results for each of the five compressors were also compared in terms of the following parameters: Adiabatic Head, Polytropic Head, Adiabatic fluid Head, polytropic Fluid Head, Adiabatic Efficiency, power consumed, polytropic head factor, polytropic exponent and isentropic exponent. The effect of reactor size and number showed that At 90% conversion the multiple CSTRS in series gave a lower volume than the single CSTR for the same conversion, and more Economical than the single CSTR for the same conversion.


Introduction
Olefins manufacturing is the third largest petrochemical industry after others like ammonia manufacturing and petroleum refining. Polyethylene has been extensively applied in industries chemicals and other related products are been manufactured all round the world [1].
Polyethylene is used as starting material for the production products such as: cosmetics, plastics, solvents etc. having a high market demand with production rate of 150,000,000 tons/year and this production rate has been predicted to rise by 3.5 percent in the next five years [2]. A large amount of polyethylene is used for producing plastics which contains polymer chains of ethylene units in its numerous chain lengths. [3] researched on non-catalytic pyrolysis of ethane to ethylene in the presence of CO 2 with or without limited CO 2 . Both presence and absence of limited CO 2 in the pyrolysis of ethane to ethylene at process conditions of 750˚C -900˚C, space velocities of 1500 -9000 per hour and CO 2 /C 2 H 6 and O 2 /C 2 H 6 nude ratios (0 -2.0 and 0 -3.0 respectively), and ethane conversion increases [4]. The activation of ethane in the presence of C0 2 increases the formation of ethylene but not oxidation of ethane.
[5] researched on Ethylene production plant design 700 metric tons per day of ethylene production plant was carried using 140,010 lb/hr of 10% butane which was fed with 100% of 8174 lb/hr ethane recycled from the furnace reactor. Other products obtained were propylene, gasoline and high-pressure steam products are subsequently sold [6]. The expectation of the ethylene plant is to profit 160 million over a 10 yrs operation period and a returned investment of 16%. The plant was expected to 8400 hrs a year of 0.96 operating factors. The capital investment of the plant was $28,000,000 and $16,000,000 per year of auxiliary equipment and gave an annual operating cost of $20,500 per yr.
[7] worked on simulation and analysis of ethane cracking process. Coiled tubular reactors were used for the processing and cracking of light hydrocarbons (Ethane, propane, n-butane and their mixtures) at high temperatures and short residence times to obtained ethylene as the main product. The simulation of the industrial reactor unit with ethane as a feedstock for the molecular reaction scheme of 8 components and five (5) reactions was done. The predicted models result using plant data were compared with the industrial data and gave small derivations interns of pressure and temperature but negligible deviation with concentration. Also, the profile of temperatures and concentration for both models results and industrial results agrees.
[8] Ethylene is of great importance to the petrochemical industry where varieties of products such as bottles, housewares, antifreeze, food containers, pipes, carpets, toys, film, etc., the various chemicals produced from ethylene as raw material include: vinyl acetate, ethylene oxide, ethyl benzene, polyethylene, ethylene dichloride etc. [9] Polyvinyl chloride which is a byproduct of polyethylene accounts for about 70% usage in construction materials, pipe fittings, windows etc. and about 30% [10] worked on how ethylene can be obtained from natural gas through the method of oxidative coupling of methane and cold energy of LNG. Liquefied Natural Gas was used as feedstock for the oxidative coupling of methane and thereafter through cryogenic distillation process, ethylene was obtained, it was discovered that production cost of ethylene greatly depends on the market price of LNG and NG which was shown through the difference in price of LNG/NG as well as other co-products affects the cost of production of ethylene.
Worldwide production rate of Polyethylene is known to be about 85 metric ton/year due to its high demand and usage [11]. Figure 1 shows the process flow diagram for the production of polyethylene, it consists of three reactors namely: polymerization reactor operating at 6.5 bar and 50.7˚C, loop reactor operating at 6.5 bar 70.9˚C and finally gas phase reactor operating at 6.5 bar and 70.9˚C.
The purpose of this research is to design and simulate a process plant for the production of polyethylene using Aspen Hsys Veesion 8.8 software.

Materials
The Materials used in this work are Data from Indorama Eleme Petrochemicals Limited which includes: 1) Detailed process flow diagram.

Methods
The methods used to accomplish this research are outlined as follows: 1) Perform material and energy balance on each equipment unit using the principles of conservation of mass and energy.
2) Build the Hysys process model of the plant.
1) The material balance equation for each equipment unit can be written as follows:

Material Balance Result
The material balance results are presented in Tables 1-6 for all the various streams and units.

Energy Balance Result
The energy balance results are presented in Tables 7-12 for all the various streams and units.

Conversion Results
The Conversion results of the reactor conditions for both the multiple CSTRs and Single CSTR is presented below.  2) Multiple Reactor Sizing

Multiple CSTR Results
The size of the multiple CSTRs are fixed and hence the size is the same throughout From Tables 13-20 above, we observed that using multiple CSTRs in series at different conversions almost equals using a single CSTR at the same final conversion except that it leads to a non-isothermal behavior as temperature is not controlled as in the case of using multiple CSTRs in series.

Parameters for Compressor Sizing at 90% Conversion
The sizing of the four compressors at 90% conversion is given in Tables 21-24 below.  From Tables 21-24, we observe that a single CSTR requires a large volume at the same conversion with a multiple CSTRs in Series. The space time of the single CSTR is higher than that of the Multiple CSTR in since it just a single reactor so more time is spent to process a given volume of feed.

Sensitivity Analysis
A sensitivity analysis was performed to determine the effect of the following functional parameters given below. Figure 4 shows that the height of reactor increases with an increase in Fractional conversion. The increment is as a result of the formation of products along the height of the rector. Figure 5 shows how Fractional Conversion is Changing with the Volume of the Reactor, as the volume of the Reactor increases so does the Fractional conversion until it reaches its maximum value of 0.9. Figure 6 shows the behavior of Temperature with Fractional conversion, As the Temperature of the Reactor increases so does the fractional conversion as a result of the Heat of reaction released to form products which can either be exothermic or endothermic. When it is endothermic heat is absorbed from the environment but when it is exothermic heat is released to the environment.

Pressure Variation with Fractional Conversion
This is also seen to behaving like the Temperature graph where an increase in the pressure of the reactor also brings about increase in Fractional conversion as shown in Figure 7. Advances in Chemical Engineering and Science

Variation of Space Time with Fractional Conversion
Space time is also an important functional parameter in the design of continuous reactors. The space time gives the information of the amount of time required to process a given volume of feed at inlet conditions.       Figure 8 shows that as the space time of the reactor increases, the fractional conversion of the propane to propene also increase. Hence, it could be concluded from the plot that the conversion is directly proportional to the total amount of time spent by the reacting species in the reactor. The more time the reacting species spend in the reactor, the more the yield of products.

Heat Load
The heat load is the amount of heat required in maintaining the temperature of

Fractional Conversion
Heat generated per unit volume