Introduction Latest

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INTRODUCTION

This chapter will be discuss about material and energy balance and mechanical design of

the equipment in Polyethylene production. The production of Polyethylene is emphasized on

slurry process by usage of Chevron-Phillips Technology. For the material balance, it involved all

the equipment in the technology meanwhile for energy balance and mechanical design are

involved only four equipments that will be mention later.

Material Balance

A mass balance, also called a material balance, is an application ofconservation of

mass to the analysis of physical systems. Material balances are important first step when

designing a new process or analyzing an existing one. They are almost always prerequisite to all

other calculations in the solution of process engineering problems. For example, mass balance

theory is used to designchemical reactors,to analyze alternative processes to produce chemicals,

as well as to modelpollution dispersion and other processes of physical systems. Closely related

and complementary analysis techniques include thepopulation balance,energy balance and the

somewhat more complexentropybalance. These techniques are required for thorough design and

analysis of systems such as therefrigeration cycle.

In the production of High Density Polyethylene (HDPE), the minimum production was

230 000 MTA and the flow of the process operation need to be control in order to achieve the

target of the operation. By accounting for material entering and leaving a system, mass flows can

be identified which might have been unknown, or difficult to measure without this technique.

Therefore, mass balances are used widely inengineering andenvironmental analyses.Material

balance can be simple, at times they can be very complicated, but the basic approach is general.

Commonly the complete equation of material balance can be performed as:

InOut + GenerationConsumption = Build up
http://en.wikipedia.org/wiki/Conservation_of_masshttp://en.wikipedia.org/wiki/Conservation_of_masshttp://en.wikipedia.org/wiki/Chemical_reactorhttp://en.wikipedia.org/wiki/Pollutionhttp://en.wikipedia.org/wiki/Population_balance_equationhttp://en.wikipedia.org/wiki/Energy_accountinghttp://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Mass_flowhttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Environmental_analysishttp://en.wikipedia.org/wiki/Environmental_analysishttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Mass_flowhttp://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Entropyhttp://en.wikipedia.org/wiki/Energy_accountinghttp://en.wikipedia.org/wiki/Population_balance_equationhttp://en.wikipedia.org/wiki/Pollutionhttp://en.wikipedia.org/wiki/Chemical_reactorhttp://en.wikipedia.org/wiki/Conservation_of_masshttp://en.wikipedia.org/wiki/Conservation_of_mass


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A material balance is an accounting for material. Thus, material balances are often

compared to the balancing of current accounts. It was used in industry to calculate mass flow

rates of different streams entering or leaving chemical or physical processes. The general form

quoted for a mass balance is that the mass enters a system must, by conservation of mass, either

leave the system or accumulate within the system. Mathematically the mass balance for a system

without a chemical reaction is stated as:

The block diagram of the production of the High Density Polyethylene (HDPE) was

shown in figure 1.1. The amount of the output produces similar to the amount of the input stream

supply to the process operation condition. The calculations of material balance for High Density

Polyethylene (HDPE) production are based on this general equation.

Basically, there are five main equipments involved in Polyethylene material balance

which are loop reactor, flash chamber, dryer, purge column and extruder. The production of

Polyethylene in a year is 230,000 Tonnes per Annum where the plant is operate for 360 days.

The rest of five days is used for plant shutdown and turnaround in the purpose of maintenance.

The Polyethylene production was emphasized on High Density Polyethylene (HDPE) by using

slurry-loop process. This slurry-loop process has been using Chevron-Phillips Technology which

capable to achieve 95 to 98% conversion of ethylene into polyethylene in the process

polymerization. This technology is very high efficiency with low cost operation as well as

requires 30 to 60 minutes of residence time to mix in chemical reactors. Thus, the compositions

of each component at inlet and outlet stream for those five equipments were assumed as

mentioned in the process descriptions of Chevron-Phillips Technology.

Input = Output + Accumulation


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TO ETYLENE

RECOVERY

TO DILUENT

RECOVERY

PRODUCT

Loop Reactor

Dryer

Vent

Scrubber

Heat

Exchanger

Distillation

Column

Compressor

Flash

Chamber

Purge

Column

Extruder

3 7

13

12

8

11

18

9

3

3

Figure 1.1: Simplified Block Diagram of Polyethylene Production


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Energy Balance

Energy can exist in several forms: heat, mechanical energy, electrical energy, and it is the

total energy that is conserved. The law of conservation of energy states that energy can neither

be created nor destroyed. The total energy in the materials entering the processing plant, plus the

energy added in the plant must be equal the total energy leaving the plant.In process design,

energy balances are made to determine the energy requirements of the process: the heating,

cooling and power required. In plant operation, an energy balance on the plant will show the

pattern of energy usage, and suggest areas for conservation and savings. Energy takes many

forms, such as heat, kinetic energy, chemical energy, potential energy. A system is termed open

or closed according to whether or not mass crosses the system boundaries during the period of

time covered by the energy balance. The derivation used for energy balance can be state as :

Final system energy - initial system energy = net energy transferred to the system ( in

out). Which:

Initial system energy: UiEki- Epi

Final system energy: UfEkfEpf

Energy transferred: QW

Where the subscripts i and f refer to the initial and final states of the system U, Ek, Ep, Q

and W represent internal energy, kinetic energy, potential energy, heat transferred to the system

from its surrounding and work done by the system on its surrounding and the equation becomes,

U + Ek + Ep= QW

In this chapter the fundamentals of energy balances are reviewed briefly, and examples

given to illustrate the use of energy balances in process design. The main equipment involved in

the calculation for energy balance of production polyethylene are reactor, distillation

column,purge column and heat exchanger as shown in figure 1.2 for polyethylene process flow

diagram below.

.


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Figure 1.2: Equipment Involved in Energy Balance and Mechanical Design


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Mechanical Design

The selection, specification and design of the equipment required to carry out the

function of the process unit which the detail design of the equipment is needed. Process design

establishes the sequence of chemical and physical operations; operating conditions; the duties,

major specifications, and materials of construction of all process equipment. The general

arrangement of equipment needed to ensure proper functioning of the plant; line sizes; and

principal instrumentation. Many factors have to be considered when selecting engineering

materials during mechanical design, but for chemical process plant the overriding consideration

is usually the ability to resist corrosion. The process designer will be responsible for

recommending materials that will be suitable for the process conditions. All factors must be

consider for mechanical design in term of sizing, volume, heat transfer, pressure drop and type of

material in the equipment used. Mechanical design is important in order to determine the suitable

measurement for each equipment in the chemical plant before plant operating well. In this

chapter, those aspects of the mechanical design have been discussed detailed in the calculation

Others consideration also must be completely design to get the detail information for the

equipment. The equipment that will be considered for mechanical design are loop reactor,

distillation column, purge column and heat exchanger.


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MATERIAL BALANCE

LOOP REACTOR

General Equation:

nC2H4 (CH2-CH2) n

Where n: 2

2C2H4 (CH2-CH2) n

C4H4 C2H4C2H4

Presence of Hydrogen in the inlet of loop reactor (reactant), thus:

C4H4+ H2 C2H4C2H4

Balance the equation:

C4H4+ 2H2 C2H4C2H4

Stream 3 Stream 7

A = F B= 27581.59 kg/hr

0 kg/hr Ethylene 26202.51 kg/hr Polyethylene

0 kg/hr Hydrogen 827.447 kg/hr Ethylene

275.816 kg/hr Isobutane 137.908 kg/hr Hydrogen

137.908 kg/hr 1-Hexene 275.816 kg/hr Isobutane

137.908 kg/hr 1-Hexene

For the comonomer and diluent chemical which are 1-hexene and isobutene have equal

mass flowrate of inlet and outlet. This is due to the equation involved in the loop reactor are only

ethylene and hydrogen. The rest were comonomer and diluents chemical are used just to enhance

the process of polymerization.

Loop Reactor


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Table 1.1: Extend of Reaction Method for Loop Reactor

Component 0 i= o+

Ethylene 27029.96 -1 - 827.447= o-

Hydrogen 52542.93 -2 -2 137.908= o- 2

1-hexene 137.908 - - 137.908

Isobutene 275.816 - - 275.816

Polyethylene - +1 26202.51=

= 26202.51

Ethylene:

i= o+

827.447 = o+

827.447 = o+ 26202.51

o = 27029.96 kg/hr

Hydrogen:

i= o+

137.908 = o - 2

137.908 = o2(26202.51)

137.908 = o52405.02

o= 52542.93 kg/hr

TOTAL(Inlet), A= 01+ 02+ 03+ 04

= 27029.96 + 52542.93 + 137.908 + 275.816

A= 79 986.61 kg/hr


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FLASH CHAMBER

The unreacted diluent (Isobutane), any monomer vapors (ethylene) and all co-monomer

(1-hexene) recovered in flash vessel typically condensed and reused in polymerization process.

Meanwhile hydrogen was completely used in this flash chamber. The product leave the flash

chamber was polyethylene contain 98% purity and small excess ethylene.

Table 6.2: Inlet and Outlet Composition of Flash Chamber

Component Composition (weight/weight)

Inlet Stream (No. 7) Outlet Stream (No.8) Outlet Stream (No.9)

Ethylene 0.03 0.68 0.02

Isobutane 0.01 0.22 -

1-Hexene 0.005 0.10 -

Hydrogen 0.005 - -

Polyethylene 0.95 - 0.98

Stream 8, T = 417.90 kg/hr

284.17 kg/hr Ethylene

91.94 kg/hr Isobutane


Stream 7, F = 27581.59 kg/hr

2602.51 kg/hr Polyethylene

827.45 kg/hr Ethylene Stream 9

275.82 kg/hr Isobutane B= 27163.69 kg/hr


137.91 kg/hr hydrogen 26620.42 kg/hr Polyethylene


Flash Chamber


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Overall Mass Balance:

F = T + B

(F) = (T)+ 27163.69 kg/hr

Species Balance of Ethylene:

To find mass flowrate for Stream 8:

F (0.03) = T (0.68) + 27163.69 kg/hr (0.02)

0.03F = 0.68T + 543.274 kg/hr

0.03 (T+27163.69) = 0.68T + 543.274 kg/hr

0.03T + 814.910 kg/hr = 0.68T + 543.274 kg/hr

814.910 kg/hr543.274 kg/hr = 0.68T0.03T

271.636 kg/hr = 0.65T

T = 417.90 kg/hr


Stream 7 = Stream 8 + Stream 9

F = T + 27163.69

F = 417.90 + 27163.69

F = 27581.59 kg/hr


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Inlet Mass Flowrate of Each Component (Stream 7):

i) Polyethylene:

= 0.95 x 27581.59 kg/hr

= 26202.51 kg/hr

ii) Ethylene:

= 0.03 x 27581.59 kg/hr

= 827.45 kg/hr

iii) Isobutane:

= 0.01 x 27581.59 kg/hr

= 275.82 kg/hr

iv) 1-Hexene:

= 0.005 x 27581.59 kg/hr

= 137.91 kg/hr

v) Hydrogen:

= 0.005 x 27581.59 kg/hr

= 137.91 kg/hr

Total Inlet Mass Flowrate (Stream 7):

= 2614.01kg/hr + 827.45kg/hr + 275.82kg/hr + 137.91kg/hr + 137.91kg/hr

= 27581.59 kg/hr

Outlet Mass Flowrate of Each Component (Stream 8):

i) Ethylene:

= 0.68 x 417.90 kg/hr

= 284.17 kg/hr


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ii) Isobutane:

= 0.22 x 417.90 kg/hr

= 91.94 kg/hr

iii) 1-Hexene:

= 0.10 x 417.90 kg/hr

= 41.79 kg/hr

Total Outlet Mass Flowrate (Stream 8):

= 284.172 kg/hr + 91.938 kg/hr + 41.79 kg/hr

= 417.90 kg/hr


i) Polyethylene:

= 0.98 x 27163.69 kg/hr

= 26620.42 kg/hr

ii) Ethylene:

= 0.02 x 27163.69 kg/hr

= 543.27 kg/hr

Total Outlet Mass Flowrate (Stream 9):

= 26620.42 kg/hr + 543.27 kg/hr

= 27163.69 kg/hr


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DRYER

The inlet components enter the dryer were polyethylene and excess ethylene. The

removal of moisture content for the product occurred here which it generally will remove about

0.2% to 0.5% moisture content by weight.

Table 6.3: Inlet and Outlet Composition of Dryer


Inlet Stream (No.9) Outlet Stream (No.17)

Ethylene 0.02 0.02

Polyethylene 0.98 0.98


Since inlet stream equal to outlet stream, hence the mass flowrate of outlet component is

equal to mass flowrate of inlet component.

Stream 9 = Stream 17

in = out

Species Balance of Polyethylene:

in(0.98) = 27163.69(0.98)

0.98 in= 26620.42

in= 27163.69 kg/hr

Dryer





Stream 9

in = 27183.69 kg/hr

Stream 17

out= 27183.69 kg/hr


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i) Polyethylene:

= 0.98 27163.69

= 26620.42 kg/hr

ii) Ethylene:

= 0.02 27163.69

= 543.27 kg/hr


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COMPRESSOR



Top

Outlet Stream (No.12)

Bottom

Ethylene 0.67 0.65 -

Isobutane 0.22 - 0.95

1-Hexene 0.1 0.35 0.05

Stream 12, TopT = 398.62 kg/hr



Stream 8,

F = 417.90 kg/hr



41.79 kg/hr 1-Hexene Stream 12, Bottom

B =19.28 kg/hr

18.316 kg/hr Isobutene

0.964 kg/hr 1-hexene


F = T + B

417.90 = T + B

T = 417.90B

COMPRESSOR


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284.17 = T (0.65) - B (0)

284.17 = (417.90B) (0.65)

284.17 = 271.640.65 B

0.65B = 19.28

B = 19.28 kg/hr

To find T

T = 417.9019.28

T = 398.62 kg/hr


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HEAT EXCHANGER



Ethylene 0.65 0.99

1-Hexene 0.35 0.01


Stream inlet = Stream Outlet

A= B

Stream 13

Ethylene: 398.62 kg/hr x 0.99

=394.63 kg/hr

1-hexene: 398.62 kg/hr x 0.01= 3.986 kg/hr

Heat Exchanger





Stream 12, Top

A = 398.62 kg/hr

Stream 13

B = 398.62 kg/hr


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VENT SCRUBBER


Inlet Stream

(No.13)

Outlet Stream (Top) Outlet Stream (Bottom)

Ethylene 0.99 1.00 -

1-Hexene 0.01 - 1.00

Overall Mass Balance:Stream inlet = Stream Outlet

A = T + B

398.62 kg/hr = T + B

Vent Scrubber

139.29 kg/hr ethylene

394.63 kg/hr ethylene


Stream 13

A = 398.62 kg/hr

Top Stream

T= kg/hr

Kg/hr 1-hexene

Bottom Stream

B= kg/hr


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Species Balance of Ethylene

398.62 (0.99) = T (1.00) + B (0)

T= 394.63

394.63 = 398.62 - B

B = 398.62394.63

B = 3.99 kg/hr


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DISTILLATION COLUMN


Inlet Stream (No.12) Outlet Stream (Top) Outlet Stream (Bottom)

Isobutane 0.95 - 1.00

1-Hexene 0.05 1.00 -

Top Product

T = 18.316 kg/hr


Stream 12, Bottom

F = 19.28 kg/hr



Bottom Product

B = 0.964 kg/hr

0.964 kg/hr1-Hexene

Overall Mass Balance

F = T + B

19.28 = T + B

Species balance of Isobutene:

F (0.95) = T (1.00) + B (0)

19.28 (0.95) = T

Distillation column


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18.316 = T

T = 18.316 kg/hr

To find T:

18.316 = 19.28B

19.2818.316 = B

B = 0.964 kg/hr


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PURGE COLUMN

The product and excess ethylene leave the dryer enter the purge column for removing

residual liquid from the polymer solids. The purge means is typically a nitrogen purge column

through which nitrogen gas is fed to remove accumulated liquid from polymer slurry. The excess

amount of ethylene as well as nitrogen will be recycled to purification column for further usage

in polymerization process.

Table 6.4: Inlet and Outlet Composition of Purge Column


Inlet Stream (No.

17)

Inlet Stream (No.

10)

Outlet Stream

(No. 18)

Outlet Stream

(No. 11)

Ethylene 0.02 - 0.05 -

Polyethylene 0.98 - - 1.00

Nitrogen - 1.00 0.95 -

Purge Column

10322.21 kg/hr Nitrogen



26620.42 kg/hr Polyethylene543.27 kg/hr Ethylene

10322.21 kg/hr Nitrogen

Stream 17, F1

Stream 10, F2

Stream 18, T

Stream 11

B= 26620.42 kg/hr


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F1 + F2= T+B

Species Balance of Polyethylene:

F1 (0.98) + F2 (Polyethylene) = T (Polyethylene) + 26620.42(1.00)

Mass flowrate for Stream 17:

F1 (0.98) = 26620.42

F1 = 27163.69 kg/hr



F1 (0.02) + F2 (ethylene) = T (0.05) + B (ethylene)

27163.69 (0.02) = 0.05T

T = 10865.48 kg/hr

Mass flowrate for Stream 10:

F1 + F2= T+B

27163.69 + F2 = 10865.48 + 26620.42

F2 = 10322.21 kg/hr


i) Polyethylene:

= 0.98 27163.69

= 26620.42 kg/hr

ii) Ethylene:

= 0.02 27163.69

= 543.27 kg/hr


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i) Nitrogen:

= 1.00 10322.21

= 10322.21 kg/hr


i) Nitrogen:

= 0.95 10856.48

= 10322.21kg/hr

ii) Ethylene:

= 0.05 10856.48

= 543.27 kg/hr


i) Polyethylene:

= 26620.42 kg/hr


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EXTRUDER

The final product which is polyethylene goes to the last equipment for pelletizing.

Extrusion process takes place where is a process used to create objects of a fixed cross-sectional

profile. A material is pushed or drawn through a die of the desired cross-section.

Table 6.5: Inlet and Outlet Composition of Extruder


Inlet Stream (No. 11) Finishing Stream

Polyethylene 1.00 1.00

Stream 11 Finishing Stream

in = 26620.42 kg/hr out =26620.42 kg/hr

26620.42 kg/hr Polyethylene 26620.42 kg/hr Polyethylene


Since inlet stream equal to outlet stream, hence the mass flowrate of outlet component is

equal to mass flowrate of inlet component.

Stream 11 = Finishing Stream

in = out

Inlet Mass Flowrate of Polyethylene (Stream 11):

= 1.00 x 26620.42 kg/hr

= 26620.42 kg/h

Extruder

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