ethylene and acetylene plant
DESCRIPTION
design of an eEthylene is one of the most important and largest volume petrochemicals in the world today. It is used extensively as a chemical building block for the petrochemical industry. The importance of ethylene results from the double bond in its molecular structure that makes it reactive. Ethylene can be converted industrially into a variety of intermediate and end products [41]. The major use of ethylene is conversion to low and high-density polyethylene. Other significant uses of ethylene include chlorination to ethylene dichloride, used in the manufacture of the polyvinyl chloride (PVC), oxidation to the ethylene oxide, an intermediate in the manufacture of polyester fibres and films, and the conversion to ethyl benzene, an intermediate in the manufacture of polystyrene [23].Ethylene has become an important industrial intermediate and various technologies have been utilized in ethylene production. Recently, ethylene has taken the place of acetylene in virtually all large-scale chemical synthesis. However, acetylene itself is a by-product of modern ethylene production [50].More than 97% of ethylene around the world is produced by pyrolysis of hydrocarbons, which is the thermal cracking of petrochemicals in the presence of steam. This process can be described as the heating of a mixture of steam and hydrocarbon to the necessary cracking temperature depending on the hydrocarbon used. This mixture is then fed to a fired reactor or furnace and heated. As a result, the original saturated hydrocarbon “cracks” into smaller unsaturated molecules. This process is extremely endothermic, and the product2must be cooled back to the original feed temperature upon leaving the reactor in order to minimize secondary reactions. [2]Chemical companies have a variety of options for feedstock as well as processes to produce ethylene. Economics and environmental issues are the dominant factors considered in the choice of feedstock and processes of ethylene production.The focus in this report will be on the steam pyrolysis of hydrocarbons mainly ethane. There are several reasons for this choice which include the cost of production, availability of raw materials and the viability of process.TRANSCRIPT
-
1
CHAPTER 1
1.0 INTRODUCTION
1.1 Relevance of work
Ethylene is one of the most important and largest volume petrochemicals in the world
today. It is used extensively as a chemical building block for the petrochemical industry.
The importance of ethylene results from the double bond in its molecular structure that
makes it reactive. Ethylene can be converted industrially into a variety of intermediate and
end products [41]. The major use of ethylene is conversion to low and high-density
polyethylene. Other significant uses of ethylene include chlorination to ethylene dichloride,
used in the manufacture of the polyvinyl chloride (PVC), oxidation to the ethylene oxide,
an intermediate in the manufacture of polyester fibres and films, and the conversion to ethyl
benzene, an intermediate in the manufacture of polystyrene [23].
Ethylene has become an important industrial intermediate and various technologies have
been utilized in ethylene production. Recently, ethylene has taken the place of acetylene in
virtually all large-scale chemical synthesis. However, acetylene itself is a by-product of
modern ethylene production [50].
More than 97% of ethylene around the world is produced by pyrolysis of hydrocarbons,
which is the thermal cracking of petrochemicals in the presence of steam. This process can
be described as the heating of a mixture of steam and hydrocarbon to the necessary
cracking temperature depending on the hydrocarbon used. This mixture is then fed to a
fired reactor or furnace and heated. As a result, the original saturated hydrocarbon cracks
into smaller unsaturated molecules. This process is extremely endothermic, and the product
-
2
must be cooled back to the original feed temperature upon leaving the reactor in order to
minimize secondary reactions. [2]
Chemical companies have a variety of options for feedstock as well as processes to produce
ethylene. Economics and environmental issues are the dominant factors considered in the
choice of feedstock and processes of ethylene production.
The focus in this report will be on the steam pyrolysis of hydrocarbons mainly ethane.
There are several reasons for this choice which include the cost of production, availability
of raw materials and the viability of process.
1.2 Objectives
The main objective of this project is to develop a simplified plant design for the production
of ethylene and acetylene which includes a thermal cracking section, quenching section, gas
compression/separation, ethylene purification, and an integrated refrigeration section.
The design is aimed at estimating the production of ethylene and acetylene using ethane as
a feedstock and also to determine the yield of ethylene and acetylene using the steam
pyrolysis process. The design is also aimed at determining the feasibility of the steam
pyrolysis process on an industrial scale.
-
3
CHAPTER 2
2.0 LITERATURE REVIEW
2.1 Chemistry of ethylene and acetylene
2.1.1 Ethylene
Ethylene (IUPAC name: ethene) is a gaseous organic compound with the formula C2H4. It
is the simplest alkene (older name: olefin from its oil-forming property). Ethylene has four
hydrogen atoms bound to a pair of carbon atoms that are connected by a double bond and
hence is classified as an unsaturated hydrocarbon. All six atoms that comprise ethylene are
coplanar. The H-C-H angle is 119, close to the 120 for ideal sp hybridized carbon. The
molecule is also relatively rigid: rotation about the C-C bond is a high energy process that
requires breaking the -bond. The -bond in the ethylene molecule is responsible for its
useful reactivity. [12]
Ethylene has a boiling point temperature of -103.7C, a melting point temperature of -
169.2C, and a flash point temperature of -136.1C. Physical properties of ethylene include:
It is colourless
It is flammable
It has a slightly sweet smell at normal condition, that is ambient temperature and
one atmosphere
2.1.2 Acetylene
Acetylene (IUPAC name: ethyne) with the chemical symbol C2H2 is a hydrocarbon
consisting of two hydrogen atoms and two carbon atoms. As an alkyne, acetylene
is unsaturated because its two carbon atoms are bonded together in a triple bond. The
-
4
carbon-carbon triple bond places all four atoms in the same straight line, with CCH bond
angles of 180. [16]
Acetylene is an extremely reactive hydrocarbon. It is moderately soluble in water or alcohol
and markedly soluble in acetone. Acetylene has a melting point temperature of -81.5C and
a boiling point temperature of -84C. Physical properties include:
It is a combustible gas
It has a distinctive odour
Acetylene is colourless
Once the gas is compressed, liquefied, mixed or heated with air, it becomes very explosive.
2.1.3 Uses of ethylene and acetylene
The major use of ethylene is conversion to low and high-density polyethylenes, which are
used in such applications such as construction, communications, packaging, and
manufacturing of industrial and domestic products.
Other significant uses of ethylene include chlorination to ethylene dichloride, used in the
manufacture of the polyvinyl chloride (PVC), oxidation to the ethylene oxide, an
intermediate in the manufacture of polyester fibers and films, and the conversion to
ethylbenzene, an intermediate in the manufacture of polystyrene [16]. In addition, ethylene
is also a major raw material to produce plastics, textiles, paper, solvents, dyes, food
additives, pesticides, and pharmaceuticals. [12]
Acetylene is used for the production of oxy-acetylene flame. The temperature of the flame
is above 3000oC. It is employed for cutting and welding of metals. Another common use of
acetylene is as a raw material for the production of various organic chemicals including
-
5
1,4-butanediol, which is widely used in the preparation of polyurethane and polyester
plastics. Acetylene is also used for artificial ripening of fruits.
2.2 Chemistry of the Ethylene Process
Ethylene, because of its double bond, is a highly reactive compound, which is converted to
multi-intermediates and end-products on a large scale industrially. The thermal cracking
process is the most interesting process to produce ethylene commercially. In general the
starting raw material for ethylene production by thermal cracking can be any kind of
hydrocarbon. In reality, the choice of starting material is narrowed by economical
considerations. [26]
As the molecular weight of the feedstock increases, the product complexity increases.
Because many reactions occur during thermal cracking, it is complicated to determine the
rate of the cracking and predict the distribution of the products. Yet, investigations have
confirmed that the primary reaction, which splits the original hydrocarbon, is unimolecular
and that conversion rates follow the first order kinetics for a wide range of molecular
weight and up to high conversion of the original reactant, if there is no distinct equilibrium
barrier [17].
2.3 Market survey
2.3.1 Global market
In the past ten years, ethylene demand and price have fluctuated based upon the economical
growth in the United Stated and the rest of the industrial world. [7]
Although many economic uncertainties surround the petrochemical industry, ethylene
production and consumption should grow because of continuing replacement of natural and
-
6
inorganic materials with organic synthetics and the further development of radically new
synthesis materials [26].
Our target market is producers of polyethylene products, PVC, and the likes. Some of the
current global market prices of ethylene are as follows: $900/tonne in Asia, 970/tonne and
840/tonne in Europe. [9]
2.3.2 Local market
A research on the local market for ethylene did not produce significant values. However
there are industries in Ghana which can use ethylene and acetylene as raw materials or
intermediates. Our target market is the food industries, plastics industries, paints, ripening
of fruits, packaging, and for use in welding. Examples of such industries are Qualiplast,
Duraplast, Interplast, Blue Skies Ghana, and Ezzy Paints.
2.4 Feedstock
A variety of feedstock can be used in a steam pyrolysis process. The feedstock for an
ethylene plant could be methane, ethane, propane and heavier paraffin. With the
development of cracking technology, it can also be cracked from crude oil fractions:
naphtha, kerosene and gas oil. Sometimes, raffinates from aromatics extraction facilities
can also be used as feedstock. The choice of feedstock is a compromise of availability,
price and yield. In selecting a process for ethylene production, the most important factor is
the hydrocarbon feedstock. Although this is controlled by conditions like quantity, quality,
and economics, studies have shown that as the molecular weight of the feed hydrocarbon
increases, ethylene yield decreases.
2.4.1 Methane (CH4)
-
7
Methane is the first member of the alkane series and is the main component of natural gas.
It is also a by-product in all gas streams from processing crude oils. It is a colourless,
odourless gas that is lighter than air. Methane is mainly used as a clean fuel gas. It is also
an important source for carbon black. Methane may be liquefied under very high pressures
and low temperatures. Liquefaction of natural gas (methane), allows its transportation to
long distances through cryogenic tankers. [17]. Methane, though an important and abundant
fuel, has not been an attractive raw material for ethylene production, because it is thermally
stable and has no carbon-carbon bonds. The carbon-hydrogen bond requires more energy to
break than the carbon-carbon bond. The C-H bond energy is 93.3 Kcal, whereas C-C
energy bond is 71.0 Kcal [6]. The net reaction for methane dehydrogenation is
2CH4 C2H4 + 2H2 (1)
2.4.2. Ethane (CH3-CH3)
It is the second member of the alkanes and is mainly recovered from natural gas liquids.
Ethane, like methane, is a colourless gas that is insoluble in water. After methane, ethane
has the second highest composition in natural gas. Ethane is separated most efficiently from
methane by liquefying it at cryogenic temperatures. Various refrigeration strategies exist,
but the most economical process presently in wide use employs turbo-expansion, and can
recover over 90% of the ethane in natural gas. [24] The principal use of ethane is in
chemical industry, mainly, in the production of ethylene by steam cracking. Ethane is
favoured for ethylene production because the steam cracking of ethane is fairly selective for
ethylene. Ethane may be cracked alone or as a mixture with propane. [2]
The net dehydrogenation reaction of ethane is
C2H6 C2H4 + H2 (2)
-
8
2.4.3 Propane C3H8
Propane is normally a gas, but it is compressible to a liquid that is transportable.
It is derived from other petroleum products during oil or natural gas processing. Propane,
also known as liquefied petroleum gas (LPG), can be a mixture of propane with small
amounts of propylene, butane and butylenes. Propane is a by-product of natural gas and
petroleum refining. Propane is used as a feedstock for ethylene production. The production
of ethylene from propane is similar to the process of ethylene production from ethane. [2]
In the dehydrogenation of propane four initial reaction steps are conceivable when
producing ethylene and propylene; however, according to Sherwood [24, 25] and Martin
[19] the first two reactions are primary. The reactions are
C3H8 C2H4 + CH4 . (3)
C3H8 C2H6 + H2 ... (4)
2C3H8 C2H8 + 2CH4 . (5)
2C3H8 C2H6 + C3H6 + CH4 .. (6)
2.4.4 Naphtha
Naphtha, an important feedstock for ethylene production, is a collective of liquid
hydrocarbon intermediate oil refining products. It is a mixture of hydrocarbons in the
boiling point range of 30-200 C. For the naphtha cracker process, typical feedstock are
light naphthas (boiling range of 30-90 C), full range naphthas (30-200 C), and special cuts
(C6-C8 raffinates) [29]. Naphtha is obtained in petroleum refineries as one of the
intermediate products from the distillation of crude oil. The processing of light naphtha to
ethylene is similar to the ethane and propane processes.
-
9
2.4.5 Kerosene
This is a distillate fraction heavier than naphtha, and is normally a product from distilling
crude oils under atmospheric pressures. It may also be obtained as a product from thermal
and catalytic cracking or hydrocracking units. Kerosene is usually a clear colourless liquid
which does not stop flowing except at very low temperature (normally below -30C).
However, kerosene containing high olefin and nitrogen contents may develop some colour
(pale yellow) after being produced. Currently, kerosene is mainly used to produce jet fuels,
after it is treated to adjust its burning quality and freezing point. Before the widespread use
of electricity, kerosene was extensively used to fuel lamps, and is still used for this purpose
in remote areas. It is also used as a fuel for heating purposes. [17]
2.4.6 Gas oil
Gas oil is a heavier petroleum fraction than kerosene. It can be obtained from the
atmospheric distillation of crude oils (atmospheric gas oil, AGO), from vacuum distillation
of topped crudes (vacuum gas oil, VGO), or from cracking and hydrocracking units.
Atmospheric gas oil has a relatively lower density and sulphur content than vacuum gas oil
produced from the same crude. The aromatic content of gas oils varies appreciably,
depending mainly on the crude type and the process to which it has been subjected. A
major use of gas oil is as a fuel for diesel engines. Another important use is as a feedstock
to cracking and hydrocracking units. Gases produced from these units are suitable sources
for light olefins and LPG. [17]
2.4.7 Natural Gas
As a feedstock, natural gas yields ethylene from its ethane or propane content and forms the
basis of a massive chemical industry. Large reserves exist in many regions of the world.
-
10
Much of the natural gas appears in regions that are remote from markets or pipe lines, and it
is called stranded gas, which is a natural gas field that has been discovered, but remains
unusable for either physical or economic reasons. Most of this gas is flared, re-circulated
back into oil reservoirs, or not produced. In addition, natural gas has a major disadvantage
in transportation. Because of the low density of natural gas, pipeline construction is very
expensive. [2]
2.4.8 Choice of feedstock
The choice of feed stock is an important economic decision as it influences other costs as
well. For the reasons following, the choice of feedstock for our steam pyrolysis is ethane.
Subject to availability, ethane is the best feedstock, as it has higher yield and selectivity of
ethylene than heavier feed stocks and its processing is relatively simple, involving lower
capital costs. Another reason for choosing ethane as feedstock is, ethylene plants based on
light hydrocarbons are much simpler and cheaper to build and operate than plants designed
to use heavy feedstock. The plant has to employ much greater control over the composition
of the final product once the heavier feedstocks are cracked and more variety of
components comes. The choice for a particular feedstock, together with processing
conditions (heat, pressure, steam dilution rate) will determine the yield of ethylene,
propylene and other co-products in steam cracking. Manufacturing plants fed with ethane
and propane can be constructed at much lower investment costs than naphtha crackers.
Table 2.1 shows how product yield varies with feedstock type. If ethane is used as
feedstock, almost no propylene, butadiene and aromatics are formed as by-products.
Our key suppliers of ethane would include Texas Gas Service, Alliance Pipeline, BP
Amoco Co., Chevron Texaco, Duke Energy Co., and Shell Oil.
-
11
Table 2.1 Approximate material balance of pyrolysis with different feed stock
Products,% mass Gaseous feed Liquid feed
Ethane Propane butanes naphtha gas-oil
H2 and methane 13.0 28.0 24.0 26.0 23.0
Ethylene 80.0 45.0 37.0 30.0 25.0
Propylene 1.1 14.0 16.4 14.1 14.4
Butadiene 1.4 2.0 2.0 4.5 5.0
Butene mixture 1.6 1.0 6.4 8.0 6.0
C5+ 1.6 9.0 12.6 18.5 32.0
Ratio propylene/ethylene 0.003 0.3 0.5 0.4 0.6
Propylene content into C3
fraction
86.7 58.3 99.0 98.3 96.7
2.5 General Processes for Ethylene Production
Commercially ethylene is obtained by (1) thermal cracking of hydrocarbons such as ethane,
propane, butane, naphtha, kerosene, gas oil, crude oil, etc, (2) autothermic cracking (partial
oxidation) of the above hydrocarbons, (3) recovery from refinery off-gas, (4) recovery from
coke-oven gas, and (5) catalytic dehydration of ethyl alcohol or ethyl ether. Occasionally,
raffinates from aromatics extraction facilities are used as a supplementary raw material. Of
the five methods above, small quantities of ethylene are recovered from coke oven gas and
gases produced from crude oil directly [5] but this route to ethylene has for a variety of
technical and economic reasons, so far not gained commercial significance.
-
12
The first step in ethylene production is thermal cracking of the hydrocarbon feedstock.
Thermal cracking of natural gas liquids (NGL) or crude oil fractions in the presence of
steam is still the dominant method for the production of ethylene. This thermal
decomposition results from adding heat to the feed to break its chemical bonds. The steam
does not enter directly into the reaction, but it enhances the product selectivity and reduces
coking in the furnace coils. The product of this thermal cracking process is a mixture of
hydrocarbons, which extends from hydrogen and methane to gasoline and gas oil [28].
Most current ethylene processes are basically similar to each other. Ethylene plants use
similar separation units.
In the following sections, each step of ethylene production will be discussed.
2.5.1 Thermal Cracking Section
The first section of ethylene production process is thermal cracking. Thermal cracking is
the heart of an ethylene plant. This section produces all the products of the plant, while
other sections serve to separate and purify the products. Additionally, this section has the
greatest effect upon the economics of the process. Various types of pyrolysis reactors have
been proposed and commercialized for the thermal cracker. These pyrolysis reactors
include (1) direct heating (2) indirect heating (3) autothermic cracking and others.
The direct heating process using fired tubular heater is the most common cracker in an
ethylene plant. In this process a variety of the hydrocarbon feedstock can be used ranging
from ethane to gas oil. Steam is added to the hydrocarbon feed for several reasons: (1)
reduce the partial pressure of hydrocarbon, (2) lower the residence time of the hydrocarbon,
and (3) decrease the rate of coke formation within the tubes by reaction of steam with
carbon to form carbon monoxide and hydrogen.
-
13
Thermal cracking of hydrocarbons by indirect heating include the pebble bed reactors, the
fluidized bed reactors, and regenerative furnace. Even though construction costs seem to be
more expensive and the operation to be more complex, ethylene yield by indirect heating is
higher than that obtained in the fired tubular heaters. One of the advantages of the indirect
heating reactors is that crude oil and heavy fuel oil can be used as feedstock because the
coke by-product can be removed continuously or intermittently in the process [6].
The pyrolysis gas leaving the cracker usually has a temperature in the range of 375C to
500C in the case of naphtha pyrolysis and typically from 500C to 600C in the case of
gas oil pyrolysis. The outlet temperature depends upon the amount of the carbon deposits in
the transfer line exchanger [26]. Quenching of the conversion product or rapid temperature
reduction is important to prevent the decrease of ethylene yields caused by secondary
reactions. This is carried out either by transfer line exchangers or by injecting water and oil.
2.5.2 Gas Compression and Treatment Section
In addition to the thermal cracking section, the sections for removal of acid gases, drying of
the cracked gases, removal of acetylenic compounds, and purification of ethylene are also
very important, because an efficient ethylene plant is the result of the integration of these
process sections and because, in respect to cost, the thermal cracking section is only about
20-30% of the whole plant. In addition, the goal is to produce ethylene with high purity
above 99.9%.
Most ethylene processes call for compression of the pyrolysis gas leaving the quench tower.
Consequently, the cooled cracked gas leaving the water tower is compressed in four to five
stages. Plants based upon gaseous feedstock generally employ four stages, while many
naphtha-and gas oil-based plants employ five stages of pyrolysis gas compression. Between
-
14
compression stages, the cracked gas is usually cooled in water-cooled exchangers. Water
and hydrocarbons condensed between stages are separated from the pyrolysis gas in inter-
stage separators.
Hydrogen sulphide and carbon dioxide are removed from pyrolysis gas between the third
stage and fourth stage of the compression system. This location is optimum because the
actual gas volume has been reduced significantly in the first three stages of compression
while acidic components are still present in the gas stream [26]. Acid gas produced in
thermal cracking must be removed before the first major fractionation step. In removing
acid gases such as carbon dioxide and hydrogen sulphide, non-regenerative caustic washing
followed by water washing is employed in the most of the existing plants and proves to be
most economic. The pyrolysis gas leaving the caustic scrubber contains less than 1 ppm of
acid gases and hence assures that the final products of the plant will meet specification in
this respect.
Compressed cracked gas usually is dried to reduce the moisture content to 1 ppm or less
and avoid problems with freezing and hydrate formation in downstream low temperature
equipment. In drying the cracked gases, alumina, silica gel, and molecular sieves are used
commercially. Among them, molecular sieves seem to have an economic advantage over
conventional desiccants because of their higher desiccant activities and lower regeneration
temperatures [17]. Recovery of acetylene and removal of acetylenic materials from the
process gas is very important in manufacturing polymer-grade ethylene.
2.5.3 Recovery and Purification Section
After the cracked gases have been quenched, compressed, freed of the acid gases, and
dried, they generally contain hydrogen and light hydrocarbons in the C1-C6 range.
Depending upon the cracking method employed, carbon monoxide and nitrogen also may
-
15
be present. Low temperature straight fractionation, absorption, and selective adsorption are
three different methods to recover and purify ethylene. The aim of this section is to separate
ethylene and acetylene from hydrogen and methane fractions, ethane and propane fractions,
and heavier hydrocarbons. Commercial separation processes operate at four ranges of
pressure: 450-600 psia, 100-150 psia, 70-90 psia, and 30-40 psia. The most popular is the
450-600 psia because it offers an attractive combination of purity, recovery, efficiency, and
investment for large ethylene plants. [41]
In ethylene purification section, demethanized process streams are introduced to the de-
ethanizer in most cases. The de-ethanizer is a simple tower refrigerated by propane or
propylene to make reflux. The net overhead from the de-ethanizer flows to an ethylene-
ethane separator. This is the second most costly separation step in an ethylene plant because
the volatility is low and a large amount of reflux is required.
2.5.4 The Refrigeration Section
The separation of pyrolysis gas through condensation and fractionation at cryogenic
temperatures requires external refrigeration and is an important part of the ethylene system.
An ethylene refrigerator has two or three stages for a total of between five and seven stages
for the entire refrigeration cascade. Reflux ratios in the columns are selected carefully to
avoid large refrigeration consumption [47].
-
16
CHAPTER 3
3.0 PROCESS SELECTION AND DESCRIPTION
The different processes in ethylene production include steam pyrolysis, catalytic pyrolysis,
recovery from fluid catalytic cracking off gas, autothermic and fluidized-bed cracking, and
membrane reactor.
3.1 Steam pyrolysis
The most commonly used process is steam pyrolysis of hydrocarbons. The feedstock,
mixed with dilution steam, enters the cracking section and is pyrolysised by heat into small
components. The pyrolysis gas enters the quench section and is cooled there to some
controlled temperature. Water enters the water quench tower, a part of quench section,
cooling down the high temperature pyrolysis gas and becoming steam. That steam, called
dilution steam, mixes with the feedstock before entering the pyrolysis section to decrease
the partial pressure of the cracked gases and slow coke formation. Finally, the pyrolysis
gas goes into the separation section to be separated into a variety of desired final products.
[33]
Figure 3.1 A simplified ethylene plant diagram sheet. [33]
QUENCH
SECTION CRACKING
SECTION
SEPARATION
SECTION Final
product Feedstock
Steam
Pyrolysis
gas
Water
-
17
3.2 Catalytic pyrolysis
A catalytic pyrolysis process for production of ethylene from heavy hydrocarbons,
comprises heavy hydrocarbons that are contacted with a pillared inter layered clay
molecular sieve or other high silica zeolite containing catalysts in a riser or down flow
transfer line reactor in the presence of steam. It is catalytically pyrolysed at a temperature
of 650 C to 750 C and a pressure of 0.15 to 0.4 MPa for a contact time of 0.2 to 5
seconds. The weight ratio of catalyst to feedstock ranges from 15:1 to 40:1 and the weight
ratio of steam to feedstock is about 0.3:1 to 1:1.
Catalytic pyrolysis combines catalytic cracking and steam pyrolysis and has the advantages
of both catalytic cracking and steam pyrolysis. It can raise the yields of light olefins,
expand the flexibility of products distribution, and simultaneously lower reaction
temperature and decrease energy consumption for the whole system; so it has broad
application prospect. The raw material is usually crude oil.
Considering that the feed that is used in catalytic pyrolysis is crude oil, this process is not
exactly feasible in Ghana for the production of ethylene and acetylene. This is due to the
shortage in supply of Ghanas crude oil. Furthermore, the crude oil which is imported
mainly from Nigeria and Equatorial Guinea is chiefly refined to produced petroleum,
diesel, kerosene, etc. which is highly useful on the market. Furthermore, this process
requires the use of catalysts in large quantities, and which would require frequent plant shut
down in case of short catalyst life.
3.3 Autothermic and fluidized bed cracking
Most of the autothermic cracking processes produce acetylene as a main product and
ethylene as a by-product. Most of these processes operate at atmospheric pressure, and
-
18
hydrocarbon feedstock, air, or oxygen and fuels are preheated to about 593C to reduce the
oxygen consumption and increase the yield. The process of ethylene production by
autothermic cracking is based upon the thermal cracking of crude oil using fluidized beds.
[2]
Fluidized bed reactors are relatively new tools in the chemical engineering field developed
for the oil and petrochemical industries. Here catalysts are used to reduce petroleum to
simpler compounds through cracking. Today fluidized bed reactors are still used to produce
gasoline and other fuels, along with many other chemicals. Many industrially produced
polymers are made using FBR technology, such as rubber, vinyl chloride, polyethylene,
and styrene. A major advantage of this process is the ability to operate the reactor in a
continuous state. However because of the expansion of the materials in the reactor, a larger
vessel is often required, which increases the cost of production. Again, the fluid-like
behaviour of fine solid particles within the bed eventually results in the wear and tear of the
reactor vessel. This requires expensive maintenance which adds to the cost of production.
3.4 Membrane reactor
Membrane reactors may be used in either batch or continuous mode, and allow the easy
separation of the enzyme from the product. Due to the ease with which membrane reactor
systems may be established, they are often used for production on a small scale (g to kg),
especially where a multi-enzyme pathway or co-enzyme regeneration is needed.
Membrane reactors combine reaction with separation to increase conversion. One of the
products of a given reaction is removed from the reactor through the membrane, forcing the
equilibrium of the reaction "to the right" (according to Le Chatelier's Principle), so that
more of that product is produced. Membrane reactors are most commonly used when a
-
19
reaction involves some form of catalyst. [8] There are two main types of these membrane
reactors (1) the inert membrane reactor and (2) the catalytic membrane reactor.
The inert membrane reactor allows catalyst pellets to flow with the reactants on the feed
side (usually the inside of the membrane). In this kind of membrane reactor, the membrane
does not participate in the reaction directly; it simply acts as a barrier to the reactants and
some products. [14]
Catalytic Ceramic Membrane is a system for the dehydrogenation of ethane to produce
ethylene and hydrogen through the use of a catalytic ceramic membrane having selective
permeability, thus permitting the separation of hydrogen from the reaction zone which
causes further dehydrogenation of ethane. The catalytic ceramic membrane tube is enclosed
within an alloy tube of suitable composition to permit heating to the temperature range of
300 to 650 C. The reactor is connected to a recovery system which permits separation of
pure ethylene and unconverted ethane. A steady stream of H2O or argon continuously
sweeps away the H2 coming out through the selective membrane, thereby further
facilitating the conversion process. [14]
The membrane reactors major advantage is its combination of reaction and separation to
produce a good amount of conversion and yield. However, the membranes (ceramic and
metallic) are poor in mechanical strength and need to be replaced at regular intervals.
Another major disadvantage is the cost of the membranes and its low resistance to harsh
environments. Also, the membrane reactors are usually used for production on small scale
(g to kg). [11] Considering the amount of ethylene and acetylene we want to produce,
which runs into thousands of tonnes, this process is not recommended.
-
20
3.5 Fluidized catalytic cracking
Fluidized catalytic cracking (FCC) is an important process in oil refineries. It upgrades
heavy hydrocarbons to lighter more valuable products by cracking, and is the major
producer of gasoline in refineries. FCC Units present challenging multivariable control
problems.
The heavy molecule cracking process occurs in a riser tubular reactor, at high temperatures,
building up fuel gas, LPG, cracked naphtha (gasoline), light cycle oil, decanted oil, and
coke. The coke deposits on the spent catalyst surface, causing its deactivation. The catalytic
activity is re-established by coke combustion in a fluidized bed reactor, dominated
regenerator. The system riser-regenerator is called the converter. Steam lifts the heated
regenerated catalyst to be combined with the oil in the riser so that the oil-catalyst mixture
rises in an ascending dispersed stream to the separator. The control valve manipulates the
quantity of hot regenerated catalyst from the standpipe to the "riser" in order to maintain a
predetermined outlet riser temperature. On the top of the separator, the catalyst particles are
separated from vapour products by cyclones. The stream transfers the reaction products
overhead to the products recovery section. The standpipe transfers spent catalyst
continuously from the separator to the regenerator by a control valve.
3.6 Choice of Process
Based on the comparisons above, steam pyrolysis shall be used in this project. Steam
pyrolysis is one of the most important processes of petrochemistry. The main advantage of
this process compared with other processes is that it is quite flexible in terms of feed stock.
In addition steam pyrolysis is the best economical solution to produce ethylene and
acetylene, because other methods are more expensive.
-
21
3.7 Process description (steam pyrolysis)
The fresh feed of ethane is combined with the recycled ethane from the ethylene column
and charged to the pyrolysis furnace. Dilution steam is added to ethane before it enters the
cracking furnace, to reduce the partial pressure of ethane and lower the residence time of
ethane in the high temperature zone, which decrease the rate of coke formation within the
tubes. The mixture of ethane and steam is preheated in the convection section of the
furnace, and the ethane cracks in vertical tubes within a residence time of 0.1 to 0.5 s. The
cracked gas leaves the furnace at 800 C and 8.0107kPa and is quickly cooled to 340 C in
the transfer line exchangers to preserve gas composition. It generates 370C steam at a
pressure of 16690kPa. The gas is then further quenched in quench towers by direct contact
with water where the gases leave as overhead to the compressor and the quench water is
separated and recycled.
The cracked gases are then compressed in four stages. Acid gases such as carbon dioxide,
are removed after the third stage of compression. The effluent gas leaves the compression
section at 42C and 3500kPa from which it is dried and cooled in a series of heat
exchangers. It is then passed to a de-methanizer where methane and hydrogen is separated
as overhead. The net bottom stream of the demethanizer is charged to the de-ethanizer.
The overhead vapour from the de-ethanizer is partially condensed by heat exchange and
propylene refrigerant. The bottom stream leaving the de-ethanizer contains C3+
hydrocarbons (mostly propane) that are stored in C3+ storage tanks. The net overhead from
the de-ethanizer is the ethylene-ethane stream with traces of acetylene. This stream is then
fed to the C2splitter to separate ethylene and acetylene from ethane. The ethane leaves as
bottoms product and is recycled back to the furnace. The overhead vapour (ethylene-
acetylene mixture) is forwarded to an acetylene absorber where acetone is used as the
-
22
extracting solvent. The gas is then finally sent to the ethylene column where high purity
polymer grade ethylene is recovered as product. The bottoms product containing mainly
acetylene is sent to the acetylene stripper where acetylene is recovered by further
separation.
3.8 Capacity
Our plant is likely to have a capacity of 100,000 tonnes per year of ethylene and about 550
tonnes per year of acetylene. This is because we are operating in a continuous process. Our
main product is ethylene. Acetylene is only a by-product, which is recovered in our quest to
produce 99.95% polymer-grade ethylene to meet market demands. The percentage yield for
acetylene is about 0.2% when using ethane.
-
23
CHAPTER 4
4.0 MATERIAL AND ENERGY BALANCES
The general material balance equation is
( Material out) = ( Material in ) + ( Material generation ) (Material consumption )-
( Material accumulation)
Assumptions:
1. Steady state, no accumulation
2. All masses are calculated on hourly basis
The quantity of ethylene produced per annum = 13888.89kg = 100000tonnes
Plant attainment is 300 days to allow for downtime for maintenances.
The calculations that resulted in the charts shown in this chapter are represented in the
appendix A
-
24
4.1 SUMMARY OF MATERIAL BALANCES
4.1.1 FURNACE
Fresh ethane
Component mass
flowrate
(kg/hr)
Mass,
%
Ethane 29189.55 100
TOTAL 29189.55 100
Temperature: 70C
Flue gases
Component mass
flowrate
(kg/hr)
Mass,
%
CO2 44 13.3
O2 9.6 2.9
N2 242.2 73
H2O(v) 36 10.8
TOTAL 331.85 100
Temperature: 250C
Material Steam 1
Component mass
flowrate
(kg/hr)
mass,
%
H2O 5259.6 100
TOTAL 5259.6 100
Temperature: 180 C
Material stream 2
Component mass
flowrate
(kg/hr)
mass (%)
Methane 1062.4992 3.08
Ethane 10216.35 29.59
Propane 2643.5992 7.66
H2O 5201.3132 15.06
Hydrogen 1018.2284 2.95
Acetylene 82.2172 0.24
Ethylene 14166.656 41.03
CO2 139.1368 0.40
TOTAL 34530.00 100.00
Temperature: 840.8 C
Pressure : 107kPa
Fuel/air
Component mass
flowrate
(kg/hr)
Mass,
%
Methane 1544 5.48
Air 25890.0922 94.52
TOTAL 27390.5322 100
Temperature: 25 C
Pressure: 101.325kPa
Pyrolysis furnace
-
25
4.1.2 TRANSFER LINE EXCHANGER
Material stream 2
Component mass
flowrate
(kg/hr)
mass (%)
Methane 1062.4992 3.08
Ethane 10216.35 29.59
Propane 2643.5992 7.66
H2O 5201.3132 15.06
Hydrogen 1018.2284 2.95
Acetylene 82.2172 0.24
Ethylene 14166.656 41.03
CO2 139.1368 0.40
TOTAL 34530.00 100.00
Temperature: 840.8 C
Pressure : 101kPa
Cooling water 1
Component Mass
flowrate
(Kg/hr)
Mass %
H2O 10620 100
TOTAL 10620 100
Temperature: 25C
Pressure: 4177 kPa
Material stream 3
Component mass
flowrate
(kg/hr)
mass
(%)
Methane 1062.4992 3.08
Ethane 10216.35 29.59
Propane 2643.5992 7.66
H2O 5201.3132 15.06
Hydrogen 1018.2284 2.95
Acetylene 82.2172 0.24
Ethylene 14166.656 41.03
CO2 139.1368 0.40
TOTAL 34530.00 100.00
Temperature: 350 C
Pressure : 150kPa
Steam at 500 C
Component Mass
flowrate
(Kg/hr)
Mass %
H2O 10620 100
TOTAL 10620 100
Temperature: 500C
Pressure: 16690 kPa
Transfer-line exchanger
-
26
4.1.3 QUENCH TOWER
Material stream 3
Component mass
flowrate
(kg/hr)
mass
(%)
Methane 1062.4992 3.08
Ethane 10216.35 29.59
Propane 2643.5992 7.66
H2O 5201.3132 15.06
Hydrogen 1018.2284 2.95
Acetylene 82.2172 0.24
Ethylene 14166.656 41.03
CO2 139.1368 0.40
TOTAL 34530.00 100.00
Temperature: 350 C
Pressure : 150 kPa
Material stream 4
Component mass
flowrate
(kg/hr)
mass
fraction
(%)
Methane 1062.4992 3.62
Ethane 10216.35 34.83
Propane 2643.5992 9.01
H2O 5.25402 0.02
Hydrogen 1018.2284 3.47
Acetylene 82.2172 0.28
Ethylene 14166.656 48.29
CO2 139.1368 0.47
TOTAL 29333.940
8
100
Temperature: 34 C
Pressure : 1930 kPa
Cooling water 2
Component Mass
flowrate
(Kg/hr)
Mass %
H2O 26864.82 1.0
TOTAL 26864.82 1.0
Temperature: 30C
Pressure: 4177 kPa
Water out
Component mass
flowrate
(kg/hr)
mass
fraction
H2O 141737.094 100
TOTAL 141737.094 100
Temperature: 80 C
Pressure : 6987 kPa
Quench tower
-
27
4.1.4 CAUSTIC TOWER
Material stream 5
Component mass
flowrate
(kg/hr)
mass
fraction
(%)
Methane 1062.4992 3.62
Ethane 10216.35 34.83
Propane 2643.5992 9.01
H2O 5.25402 0.02
Hydrogen 1018.2284 3.47
Acetylene 82.2172 0.28
Ethylene 14166.656 48.29
CO2 139.1368 0.47
TOTAL 29333.9408 100
Temperature: 35 C
Pressure : 3500 kPa
Caustic solution
Component Mass
flowrate
(Kg/hr)
Mass
fraction
NaOH(aq) 45068.4 100
TOTAL 45068.4 100
Temperature: 30C
Pressure: 101.325 kPa
Material stream 6
Component mass
flowrate
(kg/hr)
Mass, %
Methane 1062.4992 3.64
Ethane 10216.35 34.99
Propane 2643.5992 9.06
H2O 5.25402 0.02
Hydrogen 1018.2284 3.49
Acetylene 82.2172 0.28
Ethylene 14166.656 48.52
CO2 trace 0.00
TOTAL 29194.70971 100
Temperature: 38 C
Pressure : 3500 kPa
Spent caustic solution
Compon
ent
Mass
flowrate
(Kg/hr)
Mass %
Na2CO3 9631.954 20
H2O 38520.498 80
TOTAL 48152.452 100
Temperature: 40C
Pressure: 101.325 kPa
Caustic tower
-
28
4.1.5 SPRAY TOWER
Material stream 7
Component mass
flowrate
(kg/hr)
mass
(%)
Methane 1062.4992 3.64
Ethane 10216.35 34.99
Propane 2643.5992 9.06
H2O 5.25402 0.02
Hydrogen 1018.2284 3.49
Acetylene 82.2172 0.28
Ethylene 14166.656 48.52
CO2 trace 0.00
TOTAL 29194.70971 100
Temperature: 40 C
Pressure : 3500 kPa
Cooling water 3
Component mass
flowrate
(kg/hr)
mass %
H2O 87.9234 100
TOTAL 87.9234 100
Temperature: 21.5 C
Pressure : 101 kPa
Material stream 8
Component mass
flowrate
(kg/hr)
mass
(%)
Methane 1062.4992 3.64
Ethane 10216.35 34.99
Propane 2643.5992 9.06
Hydrogen 1018.2284 3.49
Acetylene 82.2172 0.28
Ethylene 14166.656 48.52
CO2 trace 0.00
TOTAL 29194.70971 100
Temperature: 40C
Pressure : 3500 kPa
Water out
Component mass
flowrate
(kg/hr)
mass %
H2O 87.9234 100
TOTAL 87.9234 100
Temperature: 80 C
Pressure : 101kPa
Spray tower
-
29
4.1.6 DEMETHANIZER
Material stream 9
Component mass
flowrate
(kg/hr)
mass %
Methane 1062.4992 3.97
Ethane 10216.35 34.28
Propane 2643.5992 9.06
Hydrogen 1018.2284 3.60
Acetylene 82.2172 0.27
Ethylene 14166.656 49.00
TOTAL 28910.6902 100
Temperature: -120.0 C
Pressure : 2000 kPa
Material stream 10
Component mass
flowrate
(kg/hr)
mass
%
Methane 1062.4992 48
Hydrogen 1018.2284 46
Ethylene 141.29108 6
TOTAL 2267.6624 100
Temperature: -127 C
Pressure : 3200 kPa
Material stream 11
Component mass
flowrate
(kg/hr)
mass %
Methane 11.448 0.04
Ethane 9909.4343 37.20
Propane 2621.0494 9.84
Acetylene 76.7424 0.29
Ethylene 14024.7251 52.63
TOTAL 26643.4 100
Temperature: 6.05 C
Pressure : 2600 kPa
Demethanizer
-
30
4.1.7 DE-ETHANIZER
Material stream 11
Component mass
flowrate
(kg/hr)
mass %
Methane 11.448 0.04
Ethane 9909.4343 37.20
Propane 2621.0494 9.84
Acetylene 76.7424 0.29
Ethylene 14024.7251 52.63
TOTAL 26643.4 100
Temperature: 6.05 C
Pressure : 3006 kPa
Material stream 12
Component mass
flowrate
(kg/hr)
mass
%
Methane 5.048 0.02
Ethane 9909.435 41.4
Acetylene 76.7416 0.32
Ethylene 13954.6 58.28
TOTAL 23945.82 100
Temperature: -34.95 C
Pressure : 3000 kPa
Propane product
Component mass
flowrate
(kg/hr)
mass %
Ethylene 110.31 0.04
Propane 2623.79 0.96
TOTAL 2734.1 100
Temperature: 40.00 C
Pressure : 3000 kPa
De-ethanizer
-
31
4.1.8 C2 SPLITTER
Material stream 12
Component mass
flowrate
(kg/hr)
Mass %
Methane 5.048 0.02
Ethane 9909.435 41.4
Acetylene 76.7416 0.32
Ethylene 13954.6 58.28
TOTAL 23945.82 100
Temperature: -34.95 C
Pressure : 3080 kPa
Material stream 13
Component mass
flowrate
(kg/hr)
mass %
Ethylene 13919.217 0.55
Acetylene 76.7416 99.45
TOTAL 13995.96 100
Temperature: -10.7 C
Pressure : 3200 kPa
Material stream 14
Component mass
flowrate
(kg/hr)
mass %
Methane 10.6528 99.53
Ethane 9909.435 0.11
Ethylene 35.385 0.36
TOTAL 9955.47 100
Temperature: -7.901 C
Pressure : 2000kPa
C2 splitter
-
32
4.1.9 ACETYLENE ABSORBER
Material stream 14
Component mass
flowrate
(kg/hr)
mass %
Ethylene 13919.217 99.45
Acetylene 76.7416 0.54
TOTAL 13995.96 100
Temperature: -10 C
Pressure : 3200 kPa
Material stream 15
Component mass
flowrate
(kg/hr)
mass %
Acetylene 1.5977 0.011
Ethylene 13900 99.98
TOTAL 13901.535 100
Temperature: 20 C
Pressure: 1722.44kPa
Material stream 17
Component mass
flowrate
(kg/hr)
mass %
Acetone 127.201 100
TOTAL 127.201 100
Temperature: 35 C
Pressure: 101.325 kPa
Material stream 16
Component mass
flowrate
(kg/hr)
mass %
Acetylene 75.2066 37.16
Acetone 127.201 62.84
TOTAL 202.4076 100
Temperature: 30 C
Pressure : 1000 kPa
Acetylene absorber
-
33
4.1.10 ACETYLENE STRIPPER
Material stream 16
Component mass
flowrate
(kg/hr)
mass %
Acetylene 75.2066 37.16
Acetone 127.201 62.84
TOTAL 202.4076 100
Temperature: 25.00 C
Pressure : 2138 kPa
Material stream 17
Component mass
flowrate
(kg/hr)
mass %
Acetone 127.201 100
Acetylene
TOTAL 127.201 100
Temperature: 31.30 C
Pressure: 40.09 kPa
Material stream 18
Component mass
flowrate
(kg/hr)
mass %
Acetylene 79.8872 100
Acetone
TOTAL 79.8872 100
Temperature: 34.09 C
Pressure : 6000 kPa
Acetylene stripper
-
34
4.1.11 ETHYLENE COLUMN
Material stream 15
Component mass
flowrate
(kg/hr)
mass %
Acetylene 1.5977 0.011
Ethylene 13900 99.98
TOTAL 13901.535 100
Temperature: 20 C
Material stream 20
Component mass
flowrate
(kg/hr)
mass %
Ethylene 13888.89 99.9
Acetylene 0.0135 0.1
TOTAL 13888.9 100
Temperature: -10 C
Pressure: 2500 kPa
Material stream 19
Component mass
flowrate
(kg/hr)
mass %
Acetylene 1.5842 12.49
Ethylene 11.1 87.51
TOTAL 12.68 100
Temperature: -30 C
Pressure : 2500 kPa
Ethylene Column
-
35
4.2 ENERGY BALANCES
4.2.1 FURNACE
Fresh ethane (stream 1)
Component Enthalpy
(kJ)
Ethane 2445111.3
TOTAL 2445111.3
Temperature: 70 C
Steam (stream 2)
Component enthalpy
(kJ)
H2O 1551719.1
TOTAL 1551719
Temperature: 180 C
Component Enthalpy
(kJ)
Methane 3101484.4
Ethane 26843935.6
Propane 6885500.9
H2O 8968631.9
Hydrogen 11856702.8
Acetylene 154495.7
Ethylene 31518168.1
CO2 140909.5
TOTAL 89469828.9
Pyrolysis furnace
-
36
4.2.2 QUENCH TOWER
Component Enthaply
(kJ)
Methane -196337745.9
Ethane 82223346.4
Propane 6876775.4
H2O 1589144.4
Hydrogen 221078167.1
Acetylene 13753328.5
Ethylene 9533474.4
CO2 75212.34
TOTAL 138791702.6
Componen
t
Enthalpy
(kJ)
Methane -44726466.4
Ethane 82218800.9
Propane 6877143
H2O 6977.6
Hydrogen 22107265.9
Acetylene 654218
Ethylene 96107084
CO2 4167.8
TOTAL 138791702.6
Cooling water (stream 9)
Component Enthalpy
H2O 187605993
TOTAL 187605993
Temperature: 30C
Pressure: 4.177 kPa
Material stream 8
Component Enthalpy
H2O 18176207.5
TOTAL 18176207.5
Temperature: 80 C
Pressure : 6987 kPa
Quench tower
-
37
4.2.3 CAUSTIC TOWER
Component Enthalpy
(kJ)
Methane -44726466.4
Ethane 82218800.9
Propane 6877143
H2O 6977.6
Hydrogen 22107265.9
Acetylene 654218
Ethylene 96107084
CO2 4167.8
TOTAL 138791702.6
Component Enthalpy
(kJ)
Methane -350957.8
Ethane -6953247.8
Propane 96371.2
H2O 711.3
Hydrogen 20846495.5
Acetylene -66039.6
Ethylene 597934.1
CO2 5414.2
TOTAL 13578747
Caustic soda
Component Enthalpy (kJ)
NaOH 159261400
TOTAL 159261400
Temperature: 30C
Pressure: 4.177 kPa
Spent caustic solution
Component Enthalpy
(kJ)
Na2CO3(aq) 166306735.6
TOTAL 166306735.6
Temperature: 40C
Caustic tower
-
38
4.2.4 DEMETHANIZER
Component Enthalpy
(kJ)
Methane -1058204.1
Ethane 5382174.6
Propane -863333.2
Hydrogen -6524474.9
Acetylene -5711589.7
Ethylene 8242969.9
TOTAL
Component Enthalpy
(kJ)
Methane 1094052.6
Hydrogen -1009895.2
TOTAL 84157.4
Component Enthalpy
(kJ)
Ethane -704981.6
Methane 420434.0
Propane -130952.4
Acetylene -7726.2
Ethylene 8086124.9
TOTAL 7662898.7
Demethanizer
-
39
4.2.5 DE-ETHANIZER
Component Enthalpy
(kJ)
Methane 4650.7
Ethane -706674.8
Propane -131409.9
Acetylene -6957.5
Ethylene -1145519.4
TOTAL -1985910.9
Component Enthalpy
(kJ)
Methane 10154.4
Ethane -1980235.4
Acetylene -19722.6
Ethylene -3058549.5
TOTAL -5048353.1
Component Enthalpy
(kJ)
Propane 110680.1
Ethylene 5306.8
TOTAL 115986.9
De-ethanizer
-
40
4.2.6 C2 SPLITTER
Component Enthalpy
(kJ)
Methane -4909.6
Ethane -2026150.9
Acetylene 36772.2
Ethylene -2986782.9
TOTAL -4981071.2
Component Enthalpy
(kJ)
Acetylene 30135.8
Ethylene -2318046.8
TOTAL -2287911
Component Enthalpy
(kJ)
Methane -2323.5
Ethane 3174817.8
TOTAL 3172494.3
C2 splitter
-
41
4.2.7 ETHYLENE COLUMN
Component Enthalpy
(kJ)
Acetylene -36.7686
Ethylene -317733.97
TOTAL -317770.7386
Component Enthalpy
(kJ)
Ethylene -430.4059
Acetylene -155.096
TOTAL -585.5019
Component Enthalpy (kJ)
Acetylene -76.4246
Ethylene -2836982.474
TOTAL -2837058.903
Ethylene column
-
42
CHAPTER 5
5.0 EQUIPMENT SPECIFICATIONS
Specifications for all processing equipment based on the operating conditions and flow
rates of the input and output streams among others form a major part in plant design.
The major considerations under equipment specification are:
1. Identification of the equipment
2. Function of the equipment
3. Basic design data
4. Material of construction
Information and data used are from Stanley M. Walas (1999), R K Sinnott (1999), Perry
and Green (1999). [49, 43]
5.1 Equipment list
5.1.1 Pyrolysis furnace
Duty: To crack the ethane feedstock into smaller hydrocarbons under carefully controlled
temperature to yield the optimum amount of ethylene and acetylene.
Type or description: cylindrical
Height: 15.16m
Operating temperature: 1200C
Heat duty: 23476.165KW
Material of construction: Stainless steel (SS 310) and Refractory brick
-
43
5.1.2 Heat exchanger
Duty: Immediately quenches the cracked gases to a lower temperature to stop further
undesired reactions and coke formation
Type or description: Transfer-line exchanger
Temperature: 700C
Quantity: 1
Material of construction: stainless steel
5.1.3 Quench tower
Duty: to further cool cracked gas and condense water vapour present by direct contact with
water
Type or description: packed tower
Operating temperature: 350C to 34C
Pressure: 150kPa
Height: 18m
Material of construction: austenitic stainless steel type 304
5.1.4 Gas compressor
Duty: increases the pressure of the gas to liquefy it for the distillation and separation
processes.
Type: centrifugal compressor
Output pressure: 3500kPa
-
44
Number: 4-stage
Material of construction: Carbon steel
5.1.5 Caustic tower
Duty: To remove the acid gases CO2, from the ethylene gas stream
Type: packed tower
Operating temperature: 35C
Height of tower: 6.846m
Tower diameter: 1.9155m
Vessel volume: 19.729m3
Material of construction: carbon steel
5.1.6 Spray tower
Duty: To dry the gas stream of water (vapour) before cooling it for distillation.
Type: spray tower
Operating temperature: 42C
Material of construction: carbon steel
5.1.7 Chilling train.
Duty: the gas is cooled and in turn is liquefied for distillation
Type: series of heat exchangers
Number: 3
Operating temperature: -120C
-
45
Material of construction: stainless steel
5.1.8 De-methanizer
Duty: To separate and remove methane and hydrogen from the ethylene gas stream
Temperature: 120C
Pressure: 30bar
Height: 15m
Diameter: 1.4m
Material of construction: carbon steel
5.1.9 De-ethanizer
Duty: To separate the C2s and C3s
Temperature: 6C
Pressure: 32bar
Height: 25 m
Diameter: 1.5 m
Number: 1
Material of construction: carbon steel
5.1.10 C2-splitter
Duty: To separate or split the C2 into ethylene and acetylene as overhead and ethane as
bottom stream to be recycled back to the furnace.
-
46
Type: continuous type tray column
Operating temperature: -24.95C
Feed pressure: 30bar
Height: 14m
Diameter: 1.7 m
Material of construction: carbon steel
5.1.11 Ethylene Column
Duty: To recover and obtain our final ethylene product.
Type: tray tower
Temperature: 20C
Height: 78.2 m
Diameter: 3.6 m
Material of construction: stainless steel
5.1.12 Acetylene Absorber
Duty: To separate the ethylene as overhead into the ethylene still and acetylene as bottoms
to the acetylene stripper.
Type: packed tower
Temperature: 21C
Height: 19.09m
-
47
Diameter: 3.248m
Material of construction: stainless steel
5.1.13 Acetylene Stripper
Duty: To strip the acetylene from the extracting solvent used in the acetylene absorption
column.
Type: packed tower
Temperature: 20C
Material of construction: carbon steel
5.2 SPECIFICATION OF STORAGE TANKS
Tanks are typically filled to 80% of capacity to function safely. [43]
See appendix C for detailed calculations
5.2.1 Ethane storage tank
Duty: To temporarily store the ethane feedstock before cracking.
Type or description: cylindrical vertical tank on concrete support.
Capacity: 7,427,160.362gal (US)
Internal diameter of ethane storage tank =
Length of ethane storage tank = 50.1667 in = 1.274m
Thickness of tank = 25mm
Material of construction: Carbon steel
-
48
5.2.2 Propane storage tank
Duty: To store propane produced from the cracking process temporarily.
Description: Vertical cylindrical tank with flat base and conical roof [43]
Capacity: 3,537,431gal (US) per week
Internal diameter =
Length = 42.376in = 1.076m
Thickness of tank = 28.18mm
Material of construction: carbon steel
5.2.3 Acetylene storage tank
Duty: To temporarily store acetylene produced
Type or description: cylindrical vertical tanks with flat base on concrete foundation.
Capacity: 3,374,331.513 gal (US) per week
Internal diameter =
Length = 42.05 in =1.068 m
Thickness of tank = 25mm
Material of construction: carbon steel
5.2.4 Ethylene storage tank
Duty: To store our polymer-grade ethylene produced for 7 days.
Type or description: flat bottomed vertical cylindrical tank on concrete foundation.
Capacity: 6,361,192.115 gal (US) per week
-
49
Internal diameter =
Length = 48in = 1.22m
Thickness of tank = 25mm
Material of construction: Carbon steel
5.3 PIPE SPECIFICATION
The most common means of transporting fluid is the pipeline. Every pipe is a long,
cylindrical, completely enclosed conduit used to transport gas, liquid, or both from one
point to another. Sizing of pipes for fluid flow in a given plant does not only depend on the
fluids physical properties, but also to some extent, on the sound economic factors. In most
engineering practices under this heading, the criterion used is the optimum diameter which
is the diameter of the pipe that gives the least total cost for annual pumping charges. The
design parameters considered are:
1) The nominal size
2) Schedule number
3) Material of construction
4) Wall thickness
Approximately, Schedule numberS
1000
Where P = Internal pressure, psig.
S = Allowable working stress in psi.
The optimum diameter is first of all estimated based on the fluid density, capacity and
viscosity depending on the nature of the fluid.
-
50
According to Sinnott, the optimum diameter of a stainless steel pipe is given as:
d,optimum = 230G 0.52-0.37
where, G = Mass flow rate in kg/s and = density in kg/m3
5.3.1 SAMPLE CALCULATION FOR PIPE SPECIFICATION
Pipe Location: from acetylene absorber to acetylene stripper
Mass flow rate = 13574.7079 kg/hr =3.85 Kg/s
Density of gas = 1.73 kgm-3
The optimum pipe diameter for turbulent flow using stainless steel pipe is given as:
dopt = 260G0.52-0.37 .. (1) [43]
Where: G = mass flow rate of feed
= density of slurry
It implies, dopt = 260(3.77075)0.52
(1.73) -0.37
= 428mm, 16.85 in
From the above calculation, a 428mm (16.85in) pipe diameter can be used.
Reynolds number, d
G
4Re (2) [43]
Where G is mass flow rate.
428.0001.077075.34
Re
-
51
Re = 11217
Re is greater than 4000 and hence flow is turbulent.
From Wallas (1990) Table A5
For optimum pipe diameter of 16.85 in
Nominal size = 16 in
Pipe schedule number = 30s
Outer diameter (do) = 16.00 in
Inner diameter (di) = 15.25 in
2222 1405.0423.044
mmD
The normal fluid velocity is given by:
. (3) [44]
1471.02686.8
77075.3
msu
Also Maximum design fluid velocity is assumed to be given by the correlation;
Maximum design fluid uu 2max 2.1 .. (4) [44]
12max 67824.0471.02.1 msu
-
52
5.3.1.1 Line Equivalent length
The pressure loss through the bends and check valves can be included in the line pressure-
loss calculations as an equivalent length of pipe. Assuming all the bends to be 90 elbows
of standard radius, and the isolation valves as plug-type valves.
Elbow equivalent length = 30D....................................... (9) [44]
= 30 x 0.428 m
= 12.84 m
Plug-valve equivalent length = 18D................................ (10) [44]
= 18 x0.428 m
= 7.704 m
Entry losses (at maximum design velocity) are calculated from the equation:
Entry loss = KPau
4.02
678.073.1
2
22
max
-
53
Table 5.1 Summary of pipe line specifications for our ethylene plant
Locatio
n
Optim
um
diamet
er
(mm)
Nomi
nal
Size(i
n)
Sched
ule
Numb
er
Material
of
Construct
ion
Outer
diamete
r
(in)
Inner
diamet
er
(in)
Cross-
sectio
nal
area(i
n2)
Nor
mal
fluid
veloc
ity
(m/s)
Maxim
um
design
fluid
velocit
y (m/s)
From
Furnace
to TLE 655.53 25.83 20
Stainless
steel 24 23.25 0.3375 2.297 3.3082
From
TLE to
quench
tower 423.24 16.66 40
Stainless
steel 16.00 15.25 0.2277 2.713 3.906
From
quench
tower
to
compre
ssor
733.00
28.87
20
Stainless
steel
29
28.80
0.422
1.464
2.108
From
compre
ssor to
caustic
tower
467.6
18.40
40
Stainless
steel
18.00
17.25
0.252
2.452
3.53
From
caustic
tower
to dryer
737.96 29.05 20
Stainless
steel
29.20 28.70 0.425 1.453 2.094
From
dryer to
cooler
729.29 28.65 20
Stainless
steel
28.70 28.40 0.421 1.434 2.077
From
cooler
to de-
methani
zer
721.28
28.40
20
Stainless
steel
28.40
28.20
0.418
1.413
2.035
From
de-
methani
zer to
de-
ethaniz
er
696.56
27.42
30
Stainless
steel
27.45
27.00
0.403
1.353
1.95
From
de-
ethaniz
er to C2
splitter 659 25.95 30
Stainless
steel 26 25.50 0.3806 1.43 1.206
-
54
From
C2
splitter
to
acetyle
ne
absorbe
r
495.15
19.5
30
Stainless
steel
19.5
0
19.25
0.193
1.89
2.73
From
acetyle
ne
absorbe
r to
acetyle
ne
stripper
428
16.85
30
Stainless
steel
16.6
0
16.00
0.1405
0.471
0.678
From
acetyle
ne
absorbe
r to
ethylen
e still
491 19.33 30
Stainless
steel
19.3
0
19.00 0.190 0.64 0.920
From
ethylen
e still to
storage
tank
44.95
17.70
30
Stainless
steel
17.7
0
17.00
0.0152
4.00
5.76
5.4 PUMP SELECTION
Centrifugal pumps will be used throughout the process. These pumps are characterised by
their specific speed which is a dimensionless variable. Different types of pumps have
different efficiency envelopes according to their specific speed. Pump selection is made
based on the flow rate and the head required, together with other process considerations,
such as corrosion or the presence of solids in the fluid. The pressure developed by a
centrifugal pumps depend on:
Fluid density
Diameter of the pump impeller
The rotational speed of the impeller
-
55
Volumetric flow rate through the pump
5.4.1 PUMP SPECIFICATION
Sample Calculation
Location: Between acetylene absorber and ethylene still
Volumetric flow rate = 836.934 m3s
-1
From the above pipe specification,
Optimum pipe diameter = 16.66in
Nominal size = 16 in
Pipe schedule number = 30s
Outer diameter (do) = 16 in
Inner diameter (di) = 15.25 in
Velocity of fluid in the pipe = 0.678ms-1
Reynolds number of fluid = 11350
5.4.1.1 Power requirement
Total pump head, scdc HHH
g
VZ
g
P
g
VZ
g
PH scsc
scdcdc
dc
22
22
Therefore,
g
VVZ
g
PPH scdcscdc
2
22
-
56
Where Pdc = Discharge pressure, 405 KPa
Psc = Suction pressure, 40 KPa
The suction velocity, 1873.20034.0
0097688.0 msVsc
The discharge velocity, 1284.3000033.0
0001084.0 msVdc
The total discharge head,
g
VZ
g
PH dcdc
dcdc
2
2
The total suction head,
g
VZ
g
PH dcdc
dcdc
2
2
But the Total pump head, scdc HHH
g
VZ
g
P
g
VZ
g
PH scsc
scdcdc
dc
22
22
Therefore,
g
VVZ
g
PPH scdcscdc
2
22
Where, mZZZ scdc 2
that is the height difference at the centre line of the pump between the suction and
discharge pipe. [44]
KPaPP scdc 365
1411.0 msVV scdc
-
57
= 790kg/m3; g = 9.81m/s2
Hence, mH 11.4981.92
411.02
81.9790
365000
Useful power, gQHPuseful
Q = 0.0097688; H = 49.11m ; = 790kg/m3;
g = 9.81m/s2
Hence, WPuseful 433.371811.4900977.081.9790
The value of specific speed represents the ratio of the pump flow rate to the head at the
speed corresponding to the maximum efficiency point. It depends primarily on the design
of the pump and impeller. The specific speed can be used to avoid cavitations or to select
the most economical pump for a given system layout.
The value of specific speed can be calculated from the relation;
4
3
H
QNNs .. (11)
Where N is in rpm (1750rev/min), Q in gpm (586.2gpm), and H in feet (15.41ft).
903.936
122.161
2.5861750
4
3sN
Specific speeds for centrifugal pumps usually lie the range 900-15000 but values above
12000rpm are considered impractical .Since the calculated value lies within the range it
suggest that the calculated value is correct.
-
58
5.4.1.2 Net positive suction head
NPSH is the absolute pressure at the pump inlet expressed in feet of liquid, plus velocity
head, minus the vapour pressure of the fluid at pumping temperature, and corrected to the
elevation of the pump centreline in the case of horizontal pumps or to the entrance to the
first-stage impeller for vertical. Thus if NPSH is zero or less, the liquid can vaporise. The
NPSH increases as the pump capacity increases. Hence it is important to consider the range
of flow requirement during the pump selection time.
Net positive suction headg
PP vapi
(12)
Where Pi = absolute static pressure at the pump inlet, N/m2
Pvap = Vapour pressure, N/m2 = 0.1233 x 10
5 Pa (Rogers and Mayhew)
satmi gHPP (13)
Patm = atmospheric pressure, N/m2
= density of pulp
Hs = Suction head
Inserting values into equation (12), it implies,
Net positive suction head m444.3481.9790
12330589.279272
-
59
Table 5.2 Summary of pump specification
Pump location Qty Power
requirement
(w)
Net positive
suction head
(m)
Specific
speed
(r/min)
Efficiency
From TLE to Quench
Tower
1 1637 21 293 65
For pumping caustic
solution into the
caustic tower
1 1142.82 15.095 192 60
For pumping acetone
into the acetylene
absorber
1 3718.433 34.44 862 72
-
60
CHAPTER 6
6.0 DESIGN OF A FURNACE
6.1 Problem statement
To design a cracking furnace to crack ethane feedstock to yield ethylene and acetylene as
products. Furnace to operate at thermal efficiency of 85 %.
Figure 6.1 A schematic diagram of a typical industrial furnace
6.2 CHEMICAL ENGINEERING DESIGN
Air
Fuel
Stack
Stack damper
Convection section
sesection
Radiant section
Cracked gas
Ethane feed
Stea
m
-
61
6.2.1 SCOPE OF DESIGN
Design constraints
Total energy absorbed
Total Energy absorbed
Heat flux across the cracking coils
Heat transfer coefficient across tube
Pressure drop across tubes
Stack height
Fuel and air requirement
6.2.2 Design constraints
Furnace geometry cylindrical
Tube diameter OD = 0.168275 m (6.625 in.)
Center-to-center spacing =0.3048 m (12 in.)
Tube thickness =0.00762 m (0.3 in.)
Diameter of the radiant section = 5 m
Number of tubes in the radiant section = 30
Number of tubes in the convective section = 16
Tube length = 10.7 m
-
62
Height of the radiant section = 11.5 m
Methane use as fuel
Excess air 10%
A single row tube alignment
6.2.3 Total energy absorbed
6.2.3.1 Reactions in the furnace:
C2H6 C2H4 + H2 Reaction 1
2C2H6 C3H8 + CH4 Reaction 2
C3H8 C2H2 + CH4 + H2 Reaction 3
C + 2H2O CO2 + 2H2 Reaction 4
Since there are a series of reaction in the furnace the four
-
63
Reference: CO2, C2H2, C2H4, C3H8, CH4, H2O, C2H6, H2 at 25 oC 1 atm
-
64
Table 6.1 Enthalpy table
Substance Nin 103 Hin KJ/mol
Nout 103
mol/hr
Hin KJ/mol
CO2 - - 3.555 H3
C2H2 - - 3.199 H4
C2H4 - - 505.869 H5
C3H8 - - 60.79 H6
CH4 - - 66.477 H7
H2O 291.896 H1 291.265 H8
C2H6 972.985 H2 340.564 H9
H2 - - 509.068 H10
Estimation of the enthalpy of the inlet stream
,
,
,
,
,
-
65
Total heat absorbed = Heat for preheating the feed + Heat absorbed for cracking of feed
Assume a furnace efficiency of 85 %
Duty of the furnace is 20.862 MW
6.2.4 Energy absorbed
Assume 70 % of the total heat absorbed in used for the cracking of Feed stock the
remaining is used for preheating of the feed stock.
6.2.5 Heat flux in the radiant coils
-
66
6.2
[2]
Where: N tube is number of tubes, Do is Outer diameter of tubes, L tube is the length of the
tube
6.2.6 Heat lost to the surroundings
The heat lost to the surroundings is in the range of 0.02 to 0.03 as a fraction of the total
released heat [Wallas, 1948].Since Q lost is an allowance and for this design we can set it to
be equal to 0.02.
6.2.7 Heat lost is the stack gas
6.2.8 TEMPERATURE PROFILE IN THE FURNACE
6.2.8.1 Temperature of the process fluid leaving the convective section
The stream entering the radiation section has absorbed 30 % of the total heat absorbed.
Qabsorbed in the convective tubes = Hout Hin
-
67
Where Qabsorbed in the convective tubes = heat absorbed in the convection section (MJ/hr)
Hout = Enthalpy of the stream entering the radiant section (KJ/mol)
Hin= Enthalpy of the feedstock (KJ/mol)
Qabsorbed in the convective tubes
=
103+0.688105 +0.76041012 2)
T = 312 C
6.2.8.2 Temperature of flue gas entering the convective section
By rule of thumb the temperature of the flue gas entering the convective section should be
150 C above the process temperature. This mean the temperature of the gas is 990.8 C.
6.2.8.3 Temperature of flue gas entering the stack section
(Waals 1990,pg. 214)
Where Ts temperature of flue gas leaving the convection section oF
a = 0.22048 - 0.35027z + 0.92344z2, b = 0.016086 + 0.29393z - 048139z
2
Where z = fraction excess air =0.1
Therefore substitute into the equation above: a= 0.1946874, b=0.0406653
Now solving for Ts,
-
68
6.2.9 STACK DESIGN
Where:
P = the suction available from a natural draft system, Pa
C = 0.0342
a = atmospheric pressure in Pa, h = height of the stack (m).,Ti=inlet temperature in K
To =ambient temperature in K (25 oC)
Setting the P = the suction available from a natural draft system to 400 Pa which is in the
acceptable range [2]
=80.04 m
=80.04 m
6.2.10 PROCESS SIDE HEAT TRANSFER
6.2.10.1 Process-side heat transfer
.. .. 6.14 [2]
Neglecting the viscosity correction factor
-
69
Where Nu = Nusselt number =
, Re =Reynolds number =
,
Pr =Prandtl number =
, hi= inside coefficient (W/m
2 oC), di= tube inside diameter (m),
ut = fluid velocity(m/s), kf = fluid thermal conductivity(W/moC), Gt = mass velocity, mass
flow per unit area(kg/m2s), = fluid viscosity at the bulk fluid temperature (Ns/m
2),
w= fluid viscosity at the tube wall temperature (Ns/m2),
Cp = fluid specific heat, heat capacity, J/kgoC.
6.2.10.1.1 In the convection tubes
=
6.2.10.1.2 For the radiative tubes
-
70
6.2.11 PRESSURE DROP
... 6.15 [2]
Where P = tube-side pressure drop, KPa,
Np= number of tube-side passes, Ut = tube-side velocity, m/s, L = length of one tube, m
jf = Friction factor
6.2.11.1 Pressure drop in the radiative tubes:
At Reynolds number of 5.872 , jf = 2 10-3
[2], w = 0.123810-3
, Np=30, L=11m
= 26.152 KPa
6.2.11.2 Pressure drop in the Convective tubes:
At Reynolds number of 2.446 , jf = 1.7 10-3
[2], w=1.69810-3
, Np=3, L=11m
=5.743KPa
6.2.12 FUEL CONSUMPTION AND REQUIRED AIR FLOW RATE
6.2.12.1 Fuel consumption
Using methane as the fuel
Q released=W fuel LHVfuel ( Mullinger et al.2008)
-
71
Where Q released= total heat released MJ / hr
W fuel = Fuel flow rate (Kg/hr), LHVfuel= Low heating value of fuel (CH4) (MJ/Kg)
LHVfuel=50.055 MJ/Kg (21520 Btu/Ibm) (Waals 1990,pg. 216 )
6.2.12.2 The flow rate of air to be required:
CH4 + 2(O2 + 3.76N2) CO2 + 2H2O + 7.52N2
-
72
Table 6.2 chemical engineering design summary of pyrolysis furnace
DESIGN PARAMETERS VALUES UNITS
Heat Duty 20.862 MW
Temperature in/out 70/840.8 o C
Pressure drop 5.737 KPa
Fuel consumption rate 0.417 Kg/s
Air required flow rate 7.192 Kg Air /s
Excess air required 10 %
Feedstock flow rate 3.858 Kg/s
Steam required 1.461 Kg/s
Outlet process flow rate 9.588 Kg/s
Operating pressure 107 KPa
-
73
6.3 MECHANICAL ENGINEERING DESIGN
6.3.1 Design Pressure
For vessels under internal pressure, the design pressure is normally 5 to 10 percent above
the normal operating pressure (Sinnott, 2005).The internal pressure in the furnace is related
to the hydrostatic head, atmospheric pressure and the pressure drop by:
Design Pressure (Pi) =hydrostatic pressure + atmospheric pressure = gh + (101325 - P),
Pa
Where = density of the flue gas =3.896 Kg/m3
g = acceleration due to gravity=9.81m/s2
h = height of furnace =15.16 m
P= negligible
H conv. =the height of the convective section = 3.66 m
H rad. = the height of the radiative section = 11.5 m
Design Pressure (Pi) = (3.8969.81 (3.66+11.5) + (101325)
= 101.904 KPa
10 % of the design pressure =1.1101.904
=112.095 KPa
-
74
6.3.2 Minimum Thickness of Cylindrical shell
For a cylindrical shell, the minimum thickness required to resist internal pressure is given
as:
Where Pi is the internal pressure = 112.095 N/m2
Di is the internal diameter = 6 m
F is design stress,
Typical design stress for stainless steel at 3500C is 100000 N/m
2 (Sinnott, 2005).
Allowing for a corrosion allowance of 0.002m, the minimum thickness is 0.005365m.
6.3.3 Design Temperature
The design temperature at which the design stress is evaluated should be taken as the
maximum working temperature of the material (Sinnott, 2005). The design temperature is
4000C (523.15K)
6.3.4 Materials of Construction
Stainless steels are the most frequently used corrosion resistant materials in the chemical
industry. Type 304 stainless steel (the so called 18/8 stainless steel) is the most generally
used stainless steel. If the equipment is being deigned to operate at high temperatures,
materials that retain their strength must be selected. The stainless steels are superior in this
-
75
respect to plain carbon steel. Stainless steel is to be used for this design (Coulson et al,
Volume 6).
6.3.5 STRESS ANALYSIS
The main sources of loads to consider are:
The internal Pressure
The total longitudinal and circumferential stresses due to internal pressure are given as:
Longitudinal stress, Lt
DP ii
2
Circumferential stress, ht
DP ii
4
KPa267.62681005365.02
0.6095.112
KPah 634.31340005365.04
0.6092.112
3.6 DEAD WEIGHT OF THE FURNACE
6.3.6.1 Weight of the refractory
The density of high alumina refractory bricks is given as 2579kg/m3 (Rotary kiln transport
phenomena and transport processes)
Where,
-
76
R= external radius of refractory shell, r = internal radius of refractory shell
6.3.6.2 Weight of steel shell
Density of steel is given as 8027Kg/m3.
Where,
R= external radius of steel shell, r = internal radius of steel shell
6.3.6.3 Weight of the content in tube
Total weight of the fluid in the tubes
6.3.6.3.1 Volume of convection section
Mass of content:
-
77
6.3.6.3.2 Volume of radiant section
6.3.6.4 Total dead weight
6.3.6.5 Choice of support
The support will be so strong enough to with stand the weight exerted by the furnace
Table 6.3 Summary of mechanical engineering design for pyrolysis furnace
PARAMETER VALUE
Design Temperature 400 oC
Design Pressure 112.095 KPa
Minimum thickness of shell 0.005365 m
Longitudinal stress 62681.267 KPa
Circumferencial stress 31340.634 KPa
Total force exerted on the surpport 1827.722 kN
-
78
CHAPTER 7
7.0 DESIGN OF HEAT EXCHANGER
7.1 PROBLEM STATEMENT
To design a heat exchanger to cool cracked gases at a flow rate of and
to cooled gases at using cooling water at to .
7.2 PARAMETERS TO CALCULATE
1) Heat transfer area
2) Bundle diameter
3) Bundle clearance
4) Heat transfer coefficient
5) Overall heat transfer coefficient
6) Tube side and Shell side fouling resistances
7) Pressure drops
7.2.1 CHEMICAL ENGINEERING CALCULATIONS
The fundamental heat transfer equation is given by,
-
79
The log mean temperature difference, for countercurrent flow is given by:
Where equations (5) and (6) are the dimensionless temperature ratios of the correction
factor.
-
80
7.2.2 Exchanger type and dimensions:
The graph of FT against S at various R values on page 9 of Perrys Chemical Engineers,
Section 11, 8th
Edition gives a corresponding
Hence the chosen Heat Exchanger is 2-4 Shell-and-Tube Heat Exchangers.
7.2.3 Heat Load
-
81
Overall Heat Balance gives,
7.2.4 Overall coefficient:
7.2.5 Heat transfer area:
-
82
7.2.6 Layout and tube size:
Using a split-ring floating head exchanger. Neither fluid is corrosive, so plain carbon steel
can be used for the shell and tubes.
From the tubing characteristics as given in Perrys, I chose the following dimensions of the
tube.
1-inch Outer Diameter (O.D) tubes with 1.25-inch Triangular Pitch, 16 BWG
Length of tube = 6m (standard length)
7.2.7 Number of tubes
-
83
7.2.8 Bundle and shell diameter
For a split ring floating head exchanger,
7.2.9 Tube-side heat transfer coefficient calculations:
-
84
The Reynolds (Re) and Prandtls number (Pr) of the cracked gas at the tube side is given
by,
-
85
Hence the Nusselt Number (Nu) is thus calculated as,
7.2.10 Shell-side heat transfer coefficient calculations:
-
86
Choose a baffle spacing (Lb) of 100 mm.
The shell side linear velocity is appreciable since it falls in the standard range 0.3 1.0 m/s.
Equivalent Diameter (De) of the triangular pitch is given by,
-
87
The Reynolds (Re) and Prandtls number (Pr) of the cooling water at the shell side is given
by,
-
88
7.2.11 Overall coefficient:
Using carbon-steel for the tube and shell side because neither fluid is corrosive and the
temperature is very high.
Thermal Conductivity of carbon-steel (KW) = 55
Taking the fouling coefficients of cracked gas = 0.00030
Taking the fouling coefficients of water = 0.00090
The overall coefficient is the reciprocal of the overall resistance to heat transfer, which is
the sum of several individual resistances.
The other parameter above were defined previously, hence
+ + +
-
89
Since the calculated U = is above the assumed value of
Hence the design with the above parameters is accepted.
7.2.12 Tube Side Pressure Drop Calculations:
The total pressure drop at tube side is given by the equation,
Where Np = number of tube passes
L Length of tube
For (friction factor at tube side)
The total tube side pressure drop is less than 70 kPa, hence within specification.
7.2.13 Shell Side Pressure Drop Calculations:
The shell side pressure drop is also related by the equation,
-
90
Where De = equivalent diameter of shell
Lb = baffle spacing
As this pressure drop on the shell side is less than 70 kPa, the design is acceptable from the
pressure drop point of view.
7.2.14 SUMMARY OF PROCESS DESIGN FOR HEAT EXCHANGER
Heat transfer area
Tube side coefficient
Shell side coefficient
-
91
Overall transfer coefficient, assumed
Overall transfer coefficient, re