CHAPTER 2

2.3 Olefins Plant Processes

2.3 Olefins Plant Processes

A big and fully petrochemical complex produces a host of products and an olefins plant is at the heart of it. A standard olefins plant thus located is of the gas cracking types in a singular feedstock style or as a combination of propane, ethane and butane. An olefins plant which is based on liquid cracking technology can handle a variety of feedstocks ranging from vacuum oils to LPG. Another unique kind of olefins plants are also seen in production today which use a feedstock of mixed butane which is a combination of normal and iso-butane. These plants have unique flow sheets and are capable of cracking such feedstocks that are a mix of gas and liquids. This chapter will explain the overall scheme of a typical olefins plant in little details. A section also discusses the olefins experience of the writer and can be valuable tool that can aid better understanding of differences in plant processes. 2.3.1 A standard Olefins Plant Process Scheme A standard scheme used in design and theory of a steam cracking plant does not account for use of a particular feedstock. The scheme is more suited for understanding the production of high-purity olefins. Irrespective of the feedstock used, a standard ethylene plant scheme consists of:

1 A Cracking process for the hydrocarbon feedstock which is processed and diluted with steam, at low pressure and high temperature;

2 Stopping cracking reactions by speedy cooling of the cracked gas; 3 Purification of products after separation by using low catalytic

hydrogenation and temperature fractionation at a very high pressure. Almost all of today’s olefins plants are base don the above principles and they only differ in certain practical processes that vary due tot technology providers specific to feedstocks used and by-products produced. The optimum levels of flow schemes and operating conditions have been arrived at by technology providers due to experience accumulated over the past few decades for various feedstocks. An efficient ethylene manufacturing process involves a great number of reactions (catalytic and non-catalytic), unit operations, adsorption and absorption, heat

exchange, compression, fractionation and phase separation with all of them arranged in a very manner with the sole objective of producing high-purity ethylene and other by-products at high efficiency. These ethylene plants make use of very high extremes of temperatures that vary from more than 900 oC at the cracking furnaces to about -165 oC at the cryogenic gas separation phase. The processes inside such an olefins plant are very complicated and involve a great many pieces of equipment controlled by an equally complex and automated control system. A typical ethylene producing plant consists of more than a thousand instruments that measure composition, temperatures, pressures, flow rates and levels within a plethora of control loops. Modern plants integrate a lot many dedicated process control systems run by computers and electronic instrumentation to simplify controls and raise the efficiency of operations. The following figure (Figure 1) illustrates the block diagram that forms the foundational steps of a typical olefins plant. These are the major operational units that are applied in an olefins plant that employs steam cracking processes. This figure is an illustration to help understand the design of a traditional olefins plant. Each of these processes integrates complicated sub-processes which will be discussed later. This figure is a bird’s eye-view of the process flow in a typical olefins plant.

2.3.1.1 The basics of Cracking Furnaces

The cracking process or the pyrolysis step is a very important one and is the heart of an ethylene plant since this very step leads to the production of all the products. All other processes and steps are only to serve as separation and purification processes in order to improve the quality of the products. This cracking process involves the furnace section which has undergone the most number of design changes in the past one decade resulting in improvement of efficiency and yield. In an olefins plant, the cracking heater design defines the product slate, which directly affects the basic commercial viability or the profitability of an olefins plant.

The importance of the cracking furnace is such that it takes the lion’s share of investment (almost 35%) that goes into equipment costs. It also consumes the maximum amount of energy input in an olefins plant since it is designed to provide temperatures of up to 900 oC used in cracking reactions.

2.3.1.1.1 The chemistry of Thermal Cracking

The most viable and widely used process for ethylene production is the thermal cracking of hydrocarbons. The thermal cracking reaction takes place in non-

catalytic tubular coils that are built into the radiant section of the cracking or fired heaters. This reaction is exothermic an requires a very high temperature in the range of 800 to 900 oC and this depends on the feedstock used as well as the radiant coil design used for cracking. The temperature requirement in a cracking reaction is inversely proportional to the carbon chain length in the feedstock used. Ethane cracking can be expressed by the simple reaction -

C2H6 →C2H4 +H2 (1) Theoretically, if only this reaction takes place, the resultant products would only be ethylene and hydrogen. Even at lower conversion levels, additional ethane would be produced in addition to ethylene and hydrogen. However, practice is different and in a practical situation, the produced cracked gas also consists of a few other products such as acetylene, methane, propenes, propane, butenes, butane, toluene, benzene and other heavier components. This implies that the above reaction is not the only reaction that is taking place in the radiant heaters.

Figure 1: The main process that take place in a conventional olefins plant The cracking process or pyrolysis of hydrocarbons is a process that been thoroughly researched for many. Mathematical models for simulating the pyrolysis reactions have been used in designing furnaces with prediction of products that could be obtained from such models. The models used can be classified under three general heads – mechanistic, molecular and empirical/ regression models [1]. A free-radical mechanism for decomposition of hydrocarbons was established in 1930s [2]. While, this mechanism does not really explain the full product distribution, even for simple ethane, it has proved to be of extreme use. This mechanism consists of three basic reactions - 1. Initial formation of radicals (Chain Initiation):

Cleavage of the C-C bonds on paraffin molecules produced two radicals. CnH2n+2 → CmH2m+1

. + C(n-m)H2(n-m)+1. (2)

2. Reaction between molecules and radicals (Chain propagation):

This is a more complex step that includes many reactions that result in hydrogen abstraction and addition, radical isomerization and radical decomposition.

3. Disappearance of radicals (Chain Terminations): This is just a reverse of the first step and results in disappearance of radicals. To understand the free radical mechanism better, we will take the example of the ethane cracking process. This is the simplest illustration we can use to understand the process of radical mechanisms. In the initiation step, ethane is split into two methyl radicals (Chain initiation, Eq. 8). The methyl radicals then react again with an ethane molecule resulting in the production of an ethyl radical (Eq. 9), which further decomposes into hydrogen and ethylene atoms (Eq. 10). The hydrogen atom thus produced, then reacts with another ethane molecule to produce a molecule of hydrogen and another new ethyl radical (Eq. 11). Initiation

C2H6 →CH3 • +CH3 • (8) Propagation

CH3 • + C2H6→CH4 +C2H5 • (9) C2H5 • →C2H4 +H• (10) H• + C2H6 →H2 +C2H5 • (11)

The reactions (9) and (10) will terminate when an ethyl radical or a hydrogen atom reacts with another radical or atom by reactions akin to: Termination

H•+H•→H2 (12) CH3 • +H•→CH4 (13) H•+C2H5 • →C2H6 (14) C2H5 • +CH3 •→C3H8 (15) C2H5 • + C2H5 • →C4H10 (16)

At the end, upon the termination of chain propagation, a new hydrogen atom or methyl or ethyl radicals have to be generated (Eqs. 8-10) in order to begin a new chain. A molecule of methane is thus formed whenever a new chain is initiated (Eq. 9) along with a molecule of ethylene (Eq. 10). More complex free-radical mechanisms result in decomposition into branched-chain or normal alkanes [3]. Thus, it is imperative that the number of possible free radicals and reactions will increase rapidly as the chain length increases.

While using a particular feedstock, the designer needs to have a deeper understanding of cracking kinetics so as to define the specific severity/selectivity levels in order to obtain an optimum and commercially viable yield. The olefins plant furnace designers use in-house tools and priority models to design their furnaces and thus modify the cracking design in order to achieve better selectivity amongst the primary products. From this point of view, it becomes necessary for a furnace designer to understand and posses the accurate furnace design tools needed in order to yield prediction models while describing the complex cracking processes involved in detail.

Most technology leader’s designs are starkly different from their competitors; all of them however, follow the design improvements based on three major aspects - increasing furnace severity, capacity and selectivity. 2.3.1.1.3 Configuration of Cracking Heaters

A cracking heater, in more general terms consists of two main sections – radiant sections and the convection section. The convection section is the upper offset arrangement of a traditional cracking heater and the radiant section is the lower end as illustrated in Figure 2. The two names (convection and radiant) are actually the type of heat transfer conducted and transferred to the process gas and are named accordingly. The radiant section cracks the gas in radiant coils with the help of the heat which is absorbed from burned fuel gas and then transferred to it by radiation. This section has a fire box which is lined with ceramic fiber or refractory brick thus enhancing the heat transfer by the process of radiation at the same time minimizing the heat loss external to the fire box.

The convection section functions as a heat recovery device by accumulating heat form the preheated hydrocarbon feeds that produce flue gas. The preheating takes place by the heated BFW entering the steam drum and then superheating the saturated steam in the steam drum. The convection coils in the convection section are fitted with fins to improve the convection process by increasing the surface area.

Combustion reaction that takes place between the fuel and oxygen (air) which is conducted by at multiple burners is the primary heat source that generates the heat for the actual cracking reactions. The burners are placed at the bottom of the fire box and/or on the walls. This reaction produces flue gas which is passed through the convection section which ultimately exits the cracking heater through a stack on top of the convection section. The short stack connected to the fan outlet flange has a draft fan at the top of the convection section and it creates the draft mechanically. The draft can also be created naturally using the long stack. Such draft fans make way for the complex convection sections to be constructed in such way as to handle high velocities of gas resulting in the convection banks operating with the optimum

heat transfer.

Quench exchangers (TLE’s) and the steam generation system are the other main parts of the cracking heater. The TLE’s integrate heat exchangers which are used to cool the cracked gas coming out of the radiant section by using vaporizing boiler feed water. These exchangers perform the function of rapidly cooling the furnace effluent by a few hundreds of degrees in a very short time which is measured in milliseconds, resulting in “freezing" the radiant coil outlet composition in order to stop further cracking reactions that result in degradation of the product yield. This process also recovers heat and is used to generate saturated high pressure steam. This increases the efficiency of the plant and the generation of steam by heat transfer from the furnace effluent (cracked gas) by reducing the plant steam requirements that comes the auxiliary boilers. Today’s ethylene furnaces use this quenching process as an integral part of plant design.

Figure 2: Cracking Heater Configuration

Before going into details of processes that occur inside the cracking heater, it is necessary to understand the process streams based on the materials - hydrocarbons, steam, boiler feed water and fuel gas. This classification based on materials is important to understand the functionality of the cracking heater and forms the basis of cracking heater design and operations. This classification is independent of any differences that exist amongst the technology providers who in turn provide own propriety designs of cracking heaters.

1- Hydrocarbons:

The term of hydrocarbons is used here as the feedstock or the process gas which is introduced into the cracking heater and comes out of it as the cracked gas. These hydrocarbon feedstocks, depending on technology used, are sometimes preheated externally before being inducted into the cracking heater by the use of the total furnace (cracking heater) effluent and subsequently in the convection section by heat absorption from the flue gas.

The use of a traditional conventional dilution steam generation system, the hydrocarbon feedstocks, after being inducted into the cracking furnace, are heated in the top level convection coils and then go down into the lower convection coils which make use of dilution steam that is injected by ratio control. The state of hydrocarbon feedstocks is a very important consideration here. Gaseous hydrocarbon feedstocks use lesser heat energy since they only need the energy necessary to heat the steam and feed to the radiant coil inlet temperature. Differentially, liquid hydrocarbon feeds need more heat energy, since the liquid needs to be vaporized in addition to the latent heat of

vaporization. In addition to these differences, the heat level at the cross over coils needs to be carefully selected for a specific hydrocarbon feedstock in order to maximise heat absorption, all the time keeping it lower than temperature required to initiate cracking reactions that could result in damaging coke deposits in the convection section.

After the convection section processes, the steam and hydrocarbon mix enters the radiant coils inside the fire box of the radiant section from the lower coils of the convection section. The heat radiation that takes place here facilitates the process of the thermal cracking of hydrocarbons in the desired conversion rate. The hydrocarbon feeds entering this section get evenly distributed across the radiant coils in such a way as to avoid coke formation (local hot spots may cause them to form) due to excessive surface temperatures. These technology ratios and distribution depend on the technology provider. The radiant coils are normally made from Chromium and Nickel alloys to withstand higher temperatures. However, despite these general considerations, the radiant coil design depends on the feedstocks used and the product slate keeping gin mind the economic criteria.

The actual cracking of hydrocarbons takes place in the radiant section of the furnace after which the resultant cracked gas is rapidly cooled in quench exchangers (TLE’s) in order to nullify any secondary cracking resulting in degraded olefins products. This cooling process takes place in two or three TLE’s arranged in a series depending on the design and technology used. Although the main concept of cooling remains the same irrespective of design, the technology changes may be in the form of the TLE design that could be shell-and-tube type or the double-pipe types and the number of steps involve din cooling (primary, secondary and tertiary).

The cracked gas thus cooled by quench exchangers is then sent to downstream operational units for purposes of separation or recycling.

2. Steam

Steam is a major source of heat control that is required at various stage sin an olefins plant. It is produced and recycles across the plant for more efficiency. The saturated steam that comes from TLE’s or quench exchangers, after performing its function is redirected to a common steam drum (one is available for each cracking heater) through the risers. All such steam accumulated in the steam drums is further sent to dedicated convection coils in order to superheat the steam by the use of hot flue gas before sending the same to the plant steam system. The use of disengagement devices that are usually installed within the steam drum is necessary to eliminate any liquid carry over that might occur when the steam leaves the steam drum. In addition to this a continuous blow-down is taken from the steam drum. This blow-down flow comes in use to control the dissolved solid concentration in the drum water which is measured by

the drum water pH and conductivity. This is necessary to avoid any accumulation of deposits in the water side of the TLE or quench exchanger. The blowdown rate that is used is usually 1% to 3% of the total boiler feed water fed to the drum. The circulation of boiler feed water from the steam drum to the TLE or quench exchanger is done by the use of the natural thermosyphon circulation method. This process is used with a typical water-to-steam ratio of more than 10. The rate of circulation is measured by the elevation difference between the quench exchangers and the steam drum and the pipe configurations.

Two points of controlled heating are required to control the final superheat temperature of the steam. As a part of this process, the superheated steam is de-superheated using boiler feed water after coming out of the first pass convection coils and before it enters the second pass of the convection section. The boiler feed water used to de-superheat the steam is required to be free of any dissolved solids in order to avoid any plugging of the steam-line downstream of the de-superheater with solids. The de-superheating boiler feed water is normally derived from the discharge of the BFW pumps at a stage before the phosphate and other dissolved solids or chemicals are added.

Depending on the technology used, the cracking heater steam pressure could be high pressure steam (45 barg). Some technology licensors also have designs where a very high pressure steam in the range of 70 to 125 barg is used. This high pressure steam is generated when propane or ethane is cracked. Alternatively, when heavier hydrocarbons or butane is cracked, a very high pressure steam is generated. The selection of very high steam pressure would depend on the plant design that takes into account the size and economy of steam piping and the pressure handling equipment required.

3. Boiler Feed Water (BFW)

As explained above, heat recovery system in an olefins plant increases efficiency and profitability. This is employed with the cracking heaters in the plant in order to recover heat from the cracked gas to produce steam from BFW.

To achieve this, the BFW is pre-heated in the convection section of a cracking furnace heater by the use of flue gas. Depending on the design of the plant, BFW is sometimes pre-heated in final quench exchanger against the cracked gas before it enters the convection section.

After this, the resultant BFW is inducted into a steam drum located above the quench exchangers by using the level control to facilitate thermosyphone boiling. The size of the steam drum is such that it can provide for six to eight minutes hold up at rates specified by the plant design. This capacity becomes necessary in order to allow a human operator some time to stop the hydrocarbon feed to the furnace in the event of the loss of BFW. This process is also called “switching to minimum fire” and it reduces the rate of production of steam in order to allow

some time for BFW to come back or initiate the process of decoking the cracking furnace. The furnace cannot be cooled with the decoking of radiant tubes since it may result in the coils getting plugged with spalled coke. A larger steam drum allows a simple control measure and is adequate. Another method is to allow for 1 or 2 minute holdups by use of a smaller steam drum by the three element drum level control which is not needed in such cases and is not justified.

The pressure of steam-generation is selectable and the corresponding boiling temperature of the water is kept above the known dew point of most heavy hydrocarbons in the furnace outlet gas. This step helps in avoiding fouling of TLE’s (traffic line exchangers).

4. Fuel Gas

Flue gas is formed as a result of the combustion of the fuel gas and oxygen (air) in the burners. This flue gas generally consists of H2, CO, CO2 and H2O vapor. The constituents of flow gas may vary depending on the burner performance and the fuel gas composition and may contain some particulates like non-reacted CH4,

nitrogen, NOx, and sulphur-based components (SOx). The actual emission of NOx depends on the combination of firebox and burner design during the live furnace operations. Initially, the fuel gas is inducted into the furnace fuel distribution header and consequently distributed into the bottom burners and/or side burners. The fuel gas is then ignited and combusted within the furnace firebox and generates the required heat of reaction to aid endothermic pyrolysis. Any excess heat that is generated by the combustion of the flue gas is recovered by the use of the furnace convection section through different streams. After the heat is recovered, the cold flue gas is then routed into the atmosphere by the use of the natural draft or the use of a draft fan. The required heat to be generated again depends on the feedstock and the required conversion rate.

The fuel burners are designed in such away that they can generate the heat flux (Duty) needed for the cracking reactions. Designers need to take some special care in selection of burners in order to comply with the environmental emission limits for NOx and it should not be exceeded in any case for operating a cracking furnace. The most commonly used are the low NOx burners and they are designed to meet the environmental emission norms for all particulates, NOx, CO etc. Another issue of flame stability inside the cracking heaters to aid combustion is achieved by maintainence of draft within the fire box by the use of a certain percent of excess air (10-20%).

Maintaining such a percentage of excess air has two distinct advantages: • The firebox becomes more efficient as the air to be heated is lesser and fuel consumed is low.

• Less formation of NOx. Adjusting the draft can result in controlling the excess air and there is no need to alter the burner air registers. The following figure (Figure 3) illustrates the burners and the flame generated inside the fire box of a cracking heater.

Figure 3: Firebox of a cracking heater 2.3.1.1.3 The key principles of Thermal Cracking and Design Factors

This section deals with the key factors and principles that are critical to thermal cracking and how plant design and operating factors come to the fore in aiding this. The reaction control required for cracking and many other important elements along with optimization factors that are required to be applied and implemented to cracking heaters will be discussed as we proceed.

2.3.1.1.3.1 Residence Time

Residence time is the time occupied by the hydrocarbon/steam mixture stream in the radiant coil of the cracking furnace. The residence thus depends mainly on the geometry of design of the radiant coil and a few other factors. The design geometry of the radiant coil would include the length, diameter and the number of passes. For example, radiant coil designed for single pass single pass have a shorter residence time than the multi-pass designs.

In addition to the above, the coil length has a bearing on residence time. The shorter the coil, the better the selectivity. Residence time normally varies in the range of 0.1 to 0.6 seconds and depends primarily on the furnace designers who look to continuously improvise their technologies in order to attain better selectivity or yield as a result of reduction in residence time of the cracking reaction.

The main advantage that comes from a shorter residence time in the cracking heater is the improvement in yield, or selectivity to valuable light olefins. Shorter residence time also allows for lesser secondary cracking and polymerization reactions resulting in higher light-olefins products being generated as a result of the primary reactions. Temperature also has a bearing here and the residence time can substantially reduced as a result of a higher cracking temperature.

The point to be remembered is that the olefins yield is not only determined by residence time in the radiant coil but by the actual residence time in the whole cracking system. Explained further, it is necessary to stop all cracking reactions as quickly as possible at the outlet of the radiant coil to avoid secondary

reactions. Since residence time also depends on the reactant vapor density, which is a function of the system pressure, hence the drop of pressure through the coil becomes important and the coil diameter to-length ratio becomes a very important factor in design.

2. .3.1.1.3.2 Steam to Hydrocarbon Ratio

As seen earlier in the section of cracking heater configuration using a conventional dilution steam system, dilution steam is introduce at the convection section via flow ration control to the heated hydrocarbon feed. Usually, each of the hydrocarbon feedstock pass is mixed with dilution steam applying the flow control ration after they come out of the main feed manifold. The number of the feed lines and the points of injection of the dilution steam depend on the cracking heater or furnace design. Additionally, the ratio value, i.e. the ration by volume of steam to hydrocarbons varying between 0.3 to 0.1 is dependant on the kind of feed and the level of severity of the cracked hydrocarbon feedstock. Usually, higher steam ration values are sought for high cracking severity and heavy feed.

While hydrocarbons are cracked thermally, the addition of steam reduces the partial pressure and changes the balance to produce lighter olefins, particularly ethylene and propylene. The dilution steam serves two purposes – lowers the hydrocarbon partial pressure and reduces the rate of carburization (or coke formation) of the radiant coil.

Generation steam, unlike the dilution steam, is another way of mixing steam with the hydrocarbon feed using the saturator system (section 2.3.2). This procedure involves the mixing of the total hydrocarbon feed inclusive of the recycles with circulated recovered process water and in turn vaporized to obtain saturated hydrocarbon feedstock at the desirable ration of steam to hydrocarbon. After that, the total saturated hydrocarbon feedstock or the steam mixture steam is directly sent to the cracking heater without any further addition of dilution steam at the cracking unit.

2. .3.1.1.3.3 Distribution of Flow

Before entering the furnace sections, the hydrocarbon feedstock is distributed in a number of feed passed through the flow control valves. Each section then is passed to the convection section followed by the radiant section of the furnace or cracking heater. The radiant section may vary from a single tube to 30 or 40 tubes per convection pass. In case there are several tubes per convection pass, venturi or critical flow orifices are set up at each radiant tube inlet. For all the operations, from the start to end, the upstream pressure of the venturi or orifice

must be high enough in order to ensure that sonic velocity is maintained at the throat of each venturi ensuring equal distribution. Equal flow in each radiant tube retards excessive heating, which can cause coke formation and hot spots.

The number of radiant coils and hydrocarbon passes is dependant on the design of the cracking heater, which takes into consideration the total rate of hydrocarbon feed to be managed by the cracking heater and other factors.

2.3.1.1.3.4 Cracking Severity

Severity or conversion is the relative measure of the disappearance to a new component with its feed concentration. In cracking heater operations, the term severity is used in place of conversion, besides other indicators like the propylene to ethylene ratio or the factor of kinetic severity. The reason is that the nature and type of the cracking heater feedstock is generally a mixture of more that one component and when one component is measured for conversion, the value is only an approximation of the actual conversion. Severity effects product distribution and yields.

For instance, high cracking severity increases ethylene yield from butane feedstock while considerably reduces propylene yield and mixed C4 recycles. It also increases benzene production. Reduction in the cracker plant throughput also happens, lessening the overall capital expense savings, by reducing the recycles into the cracking heater. Butane and propane yields larger propylene with the lowering of severity. The change in yield of propylene, in comparison to heavy liquids, is less dramatic as the cracking severity is increased from low to high. Additionally, ethylene yield can increase for constant feedstock at low conversion. The concentration of major co-products, like ethylene, propylene, mixed butenes and butanes, butadiene and pyrolysis gasoline (C5s through to C9s) also varies with cracking severity. Severity can be controlled according to the direct coil outlet temperature (COT). They are directly proportional. Fig.4 shows a typical COT (or severity) plot against propane conversion. Ethylene yield increases and propylene yield decreased as COT (and severity) increases.

Improvement of selectivity owing to a better cracking furnace is fast rewarding. Therefore, it is necessary for a furnace designer to have the correct furnace designing tools and models for yield prediction, describing the complicated cracking process in much detail.

Figure 4: Yield Curves of Ethylene and Propylene

2. .3.1.1.3.5 Sulfiding

A commonly used sulfiding agent, called dimethyle disulfide (DMDS) is introduced to the hydrocarbon feedstock to the cracking heater. A sulfiding agent minimizes the carburization inside the tube by poisoning nickel and iron catalysis inside the tube, causing coke formation, during the cracking of hydrocarbons. According to the discretion of the heater designer, the location of introducing the sulfiding agent can either be the primary hydrocarbon feedstock pass prior to entering convection section or the along the separate dilution steam lines connecting with each hydrocarbon feedstock coil before they enter the lower convection section. The flow controller controls the introduction of the sulfiding agent and aids the achievement of low level (60 -100 ppmv) H2S in the effluent of the furnace. The rate can also be lowered if sulfur is present in the hydrocarbon feedstock.

2. .3.1.1.3.6 Decoking

The cracking of the various hydrocarbon feed produces coke and tar along with olefins, which progressively form a thick layer on the inside of the quench exchangers and the radiant coils. This automatically hinders heat transfer from the radiant coil tube wall to the feedstock passing through it. As the layer thickens, the radiant coil temperature also increases till it reaches an optimum allowable metal temperature. In addition to the above, the coke formation significantly increases the pressure drop over the radiant coils, adversely affecting the pattern in cracking yield by increasing the hydrocarbon cracking pressure.

Factors like nature of hydrocarbon feed, the impurities present in it, quality of dilution steam, poor radiant coil flow distribution, conversion and cracking severity, rate of sulfiding agent injection, steam-hydrocarbon ratio, designs of the quench exchanger and the radiant coils, firebox firing, etc affects coke formation.

During cracking of the feedstock, it is important to decoke or remove coke from the inside of the rubes. Usually, decoking is done offline by using steam-air, though some designers may design the cracking heaters for online decoking by forming gas and spalling the coke formation. The formation and accumulation of coke within the radiant coils require regular removal in order to prevent overheating and restore desired yield pattern.

Decoking the radiant coils is done by controlling the combustion of the coke deposits using a steam-air mixture, a process known as steam-air decoking. The furnace has to be maintained at a temperature for carrying out this entire

procedure. The process is called hot standby, meaning

• Hydrocarbon feed is stopped to the furnace and the feed inlets are purged with steam.

• Fuel supply to the lower and side burners are reduced.

Steam-air decoking is carried out in the furnace radiant coils through the quench exchangers in way that constant steam generation is maintained during the entire procedure which partially cleans the quench exchangers also. The furnace effluents, while this entire procedure is under way, routed to the firebox or the decoke drum, in the same design. To control the combustion of the layer of coke during the decoking is done by installing a sample point for CO2 analysis on the decoke line, which will effectively provide information on the status of burning.

The effluent from the steam-air decoking procedure is primarily a mixture of steam, CO2 and air. Small amounts of coke and CO are also present. The effluent is generally redirected towards the decoke drum, where even the smallest micron coke particles are removed for solid disposal before releasing it into the atmosphere. Another route for the effluent leads towards the firebox, where complete combustion is ensured in cracking heater stack before releasing them into the atmosphere.

2.3.1.2 Quench System

After the quench exchangers, the complete cracked gas from the cracking furnace becomes the feed for the quench system. The configuration of the quench system primarily depends on the nature of feed. For liquid cracker that handle heavy feed such as gas oils and naphtha, generating massive amounts of fuel oil, the quench system uses fuel oil fractionation to recover the same followed by its integration with a quench water tower. If the fractionation is not employed, some of the fuel oil would form emulsions in the oil-water separator because some of these fuel oils have equal density as water. For gas crackers that handle light feed such as ethane, propane and butane, only a quench water tower is used, because not much fuel oil is produced in the cracking heaters with such feed. The primary use of the oil quench tower depends on the quantity of heavy elements such as gasoline and other fuel oil generated at the furnace unit effluent and contained in the cracked gas.

The entire effluent from the cracking heaters becomes the feed for the oil quench tower for cooling the cracked gas and condensing the fuel oil and other heavy hydrocarbons. The cracked gas is first brought in contact with circulating oil followed by pyrolysis gasoline fraction, obtained from the water quench tower

(Fig.5) at the top of the tower. The waste heat recovered from the circulation of the quench oil finds its use in generating low pressure steam. One use of this steam is the generation of dilution steam to be used in the furnaces. This use is however not very popular as it sets a minimum temperature for the quench oil affecting the viscosity of the quench oil. A portion of the quench oil is sometimes directly used for quenching of the cracked gas in the fitting for quench in the cracking heaters and the rest of it is used as the pump-around liquid for the underlying part of the oil fractionator.

The cracked gas leaving from the top of the oil quench tower are routed to the water quench tower for cooling the dilution steam further and condensing it and the heavy gasoline components by contacting it with direct countercurrent with recirculating the quench water tower with quench water. The circulating flow gravity runs from underneath the tower to an oil-water separator connected by an equalizing line to the tower.

The oil-water separator is generally an overflowing drum that has a high residence time, allowing the coke and heavier-than-water components to separate. The slurry (i.e. water saturated with tar and coke) are separated by a series of purges located at the separator bottom and routed to the coke removal unit. The coke removal unit usually handles he coke particles accumulated at the oil-water separator bottom.

Circulation pumps are used to circulate the quench water. The partial pumping and cooling of the quench water for heat recovery is done against some process users in the olefins plant (like, column reboiler) and the major task in cooling is achieved by the heat exchangers. The heat is liberated to cooling water and/or release into the atmosphere via air coolers. Dependant on the pressure drop within the system, the media that is used inside the tower can be packing and/or trays. Dedicated pumps separate and withdraw the condensed dilution steam (process condensate) from the oil-water separator and sent to a filtration unit, stripper column and finally to the dilution steam generation system for producing dilution steam for supplying to the cracking heaters. The fuel oil and the pyrolysis gasoline that has been separated from the water within the separator are pumped towards downstream processes for additional processing. The pH of the circulating quench needs to be maintained over 5.5 to ensure no corrosion and below 6.5 to enable successful oil-water separation. Amines or sodium hydroxide are effective in pH control or the pH of the circulating quench would drop due to CO2 presence. Amines are favored because they are less expected to cause emulsions. Usage of caustic with the dilution steam can cause the furnace tubes to crack. Amines will carry over in the dilution steam and crack to ammonia once at the cracking unit.

Some of the designers may combine the water and oil quench fractionator in a single tower with two separate sections. The combined tower performs the same functions as the two separate quench towers. The upper section is primarily used for condensing pyrolysis gasoline and the dilution steam, whereas the lower portion is used to recover the pyrolysis gasoline. The quench water circulation stream is usually also passed through the filtration unit so that no coke or tar is recycled back to the quench tower, which otherwise might lead to plant shut down because of the fouling of the tower packing and/or trays and the recycle distribution. Other design differences and configurations will be addressed in the following sections.

In each case, the final cooled cracked gas obtained from the overhead quench water tower, varying between a temperature of 40 and 45oC, is routed to the compression section for auxiliary processing. The temperature of the quench tower overhead is maintained by regulating the temperature and rate of water circulation to the tower. More than one loop for quench water circulation can be designed in order to achieve more control over the profile of temperature during the temperature changes in ambience (like day/night or summer/winter). Temperature can also be controlled by bypasses around the quench water coolers. The temperature of the cracked gas should be as low as possible so that the following compression power requirement is minimal.

To avoid any risk associated with transient operations under low pressure, the tower under low pressure control is equipped with a fuel gas injection to the vacuum breaker (quench water tower).

Figure 5: Quench System

2.3.1.3 Cracked Gas Compression

The overhead quench water tower vapor (i.e. cracked gas) is compressed to acquire a high pressure level so as to make it suitable for separation in the units downstream. A cracked gas compressor (CGC), which is a 4 – 6 step centrifugal compressor, is used to increase the cracked gas low pressure (0.3 – 0.9 barg) to the desired level. A steam turbine is usually used for driving the CGC for economic reasons and for effective speed control. The compressor’s suction pressure can be controlled by altering the speed of the compressor.

The temperatures of the maximum discharge from each of the stages in the compressor are set according to specification for ensuring that the machine does

not foul and this can determine the number of CGC steps. Usually, each step in the CGC is followed by an intercooler and a subsequent knockout drum for cooling the discharge after each step and for separating the condensed heavy hydrocarbons and water, in that order, as shown in Fig.6. The knockout drums are generally set with high competence mist eliminators to minimize the carry over of liquid to the following compressor stage. The condensed hydrocarbon components and water are then recycled and flashed from high pressure suction (or knockout) drums to the ones at lower pressure to recover the lighter hydrocarbon components. Then, the condensed steam is routed to the quench water tower.

Conditions are usually preset for the boiler feed water in each of the CGC stage suction line in order to keep the discharge temperature commonly below 100oC and to check fouling due to polymerization. If required, a provision can also be made to inject wash oil in the suction lines or in each of the impellers to clean off the internals, eliminate deposits causing fouling, maintain efficiency, and to extend on-stream time for the compressor.

Figure 6: Cracked Gas Compression Train

2.3.1.4 Acid gas Removal

The cracked gas obtained from the cracking unit contains hydrogen sulfide and carbon dioxide (known as acid gases). These components need to be removed for keeping up to product specifications. The carbon dioxide may as well solidify in the cooler sections of the plant when complete removal of methane and hydrogen are done. H2S is a poison for the hydrogenation catalyst.

The removal of these acid gases is commonly carried out in a caustic wash tower following the third or fourth step of the CGC. The gas is introduced into the tower from underneath, which gradually moves upwards passing two or three levels of caustic solutions for removal of acid gases as shown in Fig.7. After scrubbing in for, say, three levels of caustic soda, weak, medium and strong in the caustic concentration, the cracked gas free from H2S or CO2, the cracked gas is water washed with cooled BFW (at 450C) in the upper section of the caustic tower. The water wash specifically reduces caustic carryover to the downstream units. The water removed from the separator downstream to the caustic tower should not be recycled to the quench tower, because even the minimum caustic content in the water may disturb the pH balance in the tower. The weak caustic solution at the lower section of the caustic tower usually contains 2 – 5% free caustic while the strong solution at the upper section of the caustic tower contains 8 – 12% free caustic.

Basically, the following equation depicts the reaction that takes place within the caustic tower due to scrubbing of the acid gases:

H2S + NaOH → Na2S + H2O (17) CO2 + NaOH ,,, Co2Na plus H2O (18)

Just below the top water section, a caustic make-up solution is introduced to the string solution section after mixing it with BFW in order to dilute the caustic make-up to lower level so as to avoid polymerization within the caustic tower. The caustic wash tower is so designed as to utilize about 80% of the caustic with about 3% caustic in the spent caustic stream. To achieve this desired level of caustic utilization, it is necessary to keep a close check on the make-up caustic being added from the top of the caustic wash tower. Caustic make-up over the required level for acid gas removal is lost in the spent caustic stream, thereby lowering the factor of utilization and increasing the consumption of caustic and water. The cracked gas thereby leaving the caustic tower overhead is low in its acid gas content to meet the final olefin product specifications. The spent caustic containing CO2 and Na2 residual caustic and water leaves from the underneath of the caustic tower due to gravity and is led into a degassing drum which opens to a low pressure system (usually flare system or quench tower) where the dissolved hydrocarbons are removed and the spent caustic stream, which from the degassing drum is pumped towards the downstream disposal treatment.

Figure 7: Acid Gas Removal System 2.3.1.5 Drying The cracked gas with saturated water content from the caustic tower overhead is cooled to just the hydrate inception temperature, the value of which (around 12 – 15o C) depends on the overall pressure of the system to the maximum amount of condensed water carried over from the top portion of the caustic wash tower, limiting the size of the dryer. A higher temperature will increase the water content of the process gas and reduce the dryer efficiency. Some heavy hydrocarbon components and the condensed water are separated in a dryer knockout drum prior to the sending of the cracked gas into the gas dryer. In some of the designs the hydrocarbon components are removed from the dryer to feed the KO drum, which are then dried in a liquid dryer and passed to the downstream unit for subsequent processing. This is performed to reduce the content of gasoline components by several tons per hour recycling in the last step of the CGC. Both gas and the liquid dryers remove water from the

hydrocarbon streams in order to prevent hydrate formation and freezing in the following low temperature operations, and to ensure that the final products do not violet the water concentration specifications. The cracked gas from the overhead dryer KO drum is dried out to less than 1 ppm water by adsorbing it over molecular sieve present in the gas dryers. Their selectivity and capacity is what make the molecular sieves perfect as desiccating agents. Generally, an additional dryer is put in operation continuously when the online dryer is taken out for regeneration before the breakthrough of moisture occurs. Usually, each dryer system is provisioned with a guard bed (typically 20% volume of the primary dryer) located below the main bed to enable switching to the additional (standby) dryer when moisture breakthrough is sensed via a moisture prod set between the two beds. Any amine carryover is cracked by the guard bed and any ammonia made in the furnace is removed along with. Ammonia on the mol sieves will be replaced by water. Therefore, if the guard bed is used totally for water removal, any ammonia present will be displaced, thereby contaminating the product ethylene. Fig.8 shows a sketch of a typical dryer system. A portion of the tail gas generated in the downstream units (recovery sections) is heated with steam under high pressure and used to regenerate the main dryer taken offline. The regeneration effluent from the dryer is cooled down and sent to the regeneration gas KO drum for water removal before it joins rest of the tail gas and enters the fuel gas system within the olefin plant. The condensed water from the regeneration KO drum is commonly routed back to the quench water system. The dryers are usually designed for an online performance of 24 – 48 hours. The same concept as explained above is applied in case a liquid dryer is provided for the condensed hydrocarbon components. Fig.8 shows the arrangement for a typical drying system.

Figure 8: A typical arrangement of drying system

2.3.1.6 Chilling Train & Demethanizer The Chilling Train & Demethanizer separate and remove the hydrogen and methane. The dried cracked gas obtained from the drying system is sent to the recovery section of the olefins plant for fractionation and purification of the main olefins product. A number of various processes exist – condensate separation, fractionation and heat exchange (in that order). For simple and better understanding, the popular process scheme using the front-end demethanizer system will be elaborated in the following section. However, the other process schemes using the front-end deethanizer and de-propanizer are explained in separate sections of this chapter.

As shown in Fig.9, the chilling train basically involves the consecutive cooling of the cracked gas against cascade mainly ethylene-propylene refrigerants; and with cold process steams that need to vaporized or reheated. The temperature of the cracked gas is falling with every stage and the condensate being removed in a gas-liquid separator. The condensate liquid hydrocarbon components are withdrawn from underneath the particular vessels and routed to the appropriate tray in the demethanizer tower. The overhead gaseous hydrocarbon components obtained from the vessels are further cooled in the coldest parts of the chilling train, against streams of very low temperature, which have been generated through Joule-Thompson expansion of process streams (isenthalpic throttling) and/or turbo expander machine propagated gas expansion. This enables the recovery of the ethylene contained in the overhead demethanizer product, which again is returned to the demethanizer tower from the chilliest parts of the chilling train. Usually, the chilling train consists of a number of multi-stream heat exchangers made of brazed aluminum (known as the cold box heat exchangers and the core-in-drum) which employs the integration of several process streams to enable enhanced heat recovery. For providing flexibility, a balance in heat is achieved by recycling a small stream (Joule-Thomson recycle) towards destination under low pressure, like the first stage of the CGC, primarily used during transitory operations (like turbo-expander out of service!). The demethanizer tower (a normal tray-type fractionator) essentially and completely removes light gases, like hydrogen, methane, carbon monoxide, from the condensate but there is some loss of ethylene in the demethanizer overhead product. However, this loss of ethylene can be minimized and later recovered through condensing the overhead product in the coldest parts of the chilling train, which has been mentioned earlier. Other means of recovering ethylene can be achieved by the licensors by lowering the level of ethylene in tail gas. The light gas thus obtained (usually known as the tail gas), leaving the demethanizer system after heating and conveying it to the fuel gas unit of the olefins plant, primarily contains CH4, H2, traces of CO and C2H2. Purification of a portion of the tail gas can be performed in a pressures swing adsorption (PSA) unit to produce high-purity hydrogen gas (99.99 mol %) as required by the demand for hydrogen controlled by the PSA. The pressure swing adsorption package that is PSA removes methane, nitrogen and carbon monoxide to form a very pure stream of hydrogen and a reject gas. The reject gas is routed to the fuel gas system. The pure hydrogen thus obtained from the outlet of PSA can be used in the hydrogenation reactors within the olefins plant or exported for other external

uses.

Figure 9: Chilling Train and Demethnaizer System 2.3.1.7 Acetylene Hydrogenation & Deethanizer Units The C2;s and the heavy components from the demethanizer bottom are fractionated in the deethanizer tower (a normal tray-type fractionator) in the form of an overhead C2 stream and a bottom C3;s and heavier hydrocarbon products. Total removal of C2-hydrocarbon components from the bottom products is more essential that the purification of the products obtained overhead. If the bottom products contain any C2-hydrocarbon, it will reach the propylene product, which often has the strictest specifications set against the ethylene content. Minor C3-hydrocarbons contained in the overhead product will be concentrated from the ethylene fractionator in the ethane product (C2 splitter). As elaborated in the section XXX, acetylene recovery can be performed to recover acetylene product. However, when the recovery of acetylene is not desired, acetylene can be removed from the deethanizer product overhead by discriminatory hydrogenation of ethane and ethylene in the acetylene converter (with one or two beds with intercooling). Preheating of the net deethanizer overhead is done against converter effluent and/or steam before it enters the acetylene converters. Hydrogen from the PSA is added at the inlet to each reactor bed. Additionally, the reaction yields heat (exothermic) and for managing the hydrogen selectivity, several beds with intercooling are generally provisioned. The catalyst used in the acetylene converter is a silver promoted catalyst. This has two major advantages over general catalyst systems. The first advantage is that it can operate well without having to inject CO, which simplifies the controlling and allows the production of an ethylene product free from CO. Generally, the plants operate from 12 – 18 months at selectivity levels more than 50% acetylene to ethylene. The converter effluent is contacted with converter feed and cooling water, cooled and routed to a knock-out drum for removal of any condensed heavy hydrocarbon components (green oil) and in order to provide reflux to the deethanizer tower. A cold refrigerant (ethylene and propylene) slip stream from the relevant systems is usually used for washing the reactor runoff stream upstream of Knock-Out drum for aiding the removal of the green oil (polymerized C2H2 & C2H4) obtained from the reactor. The reactor effluent usually contains acetylene that is less than 1 ppm, but comes contaminated with minor traces of methane and hydrogen, which is the major disadvantage in this particular unit of acetylene hydrogenation. An analyzer is provided at the reactor feed and each of the reactor bed outlets for monitoring the reaction progress. The desired acetylene contained in the bottom product is less than 1 ppmv. The condensed

liquid is separated in the knock out drum and routed back to the deethanizer for separating the ethylene/ethane mixture from the green oil. The green oil is collected from the deethanizer bottom. The total hydrogenated C2 obtained from the knock out drum overhead is generally led to a molecular sieve guard dryer for the removal of any moisture that has leaked through the cracked gas dryers or has been formed in the C2 hydrogenation reactor if there had been any contained oxygen within the C2 cut. Generally, one dryer is setup, which is bypassed during the periodic regeneration. The scheme shown in Fig.10 is known as the back-end hydrogenation which needs hydrogen supply from an external source (generally from the PSA) for the hydrogenation reactions. Contrary to the back-end, the front-end hydrogenation scheme needs no external supply of hydrogen because the vapor feed to the reaction already contains an excess of hydrogen for the hydrogenation of acetylene.

Figure 10 2.3.1.8 C2 Splitter System The ethane-ethylene mixture obtained from the hydrogenation system, containing traces of methane and hydrogen are routed back to the C2 splitter tower. The normal C2 splitter tower operates at slightly higher pressure and contains multiple trays, requiring a higher reflux ratio which is dependant on the product specification and the operating pressure. An adjacent side vaporizer (at times more than one in number), indicating energy savings, is often employed to provision a major portion of the net reboiler heat at a less temperature than the primary boiler. Fig.11 gives an general sketch for the C3 splitter system. A section for pasteurization is present overhead of the C2 splitter for the removal of the light products from the final ethylene. The light gases thus obtained from the C2 splitter system are routed back to the system for recycling. The hydrogen for hydrogenation that comes from the PSA is almost free from any impurity. The reduction of venting is then done, thereby minimizing the light ends from getting recycled into the process (in turn, leading to a reduction in cost of operation from recycling reduction through the plant). The liquid ethylene product is extracted from the draw off tray situated below the section for pasteurization followed by pumping it to the user after it has passed heat exchangers for vaporization. Recovery of ethane is done from the bottom of the tower for recycling it to the cracking unit post vaporization and superheating.

Figure 11: C3 Splitter system

2.3.1.9 Depropanizer and Debutanizer The C3plus bottom stream from the deethanizer tower is treated in a normal depropanizer tower for separating C3 and C4 hydrocarbons and other heavy hydrocarbons, as illustrated in Fig.12. The bottom products as obtained from this tower possess high concentration of heavier diolefins and butadiene that have a tendency to polymerize at moderate temperatures. Polymerization can also occur at the internals of the tower and the reboiler tubes due to the high temperature prevalent at the lower base. This can severely bring down the capacity of the fractionation unit. There are two possible designs to overcome this crisis. In the first design, the temperature at the lower part of the unit is set at low, resulting in an operating pressure for the unit, requiring propylene coolant in place of cooling water for condensing the overhead product. For the mechanical cleaning and regeneration of the main reboiler unit every few months, a spare reboiler is provisioned as replacement. The overhead depropanizer can be directly led to the downstream units. The bottom products from the depropanizer are further processed to separate the C4 from heavy fractions and C5 hydrocarbons in the debutanizer. The debutanizer tower is usually a normal fractionator with a reboiler heated by steam and a condenser cooled by water. Although polymerization is not much of a threat in the debutanizer as it is in the depropanizer, a spare reboiler is provisioned just in case. The C4 liquid overhead product can either be routed to a hydrogenation reactor for further hydrogenation before it is sent to the cracking units or be routed to butadiene or 1-butene recovery units, as elaborated in sectionXX. The heavier fractions (pyrolysis gasoline) and C5 hydrocarbons stream are obtained from the tower bottom and are cooled before leading them towards downstream units. The routes and pyrolysis usage are explained in chapter 3.

Figure 12: Depropanizer and Debutanizer system 2.3.1.10 MAPD Reactor and C3 Splitter System The product from the depropanizer can directly be led to downstream C3 splitter tower, as shown in Fig.13. However, it needs hydrogenation of the methylacetylene content and propadiene (MAPD), by downstream propylene users if required, to meet propylene product stipulations. The MAPD reactor can be positioned either upstream of the C3 splitter system or on the entire propane

recycle stream obtained from the bottom of the C3 splitter tower, depending on the MAPD obtained by the variety of feedstock and varying levels of severity. The normal front-end demethanizer scheme using MAPD upstream reactor will be elaborated here and the other designs will be explained further in sectionXXX. The mixing of the assorted C3 stream obtained from the depropanizer overhead and pure hydrogen obtained from the PSA is done prior to entering the MAPD reactor for selectively hydrogenating MAPD over a catalyst based on palladium. A recycle stream of liquid obtained from a separator provisioned at the MAPD reactor effluent is also combined with this reactor feedstock to check the exothermic reaction by diluting the MAPD concentration and keeping it low in the entire feed stream to the MAPD reactor. It also develops the selectivity by controlling the concentration of MAPD in the total reactor feed. A regeneration system is used to periodically regenerate the hydrogenation catalyst. On these occasions, provisioning a spare reactor will allow unremitting operation of the unit. However, some plants do not have spare reactors, instead of which they directly bypass the reactor and the entire feed is directly to the cracking heaters or the flare. The former can cause carburization in the cracking radiant coils owing to the presence of diolefins, causing the cracking heater to have a reduced run length. The reactor can also be bypassed, keeping the recycle feed within the cracking units. The impurities present in the hydrogen supply (primarily CO and CH4) and the unreacted hydrogen are stripped in a stripper tower downstream of the MAPD hydrogenation unit before C3 cut stream is sent to the C3 splitter tower. Another method of achieving the removal of these light gases is done in the pasteurization section, similar to what was there overhead the C2 splitter tower. The propane and propylene stream obtained from the stripper tower draw-off tray is led to the C3 splitter tower. Due to the less relative volatility, it is more difficult to separate propane from propylene than it is to separate ethane from ethylene. If there is no provision for a stripper tower, the removal of the light gases can be achieved in the overhead pasteurization section of the tower. Here, propylene is obtained from the stripper tower draw-off tray and led to the users. Recycling the light gases into the olefins plant is done. The bottom propane stream is usually recycled to the cracking units till they are cracked to extinction.

Figure 13: MAPD Reactor and C3 Splitter system

2.3.1.11 Refrigeration Systems The separation of the cracked gas through fractionation and cooling through condensation at cryogenic temperature needs refrigeration from outside over the whole range of temperature change, i.e. from ambient to -1000C. Propylene and ethylene are the typical refrigerants used in a refrigeration system of an ethylene plant, due to their suitability and ready availability in the system. Although other multicomponent refrigerants can also be used for refrigeration, they are not in an ordinary olefins plant.

References 1 M. E. Denti, E. M. Ranzi in L. F. Albright, B. L. Crynes, W. H. Corcoran (eds.): Pyrolysis: Theory and IndustrialPr actice, Academic Press, New York 1983, pp. 133–137. 2. F. O. Rice, J. Am. Chem. Soc. 53 (1931) 1959–1972. 3. S. B. Zdonik, E. J. Green, L. F. Hallee, Oil Gas J. 65 (1967) no. 26, 96–101; 66 (1968) no. 22, 103–108.