Friday 18 March 2011

CRACKING FURNACE


Fired Heaters

Introduction and operation

Most of the fired heaters used in the petroleum refinery and petrochemical and other chemical plants is a pipe still heater, which is designed to heat process fluid in tubes effectively by burning fuel. The function of the heater is similar to that of the steam-generating boiler except that usually process fluid is heated instead of water.
Basically a pipe still heater consists of a combustion chamber for heat release, surrounded by tubes through which the process feedstock flows to absorb heat by both radiation and convection. Using a predetermined air mix ratio, the heat is supplied by the gas or oil burners provided on the floor or on the walls of combustion chamber.
The feedstock is fed into and passed through tubes inside the heater. If a convection section is provided, the feedstock is fed to the convection section first, and then introduced into the radiant section. During passage of the feedstock through the fired heater, it is subjected to both radiant and convection heat. The inside walls of the heater are refractory lined, to cope with the high temperatures generated by the firing fuel.
The feed stock is heated to the required temperature at the specified phase and fed to the next unit in the process sequence; e.g., distillation column, fractionators or reactor etc. the temperature of the feed stock when leaving the fired heater differs according to the operating requirements.
The fired heaters most generally used are the box type and the vertical cylinder type.

Types of Fired Heaters

Fired heaters are classified by their construction and purpose. There are basically two types of construction, the box type and the vertical cylinder type. These are further divided by their tube layout, combustion method, purpose and characteristics. Although there are many types of construction to meet process requirements

 Purpose of fired heaters

 Heating
Raising the temperature of a liquid
Raising the temperature of a gas
Vaporizing a liquid
 Thermal Cracking
Gas cracking
Liquid cracking
Thermal Reforming
Gas reforming

 Heat Transfer

The purpose of a fired heater is to transfer heat to the process feedstock at a predetermined temperature. This is accomplished by burning a fuel or gas, causing large quantities of flue gas to enter the heater. The heat is transferred to the feedstock by radiation, conduction and convection.

 Heat Transfer in Fired Heaters

In the radiant section, the heat is transmitted to the tubes by heat radiation from the burner flame and the heater walls. This heat is transferred to the feedstock by conduction and convection. In the convection section, the heat is transmitted to the tubes by convection of the hot flue gas and is then transferred to the feedstock by conduction and convection Mechanical drafts are induced, forced or balanced. All the four draft methods are used in fired heaters, although a natural draft is more generally applied.

 Heater Components

Burners
The following types of burners are used for the combustion of oil or gas, or both in fired heaters
1) Premix gas burner
2) Non-Premix gas burner
3) Steam atomizing oil burner
4) Combination gas and oil burner
The burners are designed to produce a uniform flame suitable to the type of firebox involved, together with the most efficient, safe and complete combustion of the fuel.
Refractory
The following kinds of refractoriness are used in fired heaters to protect the heater casing (insulating materials) from hot flue gas
·        Cast able (aluminum cement +aggregates)
·        Brick (fire bricks & insulation fire bricks)
·        Ceramic fiber (Al2O3 and SiO2)

Heating tube
Heating tube is a kind of container in which high temperature and high pressure process feed stock is contained and receives the heat of combustion.In some special heaters the tube metal temperature will be more then 8000C.  The material of heating tube is selected from among carbon steel, low alloy and high alloy steel depending upon service temperature, corrosiveness of process feed stock and others.
Generally the heating tube is classified into three types, that is, bare tube, finned tube and studded tube.

Tube Support
The tube support is literally, the component that supports the tubes. Tube supports are usually of high alloy casting.

Auxiliary Equipment
There is some auxiliary equipment for fired heaters to achieve higher heater efficiency and to keep the fired heater in proper condition.

Air Preheater
The flue gas at the exit of the fired heater still contains some available heat which is high enough to heat the combustion air.
Air Preheater is a kind of heat exchanger and is designed to exchange the heat between flue gas and combustion air efficiently. With preheated combustion air the fuel quantity required can be reduced, since preheated air has more heat than ambient air.
When the flue gas is cooled too much some trouble may occur in the air Preheater elements, fan elements and the refractory in the duct or stack, since flue gas generally contains sulfur compounds.
Soot Blower
When fuel oil is used as a fuel, a large amount of ash, carbon etc. will be generated and may accumulate onto the convection heating tubes, resulting in low heat transfer.

Cracking Furnaces

Thermal cracking of hydrocarbons is accomplished in tubular reactors commonly known as cracking furnaces, crackers, or cracking heaters etc. Modern designs use tubes arranged in verticals rows, providing superior mechanical performance and high capacity.
Reaction
          The reaction proceeds in the pyrolysis coils of the radiant section of the furnace. Since coke is also formed in pyrolysis, steam is added as dilutant for feed. The steam minimizes the side reaction forming coke, and improves the selectivity to produce the desired olefins by lowering the hydrocarbons partial pressure. The temperature of the hydrocarbon and steam mixture entering the radiant chamber (known as crossover temperature) is 500 to700°C.  The combination of low residences time and low partial pressure produces high selectivity to olefins at a constant feed conversion.   
Burners
 Side wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1-10MW), and hence 40 to 200 burners are required in a single furnace. With modern flow burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as 10 m. The capacity of these burners depends on the type of the fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NOx levels. If the reaction mixture is not cooled quickly, olefin selectivity is decreased because of the many side reactions taking place in this zone. After the transferline exchanger, further cooling is achieved by spraying the furnace effluent with quench oil. The oil and water phases are then separated from gas phase.



Efficiency
 Only 35 to 50% of fired duty is absorbed in the radiant section, the flue gas leaving the radiant chamber contains considerable energy that can be extracted efficiently in the convection section of the furnace. In the convection section, the feed is preheated along with the dilution steam to the desired crossover temperature. Residual heat is recovered by generating steam. The overall thermal efficiency of modern furnaces exceeds 93%, and the value of 95% is not uncommon.
The convection section is a series of cross-flow exchangers with flue gases on one side and process fluid is on the other tube side. Since mainly gas to gas transfer is involved, fin tubes are employed to improve the heat transfer rate where practical. The metallurgy of the tubes varies from carbon steel to high temperature alloy depending on the service. When high overall efficiency is desired, condensation of acidic flue gases must be taken into account in the selection of materials. Fouling of heat transfer surfaces both inside and outside is avoidable. Outside fouling is cleaned by seam lancing, the inside fouling is usually handled by burning.
Environmental Issues
Stringent environmental laws requires that nitrogen oxides (NOx), and sulfur oxides emission from furnaces be drastically reduced. In many parts of the world, regulations requires NOx be reduced to 70 vol ppm or lower. On a wet basis. Conventional burners usually produce 100 to 120 vol ppm of NOx. Since NOx production depends on the flame temperature and quantity of excess air, achieving required limits may not be possible through burner design alone. Therefore many new designs incorporate DENOX units that employ catalytic methods to reduce the NOx limit. Platinum containing monolithic catalysts are used. Each catalyst performs commonly for a specific temperature range, and most of them work properly on 400°C.
Product Distribution

In addition to mainly ethylene many by-products (propylene, butadiene, and butenes) are also formed. The product distribution is strongly influenced by residence time, hydrocarbon partial pressure, steam to oil ratio, and coil outlet pressure.

2 comments:

  1. Condensing boilers are specifically designed to operate on low return water temperature. In the process, further sensible and latent heat of condensation from the flue gas are given up, therefore increasing the thermal efficiency of the boiler.

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