Kittiwake Procal Ltd Page 1 of 7 Introduction Ethylene consists of four hydrogen atoms and two carbon atoms (C 2 H 4 ) with the carbon atoms connected by a highly reactive double bond. The reactivity of ethylene makes this olefin an important intermediate in the manufacture of other chemicals, especially plastics. Polyethylene, the most widely used plastic, is produced by the polymerisation of ethylene. Polyvinyl chloride, another important plastic, is produced from the chlorination of ethylene. Combining ethylene with benzene produces ethyl benzene that is subsequently used in the manufacture of Polystyrene. Ethylene can also be oxidised to produce important chemicals such as ethylene oxide, ethanol and polyvinyl acetate. The majority of ethylene manufacturing plants use a process known as steam cracking. In the steam cracking furnace gaseous or liquid hydrocarbons are quickly heated to induce cracking reactions that are then stopped by rapid cooling in a quench tower. Both the steam furnace and ethylene product recovery stages are energy intensive. Minimizing production costs and maintaining emissions to within acceptable limits requires strict control of energy consumption. Procal 2000 Continuous Emissions Monitoring system (CEMs) Continuous process monitoring enables energy efficiency to be maintained and even improved as measured data is acquired and process control is finely tuned. The Procal 2000 is a Continuous Emissions Monitoring system (CEMS) that can be used to monitor the furnace flue gases. The Procal 2000 is an infra red (IR) duct or stack mounted analyser that can monitor upto six gases including carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen oxides (NOx), nitrous oxide (N 2 O) and methane (CH 4 ). A typical CEMs system comprises an in-situ mounted analyser, an integral calibration function and a control unit with options that include a powerful in-situ heater and a stand-alone analysis software package. The Procal 2000 uses the reflective beam principle to directly measure process gas as it enters the sample cell. Unlike high maintenance extractive systems, Procal s patented, sintered metal technology removes the need for gas filtering or sample conditioning. Very minor maintenance is required throughout its operational lifetime with an up-time of over 98% in even the most demanding applications. Procal 2000 applied to emissions monitoring in Ethylene The Procal 2000 can measure the concentration of carbon monoxide in the flue gas of the steam cracker. The amount of carbon monoxide present can be used as an indication of the level of partially oxidised fuel in the furnace providing a measure of the furnace efficiency. Stationary combustion sources primarily emit CO 2, when fuel is fully oxidised, but will also emit low concentrations of nitrous oxide (N 2 O) and methane (CH 4 ) that can be measured simultaneously using the Procal 2000. Steam cracking furnaces are normally equipped with low NOx burners that operate with low NOx emissions. In addition, modern furnaces are equipped with selective catalytic reduction (SCR) systems to further control NOx emissions. The Procal 2000 Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 2 of 7 can monitor the NOx concentrations to an accuracy of +/- 2% of the full scale range and provide feedback control of the SCR system as shown in figure 1 below. SCR systems use ammonia (NH 3 ) as the reducing agent for chemical reduction of the NOx and a small emission of NH 3 (known as ammonia slip) can be expected. The ammonia slip, typically in the range of 0-20ppm, can be measured if required with the Procal 5000 ultraviolet gas analyser. Figure 1. Procal 2000s measuring efficiency of the SCR on the Steam Cracker Procal 5000 SCR Ammonia injection Procal 2000 Flue gases Water Gas Compressors Ethane Steam Cracker Steam Water Transfer line heat exchanger Fuel Oil Quench Tower To Gas Cleaning The Procal CEMS system can be easily integrated into existing process control systems with upto eight Procal CEM systems being controlled from the Procal 1000 Analyser Control Unit (ACU). The control unit is an industrial panel PC that controls and processes the raw data from the CEM systems. The ACU displays gas concentrations in customer specified units (mg/nm 3, ppm or %) on the integral touch screen display along with information on sample conditions, diagnostic data and trends. In addition, the ACU can also collect data from third party instruments measuring for example, dust (opacity), oxygen, temperature, pressure and velocity. The information can be re-transmitted in the form of 4-20mA current outputs (one per measured component), parallel printer interface and optional RS485 serial output. The ACU runs the powerful Procal Analyser Control for Windows Network software that can also be installed on a stand-alone PC. Networking facilities allow the software to be installed on multiple LAN connected computer terminals that can Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 3 of 7 be accessed through WAN remote devices. The ACWN software is designed to provide reports in the form required by regulatory authorities. Procal CEM Systems certification and Environmental Regulations The United Kingdom Environmental Agency has approved both the Procal 2000 analyser and Procal 1000 under the MCERTS scheme. To gain MCERTS approval the Procal CEM system has passed rigorous accuracy, repeatability and reliability tests both in a laboratory environment and in industrial process applications. Through the MCERTS scheme the Procal CEM system complies with the monitoring requirements specified in EU directives such as the Large Combustion Plant directive 2001/80/EC (LCPD) and the Waste Incineration Directive 2000/76/EC. The Procal 2000 is also available with ATEX & IEC approval for use in Hazardous Areas. Previously certified under CENELEC, the analyser already has a presence in the Chemical and Refining industries and achieving certification under the ATEX & IEC enables Procal to continue serving these markets. Emissions of Ethylene are regulated under the Clean Air Act (CAA) as a Volatile Organic Compound (VOC). The EPA has declared a number of regulations under CAA section 111 governing the emissions of VOCs, including ethylene. Ethylene is not listed as a Hazardous Air Pollutant (HAP) under section 112 of the CAA. Carbon dioxide (CO 2 ) emissions related to an ethylene plant are inversely proportional to the overall plant efficiency. Operators of ethylene manufacturing plants must adopt best available technologies (BAT) to reduce CO 2 emissions. The EPA recognises two acceptable methods for reducing CO 2 emissions. Carbon capture and sequestration technologies is one suitable method but this technology is still in its infancy and yet to be proven on a wide scale. The primary method adopted by operators of ethylene plants is therefore to improving energy efficiency and reduce energy consumption. Energy efficiency can be achieved through continuous process monitoring, automating process control, implementing advanced control techniques, adopting routine process cleaning and maintenance procedures and maintaining process operating rates to their design criteria. The energy efficiency of an ethylene plant is determined by the thermal efficiency of the steam cracking furnaces and the efficiency of the recovery section of the plant where the cracked gases are separated into the final products. The furnaces and the recovery processes consume approximately equal amounts of the total energy associated with ethylene production. The primary CO 2 emission sources are the steam cracking furnaces that require high temperatures in the form of heat to achieve the thermal reactions. Emissions such as NOx, CO 2, CO, and particulate matter (PM) are generated due to combustion and released to the atmosphere through the furnace stack. In addition, a decoking operation is performed on the furnace tubes to reduce carbon build-up. In the decoking operation the carbon in the furnace tubes is combusted producing further CO 2 emissions. Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 4 of 7 Steam Cracking and the Ethylene Production Process An ethylene plant may also be referred to as an olefin plant. Olefins are typically defined as ethylene (ethane), propylene (propane), butadiene (butane), isobutylene and isoprene. The feedstock for an ethylene plant can be methane, ethane, propane and heavier paraffin s. From developments in cracking technology ethylene can also be produced from crude oil fractions such as naphtha, kerosene and gas oil. Many oil refinery processes are combined with ethylene plants so that the extra low octane naphtha streams from the refinery process can be used as feedstock for the ethylene plant. The cracking process produces gaseous light end products, containing olefins that are distilled and separated using column splitters. The column splitters are comprised of a large number of separation stages. The majority of ethylene plants use a process known as steam cracking as shown in figure 1 above. In this process gaseous or light liquid hydrocarbons are heated for a very short period to induce numerous free radical reactions that are then stopped by rapid cooling or quenching. The feedstock (ethane, propane, butane, Naphtha or gas oil) is fed into the cracking furnaces and combined with steam that heats the mixture to temperatures between 790-870 C (1450-1600 F). The steam dilution lowers the hydrocarbon gas pressure and reduces the formation of coke deposits in the tubes of the furnace. The gas/steam mixture has a very short residence time in the steam cracker that is typically less than one second. The feedstock molecules undergo cracking due to the high temperature producing chemicals such as ethylene, methane, hydrogen, propylene, butadiene, benzene, toluene and other hydrocarbons. Instantaneous cooling takes place as the combined gas stream exits the steam cracker and passes through a transfer line heat exchanger. A quench tower further cools the gas stream through direct contact with water to minimise further cracking. The quench tower condenses all of the steam and partially condenses the gasoline components. After steam cracking and subsequent quenching the desired products are separated into streams by further petrochemical processes. Process steps can include distillation, compression, process gas drying, hydrogenation (of acetylenes), and heat transfer. A series of compressors are used to compress the gas to an optimum pressure for separation into various gas components. Hydrocarbons and water are separated between the compression stages and recycled. Ethylene is separated from the resulting complex mixture by repeated compression and distillation. Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 5 of 7 Gas Cleaning Impurities in the gas stream include gases such as carbon dioxide, hydrogen sulphide and acetylene. The gas cleaning process is shown in Figure 2. A caustic tower removes acid gases such as carbon dioxide and hydrogen sulphide by scrubbing them with a dilute caustic solution. Acetylene is removed in a subsequent acetylene converter. Figure 2. The Gas Cleaning Gas Compressor Caustic Tower Acetylene Converter Reprocessing of used Caustic Drier H 2 O & NaCl Refrigeration stage To Distillation Stages Hydrogen Gas CO 2 Following the acid gas removal the gas stream is cooled to a temperature in the region of -150 C so that only hydrogen remains in the vapour phase. This stage involves either drying the gas or absorbing the water present so that ice compounds that would block process pipes are prevented from forming. Once the gas has been dried it is then cooled by passing through a series of heat exchangers. Hydrogen gas is withdrawn and the liquefied gas is transferred to the distillation process. The Distillation Process Distillation is the most important and most common separation technique used in chemical engineering plants as it provides the most cost effective and best method for separating a liquid mixture into its components. Distillation separates mixtures by using the differences in the boiling points of the various components of the mixture in a process known as fractionation. In each of the stages described below the lowest boiling point gas is boiled off from the hydrocarbon mixture and collected at the top of the distillation tower. Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 6 of 7 The ethylene distillation process is shown in figure 3 and consists of several stages that include the demethanizer, the deethanizer, the hydrogenator (C2 splitter), the depropanizer, the propylene fractionator (C3 splitter) and the debutanizer. Figure 3 Distillation of Ethylene Methane Ethylene Cleaned Hydrocarbons Hydrogenator Ethylene fractionator C2 Splitter Recycling Demethanizer Depropanizer Propylene Fractionator C3 Splitter Deethanizer Recycling C4 Hydrocarbons Debutanizer Propylene Gasoline The Demethanizer The demethanizer, the first step in the fractionation process, separates the light gases (methane, hydrogen and carbon monoxide) from the ethylene and heavier components of the mixture. Methane containing hydrogen and carbon dioxide impurities are boiled off and collected at the top of the column. The ethylene and heavier components, left at the bottom of the column, are then passed to the deethanizer. The Deethanizer In the de-ethanizer, acetylene, ethane and ethylene (known as C2 hydrocarbons) are boiled off and collected at the top of the column whilst C3 and heavier hydrocarbons are left at the bottom. The C3 and heavier hydrocarbons left at the bottom of the deethanizer are passed to the depropanizer. Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13
Kittiwake Procal Ltd Page 7 of 7 Hydrogenator and ethylene fractionator (C2 splitter) The C2 hydrocarbons extracted from the top of the ethanizer are heated. Hydrogen is then added to convert acetylene to ethylene and ethane. The ethylene and ethane are separated in an ethylene fractionator, known as the C2 splitter. The ethylene product is acquired at this stage while the ethane is recycled. Depropanizer and Propylene Fractionator (C3 splitter) The depropanizer separates the hydrocarbon product collected at the bottom of the deethanizer. The C3 hydrocarbons are boiled off from the heavier C4+ hydrocarbons and sent to the propylene fractionator. The fractionator (C3 splitter) separates the heavier propylene from the other C3 hydrocarbons which are recycled. The Debutanizer The bottom product of the depropanizer - the C4+ hydrocarbons are further processed in the debutanizer where C4 product is separated from light gasoline. Procal 2000 CEMs in Ethlyene Change Note: 7009615 Date: 18 02 13