There are two methods available to manufacturers in determining the analysis of the various points in their process, laboratory and on-line. There are pros and cons for both, but in many cases laboratory analysis is too slow to maintain a dynamic process on specifications. This is indeed true for the Olefins process. Process designs are based on profiles of individual sections of a process. This profile says that at given variables, “temperature”, “pressure”, “flow rate”, “level” …… the result should be within the “specs”. This is theoretical, backed up with empirical data based on ideal conditions. As we know, the conditions change and many times the only way that is apparent is through analysis.
These articles on the Olefins process is a basic overview. It will address the major areas of analysis. External monitoring of various variables will not be addressed here (cooling water, CEMS, LEL, etc).
In order to produce or make anything, a raw material is needed. We call this material feedstock.
Feedstocks can vary based on a number of factors such as availability, cost, handling capability, and end-product demand. Some of the typical ones are ethane/propane mix, propane, naphtha, butane and gas oils. If all factors are positive, the objective is to produce the product that is currently in demand …. Ethylene or Propylene.
Feed
To produce the maximum amount of ethylene, the feedstock of choice is typically a 90/10 ethane/propane (E/P) mix. As the feedstock become heavier (propane, naphtha,etc) the ratio of ethylene to propylene become less and more propylene is produced.
Depending on the suppliers feedstock purity, a number of upstream operations (analysis) may or may not be required . For example, methanol or sulphur might prompt a drying or scrubbing process.
In the case of methanol, analysis for both inlet and outlet of a dryer may be justified for detecting breakthrough (saturation / regeneration) and efficiency.
Density of the feedstock is often an issue in custody transfer between the supplier and purchaser. On-line densitiometers provide a simple way to monitor on-line at remote locations and can easily be an input to a flow computer for calculations. Feedstock that is transported by rail, ship or truck is normally handled by grab samples. These values are used in the billing/payment of product.
Conditioning
Feedstock is received as a liquid and has to be vaporized prior to going to the cracking stage. A vaporizer / saturator is vessel where feedstock, recycled gas and steam are mixed. In order to get a good crack in the furnaces without coking up the tubes, steam has to be added to the feed based on the density. A ratio of feed to steam (orifice factor) is calculated for each type of feed and its density.
Since recycled gases are added to the incoming feedstock, and they vary, an analysis, is required, of the feed downstream of the mixing. On-line analyzers are recommended. Sampling becomes an issue since the feed is now saturated with steam. A refrigeration effect is required to knock-out the steam. This can be easily accomplished with a vortec / coiled tubing as a shell to the sample line. Downstream filtering / protection is a must in case of cooling failure. Cooling temperature –vs- flow rate must be observed. Not enough residence time through the cooler will cause liquid carryover. This analysis is relative simple for E/P and Propane feeds using a gc or mass spec.analyzer. For Naphtha and gas oils it is a little more difficult and some prefer to do lab analysis on these.
Cracking
Cracking is done with the mixed steam and furnaces operating between 1600 -1800 degrees F. On initial startup of a furnace, sulphides are injected into the tubes of the furnaces for a given period of time. This is an external, that is mixed with the cracked feed. It is injected to coat the tubes as a coke inhibitor. As a result it will combine with hydrogen and produce hydrogen sulphide (H2S). This will have to be dealt with later in the process.
The feed passes through the tubes and “cracks” or breaks up the molecular chain. After the cracking has occurred, analysis of the effluent (composition) is required to make adjustments to the furnace …. typically temperature. A profile severity number is compared to a calculate based one from the analysis. An example; E/P fed furnace cracking too hard (too high of temperature / severity) will yield excessive amounts of acetylene which will cause downstream problems later.
The normal components of interest are ethane, ethylene, acetylene, C4s (butane, butanes), methane, propane and propylene. These analysis’ are accomplished by either using Mass Specs or Gas Chromatographs. Sampling, once again, is hindered by steam that must be removed. Chilled knock-outs perform this duty using a vortex air or by circulating a refrigerant, like propylene. Specialized py-gas sampling conditioners (like the one in the picture) utilize a temperature controller that shuts down flow to the sample system if the conditioners temperature rises above the setpoint. As with the feed sampling, flow rate must be low enough to give cooler sufficient time to condense the steam to water and drop it out. Unlike the feed stream, the cracked gas / steam mix becomes a nasty emulsion that will carryover and cause sample tubing pluggage and a mess of a sample system.
Furnace Fuel
Fuel for the cracking furnaces is an external but mentioned here due to large percent of recycled hydrogen and methane tail gas. In the first section of distillation, hydrogen and methane become by-products and sent back as furnace fuel and mixed with makeup fuels. Olefins process, once up and running is often self-sufficient for fuel.Steam
On-line analysis is used to measure the components and more over to provide calculated; BTUs, specific gravity, etc. These values are also used to calculate the emissions rate (covered in another article). A gas chromatograph using a TCD is a typical approach to measuring the composition. Another method to BTU values is through direct measurement using a Calorimeter. Expected composition is made up of primarily hydrogen and methane, usually accounting for at least 94% with some fractional percentages of C2s, C3s, and C4s. Accountablity of 100% is required to accurately calculate the BTU value for both process efficiency and CEMs reporting.
more to come.....Quench, Compression, Fractional Distilation, Conversion, Purification
Steam Cracking
Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene).
In steam cracking, a gaseous or liquid hydrocarbon feed like Naphtha, LPG or Ethane is diluted with steam and then briefly heated in a furnace. Typically, the reaction temperature is very hot —around 850°C—but the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is even reduced to milliseconds (resulting in gas velocities reaching speeds beyond the speed of sound) in order to improve the yield of desired products. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line exchanger.
The products produced in the reaction depend on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature & furnace residence time. Light hydrocarbon feeds (such as ethane, LPGs or light naphthas) give product streams rich in the lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon (full range & heavy naphthas as well as other refinery products) feeds give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil. The higher cracking temperature (also referred to as severity) favours the production of ethene and benzene, whereas lower severity produces relatively higher amomunts of propene, C4-hydrocarbons and liquid products.
The process also results in the slow deposition of coke, a form of carbon, on the tube walls. This degrades the effectiveness of the furnace by coating and insulating the tubes, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings. De-coking is performed by taking the furnace off-line. The coke deposits are a combustible. Air and steam are introduced into the tubes where combustion of the coke occurs. This "burn" is continued until the coke is removed. On-line Infrared Analyzers monitor the (CO2) carbon dioxide levels indicating the progress of the de-coking process. Once reduction to 0.0% CO2, the furnace is placed in standby ready to feed.
