Z. LIU, GTC Vorro Technology, Houston, Texas
One of the interesting dimensions of chemical engineering is the management of processing operations related to the molecular composition of the raw material. Petroleum naphtha is one area where significant improvements can be achieved by changing the sequence of processing steps or altering the general scheme of processing altogether to take advantage of the natural distribution of components. Typically, naphtha refers to a fraction of oil with the distillation range of approximately 30°C–200°C. If the naphtha derives from crude oil directly, it is referred to as “straight-run naphtha.” In other cases, the naphtha takes on the descriptive name of the conversion unit associated with its processing, such as “fluid catalytic cracking (FCC) naphtha,” “hydrocracker naphtha” and “coker naphtha.”
Typical analytical procedures using gas chromatography make it easy to obtain a fully-detailed hydrocarbon component analysis of the naphtha. One methodology describes the material in terms of its content of paraffins, iso-paraffins, olefins, naphthenes and aromatics (PIONA). This is convenient to determine the optimum disposition and processing options for value upgrading of each individual molecule.
Naphtha from different sources has characteristics that are specific for the components according to the PIONA. Normally, straight-run naphtha is rich in saturates (paraffins and iso-paraffins), lean in aromatics and almost void of olefins. FCC naphtha is rich in olefins and aromatics. Hydrocracker naphtha is rich in iso-paraffins and naphthenes, lean in aromatics and void of olefins. Coker naphtha is rich in olefins. TABLE 1 shows the PIONA breakdown value for straight-run naphtha from the crude distillation unit.
Within refineries, where the main processing objectives are to produce on-spec transportation fuels, the typical processing configuration of straight-run naphtha into gasoline is shown in FIG. 1. This has four processing units: 1) naphtha hydrotreater and splitter, 2) light naphtha isomerization, 3) heavy naphtha reformer and 4) benzene removal.
If the refinery is integrated with a steam cracker complex for olefins production, there is an opportunity to route various intermediate streams from the refinery to the steam cracker complex as the feeds, and vice versa. The various liquefied petroleum gases (LPGs), naphthas and gasoil can have higher value in the other facility, as these intermediates are often products of partial conversion that are synergistic with a different type of unit operation.
In the steam cracker, light olefins are ideally produced from paraffins in the feed. The higher concentration of normal paraffins leads to a higher ethylene yield—paraffins with an even number of carbon atoms result in a higher ethylene yield than those with odd numbers of carbon atoms. The yield of propylene decreases when increasing the chain length of hydrocarbons in the feed.
Compared with normal paraffins, the iso-paraffins cracking product is lower in ethylene and propylene and produces higher amounts of methane and higher olefins. Benzene in the feed remains unchanged under normal cracking conditions. Branched aromatics in the feed mainly will have some thermal dealkylation reaction to form benzene and corresponding paraffins and olefins. However, some aromatics (benzene and branched aromatics) are still formed during the cracking process from paraffins and naphthenes. Overall, regardless of how many aromatics are in the feed to the steam cracker, there will be a net plus of aromatics formed.
Aromatics-rich naphtha is unsuitable for steam cracking. The industrial practice is to limit the aromatics content in naphtha feed to < 10%. TABLE 2 indicates the typical naphtha specification in the open market when purchased for steam cracking purposes.
It is obvious that a lower aromatics content in the naphtha feed to a steam cracker will provide significant improvements in the efficiency, economy and operability of the steam cracker. To separate the aromatics from non-aromatics in the naphtha, extraction of the aromatics by selective solvents has been proven as the most efficient and effective method for more than 50 yr. Two major design options are available for aromatics extraction: liquid-liquid extraction and extractive distillation. Neither of these two is universally preferred for aromatics extraction, since many factors determine the optimum solution, including:
In normal industrial practice, a steam cracker will receive the naphtha feed, per the specifications stipulated in TABLE 2, either from the open market or from the integrated refinery. The best naphtha feeds have a high ratio of hydrogen:carbon (H:C) atoms, which is reflective of high paraffin content, low olefins and very low aromatics (the lower, the better). Note: The processing objective is not to recover the high-purity aromatics product, but to segregate aromatics from non-aromatics in the naphtha feed as the bulk concentrate. Therefore, with relatively low aromatics content in the naphtha feed, liquid-liquid extraction is a better option for naphtha dearomatization over extractive distillation in terms of capital expenditure (CAPEX) and operational expenditure (OPEX).
FIG. 2 shows the generic liquid-liquid aromatics extraction process scheme using a solvent with high density and high boiling point. Aromatics in the feed are extracted by the solvent in the liquid-liquid extractor. The rich solvent goes to a backwash stripper with partial recycle to the extractor to enhance the aromatics extract purity. The non-aromatic raffinate portion is withdrawn from the overhead of the liquid-liquid extractor. The aromatics extract is separated from the solvent in the solvent recovery column. Lean solvent is withdrawn from the bottom of the solvent recovery column and recycled back to the liquid-liquid extractor.
Naphtha dearomatization for steam cracker feed optimization is different from recovering high-purity saleable aromatics products from an aromatics-rich feed. The purity and recovery of the aromatics can be relaxed to a large extent, as long as the overall economics are not compromised. Hence, the solvent selection, process/equipment design, operating parameters, etc., can be customized to achieve better economics. Impurities in the naphtha feed may be distributed into the raffinate or extract. The distribution ratio will be determined by the selected solvent and operating parameters; nevertheless, this will not impact the overall configuration and economics. TABLE 3 indicates the representative steam cracking yield from the naphtha feed with different aromatics content.
It can be concluded that ethylene yield increases significantly with the decrease of aromatics content in the naphtha feed. There is no significant change regarding the yield of propylene, butadiene and C4 olefins.
After naphtha dearomatization, non-aromatic raffinate from the liquid-liquid extraction is routed to the steam cracker as originally designed, but with less hydraulic load on the furnaces, and less thermal heating required. Due to the aromatics segregation in advance, there is room for supplemental feed, which could be the same dearomatized naphtha or other feed like LPG. The CAPEX required for dearomatizing naphtha feedstock into the cracker is less than what would be required for additional furnace and cracker hot section capacity. A detailed study can determine the full benefits and the optimum design basis for the naphtha dearomatization unit.
To minimize the CAPEX and OPEX of the naphtha dearomatization unit, the aromatics extract product from the unit may contain 1%–5% of non-aromatics and other impurities, such as sulfur. This aromatics extract stream can join the second stage of the pygas hydrotreating unit within the steam cracker complex to remove the impurities, and be processed together with the pygas for high-purity marketable aromatics products.
Raw pygas is a major byproduct in the steam cracker complex that contains a large quantity of unsaturated hydrocarbons, such as olefins, diolefins, acetylenes and styrene. Also, raw pygas contains impurities, such as sulfur. Due to the high concentrations of aromatics, raw pygas is one of the best sources for high-purity aromatics recovery (in particular BTX).
However, before recovering the aromatics, the unsaturates and impurities must be saturated and removed. In industrial practices, this is achieved by a two-stage hydrotreating process. The first-stage hydrotreating is utilized to saturate the diolefins, acetylenes and styrene contained. The second-stage hydrotreating is used to saturate the olefins and to remove the sulfur. Both stages of hydrotreating are catalytic hydrogenation processes that occur in separate fixed-bed reactors. Prefractionation, inter-stage fractionation and post-fractionation are evaluated and installed under “case-by-case” scenarios to customize the ultimate configuration required by the desired byproduct slate. FIG. 3 shows the configuration of the steam cracker integrated with naphtha dearomatization, pygas processing and aromatics recovery.
The following preliminary mass balance results for different scenarios are based on the configuration shown in FIG. 3.
Evaluation basis:
See TABLES 4–6 for the results of different scenarios based on the evaluation basis above.
Straight-run naphtha dearomatization is an economical approach from multiple aspects for the ethylene steam cracker complex for both grassroots projects and retrofitting existing ones. This method:
In addition to the dearomatization of straight-run naphtha feed in a steam cracker complex, another potential benefit is sulfur management—a great portion of the sulfur in straight-run naphtha is extracted along with aromatics molecules, which bypass the cracker furnaces. The exact portion of sulfur extracted depends on the sulfur speciation in the naphtha feed and the overall sulfur balance across the stream cracker complex. If the naphtha feed is sweet (< 100-ppm sulfur), then additionally injected sulfur compounds are used in the cracker furnace coil to reduce the coke formation, CO formation and enhance the light olefins yield. Sulfur in the naphtha feed and added sulfur compounds will end up in the various cracking products, waste caustic and process wastewater. FIG. 4 presents the simplified sulfur balance concept.
When the naphtha feed is sour and contains a significant fraction of sulfur—even without injecting the additional sulfur compounds—a large amount of caustic is needed to treat the charge gas to remove the acid gas, thereby generating the large amount of waste caustic. Waste caustic handling is a chronic nuisance that steam cracker operators wish they could do without. The operation is prone to complications, with many operational steps required that do not directly relate to producing ethylene. Anything that can be done to reduce the load on the caustic wash system will be advantageous for the cracker operator by improving the safety and environmental impact in the spent caustic and wastewater treatment system.
Additionally, OPEX savings are associated with less waste disposal. With the naphtha feed dearomatization implemented (particularly for the sour naphtha feed), a controllable fraction of sulfur in the feed will be extracted along with indigenous aromatics molecules; it will then bypass the cracker furnaces while directly blending with the pygas from the first-stage hydrotreating, and is sent to second-stage hydrotreating. HP
ZHEPENG LIU is the Director of technology and engineering for GTC Vorro Technology based in Houston, Texas. With more than 30 yr of experience in the gas processing, petrochemicals and refining industries, his background covers catalyst and process development, process design, plant operation, technical services and business management. He studied chemical engineering at Tianjin University, the National University of Singapore and Delft University of Technology. He also holds an MBA degree from the University of Houston-Victoria.