March 2020

Special Focus: Petrochemical Technology

Advances in the OCC process for propylene production

Driven by rising demand for polypropylene, cumene, acrylonitrile and other derivatives, increasing global demand is seen for propylene.

Teng, J., Shi, J., Xie, Z., Sinopec Shanghai Research Institute of Petrochemical Technology

Driven by rising demand for polypropylene, cumene, acrylonitrile and other derivatives, increasing global demand is seen for propylene. Over the next 5 yr, propylene demand is expected to grow at a pace of about 4%/yr,1 and 30% of the world’s propylene production is expected to come from specialized propylene technologies.

Worldwide, propylene is derived largely from naphtha steam cracking, fluid catalytic cracking (FCC), propane dehydrogenization (PDH) or olefins conversion technology (OCT). Another route for producing propylene includes methanol-to-propylene (MTP), which uses methanol as feedstock for the production of propylene. In MTP, the typical yield of propylene (based on hydrocarbon) is only 62%–64%, making MTP a small part of the propylene market.

Furthermore, chemical processes, such as naphtha steam cracking, oil refining and methanol to olefins (MTO), produce a large amount of byproduct stream containing low-value C4/C5 olefins. Converting C4/C5 olefins to high-value chemicals would significantly improve the economic efficiency of producers.

In 2000, Sinopec started developing a process for the production of propylene and ethylene from C4/C5 olefins. Several process options, including steam dilution, methane dilution and methane without dilution, were compared and evaluated according to the test results obtained from the company’s pilot plant. Steam dilution can reduce olefins partial pressure and improve the selectivity of propylene; moreover, coking tendency is efficiently suppressed. The diolefins content in the feedstock is 1 wt%, and the catalyst cycle length is greater than 1,000 hr.2 However, the addition of steam into the feed will result in high cost and high energy consumption, resulting in lower technical economy of the process.

Based on the results and experience gained from pilot plant operation, Sinopec developed the olefins catalytic cracking (OCC) process, a novel route to produce propylene and ethylene from C4/C5 olefins. Without any dilution addition, OCC converts low-value byproduct streams containing C4/C5 olefins from MTO, crackers and refineries into high-value propylene and ethylene products.

The OCC process is based on full-crystalline MFI zeolite experience, and the OCC 100 catalyst can be operated with high weight hourly space velocity (WHSV). The first commercial OCC plant started up in Puyang, Henan Province in 2009. Operational findings were reported in Science China.3 A second OCC plant started up in 2016, and a third OCC plant commenced operations in 2019.

To improve the technical economy of OCC, Sinopec Shanghai Research Institute of Petrochemical Technology has developed a new OCC technologya to obtain a higher yield of propylene and ethylene; a high-performance OCC catalystb with high selectivity to light olefins; and a new process design. The new process is characterized by high yields of propylene and ethylene, good feedstock flexibility, high WHSV and low energy consumption. These features make the technology a promising strategy for propylene production. The first commercial OCC plant designed with the new technology is scheduled to start up in 2020. An explanation of the new processa follows.

New OCC technology structure and reaction

Typical olefins distribution in feed and product is shown in Fig. 1. Whereas the ratio of C4 and C5 olefins in the feed is arbitrary, the product distribution depicted is typical for the process. The typical total yield of propylene and ethylene is about 75 wt%, and the weight ratio of propylene and ethylene is usually 3 to 5, depending on the feedstock type and reaction conditions.

Fig. 1. Typical olefins distribution in feed and product.

Feedstocks. One important characteristic of the OCC process is its flexibility of feedstocks. The C4/C5 olefins from the MTO process, steam crackers, refineries or a mixture of the above are all compatible for OCC. Both high and low content of olefins can be processed easily, and the higher ones are naturally more favourable. Paraffin, cycloalkanes, cycloalkenes and aromatics are considered as non-convertibles.

An investigation of the effect of feedstock butadiene on the OCC reaction showed that butadiene will lower the conversion of C4 olefins and propylene yield. The feedstock should be controlled to contain less than 0.1 wt% of diolefins, as diolefins contribute to coke formation. If the diolefins content in the feedstock is greater than 0.1 wt%, then a selective hydrogenation unit is needed to pretreat the feedstock.

Full crystalline zeolite catalyst. An early OCC catalystc features the utilization of a full-crystalline ZSM-5 zeolite technique.4 Conventional ZSM-5 catalyst is composed of two separate phases—binder particles and zeolite crystals. Full-crystalline ZSM-5 zeolite catalyst was prepared via re-crystallization. After re-crystallization, binder particles were almost totally converted to zeolite, and only zeolite crystals remained. Re-crystallization was found to not only release the acid sites imprisoned by binders, but also to create hierarchical pores and relieve internal diffusion limitations in microspores.

By regulating the acid strength and amount of the full-crystalline ZSM-5 zeolite catalyst, the formation of heavy hydrocarbons and paraffin can be restrained. The new OCC catalystb exhibited high selectivity and activity. In optimized process conditions, the typical yield of propylene and ethylene is approximately 75 wt% (based on the olefins content of the feedstock). Other coproducts include gasoline and small amounts of C1–C4 paraffin. The catalyst can be operated at WHSV = 10 hr–1–20 hr–1, so the reactor size and operating costs are notably minimized. The coking tendency on the catalyst in the OCC reaction is low, and the catalyst cycle length is 2 d–7 d; therefore, a fixed-bed adiabatic reactor system can be used without continuous catalyst regeneration.

For catalyst regeneration, nitrogen is mixed with air and enters into the reactor, where coke deposits are burned on the catalyst. According to industrial experience, the service life of catalyst has been exceeded for a period of more than 1 yr.

Process and characteristics. A simplified process flow diagram of a unit using the new OCC processa is shown in Fig. 2. Olefins feedstock first enters into a selective hydrogenation unit (SHU), where diolefins in the feedstock are hydrogenated to monoolefins. The olefins feedstock is then vaporized and mixed with a recycle stream, after being further heated against the reactor effluent in a fired heater. The mixture then enters into the fixed-bed adiabatic reactor at a temperature of 500°C–600°C and a pressure below 0.2 MPa.

Fig. 2. A simplified flow diagram of a unit using the new OCC process.

A swing reactor system is used for catalyst regeneration. One reactor is for reaction, and the other is for catalyst regeneration. After cooling, the reactor effluent is compressed for further separation. Light olefins from the top of the depropanizer enter the existing separation system to obtain polymer-grade propylene and ethylene. The stream from the bottom of the depropanizer is introduced into the debutanizer.

Most of the C4 fraction is recycled to the OCC reactor to increase the total propylene and ethylene yield. Part of the C4 fraction is purged to avoid the accumulation of paraffins in the system. The C5+ fraction is cut for further processing as pygas. The configuration of the separation facilities depend on how the unit is integrated into the existing process system.

The technical characteristics of the new OCC process can be described as follows:

  • High yield of propylene and ethylene. The typical yield of propylene and ethylene is about 75 wt% (based on the olefins content of the feedstock), while the ratio of propylene and ethylene is 3 to 5.
  • High WHSV. The new OCC catalyst can be operated with high WHSV at 10 hr–1–20 hr–1. The reactor size can be reduced dramatically so that investment and operating costs are minimized.
  • Good economic efficiency and low energy consumption. Without any dilute in the feedstock and with the integration of a high-efficiency heat exchanger, the new OCC process can reduce plant energy consumption, the size of the reactors and the extent of the pipeline network.

OCC integration into MTO: Commercial case study

MTO is a process to convert methanol to ethylene and propylene at about 80% carbon selectivity in a fluidized bed reactor with continuous regeneration. The main coproducts include C4 and C5 hydrocarbons, which share approximately 15% of the mixture of hydrocarbon products.

When combined with the Sinopec OCC process to convert C4/C5 hydrocarbons to propylene and ethylene, the overall yield of ethylene and propylene in the MTO process can be increased by 5–7 percentage points, and C4+ coproducts can be effectively eliminated. The MTO plant product marketing is completely focused on ethylene and propylene or their derivatives, thereby maximizing the plant’s profitability.

The integration scheme is shown in Fig. 3. Olefins feed (mainly the C4/C5 stream) from the MTO separation unit is fed into the OCC unit. After being cooled and compressed, the effluent from the reaction section is introduced into the depropanizer. The stream from the top of the depropanizer is further processed by the separation of the ethylene plant to obtain polymer-grade propylene and ethylene.

Fig. 3. OCC integration with an MTO plant in the Erdos case study.

This integration may be the most economical solution for a newbuild MTO plant, where the additional capacity required to separate the effluent from the OCC unit can be reserved in the MTO plant. Therefore, no expensive, independent, deep-cooling separation system is needed for the OCC unit.

The results of OCC unit operation load in an MTO plant in Erdos, China is shown in Fig. 4. The OCC unit ran smoothly from October 2017 to September 2018. The operation load was approximately 17 t of fresh C4/C5 hydrocarbons for 1 hr.

Fig. 4. Results of OCC unit operation load in an MTO plant in Erdos, October 2017–September 2018.

New OCC unit under construction: Case study. A case study was applied to demonstrate the new OCC process unit in an MTO plant. An MTO plant with a processing capacity of 1,300-kilotons/yr of methanol is located in Tengzhou, Shangdong Province, China. The methanol feed is supplied from the market, so the methanol price and the yields of ethylene and propylene significantly influence the economics of the plant.

Approximately 90 kilotons/yr of low-value C4 and C5+ hydrocarbons would be produced in the MTO process alone. The new OCC process will be integrated with the MTO plant to convert C4 and C5+ hydrocarbons into propylene and ethylene. As aforementioned, when MTO is combined with the basic OCC process, the overall yield of ethylene and propylene in the MTO process can be increased by 5–7 percentage points. When MTO is combined with the new OCC process, the overall yield of ethylene and propylene can be significantly increased. The new OCC unit is scheduled for startup in 2020.

Integration into steam cracker or refinery. Both the basic and new OCC processes can be integrated with a steam cracker in the same integration scheme. The C4 stream from steam cracker separation (butadiene extraction or MTBE synthesis units) is fed into the OCC unit. After being cooled and compressed, the effluent from the reaction section is introduced into the depropanizer.

The stream from the top of the depropanizer is further processed by the separation of the ethylene plant to obtain polymer-grade propylene and ethylene. For this integration, the light olefins product (C1–C3) separation is done by the separation system of the ethylene plant; therefore, a full separation chain does not need to be built.

The OCC process in a refinery is different from the OCC process in an MTO plant or a steam cracker. Olefins content in the C4 fraction from a refinery is approximately 40 wt%–45 wt%, which is much lower than the C4 fraction from an MTO plant or a steam cracker. Isobutane content accounts for 40 wt%. Accordingly, a deisodebutanizer is required to separate isobutane from the C4 fraction. The olefins content in the bottom of the deisobutanizer can be improved to above 60 wt%. Secondly, propylene is the most desirable product in a refinery (over ethylene), so the fraction (C1–C2) from the deethanizer can be recycled to an OCC reactor to improve the propylene yield. At the same time, C2 coproducts can be effectively eliminated so that the product marketing can be completely focused on propylene.

Status of OCC technology

Three commercial OCC plants are in operation. The first OCC plant, which can process 60 kilotons/yr of feedstock, was started up in Puyang, Henan Province in 2009. A 200-kilotons/yr OCC plant in Erdos and a 100-kilotons/yr OCC plant in Huainan, Anhui Province began operating in 2016 and 2019, respectively. The newest OCC plant is under construction and will be commissioned in 2020.

The basic and new OCC technologies allow highly selective production of propylene and ethylene by conversion of low-value byproduct streams containing C4/C5 olefins from MTO, steam crackers and refineries. A tailor-made, full-crystalline ZSM-5 zeolite catalyst was utilized in the new process and exhibits high selectivity to propylene and ethylene.

The new OCC process features high yields of propylene and ethylene, good feedstock flexibility, high WHSV and low energy consumption. If the new OCC process is integrated with an MTO plant, then the overall yield of ethylene and propylene in the MTO process is dramatically increased, to great economic benefit. HP

NOTES

       a OCC Plus process
          b OCC 200 catalyst
          c OCC 100 catalyst

LITERATURE CITED

  1. IHS Markit, “Propylene,” Chemical Economics Handbook, November 2019, online: https://ihsmarkit.com/products/propylene-chemical-economics-handbook.html
  2. Teng, J. and Z. Xie, “OCC process for propylene production,” Hydrocarbon Asia, Vol. 16, 2006.
  3. Teng, J. and Z. Xie, “Novel binder-less hierarchical ZSM-5 catalyst for olefins catalytic cracking to produce propylene,” Science China: Chemistry, Vol. 45, Iss. 5, 2015.
  4. Jian, Z., J. Teng, R. Liping, et al. “Full-crystalline hierarchical monolithic ZSM-5 zeolites as superiorly active and long-lived practical catalysts in methanol-to-hydrocarbons reaction,” Journal of Catalysis, Vol. 340, 2016.

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