Zhe WANG*
Yasutaka FUKUYO*
Fucai LIN*
Akio MORITA*
*
Elliott Group (Ebara Elliott)
In recent years, natural gas chemistry, especially in the Chinese market, has been booming. As a result, many plans are being made to construct PDH plants. Elliott Group offered many centrifugal compressors, which were the core equipment of PDH plants, and holds the top market share. This paper introduces the types and characteristics of the PDH process, typical configuration examples and characteristics of centrifugal compressors for PDH plant, and our technical approach to meet demands.
Keywords: PDH, Propane dehydrogenation process, Centrifugal compressor, Product gas, Reactor effluent, Lummus, UOP, Large scale motor, Ethylene plant, Ethane cracking
Propylene is a basic raw material for many chemical products, such as polypropylene (PP), phenol, acetone, acrylic acid/acrylic ester, acrylic nitrile, octanol, propylene oxide, propylene glycol, and isopropyl alcohol. These basic chemicals are indispensable for the sustainable development of mankind, as they are used to make various products such as PET bottle caps, containers, and automobile parts, etc.
In Japanese petrochemical industry, ethylene and propylene are produced by naphtha cracking of petroleum. On the other hand, the recent revival of natural gas chemistry due to the shale revolution in the U.S. has caused a shift to ethane cracking, which uses inexpensive shale gas as the feedstock resulting in high competitiveness1).
In addition, many ethylene plants in the world uses large-scale ethane cracking because it has lower environmental impact than the naphtha cracking of petroleum. However, in ethane-based refining, the yield of propylene (a by-product) is low. To make up this, the propane dehydrogenation (PDH) process is used for producing propylene from propane (a component of natural gas), hence large-scale PDH plants have been constructed2).
The Propylene recovery rate for PDH plant is 74-82%, sometimes up to 80-90%, depending on the operating conditions and catalyst. The economic efficiency of PDH is attractive, hence it’s also gaining the worldwide attention3).
Particularly in China, there has been a rapid increase in the number of new facilities for propylene production using, coal to propylene (CTP) and methanol to propylene (MTP) in addition to PDH. Among them, PDH facilities remarkably outnumber the others. China's share of the propylene market worldwide is increasing, thanks to its overwhelming low construction cost. While the price of crude oil has fallen and the development of natural gas (including shale gas) has been progressing, the fall in the market prices of PDH products in China has remained modest, compared to the price of the feedstock. Thus, the profitability of PDH is not considered to be a problem, and PDH projects are proceeding rapidly4).
The following are the various PDH plant processes: 1) Oleflex process of Honeywell UOP, 2) Catofin process of McDermott Lummus, 3) Star process of Uhde (now a subsidiary of the ThyssenKrupp Group), and 4) Linde/BASF/Snamprogetti/Statoil joint-development process. In addition, 5) FCDh process of Dow Chemical and 6) K-PRO process of KBR are newly developed and commercialized PDH processes based on their existing fluid catalytic cracking (FCC) process technologies.
As for the plants currently in operation, under construction or planning in the world, the UOP process is leading in Thailand, Malaysia, Korea, Spain, Egypt, Saudi Arabia, UAE, and the United States. Followed by the Lummus process, which has proven itself in Belgium, Mexico, Saudi Arabia, and Kazakhstan. The Uhde process is available for the EPPC in Port Said, Egypt.
In China, the UOP process is leading with more than 10 license agreements, but the first commercial plant employed the Lummus process.2)
Supported by the recent boom in the market, Elliott Group's PDH compressors have been delivered not only to China, but also to the U.S., Europe, and other parts of the world. In particular, the Elliott Group has a nearly 60% share of the market for reactor effluent compressors (RECs) for the UOP process and product gas compressors (PGCs) for the Lummus process, which are centrifugal process gas compressors.
Figure 1 shows the delivery records for the RECs in UOP process and the PGCs in Lummus process, which are typical compressors used in PDH plants with the largest capacities. According to our delivery records, the number of Chinese customers has increased sharply since 2014, which is in line with the trends in the global PDH market.
Fig. 1 Delivery records of typical compressors used in PDH plants
Feedstocks used in PDH plants are mainly natural gases such as propane, which is much lighter than those used in ethylene plants.
Process gas compressors mainly used in PDH plants are RECs for the UOP process and PGCs for the Lummus process, which allow high flow rates. Compared to cracked gas compressors for ethylene plants with a similar configuration, the size of those for propylene plants with a production capacity of 600 KTA is equivalent to that of compressors for mega ethylene plants with a production capacity of 1500 KTA. To cope with high flow rates, axial flow compressors and centrifugal compressors are sometimes used for low-pressure casings and high-pressure casings, respectively. From the viewpoint of the better robustness of blade design and performance characteristics, centrifugal compressors are the mainstream. In recent years, in order to improve production efficiency, inlet pressure is often lower than atmospheric pressure, and that results in increasing of compressor size and number of impeller stages due to an increase in volume flow rate and head.
In addition, unlike ethylene plants PDH plants do not have a cracking furnace, resulting in less steam generated. Therefore, electric motors are often used to drive compressors, which is also a feature of PDH plants.
(1) Typical compressor train configuration
Typical compressor train configurations
*
of the UOP process and the Lummus process, which the Elliott Group has been often used, are shown below (Figure 2).
Fig. 2 Typical compressor train configuration
This figure shows compressor configurations for both processes for plants with a capacity of 600 KTA. The Lummus process is similar configuration of an ethylene plant. On the other hand, the UOP process has no refrigeration compressors.
Except for heat pump compressor in the Lummus process (which is a single stage overhung centrifugal compressors), all the others used in both (UOP & Lummus) processes are multi-stage centrifugal compressors type only.
(2) Large-scale electric motor drive
Based on our experience, only steam turbine drive was used in the beginning, but since 2014, motor drive has become more common. From the perspective of improving the overall efficiency of the entire plant, variable speed motors have been introduced to other oil and gas processes (oil refinery plants, LNG plants, and LNG pipelines), according to the needs of the times5).
In case of motor drive, a VSD or a soft starter is used to reduce the starting current. As an example, Figure 3 shows our five available patterns used in the UOP and Lummus processes.
Fig. 3 Motor drive patterns
M-1
Like steam turbine drive, the low-pressure and high-pressure casings are connected in tandem. About 40 MW Max.
M-2
When the rated output of the motor exceeds about 40 MW, the plant capacity is divided into two systems.
M-3
Instead of a VSD, a soft starter is used with a constant speed motor. The operation panel of the soft starter is shared with the heat pump compressor.
M-4
The low-pressure and high-pressure casings are not connected in tandem, instead driven individually. For RECs, the inverter capacities for low-pressure casings, high-pressure casings, and heat pumps must be the same so that the inverter design is the same.
M-5
A VSD motor is used for the low-pressure casing and the constant-speed motor is used for the high-pressure casing. The operation panel of the soft starter is shared with the heat pump compressor.
The above train arrangements are mainly selected according to the customer's plant utilities and request. In actual compressor operation, Air dry out case or nitrogen regeneration case will be different from the rated operating cases. Depending on the system resistance, the power consumption for such cases may exceed the rated case power.
When VSD motors or motor soft starters are used, it is necessary to avoid discrepancies in the basic plan caused by insufficient sharing of information between the customers, the rotary machine manufacturers, and the electrical manufacturers.
We conduct dynamic simulation analysis including processes.
It is necessary to recognize that the rated point of electrical equipment such as VSD motors is the upper limit of design. When VSD motors or soft starters are used, a redundant or a triple redundant system may be required to avoid risks.
*Compressor train refers to the state in which the driven machinery (compressor) and the driving machinery (steam turbine, motor, etc.) are connected.
We are implementing the following three initiatives for centrifugal compressors for PDH plants, using expertise accumulated through our achievements at mega ethylene plants, ethylene oxide and ethylene glycol plants, and LNG plants.
(1) Impeller with an ultra-high flow coefficient and high efficiency
As the size of centrifugal compressors increases, to reduce the initial investment in the plant, such compressors must become more compact and their operation faster to the extent possible. It is effective to use high flow coefficient impellers that are relatively small and compatible with high flow rates; they are often used in mega ethylene plants.
Originally, they were developed for mega ethylene plants. The use of diagonal flow impellers with a flow coefficient higher than ever before, the structure that combines three-dimensional free-form blades with intermediate blades and an optimized return channel, minimizes losses at high flow rates and achieves both ultra-high flow rates and high efficiency. By using these impellers and rotating them at higher speed, it will be possible to reduce the compressor casing size by one or two ranks while maintaining the current efficiency6). Figure 4 shows an impeller with ultra-high flow coefficient and high efficiency.
Fig.4 Impeller with ultra-high flow coefficient and high efficiency
(2) Improved rotor stability
In order to meet the high flow rates and high head requirements for PDH processes, the bearing span of rotors tends to become longer due to the large nozzle and more number of impeller stages. In order to ensure rotor stability, some centrifugal compressor trains provided to PDH plants were configured with two - low-pressure casings in a parallel arrangement. However, the number of casings can now be reduced by using only one low-pressure casing with a double-suction structure. As a result, the customer’s initial investment can be reduced.
Figure 5 shows a comparison of our train configurations for 600 KTA PDH plants.
Fig.5 Comparison of 2-casing and 3-casing train configurations
In order to improve rotor stability, the nozzle flow path was optimized using CFD analysis technology. This was also applied to mega ethylene plants and the axial length of the nozzle of double-suction compressors is drastically shortened for the stability of the long rotor. Thereby while maintaining the flow passage area, it was possible to combine low-pressure compressors into one double-suction. When the number of casings connected is reduced, the machine installation area is also reduced.
Figure 6 shows compressor suction nozzles used in a mega ethylene plant.
Fig.6 Compressor suction nozzles
(3) High speed balancing
In order to ensure the rotor quality, including the dynamic balance of the entire rotor, high-speed balancing is performed on all rotors of machines whose first critical speeds are below the operating speed. After rotor assembly is completed, high-speed balancing is performed, and the values are corrected under vibration in an operating range exceeding the critical speed. This minimizes the risk of machine vibration in the actual plant operating environment. In addition, by eliminating the risk of vibration in the factory operation test before shipment, the delivery date can be strictly honored and delays in the plant construction schedule due to this is avoided.
In the petrochemical industry, the concept of SDGs has become widespread, and there is a greater need to reduce environmental impact than ever before. It is also expected that more and more large, highly efficient PDH plants will be required for mass concentrated production. We are going to continue to develop new products to meet the market requirements, applying our expertise accumulated at mega ethylene plants.
We also need to focus on the increasing size of driving machines, as well as compressors. The capacity of driving machines is currently about to exceed 60 MW. Since PDH plants use more electric motors than ethylene plants, large-scale motors, especially inverters with large-capacity and redundancy, will be increasingly needed to improve reliability. Thus, engineers need to be aware of the latest technological trends in electrical equipment and to understand both electrical and mechanical engineering7).
We introduced compressors used in PDH plants, which have been rapidly increasing in recent years, mainly in China, during the current boom in natural gas chemistry.
Currently, the global spread of COVID-19, the accompanying impact of the low price of crude oil, and the unstable economic and political situation due to the trade war between the U.S. and China, indicate an uncertain future. The sustainable development of mankind, however, is expected to require the continued construction of PDH plants.
We hope this paper will be helpful for young design engineers and users.
*Compressor train refers to the state in which the driven machinery (compressor) and the driving machinery (steam turbine, motor, etc.) are connected.
1) Asahi Research Center, April 2014, From “Petrochemistry” to “Natural Resource Chemistry”
2) Asahi Research Center, April 2017, Natural Gas Chemistry, Petrochemistry, Coal Chemistry
3) JPEC report, February 25, 2014, Propane Dehydrogenation Projects in China to Increase Propylene Production
4) Ministry of Economy, Trade and Industry, issued in March 2018, Future Trends in Global Demand for Petrochemical Products
5) 149th Turbomachinery seminar, July 3, 2018, Problems in Motor-driven turbomachinery, including variable speed
6) Turbomachinery, issued in July 2019, Introduction of Design Approach to Large frame, High speed, and Other Special Applications
7) Turbomachinery, issued in March 2009, Trends in VSD (Variable Speed) Motors for Large Rotary Machines and Problems of Electrical-Mechanical Coupled Torque Fluctuation
This paper is reprinted from the July/August 2020 double issue of Sangyo Kikai (Industrial Machinery).