Oil-injected screw gas compressor for hydrogen service

By Takao Koga, General Manager of Sales

Currently, the demand of Hydrogen is increasing dramatically all over the world. Hydrogen traditionally has been used for the cracking process in Oil Refineries and for material surface treatment, but now it is also needed for desulfurization in Oil Refining Processes, and also as fuel. Also, at this time, the demand for Hydrogen is strongly led by environmental requirements.

For the above processes, Hydrogen needs to be compressed for chemical reaction and transportation. Hydrogen is a gas with unique features, such as (1) Low molecular weight, (2) High k value and (3) hydrogen embrittlement. So, generally speaking, it is difficult gas to compress, especially for Centrifugals due to the difficulty to compress such a light gas and for Oil-Free Screw Compressors due to gas return in the compressor from discharge to suction through clearance of rotors. That is why Reciprocating Compressors have been used for Hydrogen services typically in case of high discharge pressure required in Oil Refinery Processes. Because the Reciprocating Compressor is a positive displacement type and it is not affected by gas composition and has the simplest compression mechanism. It also can achieve the high pressure level (200 barg or 3000 psig) required by some of Oil Refinery Processes. However, the main problem with Reciprocating Compressor is frequently required maintenance during the operating period that increases the running costs (including man-power and replacement parts) and the limited capacity in handling high gas volume. Therefore, given the case that large volumes of Hydrogen are required due to the current trend of larger capacity industrial plants, Centrifugal Compressors are sometimes used when the compression ratio is low.

On the other hand, for last 10 to 20 years, a 3^rd option has been gradually showing up in the market. It is another positive displacement type, the Oil-Injected Screw Compressor.

Originally, the Oil-injected Screw Compressor was developed for light duty services such as Air and Refrigeration, which are not heavy duty services and do not require API standards. The Oil-injected Screw Compressor has established a good reputation of high reliability and less maintenance = long term continuous operation in the markets of Air and Refrigeration. Then, based on such a good reputation, the Oil-injected Screw Compressor has also been used for process gas services since the 1970's. Some of the services were Coke Oven Gas (Hydrogen rich gas), Helium (Low molecular weight gas), Natural Gas, Hydrocarbon Gas, Carbon Monoxide and Carbon Dioxide. In addition, it was used for Hydrogen service as well. For example, they are also being used for low discharge pressure (approx. 10 barg or 150 psig) Hydrogen service in Tail Gas compression of the Pressure Swing Adsorption (PSA) Process.

Actually, the mechanical features of Oil-injected Screw Compressor are, by nature, suitable for the compression of Hydrogen, because it is a positive displacement type, the same as Reciprocating Compressors. Also, it has Lube Oil Injection to the compressor itself, which acts as a sealant at the clearance between Rotor and Casing, in addition to lubricant and coolant, which means higher efficiency in case of compression of low molecular weight gas. (Figure 1.)

Figure 1. Oil-injected Screw Compressor Cutaway

Even though the Oil-injected Screw Compressor has established a good reputation and also has suitable mechanical features for Hydrogen service, it was still not often used, because it was designed for light duty service (Non-API standard machine) and had a pressure limit of approx. 20 barg or 300 psig.  

As time passed, thanks to the well-established reputation of high reliability and less maintenance = long term continuous operation in the markets of Air and Refrigeration, Heavy Duty and High Pressure Oil-injected Screw Compressor have been utilized to replace Reciprocating Compressor in applicable services.

In the late 1990's, the High Pressure Oil-injected Screw Compressor was developed and started to penetrate the market. At that time, the maximum pressure range was approx. 60 barg or 900 psig.  After successful operation in the various process gas applications at such pressure ranges, advanced High Pressure Oil-injected Screw Compressors were developed in the early 2000's based on the market's requirements.

Now, Oil-injected Screw Compressor can achieve up to 100 barg or 1500 psig. Since the Reciprocating Compressor is used for high pressure (up to 200 barg or 3000 psig) in Oil Refinery Processes, Oil-injected Screw Compressor still cannot cover all pressure ranges of the Reciprocating Compressor. However, the Oil-injected Screw Compressor can cover more than 50% applications in Oil Refinery Processes, since the required gas pressure is trending lower.

Here are unique examples of current Oil-Injected Screw Compressor applications:

1.     Hydrogen (94%) Recycle Gas for Gasoline Desulfurization Unit (GDU) Process (Figure 2.)

2.     Hydrogen (94%) Booster for Continuous Catalytic Reformer (CCR) Process (Figure 3.)

3.     Hydrogen (90%) Recycle Gas for Heavy Catalytic Cracked Gasoline (HCCG) Process (Figure 4.)

4.     Hydrogen (99.9%) Booster for Steam Methane Reformer (SMR) Process (Figure 5.)

5.     Hydrogen (99.5%) Booster for Dissociation Process (Figure 6.)

6.     Hydrogen (91%) Booster for Linear Alkyl Benzene (LAB) Process (Figure 7.)

7.     Hydrogen (99.9%) Fuel Booster for Gas Turbine Power Plant (Figure 8.)

8.     Hydrogen (50%) Tail Gas for Pressure Swing Adsorption (PSA) Process (Figure 9.)

Since the worldwide demand of Hydrogen is increasing and reliable plant operation is most important for all heavy industries such as Oil Refinery, Petrochemical, Chemical, Steel and Power, Oil-injected Screw Compressors will be required more and more for up-coming new projects and also for replacement projects of old Reciprocating Compressors installed 20 to 30 years ago.

For discharge pressures of approx. 60 barg or 900 psig, Oil-injected Screw Compressors are already recognized as the standard compressor in the market instead of the traditional Reciprocating Compressor. From now, it will be a standard for more applications up to 100 barg or 1500 psig in various industries and fields with great benefits for the market and users.