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If you're tasked with conducting a study on selecting a process for an LPG recovery unit, and it's your first experience with such a study, this section will provide valuable insights. To start, it's essential to solidify your fundamentals. A strong grasp of the basics will be crucial in navigating the complexities of LPG recovery.

One of the first steps involves familiarizing yourself with simulation software commonly used in the industry, such as Aspen HYSYS or Chemcad NXT. These tools are integral to process evaluation and selection. Although I haven't personally used Chemcad NXT, it's known to be a highly capable tool for simulating a variety of process scenarios.

In the following sections, we'll explore the major LPG recovery processes. We'll examine the key features, benefits, and limitations of each to guide you in choosing the most suitable method for your specific requirements.

Natural gas and refinery gases contain valuable hydrocarbon liquids that can be recovered through various processes. Let's explores some of the most effective methods for LPG (Liquefied Petroleum Gas) recovery from these gas streams, focusing on commercially viable options such as C3 and C4 recovery.

Overview of LPG Recovery Processes

There are multiple commercially viable options for LPG recovery from hydrocarbon gases like natural gas and refinery gas. These processes can produce on-specification products including propane, LPG, sales gas, and NGL (Natural Gas Liquids). The main methods include:

1. Cooling-based processes:

   - Mechanical Refrigeration Unit (MRU)

   - Joule-Thomson (JT) expansion

   - Turbo-Expander

2. Absorption process:

   - Lean Oil Absorption

3. Membrane processes

Cooling-Based Processes

Mechanical Refrigeration Unit (MRU)

The MRU uses mechanical compressors and heat exchangers to cool the natural gas stream and separate hydrocarbon liquids. This method is applicable to medium or low pressure streams and is typically used for treating medium pressure associated gas streams rich in LPG.

Key points:

- Uses propane as the refrigerant

- Achieves temperatures between -10°C to -25°C

- Simple and direct process for LPG/NGL recovery

Joule-Thomson (JT) Expansion

 The Joule–Thomson expansion is an isenthalpic process, meaning it occurs at constant enthalpy, typically through a valve or throttling device. 

 During this process, the gas expands as its pressure is reduced, chilling the gas stream without any change in enthalpy or heat content. The Joule–Thomson coefficient, which measures the temperature change in response to pressure change during such expansion, becomes zero at a specific point called the inversion point (T = 1/β) for real gases. 

 For most real gases, expansion leads to cooling when the Joule–Thomson coefficient is positive, provided the gas temperature is below its inversion temperature. However, hydrogen behaves differently due to its low inversion temperature of 202K, causing it to warm rather than cool during a Joule–Thomson expansion at room temperature. In contrast, ideal gases exhibit no temperature change during expansion, making the Joule–Thomson coefficient relevant only for real gases, as it indicates whether a real gas will cool or warm based on its type and expansion conditions. 

Typical temperature drop achievable in a J-T process is 20 to 50 F.

This refrigeration method can be used individually or in combination with other methods such as MRU and Turbo-Expander.

Key points:

- Normally Applicable to high pressure gas streams

- Temperature reduction depends on initial pressure, final pressure, starting temperature, and gas composition

- Thermodynamically less efficient than other methods

- Easy to use / lower investment and OPEX

Turbo-Expander Process

The Turbo-Expander process uses an isentropic gas expansion and heat recovery.

A cryogenic turboexpander is employed to treat gas streams for enhanced liquid recovery, providing an efficient method for LPG extraction. This process achieves very low temperatures, allowing substantial liquefaction of ethane and heavier compounds in natural gas. Distillation is then used to recover the various fractions of the liquid stream.

Unlike a simple valve that only reduces pressure, the turboexpander also extracts work from gas expansion using a turbine, further lowering the gas temperature and improving the refrigeration cycle’s efficiency. The turbine’s kinetic energy (work) is absorbed by a “loading” element, such as a dyno (oil brake), electric generator, or centrifugal compressor stage, which is mechanically coupled via a spindle or shaft. In configurations using a compressor, the turboexpander not only serves as a pressure booster but also utilizes energy that would otherwise be lost with a Joule-Thomson (J-T) valve, potentially replacing the need for a separate electric or engine-driven compressor.

Typically, temperature reduction achievable in a Turbo-Expander is approximately 50 to 100 F 

Key points:

- Applicable to a wider range of pressures and gas compositions

- More efficient than JT expansion for cooling effect

- Can generate low temperature gas efficiently

- Generates shaft work which is often used for gas re-compression or electricity generation

Hybrid Configuration

Some processes utilize the combined effect of MRU and JT or Turbo Expander for enhanced efficiency:

- MRU cools the natural gas stream before entering the Turbo-Expander for further cooling.

- The Turbo-Expander shaft work is used generally for gas re-compression or sometimes power generation.

Impact of Feed Gas Composition on Process Selection

The composition of the feed gas significantly impacts process selection and performance:

1. Cooling during expansion:

   - Increasing molecular weight increases cooling effect

   - Decreasing critical temperature also increases cooling effect

2. Separation of components:

   - Separation becomes more challenging in lean gas streams due to concentration polarization and lower partial pressures .

Conclusion

The choice of LPG recovery process depends on various factors including 

-feed gas composition

-desired recovery efficiency

-operational requirements

-cost considerations. 

Cooling-based processes like MRU, JT expansion, and Turbo-Expander offer different advantages in terms of efficiency, cost, and applicability to various gas stream compositions. Hybrid configurations combining multiple methods can often achieve optimal results.

As the energy industry continues to evolve, understanding these complex processes becomes increasingly important for efficient resource utilization and environmental sustainability. Future research may focus on further optimizing these processes for even higher efficiency and lower costs.