Paramine Writeup

Introduction

The APV Paramine solution is an answer to the sealing difficulties experienced by sour gas processors. Amines including Sulfinols are employed to carry high concentrations of hydrogen sulfide and carbon dioxide in the rich stream which recovers heat from the hotter lean stream.

The Challenge

Our customers naturally employ high efficiency plate heat exchangers because of their natural advantages over shell and tubes

• Ability to achieve a high heat recovery thus minimising the running costs of these very large plants
• A far lower capital cost,
• A much smaller space requirement,
• Flexibility not available with alternatives, etc.

However, some of these customers have had frustrations with their plate heat exchangers – unavoidable until now. Here are the issues plate heat exchanger customers in the gas-processing sector have had to battle with.

1. Whereas nitrile and EPDM compound seals in plate heat exchangers will normally run at upto 150°C to 160°C in many environments, this is not so in amine service. The temperature limit for a satisfactory gasket life can be 30 to 50 degC lower.
2. The high hydrogen sulfide and carbon dioxide concentrations have an adverse effect on gasket life to the point that life is often reduced to 6 months.
3. The short life relates to an inexpensive gasket. However, our customers cannot afford downtime nor can they tolerate half-yearly or yearly plate cleaning and regasketing costs and delays, which mean that savings in capital cost are frittered away in 6 to 12 months of operation.
4. Premature failure in a sour gas environment poses a safety risk.
5. A separate but very common issue is selection of the right elastomer. Depending on the gas processing site, nitrile, hydrogenated nitrile or EPDM may be suitable. It is also often a matter of conjecture whether regasketing will be necessary in half year or five.
   

The Initial Investigation

The diagonal bridge gasket in contact with hot rich amine is in fact maintained close to the temperature of the lean stream via conduction. In other words, it runs much hotter than the rich stream bulk temperature. The bridge gasket was almost invariably the area of failure. However, examination of the failed rubbers did indicate lesser but significant degradation of other areas as shown.

It was decided to test various established and development rubber compounds and some of the results are presented below for laboratory testing in 135°C rich Sulfinol for a fixed period.

The EPDM labeled A was the control gasket compound exhibiting field failures.

We could make some basic conclusions from the numbers above as well as inspection of the samples:

The basic nitrile was quickly hardening where a seal could not be maintained. The hydrogenated nitrile suffered very high volumetric and mass gains and other test parameters not mentioned above ruled it out. Except for softening, clearly the Paramine compound performed best.

Other possibilities had to be considered.

a) What would happen if the gasket volume was adjusted to partially account for expected swelling
b) What would happen if the key gasket areas were lined with Paramine?
c) Will there be advantages of detail in switching from the existing SR14 model to the B model?
d) Will there be an advantage in using BL welded pairs?

These questions would be answered in a comparative field trial stage.

Comparison of Various Solutions

The results after 3 months are summarised below:

The Final Solution

APV agreed that BL welded pairs with complete Paramine gaskets will provide the 5 year plus life required for these exchangers. These alterations made to the design are proprietary and made specifically to suit this service. Up to July 2004, the first installation has completed 70 months of service without incident. 4 units run on MDEA and 4 on Sulfinol.

Qualitative Comparisons with Alternatives

Here are some pros and cons with alternative technologies:

Conclusion

Bearing in mind its flexibility, heat transfer performance, ability to achieve close temperature approaches, resistance to thermal gradient stresses and long term field testing in amine service, the APV Paramine System is the system of choice for large sour gas amine interchanges. It also follows that other amine service devoid of sour gas will also benefit greatly if the APV Paramine System is employed.