Heat exchanger type selection in Amine Systems

For many contractors and end-users, Plate & Frame Heat Exchangers (P&FHE) became the preferred technology for the Lean-Rich Amine Exchanger and Lean Amine Cooler (when not air cooled). P&FHE have a much smaller plotspace than Shell & Tube Heat Exchangers (S&THE), can handle temperature crosses much better and are generally a lot lower in CAPEX than S&THE. Lean amine temperature is generally in the range 125 – 135°C which should be a good fit for the elastomeric gaskets in P&FHEs and generally the pressures are mild as well. P&FHE are suitable for design pressures up to 15 Barg, depending on model and size. 

For P&FHE international standards have been developed:

• API 662
• ISO 15547-1

These international standards aid the selection of this technology for Oil & Gas, Refinery and Petrochemical projects. Standards contain guidelines about minimum shear stresses to minimize fouling, ratio of port pressure drop vs plate pressure drop to ensure proper fluid distribution etc.

Because of all the above factors and competitiveness in engineering and constructing amine systems, P&FHE have been selected widely and suppliers show long reference lists. However, there are downsides of the use of P&FHE in Amine Systems as well.

Leakages in Plate & Frame Heat Exchangers

Many end-users who have P&FHE installed in their Amine System have experienced leakages.

Causes of leakages are:

• Gasket blow-out due to pressure spike in the system
• Plate deformation due to improper maintenance
• Wrong application of glue to attach gaskets to plate
• Chemical attack of gaskets by hydrocarbons

For the elastomeric gaskets between the plates different materials exist. For compatibility with the amine solvent and for the working temperature mostly an EPDM grade is selected. However, EPDM is less resistant against hydrocarbons, which often occurs in traces in Amine Systems. Suppliers have further developed their EPDM grades to better handle hydrocarbons, significantly increasing gasket lifetime.

The other above-mentioned causes of leakages can theoretically be overcome by correct operational conditions and maintenance by the right specialists (preferably the OEM or its local representative). Practice has shown that in the operational reality this is not so easy to manage and some draw the conclusion that P&FHE simply does not offer the robustness which is required in this application.

One could still argue about the probability of leakages in P&FHE. However, risk is defined not only by probability but also by consequence. When a lean-rich amine exchanger is leaking and has to be taken out of service, it means reduced production capacity and thereby loss of revenue which can easily be big numbers when speaking of an LNG facility for instance. Spilling of amine solvent also has environmental consequences and must be avoided as much as possible. In cases where the rich amine contains high concentrations of H2S, any leakages, even small, can have severe safety consequences. In such cases P&FHE should not be selected at all.

The alternative many choose for today is Welded Plate-Block Heat Exchanger.

Welded Plate-Block Heat Exchangers

Welded Plate-Block Heat Exchangers (WPBHE) consists of a frame of four girder corners, upper and lower heads, four side-panels with four terminal fluid stream nozzle connections. A hard gasket (graphite or PTFE), similar to a flange gasket, is applied between each of the panels and the respective girders to seal the unit from atmosphere. These components are bolted together and can be assembled and disassembled for cleaning, maintenance or service. The heat transfer surface area, referred to as the plate pack, consists of a stack of welded corrugated plates without use of any gaskets between the plates. Plates generally have a thickness of 0.8-1.2mm and can be pressed in a wide variety of metals, like AISI 316L, 254SMO, Titanium and Hastelloy grades. Distance between the plates is ~5mm.

Specifications of WPBHE (depending on supplier and model):

• Design temp min/max: -100/400°C
• Design pressure: Vacuum / 42 barg
• Max heat transfer area: 845 m2
• Max liq. flow per unit: 4,000 m3/h
• Lowest temp difference : 3°C 

WPBHE is still much more compact and energy efficient than a S&THE. And of course also the CAPEX will be lower than for S&THE. Moreover, WPBHE combines the robustness of a fully welded concept with the advantage of the ability to open it for cleaning & inspection reasons. The technology is being applied widely in industry. There exist for instance hundreds of references of WPBHE being used in the crude preheat train of oil refineries. These are exactly the reasons why this technology is finding its way into Amine Systems as well. API is developing a standard for WPBHE, which will be the API 810.

As rule of thumb, the bare unit cost price of WPBHE is roughly a factor 3-4 higher than for P&FHE. For installed cost, generally estimators take a factor ~4. When assessing the incremental differences between these 2 technologies it is not justified to apply the same installation factors on both technologies individually. It can be assumed that the absolute installation cost is comparable. And that means that installed cost price difference is factor ~2.

Considering all advantages of WPBHE, reliability and safety, it is advisable to make WPBHE the preferred technology for Amine Systems always.

read original Pulse by Roy Niekerk   Manager Engineering  at SULPHURNEThttps://nl.linkedin.com/in/royniekerk