A cool recovery

Tobias Häggblom, Frans Launonen, and Lauri Bastman, Vahterus, Finland, explore the opportunities LNG regasification presents to improve energy efficiency through recovering the energy available in cold LNG.

LNG is not a substance that occurs naturally on Earth. When natural gas is liquefied at temperatures of approximately -162˚C, its mixture of simple hydrocarbons of primarily methane mixed with small quantities of ethane, propane, and butane have a boiling point well below any temperature found naturally on the Earth’s surface.

LNG represents a storage of energy, both in the sense that it is a hydrocarbon fuel that can be burned, but also in that it is an extremely cold liquid that has been artificially created at the expense of energy. During regasification, this energy is released. Sadly, it is typically entirely wasted: primary heat sources such as ambient air or seawater are often used, and after regasification are simply dumped back into the environment at a cooler temperature.

However, this energy can be tapped for beneficial purposes, to improve the total LNG cycle energy efficiency and reach energy savings at locations physically removed from the LNG liquefaction itself. On a large scale, it is a refrigeration process with a virtual pipeline, with compressors located at the liquefaction plant and the consumer located at the regasification location. This represents an additional financial benefit to the LNG purchaser: reducing operating costs for any processes requiring media cooling.

The production of LNG from natural gas requires substantial amounts of energy in a compressor-based refrigeration cycle. For example, it has been calculated that for pure methane and a process pressure of 55 bar, the compression work alone consumes approximately 800 kJ per kg of liquefied methane product. In real processes, the energy expenditure is even higher, and in assorted studies energy consumption in the range of 1000 - 1500 kJ per kg of LNG appears usual. Compared to the lower heating value of LNG, the energy required by liquefaction represents approximately 3% of the energy contained in the LNG itself. With cold recovery, the overall efficiency of this process can be substantially increased, since around half of the energy can be reused.

In a typical case, with logistics and regasification included, up to 25% of the energy content of LNG is consumed to deliver the gas to the end consumer. Much of this energy is wasted as ambient heat during the liquefaction process and transportation, but there is significant potential for energy recovery from the ‘cold energy’ stored in the LNG.

Practical applications of cold recovery

One of the most inexpensive and least complex ways of utilising the cold energy is to use it in boil-off gas (BOG) handling. When LNG is stored, the heat from the surroundings will make part of the LNG vaporise, since there is no such thing as a perfectly insulated tank. The sub-cooled LNG in the tank

can be used to recondense the natural gas. Vahterus Plate & Shell Heat Exchanger, with its ability to handle higher pressures, is considered an ideal choice for this application.

Another simple application for cold recovery from LNG is to use it as part of the cooling system for whatever engine or turbine is consuming the natural gas, using the cooling water from one part of the process as a heat source for regasifying the LNG, via an intermediate glycol circuit to avoid freezing issues. With an artificial source of cold in near proximity, less cooling capacity from the surrounding environment is required.

A more utility-focused application is to use an intermediate glycol or refrigerant loop to assist in HVAC and freezer or cold storage solutions. In these cases, the intermediate loops tend to

Figure 1. LNG is not a substance that occurs naturally on Earth. When natural gas is liquefied at approximately -162˚C, its mixture of simple hydrocarbons of methane mixed with small quantities of ethane, propane, and butane have a boiling point below any temperature found naturally on the Earth’s surface.

Figure 2.Vahterus R&D has undertaken testing with liquid nitrogen vaporisation at -180˚C to study the behaviour and effects of sub-zero temperature glycol in a heat exchanger for cold recovery purposes. operate at approximately -30˚C for cold storage, and as warm as 0˚C for HVAC systems, in order to be cold enough to assist the traditional electricity-driven systems. On the hot end of the loop, most heat exchangers can usually cope with the requirements, but the LNG vaporiser has more strict requirements that cannot be fulfilled by semi-welded or gasketed heat exchangers. In addition, depending on the facility, LNG consumption may be extremely variable, for example onboard cruise vessels. Marine engines can operate on rapidly cycling loads that present their own challenges. Vahterus Plate & Shell offers a fully welded construction with round plates that dissipate stress evenly instead of concentrating it into sharp corners, giving it superior longevity compared to many other technologies. In Vahterus’ experience, the payback time for cold recovery implementation is better for cold storage solutions because the energy demand is more constant, whereas with HVAC systems it is both smaller and more fluctuating, and dependent on ambient temperatures.

Another energy consuming process where LNG cold recovery can be used is air separation. This is an energy-intense process using approximately 0.5 kWh to produce 1 kg of product. To be able to liquefy air, it must be cooled down to at least -140˚C. In practice, this requires a mixed refrigerant and multi-stage compressor set-up. An intermediate cold recovery loop operating at -130˚C or below can provide meaningful energy savings for the process. Of course, this places a fair amount of stress on the heat exchanger, which has to be able to cope not only with cryogenic conditions but also with close approach temperatures. It is common for the intermediate circuit to cool down to temperatures below the regasified LNG outlet temperature, i.e. to have a temperature cross-over between the ‘hot’ and ‘cold’ media. With a true countercurrent flow possible over a welded plate, Vahterus heat exchangers can provide an ideal solution for handling both thermal performance and mechanical challenges offered by this application.

Unlike many other cold recovery applications, here, due to the operating temperatures, the use of mixed refrigerants is more beneficial than traditional liquid heat transfer fluids. The refrigerants can easily be used with an adapted Vahterus thermosiphon system, which is a closed loop between a refrigerant boiler and condenser driven entirely by gravity and variations in density between gas and liquid. This allows for the safe and easy use of an intermediate circuit for heat transfer, without consumption of electricity or other utilities.

Challenges to overcome

There are many options available when choosing the intermediate fluid for cold recovery. Unfortunately, there are no perfect solutions that avoid freezing, are non-toxic, and are non-flammable. Many of the refrigerants used previously have a high global warming potential (GWP). A water glycol mixture is the most commonly used fluid in cold storage. However, the freezing temperature of a 50/50 mixture of ethylene glycol and water is approximately -40˚C. This makes it challenging, since the sub-cooled LNG entering the vaporiser can be as cold as -160˚C, and the operating temperature of the glycol loop itself has to be approximately -30˚C. The heat exchange surface temperatures will be well below the freezing point of glycol, and there is little margin in the glycol temperature, so implementing a design that does not freeze easily is paramount.

Vahterus R&D has undertaken extensive testing with liquid nitrogen vaporisation at -180˚C to study the behaviour and effects of sub-zero temperature glycol in a heat exchanger for cold recovery purposes. One of the interesting points is that while the freezing temperature (when ice crystals begin to form) of 50% volumetric mix of ethylene glycol is -37˚C, the solidification temperature is even colder, at -51˚C. In the in-between state, the glycol exists as a sort of high viscosity slurry that can still be pumped, although poorly in comparison to liquid glycol.

The benefits of a heat transfer plate compared to a shell and tube design is evident in the shear stress equation:

Simplified, shear stress is the factor of pressure drop over the channel length, and with the higher pressure drop of a plate heat exchanger in comparison to a much longer tube, the shear stresses found in a plate heat exchanger are also much higher. This high shear stress acts as an anti-fouling mechanism, of which ice formation on the heat exchange surface can be considered a part. Finally, it must be recognised that while cold recovery has great potential to increase overall system efficiencies and act to reduce OPEX, it can create more complex systems and require more investments early on in terms of equipment and system design, increasing CAPEX. For this reason, it is important to consider projects holistically, and account for both investment and operating costs at the same time. In general, the payback time for cold recovery implementation tends to be relatively short, but is, of course, always dependent on project specifics.

Conclusions

Cold recovery from LNG regasification can recover substantial amounts of energy, exceeding 800 kJ per kg of LNG. Typically, this energy recovery potential is completely wasted. Various applications in cold storage, cargo BOG handling, air separation, and many more, exist where this potential could be taken into use with simple cold recovery solutions. Intermediate fluid selection between glycol, mixed refrigerants, and other alternatives must be considered on a per-project basis. With over 30 000 heat exchangers delivered for general refrigerant applications, Vahterus has extensive experience in cryogenics and LNG, and combined with its R&D investigation into cold recovery, the company has a comprehensive understanding of the benefits and challenges faced by heat exchangers in implementing these systems. The structural design of a Vahterus Plate & Shell unit with its round welded plate pack inside a pressure-bearing shell ensures a durable construction fit for cryogenics applications. Its compact size and small footprint also make it a good solution for retrofit projects.

While implementation of cold recovery does lead to more complex design requirements and additional investment costs, the reduced operating costs more than offset these and typical payback times tend to be short. With rising energy costs, global warming, and positive financial incentives from reduced operating costs, Vahterus believes cold recovery to be a valuable addition to LNG regasification processes and encourages regasification system owners to consider implementing it in their applications.