UTILIZING AVAILABLE "COLDNESS" FROM LIQUEFIED NATURAL GAS
(LNG) REGASIFICATION PROCESS FOR SEAWATER DESALINATION
Author:
Tomer Efrat
Presenter: Tomer Efrat
Team Leader, Thermal Process Dept., IDE Technologies, Israel
Abstract
With the increasing use of natural gas, Liquefied Natural Gas (LNG) is becoming more extensively used
to ease the storage and transport of gas. The process of returning the natural gas to its gaseous state
(regasification) prior to distribution to the gas pipeline absorbs large quantities of heat and therefore
provides a readily available and inexpensive source of "coldness" at low temperatures.
This available "coldness" source can constitute a challenging opportunity to return to the technology of
desalination by freezing, which was developed by IDE Technologies back in the 1960s, or to modify the
commonly used multi-effect distillation (MED) process for operation at low temperatures. The vast
experience accumulated by IDE over the last decades, in both MED and vacuum freezing vapor
compression (VFVC) processes, will now serve as an effective tool for evaluating the feasibility of
desalinating seawater by utilizing the "coldness" resulted from the LNG regasification process.
In this article a case study was considered, in which an LNG regasification plant can supply 1750 ton/hr
of water-glycol solution at -15°C to be utilized for seawater desalination. The main challenge in this
evaluation is being able to provide a desalination solution able to compete with the commonly used RO
plants.
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I.
THE TECHNICAL APPROACH
In general, there are two approaches to be considered for utilizing the available "coldness" for seawater
desalination:
•
The first approach involves using the "coldness" as a heat sink for the evaporation-condensation
process, in which seawater is evaporated at low temperature under vacuum conditions and then
condensed using the cold glycol solution.
•
The second approach is to freeze the seawater using the glycol solution, forming ice slurry, and
then separate the ice from the brine, wash the residual brine and melt the ice.
Each approach has its advantages and disadvantages: while the evaporation-condensation process is a
simple and straight-forward solution, it consumes much more thermal energy compared to the freezing
process and yields a lower gained output ratio (GOR). The reason for this is the difference in the latent
heat of evaporation and freezing: seawater evaporation requires 7.5 times more thermal energy than
freezing does. Therefore, although the freezing process is more complex than the evaporationcondensation process, it yields a higher GOR. To reduce the specific thermal energy consumption of the
evaporation-condensation approach, several stages need to be included in the process, effectively
migrating from SED (single-effect distillation) to MED.
The evaporation-condensation approach can provide much better water quality, since it basically is a
distillation process. The freezing process can desalinate seawater to drinking water quality, as was done
by IDE back in the early 1960s in a Vacuum Freezing Vapor Compression (VFVC) desalination plant in
the city of Eilat in southern Israel.
Figure 1: IDE VFVC Desalination Plant in Eilat, Israel (Early 1960's)
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1.1
The Freeze Desalination Approach
The considered process of desalination by freezing is based on using the glycol solution from the LNG
regasification plant to freeze pre-cooled seawater. To be able to efficiently separate ice from brine and
wash it to achieve the required water quality, the ice should be generated in ice slurry form. The ice
slurry is pumpable and has already been used successfully in seawater desalination by IDE. The main
difference between the proposed process and the process practiced in the 1960s is the basic ice
production principle. Thus, while IDE's original freeze desalination process was based on vacuum
freezing (heat is pumped away and the water freezes in direct contact, under triple point conditions), in
the proposed process freezing is achieved by using cold media that cannot be used to freeze the water in
direct contact. This process requires using a heat transfer surface to exchange heat between the glycol
solution and seawater.
To avoid ice formation in big chunks and limit the complexity and energy consumption required to
dislodge the ice from the heat transfer surface – as done in common freezing machines – a dynamic
slurry ice generator is considered. The principle behind the slurry generator is the water being precooled by a secondary refrigerant or cold media, then slowly starting to freeze to small icicles while
flowing inside a tube – forming ice slurry.
The ice slurry generator design is available on the market for several applications, such as thermal
energy storage (TES), fish industry and other applications that require "dynamic" ice, but for smaller
capacities. One of the challenges will be to scale up the current design of the ice slurry generator to
provide the required capacities. Utilizing water-glycol solution from -15°C up to -3C, with the given
glycol concentration, at a flow of 1750 ton/hr, will provide a cooling capacity of approximately
24.4 MW – able to produce approximately 262.5 tons of ice per hour.
The water to be introduced into the ice slurry generator is, as indicated in Figure 2, a mixture of precooled seawater and residual brine from the concentration and rinsing unit (ice concentrator). From the
ice slurry generator the slurry is pumped towards a concentration and rinsing unit, which operates on the
hydraulic-piston-counter-washer (HPCW) principle. IDE has been using this principle for more than
twenty years in its vacuum freezing technology (VIM - Vacuum Ice Maker) for the production of
ice/snow for skiing and mine cooling applications. The principle behind HPCW is using the potential
energy provided to the ice slurry by the slurry pump to lift the ice cake (generated in the concentrator)
above the water level. Naturally, ice that enters a vessel will start to float, forming an ice cake at the top
of the vessel. Owing to the difference between the densities of water and ice, ice would rise above water
level for approximately 10% of the ice cake volume, similarly to an iceberg in the Earth poles. However,
due to the capillary effect, a 10% rise will not allow the required drainage of brine from the ice cake and
will not enable washing the ice for achieving drinking water quality. By using HPCW, the ice cake can
be raised for approximately 50% of its volume above water level, providing enough leverage for the ice
cake to be drained from the brine. The ice generated in this process contains approximately 75% ice and
25% water and is very similar to spring snow, which contains small ice crystals of approximately
0.5 mm-1 mm, surrounded by a very thin layer of water. Since the ice is produced from seawater, the
thin film of water contains brine from the process. In the second step, the concentrated ice in the
concentration and rinsing unit is rinsed from the residual brine using part of the product water. The
rinsing process actually results in the "replacement" of the brine film with a film of fresh water. This
occurs as part of the brine returns to the ice slurry generator, while the rest of it is being rejected from
the plant as process brine. The amount of brine being rejected from the process is controlled by the brine
stream salinity, which actually determines the concentration factor or the yield of the process.
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Since the rinsing process is not 100% efficient, some salinity remains in the end product which,
according to IDE's experience in the VFVC unit in Eilat, can reach down to 200 ppm TDS of mainly
NaCl. The efficiency of the rinsing process is dependant mainly on the size of the HPCW unit and the
amount of washing water used, among other parameters. The ice is scraped from the top of the HPCW
unit and fed into the product tank, where it is melted to produce the product water.
The melting process in the product tank is achieved by heat exchange between the ice and the feed water
and, as a result, it also serves as a heat recovery mechanism in which the seawater is pre-cooled by
melting the ice.
Thermodynamically-wise, the freezing process can utilize the "coldness" from the water-glycol solution
most efficiently, due to the low latent heat of freezing compared to the latent heat of evaporation.
According to the specified conditions of water-glycol solution, the freezing process can produce up to
6300 m3/day with minimal energy provided to the process pumps and the ice scraper at the top of the
HPCW unit. The increased production comes at the expense of the product water quality, lower than the
one obtained through the evaporation-condensation process, which involves distillation. The main
challenge in the freezing process approach is designing the ice slurry generator to properly balance the
required capacity and the production cost. As previously mentioned, the other system components are
already proven efficient and they available today in IDE's VFVC plants around the world.
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Figure 2: Freeze Desalination Flow Diagram
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1.2
The Evaporation-Condensation Approach
An additional solution for seawater desalination using the "coldness" is the basic principle of
evaporation-condensation, which is widely used in MED/MSF plants. In this approach, the process
utilizes the cooling source from regasification as a heat sink for the evaporation-condensation process.
The temperature span in desalination is determined by the number of distillation effects in the plant and
the typical seawater temperature at the plant location. In the process, seawater is flashed to produce low
pressure steam, which is then condensed or reused in a number of effects to produce the product water.
The main challenge in the evaporation-condensation approach is to maximize the desalination plant
production with low capital expenditure (CAPEX) to be able to compete with RO plants.
The simplest and lowest cost approach for utilizing the "coldness" from the water-glycol solution is
using a seawater flash stage followed by a condenser, as indicated in Figure 3. In this type of unit, the
seawater is flashed inside a flash chamber and then condensed in a forced circulation condenser (FCC)
using the cold water-glycol solution. Before entering the FCC, the water-vapor passes through a louvertype demister that acts as a water droplet carryover separator. The demister should be designed to handle
the high volumetric flow of low pressure water-vapor (caused by the high specific volume of the vapor
at this pressure) with a low pressure drop.
Figure 3: Schematic Diagram of a Flash-Condensation Plant
Besides the carryover separator design requirements, the high specific volume of the water-vapor at this
pressure requires designing the seawater flash stage with a relatively large vessel. For the evaluated
capacity, the vessel should be 7.7 m in diameter, similar to IDE's high capacity MED plants. This size
will ensure a reasonable pressure drop along the process.
The FCC should be designed and sized for utilizing the water-glycol solution "coldness" for heat
rejection, while considering the high viscosity of the solution compared to water, which can result in a
reduced heat transfer coefficient. Figure 4 outlines the general dimensions of the evaluated flashcondensation plant.
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Figure 4: Side Schematic Diagram of the Flash-Condensation Plant
The flash-condensation design includes only direct contact heat exchange in the evaporator. Eliminating
the need for heat transfer surfaces in the evaporator contributes to a lower basic cost of the plant.
However, the fact that there are no additional effects comes at another cost: although with a straightforward design, the flash-condensation design has high thermal energy consumption due to the relatively
high latent heat of evaporation, especially when compared to freezing – approximately 7.5 higher.
As indicated in Figure 3, the evaluated flash-condensation design can produce approximately 960
m3/day with the given water-glycol parameters, compared to approximately 6300 m3/day which is
thermodynamically possible in the case of the freeze desalination plant.
In order to increase water production, the addition evaporation/condensation stages are required,
practically migrating to a single effect distillation (SED) or MED design. The major drawback of the
migration to these designs lies in the plant cost. Operating a plant with several stages requires adding
heat transfer surfaces in the form of tubes in the additional effects. Because of the high specific volume
of water-vapor, these effects cannot be built under a dense tube design, since this would minimize the
vapor flow areas, resulting in high vapor flow speed. A high flow speed of the vapor will increase its
drag force, resulting in an increased risk of product contamination from brine in the process.
This requires the evaporators to be designed in a spread-out manner, which obviously comes with
increased CAPEX. Figure 5 shows a general schematic diagram of a SED plant with one flash stage
followed by additional evaporation/condensation effect, which can be extended to two
evaporationqcondensation effects or more in an MED plant design, according to the available
temperature span (a function of the seawater temperature and pressure drop across the process). As
previously mentioned, increasing the number of effects will significantly increase the plant CAPEX, and
therefore in this evaluation only SED and MED plants with two evaporation/condensation effects is be
considered.
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Figure 5: SED Unit Design
II.
COMPARISON OF THE APPROACHES AND RESULTS
Since complete technical and commercial data is not yet available, this section will be submitted with
the next draft.
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