Cost and energy requirements of hybrid RO and ED brine concentration systems for salt production
Introduction
Globally, each year, more than 280 million tonnes of salt [1] is produced from solar evaporation of seawater, from conventional rock mining and from the solution mining of saline brines [2]. Of this, 39% is used by the chloralkali industry for the production of chlorine and sodium hydroxide, 22% for human consumption, 22% by the soda ash industry, 9% for de-icing of roads and 9% for other uses [2]. Salt is important both as a crucial nutritional source and as a crucial raw material for global chemical production. Given the strategic importance of salt for the world, producers globally are looking at ways to make salt production more sustainable and reduce production costs. Salt production costs vary significantly with the method of production, with the lowest production costs being for “solar salt” [3] (around $5-10/tonne of salt [4]) and the highest production costs being for “vacuum salt” [3] (around $30–50 /tonne-salt [4] for conventional vacuum salt produced from saturated brines). In “solar salt” production [3], seawater is collected in evaporation ponds and freely available solar insolation is used to evaporate off water leaving behind salt. Solar salt production requires significant land areas and is used only in locations where land prices are low. In conventional “vacuum salt” production [3], saturated solutions of brine (S = 260 g/kg) extracted from solution mines are sent to thermal or electrically driven evaporators and crystallizers (referred in this paper hence simply as “crystallizer”) to evaporate off water and produce salt. Vacuum salt is more expensive than solar salt production primarily because of the higher capital costs in equipment needed and the energy costs since in the latter solar energy is used for free. Despite the differing production costs, the global salt market can support both methods of salt production partly because of high transportation costs. Transporting 1 tonne of any commodity per 100 km costs around $10.34/tonne-100 km [5] by truck, 2.53 $/tonne-100 km by rail [5], and 1.14 $/tonne-100 km by barge [5]. Thus at a certain distance and between certain cities, “vacuum salt” becomes a more competitive option.
One of the most expensive types of “vacuum salt” is “vacuum salt” produced from seawater through the use of “brine concentration” and “crystallizer” systems. In such systems seawater (S = 35 g/kg) is first concentrated to near saturation levels (S = 180–260 g/kg) using a separate “brine concentration” system before being sent to a crystallizer. Separate “brine concentration” systems are needed because crystallizers are designed to be cost effective only when using nearly saturated brine. Significant energy is needed to concentrate seawater to near saturation levels with the required volume reduction being around 90% [6]. When low cost land is not available for the construction of solar evaporation ponds, separate “brine concentration” systems such as Mechanical Vapor Compression (MVC) [[7], [8], [9], [10], [11]] or Electrodialysis (ED) [[12], [13], [14], [15], [16], [17], [18]] needs to be used. To the best of our knowledge, the only commercial “vacuum salt” production systems that produce salt from seawater through a combination of “brine concentration” systems and “crystallizers” are plants in Japan, Korea and Kuwait [14,17,19] that use ED to first concentrate seawater from 35 g/kg to around 180–200 g/kg after which brine is sent to a crystallizer where salt is produced for either human consumption [19] or for chlor-alkali production [14]. A flow diagram of such a typical ED based “salt production” plant is shown in Fig. 1.
ED is a versatile electric driven membrane based desalination technology, that was first conceptually conceived for demineralizing sugar syrup in 1890 [20], and developed for saline water desalination in the 1940s and 1950s[20,21]. ED systems primarily consist of several pairs of anion exchange membranes (AEM), and cation exchange membranes (CEM) placed in between a cathode and anode (shown later in Fig. 5). Feed water is fed into channels created between CEMs and AEMs. When a voltage is applied, cations move to the cathode and anions move to the anode; however, the AEMs prevent the movement of cations and CEMs prevent the movement of anions resulting in one channel becoming concentrated in ions while the adjacent channel is depleted of ions. Consequently, these two adjacent channels are referred to as the ‘concentrate’ and ‘diluate’ channels. Depending on the type of AEMs and CEMs used, ED can be designed for several applications including city-scale brackish water desalination [17,21], village-scale water treatment [22], seawater brine concentration [[14], [15], [16], [17], [18], [19]], denitrification of water for municipal water supply [21], demineralization of wine, whey and sugar [23], in-home water treatment [[24], [25], [26]] and wastewater treatment [17,27]. For salt production from seawater, AEMs and CEMs that are additionally selective to monovalent ions are used. The resulting monovalent selective ED (MSED) systems concentrates sodium chloride preferentially over other ions in seawater leading to the production of brines of salinity 180 g/kg to 200 g/kg that are rich in sodium chloride. MSED systems for concentrating brine for salt production have been in commercial operation for more than 50 years in Japan [19].
In this paper, our objective is the production of salt from seawater and our analysis is restricted to MSED systems. For the convenience of readers, henceforth in this paper, we will be referring to MSED systems simply as ED systems. The flow diagram of a conventional salt production plant using ED is shown in Fig. 1. Seawater feed, typically at 35 g/kg, first flows in to both the diluate and concentrate channels of an ED stack. In the concentrate channel, seawater is typically concentrated from 35 g/kg to 177–200 g/kg with the diluate discharged back in to the sea at a salinity much less than seawater. Typically the salinity change happens along the length of a single membrane with the voltage kept constant along the length [14]. Such a design using a single voltage along the length of a stack is referred to in the literature as a “single electric stage” design [28].
While ED based brine concentration systems for salt production have been used in Japan for around 50 years, such systems have not been deployed widely outside of Japan and South Korea partly due to the high costs involved in concentrating seawater. For reference, Miyake et al. [29] had reported a production cost of 100 $/tonne-salt for an ED-crystallizer system with 65% of the costs being capital costs.
Another membrane based desalination technology that has become very popular now is reverse osmosis (RO). RO is a pressure driven membrane based process that separates saline water into pure product water and a saltier brine stream [30]. Today, RO has become the most widely adopted seawater desalination technology [31]. Seawater RO is also now the most energy efficient desalination technology [9,32] for seawater desalination. Thermodynamic analysis has also shown conceptually that RO can also be the most energy efficient brine concentration technology for concentrating brine from 150 to 260 g/kg [33]. Conventional seawater RO operates at a pressure of 50–70 bar recovering 40–50 % of seawater feed as pure product water with the RO brine discharged back in to the sea at around a salinity of 60–70 g/kg. RO systems for concentrating brine to 260 g/kg do not currently exist in operation due to the high operating pressures required for brine concentration (379.2 bar at NaCl saturation [33]). However, in the past 5 years, advances in RO membrane technology have enabled the development of higher pressure RO systems that can operate up to pressures of 120 bar [34] corresponding to a RO brine salinity of 120 g/kg. These higher pressure RO systems are new and have not yet been adopted widely by the seawater desalination industry.
Some key research questions are: can the cost of seawater brine concentration be reduced below that of current ED costs? If so, by how much can the cost be reduced? Can hybridizing ED with RO reduce costs? And what are the economic implications of reducing brine concentration costs? Can a new salt production industry be created out of small-scale salt production plants that produce “vacuum salt” through “brine concentration” and crystallizer systems? Our concept, explained in Section 2, seeks to answers these questions and may inspire further research and development work in this area.
Section snippets
Proposed concept: RO-ED hybrid brine concentration systems used with crystallizers for salt production
In this paper, we evaluate potential cost reduction for ED based seawater brine concentration by using ED systems that are hybridized with reverse osmosis (RO). Fig. 2a shows an illustration of our proposed RO-ED brine concentration system. Fig. 2b shows the detailed flow diagram of the RO, ED and crystallizer systems. Seawater feed (35 g/kg) first flows into an RO system where pure product water and desalination brine are produced. The brine produced by the RO system (60–120 g/kg salinity) is
Methodology
In this section we discuss the techno-economic models used to simulate the RO, ED and crystallizer systems.
Parametric design of hybrid RO-ED systems for brine concentration for salt production
This section discusses the effects of hybridizing RO with ED, ED current density on RO-ED design, electricity prices and water prices.
Optimal RO-ED hybrid designs significantly reduced the cost of brine concentration and salt production
When all the parameters discussed in Section 4 are considered together, fully hybridized and current density optimized RO-ED systems can cost 33–70 % less than standalone ED brine concentrators for salt production application. When including the crystallizer, optimized RO-ED-crystallizers can cost 19–55 % less than standalone ED-crystallizer based seawater salt production systems.
Fig. 21 shows the specific costs of brine concentration for standalone ED and optimal RO-ED designs, along with the
Global feasibility of RO-ED hybrid systems for concentrating seawater for salt production
Based on production costs discussed in Section 5 and market prices of salt listed in Table 4 obtained from salt manufacturers and national government agencies, we believe that RO-ED hybrid brine concentration systems for salt production are technically and economically feasible in parts of Japan, the Middle East, Europe and the United States. The economic feasibility of each region depends on its unique circumstances that arise from the following conditions:
- (a)
High market demand for edible salt or
Limitations and areas for future research
As we arrive at our conclusions, we felt it necessary to highlight some of the limitations of our work and areas for future research.
- •
The final salt purity produced from the RO-ED-crystallizer concept is expected to be only at least 99.8% pure and cannot be used directly in an electrolyzer for chloralkali production. Further purification of the salt is needed for use in an electrolyzer. This could be done either before the ED concentrate enters the crystallizer or on the salt produced. Future
Brine concentration
RO systems are more cost effective at concentrating seawater than ED systems, and hybridizing ED with RO reduced brine concentration costs. However, the ED systems should be fully hybridized with the emerging high pressure RO technology, up to operational limits of an RO brine salinity of 120 g/kg for significantly reducing costs (7%). Partial hybridizing of RO and ED systems, with the RO brine salinity restricted to the operational limit of conventional seawater RO of 70 g/kg brine salinity,
Acronyms
- CapEx
capital expense, $
- Capfac
capacity factor, –
- Costelec
electricity cost, $/kWhe
- Crys
crystallizer
- CSSF
crystallizer salinity scale-up factor, –
- ED
electrodialysis
- OpEx
operating expense, $
- Pricewater
water price, $/m3
- RO
reverse osmosis
- SpCapEx
specific capital expense, $/tonne-salt
- SpCost
specific cost, $/tonne-salt
- SpE
specific energy, kWhe/tonne-salt
- SpOpEx
specific operating expense, $/tonne-salt
Roman Symbols
- A
area, m2
- C
concentration, mol/m3
- D
diffusion coefficient, m2/s
- E
potential, V
- E
specific energy consumption, kWh/m3
- F
Acknowledgment
The authors would like to thank Kuwait Foundation for the Advancement of Sciences (KFAS) for their financial support through Project No. P31475EC01. The authors would like to thank various industry representatives cited in this work for sharing their expertise and perspectives with us.
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