Multistage

2.2.3 Multistage flash

Multistage flash distillation (MSF) is one of the well-established thermal technologies in which heat is utilized to separate water from brine by flashing (sudden reducing in pressure) in a vacuum chamber. In MSF desalination technology, the generated vapor in each stage is condensed while the brine released from the preceding stage is allowed to pass to the consecutive stages for further flashing as depicted in Fig. 5. The freshwater productivity of the MSF technology is greatly affected by the feed water and brine temperatures, feed water salinity, and the number of stages. MSF technology is highly suitable when the brine water is greatly contaminated by heavy metals, insoluble matters, suspended solids, gases, and oils. Typically, the number of stages of MSF varies between 15 and 30 stages while the newest MSF technology has reached 45 stages, which could compete with reverse osmosis in terms of energy consumption and capacity. MSF technology is extensively applied in the Middle East region, particularly in United Arab Emirates, Saudi Arabia, and Kuwait, with a contribution that reaches up to 40% of the global desalination capacity (Sharon and Reddy, 2015).

2.2.2 Multistage flash

MSF (refer to Fig. 2) is a water desalination process that is similar to MED, but instead of having vapor carry on successively between chambers, the brine itself is heated. To reduce corrosion and scale formation, brine temperature is encouraged to be less than 70°C, with an absolute maximum of 120°C. MSF distillation plants produce about 60% of all desalinated water in the world. MSF desalination plants could have from 4 to 50 chambers, and they are placed to satisfy large-scale water production needs, with up to 70,000 m³/day (United States Department of Interior, 2003). Similarly to MED, MSF plants are also capable of yielding high-purity water and do not require extensive pretreatment of seawater. Unfortunately, MSF comes with high energy needs, and the rate of water recovery is less than that of other competitive desalination technologies such as RO (Li, 2012). Analogously to MED, MSF plants run on both thermal and electrical energies, with average consumptions of 100 kW_thh/m³ and 3 kW_eh/m³, respectively (Legislative Council of Hong Kong, 2015). Table 2 summarizes the nominal attributes of one-through and recycling MSF technologies.

Fig. 2. General MSF process (Khuwaileh and Ishag, 2019).

Table 2. MSF nominal attributes.

Range	Max temp (°C)	Process recovery (%)	Performance ratio (kg/MJ)	Heat transfer coefficient (w/m2 K)	Concentrate concentration(Mg/L)	Electrical consumption (MJ/L)	Distillate quality (mg/L)
Once-through	90.6	10–15	3.44–4.30	2271–3407	58,000	0.24–0.29	0.5–25.0
Recycle	110	10–20	3.44–5.17	2207–3407	62,500	0.20–0.29	0.5–25

19.3.1.1 Multistage flash distillation

MSF distillation plays a considerable role in providing fresh water in several parts of the world. A multiple-effect or multieffect evaporator is a series of vessels for the efficient application for water evaporation of heat produced by steam. Water is sequentially boiled, with each container holding the pressure a lower value than the previous one. MSF distillation processes seawater by flashing some of the water to steam through multiple stages, generally of countercurrent heat exchangers.

MSF distillation units can follow either a recycling or a once-through process. In a recycling process, the feed water is transferred through the heater and flash chambers only once then is disposed; the feed water employed for cooling is also recycled in a once-through process. Fig. 19.3 is a scheme of a “once-through” MSF distillation unit. The operational energy of MSF distillation is higher than other desalination procedures for the same volume of produced water MED desalination has therefore earned much attention [11].

Figure 19.3. Multistage flash distillation.

(1) INVESTIGATION OF HIGH EFFICIENCY HEAT PUMP SYSTEM

Multi-stage condenser system is adopted in our high efficiency heat pump as stated previously. The COP is depending on many parameters including kind of cycle (with or without sub-cooler), number of compression stages, number of condensing stages, various temperature conditions, and kind of working fluid. In order to select the optimum one from these, characteristics of the compressor and the heat exchanger should be taken into consideration as well as COP.

For this reason, we intend to develop a simulation program for multi-stage condenser system to clarify various problems in utilizing multi-stage condenser system and to design entire system.

4.4.2 Solar-powered multistage flash desalination

MSF desalination was the first large-scale commercial desalination process and came to prominence in the 1970s. Since then it has a significant share of the market in the Middle East [29]. MSF accounts for 21% of the world’s installation capacity for desalination, second only to RO [49]. The production capacity of MSF may vary significantly, ranging from units that can produce 23,000 m³/day to very large units which can produce 528,000 m³/day. The production cost also varies, typically ranging between 0.52 and 1.75 US$/m³ [48]. The energy demand of MSF technology is high and varies between 13.5 and 25.5 kWh/m³. Hence, most MSF units are built next to an existing power plant [158]. This method of thermal desalination is based on a flash distillation of heated brine at reduced temperature and pressure. Seawater/brackish water feed into this process, where the flow goes through successive heating stages, and after flashing, some of the same heat exchangers are used (on the shell side) to condense the freshwater, recycling latent and sensible heat. Typically the seawater is preheated by an external heat source before entering the first stage. The brine temperature then rises to 90°C–110°C. The heated brine flows continuously in stages and at each stage, a small amount of water is vaporized. The resulting steam condenses at each stage and freshwater is produced. Finally, concentrated brine and freshwater are drained from the last stage. Fig. 4.22 shows a schematic of the MSF desalination process. MSF units usually consist of 4–40 stages, and each stage operates at a lower temperature and pressure than the previous stage. Thus the boiling point of the feedwater is reduced during successive stages, and owing to the continuous boiling of the brine, there is no need for an extra heat source in addition to the seawater preheating heat source [158]. The performance and GOR of the MSF process are enhanced by boosting the top brine temperature (TBT), the brine temperature in the first flashing stage, decreasing the intake saline water temperature, boosting the stages, and increasing the specific heat exchange area [74].

Figure 4.22. Schematic of MSF process [48].

In solar-powered MSF systems, various solar technologies, including parabolic collectors, flat plate collectors, central tower receivers, linear Fresnel reflectors, evacuated tubes, solar ponds, and PV panels, are used to combine with MSF desalination system [159]. To integrate MSF with solar energy, the TBT (between 90°C and 110°C) must be adjusted to prevent unstable operation [159]. Fig. 4.23 illustrates the MSF unit integrated with a solar collector. Since the 1980s, several solar MSF units with a capacity of 10–20 m³/day have been launched [160]. In 1983, in Safat, Kuwait, a solar-powered MSF plant using PTC collectors was installed with 10 m³/day capacity and the SEC of 81–106 kWh/m³ [161].

Figure 4.23. A solar-powered MSF desalination system [74].

Over the past 20 years, few studies have been conducted on solar-powered MSF units compared to the solar-powered MED process, most of which have focused on pilot and small-scale units. This shows that solar MSF is less technologically and economically competitive than solar MED, owing to some reasons [74]: (1) The need for a relatively high TBT in this process has made it unfavorable to combine with solar energy. (2) Higher TBT means higher fouling and scaling rates. (3) Thermodynamically, MSF is less efficient as compared with MED.

3.6 Renewable-Assisted MSF Desalination

Like MED, MSF can be coupled with different renewable energies. Fig. 28 shows schematics of an MSF integrated with a CSP system. An MSF system can also be coupled with a solar pond, which acts as the collector and storage of solar energy and provides this energy for MSF [14]. The capital cost of MSF is relatively lower than solar stills with a performance ratio 3 to 10 times higher [114]. The results of coupling MSF with a solar pond revealed that a solar pond (1500 m²) at 70°C could supply the energy for a 10-unit MSF operating at 0.9 bar with 15 m³/day capacity [115].

Fig. 28. Multistage flash desalination coupled with solar collectors.

7.1.1.2 Treatment process for highly concentrated fecal wastewater

Multistage anaerobic treatment process or conventional treatment process combining with anaerobic and aerobic treatment processes can be used to treat high-concentration fecal wastewater. Because the treatment process is simple and easy to manage, the expected treatment effect can be achieved.

Multistage anaerobic treatment process can be adopted (Fig. 7.5) when the treated high-concentration fecal sewage is discharged into an urban drainage system [20].

Figure 7.5. Multistage anaerobic treatment of high-concentration fecal wastewater.

While the combined treatment process of multistage anaerobic and aerobic treatment (Fig. 7.6) can be adopted when the treated high-concentration fecal sewage is required to meet the secondary discharge standard stipulated in the current national standard “Integrated Wastewater Discharge Standard” (GB8978-1996) [20].

Figure 7.6. Multistage anaerobic and aerobic treatment process of high-concentration fecal wastewater.

1.7.3 Multistage Interconnection Networks

Multistage interconnection networks (MINs) connect input devices to output devices through a number of switch stages, where each switch is a crossbar network. The number of stages and the connection patterns between stages determine the routing capability of the networks.

MINs were initially proposed for telephone networks and later for array processors. In these cases, a central controller establishes the path from input to output. In cases where the number of inputs equals the number of outputs, each input synchronously transmits a message to one output, and each output receives a message from exactly one input. Such unicast communication patterns can be represented as a permutation of the input addresses. For this application, MINs have been popular as alignment networks for storing and accessing arrays in parallel from memory banks. Array storage is typically skewed to permit conflict-free access, and the network is used to unscramble the arrays during access. These networks can also be configured with the number of inputs greater than the number of outputs (concentrators) and vice versa (expanders). On the other hand, in asynchronous multiprocessors, centralized control and permutation routing are infeasible. In this case, a routing algorithm is required to establish the path across the stages of a MIN.

Depending on the interconnection scheme employed between two adjacent stages and the number of stages, various MINs have been proposed. MINs are good for constructing parallel computers with hundreds of processors and have been used in some commercial machines.

1.1 Multistage MPC

Multistage MPC explicitly considers a selection of possible future scenarios along a prediction horizon to formulate its optimization problem. The scenarios are determined by propagating from the current state to the end of the prediction horizon, a finite number of uncertain parameter realizations using a scenario tree. When the prediction horizon is long the number of scenarios in the scenario tree increases exponentially resulting into an intractable problem. Lucia et al. (2013) proposed a robust horizon where the scenario tree branching is stopped before the end of the horizon, and the uncertain parameters are kept constant until the end of the prediction horizon. The robust horizon makes the problem practically feasible to solve but can still be expensive, especially for nonlinear problems, leading to a significant computational delay. In order to reduce the computational cost and computational delay of the multistage MPC, Thombre et al. (2020) proposed the sensitivity assisted multistage MPC. It has an algorithm to prune irrelevant scenarios from the scenario tree using NLP sensitivities in order to speed up computations. The sensitivity assisted multistage MPC is discussed further in Section 2.

3.2 Hybrid Thermal Desalination Processes

MSF and MED showing low water production because of the process limitations such as scaling and high energy consumption. An integrated MSF with the MED process has been designed to overcome the limitations of the single process [30]. The hybrid MSF-MED process has several merits like reducing the specific water unit price (32% lower than MSF and 20% lower than MED), decreasing heat transfer area, increasing the water production, and 57% lower SEC than MSF [30]. In addition, by dosing antiscalants into a pretreatment process, both processes can increase the temperature up to 130°C, which leads to further improvement of water productivity and reduction in SEC [13].

Adsorption desalination (AD) is one of the possible processes to operate below ambient conditions for MED process. AD relies on employing adsorbent materials like silica aerogel [31]. It mimics the evaporation by low-temperature solar collectors and condensing vapor at a high height to produce pure water. An integrated MED with AD has been studied for improved performance of thermal desalination plants in terms of the water production rate [32]. The integrated process also showed around 40% improved gain output ratio (GOR) and around 61% reduced SEC compared with a conventional MED process. As well, the MED-AD hybrid process mitigates fouling and scaling because of the low-temperature operation of AD [32].