Steam recovery from flue gas by organosilica membranes for simultaneous harvesting of water and energy

Steam recovery from the spent gases from flues could be a key step in addressing the water shortage issue while additionally benefiting energy saving. Herein, we propose a system that uses organosilica membranes consisting of a developed layered structure to recover steam and latent heat from waste. Proof-of-concept testing is conducted in a running incinerator plant. The proposed system eliminates the need for a water supply while simultaneously recovering latent heat from the waste stream. First, the long-term stability of an organosilica membrane is confirmed over the course of six months on a laboratory-scale under a simulated waste stream. Second, steam recovery is demonstrated in a running waste incinerator plant (bench-scale), which confirms the steady operation of this steam recovery system with a steam recovery rate comparable to that recorded in the laboratory-scale test. Third, process simulation reveals that this system enables water-self-reliance with energy recovery that approximates 70% of waste combustion energy.

recovery membrane unit.Conventional plants (case (a)) use as much as 74 t d -1 of fresh water to rapidly cool the stream from a combustion furnace to prevent the production of dioxins.The stream, which contains significant amounts of latent heat, is then released into the atmosphere from the stack.Another negative factor is that such a large amount of steam forms a steam condensate plume.The appearance of a steam condensate plume is a concern for the general public and often complicates the construction of incinerator plants in urban areas.This effect extends to all types of industrial plants, including power and chemical plants.Some plants re-heat the combustion gas stream simply to prevent the appearance of a steam condensate plume.
Please refer to Supplementary Table 1, which summarizes performances of (a) a conventional waste incinerator plant that uses a fresh water supply, (b) a self-reliant plant with a heat exchanger, and (c) the proposed self-reliant plant equipped with a steam recovery membrane unit.With the use of a heat exchanger (case (b)) or a steam recovery membrane unit (case (c)), steam contained in the stream after dust filtration can be recovered and reused, which amounts to a self-reliant waste incinerator plant without the need for a fresh water supply.This would be of great importance in any region with a shortage of water.
In addition, both options b and c enable the simultaneous recovery of latent heat and water.However, the water recovered via a heat exchanger is of lower quality, containing contaminants such as HCl, SOx and NOx.Recycling this water leads to a concentration of impurities, promoting corrosion within the process pipes and equipment.In contrast, water collected by a steam recovery membrane unit is clean because the membrane prevents the permeation of such contaminants (see Fig. 3).Moreover, generally the heat transfer efficiency of heat exchangers diminishes significantly when non-condensable gases exist, resulting in large heat exchanger volumes (Please see Supplementary Figure 2).In the case of option (c), the downstream of the membrane consists of almost pure steam, which results in high heat transfer efficiency and less device volume compared with the use of a heat exchanger.Another point to be compared is the steam condensate plume from the stack.When a heat exchanger is used directly, the processed stream must be cooled.The stream from the heat exchanger becomes saturated with water vapor, reaching a dew point.This results in the appearance of a steam condensate plume from the stack.
With the use of a steam recovery membrane unit, only the recovered stream (downstream of the membrane) must be cooled and the non-permeating (dehydrated) stream maintains its high temperature (150-200 °C).As a result, the humidity of the non-permeating stream should be 1-3%, which results in almost the complete absence of a steam condensate plume.Supplementary Figure1 Water balance in waste incinerator plants with a waste capacity of 30 t d -1 : (a) A conventional plant using a fresh water supply; (b) A self-reliant plant using a heat exchanger; and, (c) The proposed self-reliant plant equipped with a steam recovery membrane unit.The heat transport coefficient can be expressed as Eq.(S1).′ = − Eq.S1 In Eq. (S1), ′, , , , and are the heat flow rate, overall heat transfer coefficient, area and temperatures of high and low temperature fluids, respectively.
Supplementary Figure 2 illustrates the temperature drops across the wall of a heat exchanger (a) without and (b) with a steam recovery membrane unit where ′, and are constant.In case (a), the collection of water vapor condensation on the surface and within the boundary layer leads to an increased concentration of non-condensable gases increases due to the selective condensation of steam in the processing gas-flow side.The temperature dramatically decreases across the concentrated gas layer because it prevents steam diffusion from the high-temperature side to the low-temperature side.On the other hand, in case of (b), the process stream that permeates organosilica membranes is almost pure steam, which prevents the formation of thick non-condensable gas layer.So, a bit of only a thin temperature boundary layer and a condensed water layer are created for heat-transfer resistance in the processing from the gas-flow side.Reportedly, 1 1% of non-condensable gas reduces the heat transfer efficiency to only several-tens of % points compared with that of pure steam.Comparing Figs.(a) and (b), obviously with a steam recovery membrane unit is higher than that without it.This indicates that the steam recovery membrane unit improves the overall heat transfer coefficient, , in Eq. (S1).

Supplementary Note 2: Vapor permeation (VP)
Permeation of condensable components such as water through molecular-selective membranes can be categorized into three modes according to a combination of the upstream and downstream phases.As schematically shown in Supplementary Figure 3, in vapor permeation, both the upstream and downstream are in the gas phase.The permeating flux of component-i, [mol m -2 s -1 ], can be expressed as Eq.(S2).= , − , Eq. S2 In Eq. (S2), , , , and , , are the permeance (a constant related to membrane performance) and partial pressures in the upstream and downstream phases of component-i, respectively.Here, , − , , the partial pressure difference of component-i, is the driving force for permeation.
Since the driving force is given by the pressure difference, the upstream usually has higher pressure than the downstream.
Supplementary Figure 3 Permeation modes of condensable components through molecular-selective membranes.
Supplementary Figure 4 schematically illustrates a membrane module with three flows: feed, retentate and permeate."Feed" indicates a process stream that is fed to the membrane module for separation, which is followed by selective permeation of the water vapor via the membrane.The membrane-permeating and non-permeating component flows are referred to as the "permeate" (or downstream) and "retentate", respectively, and also are referred to as downstream and upstream, respectively.
Supplementary Figure 4 A schematic membrane module that separates water vapor from the mixture of water vapor and nitrogen.

Reverse osmosis (RO) Pervaporation (PV)
Vapor permeation (VP)  6 The feed gases are usually at low temperature with low water vapor pressure, so that downstream of the membrane is usually evacuated to promote permeation according to Eq. (S2).
Supplementary Table 2 summarizes VP membranes for water vapor/gases separation found in the literature.Organic membranes are mostly used for the dehumidification of air, fuel, and sampling gas.
They are usually operated at temperatures below 100 °C, because of the limitations in thermal stability.
Some thermally stable organic membranes such as perfluorosulfonic acid (PFSA) are applied to steam/gas separation at high temperature, but the permeance is low. 7Under high temperature, inorganic membranes such as zeolite and silica have been thoroughly studied.Although inorganic membranes are thermally stable compared with polymeric membranes, the hydrothermal stability is still challenging.One of the most attractive applications is the removal of steam by-product in an equilibrium reaction such as in Fischer-Tropsch synthesis, which can improve the conversion beyond the equilibrium.

Supplementary Note 3-2: Example of streams in membrane unit and heat exchanger
In our proposed system (Supplementary Figure 6), the steam is condensed downstream from the membrane unit.The membrane recovers steam from the upstream flue gas, and the heat exchanger recovers condensed water and heat.In the membrane unit, the upstream and the downstream flow in opposite directions in a so-called "counter-current", which is the flow pattern that maximizes the membrane unit performance.The heat exchanger is located downstream from the membrane unit where coolant flows counter-currently.This is also referred to as the "counter-current" and is the preferred flow pattern for heat exchangers in industrial applications.In counter-current mode, the flow downstream from the membrane is cooled along the heat exchanger from the inlet to the outlet.The outlet temperature is usually designed to be close to the coolant inlet temperature, typically with a temperature difference of 10-15 °C.On the other hand, the temperature of the coolant increases towards the outlet, which is designed to be close to the temperature of the inlet stream from the membrane.In Case Ⅰ, the coolant evaporates in the first region to maintain the temperature difference between the downstream of the membrane and the coolant, which reduces the amount of coolant.Then, after reaching position-x, the coolant completely evaporates.
In Case Ⅱ, the coolant remains in the liquid phase throughout the heat exchanger.Supplementary Tables 3 and 4 summarize the assumptions made for the subsequent simulations of membrane separation and the heat exchanger, respectively.
Supplementary phase transitions occur.In Case Ⅰ, liquid water at 3.17 kPa-a, which evaporates at 25 °C, is assumed as a simple coolant.In Case Ⅱ, pressurized liquid water is assumed as a coolant.Supplementary Table 3 Assumptions used for these simulations of membrane separation (for Case Ⅰ and Case Ⅱ); Tp and Te are the temperatures at the points illustrated in Supplementary Figure 6.

Feed stream Membrane
Flow rate 13,200  and the heat capacity of the coolant as a function of temperature [J mol -1 K -1 ], respectively.
When considering heat transfer via a heat exchanger, Eq. (S9) is used.
In Eq. (S9), Q and ,Q , respectively, are the overall heat transfer coefficient and the area of the heat exchanger in the vapor region, and Q/ and ,Q/ , respectively, are those in the vapor/liquid phase transition region.
Supplementary Table 5 summarizes an example of a construct of a membrane unit and a heat exchanger with the result of the simulation of Case Ⅰ.A membrane area of 1,100 m 2 is needed to recover 74 t d -1 of steam at 168 °C, which realizes a self-reliant waste incinerator plant.Energy from the latent and sensitive heat of the recovered steam reached quantities as large as 195 GJ d -1 when using a heat exchanger with a capacity of Q ,Q 20.1 kW K -1 and Q/ ,Q/ =137 kW K -1 , using 72.9 t d -1 of coolant liquid water at 25 °C.That setup resulted in 72.9 t d -1 of water vapor at 161 °C.Supplementary Table 5 An example of the construction of a simulated membrane and heat exchanger with coolant liquid water at 3.17 kPa-a (Case Ⅰ).

CaseⅡ
According to Supplementary Figure 7, with the membrane area of 1,100 m 2 , the composition of steam in downstream from the membrane is 95.8%.In the simulation, this stream is assumed to be recompressed to 101.3 kPa-a prior to entering to the heat exchanger.Therefore, the water vapor pressure at the inlet of the heat exchanger is 101.3×0.958= 97.0kPa-a, which corresponds to the dew point of 98.9°C.Therefore, : =98.9 °C is given for CaseⅡ.It should be noted that : can be controlled by tunning the downstream pressure at the inlet of the heat exchanger.The maximum is 168°C, which is the temperature at the inlet of the heat exchanger and the membrane unit.
Similar to Case I, the heat removed downstream from the membrane and the heat obtained are expressed as Eqs.(S12) and (S13).Eq.S13 In Eq. (S13), B,e is the coolant temperature at the position-x (in Supplementary Figure 6).
Considerations of the heat balances in regions where the downstream from the membrane is in the vapor phase and in vapor/liquid transition provide Eqs.(S14) and (S15), respectively.Eqs.(S17) and (S18) provide Q ,Q =3.83 kW K -1 and Q/ ,Q/ =62.9 kW K -1 , respectively.
Supplementary Table 6 summarizes an example of a construction of a membrane unit and a heat exchanger based on the result of the simulation for Case Ⅱ.The membrane unit, which has 1,100 m 2 of membrane area and recovers 74 t d -1 of steam at 168 °C, is the same with Case Ⅰ.In Case Ⅱ, energy from the latent and sensitive heat of the recovered steam reached quantities as large as 177 GJ d -1 when using a heat exchanger with a capacity of Q ,Q 3.8 kW K -1 and Q/ ,Q/ =62.9 kW K -1 .This setup produces 613 t d -1 of hot water at 161 °C.Supplementary Table 6 An example of the construction of a simulated membrane and heat exchanger with coolant liquid water at 101.T ],^_

Figure 6
Schematic of (a) counter-current flows in the membrane unit and the heat exchanger, and (b) temperatures along the heat exchanger.(a-1) and (b-1) are for Case Ⅰ and (a-2) and (b-2) are for Ⅱ.The temperatures of the downstream and the coolant are constants in the regions where

Table 4
Assumptions used for these simulations of the heat exchanger; Tp, Te, Tc,in and Tc,x are the temperatures at the points illustrated in Supplementary Figure6.