Condensation, re-evaporation and associated heat transfer in membrane evaporation and sweeping gas membrane distillation

The Application of Open Capillary Modules for Sweeping Gas Membrane Distillation

The paper presents the sweeping gas membrane distillation realised by using the capillary module (length 1.1 m and area 0.1 m2) without housing (module shell). During the tests, the feed was flowing inside the hydrophobic polypropylene membranes. The studies were performed for two variants of process: with pre-heating (313–330 K) and without heating of the feed (brines). Under low gas flow (0.005 m/s) the evaporation performance varied in the range of 0.15–0.25 L/m2h, depending on the relative humidity (42–63%) and the air temperature (293–300 K). The application of feed pre-heating to 330 K led to an increase in the evaporation performance to 2.4 L/m2h. The permeate flux increased by 60% when the air flow velocities between the capillaries increased to 1.8–2.5 m/s. Increasing the feed flow rate from 0.1 to 0.59 m/s led to increase the permeate flux about 20% for feed temperature 293–310 K, and over 55% for feed temperature higher than 323 K.

Experimental Investigation into Flue Gas Water and Waste Heat Recovery Using a Purge Gas Ceramic Membrane Condenser

The direct discharge of wet saturated flue gas from a coal-fired power plant boiler causes a lot of water and waste heat loss. An inorganic ceramic membrane condenser recovers water and waste heat from the flue gas, which has great significance to improve energy utilization efficiency and reduce water consumption. However, the flue gas temperature is relatively low; thus, it is difficult to effectively utilize waste heat. In this paper, it is attempted to use the boiler secondary air as the cooling medium of the ceramic membrane condenser to realize the flue gas waste heat reuse. Based on the above ideas, a purge gas ceramic membrane condenser experimental platform was built for the water and waste heat recovery from the flue gas, and the water and waste heat recovery characteristics and the purge gas outlet parameters were discussed. Simultaneously, the heat transfer resistance and water recovery power consumption are also analyzed. The experimental results show that the water and waste heat recovery characteristics are enhanced with the purge gas flow increases. Increasing the flue gas temperature will increase the water recovery rate and heat recovery power. The ceramic membrane transmission efficiency is a key factor in restricting the actual water recovery efficiency. The purge gas absorbs the water and waste heat from the flue gas, the purge gas temperature and moisture content are significantly increased, and the purge gas relative humidity is also close to saturation. The Biot number of the ceramic membrane condenser is about 3.2 × 10^-3 to 1.9 × 10^-2; thus, the ceramic membrane tube wall thermal resistance can be neglected. There is a temperature difference between the flue gas and the purge gas, and the entropy production value of the ceramic membrane condenser increases with the flue gas temperature increases by the irreversible process.

Review of Transport Phenomena and Popular Modelling Approaches in Membrane Distillation

In this paper, the transport phenomena in four common membrane distillation (MD) configurations and three popular modelling approaches are introduced. The mechanism of heat transfer on the feed side of all configurations are the same but are distinctive from each other from the membrane interface to the bulk permeate in each configuration. Based on the features of MD configurations, the mechanisms of mass and heat transfers for four configurations are reviewed together from the bulk feed to the membrane interface on the permeate but reviewed separately from the interface to the bulk permeate. Since the temperature polarisation coefficient cannot be used to quantify the driving force polarisation in Sweeping Gas MD and Vacuum MD, the rate of driving force polarisation is proposed in this paper. The three popular modelling approaches introduced are modelling by conventional methods, computational fluid dynamics (CFD) and response surface methodology (RSM), which are based on classic transport mechanism, computer science and mathematical statistics, respectively. The default assumptions, area for applications, advantages and disadvantages of those modelling approaches are summarised. Assessment and comparison were also conducted based on the review. Since there are only a couple of full-scale plants operating worldwide, the modelling of operational cost of MD was only briefly reviewed. Gaps and future studies were also proposed based on the current research trends, such as the emergence of new membranes, which possess the characteristics of selectivity, anti-wetting, multilayer and incorporation of inorganic particles.

Closing CO2 Loop in Biogas Production: Recycling Ammonia As Fertilizer

We propose and demonstrate a novel system for simultaneous ammonia recovery, carbon capture, biogas upgrading, and fertilizer production in biogas production. Biogas slurry pretreatment (adjusting the solution pH, turbidity, and chemical oxygen demand) plays an important role in the system as it significantly affects the performance of ammonia recovery. Vacuum membrane distillation is used to recover ammonia from biogas slurry at various conditions. The ammonia removal efficiency in vacuum membrane distillation is around 75% regardless of the ammonia concentration of the biogas slurry. The recovered ammonia is used for CO₂ absorption to realize simultaneous biogas upgrading and fertilizer generation. CO₂ absorption performance of the recovered ammonia (absorption capacity and rate) is compared with a conventional model absorbent. Theoretical results on biogas upgrading are also provided. After ammonia recovery, the treated biogas slurry has significantly reduced phytotoxicity, improving the applicability for agricultural irrigation. The novel concept demonstrated in this study shows great potential in closing the CO₂ loop in biogas production by recycling ammonia as an absorbent for CO₂ absorption associated with producing fertilizers.