(473f) Multi-Effect Membrane Distillation Process for Desalination and Concentration of Aqueous Solutions of Non-Volatile or Semi-Volatile Solutes

Membrane distillation (MD) is a separation method in which a nonwetting, microporous membrane is used with a liquid feed phase on one side of the membrane and a condensing, permeate phase on the other side. Separation by membrane distillation is based on the relative volatility of various components in the feed solution. The driving force for transport is the partial pressure difference across the membrane. Separation occurs when vapor from components of higher volatility passes through the membrane pores by a convective or diffusive mechanism.

Membrane distillation systems can be classified broadly into three categories: direct-contact membrane distillation (DCMD), vacuum membrane distillation (VMD) and air-gap membrane distillation (AGMD). Potential advantages of membrane distillation over traditional evaporation processes include operation at ambient pressures, lower temperatures as well as ease of process scale-up, and avoid of corrosion as a result of inertness of hydrophobic polymer membrane material.

Although MD has been extensively and intensively studied for nearly 40 years, it is still not use in commercial scale. As matter of a fact, MD is with extremely low thermal efficiency, even if the researcher on MD always declaimed that MD only need a heat resource with low temperature. For example, when VMD occurs, nearly 1 ton of pure water can be produced when 1 ton of steam is used to heat a cold feed; when DCMD occurs, only 0.3 ? 0.6 ton of pure water can be produced when 1 ton of steam is used to heat a cold feed. As a comparison, when traditional multi-effect distillation (MED) or multi-stage flash (MSF) is used for desalination, 1 - 15 ton of pure water can be produced when 1 ton of steam is used to heat a cold feed. Therefore, if a term of performance ratio (PR) is used to characterize the thermal energy utility, while the value of PR for MED and MSF is between 1 ? 15, unfortunately the value for traditional MD process is only 0.3 ? 1.0. Another problem with MD is wetting or fouling of microporous membrane surface, which leads to the decrease of permeate flux and leakage; leakage further leads to the contamination of the permeate product by the impurities in the feed. Wetting or fouling is a more grievous problem in a DCMD or VMD process, since the intimate contact of the permeate with the membrane in DCMD or the large trans-membrane pressure difference in VMD. On the other hand, the scale AGMD module was rarely reported in literature.

Recently, MD process with multi-effect characteristics were reported by using hollow fiber or flat sheet membrane, even though the concept of multi-effect membrane distillation was not directly used as a term. In the present study, multi-effect membrane distillation (MEMD), a new membrane distillation process, has been developed, which combines the advantages of both membrane distillation (MD) and multistage flash (MSF) by equipping air gap membrane distillation (AGMD) with internal heat recovery. A novel separation device in the form of hollow fibers is fabricated to test its separation performance, which is identified as MEMD module. Several MEMD modules with different configurations were fabricated in our company since 2005. The water vapor flux (J) and energy efficiency in term of performance ratio (PR) and thermal efficiency (η) are the most important indicators for evaluation of DCMD module performances. J indicates the productivity of the membrane module; PR tells how much energy is recovered by internal configuration and η shows how much energy is lost due to conduction according to the second law of thermodynamics. Experiments were conducted using aqueous solution of salt NaCl to investigate the influences of operating variables including inlet temperatures of two sets of different fibers and flow rate on these above three performance parameters. The value of J was usually 3 ? 10 kg/m2hr; the value of η was usually more than 0.9; and the value of PR varied between 3 and 15, which mainly depended on the characteristics of the hollow fiber used, and salt species and concentration, and the operation temperature. Flux decline together with reduction of PR and the decrease of ç were observed with the increased salt concentration (up to about 220g/L) mainly because of its high viscosity and reduction of water vapor pressure with the increase of salt concentration. Even so, the value of J and PR at a NaCl concentration of 20wt% is 50% of the value at a NaCl concentration of 3wt%. Such a MEMD operation for further concentration of 20% salt solution was operation at mild temperature (less than 100oC) and ambient pressure. It must be noted that it is impossible to further concentrate a feed of 20% by reverse osmosis, and high temperature and evacuation must be used when MED or MSF is used for the further concentration of 20% salt solution. Therefore, MEMD can be potentially used to further concentrate the brine by-produced during the routine desalination plant by using RO, MSF, or MED, to produce drainable water and salt.

Since 2006, MEMD has been tested in pilot-scale for desalination of real seawater and for the deep concentration of bring from the RO unit for the treatment of wastewater drained from a refinery plant. No leakage or decline of operation performance was observed during the 4-month long test period.

MEMD can also used to separate volatile semi-volatile and non-volatile solutes from their aqueous solution. The system tested was aqueous solution of the following solutes: variety of non-volatile salts, sugars, urea, sodium hydroxide, sulfuric acid, hydrochloric acid, hydrofluoric acid, hydrobromic acid, phosphorous acid, phosphoric acid, nitric acid, acetic acid, chloroacetic acid, dichloroacetic acid, oxalic acid, succinic acid, glutaric acid, adipic acid, fluosilicic acid, ammonia, alkyl monoamines with low molecular weight, aminoglucose, amino acids, glycerol, ethylene glycol, propylene glycol, formaldehyde, ethanol, acetone, formamide, dimethylformamide, dimethyl sulfone, solfulane, hydrogen peroxide, hydrazine hydrate, ethanolamine, diethanolamine, diamines or polyamines, ammonium carbonate, ammonia sulfide, or several practical aqueous solutions containing two or three solutes, such as hydrochloric acid + glucose, hydrochloric acid + aminoglucose, sulfuric acid + glycerol, phosphorous acid + formaldehyde, fluosilicic acid + hydrofluoric acid + nitric acid, nitric acid + oxalic acid, hydrochloric acid + ferrum(II) chloride, and so on. The currently performing pilot-scale test is the concentration of fruit juices, and the water production from the wastewater streams strained from the deionized water production unit in a power plant.

The test results demonstrated MEMD can effectively concentrate aqueous solution of non-volatile salt, most of inorganic acids, non-volatile or semi-volatile organic compounds with a boiling point of more than 180 oC, or solutes with extremely strong association with water such as hydrochloric acid, hydrofluoric acid, hydrobromic acid, hydrazine, formaldehyde from their aqueous solution. However, MEMD does not provide a high selectivity for aqueous solution containing a volatile organic compounds or a semi-volatile compound with a boiling point less than 180 oC.

The techno-economic analysis demonstrated that MEMD process is a strong competitor for desalination or concentration of aqueous solutions to RO, MSF and MED.