(606c) Supported-Gas-Membrane Process for Removal and Recovery of Amines From Water Streams

Amines are widely used in a range of industries. For example, methylamine is mainly used for medicine (hormone or caffeine), agricultural chemicals (carbaryl, rogor, and so on) and explosive; dimethylamine is used in dimethylformamide (DMF) synthesis, agricultural chemicals, pharmacy, tannery and organic chemical industries; trimethylamine is used in the field of feed additive, weedicide, ion exchange resins, and so on ; ethylamine is used in the field of synthetizing dyes, extraction, emulsifier, rubber vulcanizing, medicine and also in petroleum refining; diethylamine is used for organic synthesis and curing agent for epoxy; triethylamine is mainly used as organic solvent, synthetizing dyes, inhibitor, catalyst, preservative, and so on; butylamine can be used as rubber accelerator, pesticides and so on. On the other hand, diethylamine-containing wastewater is produced as a by-product from DMF hydrolysis in the application of DMF in artificial leather industry. With the extensive application of amines and amine-derivatives, environmental impact resulting from amine emission can not be ignored, when more and more amine-containing wastewaters from chemical plants, refinery plants and pharmaceutical plants are discharged into rivers, seas and also influence greatly on the environment of urban community especially in China.  

Amines are volatile, more basic than ammonia, more toxic than ammonia, and less biodegradable than ammonia. There are several conventional methods which have been applied to the treatment of amine-containing wastewater, such as chemical precipitation, biodegradation, ion exchange, air stripping, and so on. In these methods, air stripping is the most widely used in chemical industry, however with problems of high cost of equipment, high electric power or steam consumption, and secondary pollution of air around.

Nowadays, research on the applications of membrane-based absorption, and supported-gas-membrane (trans-membrane striping/absorption) is quite extensive for removing volatile and reactive components such as O₂, CO₂, NH₃, H₂S, SO₂, HCN, NO, Cl₂ Br₂, and I₂ from gaseous or water streams to another reactive aqueous solution. But until now, there is no report for application of gas-membrane stripping or supported gas membrane process to remove amine from its solution. Compared to conventional methods to remove amine from aqueous solution described above, supported gas membrane process has several prominent features and benefits. Firstly, the stripping of amine from the aqueous solution and the absorption of amine in another aqueous solution which is usually an acidic absorbent solution occur simultaneously in a membrane contactor by replacing a stripping (desorption) column and a absorption column, which provides a maximum driving force for ammonia removal. Secondly, the feed and stripping streams are located in each side of the microporous hydrophobic membrane and cannot influence each other, so the flexibility is greatly improved as to conventional column. Thirdly, the membrane contactor has high packing density and the hollow fiber membrane has small diameter, therefore the membrane contactor can supply much higher surface area per volume compared to conventional columns. Fourthly, compared to conventional separation contactors, the membrane contactor (hollow fiber modules) encounters no back-mixing, wall flow, by-pass flow, dead angle, flooding, foaming entrainment etc, so the overall transfer coefficient of this membrane-based separation process is much higher. And finally, when compared to air stripping or steam stripping, the operational cost of supported gas membrane separation process, almost eliminates the consumption of steam and only consume much lees amount of electricity to drive the wastewater to pass through the membrane module.

Aqueous solutions containing 500 – 10000 mg/L ammine was tested for supported-gas-membrane stripping by using aqueous solution of 5 – 20% sulfuric acid as absorbing solution. The amines tested included methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine and tert-butylamine. Aqueous solution of ammonia was also tested as a reference feed. Experimental data demonstrated that supported–gas-membrane stripping is a highly efficient separation process to remove, to recover, to concentrate and to purify amine from it aqueous solution even containing salt and other volatile organics. It was found that the overall mass transfer coefficient is in such an order:  trimethylamine > ammonia > triethylamine > diethylamine > ethylamine > dimethylamine > methylamine. The overall mass transfer coefficient is a function of the apolarity and volatility of amine (indicated by Henry’s law coefficient), and of its molecular weight (indicated by diffusion coefficient of amine in aqueous phase and in gaseous phase).

A pilot plant-scale demonstration unit was operated for removal/recovery of diethylamine from its aqueous solution containing ca. 2000 mg/L diethylamine, which was distillate drained from a typical artificial leather plant. Techno-economic anylysis demonstrated supported gas membrane process is a highly efficient method to remove amine from wastewaters, even though the ideal by-product is highly purified amine in the concentrated solution.