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Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Chinese Journal of Chemical Engineering,16(3) 461ü464 (2008)

RESEARCH NOTES

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas*

ZHAO Shunhua ( )1, SONG Xijin ( )2,**, PEI Ning ( )2, ZHANG Jingzhu ( )3 and LIU Shuqing ( )3

1 Zhejiang Hexin Industry Group Co., Jiaxing 314003, China

2 Department of Chemical & Biochemical Engineering, Zhejiang University, Hangzhou 310027, China

3 Yantai Dongjie Environmental Protection Machine Engineering Co., Yantai 264002, China

Abstract A new recovery technology is developed to recycle N,N-dimethyl formamide (DMF) in waste gas from wet type polyurethane synthetic leather industry. Given that the concentration of DMF in waste gas was as low as 325.6 688.3 mg·mˉ3, it was necessary to make sure two phases contact adequately and strengthen the mass transfer by increasing contact area and enhancing the turbulence. Therefore, two-stage countercurrent absorption and two-stage fog removing system were introduced into the technology. The top section of the absorption column was filled with structured wire-ripple stainless steel packing BX500, while the lower section with sting-ripple packing CB250Y. Total height of packing material was 6 m. In addition, there were both two-stage fog removing layer and high efficiency liquid distributor at the column top. All the operating parameters, including temperature, pressure, flow rate and liquid position, could be controlled by computers without manual operation, making sure the outlet gas achieved the national emission standard that the DMF concentration should be below 40 mg·mˉ3. The whole equipment could recover 237.6 t of DMF each year, with the profit up to CNY 521×103.

Keywords N,N-dimethylformamide, polyurethane synthetic leather, absorption, recovery

1 INTRODUCTION

There have been more than 200 polyurethane (PU) synthetic leather corporations in China equipped with over 500 dry and 300 wet type product lines on a large scale by the end of 2006. However, N,N-dimethylformamide (DMF), which is widely used as solvent during the production process, would cause serious pollution to the air and water resources if waste gas is discharged to the environment without purification. Due to its poor bio-toxicity, it would im-pede hematopoiesis or cause liver impediment [1] if people inbreathe or contact DMF for a long period.

Chinese Ministry of Health has announced GBZ2-2002 “Occupational Exposure Limit for Haz-ardous Agents in the Workplace”, demanding that the highest level of DMF in the workplace should be be-low 40 mg·mˉ3. To develop PU leather industry in a benign way, more attention should be paid to the clean production and the environment problems [2 4] and the recovery technologies [5].

Washing absorption method is applied to meet the need of recovery technology based on the charac-teristics of PU leather production line [6], such as the low temperature of waste gas, rather large amount of waste gas, the low concentration of DMF, miscibility of DMF and water. Ou [5] designed the treatment technology for DMF waste gas from dry type produc-tion line with the washing absorption method. As a result, the DMF recovery was 80.75%. Other coun-tries, like Russian, Italy, Japan, had also published many papers mainly in 1970s to 1990s [7 9], but few related to the practical technology. Otherwise, some countries have not formed imperative standard be-cause DMF concentration in wet type waste gas is comparatively low.

In the present study, a new technology was estab-lished to deal with wet type waste gas and appropriate devices were designed to achieve the task of absorp-tion. Trail operations were tested in Zhejiang Hexin Industry Group Company in spring 2007. The results showed that the DMF content in outlet gas was con-trolled below 40 mg·mˉ3, meeting the regulated DMF limitation in the workplace and the national emission standard. The recovered DMF from both waste gas and tanning waste water could be reused as raw mate-rial or sold as byproduct. The whole recovery equip-ment could recover about 237.6 t of DMF each year from product lines with the profit totaled to CNY 0.52 million.

2 TECHNOLOGICAL PROCESS

2.1 Technical specification

At present wet type production process was the most popular technique adopted in PU synthesis leather industry. In this process, polyurethane resin is dissolved by DMF, and then coat it on the base fabric, finally the fabric is sent through the DMF solution (concentration about 20%) to concrete leather. It was really difficult to prevent DMF from volatilizing into the air because of the long flowsheet and the difficulty in equipment air-tightness. If not treated well, each product line volatilized 80 t of DMF into the air each year, so the recovery treatment should be considered.

As DMF dissolves rather easily into the water, soft water was chosen the absorbent. After being col-lected, wet type waste gas was transported into the absorption column by the high efficiency variable

Received 2007-09-22, accepted 2008-03-25.

* Supported by the Science and Technology Fund of Zhejiang Province, China (2006C13072). ** To whom correspondence should be addressed. E-mail: xjsong@http://www.wendangku.net/doc/e5e8a9b9f121dd36a32d82d7.html

Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008

462frequency fan. After absorption, the gas was up to the standard and could be discharged directly into the air, while the DMF water solution was sent to the distilla-tion process for further concentration of DMF. Recov-ered DMF could be reused as raw material or sold as byproduct after condensation treatment.

This recovery system comprised 4 main parts: waste gas transportation and discharging system, ab-sorption column system, absorbent circulation system,

automatic control and safety alarm system (Fig. 1).

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Figure 1 DMF waste gas absorption column

1—liquid inlet; 2—gas inlet; 3—gas outlet; 4—top liquid dis-tributor; 5—two-stage fog removing layer; 6—packing layer BX500; 7—liquid redistributors; 8—packing layer CB250Y; 9—liquid spray implements; 10—liquid outlet; 11—automatic outlet valve; 12—circulating pump

Waste gas transportation and discharging system Being collected from wet type product lines by the gas-collecting hood, DMF waste gas went through the filtration device to get rid of dust. After purifica-tion, the gas was transported into the absorption col-umn by the high efficiency variable frequency fan.

Absorption column

The total height of absorption column was about 18 m and the diameter was 2.65 m. Given that the content of DMF in waste gas was quite low, it was necessary to enhance the mass transfer and make two phases contact adequately by increasing the contract area and enhancing the turbulent strength. Therefore, every device was specially designed to meet the de-mand. The top high efficiency slot style liquid dis-tributor [10] was installed to make sure of uniform distribution. There were 150 tiny holes per square meters in the top distributor. Below the distributor, there was a two-stage fog removing layer that was loaded with corrugated acanthopore packing CB250Y [11] to capture the mists with DMF dissolved. Two-stage countercurrent absorption packing layer was the key part of the absorption column system. The upper layer was with structured corrugated wire gauze

stainless steel packing BX500 [12], while the lower stage with packing CB250Y . Each layer was 3 m high. In addition, an authors designed liquid redistributor of self-distributing packing, about 200 mm high, was fixed between them. Waste gas went into the column from the bottom of the column, while the absorbent came from the top. When the absorption system started to run, the absorbent irrigated the packing lay-ers. In the mid column, in order to get liquid mists, there fixed a set of circulation solution spray equip-ment that consists of a central nozzle of 25 mm di-ameter with twelve small nozzles of 15 mm diameter around the center one. The total column pressure drop

was only 2400 Pa when gas flow rate was 60000 m 3·h ˉ

1.

Absorbent circulation system

The absorbent was circulated in a great flow rate

of 50 m 3·h ˉ

1 by a circulating pump. Once the concen-tration of DMF in circular solution reached 8% 12%, the online control system switched the automatic out-let valve to draw high concentration DMF solution to the storage tank until the liquid level dropped by about 50%, then fresh water was complemented through the pump. The whole process was controlled automati-cally by the online concentration measuring instru-ment, auto regulating valve and liquid level regulator.

After the treatment, waste gas met the national standard and was discharged directly into the air. The average absorption achieved 95%. 2.2 Operation

The recovery system started to run in 2007 spring. After several tests, it was found that the absorption effect was quite good, the bad smell in the air was basically eliminated, and the environment of the workshop and the surrounding was greatly improved. In successive examination tested by Jiaxing Environ-mental Protection Bureau, the concentration of DMF

in the discharged gas maintained below 40 mg·m ˉ

3that the national standard [13] requested.

The properties of the packing are decisive to the effect of absorption. Packing BX500 and CB250Y are both used to strengthen mass transfer after careful se-lection. The structured packing was manufactured by Zhejiang Jiaxing Zhongda Group [14]. The perform-ance parameters of CB250Y and BX500 were listed in Table 1.

Table 1 Packing performance parameters

Mod el Peak height /mm Specific surface area /m 2·m ˉ3Theoretical plates /piece·m ˉ1Poros-ity/%Pressure drop

/MPa·m ˉ1F-factor

/m·s ˉ

1·(kg·m ˉ

3)0.5CB250Y 12 250 2.5 3 97 3.0×10ˉ4 2.6 BX500

6.3

500 4.5 5.5 95 3.0×10

ˉ4

2

The operating results of the absorption process were summarized in Table 2. The operation was under normal temperature and pressure. As the inlet DMF concentration and waste gas flow changed with the time, water flow was modulated to optimize absorp-tivity. The final absorptivity maintained above 95%.

Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 463

At the same time, the outlet DMF concentration was another index that can be used to estimate the absorp-tion effect. According to the test results, the best prac-tical operation conditions are as follows: waste gas flow 60000 m3·hˉ1, top water flow 0.450 m3·hˉ1, cir-culated solution at 50 m3·hˉ1.

The average concentration of DMF in absorbent was 11.3%. Once the concentration of DMF in circu-lar solution reached 8% 12%, the online automatic control instrument (UR-20, Maselli, Italy) would turn on auto regulating valve to discharge DMF solution into the storage tank. To gain pure DMF, absorption solution was further treated by a self-designed energy saving distillation process with the yield being as high as 99%. The distillation technology was mainly com-posed of three columns: a vacuum condensation col-umn, an atmospheric condensation column, and a vacuum distillation column. The process could be seen in Fig. 2. The specific calculation data in the distilla-tion process was illuminated in Ref. [15].

2.3 Economic analysis

Every day it recovered 8 t solution with the av-erage DMF concentration of 10%. If calculated ac-cording to 99% distillation yield and 300 workdays a year, the outcome was 237.6 t of pure DMF equivalent to 0.52 million CNY every year.

The detail of analysis is included in Table 3. The market price of DMF was estimated by 7300 CNY·tˉ1, electricity 0.71 CNY·(kW·h)ˉ1, stream 119 CNY·tˉ1, and water 3.99 CNY·tˉ1. Every year the costs were: electricity 434520 CNY; device depreciation (calcu-lated by 10 years) 135000 CNY; manpower 25000 CNY. In DMF concentration process, total cost in-cluding water, electricity, manpower, device deprecia-

tion and tax was 366720 CNY annually. Final annual economic profit was 521384 CNY.

Table 3 DMF waste gas absorption technology economic

benefits analysis

Item Amount

Price

/CNY

Annual profit/

CNY

water recovered 0 electricity 85×24×300

kW·h

0.71 ˉ434520 equipment depreciation1 set (for 10 years) 1350000 ˉ135000

DMF condensation 1 set?ˉ366720 manpower 1

person

25000

ˉ25000 added value tax 237.6 t 1060 ˉ251856

recovered DMF 237.6 t 7300 1734480

saving/CNY 521384?

? Including water, electricity, stream, manpower, equipment depreciation and so on.

? 521384=1734480ˉ251856ˉ25000ˉ366720ˉ135000ˉ434520.

Table 2 Wet type waste gas absorption column running results

Water flow Waste gas Absorption solution

Top inlet /kg·hˉ1Cycling solution

/m3·hˉ1

Flow rate

/m3·hˉ1

Inlet DMF

/mg·mˉ3

Outlet DMF

/mg·mˉ3

Outlet DMF

/%

Flow rate

/kg·hˉ1

Absorption rate

/%

250 50 45000

325.6 13.5 9.1 150 95.4 350 50 55000

586.5 33.2 14.5 210 94.3 400 50 61000

529.8 26.1 12.8 240 95.1 450 50 60000

436.5 20.2 9.3 270 95.4 500 50 68000

653.7 35.1 14.0 300 94.6 600 50 75000

573.7 25.6 11.4 360 95.5 800 50 58000

688.3 21.3 8.1 480

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

Recovery Technology of DMF from Wet Type Polyurethane Synthetic Leather Waste Gas

96.9

Figure 2 Energy-saving DMF recovery technology flow chart

C1üvacuum condensation column; C2üatmospheric condensation column; C3üvacuum distillation column; 4üheat exchanger

Chin. J. Chem. Eng., Vol. 16, No. 3, June 2008 464

2.4 Discussion

The recovery technologies for the removal of DMF from wet type waste gas have not been found in the open literature yet. Wet type waste gas may have been ignored because of the difficulties in waste gas collection, mass transfer and condensation processes. The present technology has been put in operation with success. Some problems remain to be resolved, such as the huge fan energy consumption of 85 kW. Further development efforts should be taken.

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