CHARACTERISATION OF NON- CONDENSABLE SULPHUR CONTAINING GASES FROM KRAFT PULP MILLS

Transcription

1 Chemical Eng. Dept., ISEL From the SelectedWorks of João F Gomes 1998 CHARACTERISATION OF NON- CONDENSABLE SULPHUR CONTAINING GASES FROM KRAFT PULP MILLS João F Gomes Available at:

2 PII: SO (98)00121-O Chemosphere, Vol. 31, No. 7, pp , Elsevier Saence Ltd All rights reserved. Printed in Great Britain 004% $ CHARACTERISATION OF NON-CONDENSABLE SULPHUR CONTAINING GASES FROM KRAFT PULP MnLS Jogo C.M. Bordado (l), Jogo F.P. Gomes (2) (1) (2) Chemical Engineering Department, Technical University of Lisbon, Instituto Superior Tecnico, Av Rovisco Pais, 1096 Lisboa Codex, Portugal Environmental Technologies Centre, Instituto de Soldadura e Qualidade, Apartado 119, 2781 Oeiras Codex, Portugal (Received in Germany 4 January 1998; accepted 30 March 1998) ABSTRACT This paper describes work performed on the sampling and analysis of non-condensable gases (NCG) emitted from diffise sources of a Portuguese kraft pulp mill, which is the background information for a NCG collection, treatment and disposal system. The variability found in the composition of the gaseous compounds showed the existence of gaseous streams other than typical total reduced sulphur (TRS) compounds as usually described. in1998 Elsevier Science Ltd. All rights reserved KEYWORDS Kraft Pulp Mills ; Reduced Sulphur Compounds ; Non-condensable Gases 1. OVERVIEW OF THE PROBLEM The typical odour associated with pulp and paper production is due to the generation of reduced sulphur compounds such as hydrogen sulphide (H$), methyl mercaptan (CHSH), dimethyl sulphide (CHSCH3) and dimethyl disulphide (CHSSCH3). These compounds have very low olfactive detection levels. This explains their detection by humans even in small quantities and at great distances from emission sources. At these detection levels the toxicity of these compounds is negligible. However, being a nuisance, they are subjected to particular attention as the pulp and paper industry is continuously faced with more stringent air emission limits concerning gaseous pollutants. Table I presents olfactive detection as well as toxicity levels for these compounds. Table 1 - Olfactive detection and toxicity levels for some sulphur species Olfactive Toxicity level detection level Compound (ppm) (mg/m3) (ppm) (mg/m3) SOZ I,o-5, HZS 0,0009-0,0085 0,0013-0, CH$H 0,0006-0,040 0,0012-0,08 0,5 1 CHSSCHS O,OOOl-0,0036 0, ,009 _ * _ * CH,SSCHs (*) - (*) (*) not established, so far These reduced sulphur compounds may be found in several waste streams from various processes within the pulp mill. Undesirable sulphur compounds are found in the recovery boiler gases produced by the Krafi process, foul condensate, foul condensate tank off gases and undertlows, sludge de-watering operations, 1235

3 1236 batch and continuous tall oil acidification, brown stock washer hoods, digester blow vents, turpentine production, waste treatment areas and non-condensable gases (NCG) from the limestone kiln. These non-oxidised sulphur compounds are typically referred to as TRS (Total Reduced Sulphur). TRS typically comprises high levels of hydrogen sulphide along with lower levels of the mercaptans mentioned before [I]. The total TRS levels and composition vary considerably from each point in the Krafi pulp mill. These values also vary according to the age of the mill. Newer mills, with high efficiency recovery boilers, typically produce TRS levels of less than 20 ppm. At older Krafi mills TRS levels approaching 2000 ppm have been observed. Other significant stationary sources of TRS are the limekilns and incinerators of KraR mills. In these two cases, TRS are usually accompanied by high levels of carbon dioxide : 10 to 50%. These high levels of carbon dioxide present difficulties when TRS reduction is required and is to be achieved by wet scrubbing systems. Other alternatives exist for the abatement of the sulphur compounds which include steam stripping of gases [2], which was found to be more effective when treating condensates then when treating gaseous streams [3]. Other abatement possibility is the collection, treatment and disposal of the noncondensable gases generated during the pulping process. A recent [4] abatement system includes provisions for the collection and disposal of High Volume-Low Concentration (HVLC) gases, such as incineration systems as part of a larger project involving the addition of a new pulp washing line. The system was later modified to include a dedicated incinerator for incineration of these gases, together with other pulp mill noncondensable gases. As it is expected that the regulatory pressure will increase and result in lower emission levels for TRS, collection and incineration of these gases will continue to receive more and more detailed attention. However, the collection of dilute gases requires specialised consideration, calling for careful attention during all project phases. It is now accepted that through sound design, the safe and efficient collection of noncondensable gases can be accomplished [ DESCRIPTION OF PERFORMED WORK The work described in this paper was based on the need to evaluate the set-up of a NCG collection and disposal system at an existing Portuguese Kraf? pulp mill. This mill is the biggest Portuguese plant producing bleached eucalyptus ECF (elemental chlorine free) pulp, 430,000 t/year. The first step of the work consisted on the accurate determination of concentration of gaseous sulphur species emitted throughout the mill. The search included the most probable ones due to process characteristics and also aimed to contribute to the elimination of those sources with larger incidence on odour problems. The sources considered were the following ones, by process sections, which are shown schematically in Figure 1 : a) wood digestor section a. I. diffusion washer 1 - LD 1 A a.2. diffusion washer 1 - LDIB a.3. diffusion washer 2 - LD2 a.4. concentration unit - ADA a.5. concentration unit - ADB a.6. chip bin - SAP a.7. foul condensate tank - TCS a.8. vibratory machines - CVIB a.9. conical tank - TC I a. 10 filtrate tank from diffusion washer - TFLD2

4 1237 b) recovery section : b. 1. spill liquor tank - TO62 b.2. weak black liquor tank 5 - TLF5 b.3. weak black liquor tank 4 - TLF4 b.4. weak black liquor tank 3 - TLF3 b. 5. foul condensate tank - TO3 1 b.6. secondary condensate tank - TO32 b.7. spill liquor tank - TO30 b.8. dump tank - 321T011 b.9. black liquor tank - T115 b. 10. black liquor tank - T 150 b. 11. heavy black liquor tank - TO20 b.12. liquor feed to 1st phase tank - 321T023 b. 13 heat exchanger - 32OWO45 b. 14. heat exchanger - 22OW345 b. 15. gas scrubber - 320F043 b. 16. separating valve from evaporator II - VSBE2 b. 17. separating valve from evaporator III - VSBE3 b. 18. stripping column - STRIP Figure 1 - Process schematic showing sampled sources Gases I3,, onic :a1 [ Gas stripping - (320FC43, STRIP) Recovery Boiler Condensates and (VSBE2,VSBE3) (T062, TLF3, TLF4, TLFS) Concentration WA, AW Bleached t-l H I Pulp (Tl I 5, Tl 50, TOZO, 32lT023)

5 EXPERIMENTAL PROCEDURE The sampling and subsequent analysis of reduced sulphur species presents certain difficulties due to the unstable nature of these compounds [6], [7]. It was noted that if the period between sample collection and analysis exceeded 20 minutes, a significant decrease in the reduced sulphur species concentration occurs. This decrease was found to depend on time due to the formation of oxidised compounds by atmospheric oxygen. Therefore, it was decided to install the analytical equipment in locations quite near the sampling sites so that a maximum of 10 to 15 minutes would separate sampling from analysis. The samples were taken from the sources described above by using a anti-deflagrant vacuum pump and collected in Teflon bags or in borosilicate glass bottles. The use of Teflon bags proved to be more efficient for the sampling. The gaseous streams carry, in general, large quantities of moisture which are unavoidably collected in the bags. Precautions were taken to avoid injection of water into the gas chromatograph. The gas chromatograph system used for this purpose was a Hewlett-Packard , equipped with a flame photometric detector (FPD). The chromatograph injection circuit was modified by the introduction of a manual gas valve and a loop to allow injection of samples directly from the collection bag. The chromatographic column used was a Supelpack-S (Supelco Co.), 76.2 cm long, 45.7 cm packed of Porapak QS (a porous polymer composed of ethylvinyl benzene cross-linked with divinyl benzene to form a uniform structure, Dow Chemical Co.) x l/8 OD in Teflon. The column is conditioned at 25 C during 1 minute and then heated to 210 C at 30 C/minute. The flow rate of helium, as the carrier gas was of 30 ml/mm, and the approximate size of each sample injected into the column was of 2 ml. This column packing is specially prepared for separation of H$, COS, SO*, CH?SH, CH3SCH3 and CHsSSCH3 in kraft mill effluent and other sources [8]. The main advantages of this column are that moisture is eluted quickly and does not interfere with the sulphur gas peaks, the column heating can be programmed to 2 10 C, HzS is efficiently separated from COS and the short column minimises back pressure [S] High purity TRS gases in nitrogen obtained from Air Liquide, France, were used as calibration standards. A typical cbromatogram presents distinct signals for each species having different and defined retention times.

6 4. OBTAINED RESULTS The obtained results are presented in Table II for each source sampled, expressed in ppm and also in mg/m Table II - Measured TRS concentratic ms Ll 1 CHISCHI ICHaSSCHa._. _.,- TI FA I<5 I<7 7 ~732 ~732 <7,2 c73 LD2 Cl5 <21,6 -=30 < a Cl0 c3a >a5 >212,5 la5 462, >200 >519,5 20, >230 a614 >0, t-435 >I1525 >o ADA..I ~5 <7, AM! <5 *-r-l.i,l,*n \I 7n._ L 11 17flfiA1?AA57o lo 38>260 >663 so,03 912, >430 >ioa7,5 >0, <in.,_ <?R - '70 >175 so, :39?R" 97,l-l 4730 ii %q I I ----( I ,48 -I.- <20 Cl0 ~25 Cl0 c3a <35 c90, ~10 ~ >,43120 ( 84527,21>4, ~10000 ~ > ~ b25.47,7n, f=.1c.al-l la6675 a ~ : I.-v ,7033 T&i& _ i?ii%i$

7 DISCUSSION It can be seen that the gas chromatograph detector response is quite different for each sulphur species being analysed. This explains why the lower detection level for CHXSCHs is as high as 10,000 ppm when the sample collected at VSBE2 was being analysed. In fact, the concentration of HzS and the other gases was so high that it was quite impossible to determine lower concentrations for CHSCH3 From this data at least three types of NCG sources can be identified : i) sources where no significant HzS concentration is found but mercaptans exist (e.g. TLFS, ADA) ii) sources where both HzS and mercaptans are found in large quantities (e.g. TCl, 32OWO45, 32OW345) iii) sources without significant concentrations of H2S or mercaptans (e.g. TLF4, T030, CVIB) These results show the existence of gaseous streams other than typical TRS ones comprising high levels of hydrogen sulphide along with lower levels of mercaptans as referred usually in the literature [I], [4]. 6. CONCLUSIONS The obtained data will now be used for evaluating the possible alternatives for collection, treatment and disposal of these gases. In new pulp mills, the need for collection of NGC gases from sources must be considered in initial design of the collection points. Retrofitting to existing systems can sometimes be difficult because source equipment was not originally designed to withstand vacuum conditions. In either case, new or existing sources, it is critical to have reasonable estimates of the expected flow, pressure, temperature and composition of each NGC source. Failure to properly analyse the amount and composition of gases that will be present can result in an expensive, oversized system, or worse, a system which is undersized and not able to collect all the gases which will be generated from the various sources. REFERENCES 1. Trauffer, E.A., A New High Etficiency, Low Cost TRS Scavenging System, Proceedings of the 1994 TAPPI International Environmental Conference, Vol. 2, pp Guttierez, L., Mueller, J., Walden, C., Steam Stripping of Condensates Removes TRS Contaminants, PuIp and Paper Canada, 79(9), T280 (1978) 3. Pu, Q., Messmer, R., Smith, L., Caron, A., Steam Stripping of Kraft Foul Condensates to Reduce TRS and BOD, Proceedings of the 1994 TAPPI International Environmental Conference, Vol. 2, pp Giarde, D.K., Crenshaw, M., Collection and Incineration of High Volume-Low Concentration Pulp Mill Noncondensible Gases, Proceedings of the 1994 TAPPI International Environmental Conference, Vol. 1, pp Wright, J.M., Lund, G., Key Design Features of the DNCG System at Howe Sound Pulp & Paper Ltd., Proceedings qfthe TAPPI International k.kvironmentai Cortference, Vol. 1, pp Haunold, W., Georgii, H.W., Ockelmann, G, Gas Chromatographic Analysis of Atmospheric Sulfur Dioxide and Reduced Sulmr Compounds, I,( -<;( International, 5( IO), (1995) 7. Larson, L.J., An Investigation of the Utility of Commercial Mixtures of TRS Gases in Nitrogen for Calibrating Portable Gas Chromatographs, NCASI, New York (Jun. 1995) 8. de Souza, T.L.C., Lane, D.C., Bhatia, S.P., Analysis of Sulfur-Containing Gases by Gas-Solid Chromatography on a Specially Treated Porapak QS Column Packing, Anal. ( hem., 47 (3), (1975)