development of an efficient method for the removal of silane compounds from exhaust air

Research Article

Development of an Efficient Methodfor the Removal of Silane Compoundsfrom Exhaust Air

An overview and evaluation of different technologies is presented, which can beapplied to remove problematic silane and siloxane compounds from exhaust air.In a lab screening, 96 % H2SO4 was identified to be the most efficient and practi-cal absorbent to remove hexamethyldisiloxane (HMDSO) from exhaust air. Labscale results were transferred to pilot scale, where the superiority of 96 % H2SO4

in comparison with a previously used scrubber solution (NaOH/MeOH) wasdemonstrated. In several large scale experiments, the elimination factors ofHMDSO from different exhaust airs by 96 % H2SO4 and the capacity of theabsorbent were determined. The practicability and robustness of the developedscrubber system using 96 % H2SO4 will allow its easy applicability in API produc-tion processes.

Keywords: Active pharmaceutical ingredient, Hexamethyldisiloxane, Scale-up, Screening,Scrubber solution, Sulfuric acid

Received: April 16, 2012; revised: May 31, 2012; accepted: July 13, 2012

DOI: 10.1002/ceat.201200222

1 Introduction

In recent years, silane and siloxane protecting groups andreagents have become widespread and very attractive tools forthe selective and efficient chemical synthesis of complex activepharmaceutical ingredients (APIs). As a consequence, thesecompounds and their resulting emitted exhaust air havebecome part of chemical synthesis routes in production scale.Tight environmental regulations, e.g., BImSchG (German Fed-eral Immission Control Act), [1] require the treatment andpurification of exhaust air from chemical production process-es, which usually contains multiple organic volatile com-pounds, solvents and silane compounds. Among a variety ofdifferent possible technologies, recuperative thermal oxidationprocesses are probably the most practical and cost efficient.However, depending upon the design of the appropriate incin-eration plant, the presence of volatile silane compounds canlead to technical issues caused by the formation of fine SiO2

particles during combustion. Unscheduled technical shutdownof the incineration plant must be prevented by developingeffective methods to remove siloxane compounds from theexhaust air.

In this case study, hexamethyldisiloxane (HMDSO) is gener-ated within the synthesis process of an active pharmaceuticalintermediate by the cleavage of silane protecting groups fromthe target molecule. Due to its relatively low boiling point(101 °C) significant amounts of HMDSO migrate into theexhaust air during subsequent distillation steps together withprocess solvents n-heptane and tetrahydrofurane (THF). Com-bustion of HMDSO leads to the formation of fine SiO2 parti-cles, which tend to cover the recuperator pipes in the combus-tion chamber of the present incineration plant.

Previously, a mixture of methanol (MeOH) and aqueoussodium hydroxide (NaOH) was used as scrubber solution toabsorb volatile siloxanes via physisorption. In this equipmentsetup, HMDSO input concentrations reached 250 g m–3. Dueto the relatively low solubility of HMDSO in the MeOH/NaOH scrubber solution and the inability of MeOH/NaOH tochemically absorb HMDSO, relatively high concentrations inthe range of 10 g m–3 reached the combustion chamber. Infuture, the application of HMDSO absorption by scrubberswill require the evaluation of further scrubber solutions withan elevated HMDSO depletion potential by chemisorption orphysisorption.

The exhaust air in the presented case is characterized by avolumetric flow of 5 to 10 m3 h–1 with short-time peaks of upto 100 m3 h–1 (start of vacuum distillation). The temperatureof the exhaust air was in the range 20–35 °C and the pressureduring distillation procedures was 50 to 100 mbar. Calcula-tions showed that HMDSO concentrations below a threshold

Chem. Eng. Technol. 2012, 35, No. 11, 2023–2029 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com

Birgit Hickstein

Bernd Meynhardt

Oliver Niemeier

Dirk Weber

Boehringer Ingelheim PharmaGmbH & Co KG, Ingelheim,Germany.

–Correspondence: Dr. B. Hickstein ([email protected]), Boehringer Ingelheim Pharma GmbH & Co KG, BingerStraße 173, 55216 Ingelheim, Germany.

Active pharmaceutical ingredient 2023

of 150 mg m–3 would be necessary to ensure a safe and contin-uous operation of the incineration plant. These calculationswere based on a three shift operation of the chemical processand the forecast of API market demand.

As mentioned above, the design of the incineration plantplays a key role in the dimension of the problem caused by theformation of SiO2 during combustion. An alternative regenera-tive thermal oxidation (RTO) process has been presented byLufttechnik Bayreuth GmbH & Co. KG (LTB). ROXSiTherm isa thermal oxidation process with regenerative heat recovery em-ploying spheric ceramic material which functions both as theheat exchanger storage material and as the precipitation sur-face for SiO2 particles. The ceramic spheres are continuouslydischarged and precipitated contaminants are removed.Experiences with this technology are summarized in a reportby Rüskamp et al. [2]. For our purpose, application of theROXSiTherm technology was not considered practical sinceour siloxane (HMDSO) concentration in the range up to250 g m–3 was much higher than in the given literature example.

Adsorption on activated charcoal has become a very com-mon method for removing volatile methyl siloxanes (VSM)from landfill and digester gas (biogas) [3, 4]. Depending onthe activated carbon material, final concentrations in the rangeof 0.1 mgSi m–3 (corresponds to approx. 0.3 mgHMDSO m–3) canbe reached. It has been reported that VSM concentrations inthe range of up to 400 mg m–3 can efficiently be removed frombiogas using activated charcoal with a weekly replacement ofthe charcoal reservoir [5]. However, since siloxane concentra-tions in biogas are typically in the range of approximately24 mg m–3 [6] and therefore very low compared to the afore-mentioned HMDSO concentration in the range of up to250 g m–3, using activated charcoal to deplete HMDSO in amatrix of process solvents as described above is not prac-ticable.

Another method is selective siloxane removal by diffusionthrough a polymeric membrane [7]. Due to high costs [8] thistechnology has so far not found broad application for the pur-ification of biogas [3]. One prerequisite for the application ofthis technology would be a constant 50 m3 h–1 flow of gas. Theabove mentioned flow rates of 10 to 100 m3 h–1 of our applica-tion would not meet this requirement.

Cryocondensation is another well-established technologyand seems to be attractive for high flow rates andelevated siloxane loads [3, 9]. In general, this tech-nique is suitable for air flow rates between 10–100 m3 h–1 and volatile organic compounds (VOC)of 200-1000 g m–3 [10] and therefore matches ourrequirements. Deep chillers have been reported toremove siloxanes from digester gas (1700 m3 h–1,7–15 mgSi m–3) at –30 °C with an elimination rateof 80-90 % [4]. In our case, HMDSO concentra-tion at –70 °C would be approximately 100 mg m–3

depending on the saturated partial pressure. Thiswould be in the range of our desired target concen-tration of approximately 150 mg m–3 as alreadymentioned. However, as the saturated partial pres-sure only applies to ideal gas mixtures, effects suchas icing and aerosol formation could interfere withdepletion efficiency. For this reason, pilot plants

are available on the market, which allow the feasibility and thedepletion performance of this technology to be assessed underreal conditions [11]. Due to high expected investment coststhis technology was considered as a practical but also expen-sive alternative for the presented technical challenge.

On the basis of the discussion above, two technologies,namely the development of suitable scrubber solutions andcryocondensation, were considered potentially suitable for thetechnical challenge presented here. However, the use of scrub-bers was favored due to high practicability and low upfront in-vestments. In the following, the development of suitable scrub-ber solutions from lab scale to production scale is presented.

2 Lab Scale

2.1 Lab Scale Experiments

The most cost-efficient solution might be realized by the re-moval of HMDSO through absorption with scrubbers and anappropriate scrubber solution. As the previously applied scrub-ber solution MeOH/NaOH showed unsatisfactory eliminationresults, our evaluation of an appropriate technical absorptionmethod focused on the search for a suitable scrubber solution.In general, strong bases and acids catalyze the cleavage of Si-Obonds and lead to the formation of poly(dimethyl)disiloxaneswith high boiling points, preventing them from being furtherstripped of the scrubber solution [12]. In the literature, themost effective acids for these chemical conversions are nitricacid (> 65 %) and H2SO4 (> 48 %) [13, 14] with eliminationrates of up to 99 %. In contrast, Selexol™ (dimethyl ethers ofpolyethylene glycol), was reported as a promising solvent forthe physical absorption of siloxanes, resulting in the removalof 99 % of the present siloxanes in a pilot plant [15]. This rep-resents an interesting alternative to the use of strong acids, asfor this type of physical absorption a nontoxic and noncorro-sive solvent is used. Other solvents which have been tested forthe removal of siloxanes are methanol, hexane, tetradecane,acetone, diethylether, cyclohexane and dodecane [13, 14, 16]but no effective absorption has been observed.

The experimental set-up shown in Fig. 1 was applied to in-vestigate different scrubber solutions. A nitrogen stream was

www.cet-journal.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2012, 35, No. 11, 2023–2029

Figure 1. Lab scale experiments.

2024 B. Hickstein et al.

adjusted to approximately 200 mL min–1 and passed over thesurface of liquid HMDSO in the reservoir flask. The resultingHMDSO-enriched nitrogen gas was passed through the scrub-ber solution via a glass drip. A 250 mL double wall flaskequipped with a mechanical stirrer was used as the test scrub-ber. The flask was filled with 300 mL of the correspondingscrubber solution, which corresponds to a fill level of 4.5 cmsolution above the discharge point of the HMDSO gas. Tomeasure the different HMDSO loads in the gas stream, the gaswas analyzed with a gas mass spectrometer (GAM 300, InPro-cess Instruments, Germany) before and after passing throughthe scrubber solution. For a given flow and pressure, the ioncurrent (IC) for m/z = 147 corresponds to the concentrationof HMDSO in the nitrogen stream. To compare differentscrubber media, the elimination factor (EF) was calculated bydividing the ion current of bypassed stream through the ioncurrent of scrubbed stream.

EF � ICin

ICout� cin

cout

The investigated different types of solutions together withoperation temperatures are summarized in Tab. 1. Accordingto the literature, high elimination factors are obtained usingeither nitric acid (> 65 %) or H2SO4 (> 50 %). Nitric acid wasnot considered further as the resulting potential generation ofnitrous gases would cause significant safety issues. As NaOH/MeOH was the currently utilized scrubber solution, its behav-ior was also investigated together with KOH as an alternativebase. To further investigate the method of physical absorption,high boiling hydrocarbons and glycol compounds were consid-ered.

2.2 Lab Scale Results

The investigated scrubber solutions and their ability to de-crease the HMDSO concentration in the gas stream are sum-marized in Tab. 2. The elimination factors were determinedafter 30 min unless otherwise stated. 96 % H2SO4 showed avery high elimination factor of 5700 at 20 °C, even increasingslightly over time. Lowering the temperature to –10 °C resultedin a significant reduction in the ability to deplete HMDSO.Using slight aqueous dilutions of H2SO4 would be preferable

as this would reduce the hazard potential. However, dilutingH2SO4 with water quickly affects the elimination factor forHMDSO. An elimination factor of only 49 at 20 °C wasobtained with 80 % aqueous H2SO4, indicating that the avoid-ance of a significant dilution of scrubber solution with water isimportant for ensuring clean exhaust air. In contrast to theresults from Schweigkofler and Niessner [13], who measuredan elimination efficiency of more than 95 % for HMDSO with48 % H2SO4 at 60 °C, our measurements showed only margin-al depletion in 50 % aqueous H2SO4 within the investigatedtemperature range of –10 to 60 °C. From the small range ofsolvents miscible with H2SO4, primary and secondary alcohols


Table 1. Tested scrubber solutions in lab scale experiments.

Washing solution Variation, Parameter

H2SO4 50–96 %; mixed with different solvents(1-butanol, 1-propanol), T = –10–60 °C

NaOH/MeOH T = –10–40 °C

KOH 50 % T = 20–40 °C; mixed with 1-butanol

H2O/2-propanol (1:2) T = 20 °C

Hexadecane T = 20 °C

Glycol compounds Genosorb 300 + 1348, ethylene glycolT = 0–40 °C

Table 2. Lab evaluation of different absorbant mixtures for thedepletion of HMDSO.

Absorbant mixture T [°C] EF

96 % H2SO4 20 57001

96 % H2SO4 –10 28001

90 % H2SO4 20 4900

80 % H2SO4 20 491

50 % H2SO4 60 1.12

50 % H2SO4 40 1.12

50 % H2SO4 20 12

50 % H2SO4 –10 1.32

96 % H2SO4/1-butanol (1:1) 20 175001

96 % H2SO4/1-butanol (1:2) 20 2031

96 % H2SO4/1-propanol (1:1) 20 44001

96 % H2SO4/1-propanol (1:2) 20 2801

96 % H2SO4/2-propanol (1:2) 20 111

NaOH (4 mol L–1)/ MeOH (1:4) 40 162

NaOH (4 mol L–1)/ MeOH (1:4) 20 101

NaOH (4 mol L–1)/ MeOH (1:4) –10 91

50 % KOH 40 12

50 % KOH 20 1.12

H2O/2-propanol (1:2) 20 3.51

Hexadecane 20 661

Genosorb 1843 40 91

Genosorb 1843 20 401

Genosorb 1843 0 801

Genosorb 300 40 61

Genosorb 300 20 71

Genosorb 300 0 121

Ethylene glycol 20 1.52

1Elimination factor determined after 30 min; 2elimination factordetermined after 5 min, experiment was stopped thereafter dueto low elimination.


were investigated with regard to their suitability for increasingthe depletion potential of H2SO4. 1-butanol and 1-propanolboth showed high elimination factors when used as a 1:1mixture, with 1-butanol being far superior. The highest elimi-nation factor of 17500 was observed after 30 min using a 1:1mixture of 96 % H2SO4 and 1-butanol. Further dilutionresulted in a significant decrease in depletion ability. The per-formance of secondary alcohols was far below that of primaryones. 2-propanol only showed an elimination factor of 11 at20 °C in a 1:2 mixture of H2SO4 and 2-propanol. In spite ofthese promising results using mixtures of 96 % H2SO4 withprimary alcohols, the potential formation of carcinogenic dia-lkyl sulfates by reaction of the alcohols with H2SO4 represent-ed a significant safety issue. Therefore, these mixtures were notfurther considered.

The previously used mixture of aqueous NaOH/MeOH(1:4) resulted only in low elimination factors, decreasing atlower temperatures from 16 at 40 °C to 9 at –10 °C. The perfor-mance of other basic media was investigated with the use of50 % aqueous KOH. At 20 °C and slightly elevated tempera-tures, only a marginal depletion of HMDSO was observed.

The idea of physically absorbing HMDSO was initiallyinvestigated with a water/2-propanol mixture (1:2). However,only a minor depletion was observed at 20 °C. Furthermore,hexadecane as a high-boiling aliphatic hydrocarbon had only alow activity with an elimination factor of 66 at 20 °C. As pre-viously mentioned, dialkylethers of polyethylene glycol showedpromising results in the depletion of siloxanes. Here, Genosorb300 (tetraethylene glycol dimethyl ether) and Genosorb 1843(triethylene glycol dibutyl ether) were investigated for theirability to deplete HMDSO. Depletion increased slightly at low-er temperatures. However, even the slightly better performanceof Genosorb 1843 was still far below expectations. The highestelimination factor of 80 was observed at 0 °C. As a furtheralternative, ethylene glycol was investigated. Only minor deple-tions were observed. These results clearly showed the superior-ity of 96 % H2SO4 over all other investigated scrubber solu-tions.

In a further experiment, the capacity of 96 % H2SO4 for thedepletion of HMDSO was investigated. As the ability to de-plete HMDSO decreases very slowly over the time, the experi-ment was stopped when an elimi-nation factor of 1000 was reached,which corresponds to a depletionof 99.9 % HMDSO. It was shownthat 1.2 g 96 % H2SO4 depletes1.0 g of HMDSO, indicating thatonly small amounts of acid wouldactually be needed, even for large-scale applications.

During chemical synthesis of theAPI, the 96 % H2SO4 as the scrub-ber solution might be diluted bywater carried in the nitrogenstream. This leads to a significantdecrease in the observed elimina-tion factors as shown above. Usinga 90 % aqueous H2SO4 solution asthe scrubber solution, 2.0 g H2SO4

solution was required for the depletion of 1.0 g HMDSO(elimination factor of 200 at the end).

Dilution of 96 % H2SO4 with 25 wt % organic solvents (ace-tonitrile, tetrahydrofuran, methyl-tetrahydrofuran, methanol,methylene chloride (each 5 wt %)) yielded in a decrease of thecapacity of the resulting scrubber solution. For the depletionof 1.0 g HMDSO, 3.8 g of 96 % H2SO4 was required (elimina-tion factor of 1000 at the end). This influence was even moreremarkable by addition of 5 wt % water to the same mixture.The initial elimination factor did not exceed 350. Therefore,the subsequent experimental set-up should include a pre-scrubber to deplete organic solvents from the exhaust air priorto the HMDSO depletion.

3 Pilot Scale Experiments

In lab scale experiments 96 % H2SO4 showed the highest po-tential of all tested solvents for the elimination of HMDSOfrom a nitrogen gas flow. Subsequently, pilot scale experimentswere conducted to evaluate the potential of H2SO4 to elimi-nate HMDSO under simulated production scale conditions incomparison to MeOH/NaOH. To achieve more realistic con-ditions HMDSO was added to the process solvents THF andn-heptane. These mixtures were distilled in production-scaleequipment. The reproduction of the original process with alladditives and reagents was not possible, even under pilot scaleconditions, as it would have been too complex and expensive.

3.1 Experimental Set-up

Experiments were conducted at the Boehringer Ingelheim pilotplant station. The experimental set-up is shown in Fig. 2.HMDSO and the solvent THF or n-heptane were mixed in a100-L vessel. A permanent volumetric gas flow of about 7 m3 h–1

was ensured by nitrogen purge gas, or the application of vacu-um. Nitrogen was dispersed with a dip pipe into the solvents orintroduced over the surface of the solvents to simulate processconditions. Three scrubbers were connected in a series. Theapplied solvents and scrubber solutions are listed in Tab. 3.


Figure 2. Pilot scale experiments.


In experiment 1 (E1) THF was mixed with HMDSO andMeOH/NaOH was applied in all three scrubbers. In experi-ment 2 (E2) the same solvents as in E1 were distilled but differ-ent washing solutions were applied. Scrubber 1 was filled withH2O to remove the water-miscible organic solvents in order toprevent the dilution of H2SO4 in scrubber 2. Scrubber 3 wasfilled with MeOH/NaOH to eliminate evaporated H2SO4 fromthe exhaust air. In experiment 3 (E3), n-heptane was substi-tuted for THF.

The intention of these test assemblies E1 and E2 was to com-pare the efficiency of three MeOH/NaOH scrubbers against asingle H2SO4 scrubber. In E2, H2SO4 had to be superior toNaOH/MeOH in order to justify the more stringent employeesafety precautions. E3 was performed to determine the differ-ences between THF and n-heptane mixtures.

The composition of the exhaust air was analyzed with anonline micro gas chromatograph (CP-4900 Varian, carrier gas:helium, columns: CP-Wax 52 CB and CP-Sil 8 CB Varian,detectors: thermal conductivity detector and differentialmobility detector) before and after each washer (point 1–4).The detection limit amounts to 10 mg m–3 for HMDSO. Thisminimum value was assumed as cout for the calculation of theelimination factor of scrubber 2. Technical details of the scrub-bers are summarized in Tab. 4.

3.2 Results Pilot Scale

The performance of scrubber 2 will be predominantly dis-cussed because most depletion takes place in this part of thesystem. Thus, all presented data refer to the input and outputconcentrations (cin, cout) of scrubber 2.

Fig. 3 compares the performanceof scrubber 2 in E1 and E2. In E1,MeOH/NaOH was applied as awashing solution in each scrubber.In E2, scrubber 2 was filled withH2SO4, scrubber 1 with H2O andscrubber 3 with MeOH/NaOH. Inboth experiments a mixture ofTHF and HMDSO was distilled.

The HMDSO input concentra-tion of scrubber 2 was lower in E1than in E2. Since H2O was used in

scrubber 1 in E2, these data demonstrate that MeOH/NaOHis more suitable for reducing the HMDSO content thanwater. The elimination factor of scrubber 2 in E2 was muchhigher than in E1. With a higher input concentration, H2SO4

was able to achieve a complete depletion, as expected fromlab results. The HMDSO output concentration was reducedfrom 118 to 0 g m–3 with H2SO4. With MeOH/NaOH the out-put concentration increased from 23 to 49 g m–3 during distil-lation time. This is seen as a saturation effect of the scrubbermedium. Although the input concentration was almost tentimes higher in E2, no such effect was visible for the H2SO4

scrubber.The elimination potential of H2SO4 with regard to HMDSO,

THF and n-heptane is shown in Fig. 4. The elimination factorfor HMDSO appears to be higher within the THF distillationin E2 than in the n-heptane distillation in E3. However, asTab. 5 shows, the relatively high elimination factors forHMDSO in E2 arise due to the high input concentrations ofHMDSO which reach 342 g m–3. Within E3, it was not possibleto achieve such high HMDSO input concentrations. Thus,high HMDSO elimination rates such as in E2 would not beattainable even if the H2SO4 were able to eliminate higherinput concentrations.

Over the whole experiment E3, the HMDSO input concen-tration never exceeded 50 g m–3, whereas within E2, 340 g m–3

was achieved. The reason for this is the difference between theTHF/HMDSO and n-heptane/HMDSO systems. The boilingtemperature of n-heptane (98 °C) is comparable to that ofHMDSO (101 °C) whereas the boiling point of THF is signifi-cantly lower (66 °C). Consequently, the composition of the bi-nary HMDSO/THF system will change within the distillationprocess. Here, the THF concentration in the vapor phase will

decrease and the HMDSO concen-tration will increase over timewhereas the composition of the va-por phase of the HMDSO/n-hep-tane system will be constant.

The depletion of HMDSO byH2SO4 is also dependent on thesolvents absorbed, as it might beconsumed or diluted. Therefore,the absorption of solvents shouldbe taken into account. With regardto the elimination of THF andn-heptane, it can be said that theelimination of THF with a maxi-mum elimination factor of 424 is


Table 3. Solvents and washing solutions. Composition of MeOH/NaOH was in all cases: H2O19 wt %, 45 % NaOH 7.7 wt %, MeOH 73 wt %.

Experiment Solvent Scrubber 1 Scrubber 2 Scrubber 3

1 THF (85 kg) + HMDSO(7.8 kg)

MeOH/NaOH112 kg

MeOH/NaOH69 kg

MeOH/NaOH1725 kg

2 THF (75 kg) + HMDSO(9.3 kg)

H2O80 kg/fresh water

96 % H2SO4

35 kgMeOH/NaOH

1725 kg

3 n-heptane (60 kg) + HMDSO(35 kg)

H2Ofresh water

96 %H2SO4

35 kgMeOH/NaOH

1725 kg

Table 4. Technical scrubber details.

ScrubberDescription

scrubber ColumnStoragetank [L]

Pump[m3 h–1] Additional equipment

1

PP jet scrubber:10 m3 h–1, DN 50,

height 0.7 m

Additional packed column:material PP, height 0.9 m,

DN 150, pall packing15 × 15 mm

160 3.5

Plate heat exchanger1 m2

2Steel column with

Halar coatingpall packing 25 × 25 mm,

DN 150, height 1.4 m 165 8 –

3 2 glass columns

packing unknown,DN 225, height 2.5 m 2000 6

Graphite condensator3.6 m2, glas heatexchanger 2.5 m2


relatively good in comparison to n-heptane which is not elimi-nated by H2SO4.

The experiments were run until the output concentrationexceeded 150 mg m–3. From this point, the elimination factorwas close to 1 and it was assumed that the H2SO4 was ulti-mately saturated.

Finally, the amount of HMDSO eliminated bythe H2SO4 was calculated. Therefore, the differencebetween cout and cin was integrated over the timeunder consideration of the measured volumetricflow. Fig. 5 shows cout(HMDSO) as a function ofthe absolute amount of absorbed HMDSO. Ourthreshold of 150 mg m–3 for cout of HMDSO isshown as a dotted line. In the case of E2 (THF dis-tillation), the threshold was reached after HMDSOabsorption of 37 g kg–1 H2SO4, whilst in the case ofE3 (n-heptane distillation) it amounts to 46 g kg–1

H2SO4. This difference between E2 and E3 is con-sistent with the observed disability of H2SO4 toabsorb n-heptane (see Fig. 4). In E2, the H2SO4 isdiluted by THF over time whereas n-heptane doesnot significantly influence HMDSO elimination.

A similar elimination capacity of scrubber 2 inE1 could not be determined because the thresholdof cout = 150 mg m–3 was immediately attained(compare Fig. 3). With the scrubber cascade

from scrubber 1 to scrubber 3 in E1, the threshold of150 mg m–3 was even reached immediately behind scrubber3. Thus, one can conclude that 35 kg H2SO4 are signifi-cantly more efficient for the removal of HMDSO in com-parison to 1906 kg MeOH/NaOH (sum of MeOH/NaOHin scrubber 1–3) for the tested HMDSO solvent combina-tions.

In contrast to MeOH/NaOH the H2SO4 solution tendedto heat up during the washing process until it reached40 °C. As scrubber 2 was not equipped with any heatexchanger, the distillation was stopped at this point andrestarted as soon as the temperature reached 20 °C.

4 Conclusion

In summary, different technologies for the removal ofHMDSO from exhaust air, which originates from a chemi-cal batch process for the synthesis of an API, were evalu-ated. This evaluation process revealed that the develop-

ment of suitable scrubber solutions was most favorable for ourpurpose.

In a screening of several scrubber solutions, concentratedH2SO4 was identified as most promising in lab scale experi-ments. The lab results were transferred to pilot scale whereproduction conditions were simulated. Within the pilot scaletest, H2SO4 was shown to be superior in comparison to the

previously used scrubber solution MeOH/NaOH. The highest HMDSO eliminationfactor was 34000. An uptake capacity of37–46 g HMDSO per kg H2SO4 was deter-mined from exhaust air contaminated withTHF and n-heptane respectively. Abovethis uptake limit, the purified air exceededthe self-set threshold of 150 mg m–3 whichis required to protect our exhaust air in-cineration plant.

These results should be analogously ap-plicable to other volatile siloxanes. In this


Figure 3. HMDSO concentrations of E1 and E2 (cin and cout: input and outputconcentrations of scrubber 2); scrubber E1: MeOH/NaOH, scrubber ;E2: H2SO4;distillation parameters: E1: Ti = 24–34 °C, pi = 945–316 mbar, 50 L distillate; E2:Ti = 33–35 °C, pi = 280–332 mbar, 50 L distillate.

Figure 4. HMDSO and THF elimination factor from E2 and E3; scrubberE2 and E3: H2SO4; distilled solvent E2: HMDSO + THF, E3: HMDSO +n-heptane; distillation parameters: E2: Ti = 33–35 °C, pi = 280–332 mbar,50 L distillate; E3: Ti = 55–64 °C, pi = 375–468 mbar, 35 L distillate.

Table 5. Minimal and maximum input concentrations, corresponding output concentra-tion and elimination factor of HMDSO, THF and n-heptane of E2 and E3 as shown inFig. 4.

cin min [g m–3] EF cin max [g m–3] EF

HMDSO E2 16 640 342 34200

THF E2 57 244 90 193

HMDSO E3 15 1500 17 1700

n-heptane E3 141 1 155 1


paper, only the depletion of HMDSO was determined since itis the most relevant and volatile siloxane compound in oursystem.

Further steps will include the transfer of the scrubber pro-cess to production scale and a comparison with a cryoconden-sation process. The results of the performance of both systemswill be published in the near future.

Acknowledgment

The authors would like to thank W. Talies (Micro-GC, Curren-ta GmbH) and F. Bottlender (gas mass spectrometry) for excel-lent analytical support and fruitful discussions. Furthermore,we gratefully acknowledge the support from Dr. J. Schröderand T. Alt (both BI pilot plant) as well as from master’s stu-dent B. Yücel and T. Fachinger (lab experiments).

The authors have declared no conflict of interest.

Symbols used

cin [g m–3] input concentration of scrubber 2cout [g m–3] output concentration of scrubber 2EF [–] elimination factorIC [–] ion currentT [°C] temperature

Abbreviations

API active pharmaceutical ingredientBImSchG federal Immission Control ActE1, E2, E3 experiment number 1, 2, 3FI flow indicator

HMDSO hexamethyldisiloxaneLTB Lufttechnik Bayreuthm/z mass to charge ratioMeOH methanolNaOH sodium hydroxideRTO regenerative thermal oxidizerTHF tetrahydrofuranVMS volatile methyl siloxaneVOC volatile organic compounds

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Figure 5. cout (HMDSO) as function of absorbed HMDSO perkg H2SO4. The dotted line represents the internal threshold of150 mgHMDSO m–3 for the purified air.


development of an efficient method for the removal of silane compounds from exhaust air

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