How is the recovery of chemicals used or produced as a by-product during fibre production categorised?
The ZDHC requirements for chemical recovery rates are categorised at three levels – Foundational, Progressive and Aspirational. This encourages continuous improvement activities at a facility to achieve higher limits of recovery of chemicals used and move towards circularity.
For viscose staple fibres and modal, the recovery of sulphur compounds due to the use of CS2 in the process needs to be monitored. This is also linked to the level of kg of sulphur emitted to air per tonne of fibre produced.
Similarly, the recovery rates for sodium sulphate in viscose staple fibres and viscose filament yarn process, NMMO (N-Methylmorpholine N-oxide) in the lyocell process, ammonia and copper in the cupro process, and acetone in the acetate process at the three levels are also outlined. (Pg No. 20-26, MMCF V2.2)
The achievement of these levels is monitored through the MMCF module on the ZDHC Supplier Platform
What are the different technologies for recovery of carbon disulphide (CS2) in viscose fibre manufacturing and how it is calculated by mass balance calculation?
CaCO3: Calcium carbonate
Ca(OH)2: Calcium hydroxide
CO2: Carbon dioxide
CS2: Carbon disulphide
H2S: Hydrogen sulphide
H2SO4: Sulphuric acid
NaHS: Sodium hydrosulphide
Na2S: Sodium sulphide
SOx: Sulphur oxides
Fig– Viscose Fibre Manufacturing and S Recovery Process
Mass Balance calculation used for CS2 Recovery & Sulphur Air emission
CS2 Recovery = O1 + O2 + O3 + O4 + O5 + O6
Sulphur emission to air = I1 - (O1 + O2 + O3 + O4 + O5 + O6)
CS2 recovery techniques and a mass balance calculation
In viscose manufacturing spinning frames are one of the sources of CS2 emissions. These emissions can be avoided by the housing of spinning lines. For operations, the housing has to be equipped with leak-proof sliding windows. To avoid the accumulation of harmful and explosive gases, suction systems are installed in the housing from where CS2 is purged to a recovery facility. CS2 is mainly recovered by condensation and activated carbon adsorption process (CAP), but to achieve a higher recovery percentage various other techniques can also be implemented by the industry. A brief overview of the CS2 recovery techniques is given below.
Total CS2 recovery (Kgs/tonne of fibre) = (CS2 recovery from condensation route in Kgs/tonne of fibre) + (CS2 recovery from exhaust gases through CAP & other techniques unit in Kgs/tonne of fibre)
I1) CS2 addition to reactor including- Fresh input and CS2 recovered from the process
01) CS2 recovery by condensation recovery- CS2 and steam-saturated air pass through the condenser where the gases are extracted by a water jet, in which more CS2 is condensed by the cold water. In simple terms; the gases trapped in fibres are released by steam injection and CS2 vapours produced are condensed in indirect condensers using soft water & chilled water.
The recovered CS2 is of high purity, so can be used in the viscose process again with or without refining depending on the quality of CS2 in terms of total solids present.
O2) CS2 recovery by activated carbon adsorption (CAP)- The exhaust gases from the spinning and acid recovery section comprises of both H2S & CS2. To recover CS2 through CAP it is essential to remove the H2S from the gas stream otherwise it will damage the carbon bed. Therefore before CAP, the first step is to remove H2S. This is achieved by either alkaline scrubbing (03) in which H2S is converted to NaHS or Na2S or the redox process wherein H2S is converted into sulphur. The CS2-rich gas is then taken to the CAP unit where gases are passed through the activated charcoal bed (adsorber) wherein CS2 is adsorbed. Once the bed is saturated, the adsorber is isolated and taken for CS2 recovery and other stand-by adsorber is taken back into the production line.
O3) Removal of H2S as NaHS or Na2S by alkaline scrubbing (wash & spray)
To avoid H2S contamination of the gas stream in CAP, the gases are treated in a NaOH scrubber (washer) prior to CAP unit. In the scrubbing process, sodium hydroxide reacts with H2S dissolved in an aqueous solution to form NaHS and Na2S. This takes place in an absorption unit consisting of two stream scrubbers operating with diluted sodium hydroxide. Followed by a centrifugal scrubber, where NaOH spray fog is removed. NaHS can be valuable products.
O4) Conversion of H2S and CS2 into H2SO4 with oxidation by Wet sulphuric acid process (WSA)
With high concentrations of sulphur compounds in the exhaust air (>0.5 vol-%), there is a wider choice of techniques available. In practice, the combustion of the exhaust air by catalytic oxidation to sulphuric acid is often chosen. This process is only economically viable if the acid can be produced at desired concentrations.
The H2S gas is burnt at 350oC on a noble metal catalyst to form SO2, Then is oxidised to SO3 in one step on a wet catalyst (V2O5). The gases containing SO3 are condensed at 250oC, which leads to an approximately 88 % sulphuric acid.
O5) Conversion of H2S and CS2 into SOx by exhaust gas incineration/boiler followed by scrubbing of flue gases by lime to produce gypsum
Calculation method for O5 (gypsum)
Incineration in a coal-fired boiler
Sulphur from the viscose process and coal is fed to a boiler or incinerator and gets converted into SOx. The SOx produced is scrubbed (removal of impurities from a gas by chemical means) by lime to make gypsum. The flue gas from the boiler will have some remaining unscrubbed sulphur in SOx form. The purity of gypsum varies depending on the flue gas desulphurization process applied.
Explanation about gypsum on scrubbing of flue gases from Captive Power Plant (CPP )Boilers (O5)
O5 calculation : sulphur in (A+B) - sulphur out (C+D)
A: Exhaust flow rate from VSF in M3/hr x equivalent sulphur mg/m3 (from CS2 and H2S)
B: % sulphur in coal + quantity of coal burnt in boilers in metric tonne (MT)
C: SO2 in CPP boiler stack flue gas flow rate Nm3/hr x SO2 mg/Nm3)x32/64
D: (gypsum produced MT) x (32/120)
CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g)
Ca(OH)2(s) + SO2(g) → CaSO3(s) + H2O(l)
S=solid, l=liquid, g=gas
O6) Conversion H2S, CS2 or both to sulphur by biological or catalytic processes or redox process
Example of Biological Process:
Currently, in the Claus, Stretford or other processes, hydrogen sulphide is removed from process gas streams by a series of reactions at high temperatures to produce elemental sulphur. These physicochemical processes have high investment and operating costs, as well as often being restricted by contaminants, due to not effectively removing all the H2S. As an alternative, the anaerobic, photosynthetic bacterium, chlorobium thiosulfatophilum, has been demonstrated to convert hydrogen sulphide to elemental sulphur in a single step at atmospheric conditions. The autotrophic bacterium uses CO2 as the carbon source. Energy for cell metabolism is provided by incandescent light and the oxidation of H2S. A pilot study has been performed in a continuous stirred tank reactor (CSTR) equipped with a sulphur separator. Optimum process conditions have been achieved to maximise cell growth and elemental sulphur production. Near total conversion of H2S is achieved in a retention time of a few minutes. High concentrations of H2S or organics do not affect the culture. Sulphur recovery by settling is very efficient and near theoretical yields of sulphur are achieved. To reduce the sulphur emissions these advanced technologies might need to be adopted by the industry in the near future.
Sodium sulphate recovery in viscose staple fibre production
Recovery of sulphate from spinning baths
During the viscose fibre manufacturing process, sodium sulphate produced as a by-product can be crystallised as Glauber salt. The spinning bath solution containing sodium sulphate is led to multiple-stage thickeners to raise the concentration of sodium sulphate. There, water is vaporised to saturation point and crystallisation of sodium sulphate takes place. By recrystallising in melting pots and evaporation of the crystal water in crystallisers, a pulp of sodium sulphate is obtained.
This pulp is centrifuged and dried at 450 °C in a drying tower by direct heating with a natural gas burner. Another technique for drying is the usage of a tumble dryer and a subsequent separation in a cyclone.
Calculation of sodium sulphate recovery
Sodium sulphate recovery (%) =[ (quantity of sodium sulphate produced as by-product (tonne)/ viscose or modal fibres produced (tonne)]*100