Solutions using activated carbon for systems susceptible to corrosion
Activated carbon can be used in corrosion prevention to help protect systems and piping from corrosive chemical species. Often these chemicals are small, inorganic in nature and as such can be problematic for physical adsorption by activated carbon. This is done by using impregnated grades to capture, treat and retain harmful species on the surface of the activated carbon through chemisorption.
This document will focus on two applications where corrosion is an issue;
• in pulp-mills, where recycled air can damage metal in the electronics.
• in heat exchangers where mercury can damage metal piping.
Pulp mills:
The manufacturing of paper has 6 main process areas of which several involve pulp. The full process includes:
1. Wood preparation
2. Pulping
3. Bleaching
4. Chemical recovery
5. Pulp drying
6. Paper making
This section of the document will focus on the chemicals involved in steps 2 and 3. Pulp can be prepared either through chemical or mechanical methods. Of interest, are the chemical processes including sulfate (Kraft) and sulfite pulping processes as these employ sulfur containing chemicals that lead to the production of gaseous hydrogen sulfide and mercaptans. These are usually evolved when the treatment liquor comes in contact with wood chips and the two react chemically. Hydrogen sulfide and mercaptans are highly corrosive species and would affect any metal used in these processes. Sulfur dioxide and chlorine gas are also of major concern in the pulping process. Chlorine is used as a bleaching agent and sulfur dioxide is used as a digester of the raw material, with both giving the same corrosion issues as hydrogen sulfide and mercaptan.
For pulp mills in colder climates, the major issue is through air recirculation from the factory floor. By recirculating the process air, containing the corrosive species mentioned above, electronics in the control room are under threat from corrosion and thus affecting the running of the whole mill.
The presence of moisture is critical as the above species dissolve in water, generating hydrochloric acid (HCl) from Cl2 and sulfuric acid (H2 SO4 ) from SO2 .
Jacobi Carbons can offer activated carbons for corrosion prevention. For this application, Jacobi Carbons recommends the following grades to aid in corrosion prevention from acid gases:
| Name |
AddSorb™ VA1 |
AddSorb™ VA2 |
|
| Base |
Coal |
Coconut | |
| Impregnation |
KI/KOH |
KI/KOH |
|
| CTC (Base) | %(min) |
56.2 |
51 |
| Butane (Base) | %(min) | – |
22.8 |
| Ash (Base) | %(max) |
11.5 |
3.8 |
| Moisture | %(max) |
15.9 |
13.4 |
| Apparent Density (as recorded) |
kg/m3 (min) |
495 |
508 |
| Ball Pan Hardness | %(min) |
96.6 |
98 |
Table 1: Typical properties of VA1 and VA2.
These grades work by using simple acid/base neutralisation to remove acidic species from the process stream. The reacted products are fixed as a salt on the activated carbon surface. Potassium iodide, KI, works to catalyse the removal of hydrogen sulfide by converting it to sulfur in the presence of oxygen:
8H2 S + 4O2 → S8 + 8H2 O
Water helps facilitate the uptake of H2 S. The gases are dissolved in the liquid, allowing the gases to be in a close enough proximity to react (as gas molecules are more diffuse than liquid molecules in the same volume). KOH, potassium hydroxide, converts to K2 CO3 in air, leaving a thin layer of K2 CO3 on the surface of impregnation of the activated carbon. It is this layer of potassium carbonate that reacts with acidic species to be treated. The following reactions show the action of the impregnation on these compounds, in the presence of water;
K2 CO3 + H2 O + 2SO2 → 2KHSO3 + CO2
K2 CO3 + 2HCl → 2KCl + H2 CO3
Szabo (1986) has shown the capacity of SO2 and H2 S of using AddSorb™ VA1 and VA2. In this report, H2 S and SO2 were diluted in air to a concentration of 1 ppm.
Capacity (%)
|
Base |
Impregnation |
Grade Name |
SO2 |
H2 S |
Time for breakthrough (hrs) /compound |
|
Coal |
KI/KOH |
AddSorb VA1 |
>11.3 |
>5.3 |
>3500 / SO2 |
|
Coconut |
KI/KOH |
AddSorb VA2 |
>15.3 |
>7.1 |
>3500 / SO2 |
Table 2: Capacity and breakthrough times of activated alumina and activated carbons. Capacity is defined as adsorbed amount of gas/filter weight.
Heat Exchangers:
These are devices designed to transfer heat between one or more fluids and found in a wide range of applications, these include power stations, chemical plants, refineries, natural gas processing and sewage treatment.
The species found in sewage treatment have already been discussed in the sewage odour control TIS (Reference: JACOBI-TIS-ADDSORB-Odour Control-A4-ENG-A0815). The same grades offered there can be used for treating the same species in corrosion prevention. Natural gas-processing removes impurities and various non-methane hydrocarbons from raw-natural. Heavier hydrocarbons, acid gases, water, and mercury all occur as unwanted products in the raw natural gas mix.
Mercury, poses the biggest risk to pipelines in natural gas processing as it is highly corrosive, in the sense that it will form amalgams (an alloy formed between mercury and another metal), in this instance, an amalgam between aluminium and mercury, (Hg.Al):
2Al + 3Hg2+ + 6H2 O → 2Al(OH)3 + 6H+ + 2Hg
Hg + Al → Hg.Al
2Hg.Al + 6H2 O → 2Al(OH)3 + 2Hg + 3H2
Aluminium is naturally protected with an oxide layer. This reaction only proceeds when elemental aluminium is exposed to mercury, which be facilitated by simply scratching the surface of the aluminium. The presence of moisture is helpful as it helps generate a cycle from the above equations until the supply of aluminium is exhausted. It is for this reason that restrictions are in place when handling both aluminium and mercury, including limits on the circumstances with mercury being transported on aircraft. Natural gas processing plants use aluminium piping transport gas.
Jacobi Carbons manufactures an activated carbon grade specifically designed for the treatment of vapour phase mercury. AddSorb™ VQ1 is impregnated with sulfur, useful in efficiently converting mercury to the insoluble salt, HgS. This is only applicable to oxidised mercury, Hg2+ :
Hg2+ + S2- → HgS
This generates a water insoluble and thermally stable mercuric salt making disposal easier to manage than other mercuric salts. This grade has the following specification:
| Name |
AddSorb™ VQN |
|
| Base |
Coal |
|
| Impregnation |
QS Advanta |
|
| CTC (Base) | % | – |
| Butane (Base) | % | – |
| Ash | % |
20 |
| Moisture | % | 15 |
| Apparent Density |
kg/m3 |
560 |
| Ball Pan Hardness | % | – |
Table 4: Typical properties of AddSorb™ VQN.
Both grades are suitable for the removal of mercury in the water phase to give a low hazard end-product, HgS.