pH of scrubber at rendering plant 2

[daron1]

I am trying to build a scrubber to remove the offensive smells at rendering plants. What should the ph be? What chemicals should be added to the water to obtain that pH?

 

[bimr]

Here is AP-42 information:

9.5.3.3 Emissions And Controls
1-5 Emissions —
Volatile organic compounds (VOCs) are the primary air pollutants emitted from rendering operations. The major constituents that have been qualitatively identified as potential emissions include organic sulfides, disulfides, C-4 to C-7 aldehydes, trimethylamine, C-4 amines, quinoline, dimethyl pyrazine, other pyrazines, and C-3 to C-6 organic acids. In addition, lesser amounts of C-4 to C-7 alcohols, ketones, aliphatic hydrocarbons, and aromatic compounds are potentially emitted. No quantitative emission data were presented. Historically, the VOCs are considered an odor nuisance in residential areas in close proximity to rendering plants, and emission controls are directed toward odor elimination. The odor detection threshold for many of these compounds is low; some as low as 1 part per billion (ppb). Of the specific constituents listed, only quinoline is classified as a hazardous air pollutant (HAP). In addition to emissions from rendering operations, VOCs may be emitted from the boilers used to generate steam for the operation.

Emissions from the edible rendering process are not considered to be significant because no cooking vapors are emitted and direct heat contact with the edible fat is minimal. Therefore, these emissions are not discussed further.

For inedible rendering operations, the primary sources of VOC emissions are the cookers and the screw press. Other sources of VOC emissions include blood and feather processing operations, dryers, centrifuges, tallow pocessing tanks, and percolator pans that are not enclosed. Raw material may also be a source of VOC emissions, but if the material is processed in a timely manner, these emissions are minimal.

In addition to VOC emissions, particulate matter (PM) is emitted from grinding and screening of the solids (cracklings) from the screw press and other rendering operations such as dryers processing blood and feathers. No emission data quantifying VOC, HAP, or PM emissions from the
rendering process are available for use in developing emission factors. Only test data for a blood dryer operation were identified.

Controls —
Emissions control at rendering plants is based primarily on the elimination of odor. These controls are divided into two categories: (1) those controlling high intensity odor emissions from the rendering process, and (2) those controlling plant ventilating air emissions. The control technologies that are typically used for high intensity odors from rendering plant process emissions are waste heat
boilers (incinerators) and multistage wet scrubbers.

Boiler incinerators are a common control technology because boilers can be used not only as control devices but also to generate steam for cooking and drying operations. In waste heat boilers, the waste stream can be introduced into the boiler as primary or secondary combustion air. Primary
combustion air is mixed with fuel before ignition to allow for complete combustion, and secondary combustion air is mixed with the burner flame to complete combustion. Gaseous waste streams that contain noncondensibles are typically "cleaned" in a combination scrubber and entrainment separator before use as combustion air.

Multistage wet scrubbers are equally as effective as incineration for high intensity odor control and are used to about the same extent as incinerators. Sodium hypochlorite is considered to be the most effective scrubbing agent for odor removal, although other oxidants can be used. Recently, chlorine dioxide has been used as an effective scrubbing agent. Venturi scrubbers are often used to remove PM from waste streams before treatment by the multistage wet scrubbers. Plants that are located near residential or commercial areas may treat process and fugitive emissions by ducting the plant ventilation air through a single-stage wet scrubbing system to minimize odorous emissions.
In addition to the conventional scrubber control technology, activated carbon adsorption and catalytic oxidation potentially could be used to control odor; however, no rendering plants currently use these technologies. Recently, some plants have installed biofilters to control emissions.
No data are currently available for VOC or particulate emissions from rendering plants. The only available data are for emissions from blood dryers, which is an auxiliary process in meat rendering operations. Less than 10 percent of the independent rendering plants in the U. S. process
whole blood. Table 9.5.3-2 provides controlled emission factors in English units for particulate matter (filterable and condensible), hydrogen sulfide, and ammonia from natural gas, direct-fired blood dryers.

The filterable PM was found to be 100 percent PM-10. Emission factors are calculated on the basis of the weight of dried blood meal product. In addition to natural gas, direct-fired dryers, steam-coil, indirect blood dryers (SCC 3-02-038-12) are also used in meat rendering plants. No emission data were found for this type of dryer. The emission control system in Reference 4 consisted of a cyclone separator for collection of the blood meal product followed by a venturi wet scrubber and three packed bed scrubbers in series. The scrubbing medium for the three packed bed scrubbers was a sodium hypochlorite solution. The emission control system in Reference 5 was a mechanical centrifugal separator.

Contact:

http://www.usfilter.com/water/Business+Centers/RJ_Environmental_Products/

for further assistance.

 

[daron1]

Thank you very much for your response it has explained a lot. I am unable to add any chlorine based molecules. Would sodium hydroxide be ok to add? What about H2O2?

 

[bimr]

As you are aware, scrubbing the multiple odors present is a complex issue. You are going to need some type of multi-stage scrubber, as a single scrubber is unlikely to be effective to remove all of the compounds. Here is information on the design of a two stage scrubbing system that is typically used for odor control at a wastewater treatment plant. You probably need something similar.

It is doubtful that any scrubbing system will be effective without the use of a chlorine compound.

Where removal of ammonia and other nitrogen compounds is required, a two-stage scrubber system using a dilute sulfuric acid solution in the first stage is typically used. The ammonia reacts with the sulfuric acid to form ammonium sulfate, a soluble, non-volatile salt, which is removed from the scrubber effluent through the overflow.

1. NH3 absorption section

Gas passes through the main scrubber. In the first stage, absorption reactions are as follows:

2NH3+H2SO4 -> (NH4)2SO4-------------(1)
(CH3)3N+H2SO4 -> ((CH3)3NH)2SO4-----(2)

2. H2S absorption section
In the second stage H2S and other acid gases are absorbed. For example,

H2S+4NaClO -> Na2SO4 + 4NaCl-----(3)
H2S+2NaOH -> Na2S+2H2O-----------(4)

By using NaClO, acid odor gases can be efficiently absorbed. It is very important to control pH and ORP for the purpose of successful absorption. If not controlled, trouble such as sulfur particles and Cl2 gas will arise

The most common method of control of H2S gas is to pass the smelly gas through a vertical, packed bed wet scrubber. The air passes up the tower as the scrubbing liquid containing caustic (NaOH) and oxidizing agent (most often bleach or NaOCl, sodium hypochlorite) flows down the tower in counter-current fashion. The high pH provided by the caustic drives the mass transfer from gas to liquid phase by solubolizing H2S as HS- bisulfide and S-2 sulfide ions. Once in solution, the reaction between hydrogen sulfide and oxidizing agent is almost instantaneous (assuming sufficient oxidizing agent is present). This reaction converts the sulfide to sulfate (SO4-2) ion. The overall chemical reaction is described by the following equation:

H2S + 4NaOCl + 2NaOH Na2SO4 + 4NaCl + 2H2O

Therefore, theoretically, for each molecule of H2S destroyed, four molecules of bleach and two molecules of caustic are consumed. However, the chemistry is not quite so simple, as partial oxidation of H2S also takes place that forms elemental sulfur:

H2S + NaOCl NaCl + H2O + S?

This reaction represents about 1% of the chemistry present in a wet scrubber. The presence of excess bleach helps to minimize the formation of elemental sulfur. But bleach is an expensive chemical. The use of two stage scrubbing is often employed both to minimize chemical consumption as well as to control sulfur deposits when scrubbing H2S. The first stage operates at~ 80% efficiency and uses a caustic only scrub at high pH (~ 12.5). The air then passes to the second stage, where the remaining H2S is scrubbed with caustic / bleach solution at pH ~9.5. The H2S present is destroyed at 99%+ efficiency. The blowdown from the 2nd stage, which will contain some amount of unused NaOCl, is sent to the sump of the 1st stage. In this way, additional H2S is destroyed and maximum consumption of expensive oxidizing agent is assured.

Never the less, there are losses of chemicals which cannot be prevented, which of course raise the cost of odor control scrubbing. These losses are due to the facts that bleach, NaOCl, slowly decomposes in storage as well as the fact that some amount of caustic is constantly lost to CO2 absorption in both scrubbing stages.

http://www.eng-tips.com/viewthread.cfm?qid=72509

 

[daron1]

Thank you for the response. If i cannot use bleach. Would sulfur oxidising bacteria in a biofilter perfrom the same job as bleach on both mercuptans and H2S?

 

[bimr]

For the best results, you are going to have to find a real world application that duplicates your situation or else try to pilot test it. The odors from a rendering plant are too complex.

The latest generation of biofilers provide impressive results. However, the oxidizing bacteria function to regenerate the biofilter rather than removing the odor compounds.

Carbon filtration seems to be a better choice for mercaptan removal. Sometimes carbon units are installed after bleach/caustic scrubbers and biofilters.