- Chemical Processing
- Condensate Stripping
- Multiple Effect Evaporators
- NCG Systems
- Surface Condensers
- Turpentine Recovery
It is possible to collect the SOG as a liquid by use of an additional condenser. This allows for storage of the contaminants as a liquid, which can subsequently be used for fuel. There are capital cost and operational advantages to handling the liquid fuel.
The generally accepted optimum methanol concentration in the overheads is 50% by weight. At constant methanol removal, a product concentration of 10% methanol carries with it nine times more water vapor to incineration than a 50% methanol product would contain.
Mills have experienced a scale build-up on both sides of the heat exchanger. Fouling of the heat exchanger will result in loss of stripper efficiency. Preheaters foul more readily on the stripped condensate side of the heat transfer surfaces, and this surface should be accessible for cleaning.
Fouling of the stripping system increases at higher operating temperature and pressure.
For any given operating pressure, and any desired stripper off-gas (SOG) concentration there is fixed equilibrium temperature. Operating at the proper equilibrium temperature and pressure is necessary to control the overhead composition.
If cold condensate enters a stripping column full of steam, it can collapse the steam creating a vacuum that can buckle and dislodge the stripping trays. Avoid this condition by changing condensate feed rates very slowly.
A common cause of stripper instability is when operating pressure fluctuates, often due to poor control of the reflux condenser or fluctuations in the heat sink temperature. Therefore, flow changes should be made on a stepwise basis, and made as seldom as possible.
To avoid foaming in the stripping system, monitor conductivity of the incoming condensate and divert it away from the stripping system on high conductivity.
One technique to minimize turpentine accumulation in a stripper feed tank is to deliver condensate that may contain appreciable turpentine to the suction side of the stripper feed tank discharge pump, bypassing the feed tank entirely.
The stripper feed tank is operated as a surge tank, and turpentine in the condensate may separate and collect in a layer on the top of the condensate. The feed tank must be designed to prevent the turpentine accumulation, or include skimming or laundering provisions to remove the turpentine.
Some mills have found it advantageous to scrub the ionizable TRS from the CNCG before the gases are burned. This is typically done with a spray column using white liquor or caustic.
Consider installing hydrogen sulfide detectors in areas where leaks from the CNCG system may occur, especially inside buildings.
Mechanical cleaning and hydroblasting are more effective than chemical cleaning, however access to the exchange surfaces is necessary.
Be sure to survey the condensate level glasses on your evaporator effects ahead of the next shutdown and schedule maintenance as needed. Level glasses are an excellent troubleshooting tool but only effective when functional. Give your operators the tools needed to troubleshoot problems.
Overdosing of anthraquinone, an additive which is used to improve digester yield, can result in evaporator fouling. AQ scale can be identified as an off-white film found on the vapor side heat transfer surface of condensers and the final evaporator effect. To remove the scale wash the heat transfer service with sodium hydrosulfide.
If you find an evaporator effect operating with higher than normal condensate level check for a vacuum leak in the condensate outlet line. Alternatively, there may be a blockage in the outlet nozzle due to an impingement baffle failure. When all else fails, disconnect and inspect.
Consider weak liquor rather than condensate to boil out concentrators. Using a weak liquor reduces the volume of spills generated and can effectively increase the overall evaporator capacity in a mill that is evaporator limited.
It is recommended that you avoid the introduction of mill water into the liquor cycle. The exclusion of mill water reduces the build up of non-process elements such as calcium and manganese within the liquor cycle. Keeping the water out can significantly improve mill operations.
Consider inspecting your steam desuperheater nozzles to optimize the performance of your live steam evaporator and concentrator bodies. Excessive superheat leads to accelerated fouling and the loss of capacity
A major disadvantage of burning stripper product as a vapor is that if this stream is ever vented, it will create a severe odor problem. Most newer systems are designed without atmospheric vents, and the stripping system is effectively shut down rather than vented to the atmosphere.
The CNCG are normally saturated with water vapor, and some water vapor will condense in the lines. Therefore, it is necessary to slope the lines so that the condensate formed does not build up.
All CNCG lines after the ejector should be insulated to ensure that minimal moisture condenses.
Gaskets for NCG systems are selected for resistance to turpentine, methanol, and TRS compounds. TFE-based gaskets are typically used, as they are resistant to these CNCG components
Stainless steel, typically 304 or 316, has proven to be corrosion resistant to NCG and is the preferred material of construction.
If maintenance work is required on all or part of the NCG system, the affected part must be isolated, drained, purged with air or steam, and checked for the presence of toxic and flammable gases before entry or welding
Turpentine is able to reach a flammable concentration more readily than TRS compounds, and most explosions in NCG systems have been attributed to turpentine. Thus, it is very important to keep turpentine out of the NCG systems.
CNCG (concentrated low volume high concentration NCG) is normally not flammable because it lacks of sufficient oxygen to support combustion. Thus, CNCG systems must be designed and operated to prevent ingress of air. To be safe, always assume it is flammable.
In older evaporator systems with center NCG vents sludge can accumulate and restrict venting. Consider hydro blasting these vent lines every 3 to 5 years to a avoid a decline in performance and economy.
Excessive throttling of the NCG vent from an evaporator surface condenser can significantly reduce condensate segregation efficiency. The accumulation of NCG in the foul pass results in methanol condensing in the clean side of the unit. To control vacuum consider controlling the condenser outlet water temperature.
Clean your surface condensers annually to optimize the performance of your evaporator systems. Every inch of mercury vacuum that you lose represents a 3 to 5% capacity loss.
The performance of your inter and after condensers is critical to maintaining good vacuum in your evaporator plants. During your next shutdown be sure to inspect these vessels for fouling. Hydro blast or acid clean as necessary to keep your evaporator is running at their highest efficiency.