Reboiler clean

Internal rebollers. These are usually avoided when fitting them into the column requires a significant increase in column diameter or height, and also where reboiler cleaning is expected while the column is on-line. 

Used for small duties, clean process, and only infrequent cleaning required. Vaporization is usually less than 30%, but less than 15% if the fractionator pressure is below 50psig. The viscosity of the reboiler feed should be less than 0.5 cp. Put a butterfly valve in the reboiler inlet piping. This type is used in nearly 100% of chemical plant thermosyphon applications (70% of petrochemical). 

Abnormal Heat Input From Reboiler - Reboilers are designed with a specified heat input. When they are new or recently cleaned, additional heat input above the normal design can occur. In the event of temperature control failure, vapor generation may exceed the process system s ability to condense or otherwise absorb the buildup of pressure, which may include noncondensibles due to overheating. 

Voives Installed if Reboiler to be Removed for Cleaning, Should be Full Opening Line Size. 

Figure 10-123. Vertical thermosiphon reboilers, DAT versus AT for clean and fouled conditions. (Used by permission Hajek, J. D. Private communication. Deceased.)...

Some operations will include a scrap soap reboil to recover reclaimed soap. The soap reboil is salted out for soap recovery and the salt water is recycled. After frequent recychng, the salt water becomes so contaminated that it must be discharged to the sewer. Occasional washdown of the crutcher may be needed. The tower is usually cleaned down dry. There is also some gland water that flows over the pump shaft, picking up any minor leaks. This will contribute a very small, but finite, effluent loading. 

A little bit of entrainment is not always bad. For example, entraining a clean naphtha mist into the suction of a wet-gas centrifugal compressor is fine. The mist helps keep the rotor parts clean. If the steam from a boiler is to be used to reboil towers, entrainment is of no consequence. 

This process has completely continuous aluminum chloride addition and complex withdrawal. It tolerates somewhat more olefins and higherboiling hydrocarbons because their detrimental effect can be offset by adding make-up aluminum chloride at a higher rate. Fouling of the hydrogen chloride stripper reboiler by aluminum chloride carry-over was troublesome. This was overcome by using duplicate reboilers and cleaning one while the other was in use. 

A new permit with the state of New Mexico will require this liquid stream to meet the domestic water supply limit for dissolved NO3 of 10 mg/L measured as N (in order to comply with the federal Clean Water Act and the New Mexico Water Quality Act). A nitric-acid fractionation (distillation column) is to be installed to recover these nitrates from the discard stream prior to disposal. The column (Figure 4.22) will include a total condenser and partial reboiler and will operate with a reflux ratio of 0.4. 

Horizontal tube evaporators may be susceptible to vapor-binding, and foaming liquids cannot usually be handled. The short tube variety is seldom used today except for preparation of boiler feed water. The kettle-type reboiler is frequently used in chemical plant applications for clean fluids. 

A sufficient liquid level in a reboiler used to supply steam to the column is important from a performance and safety standpoint. The reboiler is under a slight pressure and therefore must be a pressure vessel. It usually has a gasketed and bolted lid. Welded steam outlet and stillage return lines connect the reboiler to the column. The liquid return enters the reboiler below the desired liquid level. The outflow of bottoms can be controlled by regulating the back-pressure in the bottoms line, in turn controlling the level of liquid in the pot. The pot must be designed so it can be completely drained to ease clean-out. 

Allowance for a clean reboiler. The reboiler duty may be two to three times higher with a clean reboiler because of a higher heat transfer coefficient. If not accounted for, the column may be unprotected against overpressure when the heat input controller fails and the reboiler is clean. 

A persistent impurity problem can be tackled at the source or in the column, often in both. In the air separation example, hydrocarbon impurities are often separated out at plant inlet. In addition, pockets where impurity-contaminated liquid oxygen may aggregate are avoided in the column an impurity adsorber may be installed on the oxygen liquid stream to the reboiler the oxygen side of the reboiler may be run flooded and liquid oxygen is continuously blown down from the reboiler (225). Special attention is paid to cleaning and shutdown practices, and to... 

Very low heat duty clean services, where column diameter is large due to other considerations, and the reboiler tube bundle required is small. 

In fouling services where on-line operation must be continuously maintained, the reboiler is frequently spared to facilitate cleaning while the column is on-line. An extensive discussion on decommissioning, cleaning, and recommissioning a reboiler while the column is online is described elsewhere (279). 

Forced-circulation reboilers are similar to vertical thermosiphon reboilers, but do not depend on the natural thermosiphon action, and commonly operate at high circulation rates. The pump replaces the level in the reboiler sump as the driving force that sets rehoiler circulation. Heat transfer is mainly by sensible heat, often followed hy nucleate boiling. Additional boiling occurs when the heated mixture is flashed at the reboiler outlet. Forced-circulation reboilers may be installed horizontally or vertically horizontal units are often easier to clean, but consume more plot space. 

The vapor inlet scheme may be troublesome when there is a small pressure difference between the reboiler heating medium and the condensate header (e.g., with steam reboilers using 15 to 35-psig steam). The problem is identical to that described in item 8 above, but it is caused by insiifficient AP rather them oversizing or clean siuface. In this case, the condensate outlet scheme of Fig. 11.Id is often preferred (234). The vapor inlet scheme shown in Fig. 17.1c can also be used and will provide better control, but at the expense of the additional pmnp. 

Set the highest liquid level to match the expected liquid level in the reboiler under the most severe turndown conditions required. Keep in mind that these may occur under startup conditions, when the reboiler is clean and the heat transfer coefficient is high. 

A reboiler outlet isolation valve was blinded on the reboiler side at shutdown. Liquid remained trapped in the valve botmet After the reboiler was cleaned it was water-washed, then drained. To facilitate draining, the blind was removed and the reboiler valve opened. The tr >ped hydrocarbons were released and exploded in tbe oo -- Icillinsp a warker and lift-... 

If water-soluble organic or inorganic residues have to be dealt with, water boil outs may be necessary to maintain the heat-transfer surfaces and this is especially so with external forced-circulation heat exchangers, since if a tube becomes blocked it will not wash clean and will behave as a stay tube, under stress when the reboiler is heated. 

The scrubber effluent, which contains some methanol, is fed to a distillation column to expel the methanol overhead and obtain clean water at the bottom. This distillation column operates at a slightly elevated pressure to ensure that the overhead vapors are condensed in a separate reboiler at the regeneration column and can thus provide part of the heat required by that column. The remaining regeneration heat demand may be covered by waste heat at temperatures above 90 °C. 

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