Distillation columns typical produce waste as follows:
By allowing impurities that ultimately become waste to remain in a product. The solution is better separation. In some cases, normal product specifications must be exceeded.
By forming waste within the column itself usually because of high reboiler temperatures which cause polymerization. The solution is lower column temperatures. By having inadequate condensation, which results in vented or flared products. The solution is improved condensing.
Some column and process modifications that reduce waste by attacking one or more of these three problems are outlined next.
The most common way of improving separation is to increase the reflux ratio. The use of a higher reflux ratio raises the pressure drop across the column and increases the reboiler temperature (using additional energy). This solution is probably the simplest if column capacity is adequate.
If the column is operating close to flooding, adding a new section increases capacity and separation. The new section can have a different diameter and use trays, regular packing, or high-efficiency packing. It does not have to be consistent with the original column.
Converting to a Continuous Process
The start ups and shutdowns associated with batch processes are a common source of waste and by-product
Another method of increasing separation is to retray or repack part or all of a column. Both regular packing and high-efficiency packing lower the pressure drop through a column, decreasing the reboiler temperature. Packing is no longer limited to a small column; large-diameter columns have been successfully packed.
Many columns are built with multiple feed trays, but valv-ing is seldom changed. In general, the closer the feed conditions are to the top of the column (high concentration of lights and low temperature), the higher the feed tray; the closer the feed conditions are to the bottom of the column (high concentration of heavies and high temperature), the lower the feed tray. Experimentation is easy if the valv-ing exists.
Insulation is necessary to prevent heat loss. Poor insulation requires higher reboiler temperatures and also allows column conditions to fluctuate with weather conditions (Nelson 1990).
A company should analyze the effectiveness of feed distributors (particularly in packed columns) to be sure that distribution anomalies are not lowering the overall column efficiency (Nelson 1990).
Preheating the feed should improve column efficiency. Supplying heat in the feed requires lower temperatures than supplying the same amount of heat to the reboiler, and it reduces the reboiler load. Often, the feed is preheated by a cross-exchange with other process streams.
If the overhead contains light impurities, obtaining a higher purity product may be possible from one of the trays close to the top of the column. A bleed stream from the overhead accumulator can be recycled back to the process to purge the column lights. Another solution is to install a second column to remove small amounts of lights from the overheads.
In a low-pressure or vacuum column, the pressure drop is critical. A larger vapor line reduces the pressure drop and decreases the reboiler temperature (Nelson 1990).
A conventional thermosiphon reboiler is not always the best choice, especially for heat-sensitive fluids. A fallingfilm reboiler, a pumped recirculation reboiler, or high-heat flux tubes may be preferable to minimize product degradation.
Shutting down the column because of reboiler fouling can generate waste, e.g., the material in the column (the hold up) can become an off-spec product. A company should evaluate the economics of using a spare reboiler.
Temperature reduction techniques such as using lower pressure steam, desuperheating steam, installing a ther-mocompressor, or using an intermediate heat-transfer fluid also apply to the reboiler of a distillation column.
Reducing the column pressure also decreases the reboiler temperature and can favorably load the trays or packing as long as the column stays below the flood level. The overhead temperature, however, is also reduced, which may create a condensing problem. If the overhead stream is lost because of an undersized condenser, a company can consider retubing, replacing the condenser, or adding a supplementary vent condenser to minimize losses. The vent can also be rerouted back to the process if the process pressure is stable. If a refrigerated condenser is used, the tubes must be kept above 32°F if any moisture is in the stream.
If the overhead stream is sent to another column for further separation, using partial condensers and introducing the vapor to the downstream column may be possible.
Many distillation processes use stabilizers that reduce the formation of tars as well as minimize unfavorable or side reactions. However, the stabilizers not only become large components of the tar waste stream but also make the waste more viscous. The more viscous the waste stream, the more salable product the waste stream carries with it. Upgrading the stabilizer addition system requires less stabilizer in the process.
The upgrade can include the continuous versus batch addition of a stabilizer or the continuous or more frequent analysis of a stabilizer's presence coupled with the automatic addition or enhanced manual addition of the stabilizer. Another option is to optimize the point of addition, the column versus the reboiler, along with the method of addition.
A stabilizer typically consists of a solid material slur-ried in a solvent used as a carrier. Options for waste reduction also focus on the selection of one of these two components. The addition of a stabilizer in powder form eliminates the solvent. The use of the product as a carrier component is one of the best options.
Continuous distillation processes require a means of removing tar waste from the column bottoms. Optimizing the rate at which tars are purged can reduce waste. An automatic purge that controls the lowest possible purge rate is probably best. If an automatic purge is not possible, other ways exist to improve a manually controlled or batch-operated tar purge. If a batch purge is used, more frequent purges of smaller quantities can reduce overall waste (EPA 1993).
Some processes that purge continuously are purged at excessively high rates to prevent valve plugging. More frequent cleaning or installing a new purge system (perhaps with antisticking interior surfaces) permits lower purge rates.
Treating the Column Bottoms to Further Concentrate Tars
Treating the tar stream from the bottom of a distillation column for further removal of the product by a wipe-film evaporator may be a viable option.
heat exchangers. Most techniques are associated with reducing tube-wall temperatures.
Using a Lower Pressure Steam
When plant steam is at fixed-pressure levels, a quick option is to switch to steam at a lower pressure, reducing the tube-wall temperatures.
High-pressure plant steam can contain several hundred degrees of superheat. Desuperheating steam when it enters a process (or just upstream of an exchanger) reduces tubewall temperatures and increases the effective area of heat transfer because the heat transfer coefficient of condensing steam is about ten times greater than that of superheated steam.
Another way of reducing the tube-wall temperature is to install a thermocompressor. These relatively inexpensive units work on an ejector principle, combining high- and low-pressure steams to produce an intermediate-pressure steam.
Figure 3.10.4 illustrates the principle of thermocompression. The plant steam at 235 psig upgrades 30-psig steam to 50 psig. Before a thermocompressor was installed, only 235-psig steam was used to supply the required heat.
For a distillation process, one set of operating conditions is optimum at any given time. Automated control systems respond to process fluctuations and product changes swiftly and smoothly, minimizing waste production.
If a heat-sensitive fluid must be heated, staged heating minimizes degradation. For example, the process can begin with waste heat, then use low-pressure steam, and finally use superheated, high-pressure steam (see Figure 3.10.5).
The start ups and shutdowns associated with batch processes are a common source of waste and by product formation. Converting a process from batch to continuous mode reduces this waste. This option may require modifications to piping and equipment. Before any equipment modification is undertaken, a company should do a computer simulation and examine a variety of conditions. If the column temperature and pressure change, equipment ratings should also be reexamined.
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