[pullquote]Conventional designs for binary distillation generally work well but consume a lot of energy. Usually site-generated steam boils up the liquid bottoms of the column and site-generated refrigerated liquids condense the vapor that comes off the top of the column.
The key to efficient operation is finding the optimum saturation pressure temperature offset for separating the two components. However, an optimum point that requires very low temperatures can pose significant cost issues. As temperatures get lower, the energy cost for producing them escalates. A high reflux ratio, if necessary, cascades over to a greater cost for the steam or other boil-up fluid.
In such cases, consider three alternatives: 1) raising the condensing pressure by means of a compressor in the overheads vapor line; 2) inserting a heat pump between the overheads condenser and the boil-up heat exchanger; or 3) opting for higher-pressure operation if you’re designing or refurbishing a column. Let’s explore the advantages and the difficulties of each option.
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The first option involves inserting in series a compressor that takes suction from the top of the column and discharges to a higher temperature and pressure heat exchanger using a heat sink such as cooling tower or chilled water. This avoids the use of a low temperature heat sink such as very low temperature heat transfer fluid. The reflux fluid is at a much higher temperature and so requires less heat input from the boil-up heat exchanger. Thus, energy savings accrue even after accounting for the energy cost of compression.
The drawback is finding the right compressor with compatible materials of construction. In many systems, that factor may rule out the option. There’s also the issue of where to mount the compressor. Ideally, it should be as close as possible to the discharge point on the column. Unfortunately, a compressor produces vibrations — so, mounting it in the upper reaches of the column support structure may lead to structural-integrity or noise problems. Mounting it at ground level may cause suction pressure issues and the potential for liquid flow into the compressor during upsets.
For option two, the overheads condenser would serve as the evaporator for the heat pump while a heat exchanger on the reflux or primary feed stream would serve as its condenser — enabling recycling of the energy of compression and condensing back into the column. On the plus side, this provides a self-contained unit. However, on the minus side, it requires additional capital versus using an existing plant utility. Also, a single unit might not be as reliable as the central plant.
The heat pump setup isolates the compressor from the process streams, eliminating any materials compatibility issues. To drive heat transfer, the system requires temperature differences between the cooled or heated fluids and the refrigerant fluid. This leads to a higher energy requirement — but return of the heat to the reflux stream and thus back to the column offsets that by reducing the energy necessary for boil-up.
The system still needs a boil-up heat exchanger and an overheads condenser. Anyone who has started up a highly heat-integrated pinch-designed system should appreciate the difficulty of doing without these two startup exchangers.
The third option, designing the column with a higher pressure rating, enables using a higher temperature heat sink for the overheads condenser. This only is viable if the saturation temperatures and pressures allow for higher pressures. The initial cost of the column is greater; the lower temperature refrigeration system costs less but not enough to compensate. However, the operating savings that accrue over the life of the system might offset the extra capital cost.
When you must upgrade or refurbish a column, don’t automatically default to the old standard design.
