Temperature pinch

As temperatures decrease, closer temperature approaches are needed in the heat exchangers to achieve low energy requirements. Consequendy, temperature pinches in Hquid hydrogen plants range from 1 degree K at 20 K to 6 degrees K at 300 K. 

Atkins, M. J., Walmsley, M. R. W., and Morrison, A. S. (2010). Integration of solar thermal for improved energy efficiency in low-temperature-pinch industrial processes. Energy 35, 1867-1873. 

Masso and Rudd (1969), Lee, Masso and Rudd (1970) and Pho and Lapidus (1973) start a match with the two stream inlet conditions and exchange all the heat possible, terminating when one stream reaches its target outlet temperature or when a temperature pinch occurs. Ponton and Donaldson (1974), Donaldson, Paterson and Ponton (1976) and Grossmann and Sargent (1978) start a match at the hot end of both streams (or as close to the hot end of the cold stream as possible). The idea is to introduce necessary utilities at their least cost level while exchanging as much heat as possible. Rathore and Powers (1975) do both, getting two alternative matches and obviously many more alternative networks. 

Several sub-sections are needed by the decomposition of a global reaction into several steps. For instance the sulphuric acid decomposition needs at least two or three steps, the first one to eliminate most of the water, the last one to crack the SO, . So you have to look precisely at the way you want to operate, and for instance to introduce a re-circulation circuit and separate chemical reactors. Technology means are essential as they represent an actual part of the losses. Sometimes a component should not be feasible, for instance a compressor at high temperature or a heat exchanger with no temperature pinch. Tlie way by which heat transfers and conveyance can be organized is dependent of the technology and also of the spatial organisation of the process. 

High reactor temperature pinches the reactor feed valve. 

Reboiler temperature pinches. As column pressure increases from normal operating pressure to the relieving pressure, column temperature rises. This may reduce the temperature difference (or create a temperature "pinch ) in the reboiler. The pinching that can be relied on to occur can be credited toward the relief requirement. Each situation must be anal3rzed separately. Some common situations are ... 

When the column is reboiled by a fired heaters, temperature pinches rarely (if at all) occur (60). 

Increasing column mechanical design pressure This is one of the most effective techniques in cases where credit can be taken for a temperature pinch (see Sec. 9.4). When a close-boiling mixture is distilled, and the reboiler operates at a relatively small AT, a modest increase in column pressure can drive the reboiler AT to zero... 

Temperature pinch. A temperature pinch is process temperature approaching the heating medium temperature. It therefore occurs only when the reboiler AT is small. A temperature pinch lessens reboiler heat transfer. Two mechanisms are usually responsible for temperature pinches ... 

Pinching is sometimes difficult to distinguish from fouling, because most s3unptoms are similar. Recording reboiler temperature difference (outlet minus inlet, process side), while experimenting with a temporary increase of the lights content in the column bottom or with circulation rates, can help identify a pinch. Alternatively, a computer simulation of the reboiler can often detect a temperature pinch (372). 

Figure 1S.S Temperature pinches in a reboiler, (a) Induced by pressure drop, single-component (b) induced by pressure drop, wide boiling range (c) induced by composition, wide boiling range.

Distribution. With tray towers, liquid distribution to the reboiler is uneven. With wide-boiling mixtures, this may prompt composition pinches. Further, light components can be depleted near the inlet or the top of the bundle, causing temperature pinches. In most cases, however, distribution is not a major problem because the bundle is small and internal reboiler AT is large. 

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