Heat input cracking

Ethylene Stripping. The acetylene absorber bottom product is routed to the ethylene stripper, which operates at low pressure. In the bottom part of this tower the loaded solvent is stripped by heat input according to the purity specifications of the acetylene product. A lean DMF fraction is routed to the top of the upper part for selective absorption of acetylene. This feature reduces the acetylene content in the recycle gas to its minimum (typically 1%). The overhead gas fraction is recycled to the cracked gas compression of the olefin plant for the recovery of the ethylene. 

The reference test was conducted in a stainless steel reactor assembly which was sized to duplicate the Kureha reactor geometry. The experimental operating conditions compared favorably with the actual plant conditions. In particular, the steam temperature, S F ratio, residence time, oil feed rate, and heat input were matched very closely. However, the reactor exit temperature was somewhat lower than that of the operating plant. The experimental gas yields for ethylene, ethane, propylene, and propadiene agreed very well with the plant. There were slightly lower experimental values for hydrogen, methane, acetylene, and total gas, which indicated a less severe crack. 

Decomposition. As column pressure rises, so does column temperature. When the materials distilled are heat-sensitive, decomposition or thermal cracking of the liquid may set in. Decomposition is likely to yield gaseous products which behave as noncondensables. Decomposition will therefore increase the relief requirement and may reduce the credit that can be allowed for cooling. The rate of decomposition may be particularly high upon a heat input controller failure. This problem is most severe when the reboiler is a fired heater or one that has a high temperature difference. 

Recent work indicates that for non-alloyed C-Mn weld metal, in contrast to the situation in the HAZ, the risk of weld metal hydrogen cracking can increase, particularly in multipass welds, with increasing heat input. The normally envisaged relaxation of preheat at higher heat inputs should therefore be reviewed carefully in such situations. 

One of several laboratory teks to investigate hydrogen cracking. The combined thickness (thermal severity), welding process, weld hydrogen level, preheat and heat input can be varied independently. The test is a fillet weld with a deliberate root gap and a moderate restraint level results of CTS tests provided the basis of most of the welding procedure diagrams in the present text. 

In practice, the considerable influence of the stress level can also be seen by the fact that weld seams are preferentially attacked by stress corrosion cracking on account of shrinkage stresses and the stresses caused by structural changes due to the effect of heat input during welding. 

Major uses of ethane include its application as a fuel and in organic synthesis. For example, it can be chlorinated to produce ethyl chloride, and can yield ethylene with a greater heat input than is required for obtaining ethylene by propane cracking. Ethane is also used as a refrigerant. 

---