Cracking, thermal Quenching

In fluid coking (Figure 2.2), the residium feed is injected into the reactor, where it is cracked thermally in a fluidized bed catalyst. Products other than coke leave the top of the reactor and are quenched in a scrubber, where residual coke is removed. The coke fines and some of the heavy fractions are recycled to the reactor. The lighter fractions are fed to conventional fractionating equipment. 

Crackle. (1) A multiply crazed or cracked surface on art pottery or glass. To produce the effect on pottery the glaze is compounded so as to have a higher thermal expansion than the body the craze pattern is sometimes emphasized by rubbing colouring matter, such as umber, into the fine cracks. With glass, the ware is cracked by quenching in water it is then reheated and shaped. (2) A crackled vitreous enamel - the... 

FIG. 23-3 Temperature and composition profiles, a) Oxidation of SOp with intercooling and two cold shots, (h) Phosgene from GO and Gfi, activated carbon in 2-in tubes, water cooled, (c) Gumene from benzene and propylene, phosphoric acid on < uartz, with four quench zones, 260°G. (d) Mild thermal cracking of a heavy oil in a tubular furnace, hack pressure of 250 psig and sever heat fluxes, Btu/(fr-h), T in °F. (e) Vertical ammonia svi,ithesizer at 300 atm, with five cold shots and an internal exchanger. (/) Vertical methanol svi,ithesizer at 300 atm, Gr O -ZnO catalyst, with six cold shots totaling 10 to 20 percent of the fresh feed. To convert psi to kPa, multiply by 6.895 atm to kPa, multiply by 101.3. 

One way of measuring thermal shoek resistanee is to drop a piece of the ceramic, heated to progressively higher temperatures, into cold water. The maximum temperature drop AT (in K) which it can survive is a measure of its thermal shock resistance. If its coefficient of expansion is a then the quenched surface layer suffers a shrinkage strain of a AT. But it is part of a much larger body which is still hot, and this constrains it to its original dimensions it then carries an elastic tensile stress EaAT. If this tensile stress exceeds that for tensile fracture, <7js, the surface of the component will crack and ultimately spall off. So the maximum temperature drop AT is given by... 

Reactor vapor quench. LCO, naphtha, or other quench streams can be used to quench reactor vapors to minimize thermal cracking. 

Furnace Black One of the three principal processes used for making carbon black the others being the Thermal Black and the Channel Black processes. In the Furnace Black process, aromatic fuel oils and residues are injected into a high velocity stream of combustion gases from the complete burning of an auxiliary fuel with an excess of air. Some of the feedstock is burned, but most of it is cracked to yield carbon black and hydrogen. The products are quenched with water. 

A major factor determining the importance of ceramics lies in their usefulness at high temperatures. Two types of experimental techniques are generally used in order to determine the minimum shock required to nucleate fracture (cracking), and the amount of the damage caused by thermal shock 83 the number of quenching cycles, and strength as a function of quench temperature and crack patterns. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

Equation (15.24) shows that, for high resistance to crack initiation, high strengths combined with low stiffness and CTE are required. Under mild thermal shock conditions (e.g. in a boiling water quench) the thermal conductivity also becomes important, and (15.24) is modified to give ... 

The thermal shock behaviour of a 3-D carbon fibre-reinforced SiC-matrix CMC manufactured by CVI was assessed using the air-quench method by Yin et al. (2002). Damage consisted of matrix cracks that induced a reduction in Young s modulus, strength, and work of fracture for A7 > 700°C. 

After thermal cracking the reaction mixture is quenched and cooled (nonpre-sented). The recovery of HC1 and the separation ofVCM from unreacted DCE take place in units S6 and S7, respectively. 

---