PROCESS FOR THE RECOVERY OF POWER FROM HOT GASES AND AN APPARATUS THEREFOR

Abstract: Process and apparatus for the recovery of power from hot gases, e.g. gases having a temperature in the range of from 500-700°C, in which process they are intro­ duced into a hot-gas expander turbine provided with a by-pass for said hot gases wherein the by-pass is kept at elevated tem­ perature in order to prevent or minimize thermal shock when the by-pass is taken into operation.

Description: PROCESS FOR THE RECOVERY OF POWER FROM HOT GASES AND AN APPARATUS THEREFOR The invention relates to a process for the recovery of power from hot gases, e.g. hot gases having a temperature in the 0 range of from 500 - 750 C. It also relates to an apparatus to be used in such a process. From US 3,702,308 a process is known for the utilization of the energy content of the flue gas accrued in plants used for the catalytic cracking of hydrocarbons. Oil coke which has been deposited on the catalyst during the catalytic cracking process will normally be burnt off in order to regenerate the catalyst. This burning takes place in the so-called regenerator from which a flue gas emanates having a 0 temperature of about 600 C at pressures between about 1.3 and 1.8 bar gauge and a CO content of up to 10%, in effect, a gas with a considerable energy content. Efforts are therefore made to recover as large a part as possible of the energy contained in the flue gas produced during the cracking process. For this purpose the hot flue gas originating from the regenerator is supplied to a hot-gas expander turbine via a cyclone or catalyst-separating device which can operate independently of the regenerator. In the hot-gas expander turbine, the flue gas, entering at pressures between about 1.3 and 1.8 bar gauge and at a temperature of about 6000C, is expanded to a large extent so that it is cooled off. Its energy content is thus converted into electrical or mechanical energy which can be utilized for the internal requirements of the refinery or supplied to consumers outside the refinery e.g. via the electrical power grid. In the event of troubles with the supply of electricity in the plant, or in the event of defects in the hot-gas expander turbine, the flue gas should by-pass the hot-gas expander turbine, e.g. via a by-pass line. For that purpose there are present a valve at the inlet of the by-pass line and a valve at the inlet of the hot-gas expander turbine which are connected in such a manner that closing of one valve causes immediate opening of the other valve, and vice versa. Accordingly, a zero percent by-pass is followed by a 100% by-pass in case of trouble sufficiently serious to necessitate the decoupling of the hot-gas expander turbine from the catalytic cracking process. In that case a problem arises which has not been appreciated in US 3,702,308 but which problem appears to be of great importance in actual practice. This problem lies in the high pressure drops, high temperatures and high rates of temperature rise the by-pass-inlet valve and the by-pass-line are subjected to when put into operation suddenly and, to a lesser extent, when put out of action again. The by-pass-line 0 easily cools down to a temperature of below 200 C when taken out of action temporarily and will suffer low-temperature stress corrosion cracking. Consequently, the life of these parts of the power recovery system is greatly dependent on the change of operating conditions imposed on them. In to-day's fluid catalytic cracking plants power recovery systems are applied in which regenerator-flue gas having a temperature of up to 7000C is expanded so that by-pass lines around hot-gas expander turbines are subjected to increases in 0 0 temperature from about 50 C at zero % by-pass to about 700 C at 100% by-pass. The occurrence of large temperature differences (thermal shock) which may be demonstrated by overstressing of the metal of the by-pass, comprising by-pass line and by-pass valve, may result in a drastic decrease of the lifetime of the by-pass line and by-pass valve and/or of other parts of the power recovery system. In order to avoid or to minimize thermal shock the temperature of the by-pass should preferably be kept not lower than 2000C below the temperature of the flue gas entering the hot-gas expander turbine. Another factor which decreases the lifetime of the by-pass line and valve is the corrosion activity which occurs when liquid water is present therein. Corrosive salts such as chlorides which are dissolved in the water may corrode the metal of the valve and by-pass line and this may lead to a cracking of the metal which will result in a complete destruction of the metal parts in a relatively short time. Moreover, by-pass valves will start to leak. This will cause a reduction in power recovery because (an increasing part of the) flue gas will be flowing through the by-pass line instead of flowing through the hot-gas expander turbine to allow recovery of power. It is therfore an object of the present invention to provide a solution for the above-mentioned problems and to improve the overall effectiveness of a hot-gas power recovery process and its adaptability to various operating conditions. Accordingly, the present invention relates to a process for the recovery of power from hot gases by introducing them into a hot-gas expander turbine provided with a by-pass for said hot gases wherein the by-pass is kept at elevated temperature. The present invention relates in particular to a process for the recovery of power from hot gases by using heat derived from gases having a temperature in the range of from 500 - 7500C, introduced to keep the by-pass at elevated temperature so that thermal shock will be avoided or minimized when the by-pass is taken into operation. The pressure of the hot gases is preferably in the range of 1.5-4.0 bar abs. Preferably the by-pass is kept at a temperature at which thermal shock is avoided or minimized by heat derived from the hot gases, which heat is not only the heat the hot-gases contain before entering the hot-gas expander turbine but may also be the heat which is left in the exhaust gases which leave the hot-gas expander turbine. Thus no extra (external) energy is necessary for heating the by-pass which improves the economics of the process. As the temperature of the exhaust gases from the hot-gas expander turbine is normally not more than about 2000C lower than the temperature of the hot gases entering the hot-gas expander and thus in catalytic cracking operations in the range of from about 3000600 0C, the by-pass is preferably kept within this temperature range by the heat of the exhaust gases. Although there are many possible constructional ways and means for heating the by-pass with heat derived from (the) hot gases there is a number of preferred options. In particular, the by-pass line is located inside the exhaust duct of the hot-gas expander turbine. Another preferred option is the location of the by-pass line inside the inlet duct of the hot-gas expander turbine. Still another preferred option is the location of the by-pass line partly inside the exhaust duct and partly inside the inlet duct of the hot-gas expander. In order to let down the pressure in the by-pass line to the lower level required by downstream equipment i.e. from a pressure with which the hot gases enter the by-pass line to a pressure with which the exhaust gases enter the exhaust duct of the hot gas expander turbine, an orifice chamber may be located in the inlet part and/or outlet part of the by-pass line. The orifice chamber not only serves the function of pressure let down, but also that of noise suppression. To economize the power recovery unit the by-pass line is preferably substantially an orifice chamber. Although the present invention may be used in any process which produces hot gases from which power can be recovered in a power recovery unit and which process also contains a by-pass around the power recovery unit, the present process is preferably applied in a fluid catalytic cracking process which produces hot gases in an oxidative catalyst regenerator containing one or more cyclones or separating stages for separating catalyst particles from hot gases which gases, having normally a temperature in the range of from 600-700 C, are further cleaned of catalyst particles in a next stage separator before entering a hot-gas expander turbine for the recovery of power. The present process is very suitably applied in processes in which power recovery units are used having by-pass facilities which normally operate at zero by-pass conditions. However, in order to improve the flexibility of the process from which energy is recovered by the present power recovery process the latter process may be operated in such a way that less than 100% by-pass is used. When a minor upset causes the rotating machinery of the power recovery unit - such as motor/generators and air blowers - to speed past some set speed, a flow by-pass of a few percent will be sufficient to cope with the upset whereas the by-pass should be so regulated that the flow by-pass is increased in case of upsets of a more severe order. Also under these circumstances the advantages of the present process are evident. The present invention further relates to an apparatus for the recovery of power from hot gases which comprises a hot-gas expander turbine having an inlet duct for hot gases, an exhaust duct for exhaust gases and a by-pass around the hot-gas expander turbine which is so arranged within the apparatus that the by-pass is in heat exchange with (the) hot gases and/or with the exhaust gases. Suitably the exhaust gases are led from the hot-gas expander turbine to additional heat recovery and/or stack facilities. In a preferred embodiment of the apparatus according to the invention the by-pass line is located inside the exhaust duct of the hot-gas expander turbine. In another preferred embodiment of the apparatus according to the invention the by-pass line is located inside the inlet duct of the hot gas expander turbine. Still another preferred embodiment is the location of the by-pass line partly inside the exhaust duct and partly inside the inlet duct of the hot-gas expander turbine. The present invention will now be further described with reference to the accompanying drawings and the Example. Fig. 1 represents one embodiment of the invention. 1 is an inlet duct through which hot gas is led to the hot-gas expander turbine 2 which gas after having been expanded flows through the exhaust duct 3. A by-pass line substantially consisting of an orifice chamber 7 is located inside the exhaust duct 3. A by-pass valve 4 is located between the inlet duct 1 and the exhaust duct 3. When the apparatus is on stream with zero by-pass the by-pass valve 4 is kept at operating temperature not only by the hot gases flowing through the inlet duct 1, but also by the exhaust gases flowing through the exhaust duct 3. The orifice chamber 7 is kept at elevated temperature by the exhaust gases flowing through the exhaust duct 3. Valves 5 and 6 may be used to decouple-the hot-gas expander turbine from the process which produces the hot gases by closing them completely while opening by-pass valve 4. 8 represents a turbine shaft. Fig. 2 represents another embodiment of the invention showing an exhaust duct 3 of a hot-gas expander turbine, which is not shown, through which exhaust gases from the hot-gas expander turbine flow and an inlet duct 1 through which hot gases flow to the hot-gas expander turbine. Inside the inlet duct a by-pass line substantially consisting of an orifice chamber 7 is located. Between the exhaust duct 3 and the inlet duct 1 there is a valve 4 at the outlet side of the orifice chamber 7. Example In a fluid catalytic cracking unit regenerator-flue gas 0 with a temperature of 659 C and a pressure of 3.16 bar abs. is routed via a third stage separator to a hot-gas expander turbine. 0 The clean flue gas with a temperature of 654 C and a pressure of 2.93 bar abs. passes out of the third stage separator through an inlet duct 1 and a valve 5 into a hot-gas expander turbine 2. The exhaust gases from the hot-gas expander turbine 2 having a temperature of 49!or and at atmospheric pressure (0.97 bar abs.) flow through an exhaust duct 3 inside of which a by-pass line substantially consisting of an orifice chamber 7 is located. The shaft 8 of the expander turbine is connected to an air blower from which air is supplied to the regenerator which in turn is connected to an induction motor/generator. The motor is activated to commence the start-up. During start-up, the expander absorps energy. As the start-up temperature and the flue gas flow increase, the expander turbine develops power and the electric motor draws less current. As the expander meets and exceeds blower horsepower demands, the train of hot-gas expander turbine, motor/generator and air blower increases in speed slightly and the electric motor performs as Åa generator and produces power which can be fed back into the refinery grid.

Claims: CLAIMS 1. Process for the recovery of power from hot gases by introducing them into a hot-gas expander turbine provided with a by-pass for said hot gases, characterized in that the by-pass is kept at elevated temperature. 2. Process as claimed in claim 1, characterized in that heat derived from hot gases having a temperature in the range of from 0 500-750 C is used to keep the by-pass at elevated temperature. 3. Process as claimed in claim 1 or 2, characterized in that exhaust gases leaving the hot-gas expander turbine are used to keep the by-pass at elevated temperature. 4. Process as claimed in one or more of claims 1-3, characterized in that the by-pass is kept at a temperature between 3000 and 6000C. 5. Process as claimed in one or more of claims 1-4, characterized in that hot gases are used emenating from a fluid catalytic cracking process. 6. Process as claimed in claim 1, substantially as hereinbefore described with particular reference to the Example. 7. Apparatus for the recovery of power from hot gases according to a process as claimed in one or more of claims 1-6 comprising a hot-gas expander turbine having an inlet duct for hot gases, an exhaust duct for exhaust gases and a by-pass around the hot-gas expander turbine, characterized in that the by-pass is so arranged within the apparatus that it is in heat exchange with (the) hot gases and/or with the exhaust gases. 8. Apparatus as claimed in claim 7, characterized in that the by-pass line is located inside the exhaust duct or inside the inlet duct of the hot-gas expander turbine. 9. Apparatus as claimed in claim 7, characterized in that the by-pass line is partly located inside the exhaust duct and partly located inside the inlet duct of the hot-gas expander turbine. 10. Apparatus as claimed in one or more of claims 7-9, substantially as hereinbefore described with particular reference to the drawings.

Inventors: MCKILLOP, JOHN GORDON
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1982
Main classification: F02C1
IPC codes: F02C1, F27D17, F02C9, C10G11, F02C6