It is not a hard and firm rule that once drying and firing has been accomplished, the refractory lining is capable of everything, e.g. cooling and re-heating, thermal shocks, abrupt cooling and the like.
But it may be clear that once all water is removed, heating rates can be adopted in accordance with the materials properties, e.g. thermal conductance, shock resistance, stresses, etc. Also cooling rates or thermal shocks can be adjusted.
Common rule is: • re-heating rate: • cooling down rate:
linear 90 - 120 K/h.; 60 K/h, maximum 90 K/h. 4.2 Dedicated drying and firing schedules
Although the majority of equipment used in refineries will follow the above requirements, there is equipment, such as FCC, SC/GG, SRU, which need specialised schedules resulting from their specific nature or line-up of various critical components.
Close cooperation with and integration of OP’s start-up procedure for the units as a whole should be planned, worked out and applied.
4.2.1 Fluid bed catalytic crackers
The Reaction and Regeneration section of a L/R-FCCU consists generally of the reactor, the regenerator, stripper and cat. piping connecting between the vessels, which are lined with different types of (erosion-resistant and/or insulating) refractory materials. The regenerator with its insulating refractory lining requires most of the attention with regard to drying and firing, operates at the highest temperature and is also during operation under oxidising condition, contrary to reducing condition of the reaction section. For these reasons procedures are mainly dedicated to the regenerator and the other vessels will accommodate as far as required.
FCCU's have their own auxiliary pre-heater, which preheats the regenerator air flow. With a temporary bypass the reaction section can be heated as well.
Nowadays with all the different types of refractories: hydraulically bonding, chemically bonding, low-cement type and even the use of ceramics, a harmonised, dedicated procedure is used. Additionally it takes into account the start-up requirements for final operation, i.e. catalyst loading and heating, without upsetting other lined sections.
4.2.2 Sulphur recovery Units
Figure 9 shows a dedicated schedule for a SRU unit. These units consist of a line-up of various and different equipment connected via piping. The generally line-up in a Sulphur Recovery Unit is as follows: • Main burner and Reaction Chamber; • mostly integrated with a Waste Heat Steam Boiler (WHB); • Line burners and Mixing Chambers; • Reactors, Incinerator, and/or SCOT, • followed by a Chimney or a Duct to a stack.
In figure 8 it is shown that, similar to fired equipment, the actual temperatures downstream of the main reaction chamber will tarry behind, especially because of the WHB. In first instance steam circulation through the WHB can be used to dry out the downstream equipment at ~110 °C, when necessary later on assisted with the line burner, depending on possible turn-down ratio. In SRU's it is often not possible to execute the required drying and firing procedure of the main reaction chamber using the H2
S/Fuel gas burner.
The minimum burner capacity is often too high to achieve the initial lower temperature levels. In such case dedicated auxiliary or special burner equipment have to be installed for this temperature section, for which professional companies can be consulted.
If reactor catalyst will be loaded after complete dry out, the unit shall be cooled down at 30 K/h. and re-started from the beginning, but then a hold time of 4 - 8 hours at 110 °C will only be required, mainly meant as 'stabilising' period to avoid possible trips, temperature runaways, etc.
As a further improvement for the control of temperatures of the refractory linings in individual equipment 'sacrificial' thermocouples could be installed, especially during the initial drying and firing procedure and/or after major maintenance activities.
4.2.3 Furnaces, another approach for fired equipment
Figure 7: Harmonised schedule for FCCU
Figure 8 shows a dedicated procedure for a common furnace section, e.g. HVU, CDU etc. Simultaneously it shows the actual temperatures reached in all lined-up equipment, i.e. radiant cells, convection bank and chimney, during the execution of the procedure.
Figure 7 shows a dedicated procedure for a FCC-unit. The main difference is the adopted level of integration of the 'holding time' with the subsequent period of (batch-wise) loading of the catalyst. In the past the standard procedures were used separately, because the majority of refractories were hydraulically bonding refractories and “thermal shocks” as result of the cold catalyst loading were generally neglected, or rested unidentified.
The procedure was partly executed with auxiliary burner equipment. Tie-ins were made in the central air ducts to the burners of each furnace block. With the main reason to be able to use the present burner assemblies and their capacity to distribute the heated combustion air flows ‘through all corners’.
However, as a result of limited turn-down of the high-capacity burners at the start-up of the unit, i.e. for (initial and later) drying of the refractory
14 ENGINEER THE REFRACTORIES
May 2018 Issue
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