Jan-2013

Optimising a process scheme for platforming heaters

Thermal imbalance resulting from an existing process scheme for CCR platforming furnaces is addressed by process schemes for distributing heat load more uniformly

Adil REHMAN, Suman PACHAL, Shyam K CHOUDHARY, Ugrasen YADAV and M K E PRASAD
Technip KT India

Article Summary

Concern over safety issues, especially in industries such as refining, lead operating personnel to be more and more reluctant to operate their plants near design or permissible limits, thereby avoiding the risk of mishap or industrial accident. On the other hand, they are under pressure to increase their profit margins.

CCR platforming heaters in an operating oil refinery have been in operation for the last 30 years or so. Due to the existing process scheme, a thermal imbalance in all four of the CCR platforming unit’s heaters had been observed in terms of permissible limits for tube metal temperatures and uneven bridgewall temperatures. The operator decided to engage a consultant to analyse the problem and to come up with an optimised process scheme, thereby avoiding operation of the furnaces near the design limits of the process coils.

The study was carried out based on various document and data provided for the existing system in the form of DCS output to obtain full details of the process.

Methodology of study
The feed property grid was generated using PRO II and a thermodynamic method (Grayson Streed). A reasonable process-side fouling factor in the convection zone has been considered, which is quite likely to happen over a long period of operation. The CCR platforming heaters were studied using commercial software (FRNC-5PC Version 4.18 Mod 7.6) with the following assumptions and data for modelling: radiant heat loss of 2.5%; ambient air temperature of 17°C and relative humidity of 95%, as per the meteorological data provided by the client; maintaining the oxygen percentage at the outlet of the convection section by keeping the quantity of air the same as in the existing operating case in order to account for the ingress of air, which might have taken place over a long period of operation.
The algorithm followed for the study is shown in Figure 1.

Definition of fluid
Fluid in this case is defined and identified as process feed entering the reactor. Thus, process fluid entering Reactor 1 is called “Fluid 1”, process feed entering Reactor 2 is “Fluid 2” and process feed entering Reactor 3 is “Fluid 3”. This definition of fluid is shown in Figure 2.

Comparison of existing scheme with proposed scheme
The existing scheme according to DCS outputs is summarised in the process flow diagram shown in Figure 3. The proposed scheme’s results obtained from FRNC have been summarised in the form of process flow diagrams shown in Figures 4, 5 and 6 for proposed Options I, II and III, respectively.

Results and discussion
Mechanical aspects
Option I
This scheme is identical to the existing scheme with regard to the process flow pass arrangement inside the heaters. Thus, modifications inside the heater are minimal with respect to the existing scheme. However, the north-side interconnecting process header between 501A and 501B needs to be cut in order to connect the process outlet of 501A to the inlet of Reactor 1. Similarly, the outlet of Reactor 2 needs to be connected to the inlet of 501`B. Also, the outlet of 502, which goes to Reactor 2 in the existing scheme, needs to be dismantled and rerouted to the inlet of 503. Ultimately, this scheme would require a lot of external mechanical modifications and rerouting of hot piping, cutting of large-sized process headers and so on, and thus would require a long shutdown time.

Option II
In this scheme, the interconnecting north-side header between 501A and 501B needs to be blinded/dismantled and the outlet of 501B, which goes to the inlet of Reactor 1 in the existing scheme, needs to be routed to Reactor 3. Similarly, the outlet of Reactor 2, which goes to the inlet of 503 in the existing scheme, needs to be rerouted to the inlet of 501B, and the outlet of 502, which goes to Reactor 2 in the existing scheme, needs to be connected to the inlet of 503. The interconnecting north-side header between the inlet and the outlet of 502 needs to be dismantled/blinded. This option would require changes inside the heater process flow pass arrangement (single pass for 501A and 501B in Option II, against double pass for 501A and 501B in the existing scheme). This scheme would require a lot of external rerouting of heater outlet and inlet piping to and from the reactors and thus would directionally require a longer shutdown time compared with Option I.

Option III
This scheme is similar to the existing scheme with regard to the process flow arrangement inside the heaters. The interconnecting north-side header between 501A and 501B need not be cut. However, the Packinox outlet, which goes to the inlet of 501A in the existing scheme, now needs to be routed to the inlet of 502. The Reactor 1 outlet, which goes to the inlet of 502 in the existing scheme, needs to be routed to the inlet of 501A. Similarly, the outlet of 502, which goes to the inlet of Reactor 2 in the existing scheme, needs to be routed to the inlet of Reactor 1. Apart from external rerouting, there is no cutting/dismantling of interconnecting headers and no change in heater internal pass flow arrangements, with respect to the existing scheme, in Option III. Thus, Option III would directionally require minimum mechanical modification and rerouting compared with Options I and II, and consequently would require the least shutdown time.

Maximum tube skin temperature
The maximum tube skin temperature encountered in Options I and II is 545°C, well within the permissible tube metal temperature of 559.8°C. A comparison of maximum tube metal temperature is shown in Figure 7 for various proposed process schemes compared with that encountered in the existing process scheme.