(151d) Dynamic Instabilities of Two Parallel Integral Superheaters to Vaporize Ngl Feed to Olefins Cracking Furnaces

Cracking propane and butane in ethylene producing furnaces requires that the liquid feed be first vaporized to a superheated temperature before being introduced into the olefins furnaces. Conventionally, superheating these liquid feeds (C3 and C4) were performed in a two-unit operation, namely, a vaporizer unit to generate a saturated condition, followed by a superheater unit to achieve the desired degree of superheating. The vaporizer and the superheater are thus two separate units that are in tandem, yet physically separated from each other. A new technology has been implemented in which the two units were combined in one shell-tube type heat exchanger vessel, where the inlet liquid feed (on the shell side) is vaporized and superheated in one path. When two such combined units are installed in parallel, i.e. sharing same inlet and outlet manifold headers, flow and pressure oscillations were observed which causes unstable operation in the vaporizer trains and risks the reliability of the manufacturing operation. . In fact, it was observed that such instabilities do not appear when one unit is operating alone, and that the instabilities come about when the second unit is brought on line. It was also observed that such instabilities persist indefinitely once the second unit is brought on line regardless of the process conditions.

Dynamic simulations were conducted on this new vaporizer using HYSYS, which revealed the dynamic interaction between the two units that was not easily detected with current plant instrumentation scheme. It was shown that the overall pressure and flow oscillations across the two units were induced thermally via a swinging mechanism of liquid hold-up between the two units.

Open loop analysis was used to create an instability map, indicating that certain combinations of pressure and temperature could either render the system stable or unstable. A short term solution to operate the system stably was suggested based on this map. Further, a new, improved control scheme was tested under several simulated disturbances including switching from one-unit to two-unit operations and proved stable and robust. Recommendations for operating such new technology vaporizer units are made.