TABLE 1. PRE- AND POST-SHUTDOWN OPERATING CONDITIONS Pre-shutdown
Engineering Practice brought to minimum governor speed. Steam to the reboiler was started 10 h later. The depropanizer was operated in the liquid re-injection mode for the following 12 h. Throughout this period, several moves were made on the reflux and reboil rate. Since these operations were unsuccessful in bringing the tower bottom within specification, the operation mode was transitioned to the condensate stripper scheme. Furthermore, roughly 5,000 lb/h of offspec C4’s from storage was fed to the depropanizer through the external C4’s feed point to help load the tower. The tower was still performing poorly and process engineering support was then requested by operations. Depropanizer troubleshooting. Prior process engineering work [2] done on startup conditions highlighted the risks associated with operating high-capacity trays below their minimum liquid limit. There was concern that the tray load might not be sufficient to seal the downcomers. However, an initial high-level review of the operating conditions did not reveal anything abnormal. As described in Table 1, the tower operating conditions were not significantly different from the pre-shutdown ones. Plant data also suggested that the upper section was working fine, since reflux changes and overhead composition were varying as expected. The stripping section, however, was performing poorly. Simulation work and a gamma scan on the tower were then conducted. The Operations Dept. was asked to maintain stable conditions, regardless of performance, for the duration required to conduct the gamma scan on the tower. The intent of the scan was to determine if any mechanical anomalies were present inside the tower and to assess the tray-loading profile. The gamma scan results (Figure 3) indicated no mechanical anomalies and no sign of flooding. Vapor-liquid disengagement looked better compared to a previous scan done at a normal plant rate. Many of the high-capacity trays showed a clear vapor space equivalent to the vapor space above the reboiler return and the bottom tray. Scanning across the tray could not provide any information with regard to the down50
Post-shutdown (at time of Gamma Scan)
Feed from the deethanizer, thousand lb/h
112.8
105.6
Feed from the condensate stripper, thousand lb/h
63.0
72.8
External C4’s, thousand lb/h
0
5 (off-spec C4s)
Reflux rate, thousand lb/h
98.9
94.3
LP steam to reboiler, thousand lb/h
24.2
26.3
Tower pressure, psig
123
122
Tower pressure drop (in. H2O)
111
112
TABLE 2. PROBLEM ANALYSIS Deviation: Depropanizer bottom stream C3s outside specification Is Is not Differences Changes Probable cause • Low • Feedrates Object: What Object: rates in Depropanizer Deethanizer • Properties C3 area • Process bottom conditions stream • Composition Defects: Defects: • Trays Bottom C3s outside product spec- specification ification Where
Depropanizer stripping section
Depropanizer rectifying section
When
Startup
Pre-shutdown
• Low • Tray type • High capacrates in • Composiity tray turnC3 area tion down not • Hydraulic met load • Temperature • Properties • Path to low- • Low • Stripping rates in rate operasection tray C3 area tion downcomer unseal • Stripping section tray weeping
comer operation. Based on the work done by Urbanski and others [3], it is assumed that tray weeping cannot be determined by the gamma scan. The tower was simulated with inputs reflecting operating conditions sustained at the time of the scan. The intent of the simulation was first to ensure that the trays were operating within their hydraulic capabilities. The operating point relative to flooding was calculated using the Kister and Haas correlation [4] for the sieve tray section. The operating point for the highcapacity trays was plotted on the tray operating window obtained from the tray vendor. The simulation results estimated the sieve tray operation at 72–73% of flood, which compare to 68–70% of flood for the pre-shutdown operation. The high-capacity tray operating points are shown on Figure 4 and compared against loads calculated
CHEMICAL ENGINEERING WWW.CHE.COM JUNE 2012
for pre-shutdown operation. As for the sieve tray, the startup conditions showed loads slightly higher than preshutdown. Tray loading calculations were consistent with the gamma scan results, which supported the conclusion that the trays were not flooded but rather were operating at low rates. Some of the key information gathered was structured into a problem analysis matrix, as shown in Table 2. The concept of problem analysis is to look at what, where and when the problem is — and is not — and to look at differences and changes, as well. It was clear from Table 2 that the main change was the low feedrates in the C3 area. A review of previous startups concluded that the plant had never been brought up to steady-state conditions at such low rates. A light feed slate similar to preshutdown conditions explained the lower rates to the C3 and C4 area. This