1. Inter-Office Memo
From: D. Willard Date: August 22, 1997
Subject: Slurry Tank Pressure Relief, Pressure Relief
Rationale
Location: Ashtabula Complex
To: T. Messer S. DiBacco
cc: File
A Millennium Chemicals Company

2. Inter-Office Memo
Liquid TiCl4 reacts spontaneously with water (hydrolysis)
producing HCl gas that expands rapidly causing a rapid pressure rise. A relief device is required on
the slurry tank as a countermeasure for this contingency.
Because solids may be carried out during a rupture, the discs must be oversized. A pair of rupture
discs with burst tolerance of 5% performs better than a relief valve that could plug. Once plugged,
the valve may stay closed even with additional relief flows. Set the first disc at no more than 95% of
the tank maximum allowable working pressure (MAWP). The set pressure of the first disc will be no
more than 142.5 psig for the 150 psig MAWP established under Division 1 of Section VIII of the
ASME Boiler and Pressure Vessel code.
The most credible relief scenario is operation of the TiCl4 vaporizer without oxygen provided by the
oxygen preheater. A DCS programming error could cause this scenario. The scenario could occur
during start-up, or while the train is operating at full capacity. Under all circumstances, pure,
saturated, TiCl4 vapor condenses in the flue pond. The relief flow rate of the slurry tank rupture
discs is determined by the vaporizer operating rate. The vaporizer rate is much less than the capacity
of the star valve; in 1990, United Engineers & Constructors (UE&C) used the capacity of the star
valve to size the rupture discs for 15,000 PPH. At a maximum speed of 10 rpm, a star valve can
deliver 76,000 PPH of TiCl4! This assumes no fouling of the valve pockets. At worst, condensation
will produce 47,000 PPH of TiCl4 at 10.0 TPH. This condensation generates about 5,060 PPH of gas
in the slurry tank at 10.0 TPH. Laboratory analysis for the reaction of TiCl4 liquid and pure water
was used to develop a gas generation rate. Complete calculations are attached to the memorandum
for relief flows at 6.0 TPH, 9.0 TPH, and 10.0 TPH. 6.0 TPH is the start-up rate for a class ‘S’
(Small) STAR.
The relief flows are determined assuming pure, saturated, TiCl4 vapor. The vaporizer is designed to
superheat the vapor; superheated vapor has a lower film coefficient, drastically reducing
condensation in the flue pond and Pair filter. Reactor nitrogen will dilute the TiCl4 vapor, further
reducing condensation. Nitrogen composes 10-9% of the Paire inlet gas stream. If chlorine flows to
the reactor, condensation will be reduced even more. The net effect of assuming pure, saturated
TiCl4 is to produce calculations which are reasonably conservative.
Laboratory analysis was used to estimate quantities of HCl – H2O vapor produced for a given mole
ratio of Water over TiCl4. This work was done for the 1990 expansion in Stallingborough. This work
is highly dangerous and was carried out with small volumes of water and TiCl4 which were added to
a combustion bomb. Pressure rise and temperature rise measurements were made, but, exact
calorimetry data was not collected. Because of the small volumes used during the test, the ratios used
were low compared to those expected for the reaction in the TiO2 slurry tank. Regression analysis
was used to estimate temperature and pressure effects at higher ratios. Water in the slurry tank is rich
in chlorine. The effect on TiCl4 hydrolysis is unknown. It is assumed to be negligible. Normal slurry
tank level and concentration of TiO2 showed little impact on the relief flows developed for the sizing
program.
An adequate water concentration is crucial to reducing the damage from hydrolysis. At low level, the
heat absorbed by the tank liner will be substantial. It is recommended that water flow should be
increased during a relief situation to reduce the tank temperature and gradually kill the reaction,
A Millennium Chemicals Company

3. Inter-Office Memo
provided that the relief vent is not blocked.
Program results show that the existing 2” rupture discs are adequate for at least 10.0 TPH capacity.
The potential for solids in the relief is a problem. It is best to consider increasing the relief discharge
in the future from 2” to 3”.
If there are any questions contact me.
A Millennium Chemicals Company