Overcoming high conductivity in process condensate
Analysis of design and operations enabled a refiner to eliminate contaminants from hydrogen plant process condensate by installing a high pressure stripper
TAPAS Kanti SAMADDAR and GREGORY M LILBURNE
The Bahrain Petroleum Company BSC(c)
CALUM MCINTOSH and MILETA BABOVIC
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In 2007 the Bahrain Petroleum Company’s (Bapco) Low Sulphur Diesel Production (LSDP) Complex came onstream. The complex centres on a 60 000 b/d heavy vacuum gas oil (HVGO) hydrocracking unit, which is supplied with hydrogen from the No.2 Hydrogen Plant (2H2P). 2H2P is a conventional unit, licensed from Technip USA, consisting of zinc oxide (ZnO) purification, a top-fired Steam Methane Reformer (SMR), High Temperature Shift (HTS) conversion and a pressure swing adsorption (PSA) unit. The ZnO, SMR and HTS catalysts were all supplied by Johnson Matthey.
The feed to 2H2P is sweet natural gas and the plant design capacity is 100 million scfd of 99.9% purity hydrogen. 2H2P is a large exporter of 600 psig superheated steam to the rest of the refinery.
After start-up, Bapco found that the water in the refinery condensate systems had high conductivity and had to be dumped to sewer to prevent it from entering the boilers in the Bapco operated power plant. The cause was ultimately attributed to the presence of ammonia (NH3) and methanol (MeOH) in the 2H2P process condensate, which was recycled within 2H2P for the generation of steam that was then exported, condensed and recycled to the power plant. The high conductivity masked the presence of any leaks from the seawater cooling system into the refinery condensate systems and was also a threat to the integrity of these systems due to corrosion. The 2H2P process condensate, equivalent to 200 US gal/min, therefore had to be routed to the oily water sewer (OWS). Consequently, the production of make-up water to the refinery, the flash evaporated distillate (FED) from the desalination units, had to be increased and a significant financial penalty of the order $100 000 per month was incurred. Hence a permanent solution was required.
Bapco undertook a detailed review and evaluation of different options for reducing NH3 and MeOH levels in the process condensate. This also involved discussions with several experts including the catalyst vendor, process licensor, other hydrogen plant operators and water treatment consultants. Ultimately the company decided that the most cost effective approach was to install a high pressure (HP) Steam Stripper.
The new stripper was designed by Technip USA. Johnson Matthey was again closely involved to provide catalyst performance predictions for the revamped process flow scheme. The stripper was successfully commissioned in Q1 2011.
The process engineering and catalytic issues associated with the new HP Steam Stripper, as well as the concerns, problems and limitations that arose during project development and unit start-up, and how they were overcome, are presented in this article.
Process condensate flow scheme (original design)
The original process flow scheme before the installation of the new HP Steam Stripper is shown in Figure 1. About 200 US gal/min of process condensate is produced in 2H2P during normal operation. The condensate system design that Bapco selected was the same as that used in the older No.1 H2 Plant (1H2P), which had never had any problems.
In the 2H2P flow scheme, the desulphurised natural gas is first mixed with high pressure superheated steam and this combined stream is further heated in the feed-steam superheat coil in the SMR and flows into the top of the reformer tubes. The reformer effluent is cooled to about 650°F (343°C) in the process gas boiler and flows into the HTS reactor. The HTS reactor effluent is then cooled to 110°F (43°C), which also causes the steam in the effluent to condense. The process condensate is separated from the hydrogen-rich process gas in the PSA feed knock out (KO) drum. The gas is routed to the PSA unit for final purification.
In the original as-built unit configuration, the process condensate from the KO drum was mixed with FED make-up water and routed to the deaerator for degassing, using low pressure (LP) 25 psig steam. The deaerator operating pressure is 15 psig. The condensate/FED from the deaerator was heated and recycled to the steam drum for the generation of 600 psig steam. The steam is superheated and part of this stream is routed to the SMR as feed process steam, with the remainder being exported to all parts of the refinery.
Condensate from the refinery steam users is collected in the High Grade (HG) and Low Grade (LG) condensate systems and returned to the power plant, where it is mixed with FED and flows into the boilers as boiler feed water (BFW). Conductivity meters are installed on the condensate return headers to detect leaks from the seawater cooling systems into the steam/condensate systems. When the conductivity goes high, above 25 µS, all the condensate is automatically dumped to the OWS to ensure that any hardness salts do not enter the boilers. They would cause scale formation, corrosion and hot spots. Dumping of condensate is a very costly emergency procedure because the FED production rate from refinery desalination units must be substantially increased to meet the refinery’s FED and steam demand.
The problem: impurities in process condensate
During commissioning of 2H2P, the process condensate was initially routed to sewer. However when 2H2P was brought fully onstream, the condensate was routed to the deaerator and then to the steam drum, with some of the 2H2P superheated steam being exported into the refinery steam system. After about 30 minutes, the refinery condensate conductivity shot up from a typical value of < 10 µS to 140 µS. As a result, the condensate recovery system’s high conductivity alarms were activated and all the condensate was automatically dumped. The situation returned to normal when the process condensate was again routed to OWS in 2H2P.
Through a series of plant trials, Bapco established that the high conductivity in the refinery condensate return systems was caused by the presence of NH3 in the 2H2P process condensate, at typical levels of 500 wppm, which had ended up in the superheated steam system, via the steam drum. In addition to NH3, the condensate also contained other dissolved gases like carbon dioxide (CO2), carbon monoxide (CO) and other by-products including MeOH and formic acid.
Bapco had previously experienced elevated concentrations of NH3 in the process condensate in the No.1 Hydrogen Plant (1H2P) but this was a short term phenomenon attributed to high activity fresh catalysts and it disappeared within four to six weeks after start-up. However on 2H2P, the problem persisted and even after six months of operation the NH3 had only decreased to 300 wppm. The MeOH concentration was 200 wppm.
During this period, Bapco worked very closely with Johnson Matthey, the 2H2P catalyst vendor, to identify and understand the reaction mechanisms that were occurring.
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