A major US refinery was experiencing severe fouling and
plugging problems in two shell-and-tube (S&T) heat
exchangers installed to cool desalter effluent using cooling
water. In 2008, the refinery replaced the two S&Ts with a
fouling-resistant spiral heat exchanger (SHE). The SHE gives
continuous problem-free operation, higher process reliability and more consistent
wastewater treatment temperatures than the S&Ts. Best of
all, there has been no plugging and, based on savings in maintenance and cleaning costs
alone, the payback time was less than 18 months.
In the desalting process, crude oil is contacted with hot
water to remove impurities such as chloride salts and
particulate matter before continuing through the crude preheat
train into the refinery. The solids and salts
collect in the process and must be flushed out with the
effluent water. In a process called mudwashing,
solids are mixed with the effluent water and removed. The
effluent waternow contaminated with oil, suspended solids
and dissolved solidsneeds to be cooled before traveling
downstream to wastewater treatment.
As shown in Fig. 1, Refineries typically use two stages of
cooling before wastewater treatment: 1) effluent cooling by
preheating feed water, and 2) effluent cooling with cooling
water. The first application is a heat recovery service; so
maximizing the efficiency of this heat exchanger is important.
The second heat exchanger is a trim cooler and is designed to
protect the wastewater treatment plant from excessively hot
effluent. Unfortunately, due to severe fouling in both
applications, refiners will bypass the heat-recovery service
and foul the cooling-water exchanger so rapidly that it damages
the wastewater treatment system. The consequences of the damage
include unplanned downtime and environmental penalties for
non-compliance. Fortunately, there is a solution to this
widespread problem of desalter effluent fouling with the SHE.
These facts are very evident in a specific US refinery that has
operated a SHE in desalter service since 2008.
1. Simplified desalter system
The process of cooling the effluent water has caused fouling
headaches in refineries for years, as was the case with the
major refinery covered in this article. Due to continuous,
costly fouling and plugging problems, it was necessary to shut
down the S&Ts cooling the desalter effluent water once a
month on average. The unplanned downtime caused problems for
the wastewater treatment plant, since the effluent temperature
was poorly regulated and frequently out of specification.
SHEs have a well-proven technology for fouling applications.
Its single-channel design resists plugging while the fully
counter-current flow paths allow for effective heat recovery in
a compact space.1 SHE is the only technology that
copes easily with the solids in the stream and can handle
process upsets with high solid concentrations. Due to the
design efficiency, it was possible to install just one SHE.
This only had 50% of the surface area of the S&Ts, yet it
still outperformed them in the long term.
Shortly after the SHE startup, the refinery had satisfactorily noticed
that the pressure drop through the unit remained stable.
Increasing pressure drop had frequently been one of the reasons
the refinery needed to shut the S&Ts down for cleaning. The
SHE continued to provide trouble-free operation and, over an
extended period, only a moderate reduction in thermal
performance was measured.
Cleaned once since startup.
In June 2009, after 14 months of operation, the refinery
opened the SHE for the first time to check for foulingan
operation made easy by the option of integral davits on each
cover. To their surprise, the engineers found only a thin
greasy coating on the effluent side and minor scale on the
cooling water side. With the S&T exchangers, they witnessed
many plugged tubes after only one month. The spiral was
mechanically hydroblasted and placed back in service the next
day. By December 2010, it was noted that the SHE had only been
cleaned once since startup and was still operating efficiently.
An onsite engineer stated that its doing well and there
have been no complaints.
The cost to clean the tube side of the two S&T
exchangers was estimated by the refinery in Table 1. Given the
frequency of cleaning, the investment cost for the SHE was
returned in less than 1.5 years on savings in maintenance alone (Table 1).
The SHE technology includes these
Fouling problems in refinery desalter effluent
services can be greatly reduced or eliminated.
The single-channel design and robust construction make the spiral an
ideal product for this challenging refinery service (Fig.
Often, the capital cost of the spiral equipment can
be justified with the savings in maintenance/cleaning costs alone,
not to mention the added reliability of the process and more
consistent wastewater treatment temperatures.
2. Single-channel geometry
resists plugging while counter
current flow efficiently recovers
1 Anderson, E., Minimizing refinery costs using spiral heat
exchangers, PTQ Q2 2008.
Wajciechowski is currently a business
development manager in the process technology division of Alfa
Laval. Mr. Wajciechowski is a graduate of Virginia Tech
with a BS degree in chemical engineering. Prior to his
current role, he held several positions in the refining and petrochemical industry for
Alfa Laval. Mr. Wajciechowski has been with Alfa Laval
for 12 years.