Sonatrach's GL1-Z LNG plant in Arzew, Algeria, installed an LPG-recovery heat exchanger downstream of an existing overhead condenser to alleviate problems due to changes in the plant's feed composition.
The new exchanger uses a multicomponent refrigerant (MCR) as the cooling medium.
Although operating problems in the scrub tower were resolved, the LNG product high heating value (HHV) decreased and the fractionation unit became overloaded.
This article discusses the installation and optimization of the installed LPG-recovery heat exchanger.
The plant originally processed a heavy natural gas. Currently, and since the start-up of a new neighboring LPG plant, the feed quality has become leaner with fewer heavy hydrocarbons. In fact, the propane concentration has decreased to less than 2%.
Leaner feed gas makes it increasingly difficult to recycle the required propane and butane to the scrub tower. This has consequently led to long start-up times to stabilize the scrub tower operation before feeding gas to the main cryogenic heat exchanger and a considerable amount of gas flaring.
Adding an LPG-recovery heat exchanger in the scrub tower overhead system played an essential role in solving these problems.
Starting up the trains, which was a major concern for the plant, is currently accomplished in a short time and with little flaring. This decreases production losses and minimizes autoconsumption to 14%/year from about 22 %/year.
A review of the design cooling temperature of the LPG-recovery heat exchanger helped us optimize LPG extraction, which improved the LNG's HHV by about 200 kcal/cu m. LNG quality could be further improved if the butane product were reinjected into the feed stream of the main cryogenic heat exchanger.
Keeping the cooling temperature as warm as possible while maintaining the required LNG product specification and scrub tower LPG extraction maximizes the LPG-recovery heat exchanger's efficiency.
The LPG-recovery heat exchanger enables the plant to meet the LNG product specification over a full range of potential gas compositions.
The GL1-Z plant uses the Air Products & Chemicals Inc. propane precooled-MCR process to liquefy the natural gas.
Two systems supply the refrigeration required for liquefaction: propane and an MCR.
The plant is designed to produce the equivalent of 53.03 billion kcal/day of HHV LNG measured in the storage tanks for each of six process trains. LNG is stored in tanks before being loaded into tankers.
Initial process design
Feed natural gas leaving the dryers is cooled in two feed chillers that use medium-level and low-level evaporating propane as the cooling media. A temperature-control bypass around the feed chillers controls the temperature of feed gas to the scrub tower.
Feed enters the scrub tower at either –20° C. for the light feed case or –26° C. for the heavy feed case. At those temperatures, heavy hydrocarbon components are partially condensed at an operating pressure of 39.5 barg.
The scrub tower removes heavy hydrocarbons from the natural gas to meet a required specification for the cryogenic heat exchanger feed stream.
A small recycle stream from the fractionation columns joins the scrub tower overhead vapors to increase the tower's reflux ratio. The recycle stream is a mixture of ethane, propane, and butane.
The combined stream is cooled to about –34° C. in the scrub tower condenser using low-level propane refrigerant.
Scrubbed overhead gas that is partially condensed flows to the scrub tower separator.
Liquid from the separator returns to the scrub tower as reflux. Vapor from the separator flows to the main cryogenic heat exchanger for liquefaction.
The bottoms from the scrub tower flows to the fractionation unit for separation in the de-ethanizer, depropanizer, and debutanizer columns into ethane, propane, butane, and gasoline streams.
Excess ethane, propane, and butane from the fractionation columns are reinjected into the feed stream to the cryogenic heat exchanger to increase the LNG's HHV.
Original operating difficulties
Due to lighter feedstocks, the plant had difficulties obtaining the required propane and butane to feed the scrub tower.
The time to reach stable LNG production was so long that a considerable amount of off-specification feed gas to the cryogenic heat exchanger had to be flared.
The plant consequently lost a significant number of production days; flaring overhead scrub tower gas for up to 4 days increased autoconsumption up to 22%/year.
The longer start-up times, from feed in to the scrub tower until the required overhead specification was obtained, were because:
- The scrub tower and fractionation unit took around 4 days to build up a sufficient inventory of propane and butane in order to meet a scrub tower overhead pentane specification.
- Liquid propane from the fractionation unit was insufficient to build up the necessary initial inventory and continuous makeup for the propane loop. This plant, therefore, imported liquid propane for start-up.
System modification
A plate heat exchanger using MCR as a cooling medium was installed at the scrub tower overhead circuit, downstream of the propane condenser. This modification helps increase LPG extraction to reduce start-up time and meet LNG product specification.
This exchanger can eliminate the need for propane and butane recycle during normal operations.
Vapors leaving the scrub tower overhead condenser pass through the LPG-recovery heat exchanger, where they are cooled to –50.3° C. for the lean feed case and –43° C. for the rich feed case.
The MCR refrigerant flows from the high-pressure MCR separator and returns to the MCR first-stage suction drum.
The cold scrub-tower overhead then flows to the scrub-tower separator where LPG liquids are separated and returned to the scrub tower as reflux. Process gas leaving the separator passes back through the LPG-recovery exchanger to minimize the consumption of MCR refrigeration before flowing to the main cryogenic heat exchanger.
LPG-recovery exchanger benefits
Benefits of the LPG-recovery heat exchanger on the scrub tower overhead circuit include:
- A considerably shorter start-up time with a resultant decrease in gas flaring due to more stable operation of the scrub tower. The trains' start-ups are currently accomplished with a small amount of flaring and less production losses.
- No propane and butane recycle needed during normal operations. Because sufficient reflux is created in the LPG-recovery heat exchanger, the butane recycle to the scrub tower is not required to achieve the overhead product pentane specification. Propane and butane imports were discontinued.
New problems encountered
The plant encountered some new problems when operating the new LPG-recovery heat exchanger at its design temperature.
These problems are mainly:
A decrease in the LNG HHV. A significant decrease in the HHV of the LNG product was due to the lowe
The plant normally reinjects excess propane and butane into the feed to the cryogenic heat exchanger to compensate for the deficiency of heavy hydrocarbons. This is an efficient method to control the HHV of the produced LNG.
The plant would send excess propane and butane intermittently using the fractionation reflux drum level control. This was discontinued, however, because it caused frequent plugging in the liquefaction unit due to freezing pentanes entrained with the reinjected butane.
Overload of the fractionation unit. The LPG-recovery heat exchanger, operating at low cooling tempe
In addition, the excess ethane, propane, and butane could not be reinjected due to the previously mentioned LNG product specification limits and plugging problems.
Due to these two problems, the fractionation reflux drums were overloaded, which resulted in a higher operating pressure in the depropanizer and debutanizer columns. At that time, materials causing the excess pressure were flared, causing many disturbances of fractionation unit.
- The formation of solids. Pentanes that carried over with reinjected butane caused plugging at the LNG product pump's suction and in the tubes of the main cryogenic heat exchanger. This plugging and solid formation in the liquefaction system required a shutdown of the process trains to derime the cryogenic section.
Design temperature review
GL1-Z made many efforts to satisfy the LNG quality and meet the required specification. First, it adjusted the cooling temperature of the LPG-recovery heat exchanger. This temperature was raised from the design value to about –43° C.
Sonatrach then decided to review the design cooling temperature of the LPG-recovery heat exchanger, mainly to:
- Improve the LNG's HHV while maintaining the required LPG extraction. This would satisfy the required reflux rate and avoid scrub tower disturbances.
- Eliminate the fractionator disturbances.
*Eliminate the reinjection of ethane and butane into the main cryogenic heat exchanger to avoid exceeding the LNG ethane specification and liquefaction plugging problems, respectively.
To accomplish this, the technical and operations departments conducted a field test run on one train in May 1998 to determine the optimum cooling temperature of the LPG-recovery heat exchanger. This test was conducted with real operating conditions.
Test conditions
The plant kept all the actual operating parameters of the scrub tower constant during the test.
The cooling temperature of the LPG-recovery heat exchanger, which is an important variable, was raised in 0.5° C. increments to –39° C. from – 42.5° C. The scrub tower's feed temperature was held constant at about –26° C.
Ethane and butane reinjection into the main cryogenic heat exchanger was suspended during the test. Only excess propane from the depropanizer was intermittently reinjected, depending on the level in the depropanizer reflux drum.
Propane and butane recycle to the scrub tower was maintained to minimize the amount of cooling required. Recycle butane was also used to control the scrub tower overhead pentane specification.
Test monitoring
Plant operators closely monitored the scrub tower's overhead streams and the LNG product composition using laboratory analyses. Also, these parameters were monitored using laboratory analysis and data collection:
- Natural gas feed composition.
- Operating parameters of the scrub tower and LPG-recovery exchanger.
- LNG product HHV (calculated).
The test was limited to a low temperature of –40° C. because the ethane exceeded the specified limit for the LNG product. At temperatures greater than –39° C., the recycle stream to the scrub tower overhead system was increased to avoid scrub tower operation disturbances. Higher temperatures also cause the pentane to exceed the maximum specified limit in the scrub tower overhead.
Also, during the test period, the HHV of the plant feed gas was 9.993-10.019 million cal/cu m. The propane in the feed was 1.88-1.91 mole %.
Test results
Data obtained from the run test allowed operators to improve the HHV of the produced LNG at cooling temperatures –41° C. to –40° C., while optimizing the required LPG extraction at the scrub tower's bottom.
Table 1 shows the increase in LNG HHV vs. the increase in cooling temperature of the LPG-recovery heat exchanger.
At those temperatures, however, ethane in the LNG product exceeded the maximum specification due to the scrub tower reflux temperature.
Table 1 also shows that the ethane mole fraction increased with higher cooling temperatures in the LPG-recovery heat exchanger, resulting in off-specification values at temperatures of –40.5° C. and –40° C.
Additionally, the LPG components in the LNG product were in small quantities.
The propane in the produced LNG was less than the minimum specified limit of 2.2 mole % for all the temperature ranges (Table 1). This is due to the significantly low propane concentration in the feed natural gas, which was less than 1.91 mole %.
The variation in LNG propane content is due to the intermittent reinjection of propane into the feed to the main cryogenic heat exchanger. Butane contents were within specification limits, but in small quantities.
After calculating an optimum cooling temperature of the LPG-recovery heat exchanger of –40° C. to –41° C., the plant set this new operating temperature for all the trains.
Table 2 shows how the LNG HHV varied in 1998-2002. It also shows that before the cooling temperature review, the LNG HHV was less than the minimum specified limit.
Table 3 shows that the propane in the LNG was less than the minimum specified limit during the past 5 years. Also, the butane content could improve if heavy hydrocarbons were reinjected.
The author
Abdelouahab Madouri ([email protected]) is a process engineer for Sonatrach, Arzew, Algeria. He works in the technical department of Sonatrach's GL1-Z LNG plant. He has more than 16 years' experience in the LNG industry, including plant operations and technical services. Before joining GL1-Z, he worked in Sonatrach's GL2-Z plant as a process engineer in the operations department. He holds a degree (1986) in chemical engineering, specializing in LNG, from the Algerian Petroleum Institute.
