Young Han Kim
Dept. of Chemical Engineering, Dong-A University, Pusan, 604-714 Korea
(Published in Separation and Purification Technology, vol. 139, pp. 25-35, 2015)
When the common DWC processing three products is employed in an offshore LNG (liquefied natural gas) plant of three conventional distillation columns, the DWC can replace the last two columns. If a rectifying column is connected to the prefractionator of the Petlyuk column, all four products are processed from the side-rectifier Petlyuk column as illustrated in Fig. 1. The Petlyuk column is a separate structure of the DWC, but their operating principle is identical. In this configuration most of the lightest component is separated in the rectifier, which significantly reduces the separation load of the next column. Note that the LNG feed contains over 85% of methane.
For the practical implementation of the proposed column, a single column DWC as demonstrated in Fig. 2 can be constructed. Considering the space limitation and the harsh condition of operation of the offshore LNG plant, the single column DWC is suitable to the LNG plant. Though new separation techniques were developed recently, distillation has been the most reliable process for the natural gas separation. In addition the side-rectifier DWC has high distillation efficiency, because it has the same column structure as the common DWC. The energy efficiency of the DWC has been commercially proved and implemented throughout world chemical processes, especially over 100 columns are operated in Europe. The extended DWC having three walled sections in the middle is known to produce four products, but the control of product specification is more difficult than the common DWC of two sections. The difficulty explains why the extended DWC has not been commercialized, while the common DWC is being utilized in many field operations.
The integration of multiple binary columns into the DWC raises the thermodynamic efficiency for less energy demand, but the column operating pressure has to be equal for the all sections of the DWC in spite that the binary columns are operated at different pressures. When the pressure difference is not large, the integration can be applied to the complex distillation processes. However, the pressure limitation makes the extended DWC more difficult to implement than the common DWC. The former needs a single pressure from the three column pressures of the conventional system, while the latter does from two pressures. In addition to the difficulty of the selection of column operating pressure in the extended DWC, the column operating temperature is unfavorable to result in high utility cost. The difference of condenser temperature and reboiler temperature in the DWC is very large due to the single operating pressure. The separate column pressures in the conventional distillation system are manipulated for the low-cost utilities used in the condenser and reboiler.
Reduction of Utility Cost
Because the gas separation process in the LNG (liquefied natural gas) plant handles the feed having the components of large difference of boiling points, the separation is relatively easy but cryogenic cooling and high temperature boiling are necessary. In the conventional distillation system of the LNG plant the operating pressures of the multiple columns are adjusted between 0.3 MPa and 3 MPa to make the column operating temperature close to ambient temperature. The adjustment reduces the consumption of cryogenic refrigerant and high temperature steam. However, the number of columns is limited in the offshore operation of the LNG plant, where the DWC is a good alternative for the operation due to its small column number in spite of the requirement of high-cost utility.
For the reduction of the utility cost in the DWC, diabatic distillation can be applied, which has in-tray heat exchangers to disperse the high-cost heat duties of the reboiler and condenser. The duty of cryogenic cooling was partially reduced by applying two inter-coolers to the DWC. The heat transfer at the in-tray heat exchangers is driven by lower temperature difference than the adiabatic distillation. The heat transfer not only lowers the heat duty at the condenser and reboiler, but also reduces the exergy loss in the distillation process. But in practice, the installation and maintenance of the in-tray heat exchangers in every tray are difficult.
The raw natural gas from gas well contains liquid components of oil and water, which are separated through the multiple separators having successively reduced pressures up to 20 kPa to maximize gas recovery and to stabilize the crude oil. The processed gas is compressed to high pressure to minimize the number of processing equipment in the next stage. The feed components are of wide variation of volatility, but methane is more than 86 %. The first column in the distillation system processes the most of the feed because of the large amount of methane, and therefore a specially designed distillation system can be applied in the processing. Though the onshore separation system utilizes five columns, the number of the columns is limited in the offshore operation. A three-column offshore process is conventionally utilized. The first two columns with recycle produce the mixture of methane and ethane as a typical LNG fuel. The last column produces LPG and C5+ components to be fed to hydrocarbon chemical processes.
When the prefractionator of the Petlyuk column accommodates a small column on its top, the pretreatment column for the common DWC can be eliminated to make the whole distillation system a single column. In practice the proposed column becomes a side-rectifier DWC as illustrated in Fig. 2. Because the offshore operation requires the compactness of equipment due to limited space and harsh environment, the side-rectifier DWC is a good candidate for the LNG plant if the column operating pressure is properly selected. Due to the single operating pressure in the DWC unlike the conventional system of multiple column pressures, the condenser temperature is lower than that of the conventional system and its reboiler temperature is higher. Therefore, the in-tray heat exchangers are installed to reduce the consumption of high cost utilities for the condenser and reboiler.
As indicated above the column pressure is set to the lowest pressure among the conventional three columns, which gives the condenser temperature much lower than the conventional system. However, its heat duty is minimized for the reduced use of high-cost refrigerant by employing in-tray heat exchangers. The amount of C5+ product was 15 % more than the conventional system, while LPG was produced 15 % less. Note that the price of C5+ product is higher than that of LPG product. The feed pressure in the DWC is much lower than the conventional system, and therefore the cooling load is about half of the conventional system due to less temperature elevation from the feed compression. In addition the compression of recycle stream from the depropanizer is not necessary giving less investment and utility costs in the DWC. Due to the improved thermodynamic efficiency of the DWC, the heating and cooling duties of the side-rectifier DWC including the in-tray heat exchangers are 5.9 % and 5.1 % less than the conventional system, respectively.
The economics between the conventional distillation system and the side-rectifier DWC is compared in terms of investment and utility costs. The investment cost includes the costs of column, trays, and heat exchangers. The utility costs of steam and refrigerant depend on temperature. The investment of the rectifier DWC is less than 43 % of the conventional system. The large difference comes from the high pressure operation at the demethanizer in the conventional system. The pressure requires additional cost in the construction of column and heat exchangers, and the investment of the compressor used for recycling the overhead product of the depropanizer is added. The utility cost of the DWC is 10 % more than that of the conventional system because of the lower cooling temperature requiring high-cost refrigerant. The significant reduction of the investment cost compensates the utility cost increase. The critical issue in the economics between the conventional system and the rectifier DWC is the column operating pressure. Employing high column pressure of 7 MPa in the conventional system requires a feed compressor of the cost more than 5.1 times the total investment of other equipment in the conventional system and the electricity cost of 1.8 times the total utility cost. While the feed pressure in the conventional system is 7 MPa, the pressure of the rectifier DWC is 1 MPa. As the production of raw natural gas from the well continues, the gas pressure drops significantly and the compression of the raw gas is required in the conventional system. When a payback time of 5 years is applied, the TAC (total annual cost) of the compressor is 2.5 times that of the conventional system.
The offshore operation requires compact equipment due to the severe condition of environment. The side-rectifier DWC can be constructed in a single reboiler column by combining the rectifier and DWC like a top DWC having two condensers and one reboiler. Though there are two dividing walls, one at top and the other in the middle, the whole system is placed in a column. The system of demethanizer and a DWC has two condensers and two reboliers making two complete column system. Compared with the system of demethanizer and DWC, the costs of investment and utility of the rectifier DWC are 57 % less and 3 % more, respectively. The thermodynamic efficiency of the system of demethanizer and DWC is 0.9 % point higher than that of the rectifier DWC.
Low Pressure Operation
The compact structure of distillation column for offshore LNG processing is one advantage of the proposed side-rectifier DWC. The other benefit is low pressure operation. The gas pressure at the well decreases up to 20 kPa as the production proceeds, and the pressure is boosted to 7 MPa before feeding to the conventional gas plant. High pressure and low temperature operation generates gas hydrates in the transportation lines, and hydrate inhibitors are used to prevent plugging pipes due to the solid hydrates. Moreover the well gas is almost always saturated with water. However, the plugging is less likely in the low pressure operation, and no heating is necessary before entering the separation process. The low pressure operation requires less investment cost because of thin vessel wall and small gas compressor used for the feed compression. In addition, less electricity is consumed resulting in low utility cost. The pressure of overhead product from the demethanizer in the conventional system has to be lowered for shipping to the LNG carrier. The product, mostly methane, is liquefied at a temperature about – 160 C at atmospheric pressure for the transportation by the LNG carrier. Therefore, the compression in the LNG processing is not necessary. Though the transportation and storage require smaller volume in the high pressure operation, the investment and electricity costs are greatly saved in the low pressure operation.
The application of the common DWC in field operation has been successful for many years, but the operation of the DWC is more difficult than the conventional distillation column. It is the main concern of the field operators considering the DWC adoption. The operation of the side-rectifier DWC in this study is expected to be similar to the DWC operation. The overhead product from the rectifier can be separately adjusted using the reflux flow to the rectifier. The gain of product mole fraction from the applied change of manipulated variables indicates that the conventional system shows the best control performance. However, the performance of the rectifier DWC is better than that of the existing pre-column DWC. In the control of C1 product specification the ratio of the change of methane mole fraction to the variation of reflux flow rate is the gain, which indicates the ease of the process operation. The larger the gain is, the easier the process manipulation is. Though the conventional system has much larger gain than the DWCs, the rectifier DWC has about three times the gain of the pre-column DWC. For LPG production the rectifier DWC shows the improvement by one third over the pre-column DWC. In case of C5+ production the rectifier DWC gives about twice better performance than the pre-column DWC. The difficulty of products control in the proposed DWC is alleviated by applying a sufficient margin of specification in the column design. In addition the adjustment of heat duty at the in-tray heat exchangers manipulates the internal vapor and liquid flows in the column, which provides more degrees of freedom in the column control.
Fig. 1. Schematic diagram of the side-rectifier Petlyuk column.
Fig. 2. Single column DWC for LNG plant.