Mug drying

Mug drying

While most ammonia synthesis loops have already undergone ammonia converter retrofit and are equipped with hydrogen recovery units, there are now very few ways to increase further their performance in terms of production capacity and/or energy saving. One of the possible improvement for Kellogg type plants consists in re-arranging the synthesis loop for dry loop operation and send directly the fresh synthesis gas (called Make-Up Gas or MUG in this document) from the 4th stage of synthesis gas compressor to the ammonia converter 105-D instead of passing it through the ammonia condensing unit 117-C. Because the MUG contains CO2 and H2O which are poisons for the ammonia synthesis catalyst, this requires removing these contaminants prior to the MUG feed to the ammonia converter. Molecular sieves were used on a large scale in USA and in few plants in Western Europe to remove these contaminants but they require extensive maintenance to ensure the correct operation and tightness of HP block valves and the adsorbent material needs to be changed typically every 2 years. Furthermore the molecular sieves proved to be the source of many accidents in ammonia plants and are no more regarded as the preferred option. CEAMAG offers a replacement technology to the molecular sieves with its MUG drying technology based on ammonia wash principle. The aim of this document is: To explain the benefits that can be obtained with dry synthesis loop operation and to describe the modifications involved. To describe the CEAMAG MUG drying technology WHY OPERATE WITH DRY SYNTHESIS LOOP ? Original kellogg design In original KELLOGG design for 1360 MTPD ammonia plants, the MUG is compressed to the pressure of the synthesis loop in the 4 stages of synthesis gas compressor 103-J. The compressed gas is cooled in heat exchanger 124-C and subsequently mixed with the re-circulation gas of the synthesis loop discharged by the recirculation stage of the synthesis gas compressor 103-J. The mixed gas then flows through the chilling condenser 117-C where water is condensed together with the ammonia produced in ammonia converter 105-D and CO2 is absorbed by this liquid ammonia. Dry gas is then re-heated through a series of gas-gas heat exchanger 179-C and 121-C and fed to the ammonia converter 105-D. This arrangement has following drawbacks: The chilling condenser 117C must remove the sensible heat from the Make Up Gas and cool it down from 35°C to –4°C and this loads the refrigeration compressor 105-J. Also the MUG creates pressure drop in chiller 117-C, separator 106-F and gas/gas heater 179-C and this puts on additional absorbed power on the synthesis gas circulator 103-J5. Because chiller 117-C is located at discharge of circulator 103-J5, the chiller 117-C must also remove the compression heat from the circulation gas. The synthesis loop purge is located at the outlet of KO drum 126-F and the purge gas therefore contains 7 mol% of NH3. This high ammonia content makes necessary the chilling of purge gas in heat exchanger 125-C. This chilling consumes refrigeration ammonia and loads the refrigeration compressor 105-J. benefits with dry synthesis loop operation The idea is to dry the Make Up Gas (MUG) between the second and the third compression stages of the synthesis gas compressor 103-J. The MUG is then free of elemental oxygen associated with water and carbon oxides and it can be routed directly to the ammonia converter 105-D after preheating in gas/gas heat exchanger 121-C. This solution cancels the drawbacks of original KELLOGG design listed above and brings the following benefits: The MUG is not chilled anymore in chilling condenser 117-C and the later is relieved The ammonia content in the blend gas to converter 105-D is reduced by 0.3 %mol compared to the original Kellogg design. This reduced ammonia content in inlet gas allows for a more efficient ammonia synthesis in converter 105-D and the synthesis loop operation pressure can therefore be decreased while capacity is maintained. Chiller 117-C and separator 106-F can be relocated upstream from circulator 103-J5 to carry out ammonia condensation before the circulation gas is recompressed. In this way the chiller 117-C does not need to remove the compression heat anymore and refrigeration compressor 105-J is further relieved. Because ammonia is removed from circulation gas prior to the circulator, the gas circulation rate through the synthesis converter can be maintained by the circulator 103-J5 running at lower speed. As a consequence of this lower speed the pressure at converter 105-D inlet is reduced and by way of consequence power absorbed by synthesis gas compressor 103-J is also reduced. As a result of all gains listed above, less HP steam is required to drive the synthesis gas compressor turbine 103-JT and less MP steam is required to drive the refrigeration compressor turbine 105-JT. Side benefits of MUG drying installation: Clearing of water and CO2 from MUG allows for production of Low Water Content Ammonia (LWCA) in the synthesis loop. LWCA can be sold with higher value for other applications such as pharmaceuticals... For large capacity increase projects, it must also be noted that a supplementary ammonia converter can be installed between the 2nd and 3rd stage of synthesis compressor 103-J, this converter processes synthesis gas containing very little inert gas and can therefore achieve NH3 concentration of more than 22% with reduced catalyst volume. DESCRIPTION OF CEAMAG MUG DRYING TECHNOLOGY Principle of Ceamag MUG drying system is shown below: The technology requires only static equipment and therefore does not generate maintenance costs. MUG chilling MUG coming out from the second stage of synthesis gas compressor 103-J at about 90 bar is first cooled down to 8°C in the existing inter-stage coolers 116-C and 129-C and water is separated from gas in the existing KO drum 123-C. MUG then flows to the tube side of a new heat exchanger E1 where it is chilled to –27°C. But prior to its entry in heat exchanger E1, a small quantity of liquid ammonia is sprayed in the MUG in order to avoid any ice building in E1. Mixing with liquid ammonia The chilled gas is then routed to a special high efficiency static mixer J1 where it is mixed with liquid ammonia Ammonia injection Liquid ammonia is taken from KO drum 126-F at 21°C and 220 bar and flows to the static mixer J1 with the help of the pressure difference (220 bar in 126-F and 90 bar in J1). Because of this pressure difference a major part of liquid ammonia flashes in the static mixer and the gas temperature goes further down to –30°C. The quantity of injected liquid ammonia is controlled so that a part of the injected ammonia remains in liquid phase after the flash. This excess is the guarantee that sufficient ammonia was injected and that water and CO2 have been efficiently absorbed. Mixing In theory water and CO2 in the gas are instantly absorbed by liquid ammonia, in reality a high mixing quality and a minimum residence time are necessary to allow for complete absorption of CO2 in liquid NH3. The main features of static mixer J1 are as follows: Very low pressure drop (less than 0.15 bar) – the overall pressure drop of the MUG drying system is critical because any pressure drop between the stages of compressor 103-J increases the absorbed power and reduces the benefits of dry loop operation. Mixing elements consist of special trapezoidal tabs configured to promote a “natural” mixing pattern through controlled vortex action, as a consequence the effective mixing is carried out with virtually no loss of pumping energy (minimum presssure loss) The low profile of mixing elements maintains maximum open area and reduces fouling tendencies Mixture quality higher than 99%. The mixture quality is characterized by Sigma/x with: Sigma: the standard deviation of ammonia concentration in any samples taken at different radial positions at a fixed axial location downstream of the mixer X: the volume fraction of additive (in this case the liquid ammonia) in total flow Despite the high efficiency of static mixer J1, CEAMAG knows by experience that the CO2 absorption process in liquid ammonia requires a minimum residence time. Consequently a minimum pipe length is provided downstream the static mixer before the liquid ammonia is separated from the gas. Carbamate formation Gas with low content of ammonia and carbon dioxide tends to form ammonia carbamate according to the following reaction (favoured by low temperature and high pressure) : 2NH3+CO2 <=> NH4CO2NH2 The major part of CO2 normally reacts with ammonia injected in the first stages of compressor 103-J and carbamate formed is removed with water in KO drum 123-F. Formation of carbamate after KO drum 123-F is still expected because of remaining CO2 but CEAMAG’s design accounts for the problem of carbamate formation in the MUG drying unit and this problem was never encountered with normal methanator operation. Gas / liquid separation Liquid/gas mixture enters the new KO drum S1 where the separation of phases is achieved. Separation in vane pack The KO drum is provided with a special separator installed in vertical position in front of the inlet nozzle. This separator is a vane pack made of several vanes. The gas / liquid mixture enters the separator and snakes around the vanes positioned in series. At each vane, a part of the liquid is separated from the gas and flows down to the bottom of the vane separator and from there down to the liquid via a downcomer pipe. After crossing all the vanes, the gas exits the KO drum S1 through a nozzle positioned in front of the vertical vane pack. The number of vanes is defined to achieve a high separation efficiency of 99.9%. The high separation efficiency aims to reduce the amount of ammonia carried over by the gas because the benefit of dry loop operation would be very much affected (even reduced to zero) in case of high ammonia content in MUG. Moreover each droplet of liquid ammonia carried over contains some water that may poison the synthesis converter catalyst. Protection of the third compression stage of 103-J is not a critical matter as gas coming out from KO drum S1 is reheated in heat exchanger E1 and any ammonia droplet carried over would vaporize. Liquid ammonia level in KO drum S1 Liquid ammonia recovered at the bottom of the vane pack contains the water and CO2 brought by the MUG. It flows down to the bottom of KO drum S1 through the downcomer pipe. Liquid level is controlled by extracting ammonia to KO drum 107-F operating at 17 bar. A flowmeter is installed on the liquid transfer line to ensure that liquid ammonia is supplied in excess to the MUG drying unit. Saturated gas leaving the KO drum S1 The MUG leaving KO drum S1 is cleared of any droplets, water and CO2 but is saturated with ammonia (1.6% mol/mol at –30°C). As one can see, the gas/liquid temperature in KO drum S1 is a critical parameter for the efficiency of dry loop operation because a high temperature would lead to a high ammonia content in MUG and would affect the overall energy saving. This target temperature is an important optimization parameter and may be modified by CEAMAG in regard to specific operating conditions of CUSTOMER’s plant. Gas re-heating Saturated gas leaving the KO drum S1 is re-heated to –3°C in the shell side of heat exchanger E1. Dry MUG is then directed to the 3rd compression stage of synthesis gas compressor 103-J. Oil filter before MUG injected in the loop In order to protect the synthesis catalyst, it is recommended to install a special oil filter having a very high separation efficiency. When the synthesis compressor is fit with dry sealing, this oil filter is not necessary. TYPICAL PERFORMANCE AND PROFITABILITY Energy saving For a Kellogg unit with nameplate capacity 1360 mtpd and operating between 1500 and 1700 tpd, the in the following conditions: Energy saving : 0.085 Gcal/t NH3