The invention relates to a process for preparing
natural gas by closed circulation and comprehensive use of synthetic
ammonia after-
tower gas. Low-pressure
methane synthesis system after-
tower gas isgenerated by a low-pressure
methane synthesis system and sent into a low-pressure cold box, and
nitrogen gas is injected into the low-pressure cold box at the same time, so that
methane is removed under a low pressure and at -85 DEG C to -57 DEG C; the remaining gas is compressed by a six-stage high-pressure
machine and sent to a synthetic
ammonia system for reaction to produce synthetic
ammonia,and along with the production of the synthetic ammonia, purge gas and recycle gas are combined to form synthetic ammonia after-
tower gas containing 3% of ammonia and 24% of methane; the synthetic ammonia after-tower gas is sent to a first high-pressure cold box, so that ammonia is removed under a
high pressure and at -109 DEG C to -73 DEG C; the remaining synthetic ammonia after-tower gas is sentto a second high-pressure cold box, and
nitrogen gas is injected into the second high-pressure cold box at the same time, so that methane is removed under a
high pressure and at -193 DEG C to -129 DEGC; and the methane is sent to a
natural gas storage tank for storage, and the remaining gas is sent to the synthetic ammonia
system through a circulation
machine to produce synthetic ammonia. The design is reasonable, the concept is ingenious, and the design purpose of one tower and two products is realized.