[ Diff(x[1], t) = (-1)*k[1]*k[2]*(x[1] - x[2])/piecewise(t <= k[12], 500, 400),
  Diff(x[2], t) = (k[1]*k[2]*(x[1] - x[2]) - k[1]*k[2]*(x[2] - x[3]) - k[3]*x[2]*piecewise(t <= k[12], 33/100, 33/100) - k[4]*x[2]*piecewise(t <= k[12], 33/100, 33/100))/piecewise(t <= k[12], 33/100, 33/100),
  Diff(x[3], t) = (k[1]*k[2]*(x[2] - x[3]) - k[7]*x[3]*piecewise(t <= k[12], 380, 280) - k[11]*x[3]*piecewise(t <= k[12], 380, 280))/piecewise(t <= k[12], 380, 280),
  Diff(x[4], t) = (-1)*k[9]*k[2]*(x[4] - x[5])/piecewise(t <= k[12], 500, 400),
  Diff(x[5], t) = (k[9]*k[2]*(x[4] - x[5]) - k[9]*k[2]*(x[5] - x[6]) + k[3]*x[2]*piecewise(t <= k[12], 33/100, 33/100))/piecewise(t <= k[12], 33/100, 33/100),
  Diff(x[6], t) = (k[9]*k[2]*(x[5] - x[6]) + k[7]*x[3]*piecewise(t <= k[12], 380, 280))/piecewise(t <= k[12], 380, 280),
  Diff(x[7], t) = (-1)*k[10]*k[2]*(x[7] - x[8])/piecewise(t <= k[12], 500, 400),
  Diff(x[8], t) = (k[10]*k[2]*(x[7] - x[8]) - k[10]*k[2]*(x[8] - x[9]) + k[4]*x[2]*piecewise(t <= k[12], 33/100, 33/100))/piecewise(t <= k[12], 33/100, 33/100),
  Diff(x[9], t) = (k[10]*k[2]*(x[8] - x[9]) + k[11]*x[3]*piecewise(t <= k[12], 380, 280))/piecewise(t <= k[12], 380, 280) ]